Is it really myositis? A consideration of the differential diagnosis.

PURPOSE OF REVIEW

The idiopathic inflammatory myopathies are an important and treatable group of disorders. However, the potential toxicity associated with the immune therapeutic regimens used to treat these disorders may be significant; therefore, accurate diagnosis before such treatment is essential. The differential diagnosis is potentially large. Accurate diagnosis usually depends on a combination of careful clinical assessment in conjunction with detailed laboratory investigations. Muscle biopsy remains essential in achieving an accurate diagnosis that will then guide treatment. This review describes the diagnostic approach used.

RECENT FINDINGS

There has been debate over the requirements for an accurate diagnosis of inflammatory myopathy (i.e., polymyositis and dermatomyositis). It is increasingly recognized that there can be clinical and muscle histopathologic overlap between the features of inflammatory myopathies and those of other muscle disorders, in particular, the genetic muscular dystrophies. Pathologic findings of inflammation and major histocompatibility complex upregulation, although typical of inflammatory myopathies, have been shown to occur in some muscular dystrophies, complicating the diagnostic process. Inclusion body myositis is much less responsive to immunotherapy and is now recognized as the most common acquired muscle disease in those older than 50 years of age. It is likely that genetic muscular dystrophies and inclusion body myositis account for some cases of apparently "treatment-resistant" myositis.

SUMMARY

A thorough clinical assessment, including a detailed family history, complemented by electromyography and creatine kinase measurements, should be undertaken in any patient with presumed idiopathic inflammatory myopathy. In addition, a muscle biopsy remains essential in all cases. A precise tissue diagnosis confirming features of an active inflammatory process should be achieved before immunosuppressive treatment is commenced. An increasing array of immunocytochemical and histioenzymatic stains now allows a full analysis and will help to confirm or exclude virtually all the differential diagnostic possibilities considered in this review. Electron microscopy may also be valuable in selected cases. Close collaboration between clinicians and muscle pathologists is essential in allowing the most accurate interpretation of myopathologic findings in the clinical context.