Leprosy--histopathologic aspects of nerve involvement.

The most striking single feature of the clinical manifestations of leprosy is the very wide range of appearances shown by the skin lesions. These include the vague, hypopigmented macules of indeterminate leprosy; the large, sharply defined hypopigmented anaesthetic lesions of tuberculoid leprosy; the nodules and diffuse infiltration of lepromatous leprosy; and a wide range of plaques and annular lesions of the intermediate (borderline or dimorphous) types of disease. From superficial appearances it would be impossible to say that these were manifestations of the same infection. Moreover, histologically there is a similar wide range of appearances, including minimal lymphocyte infiltration around the neurovascular bundles in indeterminate leprosy; epithelioid granulomata with Langhans' giant cells and virtually no bacilli to be seen in tuberculoid cases; and in lepromatous leprosy, histiocytic infiltration occupying the whole thickness of the dermis, with massive parasitisation by Mycobacterium leprae, of which there may be up to 10-9 per gram of tissue. Strain differences have been recognised, but appear not to be responsible for this remarkable range of manifestations; they are caused by variations in the soil, not in the seed. Leprosy is almost unique among infectious diseases in the importance that host factors play in its development. The different clinical appearances shown by different types of leprosy can be graded into a continuous series, forming a spectrum from the single or scanty lesions of tuberculoid leprosy to the total body involvement o lepromatous; and the patient with leprosy can indeed truly be said to wear his skin lesions like medals displaying his capacity to resist Myco. leprate. The histopathologic features of the developed disease have a similar spectrum, from an epithelioid granuloma to infiltration with histiocytes that are full of bacilli and are ultimately converted to foam cells (Virchow cells)...