Intranuclear inclusions in a myopathy of late onset.

Filamentous intranuclear inclusions in a case of myopathy of late onset are described. The filaments were usually in parallel array although occasionally they were haphazardly arranged in bundles ranging up to 5 mu in length. The filaments were approximately 120 A in diameter and were separated by a space measuring 180-220 A. Possible explanations of their occurrence in the nucleoplasm are discussed, including possible origin from the cytoplasm as a result of invagination of nuclear membrane and intranuclear entrapment of myofibrils from the cytoplasm. Their appearance does not suggest any recognizable structure such as myofilaments or virus. It is possible that the filaments are protein newly synthesized as a direct result of virus infection with invasion of the nuclei of the muscle cell.