An arteriovenous malformation (AVM) is a collection of abnormal blood vessels where blood flows directly from arteries into vein, without passing by small capillaries. There is no normal brain parenchyma within this tangle of vessels called the nidus. Blood flows with high velocity through the nidus and is drained by dilated veins (draining veins). These malformations are congenital in nature. Most AVMs occur sporadically although some are associated other inherited disorders such as hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu syndrome) and neurofibromatosis.

Symptoms
AVMs may remain asymptomatic and be discovered incidentally by imaging for an unrelated reason. The most common clinical presentation is intracranial hemorrhage. Depending on the localization of the AVM, hemorrhage may occur in the brain tissue (brain hematoma), in the cavities within the brain that contain CSF (intraventricular hemorrhage) or in the spaces surrounding the brain (subarachnoid hemorrhage).

In the event of a hemorrhage, presenting symptoms may be related to increased intracranial pressure such as headache, nausea, vomiting, double vision, altered level of consciousness. Focal symptoms due to a brain hematoma depend on the location of the bleed. Seizure may occur. Patients with subarachnoid hemorrhage may also describe neck stiffness, sensitivity to light (photophobia) and noice (phonophobia).

In the absence of hemorrhage, AVMs may also present with seizures by irritating adjacent brain tissue. The type of seizure depends on the AVM’s localization.

Patient may also present with progressive neurological deficit. These may be related to the mass effect of the AVM or a large draining vein on adjacent neurological structures. These slowly progressive neurological deficits may also be due to the siphoning of blood flow away from adjacent brain tissue, causing symptomatic ischemia in these regions (this is referred to as the steal phenomenon).

Imaging is required to establish the diagnosis of AVM:

     •   CT Scan: usually the best modality to identify hemorrhage

     •   MRI Scan: essential as it enables identification of the nidus and large draining veins as well as evaluation of the AVM’s location in relation to adjacent structures

     •   Magnetic Resonance Angiography: currently inadequate to delineate the morphology of feeding arteries and draining veins

     •   Cerebral Angiography: remains the gold-standard to study the AVM’s morphology. Angiography also enables identification of features potentially associated with a higher risk of hemorrhage including vascular pouches (aneurysms) arising on arteries or veins and kinking or large dilatation (ectasia) of draining veins.

In the presence of an AVM, the first question to consider is if the AVM requires treatment. Many factors must be taken into consideration including:

     •   Symtomatology

     •   The AVM’s characteristics (location, size, draining pattern, associated angiographic features)

     •   The patient’s characteristics (age, comorbidities)

If the AVM presents acutely with a hemorrhage, the patient’s clinical condition may require urgent surgery to evacuate the blood clot and relieve increased intracranial pressure. An external drain positioned in the ventricles may be required to monitor intracranial pressure and drain cerebrospinal fluid. The AVM is rarely addressed in such acute circumstances.

If the AVM presents with de novo seizure, standard anticonvulsant medication is generally sufficient in the acute setting to obtain seizure control.     

When a decision to treat an AVM is taken, three modalities may be considered: 

Surgery is the treatment of choice for easily accessible AVMs of smaller size. Through a craniotomy, the AVM is approached is a systematic fashion, coagulating the feeding arteries and at last obliterating the draining vein. The vascular conglomerate is completely removed, definitively curing the patient.

Endovascular embolization implies blocking the high-velocity shunt of blood from the arteries into the dilated veins. Numerous thrombosing agents may be used. Although embolization has succeeded in completely obliterating AVMs, it is most often used to reduce AVM and treat angiographic features potentially associated with a higher risk of bleeding.

Radiosurgery is an option to treat AVMs located in eloquent regions, measuring 3cm in diameter or less and presenting deep venous drainage. High radiation doses delivered to the AVM induce thrombosis of the abnormal vessels. Importantly, not all patients achieve thrombosis of the AVM following radiosurgery and those that do achieve thrombosis 1-4 years after treatment. During this interval, the patient remains at risk for hemorrhage from the AVM.

Angiographic follow-up is requested, regardless of the treatment modality, as AVMs have been described to recur after complete exclusion.