What is Polymyositis and Dermatomyositis

Question 1

What is it

Answer 1

Polymyositis and dermatomyositis are autoimmune disorders where there is inflammation in the muscles (myositis)

Question 2

Polymyositis

Answer 2

condition of chronic inflammation of muscles.

Question 3

Dermatomyositis

Answer 3

connective tissue disorder where there is chronic inflammation of the skin and muscles.

Question 4

Key investigation for diagnosing myositis

Answer 4

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

Question 5

Creatine kinase levels

Answer 5

usually less than 300 U/L. In polymyositis and dermatomyositis, the result is usually over 1000, often in the multiples of thousands.

Question 6

Other causes of creatine kinase raised

Answer 6

Rhabdomyolysis
Acute kidney injury
Myocardial infarction
Statins
Strenuous exercise

Question 7

What can these diseases be caused by?

Answer 7

Underlying malignancy
makes them paraneoplastic syndromes. The most common associated cancers are:

Lung
Breast
Ovarian
Gastric

Question 8

Presentation

Answer 8

• Muscle pain, fatigue and weakness
• Occurs bilaterally and typically affects the proximal muscles
• Mostly affects the shoulder and pelvic girdle
• Develops over weeks

Question 9

Dermatomyositis Skin Features

Answer 9

• 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 10

Autoantibodies

Answer 10

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

Question 11

Diagnosis based on

Answer 11

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

Question 12

Management

Answer 12

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)