Ultrasound-guided sciatic nerve block at the popliteal fossa is utilized as a diagnostic and therapeutic maneuver in the evaluation and treatment of distal lower extremity pain thought to be mediated via the sciatic nerve. The most common pain syndrome mediated via the sciatic nerve is piriformis syndrome, which is caused by compromise of the sciatic nerve by the piriformis muscle. The symptoms associated with sciatic neuralgia depend on the point at which the nerve is compromised (Fig. 149.1).

Ultrasound-guided sciatic nerve block at the popliteal fossa can be used for surgical anesthesia for distal lower extremity surgery when combined with lateral femoral cutaneous, femoral, saphenous, and obturator nerve block or lumbar plexus block. 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 at the popliteal fossa 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 popliteal fossa.

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 popliteal fossa as well as anywhere along the course of the sciatic nerve are also useful in determining the cause of sciatic nerve compromise (Figs. 149.2, 149.3 and 149.4).

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. The largest nerve in the body, the sciatic nerve, is derived from the L4, the 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. 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 (Fig. 149.5). 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.

Ultrasound-guided block of the sciatic nerve at the knee can be carried out by placing the patient in the prone position with the arms resting comfortably along the patient’s side (Fig. 149.6). A total of 8 mL of local anesthetic is drawn up in a 12-mL sterile syringe. If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. A linear


high-frequency ultrasound transducer is placed in a transverse plane ˜8 cm above the popliteal crease, and an ultrasound survey scan is obtained (Fig. 149.7). The pulsating popliteal artery should be visualized toward the bottom of the image, with the popliteal vein lying just lateral to the artery (Fig. 149.8). Just superficial and slightly lateral to the popliteal vein is the sciatic nerve, which will appear as a bright hyperechoic structure (see Fig. 149.8). Compression of the popliteal vein with pressure on the ultrasound transducer can aid in identification of the sciatic nerve, which lies just superficial to the vein (Fig. 149.9). Color Doppler can be utilized to help identify the popliteal artery and vein (Fig. 149.10). When the sciatic nerve is identified on ultrasound imaging, the skin is prepped with anesthetic solution, and a 3½-inch, 22-gauge needle is advanced from the middle of the inferior border of the ultrasound transducer and advanced utilizing an out-of-plane approach with the trajectory being adjusted under real-time ultrasound guidance until the needle tip is resting in proximity to the sciatic nerve (Fig. 149.11). When the tip of needle is thought to be in satisfactory position, a small amount of local anesthetic and steroid is injected under real-time ultrasound guidance to confirm that the needle tip is in proximity to the sciatic nerve, but not within the nerve itself. There should be minimal resistance to injection. After needle tip placement is confirmed, the remainder of the contents of the syringe are slowly injected. The needle is then removed, and a sterile pressure dressing and ice pack are placed at the injection site.