Amyloid myopathy: an underdiagnosed entity.

Amyloidosis can involve multiple organs, including kidney, heart, peripheral nerve, skin, joints, and skeletal muscle, but rarely presents as a myopathy. We studied 13 adults with muscle weakness for between 3 months and 4 years in whom the diagnosis of systemic amyloidosis was unsuspected before or until just before the time of the muscle biopsy. All muscle specimens demonstrated congophilic deposits around blood vessels and muscle fibers, some necrotic and regenerating fibers, and signs of mild denervation. Immunostains in 10 patients revealed immunoglobulin amyloidosis in 7 and gelsolin amyloidosis in 1. Apolipoprotein E co-localized with the congophilic deposits in all 10, and a C-terminal epitope of the beta-amyloid precursor protein was detected in 6. The frequency of the diagnosis of amyloid myopathy increased 10-fold when we adopted the fluorescent Congo red stain as a routine procedure in assessing muscle biopsy specimens.