Polymyositis and dermatomyositis are autoimmune disorders where there is inflammation in the muscles (myositis). Polymyositis is a condition of chronic inflammation of muscles. Dermatomyositis is a connective tissue disorder where there is chronic inflammation of the skin and muscles.
The key investigation for diagnosing myositis is a creatine kinase blood test. Creatine kinase is an enzyme found inside muscle cells. Inflammation in the muscle cells (myositis) leads to the release of creatine kinase. Creatine kinase is usually less than 300 U/L. In polymyositis and dermatomyositis, the result is usually over 1000, often in the multiples of thousands.
Other causes of a raised creatine kinase include:
- Acute kidney injury
- Myocardial infarction
- Strenuous exercise
Polymyositis or dermatomyositis can be caused by an underlying malignancy. This makes them paraneoplastic syndromes. The most common associated cancers are:
- Muscle pain, fatigue and weakness
- Occurs bilaterally and typically affects the proximal muscles
- Mostly affects the shoulder and pelvic girdle
- Develops over weeks
Polymyositis occurs without any skin features whereas dermatomyositis is associated with involvement of the skin.
Dermatomyositis Skin Features
- Gottron lesions (scaly erythematous patches) on the knuckles, elbows and knees
- Photosensitive erythematous rash on the back, shoulders and neck
- Purple rash on the face and eyelids
- Periorbital oedema (swelling around the eyes)
- Subcutaneous calcinosis (calcium deposits in the subcutaneous tissue)
- Anti-Jo-1 antibodies: polymyositis (but often present in dermatomyositis)
- Anti-Mi-2 antibodies: dermatomyositis.
- Anti-nuclear antibodies: dermatomyositis.
Diagnosis is based on:
- Clinical presentation
- Elevated creatine kinase
- Electromyography (EMG)
Muscle biopsy can be used to establish a definitive diagnosis.
Management is guided by a rheumatologist. New cases should be assessed for possible underling cancer. They may require physiotherapy and occupational therapy to help with muscle strength and function.
Corticosteroids are the first line treatment of both conditions.
Other medical options where the response to steroids is inadequate:
- Immunosuppressants (such as azathioprine)
- IV immunoglobulins
- Biological therapy (such as infliximab or etanercept)
Last updated April 2019