Dermatomyositis/Polymyositis

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Dermatomyositis/Polymyositis (Inflammatory muscle disease)

Uncommon chronic inflammatory disease of striated muscles (polymyositis) that may have a skin rash (dermatomyositis). Most cases between 4th and 6th decades (mean age of onset is about 40 yrs), but one form has peak age of onset between 5 and 10 years of age. Similar to the myositis of SLE and MCTD. F>M. Reported prevalence varies - about 0.5-8/100 000.

Classification:
Type 1 – polymyositis (most common - about 1/3rd; usually middle age females with symmetrical proximal muscle weakness)
Type 2 – dermatomyositis (about 15-30%; type 1 with skin involvement)
Type 3 – associated with underlying malignancy (up to 10% of cases; dermatomyositis may antedate the malignancy by up to several years – most common site for tumours are lungs, prostate, female pelvic organs)
Type 4 – paediatric dermatomyositis (most frequently ages 5 to 10; more severe form of the disease)
Type 5 – mixed connective tissue disease (or associated with Sjogrens syndrome)
Type 6 – dermatomyositis without muscle involvement (Amyopathic dermatomyositis)

Aetiology:
Unknown. Higher frequency of HLA-B8 and –DR3. Maybe immune related response triggered by infection  injury to muscle. 90% have significant antinuclear antibody titers.

Clinical features:
Onset may be acute (malaise, weight loss and fever) or progressive and insidious (fatigue initially  later difficulty climbing stairs, rising from a squatting position etc). Painless symmetric proximal muscle weakness is constant finding – initial feature in 50%  changes in gait, such as wider base of support and less propulsive; lower limb is more involved than upper; may have dysphagia or respiratory muscle involvement; serum enzymes are elevated (creatine kinase, aldolase, lactic dehydrogenase); EMG changes; skin rash (Gottron’s papules – raised erythematosus to violaceous plaques over bony prominences; lilac discolouration of eyelids; erythematosus rash of extensor surfaces; skin lesions can be extremely pruritic) – skin is initial complaint in 20% to 25% - eventually develop in up to 60%; arthritis in up to 50% - rarely destructive – typically in wrists, knees and small finger joints. Later, may develop gastrointestinal and cardiac problems. Raynauds is common – also skin ulcerations may develop (complicated by vasculitis). In nailfolds, develop overgrowth of cuticle, dilated capillary loops and periungual erythema. ‘Mechanic’s hand’ – symmetric hyperkeratosis with fissures on grasping aspect of hand.

Differential diagnosis – hypothyroid myopathy, myasthenia gravis, muscular dystrophies (eg Duchenne’s), polymyalgia rheumatica, fibromyalgia, metabolic disorders of muscle, electrolyte disturbances, alcohol abuse. May overlap with other connective tissue diseases.

Treatment:
Aim is to halt inflammatory process, preserve muscle strength and prevent contractures.
Search for malignancy
Corticosteroids have a good effect on symptoms in acute stages.
Immunosuppression can be used if life threatening.
High dose intravenous immunoglobulin (IVIG) can be used in those resistant to other therapy.
Physiotherapy and occupational therapy – especially range of motion activities, muscle strength and general fitness activities.

Other Inflammatory Muscle diseases:
Polymyositis and dermatomyositis are the most common of the nonsuppurative inflammatory muscle diseases, but types do occur:

Inclusion body myositis (IBM):
• sometimes referred to as treatment resistant polymyositis due to lack of response when treated as polymyositis; could account for up to 20% of inflammatory muscle diseases
• usually male over 50, but can occur at any age; slow progressive onset (signs often present for up to 5 years); falling and tripping may be first signs; symptoms are usually present for 5-6 years before diagnosis
• proximal muscle weakness and atrophy develops first – distal weakness also can occur early (especially quadriceps and wrist flexors); half develop swallowing difficulties
• can be asymmetric (polymyositis is symmetric)
• may have a peripheral neuropathy with loss of deep tendon reflexes
• muscle biopsy show invasion of nonnecrotic muscle fibres by mononuclear cells, vaculated muscle fibres and intracellular amyloid deposits.
• half have signs of circulating autoantibody – myositis specific autoantibodies (MSAs)
• unresponsive to corticosteroids; may be some short term benefits from IVIG

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