arteriovenous malformation

**Definition/Pathophysiology:** AVMs are congenital vascular lesions in which feeding arteries connect directly to draining veins through a nidus, bypassing the capillary bed. Loss of capillary resistance lets arterial pressure reach thin-walled, dilated venous channels. This causes progressive vessel enlargement and a risk of rupture. Brain tissue within the nidus is usually non-functional. Nearby parenchyma can suffer "steal" ischemia.

**Genetics/Associations:** - Most brain AVMs are sporadic. Somatic KRAS/MAPK pathway mutations have been described. - Hereditary hemorrhagic telangiectasia (HHT, Osler-Weber-Rendu) is autosomal dominant. It involves ENG (endoglin, HHT1), ACVRL1 (ALK1, HHT2), and SMAD4 (juvenile polyposis overlap). It causes mucocutaneous telangiectasias, recurrent epistaxis, and AVMs in the lung, brain, and liver. - Pulmonary AVMs cause right-to-left shunt, hypoxemia, paradoxical emboli (stroke, brain abscess), and polycythemia. - Sturge-Weber syndrome (GNAQ) involves a leptomeningeal angioma and a port-wine stain in the V1 distribution. - Von Hippel-Lindau syndrome involves hemangioblastomas. - Vein of Galen malformation can cause high-output heart failure and hydrocephalus in neonates.

**Clinical Presentation:** - Intracranial hemorrhage is the most common presentation (about 50%). Annual rupture risk is about 2-4%. Risk is higher with prior hemorrhage, deep location, deep venous drainage, and associated aneurysms. - Seizures. - Headache. - Focal neurologic deficits. - Pulsatile tinnitus or a bruit over the lesion. - Most patients present between ages 20 and 40.

**Diagnosis:** - Noncontrast CT is used first for acute hemorrhage. - MRI/MRA shows a "bag of worms" with flow voids. - Catheter (digital subtraction) angiography is the gold standard. It shows early venous filling and defines the angioarchitecture.

**Management:** - Treatment is individualized using the Spetzler-Martin grade, which is based on size, eloquent location, and deep venous drainage. - Options are microsurgical resection (definitive), stereotactic radiosurgery (for small, deep lesions), and endovascular embolization (often adjunctive). - Seizures are treated with antiepileptics. Blood pressure control matters. - Observation versus intervention for unruptured AVMs remains debated. The ARUBA trial favored conservative management, though its limitations are widely discussed.

**Board Pearls / Differentials:** - Cavernous malformation: low-flow, "popcorn" appearance on MRI, and often familial (CCM1/KRIT1). It is angiographically occult. - Berry aneurysm: subarachnoid hemorrhage ("worst headache of life"), associated with ADPKD and Ehlers-Danlos syndrome. - Capillary telangiectasia and developmental venous anomaly (a "caput medusae" appearance). - A young patient with seizures or lobar hemorrhage and no hypertension should prompt consideration of an AVM. - Dural AV fistulas are acquired, often after trauma or venous thrombosis. - Extracranial AVMs can cause high-output heart failure, and acquired AV fistulas can arise after trauma or from dialysis access.