Intraoperative Neurophysiological Monitoringv This book is based on two earlier works: Aage R. Møller: Evoked Potentials in Intraop- erative Monitoring published in 1988 by Will-

Intraoperative Neurophysiological MonitoringSecond Edition

IntraoperativeNeurophysiological Monitoring

Second Edition

Aage R. Møller, PhD

University of Texas at DallasDallas, TX

© 2006 Humana Press Inc.999 Riverview Drive, Suite 208Totowa, New Jersey 07512

www.humanapress.com

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by anymeans, electronic, mechanical, photocopying, microfilming, recording, or otherwise without written permission from thePublisher.

All papers, comments, opinions, conclusions, or recommendations are those of the author(s), and do not necessarily reflect theviews of the publisher.

This publication is printed on acid-free paper. ∞ANSI Z39.48-1984 (American Standards Institute)Permanence of Paper for Printed Library Materials.

Production Editor: Jennifer Hackworth

Cover design by Patricia F. Cleary

For additional copies, pricing for bulk purchases, and/or information about other Humana titles, contact Humana at the aboveaddress or at any of the following numbers: Tel.: 973-256-1699; Fax: 973-256-8341; E-mail: [email protected]; or visitour www.humanapress.com

Photocopy Authorization Policy:Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted byHumana Press, provided that the base fee of US $30.00 per copy is paid directly to the Copyright Clearance Center at 222Rosewood Drive, Danvers, MA 01923. For those organizations that have been granted a photocopy license from the CCC, aseparate system of payment has been arranged and is acceptable to Humana Press Inc. The fee code for users of the TransactionalReporting Service is: [1-58829-703-9/06 $30.00].

eISBN: 1-59745-018-9

Printed in the United States of America. 10 9 8 7 6 5 4 3 2 1

Library of Congress Cataloging-in-Publication Data

Møller, Aage R.Intraoperative neurophysiological monitoring / Aage R. Møller. -- 2nd ed.

p. cm.Includes bibliographical references and index.ISBN 1-58829-703-9 (alk. paper) 1. Neurophysiologic monitoring. 2. Evoked potentials (Elecrophysiology) I. Title.

RD52.N48M65 2006617.4'8--dc22

2005050259

v

This book is based on two earlier works:Aage R. Møller: Evoked Potentials in Intraop-erative Monitoring published in 1988 by Will-iams and Wilkens; and more directly by Aage RMøller: Intraoperative Neurophysiologic Moni-toring published in 1995 by Gordon and Breachunder the imprint of Harwood academic publish-ers. The present book represents an expansionand extensive rewriting of the 1995 book. In par-ticular, new chapters related to monitoring of thespinal motor system and deep brain stimulation(DBS) have been added. The anatomical andphysiological basis for these techniques aredescribed in detail as are the practical aspects ofsuch monitoring. Chapters on monitoring of sen-sory systems and monitoring in skull base sur-gery have been rewritten as has the chapter onmonitoring of peripheral nerves.

The general principles of intraoperativemonitoring are discussed in Section I whereChapter 2 describes the basis for intraoperativemonitoring and Chapter 3 discusses the variousforms of electrical activity that can be recordedfrom nerve fibers and nerve cells; near-field ac-tivity from nerves, nuclei, and muscles recordedwith monopolar and bipolar electrodes. Thischapter also discusses far-field potentials and theresponses from injured nerves and nuclei. Chap-ter 4 discusses practical aspects of recordingevoked potentials from nerves, nuclei, and musclesincluding a discussion of various stimulus tech-niques.

Section II covers sensory systems. Chapter 5covers the anatomy and physiology of the audi-tory, somatosensory and visual systems. Moni-toring of the auditory system is covered inChapter 6; Chapter 7 covers monitoring thesomatosensory system and Chapter 8, monitor-ing the visual system.

Section III discusses motor systems. Theanatomy and physiology that is of interest forintraoperative monitoring is discussed in Chap-ter 9 and practical aspects of the spinal motorand brainstem motor systems are covered inChapters 10 and 11, respectively.

Section IV is devoted to peripheral nerves,and Chapter 12 covers the anatomy and physiol-ogy, whereas Chapter 13 discusses practical as-pects of monitoring peripheral nerves.

Section V discusses different ways that intra-operative electrophysiological recordings canguide the surgeon in an operation. Chapter 14discusses methods to identify motor and sensorynerves and map the spinal cord and the floor ofthe fourth ventricle. Chapter 15 describes meth-ods that can guide the surgeon in an operation,such as microvascular decompression operationsfor hemifacial spasm and placement of elec-trodes for DBS and for making lesions in thethalamus and basal ganglia.

Section VI discusses practical aspects of in-traoperative monitoring. Chapter 16 covers therole of anesthesia in monitoring and Chapter 17discusses general matters regarding monitoringsuch as how to reduce the risk of mistakes andhow to reduce the effect of electrical interfer-ence of recorded neuroelectrical potentials.Chapter 18 discusses equipment and dataanalysis related to intraoperative monitoring.This chapter also discusses electrical stimulationof nervous tissue. The final chapter, Chapter 19discusses the importance of evaluation ofthe benefits of intraoperative neurophysiologi-cal monitoring, to the patient, the surgeon, andthe field of surgery in general.

Preface

Aage R. Møller

Acknowledgments

I have had valuable help from many individuals in writing this book. Mark Steckert, MD, PhD,provided important comments on several aspects of this second edition.

I want to thank Hilda Dorsett for preparing much of the new artwork and for revising some of theillustrations from the first edition of the book. I thank Renee Workings for help with editing themanuscript and Karen Riddle for transcribing many of the revisions of the manuscript.

I also want to thank Richard Lansing and Jennifer Hackworth, production editor, of Humana Pressfor their excellent work on the book.

I would not have been able to write this book without the support from the School of Behavioraland Brain Sciences at the University of Texas at Dallas.

Last but not least I want to thank my wife, Margareta B. Møller, MD, PhD, for her support duringwriting of this book and for her valuable comments on earlier versions of the book manuscript.

Aage R. Møller

vii

ix

Contents

Preface ..................................................................................................................................... vAcknowledgments ..................................................................................................................vii

1 Introduction ..................................................................................................................... 1

SECTION I: PRINCIPLES OF INTRAOPERATIVE NEUROPHYSIOLOGICAL MONITORING

2 Basis of Intraoperative Neurophysiological Monitoring .................................................. 93 Generation of Electrical Activity in the Nervous System and Muscles .......................... 214 Practical Aspects of Recording Evoked Activity From Nerves,

Fiber Tracts, and Nuclei ............................................................................................ 39References to Section I .......................................................................................................... 49

SECTION II: SENSORY SYSTEMS

5 Anatomy and Physiology of Sensory Systems ................................................................ 556 Monitoring Auditory Evoked Potentials ......................................................................... 857 Monitoring of Somatosensory Evoked Potentials ......................................................... 1258 Monitoring of Visual Evoked Potentials ....................................................................... 145References to Section II ....................................................................................................... 147

SECTION III: MOTOR SYSTEMS

9 Anatomy and Physiology of Motor Systems ............................................................... 15710 Practical Aspects of Monitoring Spinal Motor Systems ............................................... 17911 Practical Aspects of Monitoring Cranial Motor Nerves ............................................... 197References to Section III ...................................................................................................... 213

SECTION IV: PERIPHERAL NERVES

12 Anatomy and Physiology of Peripheral Nerves ........................................................... 22113 Practical Aspects of Monitoring Peripheral Nerves ..................................................... 229References to Section IV ..................................................................................................... 233

SECTION V: INTRAOPERATIVE RECORDINGS THAT CAN GUIDE THE SURGEON IN THE OPERATION

14 Identification of Specific Neural Tissue ....................................................................... 23715 Intraoperative Diagnosis and Guide in Operations ..................................................... 251References to Section V ...................................................................................................... 273

SECTION VI: PRACTICAL ASPECTS OF ELECTROPHYSIOLOGICAL RECORDING IN THE OPERATING ROOM

16 Anesthesia and Its Constraints in Monitoring Motor and Sensory Systems ................. 27917 General Considerations About Intraoperative Neurophysiological Monitoring .......... 28318 Equipment, Recording Techniques, Data Analysis, and Stimulation ........................... 29919 Evaluating the Benefits of Intraoperative Neurophysiological Monitoring .................. 329References to Section VI ..................................................................................................... 339

Appendix ............................................................................................................................. 343Abbreviations ...................................................................................................................... 347Index ................................................................................................................................... 349

x Contents

Surgery can generally be regarded as a risk-filled method for treating diseases, and it has apotential for causing injury to the nervous sys-tem. Because such injuries might not be detectedby visual inspection of the operative field by thesurgeon, they could occur and progress withoutthe surgeon’s knowledge. Intraoperative neuro-physiological monitoring involves the use ofneurophysiological recordings for detectingchanges in the function of the nervous systemthat are caused by surgically induced insults.

Intraoperative recording of neuroelectricpotentials makes it possible to assess functionnearly continuously throughout an operation.Although evoked potentials are important inmaking clinical diagnoses, there are often alter-native methods available to obtain the requiredinformation in the clinical setting, such as imag-ing modalities (computed axial tomography[CAT] and magnetic resonance imaging [MRI]),which have made evoked potentials and otherelectrophysiological studies less important forclinical diagnosis of neurological disorders.However, although the CAT scan is available ina few operating rooms (mainly for stereotaxicsurgery and biopsy), it is not practical for moni-toring neural injuries, at least not yet. Imagingmethods mainly detect changes in structures,whereas neurophysiological methods assesschanges in function, therefore providing obviousadvantages for intraoperative monitoring.

Appropriate use of intraoperative recordingof various types of neuroelectric potential makesit possible to assess the function of specific partsof the nervous system continuously during anoperation and detect changes in neural functionwith little delay. Early detection of such func-tional changes can reduce the risk of postopera-tive deficits caused by iatrogenic injuries to thenervous system. These methods makes it possi-ble to identify which specific surgical step has

caused a problem so that the surgeon can reversethe step that caused the injuries before theybecome severe enough to result in permanentneurological deficits.

The benefits to the patient and to the surgeonof using appropriate neurophysiological monitor-ing methods during operations in which neuraltissue is at risk of being injured are well recog-nized, and intraoperative neurophysiologicalmonitoring is now widely practiced in many hos-pitals in connection with such operations. Indi-viduals on the neurophysiological monitoringteam are now accepted as members of theoperating room team. Although monitoring ofpatients’ vital signs in the operating room hasbeen done for many years, monitoring the func-tion of the nervous system is a relatively newaddition to the operating room and it has a widerrange of applications than just the monitoringfunction.

