Peroneal mononeuropathy refers to damage affecting a single nerve, specifically the common peroneal nerve, also known as the common fibular nerve. This nerve is a branch of the sciatic nerve, originating from spinal nerve roots L4 through S2. It travels down the back of the thigh and then wraps around the outside of the knee, near the head of the fibula, a bone in the lower leg. At this point, the nerve is quite superficial, making it vulnerable to external pressure or injury.
The common peroneal nerve divides into two main branches: the superficial peroneal nerve and the deep peroneal nerve. These branches are responsible for both motor and sensory functions in the lower leg and foot. The deep peroneal nerve controls muscles that lift the foot and toes upwards, a movement called dorsiflexion, and muscles that extend the toes. The superficial peroneal nerve innervates muscles that turn the sole of the foot outwards, known as eversion. Both branches also provide sensation to different areas of the lower leg and top of the foot.
Recognizing the Signs
Damage to the common peroneal nerve causes symptoms related to movement and sensation. A common symptom is “foot drop,” which is the inability to lift the front part of the foot or the toes. This can cause a person to drag their foot while walking, often leading to frequent tripping. To compensate, individuals might adopt a “steppage gait,” where they lift their thigh higher than usual, as if climbing stairs, to avoid dragging their foot.
Beyond motor weakness, sensory disturbances are also common. Individuals may experience numbness, tingling, or a “pins and needles” sensation in the top of the foot or the outer part of the lower leg. The specific area of sensory loss can help pinpoint which part of the nerve is affected. For instance, numbness in the lower lateral distal leg suggests superficial peroneal nerve involvement, while numbness in the upper lateral distal leg points to deep peroneal nerve involvement.
Identifying the Causes
Peroneal mononeuropathy often stems from nerve compression or trauma. Prolonged pressure on the common peroneal nerve, especially where it wraps around the fibular head, is a common cause. This can occur from habitually crossing the legs, prolonged squatting, or extended periods in certain positions, such as during deep sleep or surgery. Tight casts, braces, or high boots around the lower leg can also exert sustained pressure on the nerve.
Direct trauma to the knee area, such as a direct impact, a cut, or a fracture of the fibula, can also injure the nerve. Knee dislocations are particularly associated with peroneal nerve injury, occurring in approximately 10-40% of cases. Certain medical conditions can also increase susceptibility to peroneal mononeuropathy. These include diabetes, which can degrade nerves, and inflammatory conditions like chronic inflammatory demyelinating polyradiculoneuropathy. Rapid or significant weight loss, such as that seen in anorexia nervosa, can reduce the protective fat pad around the fibular head, making the nerve more vulnerable to compression.
Diagnosis and Treatment Approaches
Diagnosing peroneal mononeuropathy typically begins with a thorough physical examination. A healthcare provider will assess muscle strength in the lower leg and foot, checking for weakness in dorsiflexion (lifting the foot) and eversion (turning the foot outwards), and evaluate sensation in the affected areas. They will also observe the patient’s gait for signs like foot drop or a “slapping” sound while walking.
Electrodiagnostic tests confirm the diagnosis and pinpoint nerve damage. Nerve conduction studies (NCS) measure how quickly electrical signals travel along the nerve, while electromyography (EMG) assesses the electrical activity of muscles. These tests can reveal nerve conduction blocks or signs of muscle denervation. Imaging studies like MRI or ultrasound may be used if a mass, such as a cyst or tumor, is suspected of compressing the nerve, or to rule out other conditions.
Treatment for peroneal mononeuropathy varies by cause and severity. Conservative management is often the first approach. Physical therapy is a common treatment, involving exercises to strengthen weakened muscles, improve range of motion, and enhance gait. Orthotic devices, such as an ankle-foot orthosis (AFO) or splints, can help manage foot drop by supporting the foot and preventing it from dragging, improving walking stability.
Pain management may involve nonsteroidal anti-inflammatory drugs (NSAIDs) or medications specifically for nerve pain. In some cases, corticosteroid injections may be considered to reduce inflammation around the nerve. If conservative measures fail or severe nerve damage is identified, more invasive options may be explored. Surgical decompression can relieve pressure on the nerve. In rare instances of severe nerve injury, procedures like nerve repair, nerve grafting using a nerve from elsewhere, or nerve transfers may be performed to restore function. Tendon transfer procedures, such as moving the posterior tibial tendon to assist with dorsiflexion, can also be used in cases where nerve recovery is unlikely.