How is foot drop evaluated and the lesion localized?

Evaluation and Localization of Foot Drop

A comprehensive neurological evaluation combined with appropriate imaging studies is essential for accurate localization of foot drop lesions, with MRI being the most sensitive imaging modality for detecting the underlying pathology. 1

Clinical Assessment for Localization

• Foot drop is a reduction in ankle dorsiflexion during the swing phase of gait that can result from lesions at various points along the motor neural pathway 1, 2
• Examination of five key muscles helps localize the lesion: tibialis anterior, peroneus longus and brevis, posterior tibial, gluteus medius, and quadriceps 3
• The most common causes of foot drop are common peroneal neuropathy at the fibular neck and L5 radiculopathy, though other causes include anterior horn cell disease, lumbar plexopathies, and partial sciatic neuropathy 1
• Habitual leg crossing is frequently implicated in peroneal nerve compression at the fibular head 1

Diagnostic Imaging Algorithm

Initial Imaging

• Radiographs should be the first imaging study when evaluating foot drop, particularly to rule out bony abnormalities or trauma 4
• Weight-bearing radiographs are preferred when possible as they may detect dynamic abnormalities not apparent on standard views 5

Advanced Imaging

• MRI is the most sensitive modality for detecting the underlying pathology causing foot drop and should be performed when the cause remains unclear after initial evaluation 1, 2

• MRI can effectively evaluate:

  • Spinal causes (L4/L5 radiculopathy) 2
  • Sciatic nerve pathology along its course 1
  • Soft tissue abnormalities in the popliteal fossa 1
  • Muscle denervation changes 4

• CT may be useful in cases of suspected bony abnormalities compressing neural structures 4

• Ultrasound can be valuable for dynamic evaluation of the peroneal nerve at the fibular head and for identifying space-occupying lesions 4

Electrodiagnostic Studies

• Nerve conduction studies and electromyography are crucial adjuncts to imaging for:
  • Precisely localizing the site of nerve injury 1
  • Establishing the degree of nerve damage 1
  • Predicting recovery potential 1
  • Differentiating between central and peripheral causes 2, 6

Special Considerations

• In patients with diabetes and neuropathy, standard clinical tests may be less reliable, and imaging should be performed more liberally 4, 5
• Multiple sclerosis should be considered in cases of exercise-induced foot drop with normal peripheral nerve studies, requiring brain MRI for diagnosis 6
• For suspected peroneal nerve compression at the fibular head, dynamic ultrasound during knee flexion/extension may reveal nerve subluxation not apparent on static imaging 4

Common Pitfalls to Avoid

• Failing to consider central causes (such as multiple sclerosis) when peripheral studies are normal 6
• Overlooking the possibility of multiple lesions in series along the neural pathway 2
• Not performing weight-bearing studies when indicated, which may miss dynamic instabilities 5
• Relying solely on imaging without correlating with clinical and electrodiagnostic findings 1, 2