Measurement of extradural relationship to other …less steel, andhas aself-tapping thread. In order to assist fixation using minimumforce on the capsule itself, a tapping screw was

Journal of Neurology, Neurosurgery, and Psychiatry, 1973, 36, 514-522

Measurement of extradural pressure and itsrelationship to other intracranial pressures

An experimental and clinical study

N. J. CORONEOS, D. G. McDOWALL, R. M. GIBSON,V. W. A. PICKERODT, AND N. P. KEANEY

From the Department of Anaesthesia, The University ofLeeds,and Department of Neurosurgery, The General Infirmary, Leeds

SUMMARY Disadvantages associated with the use of ventricular catheters for the prolongedmeasurement of intracranial pressure have resulted in the search for an alternative technique.Measurement of pressure from the extradural space is one such possibility, but widespread accept-ance of this procedure has been limited by the technical difficulties associated with this measurementand lack of information on the relationship between cerebrospinal fluid and extradural pressures.

A study to investigate this relationship and to develop a simple and effective technique for measuringextradural pressure is described.

In certain patients with head injuries and intra-cranial space-occupying lesions, acute rises inintracranial pressure producing brain shift orimpairment of the cerebral circulation-withsubsequent secondary brain damage-may occurbefore changes in vital signs are recognized.Furthermore, the reduction of intracranialpressure by means of one of the several methodscurrently available is too often unpredictable.For these reasons patient management may begreatly assisted, and more readily programmed,if intracranial pressure is directly monitored(Lundberg, Troupp, and Lorin, 1965; Johnston,Johnston, and Jennett, 1970).

Lundberg (1960) described a technique for thedirect closed measurement of intraventricularpressure which has been widely accepted. How-ever, this procedure has several clinical dis-advantages, as recognized by Lundberg (1972)himself. Because of the disadvantages, muchattention has been focused on developing asimple alternative technique which does notrequire penetration of the dura-arachnoidmembranes. Many of these techniques employimplanted electronic pressure-sensitive trans-ducers which often give technical difficulties.Furthermore, the clinical interpretation of the

data is impeded by lack of firm knowledge ofthe relationship between pressure recorded inthe extradural space and the intracranial pres-sure.The aims of this study were: (1) to explore

the relationship between extradural pressure,supratentorial subarachnoid pressure and ven-tricular pressure; (2) to assess the use of latexballoons as pressure-sensing devices in theextradural and subarachnoid spaces; (3) toevaluate a metal capsule, to be described, forclinical use in measuring extradural pressureover prolonged periods with minimal discomfortto the patient.

METHODS

ANIMAL STUDIES The experiments were carried outon 12 dogs, but two baboons were also studiedbecause the dura mater in these primates is more likethat in man, being only loosely attached to the skull.The dogs were intubated after induction with thio-pentone sodium, and anaesthesia maintained withnitrous oxide, oxygen, and halothane. Artificialventilation to normocarbia and normoxia wasachieved with the aid of muscle relaxants. Bodytemperature was maintained at 370 C. The baboonswere premedicated with phencyclidine 0 5-1 mg/kg

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Measurement of extradural pressure anid its relationship to other intracranial pressures

mnd the anaesthetic technique was the same as for.he dogs, except that the induction was inhalationalwvith nitrous oxide, oxygen, and halothane.

Arterial blood-pressure, central venous pressureand intracranial pressures were measured by elec-tronic transducers mounted at the same level as thecisterna magna, all animals being in the right lateralposition. Pressures were recorded on a multi-channelphysiological recorder. At least one hour elapsedafter the end of the operative procedures (to bedescribed) before the recording of pressures.

1. Latex balloons The compliance of small latexballoons was measured in vitro by adding incre-ments of saline, to a total of 2 ml. There was nosignificant increase in pressure recorded from theseballoons over this volume range.The balloons contained sufficient fluid to fill their

dead space, care being taken to avoid air bubbles. Infive dogs the balloons were implanted in the extra-dural space through a burr hole, after carefullystripping away the dura mater and achieving haemo-stasis. The balloon then lay in the extradural spaceaway from the skull opening. A catheter was insertedinto a lateral ventricle through a separate burr hole.All openings in the skull were closed with dentalcement. A standard volume (05 ml.) of fluid wasinjected into each balloon after insertion into theextradural space, and the pressures from both sitesrecorded.

