Myositis and malignancy

In 1975, Bohan and Peter proposed a diagnostic classification system for polymyositis (PM) and dermatomyositis (DM), a rare group of idiopathic inflammatory myopathies. They felt the presence of proximal muscle weakness, typical dermatologic manifestations, laboratory markers of muscle damage (high CK), EMG data and features on muscle biopsy would confirm a diagnosis. Today, similar clinical data is used for the diagnosis with the addition of auto-antibodies testing, however these diseases are recognized as having an increased amount of heterogeneity. In 2004, a list of categories including possible and definite DM/PM was created, also recognizing inculsion body myositis and dermatomyositis with a lack of muscle involvement. A simplified breakdown is considering if the patients presentation is highly characteristic presentation, non-specific but typical, atypical or those with skin manifestations without myositis.

An association between malignancy and myositis has been recognised since the early 20th century. It wasnt until large retrospective cohort analyses were performed that this association was confirmed. Approximately 25% of patients with DM have been found to have underlying malignancies, most of which are identified within two years of presentation. PM has a lower risk than DM, but increased from the general population.There are no guidelines on how the screening process for malignancy should take place, just that a focused assessment be performed and age appropriate cancer screening initiated. Additional risk factors for malignancy in myositis include increased age, males and severe cutaneous manifestations. The most common malignancy associated with myositis are adenocarcinomas (~70%), though there are race specific differences. Patients of south-east Asian descent with myositis were most likely to have nasopharyngeal carcinoma as the underlying cancer (~20%). Western studies have identified, ovary, breast and lung amongst the most common malignancies. Features consistent with the anti-synthetase syndrome, including fever, interstiital lung disease, Reynaud's phenomenon, arthritis and anti-Jo positivity are less associated with underlying cancer. The pathogenesis of inflammatory myopathies with underlying cancer is unclear. Case reports have suggested immune mimicry, where cell mediated response against the tumour results in cross reaction with muscle antigens. There have been conflicting results on the response of myopathy following the treatment of the underlying tumour, which may be confounded by the burden of disease at the time of diagnosis.

The main thing is to recall an association between these clinical entities, remembering to perform a focused search for malignancy in patients with inflammatory myopathy.