Polymyositis

Question 1

pathogenesis

Answer 1

proposed - T cell mediated cytotoxic process directed against unidentified muscle antigens
- CD8 T cells, macrophages surround healthy nonnecrotic muscle fibres

Question 2

what is it?

Answer 2

idiopathic inflammatory myopathy

symmetrical, proximal muscle weakness

Question 3

how does it present?

Answer 3

symmetrical, proximal muscle weakness in the upper and lower limbs
insidious onset
difficulty with particular activities - e.g. climbing stairs
some patients have myalgia
dysphagia in 1/3 patients
- secondary to oropharyngeal and oesophageal involvement
- poor prognostic sign
ILD in 5-30% of patients (especially Anti-Jo positive)
raised inflammatory markers

Question 4

Investigations

Answer 4

raised inflammatory markers
CK raised, often 10x normal
immunology
- ANA, anti-RNP: shared with other AI diseases
- Anti-Jo-I, anti-SRP: unique to myositis
MRI
- localise extent of the muscle involvement
- show signal intensity abnormalities of the muscle due to inflammation, oedema, scarring
EMG
- abnormal in 90% (staging)
Muscle biopsy - crucial
- excluding other rare muscle diseases
- muscle fibres in varying stages of inflammation, necrosis and regeneration

Question 5

how is it managed?

Answer 5

prednisolone - 40mg initially

combined with immunosuppressive drugs - methotrexate, azathioprine

Question 6

what is the prognosis?

Answer 6

usually responsive to treatment, albeit slowly
30% left with some residual weakness
older patients and late presentation - do less well