Cerebral arteriovenous malformations (CAVMs), also known as classic brain AVMs, are a common form of cerebral vascular malformation and are composed of a nidus of vessels through which arteriovenous shunting occurs.
This article corresponds to the classic form of arteriovenous malformation involving the brain parenchyma, which is also referred as a pial arteriovenous malformation, once it is related to the pial vessels 6.
These malformations are characterised by a nidus forming the transition between the feeding artery and draining vein. When this transition is made directly then it is a fistula, i.e. a separate cerebral vascular malformation (e.g. brain / pial arteriovenous fistula).
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. Despite this, a third of AVMs which are diagnosed due to haemorrhage are identified before the age of 20 years 7. Overall, AVMs 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 must be considered, including:
CAVMs are the most common symptomatic vascular malformations. Possible presentations include 3:
- incidental finding in asymptomatic patients: 15% 5
- seizures: 20%
- ischaemic events due to vascular steal from normal brain
- haemorrhage: 65% 5, incidence 2-3% per year 3
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 a number of components:
- feeding arteries
- nidus (Latin for "nest")
- 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 also referred to as venous pouches, are seen as well. It may contain dystrophic calcification, a small amount of gliotic tissue, and blood at different stages of ageing.
- supratentorial: ~85%
- superficial (two-thirds)
- deep (one-third)
- infratentorial: ~15%
- solitary AVMs (98%)
- multiple AVMs (2%)
- associated with syndromes
- flow-related angiopathy secondary to endothelial hyperplasia
- flow-related aneurysm
- intranidal: located in the nidus
- intrapedicular: located in the feeding vessel
- remote aneurysm: haemodynamically unrelated to malformation
Classification and grading
Brain AVMs can be divided into two types 4,6:
- compact (or glomerular) nidus: abnormal vessels without any interposed normal brain tissue
diffuse (or proliferative) nidus: no well-formed nidus is present, with functional neuronal tissue interspersed amongst the anomalous vessels.
- when an early venous drainage is absent, this is considered a different entity, "cerebral proliferative angiopathy" 6,9
The Spetzler-Martin AVM grading system relates morphology and location to the risk of surgery.
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. Although they might be very large in size, they do not cause any mass effect unless they bleed.
Following contrast administration, and especially with CTA, the diagnosis is usually self-evident, with feeding arteries, draining veins, and intervening nidus visible in the so-called "bag of worms" appearance. The exact anatomy of feeding vessels and draining veins can be difficult to delineate, and thus, angiography remains necessary.
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 shunting can be very rapid.
On angiography, an AVM appears as a tightly packed mass of enlarged feeding arteries that supply a central nidus. One or more dilated veins drain the nidus and abnormal opacification of veins occurs in the arterial phase (early venous drainage), represents shunting.
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 for subtracting the haematoma components when an AVM complicated by an acute haemorrhage needs to be imaged
Treatment and prognosis
Treatment options and rate of complications are dictated in part by the Spetzler-Martin grade. In general, the three options available are:
- microsurgical resection
- endovascular occlusion
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 2-3%, resulting from a 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.
Imaging differential considerations include:
- other cerebral vascular malformations
cerebral proliferative angiopathy 6,9
- absence of early venous drainage
- often, an entire lobe or even hemisphere is affected
- feeder arteries tend to be of normal size or moderately enlarged
- associated stenosis of feeder arteries is often present
craniofacial arteriovenous metameric syndrome (CAMS):
- classic locations
- association with facial AVM
- dural arteriovenous fistula (DAVF)
- developmental venous anomaly (DVA)
- cerebral proliferative angiopathy 6,9
- vascular tumour
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