Ultrasound-guided sural 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 sural nerve. Ultrasound-guided sural nerve block at the ankle can also be used for surgical anesthesia for distal lower extremity surgery when combined with deep and superficial peroneal, posterior tibial, and saphenous nerve block or lumbar plexus block. Ultrasound-guided sural nerve block at the ankle can also be used to localize the sural nerve when performing sural nerve biopsy (Fig. 161.1). Ultrasoundguided sural 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 heel fractures, and cancer pain, while waiting for pharmacologic, surgical, and antiblastic methods to become effective.

Ultrasound-guided sural 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, heel, 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 sural nerve is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the sural nerve at the ankle. Sural 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 sural nerve at the ankle is suspected. Sural 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.

Electrodiagnostic testing should be considered in all patients who suffer from sural nerve dysfunction to provide both neuroanatomic and neurophysiologic information regarding nerve function. Magnetic resonance imaging and ultrasound imaging anywhere along the course of the sural nerve are also useful in determining the cause of sural 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 lateral ankle, which can damage the sural nerve (Fig. 161.2).

The sural nerve is a branch of the posterior tibial nerve. The sural nerve passes from the posterior calf around the lateral malleolus to provide sensory innervation of the posterior lateral aspect of the calf and the lateral surface of the foot and fifth toe and the plantar surface of the heel (Figs. 161.3, 161.4 and 161.5). The sural nerve is subject to compression at the ankle and is known as boot syndrome because it is associated with compression of the nerve by boots that are too tight. Because the sural nerve is a pure sensory nerve, it is commonly sacrificed when a nerve biopsy is needed to diagnose peripheral neuropathies and is also used for nerve transplants.

Ultrasound-guided block of the sural nerve at the ankle can be carried out by placing the patient in the lateral curled-up position with the affected leg positioned up on a folded blanket (Fig. 161.6). A total of 4 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 approximately just above and behind the lateral malleolus of the ankle, and an ultrasound survey scan is taken (Fig. 161.7). The small saphenous vein is seen lying just behind the lateral malleolus, with the sural nerve lying just posterior to the vein (Fig. 161.8). Compression of the small saphenous vein with pressure on the ultrasound transducer can aid in identification of the sural nerve, which lies just posterior to the vein below the tibial vein and artery (Fig. 161.9). Color Doppler can also be utilized to help identify the small saphenous vein and the adjacent sural nerve (Fig. 161.10). When the sural nerve is identified on ultrasound imaging, the skin is prepped with anesthetic solution, and a 1½-inch, 22-gauge

needle is advanced from the anterior border of the ultrasound transducer and advanced utilizing an in-plane approach with the trajectory being adjusted under real-time ultrasound guidance until the needle tip is resting in proximity to the sural nerve (Fig. 161.11). When the tip of needle is thought to be in satisfactory position, after careful aspiration, 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 sural 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 is slowly injected. The needle is then removed, and a sterile pressure dressing and ice pack are placed at the injection site.