Meeting the challenges in the diagnosis of inflammatory myopathies.

Inflammatory myopathy (IM) is a rubric term to describe a heterogeneous group of muscle diseases typified by dermatomyositis and polymyositis. The current classifications are unsatisfactory, but IM associated with other connective tissue diseases (CTDs), such as systemic lupus erythematosus, underlying malignancy and HIV, should also be included. Although uncommon, IM should always be considered in a patient who presents with proximal weakness of gradual onset and has raised serum muscle enzymes. The diagnosis may be obvious if the patient has diagnostic skin signs such as heliotropic rash (peri-orbital discoloration) and Gottron’s lesions (typically on the extensor surfaces of the fingers). In the absence of obvious skin manifestations, other features of a CTD such as Raynaud’s phenomenon, abnormal capilloroscopy and the presence of serum antinuclear factor antibody should be searched for. Conditions that mimic IM include other causes of myopathy such as endocrine disorders, adverse effects of medication, metabolic myopathies and muscle dystrophies. Atypical features suggesting an alternative diagnosis are acute onset, severe pain, asymmetrical involvement, distal weakness and wasting. Appropriate investigations include a chest radiograph indicating interstitial lung disease or malignancy. Electromyography and muscle biopsy are useful in cases where other diagnoses are suspected.