Large intrathecal volume: a cause of true failed spinal anesthesia

A 37-year-old woman scheduled for postpartum tubal ligation received two intrathecal doses of 2% hyperbaric mepivacaine (44 mg and 40 mg) and a subsequent single dose of 5% hyperbaric lidocaine (62.5 mg). Her sensory level never extended beyond S1. She subsequently underwent an uneventful general anesthetic, and had no residual sensory or motor deficits. An examination of the patient's lumbosacral magnetic resonance imaging (MRI) scan revealed an unusually large thecal volume. A large lumbosacral intrathecal volume may result in significant dilution or poor redistribution of hyperbaric local anesthetic. The final sensory level may be reduced or absent as a result. Intrathecal volume may be the most important non-modifiable factor affecting intrathecal distribution of local anesthetics; however, it cannot be easily measured or predicted. True failed spinal anesthesia should be distinguished from technical mishap, i.e., failing to introduce the anesthetic into the intrathecal space. The differential of a truly failed single-injection spinal anesthetic may include a large thecal volume, dural ectasias, cysts, and simple anatomic sacral restriction. To minimize maldistribution and neurotoxicity, the sum dose of all intrathecal local anesthetics administered for a single procedure should not significantly exceed the maximum recommended single-dose amount.