During the late 1970s and early 1980s, theapplication of electrophysiological methods inthe operating room was primarily focused withinuniversity centers and a few large hospitals. Itsoon became evident that standard laboratorytechniques transplanted to the operating roomcould reduce the risk of inadvertently injuringneural tissue and thereby reduce the risk of per-manent neurological deficits. This new use ofstandard laboratory techniques became known asintraoperative neurophysiological monitoring.

Routine use of intraoperative neurophysio-logical monitoring developed during the 1980s,and during that time, intraoperative neurophys-iological monitoring got its own society in theUnited States (the American Society for Neuro-physiological Monitoring [ASNM]).

Although it is assumed that the era of intra-operative neurophysiological monitoringstarted in the late 1970s, electrophysiologicalmethods were used in the operating room forthe purpose of reducing the risk of permanentneurological deficits even before that time. Inthe early 1960s, monitoring of the facial nerve

1In t roduc t i on

1

From: Intraoperative Neurophysiological Monitoring: Second EditionBy A. R. Møller © Humana Press Inc., Totowa, NJ.

was mainly done to reduce the risks of facialparesis or palsy after operations for vestibularschwannoma (1,2).

Leonid Malis, a neurosurgeon, used record-ings of evoked potentials from the sensory cor-tex in his neurosurgical operations. Malis,however, fascinated by the development ofmicroneurosurgery, stated later that microneu-rosurgery had made intraoperative monitoringunnecessary (3) although others expressed theopposite opinion in support of the usefulness ofintraoperative monitoring (4).

Orthopedic surgery was one of the first spe-cialties to make systematic use of intraoperativeneurophysiological monitoring, particularly inoperations involving the spine. In the 1970s,work by Dr. Richard Brown, a neurophysiolo-gist, reduced the risk of damage to the spinalcord during scoliosis operations by using record-ings of somatosensory evoked potentials (5,6),and intraoperative neurophysiological monitor-ing has been used for several decades for manyadditional types of neurosurgical operations (5).

Monitoring of auditory brainstem evokedresponses (ABRs) was also one of the earliestapplications of intraoperative neurophysiologi-cal monitoring and was used in microvasculardecompression (MVD) operations for hemi-facial spasm (HFS) and trigeminal neuralgiapioneered by Grundy (7) and Raudzens (8) inthe early 1980s and others (9,10) thereafter.Direct recordings from the exposed intracranialstructures such as the eighth cranial nerve andthe cochlear nucleus decreased the time to get aninterpretable record (11,12). Such recordingshad been used earlier for research purposes (13).

In the 1980s, intraoperative neurophysiolog-ical monitoring was introduced in operationsfor large skull base tumors (14,15) and later byother investigators (16). Intraoperative neuro-physiological monitoring for such operationscould involve monitoring of cranial motornerves, including CN III, IV, and VI, especiallyfor tumors involving the cavernous sinus, andthe motor portion of CN V (portio minor).

Later, intraoperative monitoring of the func-tion of the ear and the auditory nerve came intogeneral use by neurosurgeons and its use

spread to other surgical specialties, such asotoneurological surgery and to plastic surgery,where it serves mainly to preserve the functionof peripheral nerves.

The spread of the use of intraoperative neuro-physiological monitoring to other types of hos-pital came in the beginning of the 1990s whenalso certification processes were established bythe American Board for NeurophysiologicalMonitoring, (ABNM) that certifies Diplomats ofthe American Board for NeurophysiologicalMonitoring (DABNM). Certification in Neuro-physiological Intraoperative NeurophysiologicalMonitoring (CNIM) is available through theAmerican Board of Registration of Electroen-cephalographic and Evoked Potential Technolo-gists (ABRET).

While the techniques that were used in thebeginning of the era of intraoperative neuro-physiological monitoring were transplantedfrom the animal laboratories, the increased useof intraoperative neurophysiological monitor-ing promoted the development of specializedtechniques to become commercially availableby several companies.

Methods for monitoring of spinal motor sys-tems advanced during the 1990s with the devel-opment of techniques using magnetic (17) andelectrical stimulation (18) of the motor cortexand stimulation of the spinal cord (19). Methodsthat provided satisfactory anesthesia and alsopermitted activation of motor system by stimula-tion of the motor cortex were developed (20,21).

Intraoperative neurophysiological monitoringis an inexpensive and effective method for reduc-ing the risk of permanent postoperative deficits inmany different operations where nervous tissueis being manipulated. It provides real-time mon-itoring of function to an extent that makes itsuperior to imaging methods that provide infor-mation about structure and that are impracticalfor use in the operating room. Intraoperative neuro-physiological monitoring relates to the spirit ofthe Hippocratic oath: namely “Do no harm.” Wemight not be able to relieve suffering from ill-ness, but we should at least not harm the patientin our attempts to relieve the patient from illness.Intraoperative neurophysiological monitoring

2 Intraoperative Neurophysiological Monitoring

provides an example in medicine and surgeryof improvements accomplished specifically byreducing failures and, thus, improving perform-ance by reducing failures, a principle that isnow regarded with great importance in the designof complex applications, such as in militaryprocedures and space exploration.

Although the greatest benefit of intraoperativeneurophysiological monitoring is that it providesthe possibility to reduce the risk of postoperativeneurological deficits, it can also be of great valueto the surgeon by providing other informationabout the effects of the surgeon’s manipulationsthat is not otherwise available. Intraoperativerecordings of neuroelectric potentials can helpthe surgeon identify specific neural structures,making it possible to determine the location ofneural blockage on a nerve. Intraoperative neuro-physiological recordings can often help the sur-geon carry out the operation and, in some cases,to determine when the therapeutic goal of theoperation has been achieved. Intraoperative neuro-physiological monitoring can often give the sur-geon a justified increased feeling of security.

We are now seeing the beginning of an eraof treatment of certain movement disorders andsevere pain that moves away from the use ofmedications and toward the use of complexprocedures such as deep brain stimulation(DBS) and other forms of functional interven-tion, some of which involve prompting theexpression of neural plasticity.

Using neurophysiological methods is criti-cal for treatments using DBS and selectivelesioning of brain tissue for treating movementdisorders and severe pain. The obvious advan-tage of such procedures as DBS and selectivelesions is that the treatment is directed specifi-cally to structures that are involved in produc-ing the symptoms, whereas other generalmedical (pharmaceutical) treatment, even whenapplied in accordance with the best knownexperience, is much less specific and often hassevere side effects and limited beneficial effect.Although any licensed physician can prescribeany drug, even such drugs that have complexactions and known and unknown side effects,procedures such as DBS can only be done, at

least adequately, by teams of experts thatinclude members with a thorough understand-ing of neuroscience and the pathophysiology ofthe disorders that are to be treated.

There is little doubt that the use of proce-dures such as DBS will expand to include disor-ders that are currently treated with medicationalone. The implementation of stimulation treat-ments will be broadened, consequently increas-ing the demands on neurosurgeons who performthese procedures, as well as neurophysiologistswho are providing the neurophysiological guid-ance for proper placement of such stimulatingelectrodes.

Neurophysiology in the operating room alsoprovides an opportunity for research and studyof the normal function of the human nervoussystem as well as the function of the diseasednervous system. In fact, use of neurophysiol-ogy in the operating room for research waspracticed before it came into general use forintraoperative monitoring. For the neurophysi-ologist, the operating room offers possibilitiesfor research that are otherwise not available.Performing studies on patients undergoing neu-rosurgical operations often makes it possible todo intracranial recordings in a unique way toexamine the normal functions of parts of thenervous system that are not affected by the dis-order for which the patient is undergoing theoperation. Electrophysiological recording dur-ing operations also offers unique possibilitiesto study the pathophysiology of diseaseprocesses, because it is possible to record elec-trical activity directly from the parts of thenervous system that are affected by the disease.

There are two kinds of research that can bedone in the operating room. The first is basicresearch, the purpose of which is to gain newknowledge but no direct benefit to patients isexpected. However, experience has taught usthat even basic research can provide (unex-pected) immediate as well as long-term benefitto patient treatment. The other kind of research,applied research, has as its aim to provideimmediate improvement of treatment, includingreduction of postoperative deficits. This meansthat both types of research can be beneficial to

Chapter 1 Introduction 3

the patients either in providing better therapeu-tic achievements or by reducing the risk of post-operative permanent neurological deficits.

There are several advantages of doingresearch in the operating room. Humans are dif-ferent from animals and the results are directlyapplicable to humans. Second, but not least, it iseasier to study the physiology of diseased sys-tems in humans than trying to make animalmodels of diseases. Humans can respond andtell you how they feel, which is an advantagewhen evaluating results of, for instance, effortsto reduce postoperative deficits.

Research in the operating room has a longerhistory that intraoperative neurophysiologicalmonitoring. One of the first surgeons-scientistswho understood the value of research in theneurosurgical operating room was WilderPenfield (1891–1976), who founded the Mon-treal Neurological Institute in 1934. Penfieldwas a neurosurgeon who had a solid backgroundin neurophysiology, inspired by Sherringtonduring a Rhodes Scholarship to Oxford. Hestated that, “Brain surgery is a terrible profes-sion. If I did not feel it will become different inmy lifetime, I should hate it,” (1921). Penfieldmight be regarded as the founder of intraopera-tive neurophysiological research and he didground-breaking work in many areas of neuro-science. His work on the somatosensory systemis especially known (22,23). In the 1950s, heused electrical stimulation to find epileptic foci,and in connection with these operations, he didextensive studies of the temporal lobe, espe-cially with regard to memory.

Other neurosurgeons have followed Penfield’stradition, such as George A. Ojemann, who hascontributed much to understanding pathologiesrelated to the temporal lobe as well as to providebasic research regarding memory and, in particu-lar, regarding the large individual variations ofthe brain. Like Penfield, he operated on manypatients for epilepsy, and during these operations,he mapped the temporal lobe and studied thecenters for memory and speech using electricalcurrent to inactivate specific regions of the brainin patients who were awake and therefore wereable to respond and perform memory tasks.

Ojemann, working with Otto Creutzfeldt fromGermany, developed methods for microelectroderecordings from the brain of awake patients.They studied neuronal activity during face recog-nition, but their studies also contributed to thedevelopment of the use of microelectrodes inrecordings from the human brain.

