Arteriovenous Malformations

Pathogenesis and Pathophysiology. Vascular malformations are congenital in origin. They are classified into several subtypes according to the predominant vasculature. The most common type is venous angiomas, which are composed of anomalous veins without any direct feeding artery. The next most common is telangiectasia, usually found deep within the brain, particularly in the brain stem. It is composed of vessels morphologically resembling capillaries but slightly larger and often found at necropsy. Another less common vascular abnormality, which also rarely causes symptoms, is the venous varix. Two other common symptomatic angiomas are AVMs and cavernous angiomas.^ , y , y AVMs are composed of clusters of abnormal arteries and veins of varying size, without intervening capillaries.

Epidemiology and Risk Factors. Vascular malformations are the second most common cause of nontraumatic SAH and ICH. Vascular malformations are one tenth as common as aneurysms; an estimated 1000 new cases are identified in the United States each year. They rupture most commonly during the second and third decades of life. Hemorrhage from malformation is most common during early pregnancy or delivery.

Clinical Features and Associated Disorders. As AVMs enlarge, symptoms are related to a number of mechanisms. They can cause bleeding, seizures, vascular headache, and chronic ischemia. Bleeding is most likely due to fragility of the abnormal vessels. The angiomas that most frequently rupture are of the AV type. Symptoms and signs depend on the location of hemorrhage. There are usually signs of meningeal irritation due to bleeding into the CSF. Not all ruptures are symptomatic but evidence of previous bleeding is often observed at necropsy. About one half of the patients present with epilepsy. Progressive neurological signs may develop secondary to a mechanism called intracerebral steal or compression of adjacent brain tissue by the pulsating blood vessels. Chronic migrainous headaches are also a frequent complaint in patients with vascular malformations. Patients with unruptured AVMs may present with increased ICP and papilledema. Angiomas in the brain stem may cause serious bleeding or progressive neurological deficits, which may be fluctuating in clinical course and may simulate multiple sclerosis. Rarely, angiomas, particularly aneurysms of the vein of Galen, present with hydrocephalus by disrupting the normal flow of CSF. Bruits may be audible either to the patient or to the examiner. If there is enough shunting through a large AVM, high-output congestive heart failure may develop, especially in children. In spinal AVMs, the patients may present with back pain, myelopathic symptoms, and root dysfunction. However, headache often accompanies spinal AVM rupture, mimicking aneurysmal SAH. A number of patients with spinal AVMs have intracranial symptoms, including headache, mental status changes, loss of consciousness, papilledema, decreased vision, nystagmus, diplopia, seizures, sixth nerve palsy, and oculomotor paresis. [1|

Some cavernous angiomas are familial. Patients with hereditary hemorrhagic telangiectasias (Osler-Weber-Rendu syndrome) have a higher than normal incidence of vascular malformations.

Differential Diagnosis and Evaluation. Diagnostic considerations include aneurysms and brain infarcts and tumors. Diagnosis of AVMs can be suspected clinically when young patients present with intracerebral hemorrhage, seizures, or frequent unilateral headaches. CT scan, MRI, mRa, and transcranial Doppler ultrasonography are helpful noninvasive tests. However, a confirmative diagnosis is made using angiography, through which therapeutic embolization can sometimes be performed at the same time. On angiography, a typical AVM shows large feeding arteries; a central tangle of vessels; enlarged, tortuous draining veins; and rapid arterial-to-venous shunting of blood.

Management. Direct surgical excision of AVMs has been improved with the use of the operating microscope and often can be carried out with low rates of morbidity and mortality. The major complications of surgical excision are loss of normal brain tissue, with additional loss of neurological function, and the so-called breakthrough phenomenon. This term describes massive brain swelling and

ICH occurring postoperatively, which is caused by redirection of the large volume of blood into small vessels that are unable to handle the large volume of blood that previously flowed into the AVM.y

Endovascular treatment of AVMs using embolization techniques can be used alone, before surgery, or at the time of surgery. It is particularly useful in treating lesions that are not surgically accessible and as an adjunct to surgical removal. y Complications include hemorrhage, ischemic stroke, and angionecrosis due to toxicity of the embolic materials.

Radiotherapy of AVMs with high energy x-rays, gamma rays, and protons induces subendothelial deposition of collagen and hyaline substances, which narrow the lumen of small vessels and shrink the nidus of the malformations by progressive occlusion of vessels during the months after treatment. Recent techniques focus the radiation beam on small regions. An example is the so-called gamma knife, a system that uses a cobalt source to generate highly collimated gamma rays that converge on a focal point. Modified linear accelerators can now deliver radiation to a defined volume of tissue with very good accuracy. Complications include radionecrosis of normal brain, bleeding, hydrocephalus, immediate post-therapy seizures, loss of body temperature regulation, and possibly long-term cognitive function deficits. y

Forms of medical management include strict control of blood pressure and avoidance of anticoagulants and antiplatelet drugs. Because pregnancy increases risk of bleeding, appropriate contraception may be recommended in fertile women with an AVM.

Prognosis and Future Perspectives. In the short term, the prognosis of ruptured AVMs and cavernous angiomas is better than that of aneurysms. The rebleeding rate is low during the first few months. Only 6 percent of patients rebleed during the first year, and vasospasm occurs only rarely. Mortality from the first hemorrhage is low, only about 10 percent. However, in the long term, the prognosis of AVMs is not as good. With subsequent hemorrhages, the mortality rate is higher, about 20 percent. With each recurrence, the chances of additional bleeding increase.

Improved noninvasive diagnosis using MRI and MRA and improved methods of treatment (endovascular and radiotherapy) promise to help reduce the rates of morbidity and mortality of this serious vascular condition.

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