Intrapartum maternal lumbosacral plexopathy.

There are many conflicting theories regarding the mechanism and prognosis of acute foot drop during labor. We report seven women who had arrested labor and foot drop. Six had short stature and one had a large newborn. All had weakness of ankle dorsiflexion, eversion, and inversion, and sensory loss in the L-5 dermatome. Superficial peroneal sensory nerve action potentials (SNAPs) were small or absent in six patients, and the sural SNAP was attenuated in one. Peroneal compound muscle action potential (CMAP) amplitude (recording from extensor digitorum brevis) was low in five, whereas the tibial CMAP was normal in all patients. Peroneal CMAP amplitude (recording from the tibialis anterior) was normal in three and small in three. Needle electromyography revealed decreased recruitment and fibrillation potentials in L-5-innervated muscles, mostly below the knee. We conclude that intrapartum foot drop occurs mostly in short women and is caused by lumbosacral trunk compression by the fetal head at the pelvic brim. The primary pathology is predominantly demyelination and recovery is complete in up to 5 months.