A neurologist, Gaston Celesia, has expandedour knowledge about the organization of thehuman cerebral cortex by recordings of evokedresponses directly from the surface of thehuman auditory cortex (24,25). Celesia mappedthe auditory cortex in humans and studiedsomatosensory evoked potentials from the thal-amus and primary somatosensory cortex (26).Other investigators have studied other structuressuch as the dorsal column nuclei, the cochlearnucleus, and the inferior colliculus in patientsundergoing neurosurgical operations wherethese structures became exposed (27–30). Themethods used to record evoked potentials fromthe surface of the cochlear nucleus by insertingan electrode into the lateral recess of the fourthventricle (28,31) became a useful method formonitoring the integrity of the auditory nerve inoperations for vestibular schwannoma, wherepreservation of hearing was attempted (32), aswell as in microvascular decompression opera-tions for trigeminal neuralgia, hemifacialspasm, and disabling positional vertigo.

Studies of the neural generators of the ABRhave likewise benefited from recordings fromstructures that became exposed during neurosur-gical operations. Recordings from the auditorynerve that were first published in 1981 by twogroups, one in Japan (Isao Hashimoto, neurosur-geon) (33) and one in the United States (13)showed that the auditory nerve is the generatorof two vertex positive deflections in the auditorybrainstem responses, whereas the auditory nerve in small animals such as the rhesus mon-key is the generator of only one (major) peak(34–36).

The neurosurgeon Fred Lenz has studied theresponses from nerve cells in the thalamus inawake humans using microelectrodes andmapped the thalamus with regard to involve-ment in painful stimulation as well as in


response to innocuous somatosensory stimula-tion (37–39).

Electrophysiological studies of patientsundergoing MVD operations for HFS have sup-ported the hypothesis that the anatomical loca-tion of the physiological abnormalities thatcause the symptoms of HFS is central to thelocation of vascular contact with the facialnerve (the facial motonucleus) (40) involvingmechanisms similar to the kindling pheno-menon (described in refs. 41 and 42), and not pri-marily caused by ephaptic transmission at thelocation of the vascular contact that caused thesymptoms as another hypothesis had postulated.The findings that a specific sign, the abnormalmuscle response (or lateral spread response),disappears when the offending blood vessel ismoved off the facial nerve (43) is now widelyused in such operations as a guide to the sur-geon in finding the vessel that is the culprit andin effectively decompressing the facial nerve. Ithas increased the success rate of the operation,decreased the operating time, and reduced therisk that a reoperation would be necessary. Thisis again an example of how studies undertakenfor pure basic science can result in practicalmethods that increase the efficacy of an opera-tion, and this case in particular essentially elim-inated the need of reoperations, which were notuncommon before that method was introduced.

These examples show clearly that there is nosharp border between basic and appliedresearch. The method used for studies of neuralgenerators for the ABR came into use for mon-itoring the auditory nerve. Research on speechand language centers in the brain has proven tobe important for epilepsy operations. Researchon hemifacial spasm provided better outcomesof MVD operations.

Although it has been difficult to use exactscientific methods for assessing the benefits ofintraoperative neurophysiological monitoring,it is my opinion based on many years of expe-rience that the skill of the surgeon togetherwith good use of electrophysiology in theoperating room can benefit the patient who isbeing operated on and it can benefit manyfuture patients by the progress in treatment

that an effective collaboration between sur-geons and neurophysiologists promotes.

REFERENCES

1. Hilger J. Facial nerve stimulator. Trans. Am.Acad. Ophth. Otolaryngol. 1964;68:74–76.

2. Kurze T. Microtechniques in neurological sur-gery. Clin. Neurosurg. 1964;11:128–137.

3. Malis LI. Intra-operative monitoring is notessential. Clin. Neurosurg. 1995;42:203–213.

4. Sekhar LN, Bejjani G, Nora P, Vera PL. Neuro-physiological monitoring during cranial basesurgery: is it necessary? Clin. Neurosurg.1995;42:180–202.

5. Nash CL, Lorig RA, Schatzinger LA, Brown RH.Spinal cord monitoring during operative treat-ment of the spine. Clin. Orthop. 1977;126:100–105.

6. Brown RH, Nash CL. Current status of spinalcord monitoring. Spine 1979;4:466–478.

7. Grundy B. Intraoperative monitoring of sensoryevoked potentials. Anesthesiology 1983;58:72–87.

8. Raudzens RA. Intraoperative monitoring ofevoked potentials. Ann. NY Acad. Sci. 1982;388:308–326.

9. Friedman WA, Kaplan BJ, Gravenstein D,Rhoton AL. Intraoperative brain-stem auditoryevoked potentials during posterior fossamicrovascular decompression. J. Neurosurg.1985;62:552–557.

10. Linden R, Tator C, Benedict C, Mraz C, Bell I.Electro-physiological monitoring during acouticneuroma and other posterior fossa surgery. J. Sci.Neurol. 1988;15:73–81.

11. Møller AR, Jannetta PJ. Monitoring auditoryfunctions during cranial nerve microvasculardecompression operations by direct recordingfrom the eighth nerve. J. Neurosurg. 1983;59:493–499.

12. Silverstein H, Norrell H, Hyman S. Simultane-ous use of CO2 laser with continuous monitor-ing of eighth cranial nerve action potentialduring acoustic neuroma surgery. Otolaryngol.Head Neck Surg. 1984;92:80–84.

13. Møller AR, Jannetta PJ. Compound action poten-tials recorded intracranially from the auditorynerve in man. Exp. Neurol. 1981;74:862–874.

14. Møller AR. Electrophysiological monitoring ofcranial nerves in operations in the skull base.

Chapter 1 Introduction 5

In: Sekhar LN, Schramm VL, Jr. eds. Tumors ofthe Cranial Base: Diagnosis and Treatment.Mt. Kisco, NY: Futura; 1987:123–132.

15. Sekhar LN, Møller AR. Operative managementof tumors involving the cavernous sinus. J.Neurosurg. 1986;64:879–889.

16. Yingling C, Gardi J. Intraoperative monitoringof facial and cochlear nerves during acousticneuroma surgery. Otolaryngol. Clin. North Am.1992;25:413–448.

17. Barker AT, Jalinous R, Freeston IL. Non-invasivemagnetic stimulation of the human motor cortex.Lancet 1985;1:1106–1107.

18. Marsden CD, Merton PA, Morton HB. Directelectrical stimulation of corticospinal pathwaysthrough the intact scalp in human subjects. Adv.Neurol. 1983;39:387–391.

19. Deletis V. Intraoperative monitoring of the func-tional integrety of the motor pathways. In:Devinsky O, Beric A, Dogali M, eds. Advances inNeurology: Electrical and Magnetic Stimualtionof the Brain. New York: Raven; 1993:201–214.

20. Sloan TB, Heyer EJ. Anesthesia for intraopera-tive neurophsysiologic monitoring of the spinalcord. J. Clin. Neurophysiol. 2002;19:430–443.

21. Sloan T. Anesthesia and motor evoked potentialmonitoring. In: Deletis V, Shils JL, eds. Neuro-physiology in Neurosurgery. Amsterdam: Else-vier Science; 2002.

22. Penfield W, Boldrey E. Somatic motor and sen-sory representation in the cerebral cortex ofman as studied by electrical stimulation. Brain1937;60:389–443.

23. Penfield W, Rasmussen T. The Cerebral Cortexof Man: A Clinical Study of Localization of Func-tion. New York: Macmillan; 1950.

24. Celesia GG, Broughton RJ, Rasmussen T,Branch C. Auditory evoked responses from theexposed human cortex. Electroenceph. Clin.Neurophysiol. 1968;24:458–466.

25. Celesia GG, Puletti F. Auditory cortical areas ofman. Neurology 1969;19:211–220.

26. Celesia GG. Somatosensory evoked potentialsrecorded directly from human thalamus and SmI cortical area. Arch. Neurol. 1979;36:399–405.

27. Møller AR, Jannetta PJ, Jho HD. Recordingsfrom human dorsal column nuclei using stimula-tion of the lower limb. Neurosurgery 1990;26:291–299.

28. Møller AR, Jannetta PJ. Auditory evokedpotentials recorded from the cochlear nucleusand its vicinity in man. J. Neurosurg. 1983;59:1013–1018.

29. Hashimoto I. Auditory evoked potentials fromthe humans midbrain: slow brain stem responses.Electroenceph. Clin. Neurophysiol. 1982;53:652–657.

30. Møller AR, Jannetta PJ. Evoked potentials fromthe inferior colliculus in man. Electroenceph.Clin. Neurophysiol. 1982;53:612–620.

31. Kuroki A, Møller AR. Microsurgical anatomyaround the foramen of Luschka with reference tointraoperative recording of auditory evokedpotentials from the cochlear nuclei. J. Neurosurg.1995;82:933–939.

32. Møller AR, Jho HD, Jannetta PJ. Preservationof hearing in operations on acoustic tumors: Analternative to recording BAEP. Neurosurgery1994;34:688–693.

33. Hashimoto I, Ishiyama Y, Yoshimoto T,Nemoto S. Brainstem auditory evoked potentialsrecorded directly from human brain stem andthalamus. Brain 1981;104:841–859.

34. Møller AR, Burgess JE. Neural generators ofthe brain stem auditory evoked potentials(BAEPs) in the rhesus monkey. Electroenceph.Clin. Neurophysiol. 1986;65:361–372.

35. Spire JP, Dohrmann GJ, Prieto PS. Correlation ofBrainstem Evoked Response with Direct AcousticNerve Potential. New York: Raven; 1982.

36. Martin WH, Pratt H, Schwegler JW. The originof the human auditory brainstem responsewave II. Electroenceph. Clin. Neurophysiol.1995;96:357–370.

37. Greenspan JD, Lee RR, Lenz FA. Pain sensitivityalterations as a function of lesion localization inthe parasylvian cortex. Pain 1999;81:273–282.

38. Lenz FA, Dougherty PM. Pain processing inthe ventrocaudal nucleus of the human thala-mus. In: Bromm B, Desmedt JE, eds. Pain andthe Brain. New York: Raven; 1995:175–185.

39. Lenz FA, Lee JI, Garonzik IM, Rowland LH,Dougherty PM, Hua SE. Plasticity of pain-related neuronal activity in the human thala-mus. Prog. Brain Res. 2000;129:253–273.

40. Møller AR, Jannetta PJ. On the origin of synki-nesis in hemifacial spasm: results of intracranialrecordings. J. Neurosurg. 1984;61:569–576.