FIG. 1. Stainless steel extradural capsule.

2. Extradural metal capsule A small stainless steelcapsule was constructed (Fig. 1). It was 9 mm indiameter and 2 mm in height, with multiple sideholes connecting with a central catheter. Side holes,rather than an end hole, were chosen to facilitaterecording and to minimize occlusion by the duramater. This device was inserted in seven dogs andtwo baboons through a burr hole, to lie in the extra-dural space under one parietal eminence.A latex balloon was inserted in the subarachnoid

space through another burr hole on the oppositeside, to lie over the anterolateral surface of theparietal cortex. Each balloon was carefully filledwith 0 5 ml. saline. All burr holes were sealed with

dental cement. A fine catheter was inserted underdirect vision through the tectorial membrane and itstip positioned to lie in the cisterna magna. A three-way tap attached to this catheter allowed either therecording of pressure or small infusions of normalsaline into the posterior fossa (Fig. 2).

Saline (0-2 ml.) was injected into the extraduralcapsule and, between three and 15 minutes later,pressures were simultaneously obtained from allthree measuring sites, while the intracranial pressurewas altered by means of small intermittent infusionsinto the cisterna magna. In each animal there wereat least four such studies over a wide range of pres-sures. All measurements were completed within sixhours of insertion of the extradural capsule. At theend of each experiment methylene blue was injectedinto the capsule to establish whether a complete sealhad been achieved around the capsule. Three dogswere excluded from the results because a leak wasdemonstrated.

CLINICAL STUDIES Patients considered to be at riskfrom increased intracranial pressure in the periodafter neurosurgery had a ventricular catheterinserted for pressure measurement. An extraduralcapsule, modified for clinical use by incorporating athreaded metal plug to allow fixation to the skull(Fig. 3) was implanted through a separate burr holeinto the extradural space.The extradural pressure capsule was sited over

the convexity of the skull in the frontal or parietalarea. An incision 3 cm long was made and the peri-cranium stripped from the skull. A skull perforatorwith matching cone and rose were used to make theskull opening. Alternatively, the opening could bemade with an electric or compressed air-driven skullperforator. It is essential that the diameter of thehole is such that the extradural pressure capsule willfit securely, both from the point of view of patientsafety and for obtaining reliable recordings. Theextradural pressure capsule is made of EN58 stain-less steel, and has a self-tapping thread. In order toassist fixation using minimum force on the capsuleitself, a tapping screw was also used to prepare theskull to receive the capsule. Histacryl tissue glue wasused in the early cases to seal the capsule in position,but this technique was given up since use of the glueis unnecessary when the capsule is properly andfirmly inserted.

Before inserting the capsule into the skull it wasnecessary to secure haemostasis in respect of thedural surface and the extradural space. If smallvessels on the surface of the dura mater bled theywere gently controlled with coagulation diathermycurrent. If extradural oozing of blood persisted, thiswas stopped by tiny pieces of Surgicel tucked gently

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516 N. J. Coroneos, D. G. McDowall, R. M. Gibson, V. W. A. Pickerodt, and N. P. Keaney

FIG. 2. Schematicdiagram of dog study.Cerebrospinal fluid pressureis altered by infusion offluid into cisterna magna.PED, pressure recorded fromextradural space. PCSF,pressure recordedfromsupratentorial subarachnoidspace. PPF, pressurerecordedfrom posteriorfossa.

into the extradural space. This was not often neces-sary unless the capsule had been placed near themidline of the skull.

