Idiopathic Inflammatory Myopathies (polymyositis, dermatomyositis)

Question 1

Definition

Answer 1

Connective tissue diseases characterised by inflammation of muscles

Question 2

Aetiology

Answer 2

• Autoimmune basis

* Viral infection has been implicated in its pathogenesis

Question 3

Epidemiology

Answer 3

• Polymyositis presents between 30-60 yrs
• Dermatomyositis can occur at any age (peak onset: 5-10 (children) and 50 (adults))
• Both diseases are 2 x more common in FEMALES

Question 4

Presenting symptoms (polymyositis)

Answer 4

o Inflammatory myopathy with onset over weeks or months

o Steady progression of symptoms

o Diffuse weakness in proximal muscles
• Causing difficulty rising from a low chair, climbing steps, lifting objects and
combing hair
• Also fatigue, myalgia and muscle cramps

o Distal muscles are spared - so fine motor coordination tends to be preserved in the early stages

o Pharyngeal weakness –> dysphagia

o NO rash

Question 5

Presenting symptoms (dermatomyositis)

Answer 5

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

Question 6

Signs on physical examination (polymyositis)

Answer 6

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

Question 7

Signs on physical examination (dermatomyositis)

Answer 7

o RASH characteristics
• Blue-purple discolouration of the upper eyelids with periorbital oedema
• Flat red rash involving the face and upper trunk
• Raised purple-red scaly patches over the extensor surfaces of joints and
fingers

o Rash may affect knees, shoulders, back and upper arms
o Rash may be exacerbated by sunlight
o Proximal myopathy
o Muscle pain and tenderness in early disease

Question 8

Investigations (polymyositis)

Answer 8

o Creatine kinase - up to 50 x higher than normal

o Electromyography (EMG)

o Muscle biopsy - DEFINITIVE TEST

o Autoantibodies (e.g. myositis specific antibody, anti-Jo-1 antibody)

o Enzymes (e.g. SGOT, SGPT, LDH)

Question 9

Investigations (dermatomyositis)

Answer 9

o Creatine kinase - not as reliable as in polymyositis

o Enzymes (SGOT, SGPT and LDH may be raised)

o Autoantibodies
• ANA
• Anti-Mi-2
• Anti-Jo-1 (more common in polymyositis)

o EMG - may be helpful but can be normal as well

o Muscle biopsy