Arteriovenous MalformationArteriovenous malformations (AVMs) are defects of the circulatory system that are generally believed to arise during embryonic or fetal development or soon after birth. Although AVMs can develop in many different sites, those located in the brain or spinal cord can have especially widespread effects on the body. Most people with neurological AVMs experience few, if any, significant symptoms
The vessels of an AVM are abnormal and so may leak or rupture (hemorrhage; that's the main problem; see 5. below). The blood flow and pressure in especially the larger vessels of an AVM are unusually high and may lead to significant shunting of blood to and from the lesion. Higher flow-pressures in addition to abnormal AVM vessel wall structure can lead to the formation of aneurysms on arteries feeding the AVM (i.e., "parent artery" or "pedicle" aneurysms) or within the AVM itself (i.e., "intranidal" aneurysms).
These can also rupture. Somewhere around 6-7% of brain AVMs have aneurysms associated with them, and when AVMs rupture, some think it may be these aneurysms which have ruptured (although the abnormal nonaneurysmal components of the AVM can rupture too). About 75% of the aneurysms associated with AVMs are found on arteries feeding the AVM (pedicle aneurysms), while 25% of aneurysms associated with AVMs are found within the core (nidus) of the AVM (intranidal aneurysms). Interestingly, with good treatment of the AVM, pedicle aneurysms can fade away or disappear entirely.
It should be noted that brain AVMs intertwine with the true brain tissue (parenchyma), but the nidus (tight central part of the AVM) usually has little or "sparse" brain tissue, and whatever brain tissue may be present in this part of the AVM is typically scarred (gliotic) and nonfunctional. AVMs tend to be surrounded by a rim of scar (gliosis) that is frequently helpful to surgeons removing these lesions.
Symptoms that may occur with an AVM that has not bled include: -
- Decreased sensation in any part of the body
- In one area (localized) or general
- Resembles a migraine headache in some cases
- Muscle weakness, any part of the body
- Generalized tonic-clonic seizure
- Partial (focal) seizure
- Vision changes
- Blurred vision
- Decreased vision
- Double vision
Additional symptoms : -
- Decreased consciousness
- Dysfunctional movement
- Ear noise/buzzing
- Eyelid drooping
- Facial paralysis
- Impaired smell
- Speech impairment
There are often no symptoms until the AVM ruptures. This results in sudden bleeding in the brain (hemorrhagic stroke). In more than half of patients with AVM, hemorrhage is the first symptom. Symptoms of AVM hemorrhage are the same as those of other intracerebral hemorrhage.
Signs And Tests
A complete physical examination and neurologic examination are needed, but they may be completely normal.
Tests that may be used to diagnose an AVM include: -
- Cerebral angiogram
- Cranial MRI
- Electroencephalogram (EEG)
- Head CT scan
- Magnetic resonance angiography (MRA)
Brain Arteriovenous Malformation
shows the the surface of the brain with an AVM originating there. Note the large and tortuous (bendy) feeding arteries (red) and draining veins (blue), which may also be deeper in the substance of the AVM. The nidus of the AVM is deeper, and the AVM usually forms a cone-shaped mass that extends from the (pial/cortical) surface of the brain down towards one of the fluid-filled cavities of the brain (ventricle).
Note that an AVM is not the same thing as a dural arteriovenous fistula (DAVF), even though people and literature sometimes fail to make the distinction between these two very different entities. DAVFs can occur in the brain (intracranial DAVF) or in the spinal canal (spinal DAVF), or at the junction between the skull and spinal column (craniocervical junction DAVF). A DAVF is an abnormal connection between an artery (usually one, but sometimes multiple) and a vein (frequently one big arterialized draining vein, but may be multiple), with the key differences between the two types of entities ("lesions" or "anomalies") being that the DAVF: (i) is intimately associated with the leathery covering (dura) of the brain or spinal cord/spinal nerve root; (ii) has no nidus and therefore pathologically is not made up of a tangle of hybrid vessels (despite possibly having abnormal structure/architecture to some of the vessels of the DAVF); (iii) has a small web of capillaries interposed between the arterial supply and venous drainage sides; and (iv) is (in the brain) frequently associated with a blockage to a major draining venous pathway (i.e., a venous sinus occlusion/stenosis) and therefore (in the brain) is typically an abnormality that is acquired rather than the person being born with it (congenital). Spinal DAVF are thought to be congenital, and not typically associated with neighboring blocked veins. A ruptured DAVF is worth treating early to prevent further neurological impairment from a rehemorrhage.
