A 28-year-old leprosy patient on MDT (MB regimen) for 8 months develops sudden onset of erythema and tenderness over existing skin lesions, fever, and peripheral nerve thickening. New skin lesions appear. Nerve function assessment shows new sensory loss in the ulnar nerve territory. This reaction pattern is MOST consistent with:

• A Erythema nodosum leprosum (ENL) — Type 2 reaction due to immune complex deposition
• B Type 1 (reversal) reaction — upgrading reaction with increased cell-mediated immunity ✓
• C Lucio phenomenon — diffuse infiltration reaction unique to Lucio leprosy
• D Lepra fallout — paradoxical worsening of pre-existing lesions during effective MDT

Explanation

Type 1 (reversal) reaction occurs in borderline leprosy (BT, BB, BL), typically during MDT as bacterial antigens release from dying bacilli trigger a sudden enhancement of delayed-type (Type IV) cell-mediated immunity. Clinical features include erythema and edema of existing lesions (not new lesions outside prior patches), nerve function impairment (acute neuritis) causing new sensory/motor deficits, and fever. It is an upgrading reaction (immune shift toward tuberculoid end). ENL (Type 2) occurs in BL/LL leprosy and presents with tender subcutaneous nodules, systemic features (fever, arthritis, orchitis), and is immune complex-mediated (Type III), not an upgrade reaction. Lucio phenomenon is rare, occurring in diffuse non-nodular LL leprosy (mainly Mexican leprosy).

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.