Polymyositis is a rare autoimmune, at times considered paraneoplastic, inflammatory connective tissue disease characterised by proximal muscle weakness (myositis). It has historically been considered a form of idiopathic inflammatory myopathy. The condition is closely related to dermatomyositis, and the term “polymyositis” is often applied clinically 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).

Terminology

Polymyositis is an increasingly rare diagnosis. With advances in medical science, many patients previously diagnosed with polymyositis are being reclassified as having other inflammatory myopathies, such as immune-mediated necrotising myopathy, inclusion body myositis, or anti-synthetase syndrome ⁵. There is conjecture among experts in the literature as to whether polymyositis still exists as a clinical entity, but if it does, it is likely very rare ⁵.

Epidemiology

There are several forms of the disease. The most common form affects those in their fourth decade. There is a recognised female predilection (F:M = ~2:1).

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 the chronic phase, may include ³:

• fatty infiltration of muscles

• muscle calcification

Treatment and prognosis

Management is with immunosuppression, similar to dermatomyositis.

Differential diagnosis

On MRI there is a broad differential:

• differential diagnosis of skeletal muscle oedema on MRI

Some differentials to consider include:

• inclusion body myositis 

  • usually involving patients over 50 years

• dermatomyositis

  • will have skin features

• anti-synthetase syndrome

  • will have skin features

• traumatic myopathy

  • usually, will not involve all the muscular compartments and will not present with a relatively symmetrical bilateral pattern

• collagen vascular diseases (e.g. systemic lupus erythematosus, Sjögren syndrome, and rheumatoid arthritis)

  • has a more focal, sometimes nodular, myositis

• drug-induced myositis (e.g. statins, antiretroviral drugs (e.g. d-drugs) used in HIV treatment, checkpoint inhibitors)

  • 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