Polymyositis & dermatomyositis (idiopathic inflammatory myopathies)

Question 1

Define polymyositis & dermatomyositis

Answer 1

Connective tissue diseases characterised by inflammation of muscles

Question 2

Aetiology of polymyositis & dermatomyositis

2

Answer 2

Autoimmune basis

Viral infection has been implicated in pathogenesis

Question 3

Epidemiology of polymyositis & dermatomyositis

age x2, gender

Answer 3

Polymyositis presents between 30-60 yrs

Dermatomyositis can occur at any age (peak onset: 5-10 yrs & 50 yrs)

Both diseases 2x more common in FEMALES

Question 4

Presenting symptoms of polymyositis

6

Answer 4

Inflammatory myopathy w/ onset over weeks or months
Steady progression of symptoms
Diffuse weakness in proximal muscles
- difficulty rising from low chair, climbing steps, lifting objects & combing hair
- fatigue, myalgia & muscle cramps
Distal muscles spared - fine motor coordination preserved in early stages
Pharyngeal weakness —> dysphagia
NO rash

Question 5

Presenting symptoms of dermatomyositis

7

Answer 5

Inflammatory myopathy w/ onset over weeks or months
RASH
Systemic upset w/ fever, arthralgia, malaise & weight loss
Possible cardiac disease (e.g. conduction blocks, tachyarrhythmias)
GI ulcers & infections
Interstitial lung disease (30-50%)
Children have more non-muscular features

Question 6

Signs of polymyositis on physical examination

7

Answer 6

Muscle weakness
NOT painful in most patients
Proximal myopathy
Extraocular muscles & distal muscles spared
Weak forced flexion of neck
Muscular atrophy
Muscles may be tender on palpation

Question 7

Signs of dermatomyositis on physical examination

5

Answer 7

RASH characteristics
- blue purple discolouration of upper eyelids w/ periorbital oedema
- flat red rash involving face & upper trunk
- raised purple red scaly patches over extensor surfaces of joints & fingers
Rash may affect knees, shoulder, back & upper arms
Rash may be exacerbated by sunlight
Proximal myopathy
Muscle pain & tenderness in early disease

Question 8

Investigations for polymyositis

5

Answer 8

Creatine kinase - up to 50x higher than normal
Electromyography (EMG)
Muscle biopsy - DEFINITIVE TEST
Autoantibodies (e.g. myositis specific antibody, anti-Jo-1 antibody)
Enzymes (e.g. SGOT, SGPT, LDH)

Question 9

Investigations for dermatomyositis

5

Answer 9

Creatine kinase - not as reliable as in polymyositis
Enzymes (SGOT, SGPT, LDH may be raised)
Autoantibodies
- ANA
- anti-Mi-2
- Anti-Jo-1 (more common in polymyositis) 
EMG - may be useful but can be normal
Muscle biopsy