Diagnosis Polymyositis (PM) and dermatomyositis (DM)

These conditions are relatively rare and diagnosis is usually made by a specialist, such as a neurologist or a rheumatologist.

For a more detailed overview of different diagnostic and genetic tests, please see our diagnostic tests factsheet.

Two important tests that can help to diagnose myositis are an electromyogram (EMG) and a muscle biopsy. The EMG, which is done in a hospital, involves a small needle being inserted into several different limb muscles. The electrical activity recorded will show whether muscle weakness is caused by a muscle disorder (as would be expected in myositis) or by the nerves supplying the muscles (suggesting an alternative diagnosis).

A muscle biopsy is a procedure in which a small sample of muscle – about the size of a pea – is taken and examined under a microscope. It is considered a ‘minor’ procedure, and is usually done under a local anaesthetic. Please see our muscle biopsies factsheet for more information.

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