Table 1.

Clinical subgroups of patients with juvenile-onset myositis. A summary of clinical JDM subgroups with reference to associated autoantibodies and how autoantibody status may influence clinical phenotype and likelihood of response to standard treatment

Description of clinical subgroup	Commonly identified autoantibodies and prevalence in JDM cohorts	Key clinical considerations and influence of autoantibody status on phenotype
Dermatomyositis ( [1–5])
Skin and muscle disease. Well characterised histological features.  Clinical outcome and response to standard treatment variable	Anti-TIFγ 18–32%	Severe cutaneous disease, ulceration, lipodystrophy, muscle disease may be milder. Chronic disease course
	Anti-NXP2 15–23%	Severe muscle disease, gastrointestinal bleeding and calcinosis. Lower remission rates
	Anti-MDA5 7–38%	Arthritis, ulceration, ILD, muscle disease may be milder. Rapidly progressive ILD and increased mortality in East Asian cohorts
	Anti-Mi2 4–10%	Low-incidence organ involvement, severe muscle disease, responds well to standard treatment
Amyopathic dermatomyositis ( [1–5])
Characteristic dermatomyositis skin manifestations with no muscle involvement.  Minimal or progressive muscle involvement is more common than an absence of muscle disease	Anti-MDA5 7–38%	See above
	Anti-TIF1γ 18–32%	See above
	Anti-SAE 1%	Muscle involvement may develop later
Immune-mediated necrotising myositis ( [2, 3, 7])
Severe muscle disease, histology shows myofibre necrosis with minimal inflammatory cell infiltrate. Note that rash has been reported in some JDM patients but can be atypical.	Anti-SRP 2%	Severe muscle disease, cardiac involvement, treatment resistance
	Anti-HMGCR 1%	Severe muscle disease, treatment resistance
Anti-synthetase syndrome ( [2, 3])
Myositis, ILD, Raynaud’s phenomenon, arthritis, fever and mechanic’s hands	Anti-Jo-1, anti-PL7, anti-PL12, anti-OJ, anti-KS, anti-EJ, anti-Zo, anti-Ha Collectively < 5%	Different anti-synthetase autoantibodies are associated with muscle-dominant or lung-dominant disease in adult patients. Phenotype data for JDM are limited.
Overlap disease ( [2, 3])
Patients fulfil classification for myositis and another rheumatic disease. Commonly, systemic sclerosis, inflammatory arthritis and systemic lupus erythematosus.	Anti-PmScl (4–5%) Anti-U1RNP (4–6%) Also reported anti-Ku, anti-U3RNP and anti-Scl70	Caution that due to overlapping clinical features patients may more appropriately fit

ILD, interstitial lung disease