Differentials
Subacute cutaneous lupus erythematosus (SCLE)
SIGNS / SYMPTOMS
Lesions generally occur between the knuckles, whereas in DM they tend to occur over bony prominences.
Pruritus is usually absent.[110]
INVESTIGATIONS
Autoantibodies: anti-Mi-2 positive in 25% of DM patients. Anti-Ro (SS-A) antibody positive in 60% to 70% of patients with SCLE.[110]
Skin biopsy: histology of DM and SCLE are often indistinguishable. Immunofluorescent microscopy should be negative in DM and positive in SCLE, but positive microscopy is found in only 50% of SCLE cases and false positives may occur on sun-exposed DM skin.[110]
Psoriasis
SIGNS / SYMPTOMS
Scalp lesions tend to be patchy rather than diffuse.
Not photo-aggravated and often improves with sun exposure.
Presence of psoriatic nail lesions.
Pruritus is less common.
INVESTIGATIONS
Skin biopsy shows intra-epidermal spongiform pustules and Munro's neutrophilic micro-abscesses within the stratum corneum.
Cutaneous T-cell lymphoma
SIGNS / SYMPTOMS
Cutaneous lesions are not photodistributed. They are also found on non-sun-exposed skin.[108]
INVESTIGATIONS
Skin biopsy has distinct pathological findings that distinguish the lesions from those of DM.
Scleroderma
SIGNS / SYMPTOMS
Presence of skin thickening and sclerosis. Erythema is not a prominent feature.
Raynaud's phenomenon is a prominent and often severe symptom in systemic sclerosis that predates the onset of skin involvement, often by several years.
INVESTIGATIONS
Anti-centromere antibodies are present in 50% to 90% of patients with limited systemic sclerosis.
Anti-Scl-70 antibodies are present in 20% to 30% of patients with systemic sclerosis and are associated with the diffuse variant.[73]
Anti-PM-Scl antibodies have been found in patients with overlapping myositis and scleroderma.[49]
Skin biopsy has distinct pathological findings that distinguish the condition from DM.
Trichinellosis
SIGNS / SYMPTOMS
Muscle pain, periorbital oedema, and macular or urticarial rashes.
INVESTIGATIONS
Serology: antibodies against the trichinella parasite can be measured 2 to 3 weeks after infection.
Muscle biopsy: identifying larvae in muscle tissue makes the definitive diagnosis.[122]
Drug-induced DM-like reactions
SIGNS / SYMPTOMS
History of medicine associated with development of skin lesions indistinguishable from DM (e.g., hydroxyurea).
The eruption resolves with withdrawal of the drug but make take up to 18 months. Patients may have residual scarring and skin atrophy.[38]
INVESTIGATIONS
Investigations for muscle involvement are typically negative.
Overlap myositis
SIGNS / SYMPTOMS
Symptoms of DM associated with some other connective tissue disorders.
Diagnostic criteria for the two different disorders are fulfilled.
INVESTIGATIONS
Additional serological features diagnostic of an underlying connective tissue disease (e.g., systemic lupus erythematosus, rheumatoid arthritis, scleroderma, anti-synthetase syndrome, or mixed connective tissue disease).
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