For Medical Professionals
An arteriovenous malformation (AVM) is a tangled mass of blood vessels with abnormal connections between arteries and veins.
Because of the risk of cerebral hemorrhage, they are particularly dangerous in the brain. About 2 to 4 percent of all AVMs each year result in hemorrhages.
Hemorrhage from a ruptured AVM can cause stroke-like symptoms or a fatal stroke. Patients aged 15-20 are at the highest risk of hemorrhaging.
AVMs affect less than 0.1 percent of the population, and their cause is unknown. They form in utero and have no known risk factors.
Following treatment, imaging tests are repeated to see if the AVM was completely removed, or if additional treatment is needed.
Patients are monitored closely with serial visits and serial neuroimaging if needed.
Congential Syndromes and AVM
Types of AVM
Congenital Syndromes and AVM
Once formed, extrinsic factors such as arterial shunting, growth factors, and intracranial hemorrhage may alter the size and shape of an AVM.
There are a small number of rare congenital syndromes, such as Sturge–Weber, Rendu–Osler–Weber, ataxia telangiectasia and Wyburn–Mason, that are associated with AVMs.
Sturge–Weber syndrome has two key pathological components:
- A facial angioma in the distribution of the trigeminal nerve
- A brain parietal–occipital vascular malformation.
In Sturge–Weber syndrome, the intracranial vascular malformation is on the surface of the brain and rarely causes intracranial hemorrhage. However, it may cause intractable epilepsy, mental retardation and progressive shrinkage of the brain.
Rendu–Osler–Weber syndrome, or hereditary hemorrhagic telangiectasia, is an autosomal-dominant syndrome of multiple visceral, mucosal and cerebral vascular malformations.
Patients with Rendu–Osler–Weber syndrome usually present with recurrent nosebleeds or stroke from pulmonary AVM emboli. Ataxia telangiectasia and Wyburn–Mason syndrome may have cerebellar or deep brain vascular malformations, respectively.
Types of AVMs
The basic types of AVMs that are of most concern to patients and physicians are:
Arteriovenous fistula are direct connections between arteries and veins without any intervening capillary networks in between and result in rapid blood flow across from artery to vein and generally result in symptoms caused by increased pressure in the venous side of the circulation.
These lesions can occur in a variety of locations and often the symptoms depend on the location of the fistula and the structures which are nearby.
For example, one of the common types of A-V fistulas is called a carotid cavernous (CC) fistula which results in an abnormal communication between the internal carotid artery in the base of the skull and the cavernous sinus which is a network of veins which surrounds the artery.
When a CC fistula forms, the veins that normally drain into the cavernous sinus (including veins in the eye) become subject to an increase in pressure and dilate and sometimes bleed in response.
Other common types of A-V fistula which can be located behind the ear can cause a bruit or noise in the ear which coincides with each heartbeat and has a characteristic "whooshing' sound.
They usually result from a prior blood clot forming in one of the large draining veins of the brain that runs behind the ear.
AV fistulas can often be treated by endovascular embolization using small catheters that are threaded up into the brain through a puncture of the femoral artery in the leg.
Cavernous malformations (CM's) (also called cavernomas) are the second most common cerebral vascular malformation that comes to clinical attention. These lesions occur throughout the central nervous system including all areas of the brain and spinal cord.
Cavernous malformations are well circumscribed, round or mulberry-shaped nests of thickly hyalinized sinusoidal vessels that are best imaged on MRI. Within a cavernous malformation, there is no intervening neural parenchyma. Extensive areas of micro-hemorrhage and hemosiderin deposition are present.
These lesions are often found incidentally on neuroimaging studies, most commonly on MRI. Occasionally cavernous malformations can be found on CT scan also but CT scanning is much less sensitive than MRI in detecting these lesions.
Cavernous malformations are not detectable by cerebral angiography. They come to medical attention if found incidentally on MRI or CT, or by producing symptoms. The most common presenting symptoms are either headaches, bleeding or seizures.
Repeated severe headaches can often be caused by localized bleeding within the cavernous malformation, or a severe headache associated with a new neurological deficit can be caused by bleeding into the surrounding brain.
Intractable seizures can also be the result of an underlying CM and lesion excision has a high success rate in controlling, or reducing the rate of the seizures.
The cause of cavernous malformations is believed to be from an genetic lesion that can either be inherited as part of a familial syndrome, or can occur spontaneously in individuals with no family history of these lesions. The lesions can occur as single lesions or can occur in multiple locations in the brain.
Treating Cavernous Malformations
The decision to treat or observe a CM is based on a number of factors including the presence or absence of symptoms, the age of the patient, the number of lesions, and the location of the lesion in the brain or spinal cord. If it is determined that the lesion should be treated, surgical excision is the most effective.
