Tuberculoid Leprosy

At the less severe end of the spectrum is tuberculoid leprosy, which encompasses TT and BT disease. In general, these forms of leprosy result in symptoms confined to the skin and peripheral nerves. The skin lesions of tuberculoid leprosy consist of one or a few hypopigmented macules or plaques that are sharply demarcated and hypesthetic, often have erythematous or raised borders, and are devoid of the normal skin organs (sweat glands and hair follicles) and thus are dry, scaly, and anhidrotic. AFB are generally absent or few in number. Tuberculoid leprosy patients may have asymmetric enlargement of one or a few peripheral nerves. Indeed, leprosy and certain rare hereditary neuropathies are the only human diseases associated with peripheral-nerve enlargement. Although any peripheral nerve may be enlarged (including small digital and supraclavicular nerves), those most commonly affected are the ulnar, posterior auricular, peroneal, and posterior tibial nerves, with associated hypesthesia and myopathy. TT leprosy is the most common form of the disease encountered in India and Africa but is virtually absent in Southeast Asia, where BT leprosy is frequent.

In tuberculoid leprosy, T cells breach the perineurium, and destruction of Schwann cells and axons may be evident, resulting in fibrosis of the epineurium, replacement of the endoneurium with epithelial granulomas, and occasionally caseous necrosis. Such invasion and destruction of nerves in the dermis by T cells are pathognomonic for leprosy.

Circulating lymphocytes from patients with tuberculoid leprosy readily recognize M. leprae and its constituent proteins, and patients have positive lepromin skin tests (see “Diagnosis,” below). In tuberculoid leprosy tissue, there is a 2:1 predominance of helper CD4+ over CD8+T lymphocytes. Tuberculoid tissues are rich in the mRNAs of the proinflammatory TH1 family of cytokines: interleukin (IL) 2, interferon γ (IFN- γ), and IL-12; in contrast, IL-4, IL-5, and IL-10 mRNAs are scarce.

Source: Harrison_s_Principles_of_Internal_Medicine_16th_Edition