Vascular Anomalies of the Brain

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Chapter 13 Vascular Anomalies of the Brain

Nils Mueller-Kronast

INTRODUCTION TO ANGIOGRAPHIC MODALITIES

Catheter Angiography

Cerebral angiography was one of the first diagnostic modalities for brain diseases, and it has remained crucially important and routinely used since its description by Egaz Moniz in 1927.¹ However, there has been significant evolution from the direct carotid cut downs and cut film technique of the 1920s to digital subtraction angiography with three-dimensional (3D) reconstruction.

With the transition to non-ionic contrast, smaller 4- and 5-French, and more flexible catheters and guidewires with hydrophilic coating, the risk for permanent neurologic injury was reduced from 1% to 2.4%^2,³ to less than 0.1% to 0.3%.⁴^–⁶ However, given risk of local pain, bruising, and hematoma in up to 25% of cases, noninvasive alternatives should be considered whenever they will provide similar diagnostic information.

Cerebral angiography remains the gold standard in the diagnosis of dural arteriovenous fistulas (dAVFs), as well as small aneurysms and arteriovenous malformations (AVMs). In addition, super-selective angiography can provide functional and physiological data important for clinical decision analysis, although pressure measurements are not commonly performed. It is still strongly recommended that a magnetic resonance imaging (MRI) study and a four-vessel angiogram be obtained to delineate the anatomy of an AVM.⁷ Computed tomography (CT) angiography can outline enlarged abnormal cortical veins but may miss subtle dural venous drainage or small arteriovenous malformations.⁸ MRI may show important indirect signs such as edema and abnormal cortical enhancement. In our practice, all patients with unexplained subarachnoid, subdural, or atypical parenchymal hemorrhage undergo digital subtraction angiography (DSA).

Although noninvasive vascular studies including carotid ultrasound, MR angiography (MRA), and CT angiography have lead to a decreased use of diagnostic digital substraction angiography (DSA), its use as part of evolving interventional treatment strategies is increasing.

Three-Dimensional Digital Subtraction Angiography

Excellent spatial resolution and lack of artifact has made 3D DSA an attractive tool for treatment planning and follow-up (Figure 13-1). It further increases the sensitivity for detection of aneurysms smaller than 3 mm⁹ and provides invaluable conformational information for endovascular treatment planning (Figure 13-2).

Figure 13-1 Surface shaded display of a three-dimensional rotational angiogram showing an 11-mm left carotid bifurcation aneurysm.

Figure 13-2 A 59-year-old woman treated with elective clipping of a right posterior communicating aneurysm and coiling of a broad-necked left posterior communicating aneurysm (A and B). Three-dimensional rotational angiography (C) allows excellent visualization of the minimal residual filling of the neck without artifact.

Computed Tomography Angiography

With the advent of helical CT scanners in the early 1990s, multidetector CT scanners in this decade, and improved 3D processing algorithms including methods to automate removal of bony structures, CT angiography has become a routine tool in the assessment of patients with cerebrovascular diseases.

Multidetector CT angiography (MDCTA) has higher spatial resolution and decreased, but not eliminated, propensity to motion artifacts (Figure 13-33) than MRA.^10,¹¹ For these reasons, MDCTA has largely replaced MR angiography in our practice.

Figure 13-33 Computed tomography (CT) angiography with motion artifact. The CT angiogram suggested moderate perianeurysmal vasospasm, which was not confirmed by digital substraction angiography (see Figure 13-34 A and B,).

The sensitivity for detection of even small aneurysms has substantially increased. One study found MDCTA to be actually more sensitive than DSA for the detection of small aneurysms.¹² Yet DSA remains the gold standard for aneurysm detection. Colen et al.¹³ found that during screening of patients with nontraumatic subarachnoid hemorrhage, MDCTA missed the primary aneurysm in 11 of 211 cases. Regions with overprojecting bone (such as the cavernous segment or supraclinoid carotid artery) may not be optimally visualized,^14,¹⁵ and MDCTA has lower sensitivity in detecting branches incorporated into the aneurysmal sac.¹⁶

The bony structures of the sella are often manually excluded on the reconstruction and thus an aneurysm can easily be “removed” in the cavernous and supraclinoid region (as seen in Figure 13-31, F). Careful analysis of the source images is crucial to preserve high sensitivity. Inadequate bolus timing with poor arterial and dense venous filling can render interpretation of CT angiography much more challenging. Still, CT angiography can be superior to DSA for aneurysms with competing flow, such as some basilar tip, basilar trunk, or anterior communicating aneurysms because it avoids the problem of contrast “washout” (see Figure 13-18).

