Brain AVMs

1 What are Intracranial Venous Malformations?

Intracranial Venous malformations occur in the brain, and could pose as a serious threat to your health.

Major classes of vascular malformations are:

Mixed lesions, of course, may occur. Vascular malformations develop early in embryonic life as a result of maldevelopment of vascular channels.

In major vascular malformations, arteriovenous shunting occurs through the path of least resistance, bypassing the brain parenchyma, resulting in tissue ischemia.

“Normal perfusion pressure breakthrough” may occur after AVM resection, resulting in edema and hemorrhage at the operative site. Distinct genetic mutations result in cavernous vascular malformations.

Epidemiology and natural history of AVMs are subjects of controversy. Clinical presentation of vascular malformations includes

  • hemorrhage,
  • seizures,
  • headache,
  • steal syndromes.

Treatment options include

  • surgical resection,
  • endovascular therapy,
  • radiosurgery,
  • a judicious combination thereof.

All blood vessel malformations involving the brain and its surrounding structures are commonly referred to as AVMs. But several types exist:

True arteriovenous malformation (AVM)

This is the most common brain vascular malformation. It consists of a tangle of abnormal vessels connecting arteries and veins with no normal intervening brain tissue.

Occult or cryptic AVM or cavernous malformations

This is a vascular malformation in the brain that doesn’t actively divert large amounts of blood. It may bleed and often produce seizures.

Venous malformation

This is an abnormality only of the veins. The veins are either enlarged or appear in abnormal locations within the brain.

Hemangioma

These are abnormal blood vessel structures usually found at the surface of the brain and on the skin or facial structures. These represent large and abnormal pockets of blood within normal tissue planes of the body.

Dural fistula

The covering of the brain is called the “dura mater.” An abnormal connection between blood vessels that involve only this covering is called a dural fistula. Dural fistulas can occur in any part of the brain covering.

Three kinds of dural fistulas are:

Dural carotid cavernous sinus fistula

These occur behind the eye and usually cause symptoms because they divert too much blood toward the eye. Patients have eye swelling, decreased vision, redness and congestion of the eye. They often can hear a “swishing” noise.

Transverse-Sigmoid sinus dural fistula

These occur behind the ear. Patients usually complain of hearing a continuous noise (bruit) that occurs with each heartbeat, local pain behind the ear, headaches and neck pain.

Sagittal sinus and scalp dural fistula

These occur toward the top of the head. Patients complain of noise (bruit), headaches, and pain near the top of the head; they may have prominent blood vessels on the scalp and above the ear.

2 Symptoms

Vascular malformations of the brain may cause

  • headaches,
  • seizures,
  • strokes,
  • bleeding in the brain (cerebral hemorrhage).

Arteriovenous malformations or AVMs affect

  • arteries,
  • veins,
  • middle- sized vessels

but not capillaries. These blood vessels are enlarged, twisted, and tangled. Arteries and veins may be connected directly instead of being connected through fine capillaries for which reason they are often referred to as “shunt lesions” since the capillaries are by-passed.

These abnormal “feeding” arteries progressively enlarge and as a result the “draining” veins dilate as well. The brain tissue between these vessels may be hardened or rigid (atrophied), full of a network of fine small fibers (fibrils) interspersed with flattened cells (gliotic), and sometimes may be calcified. Such malformations may, by drawing blood away from the brain, cause brain cell atrophy. Hemorrhages or seizures are commonly experienced with AVMs.

Cavernous malformations

CMs (also called cavernous angiomas, or cavernous hemangiomas, or cavernomas) present as abnormally enlarged collections of blood-filled spaces. A cavernous hemangioma acts like a “blood sponge” soaking up blood that has found its way between capillaries, in the spaces between tissues (sinusoids) and “larger cavernous spaces.” These are “slow-flow lesions.” There is not usually any brain tissue in these spaces in contrast with symptoms of AVMs. Hemorrhages or seizures are also common with CMs.