How Common Is An Arteriovenous Malformation?
AVMs are relatively rare lesions, much rarer than brain aneurysms. Depending on what you read, the population prevalence of AVMs [i.e., what percentage are present (in 100% of) the population at any one timepoint] is probably somewhere around 0.2% (i.e., 1 in 500 persons), i.e., 5-25 times less than the prevalence of brain aneurysms (which is somewhere between 1-5% by most reports). DAVFs are extremely rare lesions, much rarer than AVMs.
Why Does An Arteriovenous Malformation Develop?
Unlike brain aneurysms, there are no well established risk factors for AVM formation, growth and rupture. They are regarded as "developmental" or "congenital" vascular anomalies, i.e. you're born with them and they typically increase in size as the brain enlarges. There are rare instances of persons with multiple AVMs (Wyburn-Mason syndrome, which involves multiple central nervous system AVMs, including in the eye's retina) said to be nonhereditary, but probably due to some yet-unknown genetic event).
What Causes AVM?
We don't know what causes most AVMs. People are born with AVMs although they do not appear to be hereditary. AVMs occur about equally in both sexes and in different races. An estimation of 3 million people in the United States are born with vascular malformation, 10% of which are AVMs. AVMs may be caused by a rupture or clotting of a blood vessel during fetal development. It is usually not associated with any other problems at birth.
What Are The Risks Of Avms?
Most patients do not know that they have an AVM. A number of the patients with AVMs have seizures or persistent headaches. An AVM can put additional strain on the blood vessels and the surrounding tissues. For the very young (under the age of twenty) this is usually not a problem. The increased flow of blood caused by the shunt weakens the blood vessels. These weakened blood vessels can rupture.
How Is The Diagnosis Made?
AVMs can be seen on outpatient imaging studies such as CT's or MRI's. A CT, or computerized axial tomography scan, uses x-rays to create cross-sectional images of the body. An MRI or magnetic resonance imaging scan uses magnetism and radio-waves to create a detailed picture of the body. Patients will be put on a bed surrounded by a tunnel-like magnet. Unlike x-rays, there are no known side effects of MRI.
Angiograms are inpatient procedures needed to image the arteries and veins before any treatment. An angiogram is an x-ray movie of the blood flowing through the blood vessels. It is made by injecting contrast into the arteries going into the head and taking a series of x-rays films.
TreatmentA bleeding AVM is a medical emergency. The goal of treatment is to prevent further complications by controlling bleeding and seizures and, if possible, removing the AVM.
Treatments include: -
- Open brain surgery
- Endovascular treatment
Some treatment options are used together.
Open brain surgery removes the malformation through an opening made in the skull. It must be done by a highly skilled neurosurgeon.
Embolization (endovascular treatment) is the injection of a glue-like substance into the abnormal vessels to stop blood flow in the AVM and reduce the risk of bleeding. This may be the first choice for some kinds of AVMs, or if surgery cannot be done.
Stereotactic radiosurgery is another alternative. This procedure delivers radiation directly to the area of the AVM to cause scarring and shrinkinge. It can control bleeding and other symptoms. It is particularly useful for small deep AVMs, which are difficult to remove by surgery.
Anti-convulsant medications, such as phenytoin, are usually prescribed if seizures occur.
- Focal weakness
- Intracerebral hemorrhage
- Language difficulties
- Numbness of any part of the face or body
- Permanent neurologic changes such as paralysis or sensory problems
- Persistent headache
- Subarachnoid hemorrhage
- Vision changes
- Water on the brain (hydrocephalus)
Possible complications of open brain surgery include: -
- Brain swelling
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