41. Goddard GV. Amygdaloid stimulation andlearning in the rat. J. Comp. Physiol. Psychol.1964;58:23–30.

42. Wada JA. Kindling 2. New York: Raven; 1981.43. Møller AR, Jannetta PJ. Microvascular decom-

pression in hemifacial spasm: intraoperativeelectrophysiological observations. Neurosurgery1985;16:612–618.


SECTION I

PRINCIPLES OF INTRAOPERATIVENEUROPHYSIOLOGICAL MONITORING

Chapter 2Basis of Intraoperative Neurophysiological Monitoring

Chapter 3Generation of Electrical Activity in the Nervous System and Muscles

Chapter 4Practical Aspects of Recording Evoked Activity From Nerves, Fiber Tracts, and Nuclei

The basic principles of recording and stimulation of the nervous system used in intraoperativeneurophysiological monitoring resemble techniques used in the clinical diagnostic laboratory withsome very important differences. The electrical potentials that are recorded from the nervous sys-tem in the operating room must be interpreted immediately and are recorded under circumstancesof interference of various kinds. This means that the person who does intraoperative neurophysio-logical monitoring must be knowledgeable about the function of the neurological systems that aremonitored, how electrical potentials are generated by the nervous system, and how such potentialschange as a result of pathologies that occur because of surgical manipulations. This section pro-vides basic information about the principles of intraoperative neurophysiological monitoring.Chapter 3 describes how electrical activity is generated in the nervous system and how such elec-trical activity can be recorded and can be used as the basis for detecting injuries to specific parts ofthe peripheral and central nervous system. Chapter 4 provides some practical information aboutrecording of neuroelectric potentials from the nervous system and how to stimulate the nervous sys-tem in anesthetized patients. This chapter also discusses how to record very small electrical poten-tials in an electrically hostile environment such as the operating room.

INTRODUCTION

Intraoperative neurophysiological monitor-ing is often associated with reducing the risk ofpostoperative neurological deficits in operationswhere the nervous system is at risk of being per-manently injured. Although the main use ofelectrophysiological methods in the operatingroom might be for reducing the risk of postoper-ative neurological deficits, electrophysiologicalmethods are now in increasing use for otherpurposes. For example, electrophysiologicalmethods are now regarded as necessary for guid-ing the placement of electrodes for deep brainstimulation or for making lesions in specificstructures for treating movement disorders andpain. Intraoperative electrophysiological record-ings can also help the surgeon in carrying outother surgical procedures. Finding specific neuraltissue such as cranial nerves or specific regionsof the cerebral cortex are examples of tasks thatare included in the subspecialty of intraoperativeneurophysiological monitoring. Neurophysio-logical methods are in increasing use for diag-nostic support in operations such as thoseinvolving peripheral nerves. In certain operations,intraoperative electrophysiological recordings

can increase the likelihood of achieving thetherapeutical goal of an operation. Intraopera-tive neurophysiological recordings have shownto be of help in identifying the offending bloodvessel in a cranial nerve disorders (hemifacialspasm).

REDUCING THE RISK OF NEUROLOGICAL DEFICITS

The use of intraoperative neurophysiologicalmonitoring to reduce the risk of loss of func-tion in portions of the nervous system is basedon the observation that the function of neuralstructures usually changes in a measurable waybefore being permanently damaged. By revers-ing the surgical manipulation that caused thechange within a certain time will result in arecovery to normal or near-normal function,whereas if no intervention had been taken,there would have been a risk that permanentpostoperative neurological deficit would haveresulted.

Surgical manipulations such as stretching,compressing, or heating from electrocoagulationare insults that can injure neural tissue, as canischemia caused by impairment of blood sup-ply resulting from surgical manipulations orintentional clamping of arteries, that could alsoresult in permanent (ischemic) injury to neural

IntroductionReducing the Risk of Neurological DeficitsAiding the Surgeon in the OperationWorking in the Operating RoomHow to Evaluate the Benefits of Intraoperative Neurophysiological MonitoringResearch Opportunities

2Bas i s o f I n t raope ra t i ve Neurophy s i o l og i ca lMon i to r i ng

9


structures, causing a risk of noticeable postop-erative neural deficits.

The effect of such insults represents a con-tinuum; at one end, function decreases for thetime of the insult, and at the other end of thiscontinuum, nervous tissue is permanentlydamaged and normal function never recovers,thus causing permanent postoperative deficits.Between these extremes, there is a large rangeover which recovery can occur either totally orpartially. Thus, up to a certain degree of injury,there can be total recovery, but thereafter, theneural function might be affected for sometime. After more severe injury, the recovery ofnormal function not only takes a longer timebut the final recovery would only be partial,with the degree of recovery depending on thenature, degree, and duration of the insult.

Injuries acquired during operations thatresult in a permanent neurological deficit willmost likely reduce the quality of life for thepatient for many years to come and maybe fora lifetime. Therefore, it is important that theperson responsible for interpreting the resultsof monitoring is aware that the neurophysiologisthas a great degree of responsibility, togetherwith the surgeon and the anesthesiologist, inreducing the risk of injury to the patient duringthe operation.

Techniques for Reducing PostoperativeNeurological Deficits

The general principle of intraoperative neuro-physiological monitoring is to apply a stimulusand then to record the electrical response fromspecific neural structures along the neural path-way that are at risk of being injured. This canbe done by recording the near-field evokedpotentials by placing a recording electrode on aspecific neural structure that becomes exposedduring the operation or, as more commonlydone, by recording the far-field evoked poten-tials from, for instance, electrodes placed onthe surface of the scalp.

Intraoperative neurophysiological monitoringthat is done for the purpose of reducing the riskof postoperative neurological deficits makesuse of relatively standard and well-developed

methods for stimulation and recordings of elec-trical activity in the nervous system. Most of themethods that are used in intraoperative neuro-physiological monitoring are similar to thosethat are used in the physiological laboratory andin the clinical testing laboratory for many years.

Sensory System. Intraoperative neurophysi-ological monitoring of the function of sensorysystems has been widely practiced since themiddle of the 1980s. The earliest uses ofintraoperative neurophysiologic monitoringof sensory systems were modeled after theclinical use of recording sensory evoked poten-tials for diagnostic purposes.

Sensory systems are monitored by applyingan appropriate stimulus and recording theresponse from the ascending neural pathway,usually by placing recording electrodes on thesurface of the scalp to pick up far-field potentialsfrom nerve tracts and nuclei in the brain (far-fieldresponses).

It has been mainly somatosensory evokedpotentials (SSEPs) and auditory brainstemresponses (ABRs) that have been recorded inthe operating room for monitoring the functionof these sensory systems for the purpose ofreducing the risk of postoperative neurologicaldeficits. Visual evoked potentials (VEPs) arealso monitored in some operations. When intra-operative neurophysiological monitoring wasintroduced, it was first SSEPs that were moni-tored routinely (1), followed by ABRs (2–4).

Although the technique used for recordingsensory evoked potentials in the operating roomis similar to that used in the clinical diagnosticlaboratory, there are important differences. Inthe operating room, it is only changes in therecorded potentials that occur during the opera-tion that are of interest, whereas in the clinicaltesting laboratory, the deviation from normalvalues (laboratory standard) are importantmeasures. Another important difference is thatresults obtained in the operating room must beinterpreted instantly, which places demands onthe personnel who are responsible for intraoper-ative neurophysiological monitoring that differfrom those working in the clinical laboratory. In


the operating room, it is sometimes possible torecord evoked potentials directly from neuralstructures of sensory pathways (near-fieldresponses) when such structures becomeexposed during an operation.

The use of evoked potentials in intra-operative neurophysiological monitoring forthe purpose of reducing the risk of postopera-tive permanent sensory deficits is based on thefollowing:

1. Electrical potentials can be recorded inresponse to a stimulus.

2. These potentials change in a noticeable wayas a result of surgically induced changes infunction.

3. Proper surgical intervention, such asreversal of the manipulation that caused thechange, will reduce the risk that theobserved change in function develops into apermanent neurological deficit or, at least,will reduce the degree of the postoperativedeficits.

Motor Systems. Intraoperative neurophysi-ological monitoring of the facial nerve wasprobably the first motor system that was moni-tored systematically. The introduction of skullbase surgery in the 1980s (5) caused anincreased demand for monitoring of other cra-nial systems, and the use of monitoring formany cranial motor nerves spread rapidly (6,7).Intraoperative neurophysiological monitoringof spinal motor systems was delayed becauseof technical difficulties, mainly in elicitingrecordable evoked motor responses to stimula-tion of the motor cortex in anesthetizedpatients. After these technical obstacles in acti-vating descending spinal motor pathways wereresolved in the 1990s, intraoperative neuro-physiological monitoring of spinal motor sys-tems gained wide use (8). Monitoring of cranialnerve motor systems commonly relies on record-ings of electromyographical (EMG) potentialsfrom muscles that are innervated by specificmotor nerves, whereas monitoring of spinalmotor systems also makes use of recordingsdirectly from the descending motor pathways

of the spinal cord. Spinal motor systems areoften monitored by recording EMG potentialsfrom specific muscles in response to electricalor magnetic stimulation of the motor cortex(Chap. 10).

Peripheral Nerves. Monitoring of motornerves is often accomplished by observing theelectrical activity that can be recorded from oneor more of the muscles that are innervated by themotor nerve or motor system that is to be mon-itored (evoked EMG potentials). The respectivemotor nerve might be stimulated electrically orby the electrical current that is induced by astrong magnetic impulse (magnetic stimula-tion). Recordings of muscle activity that iselicited by mechanical stimulation of a motornerve or by injury to a motor nerve are impor-tant parts of many forms of monitoring of themotor system. Such muscle activity is moni-tored by continuous recording EMG potentials(“free-running EMG”). When such activity ismade audible, it can provide important feedbackto the surgeon and the surgeon, can then modifyhis/her operative technique accordingly.

Monitoring peripheral nerves intraopera-tively can be done by electrically stimulatingthe nerve in question at one point and recordingthe compound action potentials (CAPs) at adifferent location. Changes in neural conduc-tion that might occur between these two loca-tions will result in changes in the latency of theCAP and/or in the waveform and amplitude ofthe CAP. The latency of the CAP is a measure ofthe (inverse) conduction velocity, and decreasedconduction velocity is a typical sign of injury toa nerve. The latency and waveform of therecorded CAP typically increases as a result ofmany kinds of insult to a nerve.

