One of the causes of foot drop is compression of the common peroneal nerve caused by space-occupying lesions such as a synovial cyst or a ganglion cyst. Most previous reports have involved compressive common peroneal neuropathy by intraneural ganglion cysts and synovial cysts. Compression of the peroneal nerve by extraneural ganglion cysts is rare. We report a rare case of compressive common peroneal neuropathy by an extraneural ganglion cyst.

A 46-year-old man was hospitalized after he reported a right foot drop for 1 mo. Manual muscle testing revealed scores of 1/5 on dorsiflexion of the right ankle. Hypoesthesia and paresthesia on the right lateral leg and foot dorsum were noted. He was diagnosed with a popliteal cyst by using electrophysiologic study and popliteal ultrasound (US). To facilitate common peroneal nerve (CPN) decompression, 2 cc of sticky gelatinous material was aspirated from the cyst under US guidance. Electrical stimulation and passive and assisted active ROM exercises of the right ankle and strengthening exercises for weak muscles using elastic band were prescribed based on the change of muscle power. A posterior leaf spring ankle-foot orthosis was prescribed to assist the weak dorsiflexion of the ankle. Follow-up US revealed that the cystic lesion was growing and magnetic resonance imaging demonstrated compression of the CPN by the cystic mass. The cyst was resected to prevent impending compression of the CPN.

Precise diagnosis and immediate treatment are important in cases of compressive common peroneal neuropathy caused by an extraneural cyst.

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