Before inserting the capsule the burr hole openingand the exposed dura mater were irrigated thoroughlywith saline at body temperature to remove anydebris, bone dust or blood. A spray of topical anti-biotic (Polybactrin) was also used to minimize therisk of infection.The scalp was sutured in a single layer with inter-

rupted black silk sutures. When the capsule wasremoved the wound was checked under good illu-mination. No evidence of haemorrhage or infection

FIG. 3. Extradural capsule modifiedfor clinical use.

was found in our cases. The wound was alwaysclosed in a single layer, no deep suture beingemployed in the secondary closure.The practical clinical problems encountered in

this study were:

a. The capsule could not be inserted if the skullopening was too small. Similarly, if the openingwere even marginally too large the capsule wasunsafe and recordings were not valid. This difficultycan be overcome by the use of a tapering screwcapsule and work on this is in progress.

b. The head bandages had to be arranged aroundthe device in such a way as to allow access to itwithout disturbing the wound or the head dressing.

c. In very thick skulls-for example, in patientswith acromegaly or Paget's disease-the capsulemay be too small and for these patients a deepercapsule is necessary. Similarly, for children a shortercapsule was used, otherwise the skin could not beclosed over it.

d. Where extradural bleeding or oozing welled upimmediately after its insertion, the capsule had to beremoved, cleansed, and rinsed through before re-insertion.The technical aspects of the use of the capsule did

not prove difficult or troublesome in our hands, andthe insertion and removal processes are now routine.They are mentioned none the less as guidance toothers who may use the technique.



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Measurement of extradural pressure and its relationship to other intracranial pressures

EDP - IVP

mm Hg

+30

+20

+ 10

0

-10I

mm Hg

100

80

60

*1 .2 .3 .4 .5

40

20ml

VOLUME IN E D-BALLOON

FIG. 4. Graph showing the effect of altering thevolume offluid in extradural balloons on the accuracyof the pressure recorded from the system. EDP,extradural pressure. IVP, intraventricular-measuredby catheter. Upper curve shows the effect of addingfluid to the balloon; lower curve the effect of with-drawing fluid.

Over a 12 month period 25 such patients weremonitored by both techniques. The extradural andventricular catheter were connected to a miniatureelectronic transducer which was mounted externallyon the head bandage and could therefore be readilycalibrated. Pressure measurements were taken fromboth sites, three to 15 minutes after injecting thecapsule with 0-2 ml. saline. The capsule was removedunder local analgesia, usually 48 to 72 hours afteroperation, when measurements were no longerconsidered necessary for clinical management.

RESULTS

ANIMAL STUDIES 1. Latex balloons The extra-dural pressure (EDP) readings obtained fromlatex balloons were unreliable, either over-estimating or under-estimating the extraduralpressure in an unpredictable manner, whenfilled with the standard volume of 0 5 ml. Inorder to study this further, in one dog 0-1 ml.aliquots of saline, to a total volume of 0-5 ml.,were injected and removed, and pressures fromboth the balloon and ventricle were recordedafter each volume change. The extraduralpressure measured was dependent upon thevolume of fluid in the balloon, and upon whetherthe balloon was being filled or emptied (Fig. 4).There was good correlation between extradural

0

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4",4

SAP

20 40 60 80 100 mm Hg

FIG. 5. Comparison of posterior fossa pressure(measuredby catheter) andsupratentorial subarachnoidpressure (measured by latex balloons) in dogs.PFP, posterior fossa pressure. SAP, subarachnoidpressure.

and intraventricular pressure (IVP) for a shortbut inconstant time (minutes), following thetemporary distension of the balloon with 05 ml.,at an initial volume of 05 ml.

2. Extradural mnetal capsule During several

100/

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FIG. 6. Comparison ofextradural pressure (measuredby capsule) and supr-atentorial subarachnoid pressure(measured by latex balloons) in dogs. Broken line-line of identity.