Figure 13-31 A 36-year-old woman had consulted for progressive headaches over the previous 3 weeks. Computed tomography (CT) scan of the brain showed subdural hemorrhage without obvious subarachnoid hemorrhage (A and B). CT angiography was initially read as normal because the aneurysm was cropped on the three-dimensional reconstruction (C–E). The patient then developed progressive third nerve palsy. Digital substraction angiography (F) showed a 9- by 5-mm bilobed posterior communicating artery aneurysm, which was treated with coil embolization (G). A small remnant was stable on 6-month follow-up angiography.

Figure 13-18 Catheter angiography only partially outlines the extent of this giant fusiform basilar trunk aneurysm despite bilateral simultaneous vertebral injection due to slow flow (A– D). The patient was treated with endovascular Hunterian occlusion of both distal vertebral arteries and STA to PCA bypass (E).

CT angiography is valuable for follow-up of aneurysms treated with titanium alloy clips, but it is not useful when cobalt alloy clips have been used.¹⁷ Image quality is also degraded by orientation of the clip perpendicular to the imaging axis or presence of multiple clips. Streak artifacts caused by platinum coils make it an unsuitable method to assess for recanalization of embolized aneurysms. (See Figures 13-3 and 13-4.)

Figure 13-3 A 68-year-old woman, with history of pacemaker placement and atrial fibrillation on chronic anticoagulation was found to have an incidental basilar tip aneurysm, as shown on digital substraction angiography (A). The aneurysm was treated with elective coil embolization (B). Because of the risk associated with stopping anticoagulation for follow-up angiography, a CT angiography was attempted but yielded no useful information regarding recanalization because of a streak artifact (C and D).

Figure 13-4 A 59-year-old woman presented with a ruptured anterior communicating artery aneurysm, which was treated with coiling. A broad-based middle cerebral bifurcation aneurysm was later clipped. The artifact from both the coils and middle cerebral artery clip are evident on this computed tomography angiogram source image.

In our institution, patients with subarachnoid or atypical intracranial hemorrhage are generally screened with CT angiography. An aneurysmal source of bleeding can often be identified in the emergency room, allowing triage to endovascular versus surgical treatment.

If endovascular treatment is deemed feasible, diagnostic angiography and dedicated 3D DSA is then performed under anesthesia in preparation for endovascular treatment. Anesthesia is helpful because 3D DSA is particularly sensitive to movement artifacts, which are otherwise common in these often uncooperative and obtunded patients.

If the CT angiography is nondiagnostic, DSA is always performed to evaluate for a missed aneurysm or nonaneurysmal sources, such as a dural arteriovenous (AV) fistula. See Figure 13-5.

Figure 13-5 A 47-year-old man presented with isolated intraventricular hemorrhage. Computed tomography angiography (A) showed an arteriovenous malformation in the left temporal lobe, which was confirmed by magnetic resonance imaging (B). Angiography (C) and especially three-dimensional digital substraction angiography (D) outlined the posterior communicating and anterior choroidal arteries as feeders.

However, protocols relying directly on DSA are also very acceptable and may actually reduce cost.

Magnetic Resonance Angiography

MR angiography uses the signal difference between stationary tissue and flowing blood in gradient-echo sequences to generate angiographic images. For this reason, short T1 substances such as methemoglobin within a subacute hematoma may simulate flow.

