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Exploring the subdural space
Exploring the Subdural Space
A
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 myself will place an epidural catheter for labor analgesia, only to have it perform in strange and unexpected ways. Usually the block obtained is much higher than expected, spreads unpredictably, or causes symptoms such as numbness in the hands, difficulty swallowing, or shortness of breath, yet is not as dense as one would expect with a subarachnoid injection. As the resident expert in such matters I am usually called upon to explain how this could happen. I generally only have two possible explanations to offer: interindividual variation or an unintended subdural injection. Most of us have been taught that the subdural space lies between the dura mater and the arachnoid matter, the two outer membranes surrounding the spinal cord. According to my edition of Cousins’ and Bridenbaugh’s text, “. . . There is a capillary interval, called the subdural space, between the dura and the arachnoid. It contains a minute quantity of serous fluid, but has no connection with the subarachnoid space that contains the CSF.”1 Miller’s current textbook calls the subdural space “. . . a potential space between the dura mater and the arachnoid which contains only small amounts of serous fluid . . .”2 Cousins and Bridenbaugh go on to state “. . . The dura and the arachnoid are in such close contact that in the process of lumbar puncture, it is not possible to pierce the dura without piercing the arachnoid as well.”1 One hundred pages later they have apparently reconsidered, stating “. . . Subdural cannulation results from perforation of the dura without penetration of the underlying arachnoid membrane. This is a rare result of intended epidural cannulation. It occurs quite frequently during myelography 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) have assumed is responsible for the strange high blocks mentioned above. For literally decades, anesthesiology residents, myself included, have been taught “. . . Injected solutions spread slowly but quite extensively in the subdural space, and this particular anatomical error may be a cause of occasional cases of unexplained massive extradural analgesia.”4 Twenty-five years later, little seems changed: “Subdural injection of a local anesthetic leads to an unexpectedly high but patchy block. A subdural block has a variable spread that is ultimately quite extensive for the volume of local anesthetic injected.”5 An article in this issue of Regional Anesthesia and Pain Management by Clive Collier, M.D.,6 presents compelling evidence that maybe we have been wrong about the subdural space all these years. Maybe subdural injection of local anesthetics does not always lead to “exaggerated extension” of the block; perhaps it is just the opposite—maybe it more often leads to inadequate spread of the injected local anesthetic! Further, maybe subdural injection is not as rare as we have been led to believe. . . . Dr. Collier presents four cases of epidural anesthetics with inadequate spread of anesthesia and delayed onset that required additional epidural local anesthetic supplementation to be effective. After each case was ended, injection of a radiopaque dye and radiographic imaging revealed the catheters to be subdural, with limited spread of the injected contrast. This is the first report to clearly associate limited local anesthetic spread with subdural injection. In retrospect, much of the evidence has been available to us for years, although perhaps not exactly where anesthesiologists would typically look for it. It does seem perplexing that radiologists never seemed to have much difficulty getting into the subdural space, while for anesthesiologists it was a “rare result.”3 Forty years 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
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Regional Anesthesia and Pain Medicine Vol. 29 No. 1 January–February 2004
myelograms,7 although more recent reports indicate that the incidence of the complication is somewhat lower.8 Interestingly, radiologists often find that once an unintended injection into the subdural space has been recognized, it can be difficult to perform the intended subarachnoid injection. If the dura and arachnoid membranes really are so closely opposed, and injections into this “potential space” spread widely, how could this be the case? As Collier points out in his report, the subdural space may be a very different animal than we have been led to believe. Unless a clinician regularly reads anatomy and pathology journals, he or she might not have been aware that the concept of a “subdural space” had become controversial. It may not be a “potential space” at all; in fact, it may not even exist! What we have come to think of as the subdural space may actually be an iatrogenic dissection of a cellular layer between the dura and the arachnoid. Thought of as a dissection, a number of clinical observations suddenly make much more sense. The extent of a dissection can be extremely variable: it can be highly extensive, resulting in a high block, or it can be limited, resulting