Cardiac amyloid deposits in endomyocardial biopsies. Light microscopic, ultrastructural, and immunohistochemical studies.

In four patients with unexplained, abnormal thickening of the interventricular septum as demonstrated by echocardiography, right ventricular endomyocardial biopsy revealed unexpected cardiac amyloid deposits that resulted in increased myocardial thickness and rapidly progressive heart failure. Light microscopically, amyloid was observed in the subendocardial layer, interstitium, and walls of the intramural arterioles. Electron-microscopically, the amyloid fibrils were adjacent to the basement membranes of the heart muscle cells and the vascular smooth muscle cells. Immunohistochemical typing with specific antibodies against different amyloid fibril proteins on glutaraldehyde-fixed paraffin sections revealed different amyloid types. In two patients with generalized idiopathic amyloidosis and in two others with amyloidosis in multiple myeloma, the A-lambda form was diagnosed. In a fifth patient, AA-amyloidosis was found in familial Mediterranean fever with cardiac manifestation without thickening of the interventricular septum. The amyloid deposits were located almost exclusively within the walls of the myocardial arterioles. The amount of amyloid as observed in the myocardial biopsies correlates with the rapidly progressive cardiac failure. It is suggested that in patients with abnormal thickening of the interventricular septum of unknown origin the diagnosis should be clarified by endomyocardial biopsy.