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EDP /

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infusion. Subsequent infusions in this dog pro-duced results consistent with those found in theremaining animals. The mean values shown inFig. 7 demonstrate that, over the entire range ofpressures that would commonly be encountered,no appreciable pressure gradient occurredacross the dura mater under the conditions ofmeasurement described. The attainment of acompletely leak-free system was seen to beessential because any decompression of theextradural space led to underestimation ofpressure (Fig. 8). In the baboon, similar resultswere obtained for the relationship betweenextradural pressure and supratentorial sub-arachnoid pressure.

FIG. 7. Extradural pressure (means and standarderrors) observed from capsule with 5 mmHg changesin supratentorial subarachnoid pressure in four dogs.

CLINICAL STUDIES In five of the 25 patients acomparison between the extradural and ven-

separate infusions into the cisterna magna offour dogs, 103 observations were made of pres-sures recorded simultaneously from the posteriorfossa and the supratentorial subarachnoidcerebrospinal fluid. Figure 5 demonstrates theclose correlation found between these two pres-sures. Figure 6 shows similar close correlationbetween extradural and supratentorial subarach-noid pressures, based on 156 measurementsfrom the four dogs. All points lie close to theline of identity, except in one dog during one

A B

*7 i SAP

EDP

1 5 3 0MINUTES

FIG. 8. Effect on accuracy of measurement of EDPofa poorly-sealed capsule. Arrow denotes injection of0-2 ml. saline into the capsule. SAP, supratentorialsubarachnoid pressure. Horizontal bars indicate halo-thane administration.

FIG. 9. Relationship of extradural pressure tointraventricular pressure within six hours of crani-otomy. Solid line-regression line. Broken line-lineof identity.

tricular pressures could not be made post-operatively. Difficulty in locating a smallventricle prevented the insertion of a catheter inone and obstruction or displacement of thecatheter accounted for the exclusion of twoothers. Of the remaining two patients, one hada separate burr hole 5 mm away from thecapsule and it was considered that decompres-sion from the extradural space was occurringthrough this. In the last patient, the capsule was

palpably loose in the burr hole. This problem

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has not recurred since the insertion procedurehas been modified to include the use of a bone-tapping tool.

In the remaining 20 patients the extraduraland intraventricular pressures were comparedand the results have been divided into threegroups, based on time of measurement afterinsertion of the extradural capsule.

1. Readings within six hours of inserting extra-dural capsule For various reasons unrelated tothe extradural device, readings were not takenfrom four patients within six hours of operation,reducing this group to 16. Figure 9 shows therelationship between EDP (Y-axis) and IVP

-200 V-200

4001

O 200IVP (mm H20)

FIG. 10. Relationship of extradurintraventricular pressure within 6 tocraniotomy. Solid line-regression lineline of identity.

FIG. 11. Relationship of extradural pressure tointraventricular pressure within 24 to 48 hours ofcraniotomy. Solid line-regression line. Broken line-line of identity.

(X-axis) obtained from 178 observations takenbetween three and 15 minutes after flushing thecapsule. The readings form a band which crossesthe line of identity, with the extradural pressure

400 ' 0 tending to exceed the intraventricular pressureat low values of the latter. With ventricular

al pressure to pressures greater than 100 mm H20, the24 hours after converse relationship may be noted. Of the'.Broken line- seven patients in whom the intraventricular

pressure was low, all had had removal of sub-

300-0-

l-

100-

o- V FE

WW.W.h

V E V

FIG. 12. Effect of injecting 0-2 ml. saline into extradural capsule in a post-craniotomy patient.V, ventricular trace. F, time of injection. E, extradural trace. Top trace: time marker. Gaps atone minute intervals.

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of cerebrospinal fluid at

2. Readings about 24 hours after insertion ofextradural capsule Figure 10 shows the valuesobtained from 222 observations on 20 patientsafter flushing the extradural capsule. At thisstage the band of readings has moved closer tothe line of identity.

3. Readings about 48 hours after insertion of

extradural capsule All but 16 of the 220readings of extradural pressure taken from 18patients at this time lie above the line of identity,with the line of best fit being 80-100 mm H20higher than, and approximately parallel with,the line of identity (Fig. 11). In almost allpatients flushing of the capsule resulted in a

localized rise in extradural pressure. In none was

more than 50 mm H20 of this pressure increasetransmitted to the ventricle (Fig. 12).

