Polymyositis and Dermatomyositis

Question 1

What is poly vs. dermatomyositis?

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

Clinical Features of polymyositis and dermatomyositis

Answer 2

Muscle pain, fatigue and weakness

Occurs bilaterally and typically affects the proximal muscles

Mostly affects the shoulder and pelvic girdle

Develops over weeks

Question 3

Other causes of raised creatine kinase (that isn’t poly or dermatomyositis)

Answer 3

Other causes of a raised creatine kinase include:

Rhabdomyolysis
Acute kidney injury
Myocardial infarction
Statins
Strenuous exercise

Question 4

Diagnosis

Answer 4

Diagnosis is based on:

• Clinical presentation
• Elevated creatine kinase
  (Creatine kinase is usually less than 300 U/L. In polymyositis and dermatomyositis the result is usually over 1000)
• Autoantibodies
  (Anti-Jo-1 antibodies: polymyositis (but often present in dermatomyositis)
  Anti-Mi-2 antibodies: dermatomyositis.
  Anti-nuclear antibodies: dermatomyositis)
• Electromyography (EMG)
• Muscle biopsy can be used to establish a definitive diagnosis

Question 5

Clinical Features of dermatomyositis

Answer 5

Gottron lesions (scary erythematous patches) on the knuckles, elbows and knees

Photosensitive erythematous rash on the back, shoulders and neck

Purple rash on the face and eyelids: heliotropic

Periorbital oedema (swelling around the eyes)

Subcutaneous calcinosis (calcium deposits in the subcutaneous tissue)

Question 6

Management of poly and dermatomyositis

Answer 6

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)

Question 7

Differential

Answer 7

Polymyalgia Rheumatica: presents with proximal STIFFNESS and pain sometimes but NOT WEAKNESS

Whilst dermato and polymyositis both cause weakness

Question 8

Associations

Answer 8

Dermatomyositis is highly associated with malignancy (particularly lung cancer)