Necrotizing myopathy with microvascular deposition of the complement membrane attack complex.

Necrotizing myopathy without prominent inflammatory changes is increasingly being recognized as a form of inflammatory myopathy, different from dermatomyositis, polymyositis and sporadic inclusion-body myositis. We report a patient with a chronic steroid-responsive myopathy and an ill-defined overlap syndrome. The muscle biopsy showed thickened capillaries and arterioles and deposition of the membrane attack complex in the replicated mural elements of some vessels. The surface of all muscle fibers showed major histocompatibility class I immunoreactivity. Similar patients have rarely been reported, either suffering from an undifferentiated connective tissue disorder or a carcinoma. The link between the muscle fiber necrosis and the microangiopathy is unclear. Absence of prominent inflammatory changes in a diagnostic muscle biopsy does not exclude the diagnosis of a treatable autoimmune inflammatory myopathy.