Although patients with polymyositis or dermatomyositis may respond to treatment with corticosteroids alone, in many cases a combination of steroids and an immunosuppressive agent is required to achieve satisfactory disease control and remission.1 However, about 20–30% of patients with polymyositis, and a similar proportion of cases of dermatomyositis and overlap syndromes, have persisting weakness and raised serum creatine kinase (CK) activity, or continue to deteriorate. In such cases it is necessary to review the clinical and muscle biopsy findings on which the diagnosis of an inflammatory myopathy had been based, and whether the treatment administered had been appropriate, and consider other treatment options. In addition, if they have not already been done, investigations should be performed to exclude an underlying malignancy and other contributory causes of muscle weakness and raised serum CK activity, such as thyroid and other endocrine conditions and drug-induced disorders.

WAS THE INITIAL DIAGNOSIS CORRECT?

The diagnosis of an inflammatory myopathy can only be made with certainty with a muscle biopsy. A number of conditions (table) may be misdiagnosed as polymyositis if a biopsy is not performed, if the biopsy findings are non-specific and inconclusive (for example, muscle fibre necrosis without inflammation), or if the biopsy workup has been incomplete, or the interpretation incorrect. These include painful muscle conditions such as fibromyalgia in which the biopsy is usually normal, hypothyroidism, and metabolic myopathies such as McArdle’s disease, lipid storage and mitochondrial myopathies which can present with acute or subacute serum CK elevation due to rhabdomyolysis and where the full battery of histochemical stains and biochemical studies are usually necessary to make the diagnosis. Other conditions in which the presence of mild inflammatory changes in the biopsy, in association with myofibre necrosis or dystrophic features, may lead to a mistaken diagnosis of a primary inflammatory myopathy include drug-induced myopathies (for example, due …