Arteriovenous malformation (AVM) is a vascular lesion that is a tangle of vessels of varying sizes in which there is one or more direct connections between the arterial and venous circulations. In the lesion, there is no capillary bed, which is part of normal tissue. Brain AVMs are often presumed to be congenital, but there is no direct evidence that they form in utero. The distribution of age at detection for brain AVMs is normally-distributed with the mean age in the mid-30s. Although a small number of AVMs manifest themselves at or shortly after birth, most of them present later in life, and just as likely, form and progress during the later years of life. The lack of capillaries allows blood traveling through the abnormal fistulous connections to flow rapidly. The low resistance of the direct A-V connections, termed fistulas, results in very high flow rates in the vessels leading to and within the AVM. These high flow rates can lower the pressure in the arteries leading to the AVM and to surrounding relatively normal brain tissue. Further, because of the direct A-V connections, the pressure in the arteries, even if somewhat reduced, is transmitted to the veins draining the AVM and surrounding brain, which normally operate at very low pressures. AVM can occur in many different parts of the body, but those located in the central nervous system (brain and spinal cord) can cause problems that affect the brain like other forms of stroke.
Causes
AVM is often attributed as to result of an error in embryonic or fetal development, but there is no direct evidence of this assertion. No environmental risk factors have been identified for neurological AVM. AVM does not usually run in families, but somewhere on the order of 5% of AVMs may be due to autosomal dominant inheritance of a genetic mutation, most commonly hereditary hemorrhagic telangiectasia or the capillary malformation-AVM syndrome. AVM can rarely be associated with certain syndromes such as Wyburn-Mason syndrome.
Not much is known about the etiology of brain AVMs. The cause of brain AVMs is yet unknown, however, it is possibly multifactorial; apparently, both genetic mutation and angiogenic stimulation (the physiological process of formation of new blood vessels from pre-existing vessels) play roles in AVM development. Some believe that AVMs develop in utero. While others advocate an angiopathic reaction, following either a cerebral ischemic or hemorrhagic event (subtypes of stroke) as a primary factor in their development.[rx][rx][rx][rx]
AVMs can occur in various parts of the body:
- brain (cerebral AV malformation)
- spleen
- lung
- kidney
- spinal cord
- liver
- intercostal space
- iris
- spermatic cord
- extremities – arm, shoulder, etc.
AVMs may occur in isolation or as a part of another disease (for example, Von Hippel-Lindau disease or hereditary hemorrhagic telangiectasia). AVMs have been shown to be associated with aortic stenosis.[rx]
Diagnosis
A variety of imaging studies are used to diagnose AVM. Noninvasive imaging technologies that are used to diagnose AVM are computed axial tomography (CT) and magnetic resonance imaging (MRI). CT is particularly useful in identifying a hemorrhage but can identify only large AVM. MRI is necessary for the initial diagnosis of AVM. Magnetic resonance angiography (MRA) can be used to determine the pattern and speed of blood flow through AVM but can miss small lesions.
There is no intervening capillary bed, and the feeding arteries drain directly into the draining veins by one or multiple fistulae. These arteries lack the normal muscularis layer and the draining veins often appear dilated due to the shunted high-velocity arterial blood flow entering through the fistulae.
History and Physical
AVMs tend to be clinically asymptomatic in 15% of cases until the presenting event occurs.
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Approximately 41 to 79 percent present with intracranial hemorrhage. AVMs are the second most common cause of intracranial bleed after cerebral aneurysms, responsible for 10 percent of all cases of subarachnoid hemorrhage. Children are more likely to present with hemorrhage than adults. Hemorrhages are usually intraparenchymal, but can primarily occur in the subarachnoid space. Symptoms due to hemorrhage include loss of consciousness, sudden and severe headache, nausea, vomiting as the coagulated blood makes its way down to be dissolved in the individual’s spinal fluid. Reported sequelae caused by local brain tissue damage on the bleed site are also possible, including seizure, hemiparesis, a loss of touch sensation on one side of the body, and deficits in language processing. Minor bleeding may be asymptomatic. Following the arrest of bleeding, most AVM victims recover symptomatically, as the damaged blood vessel repairs itself.
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Studies report seizure as a presenting disorder in 15 to 40% of patients. The risk of seizures increases with cortically-located, large, multiple, and superficial-draining AVMs. Seizures are typically focal, either simple or partial complex, but often show secondary generalization.
