Does sciatic parasacral injection spread to the obturator nerve? An anatomic study.

BACKGROUND: The ability of parasacral sciatic nerve block to provide consistent obturator nerve and perineal blockade remains undetermined. In this anatomic work, we assessed the spread of a colored latex mimicking a parasacral injection, and observed the spread to the obturator nerve and sacral nerve roots.

METHODS: Fourteen parasacral injections were performed bilaterally on seven human cadavers. Dissection was performed in two steps. First, the posterior approach confirmed the presence of dye on the sciatic nerve at the level of the piriformis muscle to define the success of the injection. Second, the anterior endopelvic dissection assessed the presence of dye around the pelvic portion of the obturator nerve and on the sacral roots (S1-3). The same score was used for all the nerves, from 0 (total failure) to 3 (total success). For all nerves (sciatic, obturator, sacral roots), a score of 2 or 3 was considered a successful injection.

RESULTS: Eleven of 14 injections were considered successful parasacral injections. The three failed injections were excluded from further analysis. One was intravascular and two (same cadaver) in the gluteal muscles were too superficial. Of the 11 successful injections, 9 (82%) were scored as providing the spread of latex to both the obturator nerve and to the sacral roots.

CONCLUSION: We conclude from this anatomical study that successful parasacral injection consistently spreads to the pelvic portion of the obturator nerve and to the sacral roots. Therefore, parasacral block should theoretically provide obturator and perineal blockade, and eliminate the need for systematic separate obturator nerve block. These results must be confirmed by further clinical studies.