Foot Drop — Differential Diagnosis

Anatomical Framework

Cortex/UMN → Spinal Cord → Anterior Horn Cell → 
Nerve Root (L4–L5) → Lumbosacral Plexus → 
Sciatic Nerve → Common Peroneal Nerve → Muscle

1. Peripheral Nerve — Most Common Causes

Common Peroneal (Fibular) Neuropathy ⭐ Most common cause of unilateral foot drop

• Compression at the fibular head (vulnerable due to superficial location)
• Classic precipitants: leg crossing, weight loss/emaciation, prolonged squatting, tight casts, lateral decubitus positioning, total knee arthroplasty, yoga, natural childbirth ("pushing palsy")
• Also: trauma (bumper injury), intraneural ganglia from superior tibiofibular joint, vasculitis
• Deficits: foot dorsiflexion ↓, eversion ↓, toe extension ↓; sensory loss over anterolateral leg and dorsum of foot
• Key distinguishing feature: foot inversion is spared (tibialis posterior = tibial nerve, intact)
• Most cases resolve spontaneously when mechanical cause removed

Sciatic Neuropathy

• Causes: hip arthroplasty, pelvic surgery/lithotomy position, trauma, hematoma, tumor, vasculitis, idiopathic
• The peroneal division of the sciatic nerve is disproportionately vulnerable, so presentation may mimic peroneal neuropathy alone
• Full sciatic: weakness of all ankle/toe movements + knee flexion; sensory loss entire foot + distal lateral leg; absent ankle jerk

Peripheral Neuropathy (Bilateral foot drop)

• Charcot-Marie-Tooth disease (hereditary motor-sensory neuropathy, peroneal muscular atrophy) — classic distal-predominant pattern
• Chronic acquired neuropathies: diabetic, inflammatory (CIDP), toxic, nutritional
• Guillain-Barré syndrome (ascending), tick paralysis
• Amyloid neuropathy

2. Nerve Root

L5 Radiculopathy ⭐ Key mimic of peroneal palsy

• Common causes: disc herniation (L4–L5), degenerative joint disease, osteophytes
• Deficits: foot dorsiflexion ↓, eversion ↓, inversion ↓ (tibialis posterior = L5 via tibial nerve)
• May have hip abduction weakness (gluteus medius, L5)
• Back/buttock/lateral leg pain often present; paresthesias in L5 distribution (lateral leg → dorsum of foot → great toe)
• EMG: denervation in L5-innervated muscles (including tibialis posterior and gluteus medius — not peroneal territory)

L4 Radiculopathy

• Less common cause; weakness of tibialis anterior + inversion; diminished patellar reflex

3. Lumbosacral Plexopathy

• Trauma, retroperitoneal hematoma, tumor (lymphoma, sarcoma), radiation, diabetic amyotrophy (Bruns-Garland syndrome)
• L5 division of plexus involvement → foot drop + variable other lower limb deficits
• Key: pattern of weakness exceeds a single nerve or root territory

4. Anterior Horn Cell Disease

• ALS (Amyotrophic Lateral Sclerosis): can present with focal lower motor neuron foot drop early; look for UMN signs elsewhere, fasciculations, bulbar involvement
• Progressive spinal muscular atrophy
• Poliomyelitis (or post-polio syndrome)

5. Central / Upper Motor Neuron

Spasticity (UMN Foot Drop)

• Stroke (cortical or subcortical — foot area of motor cortex or internal capsule)
• MS plaques affecting corticospinal tracts
• Cerebral palsy
• Spinal cord lesion (myelopathy)
• Gait: spastic/circumduction rather than high-steppage; hyperreflexia, upgoing plantar response

6. Muscle / Neuromuscular Junction

Myopathies

• Distal muscular dystrophies (e.g., Miyoshi, Nonaka, Welander) — distal leg involvement
• Scapuloperoneal syndromes (facioscapulohumeral MD variant)
• Myotonic dystrophy (distal weakness, foot drop can be presenting feature)

Non-neuromuscular musculoskeletal

• Tibialis anterior tendon (TAT) rupture — pure foot drop, no sensory loss, no neurologic signs; pathognomonic: absent tendon palpation; spared eversion and hip abduction

7. Other / Functional

Dystonia

• Action dystonia of foot (equinovarus posture while walking); dystonia may diminish or resolve when walking backwards — a pathognomonic feature
• Sensory trick (geste antagoniste) may relieve it

Sensory Ataxia

• Mimics foot drop gait; seen in severe polyneuropathy, dorsal column disease (tabes dorsalis, subacute combined degeneration, B12 deficiency)
• Romberg positive; high-stepping due to proprioceptive loss rather than true dorsiflexion weakness

Functional (psychogenic)

• Inconsistent weakness, Hoover sign positive, non-anatomic sensory loss

Summary by Level

High-Yield Diagnostic Approach

1. Inversion intact? → Points to peroneal nerve (not L5 root)
2. Knee flexion weak? → Points to sciatic nerve (not peroneal)
3. Bilateral? → Think polyneuropathy (CMT, GBS, diabetes), ALS, myelopathy
4. Back pain + radiation? → L5 radiculopathy
5. UMN signs? → Central cause (stroke, MS, cord)
6. No sensory loss, no EMG abnormality? → TAT rupture, dystonia, functional
7. EMG/NCS — indispensable: peroneal motor conduction shows slowing/amplitude drop across fibular head in peroneal palsy; L5 denervation involves tibialis posterior (tibial nerve territory) in radiculopathy

Common vs. Deep Peroneal Nerve

The Hierarchy

Sciatic nerve (L4–S2)
    └── Common Peroneal Nerve  ← divides at fibular head
            ├── Superficial Peroneal Nerve
            └── Deep Peroneal Nerve

Comparison Table

Motor Innervation

Sensory Innervation

Clinical Syndromes by Level

Common Peroneal Neuropathy (at fibular head)

• Affects both branches → complete foot drop (dorsiflexion ↓) + eversion ↓
• Sensory loss: dorsum of foot + anterolateral leg
• Most common: compression at fibular head (leg crossing, weight loss, cast, arthroplasty)

Isolated Deep Peroneal Neuropathy (anterior tarsal tunnel syndrome)

• Compression at the inferior extensor retinaculum / talonavicular joint level
• Dorsiflexion weak (tibialis anterior, toe extensors); eversion spared
• Sensory loss confined to 1st web space
• Causes: tight footwear, ganglion cyst, osteophyte

Isolated Superficial Peroneal Neuropathy

• Eversion weakness; dorsiflexion spared (no foot drop)
• Sensory loss over dorsum of foot
• Causes: ankle inversion/plantar flexion sprains, lateral compartment hernias, fascial defects, surgery

High-Yield Bedside Distinction