Idiopathic inflammatory myopathies (incl. polymyositis, dermatomyositis)

Question 1

Define Idiopathic inflammatory myopathies

Answer 1

Heterogenous group of sub-acute, chronic, and rarely, acute diseases of skeletal muscles 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 of Idiopathic inflammatory myopathies

Answer 2

1. Polymyositis (autoimmune pathogenesis) - muscle involvement
2. Dermatomyositis (autoimmune pathogenesis) - muscle and skin involvement
3. Inclusion body myositis (autoimmune and degenerative pathogenesis)

Question 3

Aetiology and risk factors of Idiopathic inflammatory myopathies

Answer 3

Unknown

Suggested viral, HLA alleles, UV radiation, hydroxyurea, autoimmune

RF:
Children and age >40
Exposure to high intensity of global UV radiation
Genetic predisposition
Female, black ethnicity (polymyositis + dermatomyositis)
Male, white ethnicity (inclusion body myositis)

Question 4

Symptoms of Idiopathic inflammatory myopathies

Answer 4

Difficulty with motor tasks (getting up from chair, climbing steps, lifting objects, combing hair)
Muscle weakness (weakness in upper extremities with no pain)
Frequent falls
Weight loss
Fatigue and generalised malaise
SOB
Mild fever
Dysphagia, myalgia, arthralgia, palpitaitons, syncope, MI symptoms, facial rash, nail fold changes

Question 5

Signs of Idiopathic inflammatory myopathies of examination

Answer 5

Heliotrope rash with eyelid oedema: Blue-purple discoloration of the upper eyelids, with oedema | Highly suggestive of dermatomyositis

Gottrons’ papules: Erythema over the knuckles, accompanied by raised violaceous scalp eruption (dermatomyositis)

Muscle weakness: Dermatomyositis - Weakness in the upper extremities but NO pain | symmetrical peri-ungual erythema and erythema on joints | Grotton’s papules.

Muscle atrophy: Inclusion body myositis, quadriceps and distal wrist and finger flexor atrophy is common.

Shawl sign (apparent sunburn in the distribution of a shawl)

Question 6

Investigations for Idiopathic inflammatory myopathies

Answer 6

CK: Elevated (esp. polymyositis)
Muscle biopsy:
- endomysial inflammatory infiltrates, muscle necrosis, atrophy
- 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 + early recruitment

Adolase: elevated 
LDHL elevated
ALT: elevated
Myoglobin: elevated
ESR: elevated (1/2)
ANA: variable depending on cause
Myositis-specific antibodies