A 32-year-old man from rural Bihar presents with a 2-year history of progressive numbness and weakness in his hands and feet. On examination, he has multiple hypopigmented macules with diminished sensation over the lesions. He has thickened ulnar and posterior tibial nerves. Slit-skin smear examination shows acid-fast bacilli. Which of the following best describes the pathological mechanism underlying the nerve involvement in this patient?

Explanation

## Pathophysiology of Nerve Involvement in Leprosy **Key Point:** Mycobacterium leprae has a unique tropism for peripheral nerves and Schwann cells, directly invading the nerve sheaths and causing demyelination and axonal degeneration. ### Mechanism of Neural Damage M. leprae preferentially colonizes: - Schwann cells of peripheral nerves - Cooler peripheral tissues (skin, superficial nerves) - Nerve sheaths, leading to direct structural damage **Clinical Pearl:** The organism's ability to damage nerves is a hallmark feature distinguishing leprosy from other mycobacterial infections. This direct invasion causes: 1. Demyelination of nerve fibers 2. Axonal degeneration 3. Nerve enlargement and thickening (palpable nerve trunks) 4. Progressive sensory and motor loss ### Why This Mechanism Matters | Feature | Leprosy | Tuberculosis | | --- | --- | --- | | Nerve involvement | Direct invasion of Schwann cells | Rare; via granulomas | | Nerve enlargement | Characteristic and early | Absent | | Mechanism | Demyelination + axonal loss | Granulomatous inflammation | **High-Yield:** The thickened, palpable nerves (ulnar, posterior tibial, common peroneal) are pathognomonic for leprosy and result from direct bacterial invasion and demyelination, not immune-mediated vasculitis. ### Distinction from Immune Reactions While Type 2 lepra reactions (Erythema Nodosum Leprosum, ENL) involve immune complex deposition and vasculitis, the chronic progressive nerve damage in stable lepromatous leprosy is due to direct bacterial invasion. [cite:Park 26e Ch 33]