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. The infiltrate in this example is a fairly intense and many cases are more subtle then this. Immunohistochemical staining using anti-lymphoid antibodies can be very useful in borderline cases. A number of specific metabolic abnormalities can cause muscle disorders. Myopathies have also been described in association with several endocrine disorders, notably thyroid, parathyroid and pituitary disorders.