Interpretation of Neuroelectric PotentialsThe success of intraoperative neurophysiolog-

ical monitoring depends greatly on the correctinterpretation of the recorded neuroelectricalpotentials. In most situations, the usefulness ofintraoperative neurophysiological monitoringdepends on the person who watches the display,makes the interpretation, and decides what

Chapter 2 Basis of Intraoperative Neurophysiological Monitoring 11

information should be given to the surgeon. It is,therefore, imperative for success in intraopera-tive neurophysiological monitoring that the per-son who is responsible for the monitoring bewell trained. It is also important that he/she isfamiliar with the different steps of the operationand well informed in advance about the patientwho is to be monitored.

It is important that information about changesin recorded potentials be presented in a waythat contributes specific interpreted detail thatthe surgeon will find useful and actionable.Surgeons are not neurophysiologists and theknowledge of neurophysiology varies amongsurgeons. The neurophysiologist who providesresults of monitoring to the surgeon must,therefore, present their skilled interpretationof the recorded potentials. The surgeon mightnot always appreciate data such as latency val-ues because the surgeon might not understandwhat such data represent. Monitoring is of novalue if the surgeon does not take actionaccordingly. If the surgeon does not understandwhat the information provided by the neuro-physiologist means, then there is little chancethat he/she will take appropriate action.

Correct and prompt interpretation of changesin the waveforms of the recorded potentials isessential for such monitoring to be useful. Thefar-field potentials such as ABR, SSEP, andVEP are often complex and consist of a seriesof peaks and troughs that represent the electri-cal activity that is generated by successivelyactivated nerve tracts and nuclei of the ascend-ing neural pathways of the sensory system.Exact interpretation of the changes in suchpotentials that could occur as a result of variouskinds of surgical insult therefore require thor-ough knowledge of the anatomy and physiologyof the systems that are monitored and of howthe recorded potentials are generated.

The most reliable indicators of changes inneural function are changes (increases) in thelatencies of specific components of sensoryevoked potentials, and surgically inducedinsults to nervous tissue often also causechanges in the amplitude of the sensory evokedpotentials.

It must be remembered that the recordedsensory evoked potentials do not measure thefunction (or changes in function) of the sensorysystem that is being tested. For example, thereis no direct relationship between the change inthe ABR and the change in the patient’s hearingthreshold or change in speech discrimination.This is one reason why it has been difficult toestablish guidelines for how much evokedpotentials could be allowed to change during anoperation without presenting a noticeable riskfor postoperative deficits.

Interpretation of sensory evoked potentialsis based on knowledge of the anatomical loca-tion of the generators of the individual com-ponents of SSEP, ABR, and VEP in relation tothe structures that are being manipulated in aspecific operation. Interpretation of sensoryevoked potentials also depends on the pro-cessing of the recorded potentials. For exam-ple, filtering of various kinds are used and thataffects the waveform of the potentials. Theamplitude of these sensory evoked potentialsis smaller than the background noise (ongoingbrain activity [EEG potentials] and electricalnoise) and it is, therefore, necessary to use signalaveraging to enhance the signal-to-noise ratio ofelectrical potentials such as sensory evokedpotentials. Signal averaging (adding theresponses to many stimuli) is based on theassumption that the responses to every stimu-lus are identical and they always occur at thesame time following stimulation. Because thesensory evoked potentials that are recorded inthe operating room are likely to change duringthe time that responses are being averaged, theaveraging process might produce unpre-dictable results. These matters are importantto take into consideration when interpretingsensory evoked potentials. (Signal averagingand filtering are discussed in more detail inChap. 18.)

Different ways to reduce the time necessaryto obtain an interpretable recording are dis-cussed and described in Chaps. 4, 6, and 18. Thespecific techniques that are suitable for intra-operative neurophysiological monitoring of theauditory, somatosensory, and visual systems


are dealt with in more detail in Chaps. 4 and 6,respectively.

In some instances, it is possible to recordpotentials from the structures that actually gen-erate the evoked potentials in question (near-field potentials). Such potentials often havesufficiently large amplitude, allowing observa-tion of the potentials directly without signalaveraging. If it is possible to base the intraoper-ative neurophysiological monitoring on record-ing of evoked potentials directly from an activeneural structure (nerve, nerve tract, or nucleus),little or no signal averaging might be necessarybecause the amplitudes of such potentials aremuch larger than those of far-field potentials,such as the ABR and SSEP, and such near-fieldpotentials can often be viewed directly on ancomputer screen or after only a few responseshave been averaged. These matters are also dis-cussed in more detail in the chapters on sensoryevoked potentials (Chaps. 4 and 6).

The design of the monitoring system and theway the recorded potentials are processed areimportant factors in facilitating proper interpre-tation of the recorded neuroelectric potentials,as is the way the recorded potentials are dis-played (see Chap. 18). The proper choice ofstimulus parameters and the selection of thelocation along the nervous pathways where theresponses are recorded also facilitate promptinterpretation of recorded neuroelectricalpotentials.

When recording EMG potentials, it is oftenadvantageous to make the recorded responseaudible (9,10) so that the neurophysiologistresponsible for the monitoring and the surgeoncan hear the response and make his/her owninterpretation. Still, the possibilities to presentthe recorded potentials directly to the surgeonare currently few, and it is questionablewhether it would be advantageous. Few sur-geons are physiologists and most surgeonswant the results of monitoring to be presentedin an interpreted form rather than raw data.

The importance of being able to detect achange in function as soon as possible cannot beemphasized enough. Prompt interpretation ofchanges in recorded potentials makes it possible

for the surgeon to accurately identify the step inthe operation that caused the change, which isa prerequisite for proper and prompt surgicalintervention and, thus, the ability to reduce therisk of postoperative neurological deficits.

Correct identification of the step in an oper-ation that entails a risk of complications mightmake it possible to modify the way such anoperation is carried out in the future andthereby makes it possible to reduce the risk ofcomplications in subsequent operations. In thisway, intraoperative neurophysiological moni-toring can contribute to the development ofsafer operating methods by making it possibleto identify which steps in an operation mightcause neurological deficits, and it thereby natu-rally also plays an important role in teachingsurgical residents and fellows.

When to Inform the SurgeonIt has been debated extensively whether the

surgeon should be informed of all changes inthe recorded electrical activity that could beregarded to be caused by surgical manipula-tions or only when such changes reach a levelthat indicate a noticeable risk for permanentneurological deficits. The question is thus:should the information that is gained be usedonly as a warning that implies that if no inter-vention is made, there is a likelihood that thepatient will get a permanent postoperative neu-rological deficit, or should all informationabout changes in function be conveyed to thesurgeon?

If only information that is presumed to indi-cate a high risk of neurological deficits is givento the surgeon, then it must be known how largea change in the recorded neuroelectrical poten-tials can be permitted without causing any per-manent damage. This question has so farlargely remained unanswered. The degree andthe nature of the change and the length of timethat the adverse effect has lasted are all factorsthat are likely to affect the outcome, and theeffect of these factors on the risk of postopera-tive neurological deficits are largely unknown.Individual variation in susceptibility to surgicalinsults to the nervous system and many other


factors affect the risk of neurological deficits inmostly unknown ways and degrees. An individ-ual’s disposition and homeostatic condition andperhaps the effect of anesthesia are likely toaffect the susceptibility to surgically inducedinjuries.

If the surgeon is given information aboutany noticeable change in the recorded poten-tials that may be related to his/her action it isnot necessary to know how large a change inrecorded potentials can be permitted without arisk of permanent neurologic deficits. The sur-geon can use such information in the planningand the decision of how to proceed with theoperation, and intraoperative neurophysiologicalmonitoring can thereby effectively help decreasethe risk of neurological deficits. This meansthat it is beneficial to the surgeon to beinformed whenever his or her actions haveresulted in a noticeable change in the recordedneuroelectrical potentials. In that way, intraop-erative neurophysiological monitoring providesinformation rather than warnings. Changes inthe recorded potentials that are larger than the(small) normal variations of the potentials inquestion should be reported to the surgeon ifthere is reasonable certainty that these changesare related to surgical manipulations.

If the surgeon is made aware of any changein the recorded potentials that is larger thanthose normally occurring, it can help the sur-geon to carry out the operation in an optimalway with as little risk of adverse affect on neu-ral function as possible. Providing such infor-mation gives the surgeon the option of alteringhis/her course of action in a wide range of time.If the change in the recorded potentials issmall, it is likely that the surgeon would be ableto reverse the effect by a slight change in thesurgical approach or by avoiding furthermanipulation of the neural tissue affected;alternatively, the surgeon might choose not toalter the technique if the surgical manipulationsthat caused the changes in the recorded neuro-physiological potentials are essential to carry-ing out the operation in the anticipated way.However, even in such a case, the knowledgethat the surgical procedure is affecting neural

function in a measurable way is valuable tothe surgeon, and continuous monitoring of thechange can keep his/her option to modify theprocedure to remain open because monitoringhas identified which step in the operationcaused the change in function.

If information about a change in therecorded potentials is withheld until the changein the recorded electrical potentials hasincreased greatly, it would be difficult for thesurgeon to determine which step in the surgicalprocedure caused the adverse effect, and thus itwould not be possible for the surgeon to inter-vene appropriately because it would not beknown which step in the procedure caused thechange. Also, in such a situation, the surgeonwould not have had the freedom of delayinghis/her action to reverse the change because ithad already reached dangerous levels.

The more knowledge that is gathered aboutthe effect of mechanical manipulation onnerves, the more it seems apparent that evenslight changes in measures of electrical activity(such as the CAP) might be signs of permanentinjury. However, studies that relate changes inevoked potentials to morphological changesand changes in postoperative function are stillrare. Thus, relatively little is known quantita-tively about the degree to which a nerve can bestretched, heated, or deprived of oxygen beforea permanent injury results, but there is no doubtthat different nerves respond in different waysto injury because of mechanical manipulations,heat, or lack of oxygen.

Presenting information about changes in therecorded neuroelectrical potentials as soon asthey reach a level where they are detectablealso has an educational benefit in that it tellsthe surgeon precisely which steps in an opera-tion might result in neurological deficit. It isoften possible on the basis of such knowledgeto modify an operation to avoid similar injuriesin future operations.

When conveying information about earlychanges in the recorded potentials, it is impor-tant that it be made clear to the surgeon thatsuch information represents guidance details,as opposed to a warning that the surgical


manipulations are likely to result in a high riskof serious consequences if appropriate action isnot taken promptly by the surgeon. Warningsare justified, however, if, for instance, there is asudden large change in the evoked potentials orif the surgeon has disregarded the need toreverse a manipulation that has caused a slowchange in the recorded electrical potentials.

