Borderline Leprosy

Between the two extremes of tuberculoid and lepromatous leprosy, the "polar" forms of leprosy, there is the complete spectrum reflecting the balance between bacillary multiplication and cell-mediated immunity for each patient. Histologically, differentiation can be made accurately and is of value in judging the prognosis. Tuberculoid leprosy is associated with severe nerve damage; lepromatous leprosy is progressive with long-term complications. The tuberculoid variety may heal spontaneously, but this never occurs with the lepromatous pattern. Patients in the borderline state have an unstable immune response and may change the pattern of their disease in either direction. Such reactions, which are often acute, may cause severe damage, particularly to nerves, and may be fatal.

The Lepra Reaction

The "lepra reaction" is the term given to a violent but often ineffective tissue response presenting as an acute deterioration in the clinical lesions of the patient undergoing treatment for leprosy. It is important that these reactions be treated promptly, because eye and nerve damage may be rapid and extensive. They probably represent acute hypersensitivity to leprosy antigens. There are two varieties.

The type I reaction is cellular hypersensitivity and occurs in patients with the borderline or mixed pattern of disease with an unstable immunological status. The change may be in either direction, yet the clinical results may be indistinguishable. Type II is humoral hypersensitivity, due to an antigen-antibody reaction with the formation of immune complexes in various tissues and without a change in immunological status. This type of reaction occurs in lepromatous leprosy.

In the type I reaction, skin lesions become swollen and edematous, the disease progresses in the eye and in the nose, and the nerves become rapidly swollen, extremely painful, and tender. Immediate treatment is required to avoid permanent damage. Despite this, systemic illness is minimal. The histological findings are those of edema, increased lymphocytic infiltration, and necrosis. The process is best considered one of "acute inflammation" but the cause is not fully understood.

In the type II reactions new lesions predominate, particularly in the skin, where there are painful red nodules which may be superficial or deep. The lesions last for a few days, disappear, and are succeeded by others. The joints may be painful; orchitis, dactylitis, and enlargement of all the peripheral nerves and lymphatics may occur. These reactions are not of such serious significance as the type I reactions but still require vigorous anti-inflammatory treatment. In the type II reaction, the systemic illness may be considerable. Histologically there is vasculitis or vascular necrosis and hemorrhage, edema, and cellular disintegration. There may be marked cellular infiltration, particularly through the dermis. Because this is an antibody-antigen reaction, with circulating immune complexes, there may be not only eruption of skin nodules but proteinuria, arthralgia, neuritis, and even serum sickness. The type II reaction is also known as "erythema nodosum leprosum" (ENL).

Both clinically and histopathologically it may be difficult to decide whether the patient is relapsing or undergoing a reversal reaction. In the type I reversal reaction, the process may be self-healing, but the neuritis may cause severe sensory loss and it is at this stage the foot drop, claw hand, and other deformities become important to the radiologist. In the type II (erythema nodosum) reaction it is the iridocyclitis, the synovitis, and the rapidly developing skin changes which are the obvious problems, but glomerulonephritis and secondary amyloidosis are late sequelae.