Table 1.

Comparison between COVID-19, anti-MDA5 dermatomyositis and classic dermatomyositis.

		COVID-19	Anti-MDA5 dermatomyositis with ILD	Classic dermatomyositis with ILD
Epidemiology	Prevalence	More than two million cases globally	Rare	Rare
	Geographic clusters	First reports in China, (now in all continents)	Mainly reported in east Asia	None
	Sex predominance	None	None	Female predominance
	Natural history	Severe and rapidly progressive disease in about 20% of cases	Rapidly progressive	Slowly progressive
	Recurrence	Unknown	Rare	Relapsing-remitting
	Mortality rate	High	Very high	High
Pathogenesis	Association with viral infection	Proven association with SARS-CoV-2 infection	Possible trigger of picoRNA- or other viruses	Debated triggering role of viruses
	Inflammatory state	High grade systemic inflammation	High grade systemic inflammation	Low-moderate grade systemic inflammation
	Prothrombotic state and endothelial dysfunction	Hallmark of the disease	Hallmark of the disease	Hallmark of the disease
	Autoantibody mediated injury	Possible cross-reactivity of induced antibodies	Postulated direct role of anti-MDA5	Debated direct pathogenetic role
	Lung histopathology	DAD and microangiopathy	DAD and microangiopathy	NSIP and OP
Clinical manifestations	Lung disease	Almost always present	Almost always present	Common
	Myositis	Mild-absent	Mild-absent	Almost always present
	Skin and peripheral vascular involvement	Common	Almost always present	Almost always present
	Fever	Almost always present	Very common	Uncommon
	Association with cancer	Absent	Rare	Possible
Diagnosis and monitoring	CK	Mild-moderate high	Mild-moderate high	Very high
	Ferritin	High	High	Normal or slightly increased
	Lymphocytes	Commonly low	Occasionally low	Occasionally low
	CRP	Very high	Very high	Usually normal
	ESR	High	High	High
	Antinuclear Antibodies	Unknown	Negative	Usually positive
	Antiphospholipid antibodies	Possibly positive	Possibly positive	Possibly positive
	CT scan of the chest	Bilateral GGO or consolidation in posterior and peripheral lungs	Bilateral GGO or consolidation in posterior and peripheral lungs	Bilateral peribronchovascular GGO or consolidation
	Nailfold capillaroscopy	Unknown	Enlarged capillaries, hemorragias, neovascularization	Enlarged capillaries, hemorragias, neovascularization
Treatment	Corticosteroids	Under investigation	Commonly used	Commonly used
	Anti-IL6	Under investigation	Unknown efficacy	Unknown efficacy
	Anti-IL1	Under investigation	Unknown efficacy	Unknown efficacy
	JAK-inhibitors	Under investigation	Under investigation	Under investigation
	Anti-CD20	Not suitable	Rescue therapy	Rescue therapy

ILD interstitial lung disease, DAD diffuse alveolar damage, NSIP nonspecific interstitial pneumonia, OP organizing pneumonia, CK creatine kinase, CRP C reactive protein, ESR erythrocyte sedimentation rate, CT computed tomography, GGO ground glass opacities, IL interleukin, JAK Janus kinase.