While there may be a tendency to associate anesthetic intervention with neurological pathology in the practice of obstetrics, the events surrounding pregnancy and labor are most commonly responsible for such neuropathies. Obstetric palsies are estimated to be more common than neurological injuries caused by regional anesthesia. Thus, understanding the spatial relationship between nerves and surrounding anatomical structures is paramount to properly identify the site of obstetrical palsy, its presentation relative to neurological dysfunction, and subsequent treatment options.
Foot drop is one type of obstetrical palsy. First described in 1838, a foot drop can have pronounced adverse effects on quality of life if not recognized, diagnosed, and treated appropriately. It can be caused by three separate but similar etiologies: lumbar radiculopathy, lumbosacral (LS) plexopathy, or common fibular (CF) neuropathy. The location of the neural injury should be assessed by a combination of careful neurological examination, MRI imaging, and electrophysiological studies.
Foot drop manifests primarily as a loss of dorsiflexion and eversion of the foot at the ankle. This can produce a slapping sound when the foot hits the ground. To avoid scraping the foot on the ground, the patient learns to elevate it higher than normal, leading to the classic steppage gait. Patients suffering from foot drop can also present symptoms including but not limited to paresthesia, hypoesthesia, numbness, weakness, atrophy, and diminished or absent deep tendon reflexes.
The presentation of foot drop is most commonly described as unilateral, except for the case of common fibular neuropathy, which may be bilateral. This paper aims to elucidate the individual mechanisms underlying each etiology of obstetric foot drop and to use relevant clinical anatomy to guide clinicians in the appropriate diagnosis and management of such nerve palsies.
Anatomy of the Roots of the Lumbosacral Plexus
The lumbosacral (LS) plexus is located in the psoas muscles and is formed by the L1-S4 nerve roots, which provide motor and sensory innervation to the pelvis, thigh, leg, and foot. Its dorsal (sensory) and ventral (motor) roots emerge from the spinal cord and stream down in the cauda equina to exit the intervertebral foramina to join to form the mixed spinal nerves.
Each mixed spinal nerve then divides into a ventral primary ramus and a dorsal primary ramus. The muscles of the pelvis and lower limbs are innervated by the ventral primary rami of the lumbosacral plexus. Damage to one or more of the dorsal nerve roots before the fibers join causes a loss of sensation in a dermatomal distribution. Meanwhile, damage to the ventral roots will result in paresis of all muscles innervated by that root level. Regarding obstetrical neural lesions, the L5 and S1 ventral rami of the plexus are the most important.
Anatomy of the Lumbosacral Trunk Component of the Lumbosacral Plexus
The nerve roots of L1-L4 form the upper portion of the plexus, while the roots of the L4-S4 constitute the lower portion of the lumbosacral plexus. Between these two portions of the lumbosacral plexus, the L4 and L5 rami unite to form the lumbosacral trunk. The lumbosacral trunk must pass inferiorly on the anterior surface of the sacral ala, where it is liable to injury by the head of the descending fetus.
The lumbosacral trunk is part of the lumbosacral plexus but is considered separately in this paper, as its exposed position renders it especially liable to injury. Damage to the L5 component of the lumbosacral trunk is sufficient to cause foot drop.
Anatomy of the Peripheral Nerves formed from the Lumbosacral Plexus
Branches of each portion of the lumbosacral plexus divide to form the distinct peripheral nerves associated with the lower limbs. The upper portion of the plexus gives rise to the iliohypogastric nerve (T12-L1), the ilioinguinal nerve (L1), genitofemoral nerve (L1-L2), lateral femoral cutaneous nerve (L2-L3), the femoral nerve (L2-L4), and obturator nerve (L2-L4).
The lower part of the plexus gives rise to the superior gluteal nerve (L4-S1), inferior gluteal nerve (L5-S2), posterior femoral cutaneous nerve (S1-S3), the pudendal nerve (S1-S4), and the large sciatic nerve (L4-S3).
Anatomy of the Common Fibular Nerve
The common fibular nerve arises from the sciatic nerve above the popliteal fossa. It passes over the head of the fibula to curve around the posterior aspect of the fibula. It divides into the superficial fibular nerve and the deep fibular nerve.
The superficial fibular nerve supplies the fibularis longus and brevis and the skin over the lateral side of the leg and dorsum of the foot. The fibularis longus and brevis are the evertors of the foot at the ankle.
The deep fibular nerve then enters the anterior compartment of the leg, where it innervates the tibialis anterior, extensor hallucis longus, and extensor digitorum longus muscles. These muscles are extensors of the foot at the ankle (the dorsiflexors in clinical terminology). Lesion of the common fibular nerve or the deep fibular nerve will produce foot drop.
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