Basophilic inclusions in skeletal muscle from two cases of hypothyroid myopathy

We have been able to examine muscular tissue from two females showing hypothyroid myopathy. Under light microscopy, certain fibres contained basophilic inclusions. Histoenzymology showed that these inclusions were found exclusively in certain type 1 fibres. Ultrastructural examination showed that the inclusions were accumulations without a limiting membrane. These accumulations contained straight fibrils which were randomly distributed. Their mean diameter was 70 A. The neighbouring myofibrils were usually unaltered. We have found similar images described for five cases of hypothyroid myopathies. They are more common in myocardium than in skeletal muscle. These images are similar to those observed in some case of type IV glycogenosis and Lafora's disease.