What is the recommended initial imaging study for foot drop?

Recommended Initial Imaging Study for Foot Drop

MRI of the lumbar spine is the recommended initial imaging study for foot drop, as the most common causes are L5 radiculopathy and peroneal nerve injury, which require different anatomical localization strategies. 1, 2

Clinical Localization First

Before ordering imaging, perform a focused neurological examination to localize the lesion along the motor pathway:

• Check reflexes: Hyperreflexia (3+ reflexes with clonus) suggests a central/upper motor neuron lesion, while hyporeflexia or areflexia suggests peripheral nerve pathology 3
• Assess sensory distribution: L5 radiculopathy causes numbness in the dorsum of the foot and lateral leg, while common peroneal neuropathy causes numbness limited to the dorsum of the foot between the first and second toes 1
• Examine for pathological reflexes: Presence of Babinski sign indicates central pathology 4
• Palpate the fibular head: Tenderness or mass at the fibular neck suggests peroneal nerve entrapment 1

Imaging Algorithm Based on Clinical Findings

For Suspected Peripheral Nerve Lesion (Normal Reflexes, Localized Sensory Loss)

Electrodiagnostic studies (EMG/nerve conduction) should be performed first to localize the site of nerve injury and assess severity before imaging 1, 5, 2

• Ultrasound of the common peroneal nerve can identify nerve enlargement, fascicular changes, or compressive lesions at the fibular head 5
• MRI of the knee/fibular head is indicated if ultrasound shows abnormalities or if a mass lesion is suspected 5

For Suspected Radiculopathy (Diminished Reflexes, Dermatomal Pattern)

MRI of the lumbar spine is the primary imaging modality to evaluate for L5 nerve root compression from disc herniation or foraminal stenosis 5, 2

• CT myelography is an alternative if MRI is contraindicated or if surgical planning requires better bony detail 5

For Suspected Central Lesion (Hyperreflexia, Clonus, No Sensory Loss)

MRI of the brain with gradient echo (GRE) sequences is essential, as standard CT may miss microhemorrhages or contusions in the motor cortex 3

• This is particularly important in post-traumatic foot drop with upper motor neuron signs 3

For Suspected Thoracic Pathology (Bilateral Symptoms, Sensory Level)

MRI of the thoracic spine (T11-L1 level) should be obtained if there are signs of myelopathy or if lumbar imaging is negative 4

• Look for calcified disc herniations with >40% canal occupation, which can present with isolated foot drop 4

Critical Pitfall: Double Crush Syndrome

Always consider multiple compression sites along the same neural pathway 5

• If foot drop persists after treating one lesion (e.g., peroneal nerve decompression), repeat EMG and obtain imaging of the lumbar spine 5
• Combined L5 radiculopathy and common peroneal neuropathy can coexist and requires treatment of both sites 5

When Advanced Imaging Changes Management

Surgical decompression is reasonable when there is realistic chance of nerve recovery, making accurate localization critical for treatment decisions 2

• For peroneal nerve compression at the fibular head: Ultrasound or MRI guides surgical decompression 5, 2
• For L5 radiculopathy with severe foraminal stenosis: MRI or CT myelography guides spinal decompression and fusion 5, 2
• For thoracic disc herniation: MRI showing >40% canal occupation warrants surgical decompression with fixation 4