In general, the magnitude and duration of theextradural pressure rise after flushing dependedon the time that had elapsed after insertion ofthe capsule.The correlation coefficients and values for

intercept and slope of the calculated regressionlines are shown in the Table.

DISCUSSION

The direct closed measurement of intraventricu-lar pressure described by Lundberg (1960) was a

significant step forward in the management ofhead injuries, and allowed a quantitative assess-

ment of the efficacy of the measures designed toreduce cerebral oedema (Lundberg, et al., 1965;Coe, Nelson, Rudenberg, and Garza, 1967;Johnston, et al., 1970). However, this techniquehas serious disadvantages-namely, the diffi-culty of locating a displaced or compressed

ventricle, alteration in intracranial hydro-dynamics with possible disastrous effects whencerebrospinal fluid is withdrawn or inadvertentlylost (Schettini, McKay, Majors, Mahig, andNevis, 1971; Nornes, personal communication)the possibility of introducing infection directlyinto the ventricular system, and difficulty inmaintaining an unobstructed catheter/mano-meter tube system over a prolonged period(Lundberg, 1960; Lundberg et al., 1965;Jacobson and Rothballer, 1967). In our ownseries of 25 patients, the ventricle could not belocated in one, and obstruction or displacementof the catheter occurred in two. In no case wasthere clinical evidence of infection.

In addition to intraventricular pressuremeasurement, a number of pressure-sensingdevices have been implanted intracranially. Inthe subdural space, fluid-filled latex bal-loons (Hoppenstein, 1965), pneumatic switches(Numoto, Slater, and Donaghy, 1966) andelectronic transducers (Hulme and Cooper,1966; Coe et al., 1967; Richardson, Hide, andEversden, 1970) have been used. This approach,however, involves breaching the integrity of thedura mater, and therefore miniature transducersof varying design (Nornes and Serck-Hanssen,1970; Schettini, McKay, Majors, Mahig, andNevis, 1971) have been designed for implantingin the extradural space. These have not beenwidely accepted for clinical monitoring becauseof technical difficulties concerning zero drift,calibration, and hysteresis.

It was for these reasons that we decided toevaluate a simple system for measuring extra-dural pressure. The first approach was to testlatex balloons in the extradural space of experi-mental animals, where they were found to be un-reliable; pressures recorded were dependent onthe contained volume and on the in situ hysteresisof individual balloons. Similarly unreliableresults were obtained in a few post-craniotomypatients in whom small balloons were implantedin the extradural space. A possible explanationof the unreliability of the balloon readings isthat adherence of the dura mater to the skulllimits the free expansion of the balloon.The difficulties associated with latex balloons

in the extradural space were not found when theballoons were inserted into the subarachnoidspace, where a close correlation was seen

TABLE

Period Readings Correlation Slope of Intercept ofafter (no.) coefficient regression regression

operation line line(hr) (ntm H20)

6 178 0 48 0o59 +4824 222 0-64 0 67 +4848 220 0-79 1 06 + 84

stantial quantitiesoperation.



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between the posterior fossa pressure and thesupratentorial subarachnoid pressure. This agreeswith the finding of Langfitt, Weinstein, Kassell,and Gagliardi (1964) and justifies the use ofballoons in the supratentorial subarachnoidspace as a means of measuring accurately cere-brospinal fluid pressure in this region.

Balloons having proved unsatisfactory in theextradural space, a metal capsule was designedfor the implantation in this site and consistentrepeatable results were obtained. In order toensure patency, the capsule was flushed inter-mittently with 0-2 ml. saline, this being itsinternal volume. This metal capsule demon-strated that the pressure in the extradural spaceof experimental animals was identical with thesubarachnoid pressure over a range of intra-cranial pressure up to 100 mmHg (Fig. 6). Thisimplies that the dura mater fully transmits thesubarachnoid pressure to the artificial extra-dural space. The results of Langfitt et al. (1964)would appear to be in line with this conclusion.These workers also investigated the stretching ofdura mater and concluded that in vitro it is avirtually non-elastic material. Thus, our findingin animal experiments, that no pressure gradientexisted, must imply that the dura mater is notfully distended in vivo. This non-distensibility ofthe dura mater accounts for the need to seal thecranial cavity completely.