The surgeon should be informed of the pos-sibility of a surgically induced injury even incases in which the change (or total disappear-ance of the recorded potentials) could becaused by equipment or electrode malfunction.Thus, only after assuming that the problem isbiological in nature can equipment failure beconsidered as a possible cause.

False AlarmsThe question of false-positive and false-

negative responses in intraoperative neurophysio-logical monitoring has been extensively debated.In some of these discussions, a false-positiveresponse meant that the surgeon was alerted of asituation that would not have led to any notice-able risk of neurological deficits if no action hadbeen taken.

Before discussing false-positive and false-negative responses in intraoperative neurophysio-logical monitoring, the meaning of false-positiveand false-negative responses should be clarified.A typical example of a false-positive result of atest for a specific disease occurs when the testshowed the presence of a disease when therewas, in fact, no disease present. Using the sameanalogy, a false-negative test would mean thatthe test failed to show that a certain individualin fact had the specific disease. In the clinic orin screening of individuals without symptoms,false-negative results are more serious thanfalse-positive results: false-positive resultsmight lead to an incorrect diagnosis orunnecessary treatment, whereas false-negativeresults might have the dire consequence of notreatment being given for an existing disease.

These definitions cannot be transposeddirectly to the field of intraoperative neuro-physiological monitoring. One reason is thatthe purpose of intraoperative neurophysiological

monitoring is not to detect when a certainsurgical manipulation will cause a permanentneurological deficit. Instead, the purpose is toprovide information about when there is a(noticeable) risk that a permanent neurologicaldeficit might occur. In fact, in most caseswhen intraoperative neurophysiological moni-toring shows changes in function that indi-cates a risk of causing neurological deficits, nopermanent deficits occur. There is no seriousconsequences associated with this kind of false-positive responses in intraoperative neuro-physiological monitoring. A situation in whichthe surgeon was mistakenly alerted of a changein the recorded potentials that was afterwardshown to be a result of a technical fault or aharmless change in the nervous system ratherthan being caused by surgical manipulationsmight be regarded as a true false-positiveresponse.

The occurrences of false-negative results,which mean that a serious risk has occurredwithout being noticed, indicate a failure inreaching the goal of intraoperative neurophysi-ological monitoring and it might have seriousconsequences.

Therefore, the conventional definition offalse-positive and false-negative results cannotbe applied to intraoperative neurophysiologicalmonitoring because the purpose of monitoringis not to identify an individual with aneurological deficit but to identify signs thathave a certain risk of leading to such deficits ifno action is taken.

Nonsurgical Causes of Changes in Recorded Potentials

Alerting the surgeon as soon as a changeoccurs naturally always implies a faint possibilitythat a change in evoked potentials might becaused by technical problems that affectedsome part of the equipment that is used or by aloss of contact of one or more of the electrodes.The characteristics of changes caused by tech-nical problems are usually so different fromthose of changes caused by injury from surgicalmanipulations that these two phenomena caneasily be distinguished by an experienced


neurophysiologist. It is possible that a total lossof recorded potentials can be caused by a tech-nical failure, but it could also be caused by amajor failure in the part of the nervous systemthat is being monitored. However, if such anevent should occur, it is much better to firstassume that the cause is biological and topromptly alert the surgeon accordingly andthen do trouble-shooting of the equipment. Ingeneral, when something unusual happens, it isadvisable to alert the surgeon promptly thatsomething serious could have happened insteadof beginning to check the equipment andelectrodes. It is highly unlikely that a technicalfailure will occur and cause a change in therecorded potentials that might be confused witha biological cause for the change. Theneurophysiologist should explain to the surgeonthat a potentially serious event has occurredand then check the equipment and the elec-trodes for malfunction. The surgeon, not wait-ing for the completion of this equipment check,should immediately begin his/her own investi-gation to ascertain whether a surgically inducedinjury has occurred. If it is discovered that thechange in the recorded potentials was caused byequipment malfunction, the surgeon can then beapprised of this; thus, the only loss that the inci-dent would cause is a few minutes of thesurgeon’s time. If such an occurrence isregarded as a “false alarm,” then the price fortolerating such “false alarms,” namely that theoperation might be delayed unnecessarily for abrief time, seems small compared to what couldoccur if one chose to check the equipment beforealerting the surgeon.

If the cause of the change in the recordedneuroelectrical potentials was indeed a result ofan injury that was caused by surgical manipu-lation of neural structures and appropriateaction was not taken immediately by the sur-geon, precious time would have been lost. Thiswould occur if the neurophysiologist hadassumed that the cause of the change was tech-nical in nature. Not only would the opportunityto identify the cause of the change be missed bytaking the time to check the equipment first, butsuch a delay could also have allowed the

change in function to progress, thus increasingthe risk of a permanent neurological deficit. Theopportunity to properly reverse the cause of theobserved change in the recorded neuroelectricalpotentials might be lost if action is delayedwhile searching for technical problems.

In accepting this way of performing intraop-erative neurophysiological monitoring, it mustalso be assumed that everything is done thatcan be done to keep technical failures thatcould mimic surgically induced changes in therecorded potentials to an absolute minimum.Actually, high-quality equipment very seldommalfunctions, and if needle electrodes are usedin the way described in the following chaptersand care is taken when placing the electrodes,incidents of electrode failure will be rare.

There are factors other than surgical manipu-lations or equipment failure that can causechanges in the waveform of the recordedpotentials (e.g., changes in the level of anesthe-sia, blood pressure, or body temperature of thepatient). It is therefore important that the personwho is responsible for the intraoperativeneurophysiological monitoring be knowledge-able about how these factors could affect theneuroelectric potentials that are being recorded.The physiologist should maintain consistent andfrequent communication with the anesthesiolo-gist to keep informed about any changes in thelevel of anesthesia and changes in the anesthesiaregimen that could affect the electrophysiologi-cal parameters that are to be monitored.

How to Evaluate Neurological DeficitsTo assess the success of avoiding neurologi-

cal deficits, it is important that patients beproperly examined and tested both preopera-tively and postoperatively so that changes canbe verified quantitatively. In some cases, aninjury is detectable only by specific neurologicaltesting, whereas in other cases, injury causesimpaired sensory function that is noticeable bythe patient. Other patients might suffer alter-ations in neural function that are noticeable tothe patient as well as others in everydaysituations. It is therefore important that carefulobjective testing and examination of the patient


be performed before and after operations tomake accurate quantitative assessments of sen-sory or neurological deficits.

There is no doubt that the degree to whichdifferent types of neurological deficit affectindividuals varies, but reducing the risk of anymeasurable or noticeable deficit as much aspossible must be the goal of intraoperativeneurophysiological monitoring.

AIDING THE SURGEON IN THE OPERATION

In addition to reducing the risk of neurologicaldeficits, the use of neurophysiological tech-niques in the operating room can provide infor-mation that can help the surgeon carry out theoperation and make better decisions about thenext step in the operation. In its simplest form,this might consist of identifying the exactanatomical location of a nerve that cannot beidentified visually or it might consist of identi-fying where in a peripheral nerve a block oftransmission has occurred (11). In operations torepair peripheral nerves, intraoperative diagnosisof the nature of the injury and its exact locationusing neurophysiological methods haveimproved the outcome of such operations.

An example of a more complex role of intra-operative recording is the recording of theabnormal muscle response in patients undergo-ing microvascular decompression (MVD) oper-ations to relieve hemifacial spasm (HFS)(12,13). This abnormal muscle response disap-pears when the facial nerve is adequatelydecompressed (14), and by observing thisresponse, it is possible to identify the bloodvessel or blood vessels that caused the symptomsof HFS as well as to ensure that the facial nervehas been adequately decompressed.

Electrophysiological guidance for place-ment of lesions in the basal ganglia and thethalamus for treatment of movement disordersand pain is absolutely essential for the successof such treatment. More recently, makinglesions in these structures has been replaced byelectrical stimulation deep brain stimulation

(DBS) and electrophysiological methods areequally important for guiding the placement ofelectrodes for DBS.

Implantation of electrodes for DBS and forstimulation of specific structures in the spinalcord no doubt will increase during the comingyears. Such treatments are attractive in compar-ison with pharmacological (drug) treatment inthat it has fewer side effects. Whereas aphysician with a license to practice medicinecan prescribe many complex medications,procedures such as electrode implantation forDBS require expertise in both surgery andneurophysiology and it must involve intraoper-ative neurophysiological recordings being per-formed adequately. This means that the need ofpeople with neurophysiological knowledge andskills of working in the operating room will be inincreasing demand for the foreseeable future.

There is no doubt that in the future we willsee the development of many other presentlyunexplored areas in which intraoperative neu-rophysiological recording will become an aidto the surgeon in specific operations, and theuse of neurophysiological methods in the oper-ating room will expand as a means to studynormal as well as pathological functions of thenervous system.

WORKING IN THE OPERATINGROOM

Intraoperative neurophysiological monitoringshould interfere minimally with other activitiesin the operating room. If it causes more thanminimal interference, there is a risk that it wouldnot be requested as often as it should. There is somuch activity in modern neurosurgical, otologic,and orthopedic operating rooms that addingactivity that consumes time will naturally be metwith a negative attitude from all involved andmight result in the omission of intraoperativeneurophysiological monitoring in certain cases.Careful planning is necessary to ensure thatintraoperative neurophysiological monitoringdoes not interfere with other forms of monitoringand the use of life-support equipment.


How to Reduce the Risk of Mistakes in Intraoperative NeurophysiologicalMonitoring

The importance of selecting the appropriatemodality of neuroelectric potentials for moni-toring purposes cannot be overemphasized andmaking sure that the structures of the nervoussystem that are at risk are included in the mon-itoring is essential. Thus, monitoring SSEPelicited by stimulating the median nerve whileoperating on the thoracic or lumbar spine natu-rally could lead to a disaster, because it is thethoracic lumbar spinal portion of thesomatosensory pathway that is at risk of beinginjured when only the cervical portion of thesomatosensory pathway is being monitored.

Monitoring the wrong side of the patient’snervous system is also a serious mistake. Anexample of this is presenting the sound stimulusto the ear opposite the side on which the opera-tion is being done while monitoring ABR. Thiskind of mistake could occur when earphones arefitted in both ears and selection of which ear-phone to be used is controlled by the neurophys-iologist. A user mistake can cause the wrongearphone to be used. Because the ABR is notfundamentally different when elicited from theopposite side, such a mistake will not be imme-diately obvious, but it will naturally prevent thedetection of any change in the ear or auditorynerve as a result of surgical manipulation. Thepossible catastrophic consequence of failing todetect any change in the recorded potentialswhen the auditory nerve is injured by surgicalmanipulation is obvious.

