Capillary malformation-arteriovenous malformation (CM-AVM) is a syndromic disorder of the vascular system characterized by development of capillary malformations (CM), arteriovenous malformations (AVM), and arteriovenous fistulas.

Patients most often present with multiple erythematous to violaceous patches on the skin secondary to superficial CMs and AVMs. The AVMs are typically warm to touch and often have a peripheral halo of vasoconstriction leading to surrounding pallor. Lesions may be present at birth or develop during childhood.

Diagnosis is usually suspected when multiple atypical CMs and/or AVMs are seen on physical examination.

In rarer cases, patients may present with recurrent epistaxis or multiple telangiectasias. More acutely, one-third of patients can develop internal high-flow lesions, which may lead to bleeding secondary to rupture of an intracranial arteriovenous malformation (vein of Galen) or congestive heart failure from an arteriovenous fistula.

CM-AVM is caused by a RASA1 mutation leading to production of a nonfunctional p120-RasGAP protein, a signaling molecule important for the appropriate development of the vascular system. It is inherited in an autosomal dominant manner, but around one-third of RASA1 cases of CM-AVM are de novo variants.

CM-AVM occurs in approximately 1 in 100 000 patients of Northern European descent.