Peroneal mononeuropathy is the most common isolated mononeuropathy of the lower extremity. It usually results from compression or trauma to the common peroneal nerve at the fibular head, which can be damaged by frequent leg crossing. Other etiologies include leg casting, inflammatory disease, nerve ischemia, or infection. The clinical result is foot drop with weakness of ankle dorsiflexion and/or ankle eversion, depending on where the nerve is injured.

Patients may present with frequent tripping. There may also be numbness or tingling in the lateral lower leg and/or dorsum of the foot. Pain is rarely present. Prognosis depends on the underlying etiology and degree of nerve injury, although most patients will experience gradual improvement over time.

Risk factors include slim body habitus, recent loss of weight, and prolonged or frequent pressure on the lateral aspect of the knee or prolonged extreme flexion. Patients with hereditary neuropathy with liability to pressure palsy are at particular risk.