What causes foot drop?

Foot Drop Causes

Foot drop results most commonly from common peroneal nerve injury at the fibular neck, but can also arise from L5 radiculopathy, lumbar plexopathy, sciatic nerve injury, anterior horn cell disease, or central nervous system lesions including brain tumors and stroke. 1, 2

Peripheral Nerve Causes (Most Common)

Common Peroneal Nerve Injury

• Peroneal neuropathy at the fibular neck is the single most frequent cause of foot drop 2
• Habitual leg crossing is often the culprit and most patients improve when they stop this habit 2
• Other mechanisms include direct trauma, surgical nerve injury, and compression at the fibular head 1, 3
• Compartment syndromes of the lower leg can damage the nerve and lead to foot drop 1

Sciatic Nerve Lesions

• Proximal sciatic nerve injuries can present with foot drop, particularly affecting the peroneal division 1
• Partial sciatic neuropathy should be considered when foot drop occurs with additional findings beyond peroneal distribution 2

Spinal and Radicular Causes

Lumbar Degenerative Disease

• L5 radiculopathy from lumbar degenerative disease represents one of the top two most common etiologies for foot drop 4
• Lumbar spinal stenosis can compress nerve roots and cause foot drop 3
• L4/L5 radiculopathy specifically affects the dorsiflexor muscles 1
• Preoperative motor strength and time to surgery are the most important predictors of improvement in foot drop from degenerative lumbar disease 4

Lumbar Plexopathy

• Lumbar plexus lesions can manifest as foot drop and should be distinguished from isolated nerve injuries 1, 2

Central Nervous System Causes (Uncommon but Critical)

Brain Lesions

• Brain tumors, particularly parasagittal parietal tumors, can present with isolated foot drop as the only neurologic finding 5
• Brain metastases may manifest as foot drop with minimal other symptomatology, potentially causing diagnostic delays 3
• Stroke affecting motor pathways can result in foot drop 3

Spinal Cord Pathology

• Anterior horn cell disease (motor neuron disease) affects the lower motor neurons supplying dorsiflexors 1, 2
• Tethered cord syndrome in children and adults can cause progressive muscle weakness, gait disturbances, and foot deformities including foot drop 6

Systemic and Metabolic Causes

Neuropathies

• Diabetic peripheral neuropathy can contribute to foot weakness and deformity 6
• Connective tissue diseases and vasculitides may cause neuropathic foot drop 3
• Drug toxicities can damage peripheral nerves leading to foot drop 3
• Charcot-Marie-Tooth syndrome and other inherited neuropathies may present with progressive foot weakness 6

Diagnostic Approach

Clinical Localization

• A meticulous neurological examination is essential to determine the lesion site 2
• Examine for weakness isolated to ankle and toe dorsiflexion (peroneal nerve) versus broader involvement suggesting more proximal lesions 2
• Assess for sensory deficits: peroneal nerve injury causes numbness over the dorsum of the foot, while L5 radiculopathy affects the lateral leg and dorsal foot 2
• Check for associated findings like foot deformities, muscle atrophy, or signs of systemic disease 6

Electrodiagnostic Studies

• Nerve conduction studies and electromyography are useful for localizing the injury site, establishing severity, and predicting recovery 2
• These studies help distinguish between nerve, plexus, root, and central lesions 2

Imaging

• When peripheral workup fails to disclose the etiology, the central nervous system must be investigated 3
• MRI of the lumbar spine is indicated when radiculopathy or spinal stenosis is suspected 4
• Brain imaging (MRI preferred) should be obtained when central lesions are possible, particularly with atypical presentations or progressive symptoms 3, 5
• Ultrasonography, CT, and MR imaging are all useful along the course of the sciatic nerve and in the popliteal fossa 2

Common Pitfalls

• Do not assume all foot drop is peripheral in origin—central lesions, though rare, require prompt recognition and treatment 3, 5
• Failure to ask about leg-crossing habits misses a readily reversible cause 2
• Delayed imaging of the spine or brain when indicated can result in missed opportunities for intervention 3, 4
• In diabetic patients, foot drop may be multifactorial, involving both neuropathy and structural deformities that increase fall risk 6