Polymyositis and dermatomyositis - Connective tissue disorders

Polymyositis and dermatomyositis

Definition

Myositis is an inflammatory disorder of striated muscle. When associated with skin manifestations it is termed dermatomyositis.

Prevalence

1 per 100,000 population.

Age

Any, peaks 40–50 years.

Sex

2F: 1M

Aetiology/pathophysiology

Lymphocytic infiltration, autoantibody production and HLA-DR3 association suggests an immune-mediated pathogenesis. There may be environmental trigger factors. Dermatomyositis is associated with malignancy (e.g. lung, stomach carcinoma) predominantly over the age of 40 years. The inflammation within muscle causes necrosis and patchy regeneration with fibrosis and atrophy in later stages. The skin shows collagenous thickening of the dermis with chronic inflammatory cell infiltrates.

Clinical features

Gradual onset of non-specific systemic features followed by symmetrical, progressive, proximal muscle weakness. In more chronic presentations there is muscle atrophy, contractures and calcinosis. Skin manifestations include a purple rash on the eyelids often with oedema (he-liotrope rash) and scaly vasculitic patches on the knuckles (Gottron’s papules). There may be skin ulceration and Raynaud’s phenomenon. Occasionally there is cardiac involvement leading to heart failure, respiratory involvement, including nonspecific interstitial pneumonia, and oesophageal involvement, which may be sufficiently severe so as to require gastrostomy feeding.

Investigations

Inflammatory markers: The ESR is raised.

Autoantibodies: Anti-histidyl-tRNA synthetase (anti-Jo1) are seen in 30%, and rheumatoid factor is positive in 30%.

CK-MB fraction and troponin I can be used to assess cardiac involvement.

Raised muscle enzymes such as creatine kinase (prior to biopsy).

EMG: Spontaneous fibrillations at rest, abnormal low-amplitude, short-duration polyphasic motor potentials on voluntary contraction and high-frequency discharges.

Muscle biopsy is definitive.

MRI of thighs is routinely performed.

Management

Acute phases are treated with corticosteroids, which should be reduced gradually to a low-maintenance dose. Methotrexate, azathioprine or cyclophosphamide are used in resistant cases.

Prognosis

The condition may last several years. The disease is of variable severity, and spontaneous remissions can occur.