Peroneal neuropathy caused by an extraneural ganglion cyst in the supracondylar area of the femur: A case report

RATIONALE: Peroneal neuropathy is the most common type of peripheral neuropathy in the lower extremities. The peroneal nerve is usually compressed at the lateral aspect of the fibular head. Compression by ganglion cysts are one of the numerous underlying etiologies for peroneal nerve neuropathy and are most frequently located around the fibular neck and proximal tibiofibular joint. To the best of our knowledge, this is the first report of an extraneural ganglion cyst located at the level of the distal thigh that resulted in compressive peroneal neuropathy. PATIENT CONCERNS: We report a case of a 56-year-old man with sudden onset of left foot drop and gait disturbance caused by an extraneural ganglion cyst located in the popliteal fossa. DIAGNOSIS: Electrodiagnosis (EDX) suggested a peroneal nerve lesion. Subsequently, diagnostic ultrasonography (USG) revealed a cystic mass located within the left side of the supracondylar area of femur. Further magnetic resonance imaging confirmed that the mass was located at the proximal of popliteal fossa. INTERVENTIONS: Surgical excision was performed using a direct posterior approach. The cystic mass was compressing the common peroneal nerve, and was carefully and completely removed ensuring that all nerve branches were protected. OUTCOMES: A histopathologic evaluation confirmed the diagnosis of a ganglion cyst. There were no postoperative complications. Two months after the surgery, follow-up USG revealed no evidence of cyst recurrence or residual lesions. Six months after the surgery, the ankle dorsiflexor motor power improved and he experienced less pain and hypoesthesia. LESSONS: Physicians should bear in mind that the peroneal neuropathy can occur because of the ganglion cyst in the distal thigh. The thorough evaluation of EDX and USG is crucial for the early diagnosis and surgical intervention, although there is no abnormal finding around the fibular neck.