Polymyositis

Polymyositis is a rare autoimmune, at times considered paraneoplastic, inflammatory condition characterised by proximal muscle weakness (myositis). It is considered a form of idiopathic inflammatory myopathy(IIM). The condition is closely related to dermatomyositis, and the term “polymyositis” is applied when the condition spares the skin.

It principally affects the skeletal system but can also affect other organ systems, such as the lungs (see polymyositis - lung involvement).

Epidemiology

There are several forms of the disease. The most common form affects those in their fourth decade. There is a recognised female predilection (~2F:M).

Clinical presentation

Polymyositis usually presents insidiously over a few months with symmetrical painless muscular weakness with no cutaneous manifestations (cf. dermatomyositis). Myalgia has been reported in less than one-third of the patients. 

Radiographic features

Muscle involvement is usually symmetric, but not necessarily evenly distributed.

MRI

MRI has been used for diagnosis, follow-up, and to identify the best site for muscle biopsy. 

• T2/STIR: affected regions during active disease have high signal intensity, reflecting muscle oedema

Other features, in a chronic phase, may include ³:

• fatty infiltration of muscles
• muscle calcification

Treatment and prognosis

This condition is usually progressive.

Differential diagnosis

On MRI there is a broad differential:

• differential diagnosis of skeletal muscle oedema on MRI

Some differentials to consider include:

• traumatic myopathy
  • usually, will not involve all the muscular compartments and will not present with a relatively symmetrical bilateral pattern
• inclusion body myositis 
  • usually involving patients over 50 years
• collagen vascular diseases (e.g. systemic lupus erythematosus, Sjogren syndrome, and rheumatoid arthritis)
  • has a more focal, sometimes nodular, myositis
• drug-induced myositis (e.g. statins, antiretroviral drugs used in HIV treatment)
  • reversible after discontinuation of the medication
• muscular dystrophy (e.g. Duchenne)
  • muscles evolve from an oedematous phase to become atrophic 
  • there is no associated skin and subcutaneous oedema