Often stereotactic guidance with microsurgical excision can be highly effective in removing the lesion to prevent mass effect or future bleeding and also to control intractable seizures, Radiosurgery is not recommended in most cases.
True Arteriovenous Malformations
Arteriovenous malformations of the brain are high-flow cerebrovascular lesions that may occur in any intracranial location and range in size from microscopic to more than 10 cm in diameter.
There are three distinct components to each AVM: (1) arterial feeding vessels, (2) a central nidus where the arterial to venous shunts are located, and (3) draining veins.
The AVM nidus is a compact tangle of dysplastic, thin-walled vessels of varied length connecting feeding arteries to draining veins. An AVM nidus can either be globular or conical in shape.
Within the AVM nidus, arterial blood is shunted directly into draining veins without passage through a normal capillary network. The amount of arteriovenous shunting varies among AVMs and is determined by vessel impedance. There is usually little brain parenchyma within an AVM nidus.
However, functional brain material may occasionally be found among the vessels of a diffuse AVM. Although the weak, dysplastic nidal vessels are usually the source of AVM hemorrhage, aneurysms of the feeding vessels also represent an independent risk factor for AVM hemorrhage.
These flow-related aneurysms which occur in a small but significant number of AVM's occur on the proximal intracranial vessels and are pathologically similar to saccular aneurysms. Veins draining an AVM are often large and dilated.
In addition, direct arterial blood flow into draining veins may promote wall thickening and stenosis. Venous drainage can be directed into deep into the brain or superficial, along the cortical venous systems.
Developmental Venous Anomalies
Developmental venous anomalies (DVA) are benign abnormal developmental anomalies of the cerebral venous system that rarely hemorrhage and therefore are not treated by either surgery or radiosurgery.
They are comprised of numerous small veins that drain in a centripetal pattern into a large venous trunk, analogous to the spokes of a wheel.
The venous trunk subsequently drains into a superficial vein that connects to a dural sinus. These lesions are thought to be primitive venous elements that fail to regress during fetal development and so function like normal subcortical veins.
Consequently, surgical resection is not indicated. Histologically, DVAs comprise normal, thin-walled venous structures.
Vessel wall hyalinization may occur; this, however, is not associated with a worse prognosis. Developmental venous anomalies may be associated with cavernous malformations.
Therefore, if a patient with a DVA presents with a brain hemorrhage, it is important to look for a cavernous malformation which is often the cause of the bleeding. MRI and angiogram of DVA below.
Capillary telangiectasias are benign focal collections of dilated capillaries with normal intervening brain structures. Capillary telangiectasias are incidental lesions usually found at autopsy or now more commonly are found during neuroradiographic evaluation of other CNS disorders.
They are most commonly found to appear as a bright signal on MRI scans of the brain which are performed with intravenous contrast (see below). Capillary telangiectasias may occur throughout the brain and central nervous system.
The vast majority are found however in the posterior fossa, particularly the pons. The capillary walls are histologically normal and hemosiderin staining of the adjacent brain around a capillary telangectasia is not present. These lesions do not hemorrhage and do not require treatment.
Most AVM patients are asymptomatic, with AVM discovered when receiving imaging for another condition. About 12 percent present with symptoms that can include some combination of:
- sudden and severe headaches or seizures.
- muscle weakness or paralysis
- numbness and tingling
- problems with vision, language use, coordination, or memory
Some of these problems result from the "steal effect," where a very large AVM with a high blood flow will steal blood from other areas of the brain, causing decreased function in those areas. Symptoms from steal effect are rare however.
It is important to plan an appropriate diagnostic workup, and plan to get the proper diagnostic studies that allow complete evaluation of each AVM to guide proper decision making regarding treatment. MRI may include functional imaging to determine if the AVM is near critical brain areas.
Below is a functional MRI obtained at the Swedish Neuroscience Institute showing an AVM and its relationship to functional brain containing speech function.
Treatments for AVM
Treatment plans are based on the AVM size, location and blood vessels involved as well as patient age and medical history.
The preferred treatments – using state-of-the-art technology in the hands of highly skilled physicians – are microsurgery, selective vessel AVM embolization, and radiosurgery.
Microsurgery involves special techniques to close the blood vessels, remove the AVM, and take away the risk of future bleeding. This is often a staged procedure and is done in conjunction with staged embolization with specially trained physicians and staff in interventional neuroradiology.
Other specialized treatment techniques including strereotactic navigation and C02 laser techniques are very useful in achieving optimal results for the surgical resection of cavernous malformations.
Radiosurgery involves delivery of high dose of radiation directly to the AVM. This causes the blood flow to slowly close off over a period of time, until it eventually stops. This treatment is often selected if AVMs are in deep locations in the brain, making surgery difficult, or they are too close to eloquent brain regions.
For referrals to the Cerebrovascular Center, call: 206-320-3470.