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fiber necrosis and infiltration. Vasculitis may be observed, but is less common than in dermatomyositis. Regeneration is variable. Endomysial infiltrates of CD8+ T-cells, and macrophages invading non-necrotic muscle fibers that express MHC-1 antigen, are the distinguishing features. Interleukin-2 receptors indicative of activation are expressed on T-cells in peripheral blood.

The incidence is 2 to 9 cases per million of the population per year. Possible causes include picorna viruses such as coxsackie virus.

Magnetic resonance imaging can be very useful, both in showing the site and extent of muscle involvement, and in differentiating myositis from edema, fibrosis and calcification.

(c) DERMATOMYOSITIS

The appearance of the characteristic rash on the knuckle pads (Gottron's sign), elbows, shoulders, neck, knees or thighs and the changes in the face (heliotrope eyelids) is the only real point of clinical distinction from polymyositis. Biopsy of the rash can be histologically similar to discoid lupus. Electromyography will be similar to that for polymyositis.

The muscle histopathology may help make the diagnosis. Whereas in polymyositis the cellular infiltrate is mainly within the fiber or fascicle, and abnormal fibers are scattered throughout, in dermatomyositis there is a perifascicular infiltrate, the primary lesion being located in blood vessels, as C5b-9 membrane attack complex, before the appearance of an infiltration of inflammatory cells. This is mainly a B-cell infiltrate, and there is a higher ratio of CD4+ to CD8+ T-cells. Abnormal muscle fibers are grouped in one portion of the fascicle, suggesting a microinfarct. Vasculitis is common in dermatomyositis.

Malignancy may occur in 20% of adults over the age of 45 with dermatomyositis, as opposed to 13% of adults over the age of 45 with polymyositis. It is unlikely to be present in younger adults, and there is no link between juvenile dermatomyositis/polymyositis and malignancy. Another feature of juvenile dermatomyositis is prominent calcinosis, which can precede the other features of the disorder, and may occur because of vasculitis.

(d) INCLUSION BODY MYOSITIS

Sporadic inclusion body myositis (s-IBM) is the most common muscle disease of older persons. It is of unknown aetiology, there is no successful treatment, but advances have made in its pathogenesis in recent years. The term for the hereditary form of the myopathies, h-IBM, was introduced in 1993 to designate familial diseases with pathological features strikingly resembling those of s-IBM except for the lack of lymphocytic inflammation. There are several autosomal recessive and autosomal dominant syndromes of progressive muscle weakness, with various clinical presentations.

The s-IBM muscles show accumulation of amyloid beta precursor protein (ABETAPP), amyloid beta (ABETA), and phosphorylate tau and presenilin-I, similar to the brain in Alzheimer's disease. However, these diseases are still completely organ-specific, involving either muscle fibers or neurons.

The clinical findings include postproximal and distal muscle involvement with wasting and weakness that can become quite striking. The muscle biopsy specimens from s-IBM have an inflammatory component, at least in the early stage, but unlike polymyositis and dermatomyositis, this responds poorly to immunosuppresants.

References:
1 . Therapeutic Guidelines: Rheumatology, Version 1, 2006.
2. Therapeutic Guidelines: Palliative Care, Version 2, 2005.
3. Therapeutic Guidelines: Neurology, Version 3, 2007.

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