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s an obstetric anesthesiologist, I am very familiar with the “subdural”space . . . or, so I thought. Every few months or so, a colleague, resident, or

yself will place an epidural catheter for labor analgesia, only to have it performn strange and unexpected ways. Usually the block obtained is much higher thanxpected, spreads unpredictably, or causes symptoms such as numbness in theands, difficulty swallowing, or shortness of breath, yet is not as dense as oneould expect with a subarachnoid injection. As the resident expert in suchatters I am usually called upon to explain how this could happen. I generally

nly have two possible explanations to offer: interindividual variation or annintended subdural injection.Most of us have been taught that the subdural space lies between the duraater and the arachnoid matter, the two outer membranes surrounding the

pinal cord. According to my edition of Cousins’ and Bridenbaugh’s text,. . . There is a capillary interval, called the subdural space, between the dura andhe arachnoid. It contains a minute quantity of serous fluid, but has no connec-ion with the subarachnoid space that contains the CSF.”1 Miller’s current text-ook calls the subdural space “. . . a potential space between the dura mater andhe arachnoid which contains only small amounts of serous fluid . . .”2 Cousinsnd Bridenbaugh go on to state “. . . The dura and the arachnoid are in such closeontact that in the process of lumbar puncture, it is not possible to pierce the duraithout piercing the arachnoid as well.”1 One hundred pages later they have

pparently reconsidered, stating “. . . Subdural cannulation results from perfora-ion of the dura without penetration of the underlying arachnoid membrane. Thiss a rare result of intended epidural cannulation. It occurs quite frequently during

yelography and in spinal anesthesia, with an incidence of up to 1 in 100.”3

It is cannulation of this subdural space that I (and many other anesthesiologists)ave assumed is responsible for the strange high blocks mentioned above. For

iterally decades, anesthesiology residents, myself included, have been taught “. . .njected solutions spread slowly but quite extensively in the subdural space, andhis particular anatomical error may be a cause of occasional cases of unexplainedassive extradural analgesia.”4 Twenty-five years later, little seems changed:

Subdural injection of a local anesthetic leads to an unexpectedly high but patchylock. A subdural block has a variable spread that is ultimately quite extensive forhe volume of local anesthetic injected.”5

An article in this issue of Regional Anesthesia and Pain Management by Cliveollier, M.D.,6 presents compelling evidence that maybe we have been wrongbout the subdural space all these years. Maybe subdural injection of localnesthetics does not always lead to “exaggerated extension” of the block; perhapst is just the opposite—maybe it more often leads to inadequate spread of thenjected local anesthetic! Further, maybe subdural injection is not as rare as weave been led to believe. . . .Dr. Collier presents four cases of epidural anesthetics with inadequate spread of

nesthesia and delayed onset that required additional epidural local anestheticupplementation to be effective. After each case was ended, injection of a ra-iopaque dye and radiographic imaging revealed the catheters to be subdural,ith limited spread of the injected contrast. This is the first report to clearly

ssociate limited local anesthetic spread with subdural injection.In retrospect, much of the evidence has been available to us for years, although

erhaps not exactly where anesthesiologists would typically look for it. It doeseem perplexing that radiologists never seemed to have much difficulty gettingnto the subdural space, while for anesthesiologists it was a “rare result.”3 Fortyears ago, subdural injection was reported to complicate over 10% of attempted

Accepted for publication November 10, 2003.doi:10.1016/j.rapm.2003.11.003

See Collier page 45

Regional Anesthesia and Pain Medicine, Vol 29, No 1 (January–February), 2004: pp 7–8 7

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8 Regional Anesthesia and Pain Medicine Vol. 29 No. 1 January–February 2004

yelograms,7 although more recent reports indicate that the incidence of theomplication is somewhat lower.8 Interestingly, radiologists often find that oncen unintended injection into the subdural space has been recognized, it can beifficult to perform the intended subarachnoid injection. If the dura and arach-oid membranes really are so closely opposed, and injections into this “potentialpace” spread widely, how could this be the case?

As Collier points out in his report, the subdural space may be a very differentnimal than we have been led to believe. Unless a clinician regularly readsnatomy and pathology journals, he or she might not have been aware that theoncept of a “subdural space” had become controversial. It may not be a “potentialpace” at all; in fact, it may not even exist! What we have come to think of as theubdural space may actually be an iatrogenic dissection of a cellular layer betweenhe dura and the arachnoid. Thought of as a dissection, a number of clinicalbservations suddenly make much more sense. The extent of a dissection can bextremely variable: it can be highly extensive, resulting in a high block, or it cane limited, resulting in a low, inadequate block. Low or inadequate blocks areertainly a much more common clinical problem than overly extensive blocks. Aommon scenario in obstetric anesthesia involves the case in which the epiduraloes in smoothly, the initial bolus dose provides some analgesia, but whenssessed, levels are clearly inadequate. Redosing the catheter two or even threeimes fails to raise the patient’s level sufficiently, and finally, after the patient hasndured her discomfort for 40 or 50 minutes more than either you or shenticipated, you give up and replace the catheter, despite the fact that there werelearly demonstrable, yet insufficient, levels.

What went wrong? As I tell the patient when this happens, “I’ve given younough drugs that if the catheter were in the correct place, you would beomfortable. All I can say is it must not be in the right place.” I often wonderedut never knew where all that local anesthetic was going, but Collier’s report mayrovide the answer—perhaps it was a subdural injection, a subdural dissection,ith very limited spread. Transient pain on injection may be a marker of such

njections, and I will look more closely for it in the future.It is surprising at times how much dogma resides within our science of anes-

hesiology. Sometimes it becomes so ingrained that it goes unchallenged forecades, for the simple reason, (to paraphrase Collier) “. . . nobody (is) lookingor it.” Maybe it is time to take a closer look at this thing called the subdural space,nd our understanding of it.

Craig Palmer, M.D.Department of Anesthesiology

University of Arizona Health Sciences CenterTucson, Arizona

References

. Bridenbaugh PO, Kennedy WF Jr. Spinal, subarachnoid neural blockade. In: CousinsMJ, Bridenbaugh PO, eds. Neural Blockade. Philadelphia, PA: J.B. Lippincott Company;1980:146-175.

. Brown DL. Spinal, epidural, and caudal anesthesia. In: Miller RD, ed. Anesthesia. 5th ed.Philadelphia, PA: Churchill Livingstone; 2000:1492-1493.

. Cousins MJ. Epidural neural blockade. In: Cousins MJ, Bridenbaugh PO, eds. NeuralBlockade. Philadelphia, PA: J.B. Lippincott Company; 1980:176-274.

. Bromage PR. Epidural Anesthesia. Philadelphia, PA: W.B. Saunders Company; 1978:20.

. Rosen MA, Hughes SC, Levinson G. Regional anesthesia for labor and delivery. In:Hughes SC, Levinson G, Rosen MA, eds. Anesthesia for Obstetrics. 4th ed. Philadelphia,PA: Lippincott Williams and Wilkins; 2002:42.

. Collier CB. Accidental subdural injection during attempted lumbar epidural block maypresent as a failed or inadequate block: Radiographic evidence. Reg Anesth Pain Med2004;29:45-51.

. Jones MD, Newton TH. Inadvertent extra-arachnoid injection in myelography. Radiology1963;80:818-822.

. Ajar AH, Rathmell JP, Mukherji SK. The subdural compartment. Reg Anesth Pain Med2002;27:72-76.