Prognosis of polymyositis and dermatomyositis

Excerpt

There is limited information on the long-term outcome of patients with inflammatory myopathy; Airio et al. reported that the 5-year survival rates ranged from 63 to 75% [1]. In this large series from Finland, 176 of 248 patients (71%) had polymyositis. This is a high percentage compared with similar series that reported consecutive mixed dermatomyositis and polymyositis cases, where the corresponding proportion is usually less than half [2‐6]. In one detailed clinical and histological review of patients with myositis and proximal myopathy, van der Meulen showed the incidence of idiopathic polymyositis to be low, at 2% (4/160 patients) [6]. Excluding those cases that are associated with connective tissue disorders and malignancy, it is increasingly recognised that idiopathic polymyositis is an overdiagnosed condition [6‐8]. The traditional diagnostic criteria from Bohan and Peters used by the authors, although widely cited, do not distinguish this from other myopathies such as dermatomyositis sine dermatitis, sporadic or hereditary inclusion body myositis (IBM). Other clinical mimics include limb-girdle and fascioscapulohumeral dystrophies, which may be erroneously diagnosed as polymyositis [7]. In the context of patients with proximal weakness, auxillary investigations are not specific: for example, serum creatine phosphokinase may be elevated in a number of other disorders, such as dysferlinopathy and muscular dystophies, while myositis-specific antibodies are not that specific. On the electromyogram, myotonic discharges may be found in proximal myotonic dystrophy. Biopsy abnormalities can be non-specific and may not reliably distinguish polymyositis from dermatomyositis or from certain muscular dystrophies. Even with routine muscle biopsies, IBM can be missed, as the rimmed vacuoles and inclusions that support the diagnosis may be absent. In light of this recent information, the retrospective nature of this series (going back to 1969), the large percentage of polymyositis and the scarcity of IBM cases raise the possibility of misclassification. More rigorous diagnostic standards including detailed immunopathology have been proposed [9]. A prospective evaluation using these new criteria may shed more light on the contributions of the different types of myopathies and their response to treatment. …