Description and Causes
AVMs are compact collections of abnormal, thin-walled vessels (most commonly called the nidus, this is the true malformation) that connect dilated arteries to veins without an intervening capillary network. These persistent primitive connections between arteries and veins are formed during the late somite stages of the fourth embryonic week; they develop from the sinusoidal vascular network that perfuses the early developing telencephalon. The absence of capillaries produces a low resistance shunt, which results in rapid arteriovenous shunting within the malformation.
Patients with cerebral AVMs most commonly present as a result of spontaneous intracerebral hemorrhage (up to 61% of cases) during late childhood (43
) or as a young adult (44
), but affected patients can present at any age or, rarely, with seizures (29
). Intraparenchymal hemorrhage can cause significant injury at any age, with 30% to 50% neurological morbidity and 10% mortality (47
). Consequently, a premium has been placed upon identification and treatment of AVMs at major neurosurgical centers throughout the world.
The prevalence (total number of cases in a population at a given time) of AVMs has been estimated in autopsy series between 0.06% and 0.11% (44
AVMs were detected in 1 per 100,000 person-years, and prevalence was inferred from the incidence data to be less than 10 per 100,000 in a US population study (49
) and 5 per 100,000 in Australia (51
). Thus, although the precise numbers are not known, it seems that a significant percentage of AVMs become symptomatic during the average human lifetime, usually due to hemorrhage (44
The vascular channels of AVMs that develop instead of normal capillaries are characterized by fibrointimal thickening and elastic tissue disorganization. These features probably take time to develop, and thus, classic brain AVMs (those with the classic tangle of vessels that compose the nidus) are not typically seen before birth or in early infancy. Flow-related angioarchitectural features in pediatric AVMs also appear to take time to develop postnatally: there is a paucity of feeding artery aneurysms and venous outflow stenosis in young children with AVMs (Fig. 12-1
) as compared to older children and adults (39
). No classic AVMs have been described prenatally, although several fetal cases with NGAVFs (direct connections between artery and vein without the tangle of abnormal vessels) have been identified (47
). In neonates, AVMs are generally seen as direct arteriovenous communication without intervening nidus. Thus, AVMs seem to evolve, not becoming fully developed until later in life (53
). De novo development of AVMs has rarely been documented in children or adults (54
). Although most brain AVMs are thought to be congenital, a growing number of case reports of de novo brain AVMs has challenged this view in the past decade (55
). In these apparent de novo
cases in children and adults, there may well have been microscopic arteriovenous abnormalities below the detection threshold of MRI since early in development, but only later in life did the abnormality enlarge enough to be seen on noninvasive imaging.
No definite genetic linkage has been identified with the exception of Rendu-Osler-Weber (58
) and Wyburn-Mason syndromes (59
). Dysautoregulation of vascular growth factors has been implicated in the subsequent development and maturation of the vascular malformation (60
AVMs often increase in size with age, as a result of increase in size of the nidus and progressive dilation of the feeding arteries and draining veins. Astrogliosis and atrophy are often seen in surrounding brain tissues. This was at one time thought to be a consequence of low resistance shunting of blood through the nidus, causing parenchymal hypoperfusion (vascular steal). Others suggest that edema and subsequent parenchymal injury results from venous stenosis/venous hypertension, caused by high pressure and shear stresses from turbulent blood flow and consequent stenosis in draining veins. Rapid flow in the feeding arteries may result in the development of arterial aneurysms, both in the same distribution as classic berry aneurysms (circle of Willis) and in feeding arteries (pedicle and nidal aneurysms) close to the malformation (Fig. 12-1
). In addition, varices (venous aneurysms) can develop upstream to venous outflow stenosis and cause hemorrhage if they rupture (61
). As a result of all these factors, AVMs are the vascular malformations that most commonly cause neurological defects other than seizures, despite the fact that they are not the most common form of vascular malformations.
Patients with AVMs usually present with seizures, recurrent headaches, progressive neurological deficits, hydrocephalus, or hemorrhage. Approximately 20% become symptomatic before the age of 20 years (43
). The mortality associated with the initial rupture of an AVM is 10%, with morbidity between 30% and 50% (38
). Morbidity and mortality increase with each subsequent hemorrhage.
AVMs are the cause of up to 40% of spontaneous intracranial hemorrhages
), most of which occur in the cerebral parenchyma. However, superficial malformations can rupture into the subarachnoid space and deep malformations may cause intraventricular hemorrhage. Symptomatic vasospasm and rebleeding are relatively rare unless the source of hemorrhage is an associated aneurysm. The risk of rehemorrhage from an AVM has been estimated to be higher in children than adults (62
). In children less than 15 years of age, AVMs are the most common cause of spontaneous intracranial hemorrhage and account for 20% of all strokes (2
Figure 12-1 AVM with hemorrhage from a feeding artery aneurysm. A. Coronal reformatted CTA demonstrates dense vascular enhancement adjacent to the right lateral ventricular intraventricular hemorrhage. There is associated hydrocephalus. B. Axial T2-weighted image shows a circular structure within the lateral aspect of the intraventricular hemorrhage consistent with an aneurysm associated with a tangle of AVM nidus vessels in the corpus striatum just lateral to the right lateral ventricle. C. RAO image from a right internal carotid artery DSA shows the posterior corpus striatum AVM being fed by a medial lenticulostriate artery (recurrent artery of Heubner) that itself harbors the feeding artery aneurysm (white arrow) that hemorrhaged. D. The feeding artery aneurysm is more easily seen on a surface-rendered reformat of a 3D rotational angiogram of the right ICA.
occur in approximately 70% of patients with AVMs, with one-half of the seizures being generalized (65
); most patients are well controlled with anticonvulsants. Seizures are most common in AVMs that are located in the cerebral cortex and have associated varices. Their cause is probably cortical injury and gliosis from previous hemorrhage (66
) or venous ischemia (67
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