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The progressive neurological deficit may occur in 6 to 12% of patients over a few months to several years. A vascular steal syndrome has been hypothesized to cause this presentation, but in most cases, this is related to mass effect, hemorrhage, or seizure. These include seizure, hemiparesis, visual disturbances, loss of sensation in one-half of the body, and aphasia. Minor bleeding can occur with no noticeable symptoms.
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A headache – There are no specific headache features that associate with AVM, which may be incidental to the headaches.
Brain AVMs are typically first identified in cross-sectional imaging – computed tomogram (CT) or magnetic resonance imaging (MRI). A combination of MRI and angiography are often helpful to plan therapy and predict the likely success and associated risks of surgical, endovascular, or radiological therapy.
Computed tomography — On non-contrast CT the nidus is blood density and therefore usually somewhat hyperdense compared to the adjacent brain, enlarged draining veins, and calcification may be evident. Although many of them are large, however, no mass effect or edema is present unless they bleed. On postcontrast CT especially with CT angiogram, the diagnosis is easily derived with visible feeding arteries, nidus, and draining veins apparent in the so-called “bag of worm” appearance. The exact anatomy of feeding vessels and draining veins can usually be delineated with angiography. The sensitivity of CT to identify brain AVMs in the acute setting of hemorrhage is reduced owing to compression of the nidus by the hematoma so more sensitive techniques such as MRI or angiography are required.
Magnetic resonance imaging — MRI is very sensitive for plotting the location of the brain AVM nidus and often an associated draining vein or any distant bleeding event. Fast flow in a conglomerate of tangled blood vessels generates serpiginous and tubular flow voids seen on bothT1 and T2, however mostly evident on T2 weighted images. Complications like the previous hemorrhage, adjacent brain edema, and atrophy may be seen. After radiosurgery, MRI can evaluate the regression of the nidus volume, post-therapy edema as well as radiation necrosis in the radiation field.
Angiography — It remains the gold standard for diagnosis and treatment planning. Nidus configuration, its relationship, and drainage to surrounding vessels are precisely evaluated. The presence of an associated aneurysm suggests a higher risk for hemorrhage. Contrast transit time, which relates to the flow state of the lesion, can provide critical information for endovascular treatment planning.
The current gold standard for diagnosis is X-ray angiography. This test is usually performed by placing a small tube (catheter) in the femoral artery, a large artery in the groin. A contrast agent that highlights blood vessels is injected into the blood vessels that supply the brain, then X-rays can reveal the structure of blood vessels in and around the lesion. The results of the angiogram help to determine the most appropriate treatment. An angiogram is often needed for treatment planning.
Spetzler-Martin Grade (SMG) scale is commonly employed for the assessment of the risk of surgical morbidity and mortality with brain AVMs. It is a composite score of nidus size (<3 cm, 3-6 cm, >6 cm; 1-3 points), the eloquence of adjacent brain (1 point for lesions located in the brainstem, cerebellar peduncles, thalamus, hypothalamus, or language, sensorimotor, or primary visual cortex), and venous drainage (1 point if any or all of venous drainage is via deep veins, such as basal veins, internal cerebral veins, or precentral cerebellar veins). The higher the score, the higher the associated surgical morbidity and mortality risk.
Treatment
There is no specific medical therapy currently available, but this is an area of active research. There are some promising medical therapies for AVMs outside of the brain that might one day be adapted for use to treat brain AVMs. Medications can be used to control the headaches, pain or seizures associated with AVM. Surgery may or may not be recommended on a case-by-case basis based on the estimated risks and benefits. Surgery is often necessary because if an AVM is left untreated there is a risk for hemorrhage. Three types of surgery are used for AVM, either alone or in combination. Conventional surgery (microsurgical resection) to remove the AVM is appropriate if the lesion is located in an accessible area that does not involve critically important functional areas and is relatively small in size. Endovascular embolization is a surgical technique in which the AVM is blocked off so that blood can no longer flow through it. This technique is sometimes all that is necessary to cure the AVM but often it is used as a first step prior to other types of surgery. Most North American practitioners do not use embolization as a sole therapy. Radiosurgery is a procedure in which a high dose of radiation is focused on the AVM, which sets off a gradual process that eventually closes the vessels in the lesion. Patients are not protected from spontaneous bleeding during the period of several years that it takes for closure.
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