Myositis

Myositis is a heterogenous group of systemic immune mediated disorders.

Classification

The original Bohan and Peter classification which includes 5 groups from 1975 has been the most widely used classification criteria. The criteria are sensitive but not specific for diagnosing myositis.Over the years there has been increasing recognition of involvement of other organ systems such as interstitial lung disease. In 2017 EULAR-ACR developed a new classification criteria. These new criteria are more specific.

There are four main subtypes of myositis: dermatomyositis, polymyositis, immune-mediated necrotising myopathy, and inclusion body myositis. The polymyositis category is controversial as to whether it actually exists as many of these patients evolve to develop more typical features of another subtype or actually have a genetic neuromuscular disease such as facioscapulohumeral dystrophy or limb girdle muscular dystrophy.

The clinical subclassification schema is limited by overlapping clinical and histopathological features. Subgrouping by myositis-specific autoantibodies is a newer classification method to overcome this. These autoantibodies are very specific and can help diagnose myositis. The autoantibodies are associated with distinct clinical features, including arthritis, lung disease, and skin manifestations.

Dermatomyoisitis

This is the easiest subtype to diagnose due to the skin manifestations. Gottron's papules are found over the dorsal aspects of the MCPJs and PIPJs and even over the DIPJs. These can be itchy. The patient may have a very photosensitive rash, very similar to lupus. The characteristic areas are the V neck area, proximal arms, and proximal limbs.