Doctor Appleyard deftly deflates Ganapathy et al.’ s 1inference—x-ray vision in positioning the spinal needle tip away from the conus medullaris—by cautioning practitioners that Tuffier's intercrestal line is an approximation to the L4–L5 interspace at best, and that the conus medullaris often ends well caudad of the L1 vertebral body. In other words, even in the hands of experienced clinicians, the spinal needle tip as often as not approaches the conus closer than we sometimes appreciate. 2
The authors 3may take heart from Reynolds’ analysis 4of 7 instances of conus medullaris injury from spinal needle insertion. As she points out 4: although the spinal cord is said to terminate at L1, it extends to L2 in nearly one-half (43%) of women; likewise, Tuffier's intercrestal line is a patently unreliable method of identifying lumbar interspaces, such that “…anaesthetists commonly select a space that is one or more segments higher than they assume.”
To be sure, as Ganapathy et al. 1point out, conus trauma ought to elicit a paresthesia but then, as Doctor Appleyard notes, that response may have been muted by preoperative sedation. Every other symptom and neurologic sign—intraoperative sacral pain despite solid surgical anesthesia to T4, lower back pain for 3 weeks, numbness of the soles of the feet, ataxia, and asymmetric ankle reflexes— tellingly typifies a spinal cord (rather than a spinal rootlet) mechanical injury secondary to lumbar puncture. 5,6
Whatever, the core issue remains whether Ganapathy et al.’ s original Report 3made a compelling case for ropivacaine neurotoxicity. I would say not and Dr. Appleyard, the primary correspondent, concurs. Rather, the Report's authors once again validated that needling in the dark can lead to an inadvertent stab in the back.