in a low, inadequate block. Low or inadequate blocks are certainly a much more common clinical problem than overly extensive blocks. A common scenario in obstetric anesthesia involves the case in which the epidural goes in smoothly, the initial bolus dose provides some analgesia, but when assessed, levels are clearly inadequate. Redosing the catheter two or even three times fails to raise the patient’s level sufficiently, and finally, after the patient has endured her discomfort for 40 or 50 minutes more than either you or she anticipated, you give up and replace the catheter, despite the fact that there were clearly demonstrable, yet insufficient, levels. What went wrong? As I tell the patient when this happens, “I’ve given you enough drugs that if the catheter were in the correct place, you would be comfortable. All I can say is it must not be in the right place.” I often wondered but never knew where all that local anesthetic was going, but Collier’s report may provide the answer—perhaps it was a subdural injection, a subdural dissection, with very limited spread. Transient pain on injection may be a marker of such injections, and I will look more closely for it in the future. It is surprising at times how much dogma resides within our science of anesthesiology. Sometimes it becomes so ingrained that it goes unchallenged for decades, for the simple reason, (to paraphrase Collier) “. . . nobody (is) looking for it.” Maybe it is time to take a closer look at this thing called the subdural space, and our understanding of it. Craig Palmer, M.D. Department of Anesthesiology University of Arizona Health Sciences Center Tucson, Arizona
References 1. Bridenbaugh PO, Kennedy WF Jr. Spinal, subarachnoid neural blockade. In: Cousins MJ, Bridenbaugh PO, eds. Neural Blockade. Philadelphia, PA: J.B. Lippincott Company; 1980:146-175. 2. Brown DL. Spinal, epidural, and caudal anesthesia. In: Miller RD, ed. Anesthesia. 5th ed. Philadelphia, PA: Churchill Livingstone; 2000:1492-1493. 3. Cousins MJ. Epidural neural blockade. In: Cousins MJ, Bridenbaugh PO, eds. Neural Blockade. Philadelphia, PA: J.B. Lippincott Company; 1980:176-274. 4. Bromage PR. Epidural Anesthesia. Philadelphia, PA: W.B. Saunders Company; 1978:20. 5. 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. 6. Collier CB. Accidental subdural injection during attempted lumbar epidural block may present as a failed or inadequate block: Radiographic evidence. Reg Anesth Pain Med 2004;29:45-51. 7. Jones MD, Newton TH. Inadvertent extra-arachnoid injection in myelography. Radiology 1963;80:818-822. 8. Ajar AH, Rathmell JP, Mukherji SK. The subdural compartment. Reg Anesth Pain Med 2002;27:72-76.
A
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 myself will place an epidural catheter for labor analgesia, only to have it perform in strange and unexpected ways. Usually the block obtained is much higher than expected, spreads unpredictably, or causes symptoms such as numbness in the hands, difficulty swallowing, or shortness of breath, yet is not as dense as one would expect with a subarachnoid injection. As the resident expert in such matters I am usually called upon to explain how this could happen. I generally only have two possible explanations to offer: interindividual variation or an unintended subdural injection. Most of us have been taught that the subdural space lies between the dura mater and the arachnoid matter, the two outer membranes surrounding the spinal cord. According to my edition of Cousins’ and Bridenbaugh’s text, “. . . There is a capillary interval, called the subdural space, between the dura and the arachnoid. It contains a minute quantity of serous fluid, but has no connection with the subarachnoid space that contains the CSF.”1 Miller’s current textbook calls the subdural space “. . . a potential space between the dura mater and the arachnoid which contains only small amounts of serous fluid . . .”2 Cousins and Bridenbaugh go on to state “. . . The dura and the arachnoid are in such close contact that in the process of lumbar puncture, it is not possible to pierce the dura without piercing the arachnoid as well.”1 One hundred pages later they have apparently reconsidered, stating “. . . Subdural cannulation results from perforation of the dura without penetration of the underlying arachnoid membrane. This is a rare result of intended epidural cannulation. It occurs quite frequently during myelography 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) have assumed is responsible for the strange high blocks mentioned above. For literally decades, anesthesiology residents, myself included, have been taught “. . . Injected solutions spread slowly but quite extensively in the subdural space, and this particular anatomical error may be a cause of occasional cases of unexplained massive extradural analgesia.”4 Twenty-five years later, little seems changed: “Subdural injection of a local anesthetic leads to an unexpectedly high but patchy block. A subdural block has a variable spread that is ultimately quite extensive for the volume of local anesthetic injected.”5 An article in this issue of Regional Anesthesia and Pain Management by Clive Collier, M.D.,6 