With phase-contrast (PC) MR angiography, flow is detected and imaged on the basis of the shift in the phase of magnetization that occurs when material moves in the presence of a magnetic field gradient. PC imaging has excellent background suppression and provides directional flow information. It suffers from the need to apply multiple gradients, thus lengthening acquisition time, and to preselect a range of velocities to be sampled. Therefore, PC MR angiography has become much less common than time-of-flight (TOF) MR angiography.

TOF techniques exploit the contrast between the high signal intensity of inflowing, fully magnetized blood and the low signal intensity of saturated stationary tissue. Multiple thin imaging slices (two-dimensional [2D]) or larger slabs (3D) perpendicular to the axis of flow are acquired with a flow-compensated gradient-echo sequence. Three-dimensional TOF angiography allows greater spatial resolution and is more resistant to signal loss from disturbed flow than 2D TOF. However, PC MRA and TOF MRA are sensitive to artifacts caused by turbulent or slow flow (Figure 13-6, B). TOF is also affected by in-plane artifacts.

Figure 13-6 A 42-year-old man with a headache was diagnosed with a 16-mm basilar tip aneurysm seen on computed tomography scan (A). Magnetic resonance angiography was unable to define the relation of the P1 segments to the aneurysm sac because of flow turbulence (B). Angiography (C) and three-dimensional digital substraction angiography (D) confirmed origin of the right P1 segment from the aneurysm neck, requiring stent-assisted coil embolization.

Contrast-enhanced (TOF) MR angiography has been employed to overcome these problems. It is performed by intravenously injecting a contrast agent to shorten selectively the T1 of the blood to produce the signal, rather than relying on flow. By implementing a T1-weighted imaging sequence during the first pass of the contrast agent, images can be produced showing arteries in striking contrast relative to surrounding stationary tissues and veins. However, synchronizing the acquisition with the arrival of the contrast agent is critical to ensure image quality. For a more detailed discussion on these MR angiography techniques, the reader is referred to the review authored by Aygun.¹⁸

MRA has limited sensitivity in the evaluation of the anatomy of large aneurysms with significant turbulence. Also, detailed characterization of the aneurysm neck and adjacent vessels is often suboptimal on MRA.

Whereas MRA for follow-up of clipped aneurysms is limited by severe “blooming” artifact, it is usually adequate to monitor coiled aneurysms because it has a sensitivity of 75% for detecting a residual neck.¹⁹ See Figure 13-7. However, problems related to signal drop-out may occur (Figure 13-8).

Figure 13-7 A 3-mm recanalization of a basilar tip aneurysm with good correlation between magnetic resonance angiography (A) and digital substraction angiography (B).

Figure 13-8 A 41-year-old female with fibromuscular dysplasia presented with a ruptured paraophthalmic aneurysm (A and B). After coil embolization, magnetic resonance angiography showed partial signal dropout in the supraclinoid internal cerebral artery from coil artifact (arrows) (C and D).

No clear information has emerged on whether contrast-enhanced MR angiography is superior to conventional 3D TOF.^19,²⁰ Where available, contrast-enhanced 3-Tesla MRA may a superior diagnostic option.²¹

DSA remains the gold standard for the detection of small AV malformations and dural AV fistulas. Although specialized sequences such as 3T four-dimensional MRA ²² and others²³ show good correlation for size of feeders and AVM classification, DSA is still more reliable in the diagnosis of small AVMs, which can be missed even by repeated MRA studies.²⁴ Contrast-enhanced MRA can be used to follow dural AV fistulas, but it is less sensitive than DSA (Figure 13-9).²⁵

Figure 13-9 A 54-year-old patient presented with acute severe headache. Plain CT shows the venous varix without definite hemorrhage (A), which was confirmed by gradient-recalled echo. Magnetic resonance angiography (B) failed to show the arteriovenous malformation (AVM), and magnetic resonance imaging (C) showed the typical flow voids. Angiography (D and E) showed a Spetzler-Martin Grade III AVM, which was treated surgically.

COMPUTED TOMOGRAPHY AND MAGNETIC RESONANCE IMAGING

CT without contrast is poorly sensitive for brain aneurysms. However, unruptured aneurysms may present with rim calcification or slight hyperdensity due to the blood contained in the aneurysm. Only relatively large (≥10mm) aneurysms can be seen on plain CT scan, however (Figure 13-10).

