Flaccid leg monoparesis is rarely of central nervous system origin. Flaccid leg monoparesis is usually due to peripheral nervous system damage. The clinical manifestation of flaccid leg monoparesis is foot drop. Foot drop due to peripheral nerve system damage occurs with involvement of the L5 root fibers at any level of the peripheral lumbosacral somatic motor system.Lesions in the cauda equina usually occur in association with meningocele (Figure 236.1). Lesions of the cauda equina and lumbosacral plexus usually produce, in addition to foot drop, weakness of hip flexion and leg adduction.

A	B

Figure 236.1.— Neonate with a left foot drop at rest [A]. During action he is unable to lift the left foot up [B]. The patient had a lumbar meningocele and a lipoma of the phylum terminalis.

Sciatic nerve (Figure 236.2 [SN]) lesions may be complete or fascicular.A complete sciatic nerve lesion produces foot drop, but unlike a lumbosacral plexus lesion, it spares hip movements (iliopsoas nerve), knee extension (femoral nerve), and adduction (obturator) of the leg. A complete sciatic nerve lesion involves all muscles of the thigh. Fascicular sciatic nerve lesions are more frequent than complete sciatic nerve lesions. Fascicular lesions of the sciatic nerve may involve the lateral or the medial fascicles. Lateral fascicle sciatic nerve injury occurs more frequently than medial fascicle sciatic nerve injury. Lateral fascicles sciatic nerve injury spares all the muscles of the thigh except the short head of the biceps (Figure 236.2 [BshM]). This finding (the sparing or involvement of the short head of the biceps) is an important EMG finding to distinguish a lateral fascicle sciatic nerve injury from a common peroneal nerve lesion (both present with foot drop). A neonate with a lateral fascicular sciatic nerve lesion shows evidence of denervation of the short head of the biceps femoralis, whereas a neonate with a common peroneal nerve lesion does not.
Common peroneal nerve (Figure 236.2 [PN]) lesions produce foot drop. Common peroneal nerve lesions spare all the muscles of the thigh, including the short head of the biceps, and the muscles innervated by the tibial nerve. Peroneal nerve lesions produce weakness of the tibialis anterior muscles.
Tibial nerve (Figure 236.2 [TN]) lesions are extremely rare in neonates. Tibial nerve damage produces plantar flexion weakness. Tibial nerve injury does not produce foot drop. Obturator nerve (Figure 236.2 [ON]) lesions produce inability to adduct the thigh. Femoral nerve (Figure 236.2 [FN]) lesions produce inability to extend the knee. Femoral and obturator nerve lesions do not produce foot drop.

Figure 236.2.— Schematic representation of the lumbosacral plexus and most important intermedial nerves. IPN: iliopsoas nerve; SGN: superior gluteal nerve; IGN: inferior gluteal nerve; ON: obturator nerve; FN: femoral nerve; LST: lumbosacral trunk; SN: sciatic nerve; TN: tibial nerve; CPN: common peroneal nerve; AdM: adductor muscle of the thigh; HSM: hamstring muscles; PostTM: posterior tibialis muscle; B(sh)M: short head of the biceps femoralis muscle; PlNs: plantar nerves; DPN: deep peroneal nerve; SPN: superficial peroneal nerve.