presents compelling evidence that maybe we have been wrong about the subdural space all these years. Maybe subdural injection of local anesthetics does not always lead to “exaggerated extension” of the block; perhaps it is just the opposite—maybe it more often leads to inadequate spread of the injected local anesthetic! Further, maybe subdural injection is not as rare as we have been led to believe. . . . Dr. Collier presents four cases of epidural anesthetics with inadequate spread of anesthesia and delayed onset that required additional epidural local anesthetic supplementation to be effective. After each case was ended, injection of a radiopaque dye and radiographic imaging revealed the catheters to be subdural, with limited spread of the injected contrast. This is the first report to clearly associate limited local anesthetic spread with subdural injection. In retrospect, much of the evidence has been available to us for years, although perhaps not exactly where anesthesiologists would typically look for it. It does seem perplexing that radiologists never seemed to have much difficulty getting into the subdural space, while for anesthesiologists it was a “rare result.”3 Forty years 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
8
Regional Anesthesia and Pain Medicine Vol. 29 No. 1 January–February 2004
myelograms,7 although more recent reports indicate that the incidence of the complication is somewhat lower.8 Interestingly, radiologists often find that once an unintended injection into the subdural space has been recognized, it can be difficult to perform the intended subarachnoid injection. If the dura and arachnoid membranes really are so closely opposed, and injections into this “potential space” spread widely, how could this be the case? As Collier points out in his report, the subdural space may be a very different animal than we have been led to believe. Unless a clinician regularly reads anatomy and pathology journals, he or she might not have been aware that the concept of a “subdural space” had become controversial. It may not be a “potential space” at all; in fact, it may not even exist! What we have come to think of as the subdural space may actually be an iatrogenic dissection of a cellular layer between the dura and the arachnoid. Thought of as a dissection, a number of clinical observations suddenly make much more sense. The extent of a dissection can be extremely variable: it can be highly extensive, resulting in a high block, or it can be limited, resulting in a low, inadequate block. Low or inadequate blocks are certainly a much more common clinical problem than overly extensive blocks. A common scenario in obstetric anesthesia involves the case in which the epidural goes in smoothly, the initial bolus dose provides some analgesia, but when assessed, levels are clearly inadequate. Redosing the catheter two or even three times fails to raise the patient’s level sufficiently, and finally, after the patient has endured her discomfort for 40 or 50 minutes more than either you or she anticipated, you give up and replace the catheter, despite the fact that there were clearly demonstrable, yet insufficient, levels. What went wrong? As I tell the patient when this happens, “I’ve given you enough drugs that if the catheter were in the correct place, you would be comfortable. All I can say is it must not be in the right place.” I often wondered but never knew where all that local anesthetic was going, but Collier’s report may provide the answer—perhaps it was a subdural injection, a subdural dissection, with very limited spread. Transient pain on injection may be a marker of such injections, and I will look more closely for it in the future. It is surprising at times how much dogma resides within our science of anesthesiology. Sometimes it becomes so ingrained that it goes unchallenged for decades, for the simple reason, (to paraphrase Collier) “. . . nobody (is) looking for it.” Maybe it is time to take a closer look at this thing called the subdural space, and our understanding of it. Craig Palmer, M.D. Department of Anesthesiology University of Arizona Health Sciences Center Tucson, Arizona
References 1. Bridenbaugh PO, Kennedy WF Jr. Spinal, subarachnoid neural blockade. In: Cousins MJ, Bridenbaugh PO, eds. Neural Blockade. Philadelphia, PA: J.B. Lippincott Company; 1980:146-175. 2. Brown DL. Spinal, epidural, and caudal anesthesia. In: Miller RD, ed. Anesthesia. 5th ed. Philadelphia, PA: Churchill Livingstone; 2000:1492-1493. 3. Cousins MJ. Epidural neural blockade. In: Cousins MJ, Bridenbaugh PO, eds. Neural Blockade. Philadelphia, PA: J.B. Lippincott Company; 1980:176-274. 4. Bromage PR. Epidural Anesthesia. Philadelphia, PA: W.B. Saunders Company; 1978:20. 5. 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. 6. Collier CB. Accidental subdural injection during attempted lumbar epidural block may present as a failed or inadequate block: Radiographic evidence. Reg Anesth Pain Med 2004;29:45-51. 7. Jones MD, Newton TH. Inadvertent extra-arachnoid injection in myelography. Radiology 1963;80:818-822. 8. Ajar AH, Rathmell JP, Mukherji SK. The subdural compartment. Reg Anesth Pain Med 2002;27:72-76.