What imaging studies should be obtained for a patient presenting with foot drop?

Imaging for Foot Drop

For patients with foot drop, MRI is the recommended first-line imaging modality as it can evaluate both central and peripheral causes with excellent soft tissue contrast and multiplanar capabilities. 1

Diagnostic Approach

Initial Evaluation

• MRI should be targeted based on clinical suspicion of the anatomical location:
  • Brain MRI: When upper motor neuron signs are present or when peripheral causes have been ruled out 2
  • Cervical and thoracic spine MRI: When myelopathy is suspected or when lumbar imaging is negative 3
  • Lumbosacral spine MRI: To evaluate for L5 radiculopathy (most common spinal cause) 4
  • MR neurography: To visualize the sciatic nerve and common peroneal nerve pathway 4

Imaging Protocol Considerations

• MR neurography should be tailored and focused after review of clinical notes and electrodiagnostic findings 4
• For suspected central causes, brain MRI should focus on the parasagittal region (location of the foot homunculus) 2
• For suspected peripheral causes, imaging should follow the course of the peroneal nerve from the sciatic bifurcation to the fibular head 4

Common Causes and Specific Imaging

1. Peripheral Causes (most common):

   • Common peroneal nerve compression at fibular head: MR neurography showing nerve enlargement, signal change, or compression
   • Sciatic neuropathy: MRI of the pelvis and thigh to visualize the sciatic nerve

2. Central Causes (rare but important):

   • Parasagittal brain lesions: Brain MRI focusing on the medial precentral gyrus 2
   • Cervical or thoracic spinal stenosis: MRI of the entire spine may be necessary 3
   • Multiple sclerosis: Brain MRI showing demyelinating lesions 5

Clinical Pitfalls to Avoid

• Focusing only on the most common site: While peroneal neuropathy at the fibular neck is the most frequent cause, failure to consider central causes can lead to missed diagnoses 3, 6
• Relying solely on lumbar spine imaging: When lumbar imaging is negative, consider imaging the entire neuraxis 3
• Overlooking intermittent symptoms: In cases of exercise-induced foot drop, brain MRI may reveal demyelinating disorders like multiple sclerosis 5

Practical Recommendations

• When clinical presentation is atypical (bilateral symptoms, progressive course, or absence of trauma/compression history), prioritize imaging of the central nervous system 3, 2
• Correlate imaging findings with electrodiagnostic studies for optimal diagnostic accuracy 4
• For patients with negative initial imaging but persistent symptoms, consider expanding the imaging evaluation to include other areas of the neuraxis 3, 5

MRI's ability to visualize both neural and soft tissue structures makes it the superior modality for evaluating the complex and potentially multifocal causes of foot drop, leading to better outcomes when the correct diagnosis is established early.