Schettini and co-workers (1971)-using anextradural transducer-found that 'brain sur-face pressure' exceeded posterior fossa pressurein dogs, which apparently contradicts ourfindings and those of Langfitt et al. (1964). Theypostulated that there exists at the brain surface atissue pressure which exceeds subarachnoidpressure, due to the presence of a gradientacross the pia mater. They further believe thattheir extradural transducer, which incorporateda non-sensitive coplanar ring, pushed on to thebrain through the intact dura mater, detectedthis 'brain surface pressure'. As we are notcomparing the same pressure, their results donot necessarily conflict with ours.Turning now to the clinical results, and

beginning with measurements made in the firstsix hours after operation for neurosurgical pro-cedures, it was found that in approximately 60%of the patients the extradural capsule recordedpressures lower than the intraventricular pres-

sure during the 12 minute observation period. Asimilar picture had been seen in dogs known tobe decompressing around a poorly-sealed cap-sule (Fig. 8). In the case of patients, however,injection of methylene blue through the capsuleinto the extradural space did not result in stain-ing of the scalp tissues. This suggests that de-compression may have been occurring else-where, possibly through the mobile bone flap. Inthe remaining patients in whom extraduralpressure exceeded the intraventricular pressure,the ventricular pressure was usually found to below and associated with removal of substantialamounts of cerebrospinal fluid in the course ofthe surgical procedure. On the next day the slopeof the regression line had moved closer to theline of identity, and this may have been due tosealing of the bone flap and expansion of thecerebrospinal fluid volume. By 48 hours aftersurgery the regression line was virtually parallelwith the line of identity, and measurements ofextradural pressure correlated well with intra-ventricular pressure readings but systematicallyover-read by 80-100 mm H2O for reasons thatare still not clear. Possible explanations includetissue reaction to the capsule, accumulation offlushing fluid, and slow venous oozing. Withregard to this last possibility, no clinicallyimportant amount of blood clot was discoveredwhen the capsule was removed.As mentioned previously, it was considered

necessary to flush the capsule intermittently.This procedure produced small and transientelevations of extradural pressure immediatelyafter insertion. With the progression of timeafter surgery, the rise in extradural pressureproduced by the same volume of flushing fluidwas higher and longer sustained. However, onlyan insignificant part of this was transmitted tothe intraventricular pressure. This differencebetween extradural and intraventricular pres-sures after flushing can be ascribed to the non-distensibility of the dura mater already discussedand may indicate that the space in which thecapsule is sited and the channels for absorbingthe injected fluid are becoming obliterated byfibrin and organizing blood clot.

Previous evidence concerning the relationshipbetween extradural pressure and intraventricularpressure in man is limited to a comment byLundberg (1972), that: 'extradural pressure

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measurement may give adequate informationabout variations in ICP and, within certainlimits, permit approximate quantitative assess-ment of the actual pressure level', and a figure ofthe comparison of the two pressures in onepatient published by Nornes and Magnaes(1971). The results from this latter case showedextradural pressure to be higher than intra-ventricular pressure by a margin similar to thatreported here.From the results of this study it would seem

that the use of a perforated metal capsule in theextradural space for the recording of intra-cranial pressure after neurosurgery has theadvantage that the dura mater need not bebreached, and thus avoids producing changes inintracerebral hydrodynamics and minimizes therisk of infection in the cerebrospinal fluid. Inaddition, a check on base-line drift of calibrationcan be achieved accurately and simply becausethe transducer is mounted externally on thehead bandage. The animal studies indicate that,at least in the six hours after insertion- of thedevice, the dura mater provides no barrier to theaccurate transmission of pressure from thecerebrospinal fluid to the extradural space, pro-vided the volume of fluid injected into the cap-sule is not excessive.