Generally speaking, if a mistake can be madeby the action of the user (neurophysiologist), itwill be made; it might be rare. Mistakes mightbe tolerated, depending on the consequencesand the frequency of its expected occurrence.Mistakes can only be avoided if it is physicallyimpossible to make the mistake. Thus, only byplacing an earphone solely in the ear on theoperated side can the risk of stimulating thewrong ear be eliminated. If earphones areplaced in each ear, the risk of making mistakescan be reduced by clearly marking the right andleft earphone and only having properly trained

personnel operate the stimulus equipment. Thiswill reduce the risk of mistakes but not elimi-nate mistakes.

In a similar way, monitoring the wrong sideof the spinal cord could cause serious neuro-logical deficits without any change in therecorded neuroelectrical potentials beingnoticed during the operation. When an operationinvolves the spinal cord distal to the cervicalspine and stimulating electrodes are placed inthe median nerve as well as in a nerve on thelower limb, the median nerve might mistakenlybe stimulated when the intention was to elicitevoked potentials from the lower limb. Thiscould happen if the stimulation is controlled bythe user. The considerable difference betweenthe waveform of the upper limb SSEP and thatof the lower limb SSEP might make this mis-take more easily detectable than when elicitingABR when the wrong ear is being stimulated orwhen eliciting SSEP from the wrong side.

Reliability of IntraoperativeNeurophysiological Monitoring

Like any other new addition to the operatingroom armamentarium, intraoperative neuro-physiological monitoring must be reliable inorder to be a tool that is used routinely. It is notunreasonable to assume that if intraoperativeneurophysiological monitoring cannot alwaysbe carried out and, consequently, operations aredone without the aid of monitoring, it might beassumed by the surgeon that it is not necessaryat all to have such monitoring.

Reliability can best be achieved if only rou-tines that are well thought through and that havebeen thoroughly tested are used in the operatingroom. The same methods that have been foundto work well over a long time should be usedconsistently. New routines or modifications ofold routines should only be introduced in theoperating room after thorough considerationand testing. Procedures of intraoperative neuro-physiologic monitoring should be kept as simpleas possible. The KISS Principle (Keep it Simple[and] Stupid) (or Keep it Simple and Straight-forward) is applicable to intraoperative neuro-physiological monitoring.


Electrical Safety and IntraoperativeNeurophysiological Monitoring

A final, but not inconsiderable, concern is thatintraoperative neurophysiological monitoringshould not add risks to the safety, particularlyelectrical safety, of any operation. Intraoperativeneurophysiological monitoring requires theaddition of complex electrical equipment to anoperating room already crowded with a varietyof complex electrical equipment. Electricalsafety is naturally of great concern wheneverelectronic equipment is in direct galvanic contactwith patients, but this is particularly true in theoperating room, where many pieces of electricalequipment are operated together, often incrowded conditions, and frequently under wetconditions. The equipment and procedures usedfor intraoperative neurophysiological monitoringmust, therefore, be chosen with consideration forthe protection of the patient as well as of the per-sonnel in the operating room from electricalhazard. Accidents can best be avoided whenthose who work in the operating room and whouse the electronic equipment are knowledgeableabout the function of the equipment and howrisks of electrical hazards that are associatedwith specific equipment could arise. For theneurophysiologist, it is important to have a basicunderstanding about how electrical hazardscould occur and to specifically have an under-standing of the basic functions of the variouspieces of equipment used in electrophysiologicalmonitoring. The area of greatest concern inmaintaining electrical safety for the patient is,naturally, the placement of stimulating andrecording electrodes on the patient. It is particu-larly important to consider the safety of theequipment that is connected to electrodes placedintracranially for either recording or stimulation.

HOW TO EVALUATE THE BENEFITSOF INTRAOPERATIVE

NEUROPHYSIOLOGICALMONITORING

Naturally, it is the patient who can gain themost from intraoperative neurophysiological

monitoring. Many of the severe postoperativeneurological deficits that were common beforethe introduction of intraoperative neurophysio-logical monitoring are now rare occurrences. Itis not only the use of intraoperative neurophys-iological monitoring that has caused theseimprovements of medical care, but also bettersurgical techniques and various technologicaladvancements have provided significantprogress. There is no doubt that the introduc-tion of microneurosurgery and, more recently,minimally invasive surgery has made opera-tions that affect the nervous system less brutalthan it was 25 yr ago, and even the last decadehas seen steady improvements regarding reduc-ing complications.

Assessment of Reduction of NeurologicalDeficits

It has been difficult to accurately assess thevalue of intraoperative neurophysiologicalmonitoring with regard to reducing the risk ofpostoperative neurological deficits. One of thereasons for these difficulties is that it has notbeen possible to apply the commonly usedscheme, such as double-blind methods, todetermine the value of intraoperative neuro-physiological monitoring. Surgeons who haveexperienced the advantages of intraoperativeneurophysiological monitoring are reluctant todeprive their patients of the benefits providedby an aid in the operation that they believe canimprove the outcome. The use of historical datafor comparison of outcomes before and after theintroduction of monitoring has been describedin a few reports, but such methods are criticizedbecause advancements in surgical techniqueother than intraoperative neurophysiologicalmonitoring might have contributed to theobserved improvement of outcome. Even moredifficult to evaluate is the increased feeling ofsecurity that surgeons note while operating withthe aid of intraoperative neurophysiologicalmonitoring.

For the sake of evaluating future benefitsfrom monitoring, it is important that all patientswho are monitored intraoperatively be evalu-ated objectively before and after the operation


and that the results obtained during monitoringbe well documented.

Which Surgeons Benefit Most From Intraoperative Monitoring?

Surgeons at all levels of experience couldbenefit in one way or another from the use ofintraoperative neurophysiological monitor-ing, but the degree of benefit depends on theexperience of the surgeon in the particularkind of operation being performed. Whereasan extremely experienced surgeon might bene-fit from monitoring only in unusual situationsor for confirming the anatomy, a surgeon withmoderate-to-extensive experience might feelmore secure and might have additional help inidentifying specific neural structures whenusing monitoring. A surgeon with moderate-to-extensive experience will also benefit fromknowing when surgical manipulations haveinjured neural tissue. A less experienced surgeonwho has done only a few of a specific type ofoperation is likely to benefit more extensivelyfrom using intraoperative neurophysiologicalmonitoring, and surgeons at this level of experi-ence will learn from intraoperative monitoringand through that improve his/her surgical skills.

Even some extremely experienced surgeonsdeclare the benefit from neurophysiologicalmonitoring and appreciate the increased feelingof security when operating with the assistanceof monitoring. Many very experienced sur-geons are in fact not willing to operate withoutthe use of monitoring.

In fact, most surgeons can benefit fromintraoperative neurophysiological monitoring

mainly by its help in reducing the risk of post-operative neurological deficits as well as by itsability to provide the surgeon with a feeling ofsecurity from knowing that he/she will knowwhen neural tissue is being adversely manip-ulated. Most surgeons will appreciate the aidthat monitoring can provide in confirming theanatomy when it deviates from normal as aresult of tumors, other pathologies, or extremevariations.

RESEARCH OPPORTUNITIES

The operating room offers a wealth ofresearch opportunities. In fact, many importantdiscoveries about the function of the normalnervous system as well as about the function ofthe pathological nervous system have beenderived from research activities within theoperating room. Neurophysiological recordingis almost the only way to study the pathophys-iology of many disorders. Many importantdiscoveries were made by applying neuro-physiological methods to work in the operatingroom, but many discoveries were made beforethe introduction of intraoperative neurophysio-logical monitoring (15,16) and many studieswere made in connection with intraoperativeneurophysiological monitoring (14,17,18).Some studies have concerned basic research(19), whereas other studies have been directlyrelated to the development of better treat-ment and better surgical methods (14,17,18);some studies have served both purposes(15,17,19–24).


INTRODUCTION

To understand why and how neuroelectricalpotentials, such as evoked potentials, mightchange as a result of surgical manipulations, it isnecessary to understand the basic principlesunderlying the generation of the neuroelectricalpotentials that can be recorded from various partsof the nervous system. In this volume, we discusselectrical potentials that are generated inresponse to intentional stimulation and wedescribe how the waveform of such recordedpotentials might change as a result of injury tonerves or nuclei. It is also important to under-stand the nature of the responses that might beelicited by surgical manipulations of neural tis-sue and from surgically induced injuries. Fur-ther, it is important to know where in the nervoussystem specific components of the recordedevoked potentials are generated, so that the exactanatomical location of an injury can be identifiedon the basis of changes in specific components ofthe electrical potentials that are being monitored.

The potentials that can be recorded fromnerves and structures of the central nervoussystem can be divided into three large cate-gories: unit (or multiunit), near-field, and far-field potentials.

Unit potentials are potentials recoded fromsingle nerve fibers, nerve cells, or from smallgroups of nerve fibers or nerve cells (multiunitrecordings). Such potentials can be either spon-taneous activity that occurs without any inten-tional stimulation or evoked by some form ofstimulation. Unit or multiunit responses arerecorded by placing small electrodes (micro-electrodes) in indirect contact with nerve fibersor nerve cells. Recording of such potentialshave played important roles in animal studiesof the function of the nervous system. Thesetechniques have only recently been introducedfor use in the operating room.

Near-field evoked potentials are recorded byplacing a much larger recording electrodedirectly on a nerve, a nucleus, or a muscle, andthese potentials represent the sum of the activ-ity in many nerve cells or fibers in one of onlya few structures. It is not always possible torecord near-field potentials because it is notpossible to place a recording electrode directlyon the structure in question; instead, one oftenhas to rely on far-field potentials.

Far-field potentials are recorded from elec-trodes that are placed at a (long) distance fromthe structures that generate the potentials thatare being recorded. Whereas near-field poten-tials, such as those recorded by placing an elec-trode directly on a nerve, nucleus, or muscle,reflect electrical activity in that specific struc-ture, far-field potentials are usually mixtures of

IntroductionUnit ResponsesNear-Field ResponsesFar-Field PotentialsEffect of Insults to Nerves, Fiber Tracts, and Nuclei

3Genera t i on o f E l e c t r i c a l Ac t iv i t y i n t he Nervous Sy s t em and Musc l e s

21


potentials that are generated by several differ-ent structures.

