Vascular malformations of the brain are divided into 3 categories: arteriovenous malformations (AVMs), cavernous malformations, and venous malformations. The first two have the potential to bleed, whereas venous malformations (which represent developmental variations of the brain’s normal venous anatomy) do not generally bleed.

AVMs are high-flow lesions in which arteries of the brain communicate with the veins of the brain through fragile low-resistance AVM vessels. AVMs carry a significant risk of bleeding, and the bleeding can be quite significant because of the high blood flow through these lesions.

Cavernous malformations are low-flow lesions which usually hemorrhage into their own cavity rather than into the adjacent brain. As such they cause less brain destruction when they hemorrhage and are less dangerous and easier to remove than AVMs.