Idiopathic inflammatory myopathies

Question 1

What are idiopathic inflammatory myopathies?

Answer 1

Idiopathic inflammatory myopathies constitute a heterogeneous group of sub-acute, chronic, and, rarely, acute diseases of skeletal muscle that have in common the presence of moderate-to-severe proximal muscle weakness and inflammation on muscle biopsy.

Question 2

What are the 3 groups?

Answer 2

Based on distinct features, they are divided into 3 groups: polymyositis, dermatomyositis, and inclusion body myositis. Polymyositis and dermatomyositis seem to have an autoimmune pathogenesis, whereas inclusion body myositis seems to have an autoimmune and degenerative pathogenesis

Question 3

What are the risk factors?

Answer 3

children and age >40 years
exposure to high intensity of global UV radiation
genetic predisposition
female sex and/or black ethnicity (polymyositis and dermatomyositis)
male sex and/or white ethnicity (inclusion body myositis)

Question 4

What are the signs and symptoms?

Answer 4

> Difficulty with motor tasks
> Muscle weakness
> Muscle atrophy
> Frequent falls
> Fatigue and generalised malaise
> Weight loss
> SOB
> Mild fever
> Abnormal breath sounds

Question 5

What investigations would you do?

Answer 5

Bloods (all elevated)
> CK
> Aldolase
> LDH
> Alanine transaminases
> Myoglobin

Muscle biopsy (Polymyositis: endomysial inflammatory infiltrates, muscle necrosis, atrophy, muscle fibre regeneration 
Dermatomyositis: perifascicular atrophy, perivascular/perimysial inflammation 
Inclusion body myositis: endomysial inflammatory infiltrate, fibre size variability, fibre necrosis, rimmed vacuoles)

EMG
short duration, low amplitude, polyphasic units with early recruitment on voluntary activity; diffuse spontaneous activity with fibrillation and positive sharp waves at rest