Systemic amyloid myopathy--light-microscopic and fine-structural study of the skeletal muscles with histochemical and immunohistochemical study of amyloid.

Studies were made on skeletal muscles obtained by biopsy and autopsy from a 59-year-old man with immunocyte dyscrasia and amyloidosis; he had progressive muscular stiffness with palpable tumors and chi-type Bence-Jones protein, but no overt myeloma. Histologically, much amyloid was seen deposited preferentially in the skeletal muscles, especially in the endo- and perimysia, forming tumors of various sizes and occasionally associating influx of amyloid into some muscle fibers, and in the smaller-sized blood vessel walls, frequently associated with extreme narrowing or occlusion of their lumina. Most muscle fibers were replaced by much amyloid or showed extensive atrophy, having a characteristic circular space under the endomysium, which was concluded to be not due to compression atrophy but to ischemic atrophy. Ischemic neuropathy was observed and seemed to be responsible for the diffuse atrophy without spaces seen exceptionally in some parts. Amyloid was shown histochemically to contain AL as the major protein component, immunohistochemically to have a close relation with chi-type light chain, and fine-structurally to be formed just outside the basement membrane of the blood vessels and deposited in the endo- and perimysia through perivascular infiltration. The clinicopathologic features of nine similar autopsy cases in the literature were compared with those in the present case. The causes of damage in muscle fibers and of preferential deposition of amyloid in the skeletal muscles were discussed.