Lower Limb Nerve Injuries — Common Peroneal Foot Drop, Sciatic, Gluteal, Femoral, Obturator, Meralgia Paraesthetica and Tarsal Tunnel

The lower limb is supplied by the lumbar plexus (anterior rami of L1 to L4: obturator, femoral, lateral femoral cutaneous) and the sacral plexus (sciatic nerve and its two divisions, the gluteal nerves). Most exam questions ask you to match a lesion to its clinical deficit, so it helps to learn one pattern per nerve.

The common peroneal (fibular) nerve is one of the two terminal branches of the sciatic nerve, which divides into the tibial and common peroneal nerves just above the popliteal fossa. Its fibres come from the posterior divisions of L4–S2. It runs behind the biceps femoris, winds around the neck of the fibula where it lies subcutaneously, and then divides into the deep and superficial peroneal nerves.

Peroneal neuropathy is the most common compressive neuropathy of the lower extremity and, after median and ulnar neuropathies, the most common focal neuropathy. A classical presentation is foot drop: weakness of ankle and toe dorsiflexion plus weak eversion, so the patient catches the toes while walking and lifts the knee high — the high-steppage gait — with numbness over the lateral leg and dorsum of the foot.

Foot drop is weakness of the dorsiflexors, producing a high-steppage walking pattern. It is a sign, not a diagnosis. A lesion of the L5 root, lumbar plexus, sciatic nerve, common peroneal nerve or deep peroneal nerve can cause it, plus muscle disease and central causes.

Management of peroneal foot drop is usually conservative: an ankle-foot orthosis (AFO), physiotherapy and removal of the cause (loosening a cast, stopping leg crossing). Surgery is considered for a rapidly deteriorating lesion, no improvement after about 3 months, or an open injury with suspected laceration, which should be explored and repaired within 72 hours. Foot drop after knee dislocation has a poorer prognosis, and tendon transfers are often needed for refractory palsy.

The sciatic nerve (L4–S3) is the largest nerve of the body. It leaves the pelvis through the greater sciatic foramen below the piriformis and splits in the popliteal fossa into the tibial (medial) and common fibular (lateral) divisions; the tibial division also supplies the hamstring part of adductor magnus, semitendinosus, semimembranosus and the long head of biceps. In the commonest anatomical variant the nerve leaves undivided below the piriformis, but in some people the fibular division passes through the muscle and can be compressed there.

• Sciatic neuropathy (most often from traumatic hip injury or surgery) is the second most common mononeuropathy of the lower limb and usually causes foot drop with loss of plantar flexion and inversion, sensory loss over the entire foot and weak knee flexion.
• Sciatica is a different entity — buttock-to-leg pain from nerve root irritation, most often a disc herniation at L5 or S1, not a peripheral nerve injury.
• The superior gluteal nerve (L4, L5, S1) leaves the pelvis above the piriformis — the only nerve to do so — and supplies gluteus medius, gluteus minimus and tensor fasciae latae.
• It can be injured by hip dislocation or fracture, hip arthroplasty and intramuscular injection into the buttock.

For the femoral nerve, the nerve lies between the psoas major and iliacus, so a growing haematoma or abscess there compresses it. Most patients recover satisfactorily, though some need nerve repair or grafting. For the obturator nerve, an obturator hernia typically presents in elderly, thin women or patients with raised intra-abdominal pressure and causes small-bowel obstruction; entrapment in athletes causes exercise-induced medial thigh pain.

Meralgia paraesthetica (Bernhardt-Roth syndrome) is compression of the lateral femoral cutaneous nerve (LFCN) — a pure sensory nerve from the posterior divisions of L2 and L3. It crosses the iliacus toward the anterior superior iliac spine and passes under, through or over the inguinal ligament, where it is most vulnerable. The anterior branch supplies the anterior thigh to the knee and the posterior branch the lateral thigh to the greater trochanter.

Posterior tarsal tunnel syndrome is an entrapment of the tibial nerve in the fibro-osseous tunnel behind and below the medial malleolus. The roof is the flexor retinaculum (laciniate ligament, running from the tip of the medial malleolus to the medial calcaneus); the floor is formed by the medial tibia, talus and calcaneus. It is often compared with carpal tunnel syndrome but is considerably less common.

Within the tunnel the posterior tibial nerve divides into the medial plantar nerve (sensation to the medial foot and plantar first three and a half toes; abductor hallucis, flexor digitorum brevis, flexor hallucis brevis and first lumbrical) and the lateral plantar nerve (plantar lateral one and a half toes; most intrinsic muscles). The medial calcaneal nerve to the heel usually arises before the tunnel.

• Symptoms: medial ankle pain, plantar dysaesthesia (burning, tingling), worse with walking; sensory symptoms outweigh weakness.
• Risk factors: a history of ankle fracture or sprain (17–43% of patients), runners with hyperpronation, diabetes, hypothyroidism, gout, mucopolysaccharidosis, hyperlipidaemia, and mass lesions such as tibial nerve schwannoma.
• Diagnosis: clinical, supported by electrodiagnostic studies and imaging for structural causes.
• Anterior tarsal tunnel syndrome is a separate, rare entity: the deep peroneal nerve is compressed under the extensor retinaculum against the navicular and talus.

• Foot drop with eversion weakness and dorsal foot numbness — common peroneal nerve at the fibular neck (the commonest answer).
• Foot drop after hip surgery with plantar flexion and inversion weakness — sciatic nerve; after a knee dislocation — common peroneal nerve.
• Hip sags on the opposite side during stance — superior gluteal nerve; check for gluteal injection or hip surgery.
• Quadriceps wasting, absent knee jerk — femoral nerve; remember the saphenous branch for the medial leg.
• Adductor weakness — obturator nerve; adductor jerk during TURBT is an anaesthetic complication.
• Numb outer thigh in an obese or pregnant patient — lateral femoral cutaneous nerve (meralgia paraesthetica).
• Burning sole, medial ankle — tarsal tunnel; know the order of structures under the flexor retinaculum.