Radiographic Anatomy: CT/MRI/Angiography and Risks

Arterial feeder(s)

Identification of the number, location, and type of feeding arterial branches is important for determining optimal management. Arterial feeders are classified as pial (i.e., superficial/cortical) or perforator (i.e., deep) in type. While the vast majority of iAVMs are the pial type, AVMs in deep or ventricular locations will recruit arterial supply from the perforator (lenticulostriate/thalamoperforator branches) and/or choroidal (anterior, medial, and lateral posterior) arteries. Arterial feeders are often enlarged and tortuous, with the degree of dilatation dependent upon shunt volume. Flow-related angiopathy, ranging from simple dilatation through endothelial thickening to stenosis or even occlusion, may be present.

Complications associated with arterial feeders include arterial aneurysms, which are classified as intranidal (situated within the nidus; Fig. 3.2 ), flow-related (involving AVM feeding arteries in a location proximal to the nidus and therefore may regress upon AVM treatment), or unrelated (do not supply AVM and are likely coincidental). The presence of intranidal or feeder aneurysms is thought to indicate high AVM inflow and is considered a predictor of hemorrhage. Approximately 25% of iAVMs may have transdural arterial contributions; therefore a thorough evaluation of the dural vasculature should be included in the complete angiographic delineation of AVM arterial supply.

Nidus

The nidus of the iAVM is the conglomeration of abnormal tortuous vascular channels, often interspersed with nonfunctional gliotic tissue. As the nidus lacks a capillary bed, blood from supplying arteries is shunted directly into the draining vein(s) ( Figs. 3.1–3.3 ). Intranidal vascular components may range from well-differentiated arteries and veins to dysplastic vascular channels of varying diameters and wall thickening, histologically neither artery nor vein. Intranidal aneurysms have been reported to occur in up to 50% of iAVMs. Nidal compactness is an additional prognostic feature, included in the Lawton-Young grading scale ( Table 3.1 ). Compact iAVMs have distinct borders and contain tightly woven arteries that separate cleanly from adjacent parenchyma, whereas diffuse iAVMs have ragged margins and are intermixed with parenchyma.

The presence and compactness of a discrete nidus with intervening gliotic parenchyma distinguishes a classic iAVM from other cerebrovascular malformations, such as a pure arterial malformation, venous-predominant AVM, proliferative-type AVM, cerebral proliferative angiopathy (CPA), pial arteriovenous fistula (AVF), or dural AVF. Fig. 3.4 illustrates a diagnostic approach to cerebrovascular malformations based on the imaging characteristics of the nidus as well as the presence of enlarged pial arteries. Pure arterial malformation, a newly recognized entity, is defined as dilated overlapping and tortuous arteries with a coil-like appearance and/or mass of arterial loop without an associated nidus or venous component ( Fig. 3.5 ). They are rare lesions, typically detected incidentally and with a benign natural history. Another incompletely understood lesion, characterized by early contrast filling of dilated branching medullary veins draining into a dominant vein in the absence of a nidus, has been referred to as a venous-predominant AVM; these lesions can mimic the common benign developmental venous anomaly (DVA) but are distinguished by the presence of arteriovenous shunting. CPA, formerly known as diffuse nidus type AVM, is a vascular malformation characterized by the presence of normal parenchyma interspersed throughout its nidus. CPA is distinguished from proliferative-type AVM by its absence of an early venous drainage. Additional characteristic features of CPA include large areas of parenchymal involvement (e.g., lobar or hemispheric), a nidus fed by multiple arteries in the absence of a dominant feeder, normal to only modestly enlarged feeder arteries, and associated stenosis of feeder arteries. On angiography, CPA is also characterized by “puddling” of contrast involving the nidus, which persists into the late arterial and early venous phases ( Fig. 3.6 ). A pial AVF is a rare lesion composed of a direct arteriovenous connection in the subpial space without an intervening nidus ( Fig. 3.5 ), whereas a dural AVF is an abnormal arteriovenous connection that occurs within the dura mater, most frequently involving the dural venous sinuses.

The presence of multiple classic iAVMs may be indicative of an underlying syndrome, such as hereditary hemorrhagic telangiectasia or cerebrofacial arteriovenous metameric syndrome (CAMS). CAMS, also known as Wyburn-Mason syndrome or Bonnet-Dechaume-Blanc disease, is a segmental neurovascular syndrome with three forms—CAMS type I, involving the medial prosencephalon, with AVMs located at the corpus callosum, hypothalamus, and nose; CAMS type II, involving the lateral prosencephalon, with AVMs located at the occipital lobe, optic chiasm, optic tract, thalamus, retina, and maxilla; and CAMS type III, involving the rhombencephalon, with AVMs located at the cerebellum, pons, and medulla.