To the Editor:-I read with great interest the three recent articles relating to neurotoxicity after spinal lidocaine administration. [1,2,3]The mechanisms proposed by the authors are plausible. I would, however, like to propose an additional factor predisposing to neurotoxicity: slow rate of injection.
Rate of injection was elegantly demonstrated by Rigler and Drasner to be a critical factor in the maldistribution of local anesthetic that was proposed to occur with 28-gauge microcatheters: the faster the injection, the more turbulent the flow of hyperbaric solution out of the catheter, and the more thorough the mixing. They also demonstrated that physicians voluntarily choose to inject slowly through a 25-gauge spinal needle, taking approximately 10 s to inject 1 ml (presumably out of fear that rapid injection will lead to a high block, although the reason is not stated). At this slow rate, the hyperbaric fluid was seen to layer out in the dependent portion of their spinal model.
All three of the recent articles about transient neurologic symptoms reported using either 25- or 27-gauge pencil-point needles. Liguori et al. reported injecting the 3-ml test solution over approximately 30 s, which is equal to the rate reported by Rigler and Drasner ; the other authors do not report speed of injection. I would like to ask Martinez-Bourio et al. and Hampl et al. whether comparable rates of injection were used in their respective studies.
I think we should reassess whether we need to inject slowly through the newer 25- and 27-gauge pencil-point needles. Is the risk of a high spinal lower with smaller needles? Does injection in the sitting position (vs. lateral) protect against a high spinal? Does slower injection lead to areas of highly concentrated local anesthetic? Are we seeing so many articles about neurotoxicity lately because we have made the switch to smaller needles?
Elizabeth J. Youngs, M.D.
Anesthesiology Service; St. Rose Hospital; Hayward, California
(Accepted for publication August 14, 1998.)