Manson’s foot drop for 10 marks

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Manson's foot drop leprosy lateral popliteal nerve clinical features surgery

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Manson's Foot Drop

Definition

Manson's foot drop (also called leprous foot drop) is the inability to dorsiflex the foot and toes, resulting from paralysis of the muscles supplied by the lateral popliteal (common peroneal) nerve due to leprosy (Hansen's disease). It is named after Sir Patrick Manson, who described the association between leprosy and this deformity. It is the classic lower-limb deformity of leprosy.

Anatomy of the Nerve Involved

The lateral popliteal (common peroneal) nerve is a branch of the sciatic nerve. It winds around the neck of the fibula, where it is superficially placed and vulnerable to damage by Mycobacterium leprae. It divides into:

Deep peroneal (anterior tibial) nerve - supplies tibialis anterior, extensor digitorum longus, extensor hallucis longus (dorsiflexors and toe extensors)
Superficial peroneal (musculocutaneous) nerve - supplies peroneus longus and brevis (evertors)

The nerve can be palpated and felt thickened behind the upper end of the fibula (at the fibular neck) in leprosy patients.

Pathogenesis

M. leprae has a predilection for peripheral nerves, particularly at sites where nerves are superficial, cool, and subject to repeated minor trauma. The lateral popliteal nerve at the fibular head is one such site. The organism invades Schwann cells, causing:

Granulomatous inflammation within the nerve
Demyelination followed by axonal destruction
Fibrosis of the nerve - leading to the characteristic "cord-like" thickened nerve on palpation
"Silent neuritis" can occur - progressive nerve damage without pain or obvious reaction signs

The resulting motor paralysis affects dorsiflexion and eversion of the foot.

Clinical Features

Motor deficit:

Weakness/paralysis of dorsiflexion of the ankle (tibialis anterior)
Weakness of toe extension (extensor digitorum longus, extensor hallucis longus)
Weakness of eversion (peroneus longus and brevis)
This produces talipes equinovarus deformity (foot held in plantarflexion and inversion)

Gait abnormality - "Steppage gait" (high-stepping gait):

Patient lifts the knee high with each step to prevent the dropped foot and toes from dragging on the ground
The foot slaps down on the ground - characteristic "slapping" sound

Sensory loss:

Loss of sensation over the anterior and lateral aspect of the leg, dorsum of the foot, and toes (sparing the sole)

On examination:

Thickened lateral popliteal nerve behind/below the fibular head (pathognomonic of leprosy)
Other signs of leprosy may be present - hypopigmented anesthetic patches, claw hands (ulnar/median nerve), claw toes (posterior tibial nerve), trophic ulcers

Other Lower-Limb Deformities in Leprosy (for context)

Nerve	Deformity
Lateral popliteal (common peroneal)	Foot drop (Manson's foot drop)
Posterior tibial	Claw toes, trophic ulceration of sole
Both	Combined paralytic + trophic deformities

Investigations

Slit-skin smear - for acid-fast bacilli (AFB, Ziehl-Neelsen stain) - graded as Bacteriological Index (BI) 0-6
Skin biopsy - gold standard for classification; shows granulomas, AFB, nerve infiltration
Nerve conduction studies / EMG - assess extent of motor axonal loss, prognosis
Lepromin test (Mitsuda reaction) - assesses cell-mediated immunity; positive in tuberculoid, negative in lepromatous

Management

Medical (Primary Treatment)

WHO multidrug therapy (MDT) is the cornerstone:

Paucibacillary (1-5 lesions): Rifampicin 600 mg monthly + Dapsone 100 mg daily for 6 months
Multibacillary (>5 lesions): Rifampicin 600 mg monthly + Dapsone 100 mg daily + Clofazimine 300 mg monthly and 50 mg daily for 12 months

Steroids (prednisolone): Given during lepra reactions (Type 1 - reversal reaction, Type 2 - erythema nodosum leprosum) to reduce nerve inflammation and prevent further nerve damage. Also used for acute neuritis.

Conservative Management of Foot Drop

Physiotherapy: Passive and active range-of-motion exercises to prevent joint contractures, strengthen remaining muscle function
Foot drop splint / Ankle-foot orthosis (AFO): Holds the ankle in neutral position, prevents equinus contracture, enables walking without steppage gait
Patient education: Protecting the insensate foot from burns, injuries, and pressure ulcers; appropriate footwear

Surgical Management

Surgery is indicated when the foot drop is established, the nerve paralysis is irreversible, and conservative measures (splint) are inadequate.

Tendon transfer - Tibialis posterior transfer:

The tibialis posterior tendon (posterior tibial nerve supply - usually spared in common peroneal palsy) is transferred through or around the interosseous membrane onto the dorsum of the foot
This converts a plantarflexor/invertor into a dorsiflexor, correcting the foot drop
Route: Interosseous route is preferred in leprosy (circumtibial route has higher risk of recurrent inversion and ulceration of the lateral border)
Prerequisite: The muscle to be transferred must have MRC Grade 4 or 5 power; foot should be passively correctable

Other surgical options:

Triple arthrodesis - for fixed deformity with arthritis; fuses subtalar, calcaneocuboid, and talonavicular joints in neutral position
Nerve decompression - rarely done in leprosy given the fibrotic nature of the lesion
Amputation - in advanced disease with non-healing ulcers, extensive bone destruction, or secondary infection

Prerequisites for tendon transfer:

Disease must be under treatment with MDT
No active neuritis or lepra reaction
Correction must be achievable passively (no fixed bony deformity)
Adequate muscle strength in donor muscle (tibialis posterior)
Performed as part of a multidisciplinary team (orthopaedic/hand surgeon, plastic surgeon, physiotherapist)

Prognosis

Early diagnosis and prompt MDT initiation prevent nerve damage
Established nerve paralysis may be irreversible
Tendon transfer gives good functional results in properly selected patients - studies show average dorsiflexion of ~10° post-transfer, with most patients able to walk without an AFO
The deformity can be prevented by early detection and nerve decompression/steroid treatment during neuritis phase

Summary (Key Points for 10-Mark Answer)

Point	Detail
Definition	Inability to dorsiflex foot due to lateral popliteal nerve palsy from leprosy
Nerve	Lateral popliteal (common peroneal) nerve at fibular neck
Pathogen	Mycobacterium leprae
Gait	Steppage gait (high-stepping)
Deformity	Talipes equinovarus, slapping foot gait
Sensory loss	Dorsum of foot, anterior/lateral leg
Key sign	Thickened lateral popliteal nerve behind fibular head
Medical treatment	WHO MDT (rifampicin + dapsone ± clofazimine)
Conservative	AFO/foot drop splint + physiotherapy
Surgery	Tibialis posterior tendon transfer into dorsum of foot

Sources: Bailey and Love's Short Practice of Surgery 28th Ed.; Harrison's Principles of Internal Medicine 22nd Ed.; Park's Textbook of Preventive and Social Medicine; S. Das Manual on Clinical Surgery 13th Ed.; Adams and Victor's Principles of Neurology 12th Ed.

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