Inadvertent subdural injection: a complication of an epidural block.

Twenty-one hundred eighty two consecutive lumbar epidural injections were studied to determine the incidence of inadvertent subdural block retrospectively. A subdural block is defined as an extensive neural block in the absence of subarachnoid puncture, that is out of proportion to the amount of local anesthetic injected. Subdural injection is a complication of epidural block that probably occurs more frequently than previously recognized. An earlier report has estimated the incidence of subdural block to be 0.1%. This study, however, reports an incidence of 0.82% from a sample size of 2182 patients. Cadaveric dissection was also performed, further clarifying the presence and anatomic position of the subdural space.