Far-field potentials have smaller amplitudesthan near-field potentials and their waveformsare more difficult to interpret because they repre-sent more than one generator. The generation offar-field potentials is complex and it is not com-pletely understood. The contribution from suchdifferent structures depends on the distance fromthe recording electrode(s) as well as the proper-ties of the sources. For example, only under cer-tain circumstances can propagated neural activityin a long nerve generate stationary peaks inpotentials recorded at a distance from the nerve.The far-field potentials generated by nucleidepends on the orientation of the dendrites of thecells in the nuclei. The contributions from differ-ent structures to recorded far-field potentials aretherefore weighted with regard to factors such asthe distance from the source and the rate at whichthe amplitudes of the recorded potentialsdecrease with distance to the source, whichdepends on the properties of the source.

Components of the evoked potentials fromdifferent sources might overlap, depending onwhether they appear with the same, or different,latencies from the stimulus that was used toevoke the response. Therefore, the waveform offar-field potentials is usually different from thatof near-field potentials and are generally moredifficult to interpret than near-field potentials.

Because of their small amplitude, far-fieldevoked potentials are usually not directly dis-cernable from the background noise that alwaysexists when recording neuroelectrical potentials;therefore, it is necessary to add many responsesusing the method of signal averaging (describedin Chap. 18) so that an interpretable waveformcan be obtained. The use of signal averaging toenhance a signal (evoked response) that is cor-rupted by noise assumes that the waveforms ofall the responses that are added are the same andoccur in an exact time relation (latency) to thestimulus. This might not be the case when theneural system that is being monitored is affectedby surgical manipulation, excess heat, or anoxia.The necessity to average many responses mightdistort the waveform if the responses being

added change (slowly) over the time duringwhich the data are being collected and averagedand, therefore, make the added response difficultto interpret. This is another reason why changesin far-field evoked potentials are more difficult tointerpret than are changes in near-field potentials.

In this chapter, we discuss in greater detail thethree categories of neuroelectrical potentials thatare often recorded in the operating room: unit(multiunit), near-field, and far-field potentials.

UNIT RESPONSES

Unit potentials reflect the activity of a singleneural element or from a small group of ele-ments (multiunit recordings). Action potentialsfrom individual nerve fibers and from nervecells are recorded by placing microelectrodes,the tips of which could be from a few micro-meter to a fraction of a micrometer in diameter,in or near individual nerve fibers. The wave-form of such action potentials is always thesame in a specific nerve fiber or cell body,regardless of how it has been elicited. Infor-mation that is transmitted in a nerve fiber iscoded in the rate and the time pattern of theoccurrence of such action potentials. Thatmeans that it is the occurrence of nerveimpulses and their frequency (rate) that isimportant rather than their waveform.

The action potentials of nerve fibers are theresult of depolarization of a nerve fiber. Usu-ally, the electrical potential inside a nervefiber is about –70 mV. When this intracellularpotential becomes less negative (broughtcloser to zero, or “depolarized”), a complexexchange of ions occurs between the interiorof the nerve fiber and the surrounding fluidthrough the membrane. When the electricalpotential inside an axon becomes sufficientlyless negative than the resting potential, anerve impulse (action potential) will be gener-ated and the depolarization propagates alongthe nerve fiber. This depolarization and subse-quent repolarization is associated with thegeneration of an action potential (also known


as a nerve impulse, nerve discharge, or nervespike). In myelinated nerve fibers (such asthose in mammalian sensory and motornerves), neural propagation occurs along anerve fiber by saltatory conduction betweenthe nodes of Ranvier, which can be recognizedas small interruptions in the myelin sheaththat covers the nerve fiber. Unit potentialshave the character of nerve discharges(spikes) and are recorded by fine-tipped metalelectrodes that are insulated except for the tip.

The main use intraoperatively of recording ofunit potentials is for guiding the surgeon in theplacement of lesions in brain structure, such asthe basal ganglia or thalamus, for treatment ofmovement disorders and pain. More recently,lesions have been replaced by implantation ofelectrodes for electrical stimulation (deep brainstimulation [DBS]), which have a similar benefi-cial effect as lesions but with the advantage ofbeing reversible. The responses that are observedin such operations are either spontaneous activitythat occurs without any intentional stimulation,or by natural stimulation of the skin (touch), orfrom voluntary or passive movement of thepatient’s limbs. For such purposes, usually multi-unit recordings are made, using electrodes withslightly larger tips than those used for recordingof the responses from single fibers or cell bodies.These responses represent the activity of smallgroups of cells or fibers.

NEAR-FIELD RESPONSES

Near-field evoked potentials are defined aspotentials recorded with the recording elec-trode(s) placed directly on the surface of a spe-cific neurological structure. Responses recordedfrom fiber tracts and nuclei are the most impor-tant for intraoperative monitoring, but record-ings from specific regions of the cerebral cortexare also regarded as near-field evoked potentials.

Near-field evoked potentials are recorded byplacing recording electrodes that are much largerthan microelectrodes (gross electrodes) on thesurface of a nerve, fiber tracts, a nucleus or a

specific part of the cerebral cortex. Such poten-tials reflect neural activity in many nerve fibersor cells, but typically only in a single structure.The responses are usually elicited by transientstimuli that activate many fibers of cells at aboutthe same time. Such responses are known as com-pound action potentials (CAPs) because they arethe sum of many action potentials. The potentialsare graded potentials and their waveforms arespecific for nerves and nuclei; the waveformchanges in a characteristic way when the struc-ture, from which recordings are made, is injured.

Responses From NervesNear-field potentials from nerves reflect the

activity in many nerve fibers; hence, it is obtainedas a sum of the action potentials of many nervefibers. The CAPs recorded from a nerve or fibertract reflect the propagation of action potentialsalong individual nerve fibers (axons). When adepolarization is initiated at a certain point alonga nerve fiber, the depolarization propagates alongthe nerve fiber with a (propagation) velocity thatis approximately proportional to the diameter ofthe axons of the nerve. The relation betweenneural conduction velocity (in meters per second[m/s]) and fiber diameter (in micrometers [μm])is approx 4.5 m/s/μm (25). Older data (26) indi-cate a slightly higher velocity: 6 m/s/μm. Theconduction velocity of peripheral sensory andmotor nerves typically ranges from 40 to 60 m/s.The auditory nerve has an unusually low propa-gation velocity of about 20 m/s (27). Normally,depolarization of nerve fibers is initiated at oneend of a nerve fiber (peripheral end of sensoryfibers and central end of motor fibers), but neuralpropagation can occur in both directions of anerve fiber, and it does so with about the sameconduction velocity.

Initiation of Nerve Impulses. Initiation ofnerve impulses in sensory nerves normallyoccurs through activation of sensory receptors(28), and motor nerves are activated throughmotoneurons either in the spinal cord forsomatic nerves or in the brainstem for cranialmotor nerves (29). In the operating room, sen-sory nerves are almost always activated by

Chapter 3 Electrical Activity in the Nervous System 23

sensory stimuli and motor nerves might be acti-vated by (electrical or magnetic) stimulation ofthe motor cortex or the brainstem. Peripheralnerves and cranial motor nerves are also acti-vated by electrical stimulation. Such stimula-tion depolarizes axons at the location ofstimulation of a nerve.

Natural Stimulation. Nerve impulses in sen-sory nerves are normally initiated by an activa-tion of specialized sensory receptor cells thatrespond to a specific physical stimulation (28).The frequency of the elicited action potentials inindividual nerve fibers (discharge rate) is a func-tion of the strength of the sensory stimulation.The time pattern of the occurrence of actionpotentials in a fiber of a sensory nerve also car-ries information about the sensory stimulus in thesomatosensory and the auditory nerves, becausethe discharge pattern is statistically related to thetime pattern of the stimuli, which means that theprobability of the occurrence of a dischargevaries along the waveform of the stimulus (28).This neural coding of the stimulus time pattern isof particular importance in the auditory system,in which much information about sound is codedin the time pattern of the discharges in auditorynerve fibers. The ability of the auditory nervoussystem to use the temporal coding of sounds forinterpretation of complex sounds, such as inspeech, is important for the success of cochlearand cochlear nucleus prostheses (30). In thevisual system, the temporal pattern of nerveimpulses seems to have little importance, as isalso the case in the olfactory and gustatory sen-sory systems.

When sensory nerves are stimulated with nat-ural stimuli, the latency of the response from asensory nerve decreases with increasing stimulusintensity, and this dependence exists over a largerange of stimulus intensities. One reason for thisstimulus-dependent latency is the neural trans-duction in sensory cells (such as the hair cells inthe auditory system), where the excitatory post-synaptic potential (EPSP) increases from belowthreshold at a rate that increases with increasingstimulus intensity and the EPSP thereby reachesthe threshold faster when the stimulus intensity is

high, as compared to when it is low (31). Anotherreason for stimulus-dependent latency is the non-linear properties of the sensory organs such asthe cochlea (see Chap. 5) (32).

Electrical Stimulation. Although sound stim-uli (click sounds) is the most common stimula-tion for monitoring the auditory system,electrical stimulation of peripheral nerves isthe most common way of stimulating thesomatosensory system and for monitoring andintraoperative diagnosis of peripheral nerves.Electrical stimulation is also in increasing usefor stimulation of the motor cortex for monitor-ing motor systems (transcranial electrical stim-ulation [TES]).

The electrical stimulation that is used to depo-larize the fibers of a peripheral nerve use brief(0.1–0.2 ms long) electrical current impulses thatare passed through the nerve that is to be stimu-lated. A negative current is excitatory because itcauses the interior of the axons to become lessnegative, thus causing depolarization. This mightsound paradoxical, but, in fact, a negative electri-cal current flowing through the cross-section of anerve fiber will cause the outside area of thatnerve fiber to become more negative than theinside area and, thereby, the interior of the axonwill become more positive (less negative) than itsouter surface—thus, depolarization occurs.

When a nerve is stimulated by placing twoelectrodes on the same nerve a small distanceapart, the negative electrode (cathode) is theactive stimulating electrode and the positive(anode) electrode might block propagation ofnerve impulses (known as an anodal block) sothat depolarization will only propagate in onedirection, namely away from the negativeelectrode.

The amount of electrical current that is neces-sary to depolarize the axons of a peripheralnerve and initiate nerve impulses depends on theproperties of the individual nerve fibers. Large-diameter axons have lower thresholds than nervefibers with small diameters. The threshold alsodepends on the duration of the electricalimpulses that are used to stimulate a nerve. Thenecessary current to activate nerve fibers


Documents

Intraoperative Neurophysiological Monitoringv This book is based on two earlier works: Aage R. Møller: Evoked Potentials in Intraop- erative Monitoring published in 1988 by Will-