The natural history of unruptured AVMs has been extensively studied. One group followed 168 patients for an average of eight years and reported an annual hemorrhage rate of 2.2 percent, with an average annual morbidity and mortality of 2.8 and 0.7 percent, respectively. Another study reported a somewhat higher rate of bleeding (4 percent per year), with similar rates of major morbidity and mortality

The rate of rebleeding is higher during the first year after the initial hemorrhage (6 to 7 percent) and then remains relatively constant. Otherwise, the risk of hemorrhage does not appear to depend upon the manner of presentation

DIAGNOSIS — Arteriovenous malformations (AVMs) are typically first discovered on cross-sectional imaging (CT or MRI), although angiography is required to plan therapy. A combination of MRI and angiography also is often used to assess the success of surgical, endovascular, or radiosurgical therapy.

Computed tomography — Flow voids may be identified on CT with contrast administration in and around the region of the nidus of the AVM. CT characteristically demonstrates intraparenchymal hemorrhage without significant edema in patients who present with hemorrhage. However, compression of the nidus by the hematoma often precludes CT diagnosis of AVM in the setting of acute intracerebral hemorrhage; more sensitive techniques such as MRI or angiography are required in such cases. The sensitivity of CT to identify AVMs in the acute setting may be improved by CT angiography.

Magnetic resonance imaging — MRI is very sensitive for delineating the location of the nidus and often an associated draining vein. It also has unique sensitivity in demonstrating remote bleeding related to these lesions. Dark flow voids are appreciated on T1 and T2-weighted studies

MRI also is particularly valuable in following patients after treatment. After radiosurgery, for example, the regression of the nidal volume can be accurately measured and followed. In addition, surrounding tissue that is subjected to the radiation field can be accurately monitored for post-therapy edema or, in some cases, radiation necrosis.

Angiography — Angiography is the gold standard for the diagnosis, treatment planning, and follow-up after treatment of AVMs Anatomical and physiological information such as the nidus configuration, its relationship to surrounding vessels, and localization of the draining or efferent portion of the AVM are readily obtained with this technique. Contrast transit times provide additional useful information regarding the flow state of the lesion; this is critical for endovascular treatment planning.

TREATMENT — A number of variables are factored into the decision to treat cerebral arteriovenous malformations (AVMs) and the therapy chosen. Surgery is the mainstay of treatment; radiosurgery is a useful option in lesions deemed at high risk for surgical therapy, and endovascular embolization has become a useful adjunct to these techniques.

Who should be treated? — Age is probably the most important factor in the decision to treat AVMs since the annual risk of hemorrhage and associated morbidity and mortality statistics are weighed against life expectancy. Thus, children and young adults are usually recommended for therapy, while older individuals with shorter life expectancies may be managed more conservatively.

Another important factor in recommending therapy is an assessment of surgical risk. One group used anatomic considerations to construct a classification system (now referred to as the Spetzler-Martin grading scale) that takes into account the lesion size, location, and venous drainage pattern Patients are graded in accordance with these variables; outcomes have consistently been directly related to these factors. High-grade lesions are those of larger size in eloquent brain areas (eg, areas of critical cortical function, deep nuclei, brain stem) with deep venous drainage patterns that make them the most difficult to safely resect. Female gender may also increase surgical risk

A simple formula has been devised that considers both age and the risk of morbidity and mortality of treatment versus natural history:

Evidence concerning the hemorrhage risk during the latency period between treatment and obliteration is conflicting. This is an important consideration, particularly for lesions with an aggressive clinical course, such as those presenting with a previous hemorrhage and afferent pedicle or intranidal aneurysms. There are no randomized controlled trials of radiosurgery for AVMs, and none are likely to be performed as the benefit of radiosurgery for radiographic obliteration of AVMs is widely accepted. Existing studies are generally limited by retrospective and observational design.

   Earlier studies had suggested that the bleeding rate during the latency period compared with an untreated AVM was unchanged, was decreased or was increased. Most of these studies analyzed the risk of hemorrhage by comparing selected patients who had radiosurgery with those who did not have radiosurgery.

   Stronger evidence supporting a reduced AVM hemorrhage rate in the latency period comes from a subsequent study that analyzed changes in the rate of hemorrhage relative to the timing of radiosurgery in a cohort of 500 consecutive patients. The risk of hemorrhage declined by 54 percent during the latency period (hazard ratio (HR) 0.46, 95% CI 0.26-0.80) and by 88 percent after obliteration (HR 0.12, 95% CI 0.05-0.29). The risk reduction was greater among patients who presented with hemorrhage than those who presented without hemorrhage.

  Complications — Both the success rate and incidence of complications are related to the AVM location and the volume treated. Thalamic, basal ganglionic, and brainstem locations are particularly prone to development of deficits after radiosurgery. The risk of complications is also related to the radiation dose directed to the surrounding tissue.

The risk of radiation necrosis with permanent neurologic deficit is 2 to 3 percent in most reports. In a multinational study that included 1255 patients undergoing radiosurgery for cerebral AVMs, therapy-related complications developed in 102 (8 percent) and included radiographic parenchymal lesions, cranial nerve deficits, seizures, headaches, and cyst formation. Symptoms were disabling in 21, fatal in two, and resolved completely in 42 (41 percent). In a second series that included 227 patients, only two patients developed treatment-related permanent new neurologic deficits.

The increased risk of complications in large AVMs that require larger treatment volumes was illustrated in a series of 73 patients, in whom one-half of the AVMs were >3 cm in diameter. The incidence of post-treatment imaging abnormalities and clinical complications rose with increasing treatment volume. In patients whose treatment volumes were >14 mL and who received a dose 16 Gy, the incidence of post-treatment MRI abnormalities was 72 percent, and 22 percent required resection for radiation necrosis. The rate of post-treatment hemorrhage was also higher for treatment volumes 14 mL (7.5 versus 2.7 percent per person-year).

In contrast to standard fractionation cranial irradiation, radiosurgery does not appear to impact on cognitive function. One study of 10 patients found no effect of radiosurgical treatment of AVMs upon neuropsychological performance 11 months after treatment.

Endovascular techniques — Despite initial optimism that embolic agents such as microparticles and cyanoacrylates could cure AVMs, less than 5 percent of lesions are cured by this approach alone. In addition, flow studies have demonstrated that partial embolization may increase the pedicle pressure and the risk of hemorrhage

Nevertheless, embolization is an effective adjunct to radiosurgery and surgery. Embolization prior to radiosurgery is employed to reduce the nidus size of large AVMs to less than 10 cm3, as these large AVMs have a lower cure rate with radiosurgery alone. Embolization prior to surgery is employed to reduce blood loss and to occlude vessels that may be difficult to control during surgery. Furthermore, endovascular treatment appears to be associated with a relatively low risk of disabling treatment complications

A meticulous analysis of angiographic information determines the suitability for embolization. Generally, only afferent pedicles to the nidus are embolized in an attempt to avoid occlusion of branches irrigating normal brain.