6 Vascular Malformations and Other Vascular Lesions
6 Vascular Malformations and Other Vascular Lesions
An arteriovenous malformation (AVM) consists of a nidus (tangle) of tightly packed dilated, tortuous arteries and veins, without an intervening capillary network, with the result being arteriovenous shunting. It is the most common symptomatic vascular malformation of brain. The risk of hemorrhage is 2 to 4% per year, with each episode having a 30% risk of death. Most lesions present clinically between 20 and 40 years of age and involve peripheral branches of the ACA or MCA. Aneurysms of the feeding arteries (perinidal aneurysms), due to high flow, are seen in less than 10% of cases. AVMs are considered to be congenital in origin; they are one-tenth as common as aneurysms. Hemodynamically, AVMs have high flow and low resistance.
The nidus of an AVM may be compact or somewhat diffuse. They are often pyramidal in shape, with their base along a cortical surface and their apex directed toward a ventricle (Fig. 6.1). Although not common, there may be hemosiderin staining in the adjacent brain parenchyma due to previous hemorrhage. A total of 65% involve the cerebral hemispheres, 15% the deep midline structures, and 20% the posterior fossa. Most are sporadic in occurrence. Intracranial AVMs occur in about 10% of cases of hereditary hemorrhagic telangiectasia (Rendu-Osler-Weber syndrome).
AVMs are well depicted on conventional, crosssectional MR imaging (due to flow phenomena), with TOF MRA used to better demonstrate the nidus, enlarged arterial feeding vessels, and enlarged draining veins. On occasion, a small AVM will be visualized only on MR angiography and not well seen on other MR sequences. On precontrast conventional MR scans, multiple serpiginous vessels, most with low SI due to rapid flow, are typically visualized. Contrast enhancement often provides improved visualization of the nidus, together with the enlarged draining veins (Fig. 6.2; Fig. 6.3).
Between the large draining veins, there will be preserved normal brain parenchyma. Gliosis is uncommon. There is usually little mass effect, with vasogenic edema unusual. Acute hemorrhage is well visualized on unenhanced CT; however, on such scans, even large AVMs may not be detected. Calcification is seen in the minority of cases. Enhancement on CT (together with CTA) provides visualization of the nidus and large draining veins (Fig. 6.4). DSA remains the gold standard for evaluation of an AVM, with one major advantage being the clarification of feeding vessels and draining veins (Fig. 6.5). For example, for a convexity lesion, contributions from the ACA and MCA can be distinguished. This can also be done currently by MR, but remains a topic for further research and development.
The risk of hemorrhage from an AVM, from the literature, is 2 to 4% per year. The risk of re-bleeding is increased for several years following a prior hemorrhage. Hemorrhage is the most common presenting symptom (seen in half of all cases) (Fig. 6.6; Fig. 6.7, Parts 1 and 2), followed by seizures (seen in one-quarter).
Treatment includes surgery, radiosurgery, and embolization. Asymptomatic lesions, difficult to treat lesions, and patients at high risk for complications warrant conservative treatment. Lesions are stratified according to surgical risk by the Spetzler-Martin grading system, which assigns points relative to size, location, and venous drainage. Lower grade lesions have lower permanent morbidity and mortality following surgery (for example, with permanent morbidity < 5% and mortality < 4% in Spetzler-Martin grades I-III). Surgery can be delayed following hemorrhage, given that AVMs do not have the high, immediate risk of rehemorrhage that aneurysms do. Either intraoperative or postoperative DSA should always be performed to confirm complete obliteration of the lesion. Although uncommon, cerebral edema can occur after surgery and can also be seen with embolization. Surgery may carry a higher cure rate and a lower rehemorrhage risk when compared to radiosurgery.
Radiosurgery delivers a high radiation dose to the isocenter, with a substantially lower dose to nontargeted structures. Current treatment systems include the Gamma Knife and linear accelerator platforms (e.g., the X-Knife and the CyberKnife). Radiotherapy causes endothelial damage, leading eventually to stenosis of the vessels in the treated area and subsequent occlusion. This approach is minimally invasive, low risk (but specifically not free of complications, with permanent neurologic deficits seen in 5%), and effective for smaller lesions (≤ 3 cm). Its disadvantage is that obliteration is delayed, occurring over 2 to 3 years following treatment.
Embolization can be performed for palliation (treatment of part of the lesion) or prior to surgery (Fig. 6.8, Parts 1 and 2). The latter is performed to reduce the volume of the nidus and to occlude feeders that might be difficult to reach by surgery. Cure rates (complete obliteration) are low for treatment of AVMs with embolization alone (5–10%) (Fig. 6.9). This is likely due to the fact that few AVMs have a single pedicle, or just a few pedicles, that can be safely embolized. Embolization materials include polyvinyl alcohol, N-butyl2-cyanoacrylate, and Onyx.
Associated aneurysms are found in less than 10% of AVMs. These may involve feeding arteries (perinidal) (Fig. 6.5) or be intranidal, with the latter distinguished from venous varices on DSA by their visualization in the arterial phase. The risk of hemorrhage from an AVM is increased by the presence of an associated aneurysm.
Large (Spetzler-Martin grade IV-V), as well as giant (> 6 cm), AVMs are difficult to treat. Morbidity and mortality with surgery are high. Partial treatment appears not to reduce the risk of hemorrhage; thus, any treatment should be aimed at eventual complete obliteration.
Cerebral proliferative angiopathy (diffuse cerebral angiomatosis) is a rare vascular malformation with several distinguishing characteristics from a classic AVM. These include large size (lobar or hemispheric), absence of dominant feeders and large (and/or early) draining veins, additional meningeal artery involvement, and normal brain intermingled between vessels.