Venous angiomas (VAs)

Involve enlarged, tangled, and twisted veins that vary in size but do not involve the arteries. The site of these “growths” is most often just after the capillary stage of the vessel (post-capillary malformation). They may be isolated defects or associated with cavernous malformations. The defect shows itself as a “crown” of small veins (venules) that meet to form part of a larger vein (trunk).

Telangiectasias are the malformations that arise as a result of the enlarging (dilation) of the tiny capillaries. These dilated capillaries make themselves known as small pink-red spots in various parts of the body such as the face, eyes, membranes that cover the brain (dura) and spinal cord (meninges), and mucous membranes (the thin moist layer lining the body’s internal surfaces).

Vein of Galen malformations

VGMs begin while the embryo is developing. The vein of Galen is located under the cerebral hemispheres and drains the forward (anterior) and central regions of the brain into the proper sinuses.

The malformations occur when the vein of Galen is not supported within the head by surrounding tissue and lacks the normal fibrous wall. Thus, the vein of Galen appears free-floating within the fluids of the cerebral spaces (sinuses).

Should the pressure increase within the vein of Galen, its shape changes from a cylinder to that of a sphere. Such changes are accompanied by abnormal fetal blood circulation. In extreme cases, there may be cardiac failure or swelling of the brain (hydrocephalus).

Mixed malformation is a phrase used to include any of several multiple-mixed malformations. Frequently, these malformations appear to be mixes of arteriovenous malformations with telangiectasias.

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3 Causes

Causes of intracranial venous malformations vary, and must be determined by your doctor or specialist.

Three types or forms of VMB have a genetic component. The evidence for a genetic cause is strong in the case of cavernous hemangiomas and telangiectasias.

The case is much weaker for arteriovenous malformation of the brain (AVM). In each of these cases, the condition is transmitted as an autosomal dominant trait.

The malfunctioning gene in the case of cavernous malformations has been tracked to gene map locus 7q11.2-q21, and in the case of telangiectasia to gene map locus 9q34.1.

4 Making a Diagnosis

Making a diagnosis of intracranial venous malformations is done by performing several tests.

Brain AVMs can be divided into two types:

  • 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"

The Spetzler AVM grading system relates morphology and location to the 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. 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, nidus and draining veins visible in the so-called "bag of worm" appearance. The exact anatomy of feeding vessels and draining veins can be 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

Angiography (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.

On angiogram, AVM appears as a tightly packed mass of enlarged feeding arteries that supply central nidus. One or more dilated veins drain the nidus and there is abnormal opacification of veins occurs in arterial phase (early venous drainage), represents shunting.

5 Treatment

Complete obliteration is the goal of treatment for intracranial venous malformations as partial obliteration does not affect the rate of hemorrhage. Treatment includes the following:

Surgery (craniotomy)

Advantages

Cure is immediate and permanent after complete resection by craniotomy. Surgery is generally recommended for grade 1, 2, and 3 lesions, sometimes for grade 4 lesions, and not for grade 5 lesions.

Disadvantages

The potential for significant intraoperative bleeding, damage to adjacent neural tissue and ischemic stroke are disadvantages. The "arteries of passage" supply intact neural tissue and must not be destroyed while attempting to interrupt the arterial supply to the AVM.

A risk also exists for perfusion-breakthrough bleeding (i.e., hemorrhage into the healthy part of the brain caused by sudden hemodynamic shifts), which results from the removal of a large AV shunt and the subsequent increased flow to previously underperfused vessels.

Endovascular neurosurgery (obliterating vessels with glues or particles delivered via arterial catheter in the angiography suite)

Advantages

Significant reduction of pathologic blood flow through the lesion can be achieved. Its main use is as adjuvant therapy prior to craniotomy to decrease intraoperative bleeding and technical difficulty. It has also been used to decrease the size of an AVM to make it sufficiently compact for effective targeting by stereotactic radio surgery.

