Sciatic Nerve

Clinical Presentation

The sciatic nerve is the largest nerve in the body, measuring 0.8 cm to 1.5 cm in width. Disturbances along the sciatic nerve may lead to sciatic neuropathy. Among neuropathies of the lower extremity, sciatic neuropathy is the second most common, after peroneal, or fibular, neuropathy. Sciatic nerve compression may occur in the sciatic notch (lower pelvis, buttock, gluteal fold), or more distally in the midthigh (biceps femoris muscle). The most common cause of sciatic nerve injury in the gluteal region is trauma, commonly secondary to hip trauma, including hip dislocation or fracture. It can also be caused by total hip arthroplasty, compression by external sources such as prolonged bed rest, intragluteal injections, regional block, nerve infarction, or pelvic masses such as endometriosis, arterial aneurysms, or hematomas. Piriformis syndrome is a controversial condition that may also lead to sciatic nerve injury in the gluteal region. Injury in the midthigh is less common but may occur from femur fractures or masses.

Of note, although “sciatica” can occur anywhere along the course of the sciatic nerve, the term is often misleading because it usually refers to a lumbosacral nerve root compression. This is important to distinguish when evaluating for sciatic neuropathy.

Patients with injury to the sciatic nerve usually report pain along the path of the nerve, most commonly in the posterior thigh with radiation down below the knee. Patients may also report weakness in musculature innervated by the sciatic nerve and its distal components, including weakness in the knee flexors, ankle dorsiflexors, and ankle plantar flexors. Some hamstring function may be spared if compression occurs more distally in the midthigh. Proximal lower extremity motor strength is usually always spared.

On clinical exam, patients may also report sensory loss of areas innervated by the peroneal, tibial, and sural nerves. The entire lower leg may be involved, with the exception of the medial calf and arch of the foot because these areas are innervated by the saphenous nerve, a branch of the femoral nerve. Reflex evaluation of the lower extremities may reveal reduced or absent Achilles reflexes, with normal knee jerks.

Electrodiagnostic testing may be pursued to further evaluate sciatic neuropathy. Findings may show a denervation pattern with reduced tibial and peroneal motor amplitudes as well as reduced peroneal and sural sensory responses. However, significant abnormalities may not be seen, unless neuropathy is severe.

When sciatic neuropathy is suspected, it is important to evaluate for other etiologies of symptoms. The differential diagnosis is largely limited to a lumbosacral radiculopathy, lumbosacral plexopathy, or a distal neuropathy such as a common peroneal neuropathy. Less commonly, symptoms may be caused by a motor neuron disease, polyneuropathy, or a distal myopathy. Careful attention to patient symptoms as well as imaging and electrodiagnostic studies can help differentiate a sciatic neuropathy from other common diagnoses.

Anatomy

The sciatic nerve originates from the L4 to S2 nerve roots of the lower lumbar plexus and travels down the posterior aspect of the lower extremity, medial and posterior to the hip joint, between the ischial tuberosity and the greater trochanter of the femur. The sciatic nerve exits the pelvis through the sciatic foramen, after which it emerges inferior to the piriformis muscle and travels down the posterior thigh, lying deep to the long head of the biceps femoris muscle, superficial to the short head of biceps femoris and adductor magnus muscles, and lateral to semitendinosus and semimembranosus muscles. The sciatic nerve remains covered by overlying musculature as it runs down the lower extremity until it reaches the popliteal fossa, where it bifurcates into the tibial and common peroneal nerves ( Fig. 25.1 ). The two divisions of the sciatic nerve are distinctly separate for the entire length of the sciatic nerve, but proximal to this bifurcation the two nerve are combined into one trunk by a common connective tissue sheath. The sciatic nerve branches off to innervate the hamstring muscles including the semimembranosus, the semitendinosus, the long and short heads of the biceps femoris, and the lateral division of the adductor magnus. Although the sciatic nerve does not have any direct cutaneous function, it provides indirect sensory innervation to the lower leg and foot through the terminal branches of the tibial and peroneal nerves.