Neurology · Peripheral Neuropathy
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The common peroneal nerve is most frequently injured at the fibular neck due to its superficial location and susceptibility to external compression.
Patients classically present with foot drop, characterized by an inability to perform dorsiflexion and eversion of the foot.
Physical examination reveals a steppage gait and sensory loss over the dorsum of the foot and the lateral shin.
The deep peroneal nerve branch is responsible for dorsiflexion of the foot, while the superficial peroneal nerve branch is responsible for eversion.
Inversion of the foot remains intact because the tibial nerve (which innervates the posterior compartment) is spared.
Risk factors for compression include prolonged leg crossing, tight casts, or prolonged bed rest in patients with thin body habitus.
Diagnosis is primarily clinical, but electromyography (EMG) and nerve conduction studies (NCS) are the gold standard to confirm the site of injury and rule out L5 radiculopathy.
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A 45-year-old male presents to the clinic complaining of difficulty walking. He reports that he has been working from home for the past three weeks, spending 10 hours a day sitting with his legs crossed. On physical exam, he has weakness in dorsiflexion and eversion of the right foot. He exhibits a steppage gait when walking. There is decreased sensation on the dorsum of the right foot. Inversion of the foot and plantar flexion are preserved.
What is the most likely diagnosis?
Common peroneal neuropathy
The patient's presentation of foot drop, sensory loss on the dorsum of the foot, and preserved inversion is classic for common peroneal nerve compression at the fibular neck, which is supported by his history of prolonged leg crossing.
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Etiology / Epidemiology
Most common mononeuropathy of the lower extremity. Prolonged leg crossing, tight casts, and prolonged bed rest are primary risk factors.
Clinical Manifestations
Presents with foot drop and steppage gait. Sensory loss occurs over the dorsum of the foot and lateral calf.
Diagnosis
Electromyography (EMG) and nerve conduction studies (NCS) are the gold standard to confirm focal demyelination.
Treatment
Conservative management with ankle-foot orthosis (AFO). Avoid further compression of the fibular head.
Prognosis
Most cases resolve with conservative care; surgical decompression is reserved for refractory cases.
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Epidemiology & Etiology
Common in patients with prolonged immobilization, weight loss, or occupations requiring frequent squatting. External compression at the fibular head is the most frequent mechanism. It is frequently seen in patients with diabetes mellitus due to increased nerve susceptibility.
Pertinent Anatomy
The common peroneal nerve winds around the fibular neck, making it highly vulnerable to external pressure. It bifurcates into the superficial and deep peroneal nerves, controlling dorsiflexion and eversion.
Pathophysiology
Compression leads to focal ischemia and demyelination at the fibular head. Prolonged pressure causes axonal degeneration, which significantly worsens the prognosis. The loss of function in the anterior and lateral compartments results in the classic foot drop presentation.
Clinical Manifestations
Patients exhibit weakness in dorsiflexion and eversion, leading to a steppage gait to clear the toes. Sensory deficit is localized to the first dorsal web space and lateral lower leg. Red flags include sudden onset following trauma, which may indicate a fibular head fracture or compartment syndrome.
Diagnosis
Nerve conduction studies (NCS) demonstrate slowed conduction velocity across the fibular head. Electromyography (EMG) shows denervation potentials in the tibialis anterior. Imaging like MRI is indicated if a mass or cyst is suspected.
Treatment
Initial management focuses on physical therapy and the use of an ankle-foot orthosis (AFO) to prevent tripping. Avoid crossing legs and tight-fitting garments. If symptoms persist beyond 3 months, surgical decompression of the peroneal nerve may be required.
Prognosis
Prognosis is excellent for compression-related cases once the offending pressure is removed. Axonal loss on EMG indicates a poorer prognosis and longer recovery time. Patients require monitoring for gait instability and fall risk.
Differential Diagnosis
L5 Radiculopathy: involves weakness in hip abduction and gluteal muscles
Sciatic Neuropathy: involves weakness in both peroneal and tibial nerve distributions
Compartment Syndrome: presents with severe pain and tense, swollen compartments
Stroke: usually presents with upper motor neuron signs and hyperreflexia
Charcot-Marie-Tooth: typically bilateral and presents with pes cavus deformity