Polymyositis and dermatomyositis

Question 1

What is it?

Answer 1

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.

Question 2

What happens to CK?

Answer 2

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:

Rhabdomyolysis
Acute kidney injury
Myocardial infarction
Statins
Strenuous exercise

Question 3

Causes?

Answer 3

Polymyositis or dermatomyositis can be caused by an underlying malignancy. This makes them paraneoplastic syndromes. The most common associated cancers are:

Lung
Breast
Ovarian
Gastric

Anti-Jo-1 antibodies: polymyositis (but often present in dermatomyositis)
Anti-Mi-2 antibodies: dermatomyositis.
Anti-nuclear antibodies: dermatomyositis.

Question 4

Presentation?

Answer 4

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)

Question 5

Diagnosis?

Answer 5

Diagnosis is based on:

Clinical presentation
Elevated creatine kinase
Autoantibodies
Electromyography (EMG)
Muscle biopsy can be used to establish a definitive diagnosis.

Question 6

Management

Answer 6

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)