Cerebral arteriovenous malformation

Cerebral arteriovenous malformations (CAVM) are a common form of cerebral vascular malformation and are composed of a nidus of vessels through which arteriovenous shunting occur. Three types are described:

1. parenchymal (pial) AVM
2. dural AVM
3. mixed AVM

Epidemiology

Although arteriovenous malformations are thought to represent a congenital abnormality, they are thought to develop over time and are rarely found incidentally in the very young. Having said this, a third of AMVs which are diagnosed due to haemorrhage are identified before the age of 20 years of age ⁷. Overall AMVs are diagnosed at a mean age of 31 years ⁸. 

Overall AVMs are thought to occur in approximately 4% of the population, but become symptomatic in only 12% of affected individuals ⁸. There is no gender predilection ⁸. 

Arteriovenous malformations tend to be solitary in the vast majority of cases ( > 95 % ). When multiple, syndromic associations include

• Wyburn-Mason syndrome
• Osler-Weber-Rendu syndrome

Clinical presentation

Possible presentations include ³ :

• incidental finding in asymptomatic patients : 15% ⁵
• seizures : 20%
• headaches
• ischaemic events due to vascular steal from normal brain
• haemorrhage : 65% ⁵, 2 - 3% per year ³
  • parenchymal
  • subarachnoid
  • intraventricular

Pathology

The origin of arteriovenous malformations remains uncertain, although they are thought to be congenital ³ and perhaps involves dysregulation of vascular endothelium growth factor (VEGF) ¹.

AVMs comprise of a number of components :

• feeding arteries
• nidus
  • shunting arterioles: the true culprit
  • interconnected venous loops
• draining veins

The nidus is fed by one or more arteries and drained by one or more veins. The feeding arteries are enlarged due to the increased flow and flow related arterial aneurysms are encountered ³. Venous aneurysms are also seen.

Location

• supratentorial : ~ 85 %
• infratentorial : ~ 15 %

Classification and grading

AVMs can be divided into two types ⁴:

1. compact : nidus contains little if any neuronal tissue, which is non-functioning
2. diffuse : no well formed nidus is present, with functional neuronal tissue interspersed amongst the anomalous vessels. 

The Spetzler AVM grading system relates morphology and location to risk of surgery.

Radiographic features

CT

Diagnosis can be difficult on non contrast CT. The nidus is blood density and therefore usually somewhat hyperdense compared to adjacent brain. Enlarged draining veins may be seen.

Following contrast administration, and especially with CTA the diagnosis is usually self evident with feeding arteries, nidus and draining veins visible. The exact anatomy of feeding vessels and draining veins is often difficult to delineate, and thus angiography remains necessary.

MRI

Fast flow generates flow voids easily seen on T2 weighted images. Complications including previous haemorrhage and adjacent oedema may be evident.

MRA : phase contrast MR angiography is often useful to subtract the haematoma components when an AVM complicated by an acute haemorrhage needs to be imaged.

DSA

Remains the gold standard, able to exquisitely delineate the location and number of feeding vessels and the pattern of drainage. Ideally angiography is performed in a bi-plane system with a high rate of acquisition, as the shunts can be very rapid.

Treatment and prognosis

Treatment options and rate of complications are dictated in part by the Spetzler grade. In general the three options available are :

1. microsurgical resection
2. endovascular occlusion
3. radiosurgery

Occasionally AVMs have been known to spontaneously resolve ², usually in the setting of intracranial haemorrhage resulting presumably in venous compression and thrombosis. The annual risk of haemorrhage for an untreated AVM is ~ 3%. It results from flow-related aneurysm, intra-nidal aneurysm or venous thrombosis (rarely).

Following haemorrhage the risk of a further bleed in the next 12 months is up to 18% ⁵.

Differential diagnosis

• other cerebral vascular malformations
  • dural arteriovenous fistula (DAVF)
  • developmental venous anomaly (DVA)
• vascular tumour