Drop Foot Secondary to a Peroneal Intraneural Ganglion Connected to the Superior Tibiofibular Joint: A Case Report

Peroneal neuropathy is the most frequent mononeuropathy of the lower extremity. Intraneural ganglion cysts (INGCs) are among rare causes of peroneal nerve palsy. According to the articular (synovial) theory, the articular branch plays the key role in the pathogenesis. Patients present with pain around the fibular head and neck, motor weakness resulting in foot drop and paresthesia in the anterolateral calf and foot. Ultrasonography (US) and MRI are both useful in the diagnosis, but MRI is the best imaging modality in the demonstration of the articular connection and the relation of the cyst with adjacent structures, even without special neurography sequences. We present a 32-year-old male patient referred to our neurosurgery clinic with suspicion of lumbar radiculopathy. He presented with right foot drop which began 3 weeks prior. On examination, there was 90% loss in the ankle dorsiflexion and finger extension. Ankle eversion was also weakened. There was no low back or posterolateral thigh pain to suggest L5 radiculopathy and sciatic neuropathy. Following negative lumbar spine MRI, peripheral neuropathy was concerned. Electrodiagnostic evaluations findings were consistent with acute/subacute common peroneal nerve (CPN) axonal neuropathy. Subsequent MRI of knee showed a homogeneous, thin-walled tubular cystic lesion, extending along the course of the CPN and its articular branch. Full recovery of the neuropathy was achieved with early diagnosis and decompression via microsurgical epineurotomy. The diagnosis of INGC was confirmed by histopathologic examination. INGCs, although rare, should also be considered in the differential diagnosis of peripheral mononeuropathies.