Unusual Case of Peroneal Nerve Palsy Due to Compression of an Extraneural Intramuscular Synovial Cyst: A Case Report

Peroneal neuropathy is the most common neuropathy in the lower limb, typically caused by external compression over the peroneal head due to poor soft tissue protection, while internal compression by cysts is unusual. We present a case of an unusual compression neuropathy of the deep peroneal nerve caused by an intramuscular extraneural synovial cyst with acute symptomatology. A 45-year-old male experienced a sudden onset of foot drop following knee distortion, with neurophysiological studies demonstrating severe axonal injury and MRI revealing a 7.8×2×2.3 cm multilobular cystic mass within the peroneus longus muscle originating from the tibioperoneal joint. Surgical exploration revealed the peroneal nerve with external damage changes through an intramuscular mucinous cyst, which was completely excised, including its stalk, with histological examination confirming a ganglion cyst without malignancy. Progressive recovery of sensibility and motor function occurred within five days postoperatively. Focal nerve entrapment of the peroneal nerve by a synovial cyst is not common but must be considered as a differential diagnosis, and in acute peripheral nerve neuropathies with severe dysfunction, immediate MRI is recommended to rule out compressive masses requiring surgical intervention, as delay in correct diagnosis can be catastrophic for nerve function prognosis.