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The most common compressive neuropathies of the lower extremity are peroneal neuropathy (compression at the fibular head) and sciatic neuropathy (compression in the buttock). The nerves of the lumbar plexus (femoral, obturator, lateral femoral cutaneous) and the sciatic nerve can be injured by pelvic or hip trauma or surgery, or compressed by pelvic malignancy. The peroneal and tibial nerves can be injured due to knee trauma or surgery.
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The femoral nerve innervates two muscles associated with the femur: iliopsoas for hip flexion and quadriceps for knee extension (including the patellar reflex). The sensory coverage of the femoral nerve includes the anterior and medial thigh (intermediate and medial cutaneous branches) and the medial leg and foot by way of the saphenous nerve (Mnemonic: the saphenous nerve arises from the femoral nerve). The femoral nerve passes through the iliopsoas and then beneath the inguinal ligament and can be injured at either site. Since the femoral nerve innervates the iliopsoas proximal to passing beneath the inguinal ligament, the presence of hip flexion weakness in addition to knee extension weakness and diminution/loss of the patellar reflex localizes a femoral neuropathy to the pelvis or retroperitoneum, whereas isolated knee extension weakness and loss of the patellar reflex with spared hip flexion strength suggests a more distal localization along the nerve.
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An important aspect of the examination in patients with hip flexion and/or knee extension weakness is testing of hip adduction. The hip adductors are supplied by the obturator nerve, which is supplied by the L2 through L4 roots as is the femoral nerve. Therefore, hip flexion and/or knee extension weakness with spared hip adduction suggests femoral neuropathy (since the L2-L4 roots must be intact if obturator nerve function is intact), whereas hip flexion and/or knee extension weakness and hip adduction weakness suggest L2-L4 polyradiculopathy or lumbar plexopathy.
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Causes of femoral neuropathy include pelvic or hip surgery or trauma, pelvic malignancy, and femoral catheterization procedures (which can injure the nerve in the inguinal region either directly or due to hematoma formation). In anticoagulated patients who develop back and leg pain and difficulty walking, signs of a femoral neuropathy should be sought on examination, which would suggest a psoas/retroperitoneal hematoma. The diagnosis of psoas/retroperitoneal hematoma can be made by CT of the pelvis (Fig. 17–5). Weakness and sensory changes beyond the femoral distribution in this context can occur with a more proximal retroperitoneal hematoma causing a lumbar plexopathy.
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Isolated obturator nerve injury is rare, but when it occurs, it causes weakness in hip adduction. Pelvic or hip trauma or surgery and childbirth are common etiologies. As described above, the obturator nerve shares root supply (L2-L4) with the femoral nerve, so weak hip adduction with sparing of hip flexion and knee extension suggests obturator neuropathy, whereas involvement of all of these actions (hip adduction, hip flexion, and knee extension) suggest L2-L4 polyradiculopathy or lumbar plexopathy.
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Lateral Femoral Cutaneous Neuropathy (Meralgia Paresthetica)
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The lateral femoral cutaneous nerve is a pure sensory nerve supplying the lateral thigh. Injury to the nerve causes numbness, paresthesias, and/or pain in this region, known as meralgia paresthetica. The nerve’s position in the inguinal region adjacent to the anterior iliac spine makes it susceptible to injury from tight belts/pants especially in patients who are obese, rapidly gain weight (e.g., pregnancy), or rapidly lose weight.
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The sciatic nerve is really the peroneal nerve and tibial nerve bundled together. It innervates the hamstring muscles (biceps femoris, semimembranosus, semitendonosus) that flex the knee, and then divides into the peroneal and tibial nerves in the popliteal fossa. The peroneal and tibial nerves control all movements of the foot and toes.
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If there is a complete sciatic neuropathy, knee flexion and all movements of the foot will be weak. However, the sciatic nerve is often only partially affected, and in these patients, the peroneal-innervated muscles are often affected in isolation (or more affected than tibial nerve–innervated muscles). In such cases, sciatic neuropathy may be clinically indistinguishable from peroneal neuropathy. Peroneal neuropathy is most commonly due to compression at the fibular head (see “Peroneal Neuropathy” below). The short head of the biceps femoris is the only hamstring muscle innervated by the peroneal division of the sciatic nerve (the rest are innervated by the tibial portion), and so involvement of this muscle above the knee localizes the problem to the peroneal division of the sciatic nerve (as opposed to the most common site of compression of the peroneal nerve at the fibular head). However, the short head of the biceps femoris cannot be isolated clinically, so if a patient appears to have a peroneal neuropathy, the short head of the biceps femoris should be examined with electromyography (EMG) to look for denervation changes that would suggest localization to the peroneal division of the sciatic nerve proximal to the knee (see also “Approach to Foot Drop” below).
