Acute viral polymyositis with predominant diaphragm involvement.

A 20-month-old mentally retarded girl developed an acute upper respiratory infection, followed by breathing difficulties, leading to death. Picorna virus-like particles were demonstrated in a mildly affected quadriceps femoris muscle biopsy, while the necropsy findings demonstrated an acute polymyositis with predominant diaphragm involvement. The mental retardation was due to micropolygyria of the brain. Initial respiratory difficulties are an unknown feature of polymyositis. In this case the upper respiratory infection, possibly caused by a Coxsackie B2, is evoked as responsible for this unusual distribution of the acute viral polymyositis.