Inclusion body myositis presenting as treatment-resistant polymyositis.

Inclusion body myositis (IBM) has been viewed as a distinct and rare form of inflammatory myopathy. Previously reported findings from series of IBM patients have suggested that clinical and pathologic features are present which readily distinguish it from idiopathic polymyositis. We report 4 cases of IBM presenting clinically and pathologically as polymyositis, each of which was refractory to therapy. Our data suggest that IBM may be a more common and heterogeneous form of inflammatory myopathy than has been previously suggested. Furthermore, IBM may be clinically and electrophysiologically indistinguishable from polymyositis. Reasons for failing to recognize IBM by pathologic studies appear to include: the skip lesion nature of the pathologic findings, failure to examine tissues by electron microscopy, and a low level of suspicion or lack of recognition. Because of its insidious clinical course and its failure to respond to immunosuppressive therapy, IBM may be an important variant of treatment-resistant polymyositis.