Inclusion body myositis: a chronic persistent mumps myositis?

Among the generalized chronic idiopathic inflammatory myopathies, inclusion body myositis (IBM) has emerged as a clinico-pathologic variant during the past two decades. It occurs primarily in elderly persons (in approximately the sixth decade of life), but young adults (in approximately the second decade of life) may also be affected. Slowly progressive weakness of distal as well as proximal muscle groups in IBM is usually not associated with skin rash, malignancy or collagen-vascular disease, and is refractory to treatment with steroids or other immunosuppressants. Exceptions to each of these general rules have been found. Muscle biopsy and electromyography may suggest a neurogenic process mixed with myopathic features. Rimmed vacuoles with basophilic granules in cryostat sections stained with hematoxylin-eosin are strongly suggestive of IBM if accompanied by the histopathologic triad of polymyositis. The presence of eosinophilic intranuclear or cytoplasmic inclusions in affected myofibers is further suggestive of IBM. The ultimate diagnosis, however, depends on ultrastructural demonstration of characteristic microtubular filaments resembling the nucleocapsids of the paramyxovirus group. Recent reports of immunostaining of the inclusions for mumps virus antigen strongly suggest a chronic persistent mumps virus infection as the cause of IBM. IBM is considered to be pathologically related to both distal myopathy (DM) and oculopharyngeal muscular dystrophy (OPMD).