Ultrasound-guided superficial peroneal nerve block at the ankle is utilized as a diagnostic and therapeutic maneuver in the evaluation and treatment of distal lower extremity pain thought to be mediated via the distal superficial peroneal nerve. Ultrasound-guided superficial peroneal nerve block at the ankle can also be used for surgical anesthesia for distal lower extremity surgery when combined with deep peroneal, tibial, and saphenous nerve block or lumbar plexus block. Ultrasound-guided superficial peroneal nerve block at the ankle with local anesthetic may be used to palliate acute pain emergencies, including postoperative pain, pain secondary to traumatic injuries of the lower extremity including foot and ankle fractures, and cancer pain, while waiting for pharmacologic, surgical, and antiblastic methods to become effective.

Ultrasound-guided superficial peroneal nerve block at the ankle can also be used as a diagnostic tool when performing differential neural blockade on an anatomic basis in the evaluation of ankle and foot pain as well as in a prognostic manner to determine the degree of neurologic impairment the patient will suffer when destruction of the superficial peroneal nerve is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the superficial peroneal nerve at the ankle. Superficial peroneal nerve block at the ankle with local anesthetic and steroid is occasionally used in the treatment of persistent ankle and foot pain when the pain is thought to be secondary to inflammation or when entrapment of the superficial peroneal nerve at the ankle is suspected. Superficial peroneal nerve block at the ankle with local anesthetic and steroid is also indicated in the palliation of pain and motor dysfunction associated with diabetic neuropathy and to provide anesthesia for the nerve when it is being biopsied or harvested for nerve graft.

Electrodiagnostic testing should be considered in all patients who suffer from superficial peroneal nerve dysfunction to provide both neuroanatomic and neurophysiologic information regarding nerve function. Magnetic resonance imaging and ultrasound imaging anywhere along the course of the superficial peroneal nerve are also useful in determining the cause of superficial peroneal nerve compromise. Plain radiographs and/or computerized tomography of the ankle should be obtained in all patients who have trauma to the ankle to rule out fractures of the medial ankle, which can damage the superficial peroneal nerve (Fig. 160.1).

The common peroneal nerve is one of the two major continuations of the sciatic nerve, the other being the tibial nerve. The common peroneal nerve provides sensory innervation to the inferior portion of the knee joint and the posterior and lateral skin of the upper calf. The common peroneal nerve is derived from the posterior branches of the L4, L5, and S1 and S2 nerve roots. The nerve splits from the sciatic nerve at the superior margin of the popliteal fossa and descends laterally behind the head of the fibula (Fig. 160.2). The common peroneal nerve is subject to compression at this point by such circumstances as improperly applied casts and tourniquets. The nerve is also subject to compression as it continues its lateral course, winding around the fibula through the fibular tunnel, which is made up of the posterior border of the tendinous insertion of the peroneus longus muscle and the fibula itself. Just distal to the fibular tunnel, the nerve divides into its two terminal branches, the superficial and the superficial peroneal nerves. Each of these branches is subject to trauma and may be blocked individually as a diagnostic and therapeutic maneuver.

The deep branch continues down the leg in conjunction with the tibial artery and vein to provide sensory innervation to the web space of the first and second toes and adjacent dorsum of the foot (Fig. 160.3). Although this distribution of sensory fibers is small, this area is often the site of Morton neuroma surgery and thus is important to the regional anesthesiologist. The superficial peroneal nerve provides motor innervation to all of the toe extensors and the anterior tibialis muscles. The superficial peroneal nerve passes beneath the dense superficial fascia of the ankle, where it is subject to an entrapment syndrome known as the anterior tarsal tunnel syndrome. The superficial peroneal nerve also provides sensory innervation to the toes except for the area between the first and second toe, which is supplied by the deep peroneal nerve (Fig. 160.4).