In Reply:-Dr. Standl is right in highlighting the fact that several clinical studies have described techniques destined to decrease the incidence of caudally directed spinal catheters. However, it is important to note that these studies were performed when the role of the catheter's sacral direction in the occurrence of maldistribution was but an experimental hypothesis. [1,2]The point of our work was to objectively identify the clinical causes of maldistribution. Using 19-gauge, end-holed catheters, the study showed that the caudal orientation of the catheter tip is a factor of maldistribution rather than the caudal direction of the catheter. As such, the sacral flow of local anesthetics seems to be the most important factor of maldistribution; a cranially directed catheter can have a distally oriented catheter tip if a loop is created during catheter insertion, leading to a distal flow of local anesthetics.
Second, the role of injection speed, lower when local anesthetics are administered via microcatheters and experimentally evoked as being associated with the maldistribution of hyperbaric solutions, has already been debated in the literature. [3-5]According to Wendell and Cianci and Erian, neither the catheter diameter nor the baricity of the injected solution was a factor of maldistribution. However, once again, these results were derived from experimental models. Using 19-gauge catheters, we demonstrated that maldistribution did not occur more often with either isobaric or hyperbaric bupivacaine. Nevertheless, the comparison has not been clinically studied using microcatheters. As such, I find it difficult to advise against the use of hyperbaric solutions via microcatheters before clinical evaluation. In one study, although retrospective, the required doses of hyperbaric lidocaine, 5%, administered via microcatheters were not greater than those using macrocatheters. Finally, Horlocker et al. reported, also in a retrospective study, that the incidence of inadequate anesthesia was no greater when using microcatheters rather than macrocatheters. As such, in light of these experimental [3,4]and clinical results, [6,7]we cannot conclude that microcatheters and hyperbaric solutions are factors of maldistribution. The only current, clinically demonstrated factor of maldistribution is the caudal orientation of the catheter tip. 
It is important to note, however, as highlighted in our manuscript, that the danger of maldistribution does not lie in its occurrence but rather in its not being diagnosed, leading to the administration of high doses of potentially neurotoxic local anesthetics. The diagnosis and early management of maldistribution, as well as abandoning the administration of high doses of local anesthetics (lidocaine, 5%), should limit the occurrence of cauda equina syndrome after continuous spinal anesthesia.
Philippe Biboulet, M.D.
Department of Anesthesiology; Hopital Lapeyronie; Giraud; 34295 Montpellier Cedex 5; France
(Accepted for publication November 23, 1998.)