Sural Nerve Lipoma: An Anatomy Review

Sural nerve lipoma is a rare cause of sural nerve entrapment with an unknown etiology. The sural nerve is formed as a compilation of the median sural cutaneous and lateral sural cutaneous nerves, which arise from the tibial and peroneal nerves, respectively. It provides sensation to the posterolateral lower third of the leg and lateral aspect of the foot and heel. Sural nerve entrapment can result from direct trauma, fracture of the calcaneus, fifth metatarsal, posterolateral process of the talus, or osperoneum. Entrapment due to a lipoma has not been previously described in the literature. The case presented here describes sural nerve entrapment due to a lipoma. Surgical exploration with successful decompression of the sural nerve leads to improvement in the patient’s ankle symptoms. Hardeep Singh1*, Andreas Voss2 and Vinayak Sathe2 1Department of Orthopaedic Surgery, University of Connecticut School of Medicine, USA 2Department of Orthopaedic Sports Medicine, Technical University of Munich, Germany Hardeep Singh, et al. Annals of Clinical Case Reports Orthopedics Remedy Publications LLC., | http://anncaserep.com/ 2016 | Volume 1 | Article 1195 2 base of the fifth metatarsal forming the dorsal and plantar branches. The dorsal branch supplies the dorsal digital nerve providing sensation to the lateral side of fifth toe. It provides anastomoses to other digital nerves to supply the medial border of the fifth and lateral border of the fourth toe [2,4]. Cahill et al. [1] described the sural nerve as arising from the combination of MSCN and LSCN in 80% of cases, and as a direct continuation of MSCN in 20% of cases. The site of union of the MSCN and LSCN varies as well, forming anywhere from 11-20 cm proximal to the lateral malleolus [3]. Cahill et al. [1] described the average length of the sural nerve to be 11-20 cm, with its diameter ranging from 2.5 to 4.0 mm proximally and 2.0 to 3.0 mm distally; and a central nutrient artery and vein.1 Coert et al. [5] described this formation occurring in the popliteal fossa in approximately 12% of cases, and in lower third of leg in 84% of cases. Percutaneous sutures should be avoided in the lateral half of Achilles tendon repairs due to its close proximity to the sural nerve [3,6,7]. Hockenbury et al. [6] described a 60% incidence of sural nerve injury during percutaneous repair of cadaveric Achilles tendon. Non-surgical injury can occur as a result of entrapment by hematoma as well as following Achilles tendon rupture [8,9]. Traction injury to the sural nerve can occur as a result of Achilles rupture, which when intact acts as a checkrein to prevent nerve traction in physiologic conditions [8]. O’Brien et al. [10] described sural nerve entrapment by scar tissue after rupture of the gastrocnemius muscle. EMG studies showed latency in the sural nerve distribution along with reduction of sensory nerve action potential amplitude. MRI revealed abnormal signal in the gastrocnemius consistent with a subacute hematoma. Surgical exploration revealed encasement of the sural nerve in scar tissue underneath deep fascia at the level of the gastrocnemius. The release of the nerve resulted in restoration of sensation at the 3 month follow-up period [10]. Trevino et al. [11] reported on three cases in which an avulsion fracture of the base of the fifth metatarsal resulted in sural nerve entrapment. Sural nerve function was restored following removal of the fragments causing entrapment [11]. Sural nerve entrapment can occur from direct traumatic contusions to the nerve, fractures of the calcaneus, fifth metatarsal, posterolateral process of the talus, or from os perineum with the most common site of entrapment being the fibrous arcade of the Achilles musculotendinous junction [2,4]. Entrapment can lead to chronic neuropathic pain and diagnosis is based on clinical examination. A thorough history and physical examination is important to rule out exertional compartment syndrome and popliteal artery entrapment syndrome [12]. Neuropathy can present as constant, sharp, achy pain unresponsive to analgesics, dysesthesias, associated with numbness and tingling in the nerve distribution [13,14]. Characteristic signs in patients with sural nerve entrapment includes tenderness in the middle part of the calf, lateral to the musculotendinous junction of the Achilles tendon; with a positive tinel's sign [12,14]. In cases of long-standing nerve entrapment EMG studies show a decrease in nerve conduction velocity and reduction in sensory potential amplitudes, however, these studies can be negative [12]. Conservative treatment for neuropathic pain includes using tricyclic antidepressants, serotonin and norepinephrine reuptake inhibitors, and calcium channel alpha2-delta agonists [13]. Todorov et al. [13] reported complete relief of pain following pulsed radiofrequency for neuropathic Sural nerve pain. Fabre et al. [12] demonstrated sural nerve entrapment by an increase in sural muscle mass and development of local fibrous scar tissue. Thirteen patients (18 limbs) were retrospectively analyzed with calf tenderness along the course of the sural nerve distribution. EMG studies were positive in 10 patients. After failure of nonoperative treatment, patients underwent neurolysis with release of the superficial sural aponeurosis and fibrous band in which the nerve traversed. Excellent results were reported in 9 limbs, good in 8 limbs, and fair in 1 limb after 14 months follow-up [15].