This is a heterogeneous group of diseases of which an example would be nemaline myopathy. The myofilaments are stained green and the abnormal nemaline inclusions are stained deep red. The electron photomicrograph shows the ultrastructure appearance of the nemaline inclusions. They are characterized by inflammatory infiltrates of lymphocytes and macrophages. There is rhabdomyolysis and rhabdomyophagia in the acute phase with a raised serum creatine phosphokinase level.
Later stages show regenerating fibres with central nuclear chains and basophilic sarcoplasm.
There is usually a perifascicular atrophy present. The disease can be quite focal within a muscle and so sampling can be a problem. A normal biopsy does not exclude polymyositis. Treatment is with steroids such as prednisolone. This photomicrograph of a quadriceps femoris biopsy shows transversely sectioned skeletal muscle fibres with those in the centre surrounded and infiltrated by a chronic inflammatory cell infiltrate composed mostly of lymphocytes.