Ultrasound-guided sciatic nerve block at the hip is useful in the management of the hip and distal lower extremity pain subserved by the sciatic nerve. This technique serves as an excellent adjunct to lumbar plexus block and for general anesthesia when performing surgery at the hip and below. It can be used for surgical anesthesia when combined with lateral femoral cutaneous, femoral, saphenous, and obturator nerve block or lumbar plexus block. It is used for this indication primarily for patients in low-resource settings or those patients who would not tolerate the sympathetic changes induced by spinal or epidural anesthesia and who need distal extremity amputations or debridement (Fig. 127.1). Ultrasound-guided sciatic nerve block at the hip with local anesthetic may be used to palliate acute pain emergencies, including postoperative pain, pain secondary to traumatic injuries of the lower extremity, and cancer pain, while waiting for pharmacologic, surgical, and antiblastic methods to become effective.

Ultrasound-guided sciatic nerve block can also be used as a diagnostic tool when performing differential neural blockade on an anatomic basis in the evaluation of lower extremity pain as well as in a prognostic manner to determine the degree of neurologic impairment the patient will suffer when destruction of the sciatic nerve is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the sciatic nerve at the level of the hip. This technique may also be useful in those patients suffering symptoms from compromise of the sciatic nerve in patients suffering from piriformis syndrome. Ultrasound-guided sciatic nerve block at the hip may also be used to palliate the pain and dysesthesias associated with stretch injuries to the sciatic nerve.

Electrodiagnostic testing should be considered in all patients who suffer from sciatic nerve dysfunction to provide both neuroanatomic and neurophysiologic information regarding nerve function. Magnetic resonance imaging and ultrasound imaging of the lumbar plexus and the pelvis anywhere along the course of the sciatic nerve are also useful in determining the cause of sciatic nerve compromise.

The sciatic nerve provides innervation to the distal lower extremity and foot with the exception of the medial aspect of the calf and foot, which are subserved by the saphenous nerve (Fig. 127.2). The largest nerve in the body, the sciatic nerve, is derived from the L4, L5, and the S1-S3 nerve roots. The roots fuse in front of the anterior surface of the lateral sacrum on the anterior surface of the piriformis muscle. The nerve travels inferiorly and leaves the pelvis just below or through the piriformis muscle via the sciatic notch (Fig. 127.3). Just beneath the nerve at this point is the obturator internus muscle. The sciatic nerve lies anterior to the gluteus maximus muscle; at this muscle’s lower border, the sciatic nerve lies halfway
between the greater trochanter and the ischial tuberosity. The sciatic nerve courses downward past the lesser trochanter to lie posterior and medial to the femur. In the midthigh, the nerve gives off branches to the hamstring muscles and the adductor magnus muscle. In most patients, the nerve divides to form the tibial and common peroneal nerves in the upper portion of the popliteal fossa, although in some patients these nerves can remain separate through their entire course. The tibial nerve continues downward to provide innervation to the distal lower extremity, whereas the common peroneal nerve travels laterally to innervate a portion of the knee joint and, via its lateral cutaneous branch, provide sensory innervation to the back and lateral side of the upper calf.

The piriformis muscle has its origin from the anterior sacrum. It passes laterally through the greater sciatic foramen to insert on the upper border of the greater trochanter of the femur (see Fig. 127.3). The piriformis muscle’s primary function is to externally rotate the femur at the hip joint. The piriformis muscle is innervated by the sacral plexus. With internal rotation of the femur, the tendinous insertion and belly of the muscle can compress the sciatic nerve and, if this persists, cause entrapment of the sciatic nerve.