Diagnostic criteria:
1) Proximal muscle weakness (shoulders, pelvic girdle muscles)
2) Increased CK
3) Myopathic changes on EMG
4) Muscle biopsy showing inflammation
5) Dermatologic involvement as outlined below
#5 + 3 others: Definite
#5 + 2 others: Probable
#5 + 1 other: Possible
Presentation:
Usually insidious onset of proximal weakness affecting shoulders, pelvic girdle. Neck flexor weakness also prominent. Dysphagia/dysphonia may occur. Synovitis is rare.
Pulmonary involvement with fibrosis/pneumonitis may occur. Cardiac involvement (myocarditis) occurs, but is relatively uncommon
Dermatologic findings:
Gottron's papules are lacy, pink/violaceous, raised or macular, and symmetric over dorsal IP jts, elbows, knees.
Heliotrope rash is violaceous discolouration of lids with periorbital edema.
Erythema of shoulders/neck (Shawl sign) or neck/chest (V-sign)- shown above
Periungal telangiectasias, nail changes (cuticular hypertrophy)
Raynaud's may be present
Mechanic's hands- coarse, fissured, scaly, hyperkeratotic hands
Investigations:
CK is increased during disease course, but not necessarily early. ASL/ALT/LDH may all be increased. ESR is >50 in 20%, normal in 50%.
EMG shows characteristic (but not pathognemonic) changes. Bx shows focal infiltration, CD8 T cells
Therapy usually consists of steroids, and possible addition of steroid-sparing agents such as methotrexate or azathioprine.
Malignancy association:
20-30% of patients have an underlying malignancy; dermatomyositis may pre-date the malignancy as in other paraneoplastic syndromes
It is controversial how aggressively to search for malignancy, and there is little high quality evidence to guide.
At the very least, everyone should get age-appropriate screening. Many experts would also measure tumor marker levels, and consider additional screening such as thoracic or abdominal imaging.
Link:
Click here for a good online review of dermatomyositis from American Family Physician
