How to diagnose peroneal nerve injury?

Diagnosing Peroneal Nerve Injury

The most effective approach to diagnose peroneal nerve injury is to perform a systematic clinical examination including assessment of foot dorsiflexion strength, sensation testing with a 10-g monofilament, and evaluation of ankle reflexes, followed by electrodiagnostic studies if clinical findings suggest nerve damage. 1

Clinical Examination

Motor Assessment

• Test ankle dorsiflexion strength (tibialis anterior muscle)
  • Have patient attempt to dorsiflex foot against resistance
  • Grade strength using BMRC scale (0-5)
• Test foot eversion strength (peroneus longus and brevis muscles)
  • Have patient attempt to evert foot against resistance
  • Weakness in both dorsiflexion and eversion strongly suggests common peroneal nerve injury
• Test toe extension (extensor digitorum longus)
  • Weakness indicates involvement of deep peroneal nerve branch

Sensory Assessment

• Check sensation in the following areas:
  • Dorsum of foot and lateral lower leg (superficial peroneal nerve distribution)
  • First web space between great and second toe (deep peroneal nerve distribution)
• Use 10-g monofilament for standardized sensory testing 2
• Assess for paresthesias, dysesthesias, or numbness in the distribution

Reflex Testing

• Check ankle reflexes (may be normal in isolated peroneal nerve injury)
• Compare with unaffected side

Advanced Diagnostic Testing

Electrodiagnostic Studies

• Nerve conduction studies and electromyography are the gold standard for confirming diagnosis 1
• These studies can:
  • Confirm the diagnosis
  • Localize the site of injury (common peroneal nerve at fibular head vs. deep or superficial branches)
  • Determine severity (demyelinating vs. axonal injury)
  • Provide prognostic information

Imaging Studies

• MRI is highly sensitive for identifying nerve compression, particularly at the fibular head 1
  • Can visualize nerve swelling, compression, or disruption
  • Helps identify space-occupying lesions causing compression
• Ultrasound can provide dynamic assessment and may guide interventions 1
  • Particularly useful for visualizing superficial nerve segments
  • Can identify nerve swelling, compression, or discontinuity

Differential Diagnosis Considerations

When examining for peroneal nerve injury, consider these alternative diagnoses:

• L5 radiculopathy (will also affect hip abduction and knee flexion)
• Sciatic neuropathy (will affect hamstrings and all ankle/foot muscles)
• Lumbar plexopathy (more extensive motor and sensory deficits)
• Central nervous system disorders (upper motor neuron signs present)

Common Pitfalls and Caveats

• Don't miss bilateral involvement: Always compare to the contralateral side
• Identify the cause: Common causes include:
  • Compression at fibular head (most common site) 3
  • Trauma/laceration
  • Knee dislocation
  • Iatrogenic injury (e.g., during surgery) 4
• Consider timing: Early diagnosis improves outcomes, as surgical decompression within 3-4 months of injury has better prognosis if no spontaneous recovery occurs 1, 5
• Document baseline function: This is crucial for monitoring recovery and determining treatment success
• Check for fixed deformities: Longstanding peroneal nerve injury can lead to equinus deformity due to unopposed ankle plantar flexion

By following this systematic approach to diagnosing peroneal nerve injury, clinicians can accurately identify the condition, determine its severity, and initiate appropriate management to optimize patient outcomes.