Bilateral Foot Drop — Causes

1. Central / Upper Motor Neuron (Spinal Cord)

• Spastic paraparesis from myelopathy (cervical spondylosis, MS, transverse myelitis, tumor, AVM)
• Scissor gait / paraparetic gait with bilateral foot drop, extended knees, and adductor crossover

2. Anterior Horn Cell (Lower Motor Neuron — Spinal)

• Amyotrophic lateral sclerosis (ALS) / Motor Neuron Disease — may begin distally and asymmetrically in the legs
• Progressive Spinal Muscular Atrophy (PSMA)
• Poliomyelitis

3. Lumbosacral Roots

• Bilateral L4–L5 radiculopathy — disc disease, spinal stenosis, cauda equina lesion
• Lumbar plexopathy (bilateral, e.g., retroperitoneal disease)

4. Peripheral Nerve (Polyneuropathy) — Most Common Cause

5. Neuromuscular Junction

• Typically does not produce isolated foot drop

6. Muscle (Myopathy)

7. Other

• Dystonia — action dystonia of the foot can mimic foot drop
• Sensory ataxia (severe deafferentation, e.g., tabes dorsalis, subacute combined degeneration) — steppage gait with pseudo-foot drop due to proprioceptive loss

Key Distinguishing Points

• Adams and Victor's Principles of Neurology, 12th Ed. — Steppage gait, distal bilateral limb palsies
• Bradley and Daroff's Neurology in Clinical Practice — Box 25.2: Causes of Foot Drop
• Localization in Clinical Neurology, 8th Ed. — Steppage gait, bilateral foot drop
• Adams and Victor's Principles of Neurology, 12th Ed. — Distal muscular dystrophies, scapuloperoneal dystrophy

Wrist Drop — Causes

1. Radial Nerve Palsy — By Site of Lesion (Most Common Overall Cause)

A. Axillary (High) Radial Nerve Lesion

Triceps is affected here because branches to triceps arise proximal to the spiral groove.

B. Spiral Groove (Mid-Humeral) Lesion — Most Common Site

C. Posterior Interosseous Nerve (PIN) Entrapment — Distal to Elbow

• Entrapment in the radial tunnel (at the arcade of Frohse / supinator muscle)
• Trauma or dislocation of radial head
• Lipoma, ganglion, synovial cyst compressing the nerve
• Features: Finger drop (extensors of wrist and fingers weak) but no sensory loss and no true wrist drop — wrist extensors (extensor carpi radialis longus) are partially spared, so the wrist deviates radially on extension rather than fully dropping.

D. Distal / Superficial Radial Nerve Lesion

• Tight wristbands, handcuffs, IV catheter — sensory loss only (cheiralgia paresthetica), no wrist drop

2. C7 Nerve Root Radiculopathy

• C6–C7 disc herniation — the most common cervical root involved
• Wrist drop + triceps weakness + triceps reflex loss + sensory loss over 2nd and 3rd digits
• Causes: cervical disc prolapse, cervical spondylosis, tumour, trauma

3. Brachial Plexus (Posterior Cord / Middle Trunk)

• Posterior cord lesion of the brachial plexus — wrist drop + deltoid weakness (axillary nerve also from posterior cord)
• Erb-Duchenne palsy (C5–C6) — affects upper trunk; does NOT typically cause wrist drop
• Pancoast tumour (apical lung tumour) invading lower plexus — predominantly C8/T1 but can extend

4. Cortical / Central (Rare)

• Cerebral cortical infarct in the "hand knob" area of the precentral gyrus can mimic wrist drop (pseudo-radial nerve palsy) — but the weakness is upper motor neuron (spastic, hyperreflexic, no sensory loss in radial distribution)

5. Toxic / Systemic Causes (Bilateral Wrist Drop)

6. Summary by Level

• Harrison's Principles of Internal Medicine, 22nd Ed. — Radial neuropathy
• Neuroanatomy through Clinical Cases, 3rd Ed. — Radial neuropathy, upper extremity nerve injuries
• General Anatomy & Musculoskeletal System (THIEME Atlas) — Wrist drop; radial nerve lesion sites
• Bradley and Daroff's Neurology in Clinical Practice — Radial neuropathy; lead neuropathy
• Localization in Clinical Neurology, 8th Ed. — Pseudo-radial nerve palsy (cortical)
• Katzung's Basic & Clinical Pharmacology, 16th Ed. — Lead-induced wrist drop

Wasting of the Small Muscles of the Hand — Differential Diagnosis

Anatomical Framework

Cortex / UMN → Spinal cord (C8–T1) → Anterior horn cells
→ Nerve roots (C8, T1) → Lower trunk brachial plexus
→ Median + Ulnar nerves → Intrinsic hand muscles

1. Peripheral Nerve

Ulnar Nerve Palsy

• Compression at elbow (cubital tunnel / ulnar groove) — most common cause of intrinsic wasting
• Compression at wrist (Guyon's canal) — hypothenar + interossei, sparing forearm muscles
• Chronic pressure from air hammers, prolonged leaning on elbow
• Wasting of hypothenar eminence, first dorsal interosseous, interossei → claw hand (4th & 5th fingers)
• Associated: sensory loss over 4th–5th fingers and ulnar border

Median Nerve Palsy

• Carpal tunnel syndrome — most common peripheral nerve entrapment; thenar wasting (LOAF muscles)
• Proximal median nerve compression (pronator teres, anterior interosseous nerve)
• Features: thenar (lateral) wasting; sensory loss over thumb/index/middle finger; positive Tinel/Phalen

Combined Ulnar + Median = Complete Intrinsic Wasting

• Both nerves injured → "all-intrinsic" wasting

2. Brachial Plexus (Lower Trunk — C8, T1)

3. Nerve Roots (C8–T1)

4. Spinal Cord (C8–T1 Anterior Horn / Cord)

5. Distal Myopathy / Muscle Disease

6. Vasculitic / Multifocal Neuropathy

Summary Table by Distribution

• Adams and Victor's Principles of Neurology, 12th Ed. — Lower brachial plexus paralysis; thoracic outlet syndrome
• Bradley and Daroff's Neurology in Clinical Practice — Ulnar neuropathy; thoracic outlet; ALS
• Eric Kandel Principles of Neural Science, 6th Ed. — ALS and hand wasting
• Gray's Anatomy for Students — Cervical rib; intrinsic wasting
• Harrison's Principles of Internal Medicine, 22nd Ed. — Thoracic outlet syndrome
• Localization in Clinical Neurology, 8th Ed. — Cervical cord; syringomyelia