Manson's Foot Drop

Definition

Anatomy and Pathogenesis

Nerve involved

• Endoneurial inflammation and granuloma formation
• Epineurial fibrosis and nerve thickening (the palpable "cord-like" nerve)
• Progressive axonal degeneration affecting motor, sensory, and autonomic fibres

Why the common peroneal nerve?

• Located superficially at the fibular neck - susceptible to pressure and ischaemia
• Cool peripheral nerve temperature favors M. leprae growth (the organism thrives at lower temperatures, explaining its predilection for peripheral nerves and skin)

Muscles paralyzed

• Tibialis anterior (primary dorsiflexor - the main muscle lost) - "paralysis of tibialis anterior" is the proximate cause of foot drop in leprosy (Bailey and Love's Short Practice of Surgery, 28th Ed.)
• Extensor digitorum longus
• Extensor hallucis longus
• Peroneus tertius

Clinical Features

The Deformity

• Foot drop - the foot hangs with toes pointing downward; the patient cannot lift the forefoot
• Equinovarus posture - the foot may develop an inverted, plantarflexed position if untreated
• Associated claw toes from posterior tibial nerve involvement (paralysis of intrinsic foot muscles)

Gait - Steppage Gait (Equine Gait)

• The patient lifts the knee abnormally high to prevent the dropped foot from catching on the ground
• The foot slaps the floor on landing
• Regular, even steps but with characteristic high-stepping pattern
• The advancing foot hangs with toes pointing toward the ground
• Patient trips on carpet edges and uneven surfaces (Adams and Victor's Principles of Neurology, 12th Ed.)

Sensory Loss

• Anaesthesia of the dorsum of the foot (deep peroneal: first web space; superficial peroneal: rest of dorsum)
• Anaesthesia of the sole from posterior tibial nerve involvement - leads to plantar trophic ulcers
• Loss of protective sensation leads to repeated unnoticed injuries

Other associated leprosy nerve deformities (context)

Disability Grading (WHO)

• Grade 0: Normal sensation, normal muscle power
• Grade 1: Sensation absent, muscle power normal
• Grade 2: Sensation absent + muscle power weak or paralyzed (Park's Textbook of Preventive and Social Medicine)

Stages of Nerve Involvement (Park's)

• Stage I: Nerve swollen and tender - NO loss of function - reversible
• Stage II: Thickened, painful nerve + loss of function - reversible if treated within 6 months
• Stage III: Irreversible nerve damage with established deformity

Prevention

• Early diagnosis and prompt MDT (multi-drug therapy) to halt M. leprae multiplication
• Nerve function monitoring (voluntary muscle testing + sensory testing) at every clinic visit
• Corticosteroids for lepra reactions (Type 1 reversal reactions cause rapid nerve damage - a medical emergency)
• Splinting and physiotherapy to prevent fixed contracture during nerve function impairment (NFI)
• Footwear modification and patient education to prevent trophic ulcers

Treatment

Conservative

1. Ankle-foot orthosis (AFO) / foot-drop splint - first-line management; holds the foot in neutral dorsiflexion during gait
2. Physiotherapy - passive stretching to prevent Achilles tendon contracture; strengthening of any residual tibialis anterior function
3. Anti-leprosy MDT (dapsone, rifampicin, clofazimine) - must continue during all surgical procedures

Surgical - Tendon Transfer

• The tibialis posterior (innervated by tibial nerve, not affected) is rerouted to the dorsum of the foot to restore active dorsiflexion
• Two routes:
  • Interosseous (IO) route: Through the interosseous membrane - better biomechanical alignment for pure dorsiflexion
  • Circumtibial (CT) route: Around the medial border of the tibia - technically simpler
• The tendon is split and attached to both the tibialis anterior tendon and the peroneus brevis/tertius to balance dorsiflexion and eversion
• Post-op: Above-knee plaster cast for 3 weeks in dorsiflexion, then below-knee cast for 3 weeks, followed by intensive physiotherapy and re-education

• Peroneus longus transfer (when peroneal muscles are intact)
• Triple arthrodesis (subtalar + talonavicular + calcaneocuboid joints) for fixed rigid deformity - gives a stable, plantigrade foot but sacrifices mobility
• Ankle arthrodesis for severe cases
• For associated ulcers: thorough debridement + total contact cast/plaster cast

Postoperative outcomes (leprosy series)

Summary Points (for exam)

1. Manson's foot drop = leprosy deformity from common peroneal nerve destruction by M. leprae
2. Key muscle lost: tibialis anterior - cannot dorsiflex the foot
3. Gait: steppage/equine gait - high knee lift, foot slap
4. WHO Grade 2 disability (worst grade)
5. Prevention: early MDT, corticosteroids for reactions, nerve monitoring
6. Conservative treatment: AFO splint
7. Surgery: tibialis posterior tendon transfer (interosseous or circumtibial route)
8. Additional complications: plantar trophic ulcers (anaesthetic foot), claw toes, Achilles shortening