Deficiency of Adenosine Deaminase 2 (DADA2) – aka Fever with Early Onset Stroke (FEOS)
ADA2 (aka CECR1)
Unknown but rare.
Livedo reticularis rash is common. A few with polyarteritis nodosa (PAN). Diffuse vasculopathy, with impaired endothelial integrity and endothelial cellular activation.   Predominant levels of neutrophils and macrophages in the interstitium; perivascular T lymphocytes, without obvious vasculitis. 
Recurrent fevers, and early-onset lacunar strokes is common. Some patients do not have strokes.  Possible adult stroke risk. Brain biopsies: diffuse vasculopathy, with impaired endothelial integrity and endothelial cellular activation.   The first stroke often occurs before 5 years of age. 
High: CRP, ESR with flares. Cytopenia. Blood: 10-fold decrease in ADA2. Low: IgM , ADA2-specific adenosine deaminase activity: blood & CD14+ monocytes.   Absence of hypercoagulability.  Negative for antiphospholipid antibodies (APLAs). Some patients later develop a positive lupus anticoagulant (LA) level. Some have mild immunodeficiency.
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Diepgen TL, Yihune G et al. Livedo Reticularis.. Dermatology Online Atlas. DermIS Dermatology Information System. Published online at: http://www.dermis.net/dermisroot/en/42791/image.htm.