OBJECTIVE--To define the natural history of primary systemic amyloidosis when muscle involvement is prominent at presentation. METHODS--A retrospective review was carried out of all patients seen at the tertiary referral practice of the Mayo Clinic between 1 January 1960 and 31 December 1994. All patients with primary systemic amyloidosis and proof of amyloid deposits by muscle biopsy were included for analysis. No patients were lost to follow up. RESULTS--Twelve patients were identified with amyloidosis in muscle. Muscle involvement was the most prominent symptom in patients who had widespread visceral involvement, which included the heart, peripheral nerve, and tongue. Of the 12, three had skeletal muscle pseudohypertrophy. All patients had a demonstrable dysproteinaemia by the finding of free light chain in the serum or urine, a discrete monoclonal peak on serum or urine protein electrophoresis, or a monoclonal population of plasma cells in the bone marrow. Measurement of creatine kinase was not a useful test. Of eight patients treated with chemotherapy based on alkylating agents, three responded. The median survival for the entire group was 12 months. CONCLUSIONS--The finding of a monoclonal protein in a patient with muscle weakness is an important clue to the diagnosis of primary systemic amyloidosis. Most patients have visceral involvement outside the musculoskeletal system. A subset of patients seems to respond to systemic chemotherapy. The overall survival, however, remains poor, with most patients dying of cardiac failure. Immunoelectrophoresis of serum and urine should be a routine diagnostic test during the evaluation of myopathy of unknown cause.
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