ABC Medical Notes.com

Dermatomyositis is an idiopathic inflammatory myopathy with characteristic skin manifestations. An immune complex vasculitis of various severity is the pathological hallmark.

About

• Similar to adult disease but with some other features

Incidence

• 2-3 per million per year
• Average age of onset is 7 years old
• Commoner in caucasions

Aetiology

• TNF alpha 308A allele has been associated with increased disease severity, calcinosis, ulceration and duration
• JDM patients show overexpression of MHC-I and beta 2-microglobulin in the affected muscle
• Postmeiotic genetic events, different epigenetic modification, epistatic protein interactions and environmental exposure influence the disease pathology

Diagnostic criteria

• 1. Symmetrical weakness of the proximal muscles
• 2. Characteristic cutaneous changes consisting of heliotrope discolouration of the eyelids, which may be accompanied by periorbital oedema, and erythematous papules over the extensor surfaces of joints, including the dorsal aspects of the metacarpophalangeal and proximal interphalangeal joints, elbows, knees or ankles (Gottron papules)
• 3. Elevation of CK or ASK or ALT or LDH or Aldolase
• 4. Electromyographic demonstration of the characteristics of muscle irritability and denervation
• 5. Muscle biopsy documenting histological evidence of myositis. The most typical histologic features of JDM are perifascicular atrophy, centralization of nuclei, degenerating fibres, regenerating fibres, and a scattered inflammatory infiltrate (often around vessels). Specialized staining will often show a decrease in the number of capillaries in the muscle, and on electron microscopy, tubuloreticular inclusions are often seen

Clinical

• Fever, anorexia and adenopathy, Sleep disturbance and fatigue. Gower's sign and the Trendelenburg sign
• Rash which may be discoid, erythematous, violaceous rash involving the dorsum of her hands and fingers consistent with Gottron's papules
• Bilateral depigmentation and dermal thickening of her knuckles were present. A generalized erythema
• A 'heliotrope', a macular rash with periorbital oedema, is considered a characteristic finding of dermatomyositis, as are periungual telangiectasias
• Bilateral weakness of the proximal muscles of the lower limbs with difficulty standing
• Mechanic's hand may be associated with an increased risk of interstitial lung disease
• Affected muscles may, on occasion, be tender, edematous or indurated

Investigations

• FBC, U&E, LFT, CRP, ESR, Autoantibodies
• Elevated CK and LDH
• Electromyography
• Muscle biopsy is the definitive test but MRI is being used more
• MRI findings suggestive for muscle inflammation include symmetrical muscle oedema in the thigh muscle on fat-suppressed T2 -weighted or short tau inversion recovery sequences
• Nailfold capillaroscopy, which shows a high correlation with disease activity and recovery during follow-up

Management: specialist referral

• Involves a paediatrician with expertise in the condition, physical therapist, dermatologist and family physician. Functional impairment is present in 50% of juvenile dermatomyositis patients years after disease onset.
• Treatment with steroids may be needed to be supplemented with methotrexate. After the introduction, the mortality rate dropped below 10% and the functional outcome has improved markedly. Some use IV methylprednisolone or IV Immunoglobulin
• Others have used Mycophenolate mofetil and Ciclopsorin and Rituximab

References

• Juvenile Dermatomyositis