Embolization may be curative in lesions less than 1 cm in diameter that are fed by a single artery. Improved obliteration rates of approximately 20% have been reported when using the embolic agent Onyx.

Disadvantages

This is an invasive procedure, and its major risks are similar to those for open surgery, i.e., ischemia and hemorrhage. The main risk is causing ischemic stroke by occluding a feeding vessel that also supplies normal brain. Postembolization hemodynamic alterations can cause rupture of the AVM, resulting in new neurologic deficit from subarachnoid and/or intraparenchymal hemorrhage, analogous to the perfusion-breakthrough bleeding described above.

This technique is not normally used by itself, as it rarely achieves complete eradication of the lesion and the pathologic vessels usually recanalize over time.

Stereotactic radiosurgery

Advantages

Stereotactic radiosurgery is noninvasive and can access all anatomic locations of the brain. New techniques available include staged radiosurgery for larger lesions (Grade IV and V), which has promising results.

Disadvantages

It is used to treat smaller lesions (< 3 cm in diameter) and requires 2 or more years for a full destructive effect. The risk for hemorrhage is not reduced during this lag time. The risks of radiation necrosis of adjacent healthy tissue or cyst formation also exist. The cure rates for lesions smaller than 3 cm range from 81-90%. Therefore, a small subset of these lesions still hemorrhages after treatment.

The current gold standard for assessing AVM obliteration after stereotactic radio surgery is digital subtraction angiography (DSA). In one study, MRI/MRA predicted AVM obliteration after SRS in most patients, but DSA should still be performed to confirm AVM nidus obliteration after SRS.

Integration of 3-dimensional rotational angiography into stereotactic radiosurgery treatment planning was recently reported.

Combination therapy

  • Total eradication of the lesion may require more than one modality. Partial treatment may increase the risk of hemorrhage.
  • Endovascular neurosurgery can be performed before surgical excision to reduce the difficulty of surgery or before radiosurgery to bring the size of the lesion to within the limits of the machine.
  • Radiosurgery may be used to eradicate small residual disease left after craniotomy (due to technical difficulty or involvement of eloquent structures).
  • Aneurysms on an artery that does not feed the AVM can be managed as any unruptured intracranial aneurysm.
  • Aneurysms less than 5 mm in size have been reported to regress after treatment of AVM; in other cases, they have ruptured after treatment.
  • Given concern about aneurysms greater than 5 mm, treatment via microsurgical clipping or endovascular coiling is generally done prior to the treatment of the AVM.

6 Risks and Complications

The most dreaded complication of intracranial venous malformations is intracerebral hemorrhage.

Treatment decisions are based on the natural history-risk of first or subsequent hemorrhage versus the risk-benefit ratio of treatment.

Surgical complications

  • Surgical complications may include persistent neurological deficits associated with hemorrhage and stroke.
  • Complications of endovascular embolization
  • Complications of endovascular embolization include persistent neurological deficits related to inadvertent embolization of arteries supplying normal brain tissue or obliteration of the venous outflow leading to intracerebral hemorrhages. The procedure carries an associated risk for morbidity and mortality in the range of 9-22% and 0-9%, respectively.
  • No long-term outcome studies are yet available; however, as endovascular techniques continue to improve, complication rates are likely to diminish.

Complications of radiosurgery

  • Complications depend on the size and location of the AVM. AVMs located in eloquent areas and in central locations are more prone to radiation-induced complications.
  • White matter edema and radiation-induced necrosis may occur during the 1- to 3-year treatment period. Persistent neurological deficits after radiation have been reported in 8% of treated patients. Patients with hemorrhagic presentation have a higher mean annual risk for hemorrhage until radiation-induced obliteration of the AVM is achieved compared to patients with a nonhemorrhagic presentation.
  • Seizure frequency may increase in the first days to weeks after radiosurgery.
  • The potential for late effects from radiation, such as accelerated atherosclerosis in surrounding blood vessels, does exist.
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