Observations from the clinical trial suggestthat this system may be a valuable aid in clinicalmanagement 24 and 48 hours after operation.However, immediately after craniotomy, pre-sumably because of continuing decompressionof the extradural space, the method is a lessreliable guide to intracranial pressure. In casesof closed head injury, where there is neither amobile bone flap nor loss of cerebrospinal fluid,measurement of extradural pressure may give anaccurate indication of intracranial pressure fromthe moment of capsule insertion, as was found inour animal experiments. A study of this extra-dural pressure measuring system in patients withhead injury is in progress.

The authors wish to thank Dr. K. I. Gouda and thenursing staff of the Leeds General Infirmary and ofChapel Allerton Hospital for their assistance duringthe clinical trial. They also wish to thank Messrs.Michael Standen, Kahl Horner, James Allen,Michael Maher, Adrian Russell, and Brian Goughfor technical assistance, and Mrs. M. Albers forsecretarial help. During this study N.J.C. was inreceipt of a Commonwealth Medical Fellowshipsponsored by the British Council, and N.P.K. was inreceipt ofa grant from the Medical Research Council.

REFERENCES

Coe, J. E., Nelson, W. J., Rudenberg, F. H., and Garza, R.(1967). Technique for continuous intracranial pressurerecording. Journal of Neurosurgery, 27, 370-375.

Hoppenstein, R. (1965). A device for measuring intracranialpressure. Lancet, 1, 90-91.

Hulme, A., and Cooper, R. (1966). A technique for theinvestigation of intracranial pressure in man. Journal ofNeurology, Neurosurgery, and Psychiatry, 29, 154-156.

Jacobson, S. A., and Rothballer, A. B. (1967). Prolongedmeasurement of experimental intracranial pressure using asubminiature absolute pressure transducer. Journal ofNeurosurgery, 26, 603-608.

Johnston, I. H., Johnston, J. A., and Jennett, B. (1970).Intracranial-pressure changes following head injury.Lancet, 2, 433-436.

Langfitt, T. W., Weinstein, J. D., Kassell, N. F., andGagliardi, L. J. (1964). Transmission of increased intra-cranial pressure. 2. Journal of Neurosurgery, 21, 998-1005.

Lundberg, N. (1960). Continuous recording and control ofventricular fluid pressure in neurosurgical practice. ActaPsychiatrica Scandinavica, 36, Suppl. 149.

Lundberg, N. (1972). Monitoring of intracranial pressure.Proceedings of the Royal Society of Medicine, 65, 19-22.

Lundberg, N., Troupp, H., and Lorin, H. (1965). Continuousrecording of the ventricular-fluid pressure in patients withsevere acute traumatic brain injury. Journal of Neuro-surgery, 22, 581-590.

Nornes, H., and Magnaes, B. (1971). Supratentorial epiduralpressure recorded during posterior fossa surgery. Journalof Neurosurgery, 35, 541-549.

Nornes, H., and Serck-Hanssen, F. (1970). Miniaturetransducer for intracranial pressure monitoring in man.Acta Neurologica Scandinavica, 46, 203-214.

Numoto, M., Slater, J. P., and Donaghy, R. M. P. (1966).An implantable switch for monitoring intracranial pres-sure. Lancet, 1, 528.

Richardson, A., Hide, T. A. H., and Eversden, I. D. (1970).Long-term continuous intracranial-pressure monitoring bymeans of a modified subdural pressure transducer. Lancet,2, 687-690.

Schettini, A., McKay, L., Majors, R., Mahig, J., and Nevis,A. H. (1971). Experimental approach for monitoring sur-face brain pressure. Journal of Neurosurgery, 34, 38-47.



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Documents

Measurement of extradural relationship to other …less steel, andhas aself-tapping thread. In order to assist fixation using minimumforce on the capsule itself, a tapping screw was