To the Editor:—
The article by Malinovsky et al. was quite interesting. 1I am writing not because of any criticism of the paper, but to revisit the issues of lidocaine neurotoxicity, “transient radicular irritation (TRI),” and continuous spinal anesthesia.
The portions of Malinovsky's paper that refer to lidocaine show once again that intrathecal 5% lidocaine can be neurotoxic. The neurotoxicity of lidocaine in animals has recently and very nicely been reviewed. 2There is no question that in some circumstances lidocaine can damage the spinal cord of patients during spinal or epidural anesthesia. 3–8
More controversial is the TRI that occurs after otherwise uncomplicated single injection lidocaine spinal anesthesia. It is my opinion that TRI is a manifestation of lidocaine neurotoxicity. Others believe that because TRI is self-limited (lasting only a few days) that it is not toxic. 9But, Malinovsky et al. found in their study that while only two of ten rabbits injected with lidocaine showed “behavioral disturbances,” at least four (maybe all, but this is unclear from the paper) of the ten lidocaine-treated rabbits had neural histopathologic changes:
“Rabbits receiving intrathecal 5% lidocaine presented with signs of local neurotoxicity. Two rabbits presented with areas of loss of myelin or with necrosis in spinal cord and two others presented with axonal degeneration, endoneuronal edema, and perivascular lymphocytosis infiltration in spinal nerves.”
This raises the important question of whether something similar is happening with TRI. While TRI patients have no long-lasting symptoms or “behavioral changes,” other than pain and dysesthesias for a few days, do they have axonal degeneration, endoneural edema, and perivascular lymphocytosis infiltration like Malinovsky's rabbits?
The reams of publications that followed the reports of cauda equina syndrome associated with continuous spinal anesthesia 4,5fail to show that spinal microcatheters caused the neural injury, per se . In fact, cauda equina syndrome has since been reported after single shot spinal anesthesia 7,8and accidental intrathecal injection with intended epidural anesthesia. 5,6The cause is the injection of large amounts or poor distribution of certain neurotoxic local anesthetics (principally lidocaine and tetracaine). Yet, there is still a ban on the use of spinal microcatheters, and the method of continuous spinal anesthesia (with larger catheters) has essentially been eliminated from the anesthesiologist's armamentarium.
This is too bad, because epidural anesthesia is the only alternative. When compared with continuous spinal anesthesia, epidural anesthesia is more difficult to perform, requires the use of much larger doses of local anesthetics and narcotic, which increases the risks for systemic toxicity, and it is less reliable. In fact, nearly 30% of epidural anesthetics used for postoperative analgesia have problems. 10Furthermore, the target for the drugs injected epidurally is in the intrathecal space. Doesn't it make more sense to make the injection where the receptors are and avoid the barriers that diminish effectiveness?
Continuous spinal anesthesia is a valuable technique. I believe it deserves another chance. The reams of publications that I mention herein demonstrate that continuous spinal anesthesia can be safe. We need only avoid lidocaine and tetracaine, use isobaric solutions whenever possible, and avoid repeated injections when the desired effect is not achieved after injecting an amount that would be sufficient with the single shot method.