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The sciatic nerve can be injured by pelvic or hip trauma or surgery, by an inappropriately placed gluteal injection, or by prolonged pressure on the buttock (e.g., supine due to intoxication or critical illness; pressure from sitting on the toilet seat or something in the back pants pocket). “Sciatica,” referring to radiating pain down the back of the leg, is more commonly caused by radiculopathy affecting the S1 root rather than pathology of the sciatic nerve.
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Peroneal neuropathy is the most common lower extremity mononeuropathy because the peroneal nerve is the lower extremity nerve most prone to compression. The most common site of compression is the head/neck of the fibula, where the nerve can be compressed due to frequent leg crossing (the leg on top is affected), prolonged bed-bound state, prolonged kneeling (usually occupational; for example, flooring work), Baker’s cyst, injury due to knee trauma or surgery, or rapid weight loss.
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The peroneal nerve innervates the musculature of the anterior and lateral shin and foot: tibialis anterior (dorsiflexes the foot), peroneus longus and brevis (everts the foot), and the extensors of the toes. Mnemonic: The peroneal nerve brings the foot up and out (compared to the tibial nerve, which brings the foot down and in). With respect to motor function of the foot, the peroneal nerve can be thought of as the lower extremity analogue of the radial nerve in the upper extremity in that it innervates all foot extensors. The peroneal nerve provides sensory supply to the lateral shin and dorsum of the foot.
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The common peroneal nerve divides into the superficial and deep peroneal nerves that perform the individual components of the peroneal nerve functions listed above:
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The superficial peroneal nerve innervates the peroneus muscles (eversion) and provides sensation to the lateral calf and dorsum of the foot except the web space between the first two toes.
The deep peroneal nerve supplies the tibialis anterior (dorsiflexion), the toe extensors, and sensation over the web space between the first two toes on the dorsum of the foot (mnemonic for the motor functions supplied by the deep peroneal nerve: deep: dorsiflexion and extensors).
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The superficial and deep peroneal nerves may be affected separately, impairing their individual functions, or together, impairing all peroneal nerve functions (common peroneal neuropathy). Common peroneal neuropathy leads to impaired dorsiflexion (foot drop) and foot eversion, but with spared plantarflexion and foot inversion (both tibial nerve functions). Numbness in common peroneal neuropathy may be present over the lateral calf and dorsum of the foot, although may be more limited in distribution over just the dorsum of the foot (since the sural nerve, which supplies the lateral calf, usually receives both peroneal and tibial input).
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The tibial nerve innervates the muscles of the posterior calf and plantar foot: gastrocnemius/soleus (plantarflexor), tibialis posterior (foot inversion), and all flexors of the foot. Mnemonic: The tibial nerve brings the foot down and in (compared to the peroneal nerve, which brings the foot up and out). The tibial nerve innervates the functions in the foot analogous to the combined functions of the median and ulnar nerves in the hand (flexors and intrinsic muscles of the hand/foot).
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The tibial nerve is less commonly affected than the peroneal nerve since is not as exposed as the peroneal nerve, but it can rarely be injured by knee trauma or a Baker’s cyst in the popliteal fossa.
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Tarsal Tunnel Syndrome
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The most common site of tibial nerve compression (which is still rare) is at the tarsal tunnel, where the tibial nerve enters the foot with the flexor tendons of the toes under the medial malleolus (tarsal tunnel syndrome). Tarsal tunnel syndrome occurs most commonly in patients with prior ankle injury or arthritis of the ankle, but can rarely be caused by a ganglion cyst in the tarsal tunnel. The syndrome causes neuralgic pain in the plantar foot, which may radiate proximally (just as patients with carpal tunnel syndrome sometimes report pain/paresthesias proximal to the site of entrapment). On examination, sensation may be decreased on the plantar surface of the foot and there may be a Tinel sign when tapping over the posterior medial malleolus (provocation of paresthesias over the plantar surface of the foot). There is usually no obvious weakness since the main toe flexors of the plantar foot (whose tendons pass through the tarsal tunnel) are innervated proximal to the tarsal tunnel.
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If a patient with tarsal tunnel syndrome does not respond to conservative treatment with NSAIDs, steroid injections can be considered. Surgery is reserved for intractable cases confirmed by EMG/nerve conduction studies demonstrating slowing across the tarsal tunnel and denervation of tibial nerve–innervated intrinsic foot muscles (e.g., adductors of toes).