Vascular Malformations

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Central nervous system (CNS) vascular malformations are grouped into 4 categories: arteriovenous malformations (AVM), cavernous angiomas (or, cavernous malformations), capillary telangiectasias and developmental venous anomalies (DVA). AVMs are the most important to recognize because of their propensity to hemorrhage. Children can also present with headaches, seizures, hydrocephalus or progressive neurological deficits. AVMs are congenital vascular malformations in which abnormally dilated arteries and veins are directly connected to each other, bypassing any intervening capillaries. As a consequence, there is rapid arteriovenous shunting, which can lead to a vascular "steal" phenomenon and chronic hypoperfusion of adjacent brain parenchyma. Conventional cerebral angiography is the modality of choice for initial evaluation of AVMs. Scans must also be scrutinized for associated aneurysms and evidence of stenoses involving the draining veins since these features increase the risk of hemorrhage. On CT and MRI, AVMs appear as a tangle of enhancing, enlarged vessels (Fig. 7). Hemorrhage may be present. Volume loss occurs in any previously injured adjacent brain parenchyma, which will be hypodense and T2 hyperintense. Newer techniques such as MR and CT angiography are noninvasive methods used to follow vascular malformations.

Vein of Galen malformations are an unusual subset of AVMs in which direct arteriovenous connections exist between the vertebrobasilar system and the vein of Galen. They can be divided into "choroidal" (~90%) and "mural" (~10%) subtypes. Choroidal malformations demonstrate numerous small arteriovenous connections and significant shunting, which frequently leads to neonatal congestive heart failure and a poorer prognosis. In contrast, mural malformations have much fewer but larger arteriovenous conduits, and patients present later in infancy with hydrocephalus, seizures or hemorrhage. The imaging appearance is characteristic: large, enhancing, dilated vessels along the posterior midline centered in the region of the vein of Galen and straight sinus. Thrombus within the dilated vascular structures may also be present. Adjacent areas of brain injury can appear atrophic and have dystrophic calcifications. Neonatal head ultrasound is a useful method for demonstrating the enlarged vessels and arteriovenous shunting associated with these malformations.

Figure 7. Tangle of hypointense serpentine flow voids along the paramedian posterior left frontoparietal region, which enhance with contrast. (left to right: T1-WI, T2-WI, T1-WI with contrast)

Cavernous angiomas contain dilated sinusoidal capillaries without intervening normal brain parenchyma. They are well-delineated, lobulated, hyperdense, mildly enhancing lesions that have heterogeneous central T1 and T2 signal but a classic rim of T2 hypointensity representing hemosiderin from prior hemorrhages (Fig. 8). They are rare causes of seizures and hemorrhage. Cavernous angiomas are not infrequently seen in association with developmental venous anomalies and capillary telangiectasias, suggesting that these three entities represent a spectrum of lesions possibly caused by impaired outflow of DVAs. Developmental venous anomalies are felt to be normal variants of venous drainage. In isolation, they are rarely symptomatic, and are usually incidentally discovered on contrast-enhanced CT and MR studies. They appear as a "spider-like" collection of small enhancing vessels that drain into a larger vein that feeds a venous sinus. Capillary telangiectasias are composed of dilated capillaries separated by normal brain tissue. They are most commonly detected in the pons as subtle small areas of ill-defined enhancement and T2 hypointensity on MRI. They are very uncommon causes of hemorrhage.

Stroke

Stroke occurs rarely in children and can have numerous causes such as emboli from a cardiac source (e.g., congenital right-to-left shunts), arterial dissections, hy-percoagulable states, meningitis, venous sinus thrombosis and moyamoya disease. The exact origin of most pediatric strokes is never found. Arterial dissections are characterized by post-traumatic or spontaneous development of an intimal cleft that

allows blood to dissect into the arterial wall, creating a pseudoaneurysm. The false lumen associated with a dissection can expand and cause narrowing of the true vessel lumen, and serve as a source of emboli. The most frequent sites of dissection involve the distal cervical segments of the internal carotid and verterbral arteries in the upper neck just below the skull base. Intracranial dissections are more uncommon. Conventional angiography is considered the most sensitive technique for detecting the intimal irregularities, pseudoaneurysms, and stenoses associated with arterial dissections. However, a Tl-weighted transaxial MR sequence with fat saturation through the skull base and neck is usually the modality of choice because of its relative convenience and high sensitivity in detecting blood within the crescentic false lumen lining the injured artery.

Venous infarcts are a consequence of thrombosis of dural venous sinuses, deep or cortical veins. They occur in the setting of dehydration or other causes of hyper-coagulability, and as a complication of meningitis. Venous infarcts appear as ill-defined areas of edema, and approximately 25% have concomitant hemorrhage. Thrombosis of the superior sagittal sinus (SSS) leads to infarcts along the paramed-ian frontal or parietal lobes, and occlusion of the deep venous system leads to infarcts involving the thalami. In the acute setting, a thrombus within the vein can appear hyperdense on nonenhanced CT. The classic "empty-delta" sign is seen on contrast-enhanced CT studies when a central clot within the SSS appears as relatively hypodense to the contrast-containing blood flowing around it. Subacute thrombi will also appear as Tl-hyperintense material within and occasionally expanding the venous sinus. MR venography is usually very helpful in delineating narrowing or occlusion of the involved venous structure and should always be performed if possible. It should be noted that venous infarcts have a more variable appearance on diffusion-weighted imaging and may not always demonstrate a net decrease in diffusion as seen in acute arterial infarcts.

Moyamoya disease results in progressive bilateral or unilateral narrowing and occlusion of the supraclinoid internal carotid arteries and their proximal branches (Fig. 9). There is compensatory enlargement of collateral perforating vessels, most commonly the lenticulostriate arteries. CT and MR studies will reveal acute infarcts and/ or encephalomalacia related to remote is-chemic injuries. Prominent signal voids can often be seen in the bilateral basal ganglia and reflect hypertrophied arterial collaterals. These vessels are best seen on conventional angiography, which will also reveal stenosis of the supraclinoid arteries, and occasional associated aneurysms and arteriovenous malformations. Pedi-atric patients typically present with recurrent headaches, transient ischemic attacks or strokes. Moyamoya syndrome is associated with many conditions, including k k

Figure 9. Moya-moya. Marked narrowing of the bilateral supraclinoid internal carotid arteries and proximal middle and anterior cerebral arteries. (3D time-of-flight MR angiography)

sickle-cell disease, neurofibromatosis type 1, Down syndrome and tuberculous meningitis and can also occur after radiation theapy. If no such cause can be found, the child is given the diagnosis of moyamoya disease.

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