Figure 13-10 A right 9-mm posterior communicating artery aneurysm with only high density signs (arrow) from circulating blood (A). A 10-mm left carotid T aneurysm with obvious circumferential calcification (B).

CT without contrast is the standard screening test when subarachnoid hemorrhage is suspected (see Chapter 12), but almost all aneurysm locations in the circle of Willis can produce diffuse and symmetric subarachnoid hemorrhage and a dedicated vascular study is necessary to define the source of bleeding. Sometimes an aneurysm can be seen as a hypodense region within dense subarachnoid hemorrhage, a finding often referred to as a “ghost sign.” The distribution of the blood can be helpful to predict the location of the ruptured aneurysm, especially when multiple aneurysms are present at the time of hemorrhage. The location of parenchymal hemorrhage is typically adjacent to the point of rupture (Figure 13-11).

Figure 13-11 A 56-year-old woman with multiple intracranial aneurysms, including a ruptured 6-mm complex anterior communicating artery aneurysm (A and B). The aneurysm was visible as a “ghost sign” (arrow). The location of parenchymal hemorrhage indicated that this aneurysm was the source of hemorrhage. Note the different gantry angulations for computed tomography (CT) and CT angiography. Subarachnoid hemorrhage with “ghost sign” (arrow) caused by a 8-mm basilar tip aneurysm (C).

Unenhanced CT of the brain has poor sensitivity for vascular malformations but sometimes outlines enlarged draining veins of AVMs (Figure 13-12, A). Administration of contrast significantly increases the sensitivity of CT scans for the diagnosis of vascular abnormalities.

Figure 13-12 High density from a draining vein in the white matter and lateral ventricle on computed tomography scan (arrow) (A). Magnetic resonance imaging shows obvious flow voids on the axial fluid-attenuated inversion recovery (B) and coronal view of contrast-enhanced T1 sequences (C).

MRI

MRI of the brain, although more sensitive than CT, is not adequate to rule out the presence of an aneurysm. However, given the frequent use of this technique as screening test for many neurologic conditions, most of the incidental aneurysms treated in our institution are identified by this technique. Indirect markers are additional flow voids adjacent to vessels or, less commonly, perianeurysmal gliosis (see Figure 13-39 B,).

Figure 13-39 Hyperdense outline of the anterior communicating aneurysm on computed tomography (A) and flow-void and perianeurysmal gliosis on fluid-attenuated inversion recovery magnetic resonance imaging (B).

Rarely, slow flow within the aneurysm can cause high signal of fluid-attenuated inversion recovery (FLAIR)/T2, which can be confused with focal thrombus or hemorrhage (Figure 13-13).

Figure 13-13 A 62-year-old man presented for evaluation after a fall. Slow flow (corresponding with delayed filling on subsequent angiography) produced an area of high fluid-attenuated inversion recovery signal (A, arrow) rather than the usual low signal “flow void.” This finding caused by an aneurysm was initially misinterpreted as a focal contusion. The middle cerebral aneurysm was treated with coil embolization (B and C).

MRI is indispensable in the diagnosis and management planning of AVMs and is part of the routine diagnostic workup preceding the treatment of these lesions (see Figures 13-12 B–C, and 13-43 A–C,).

Figure 13-43 A 38-year-old man had a posterior circulation arteriovenous malformation diagnosed at age 12 after developing a cerebellar hemorrhage. He recovered with persistent right-gaze nystagmus, mild diplopia, and right-sided dystaxia. Magnetic resonance imaging (A–C) and digital substraction angiography (D–F) showed a complex Spetzler-Martin Grade V cerebellar and midbrain vascular malformation measuring approximately 5.6 cm in maximum diameter supplied by the posterior circulation vessels, posterior communicating arteries, and external branches. He continued to be treated conservatively because of the unacceptable risks associated with any form of treatment.

ANEURYSMS

General Concepts

♦ Cerebral aneurysms are most commonly located near the branches of the carotid artery, including the posterior communicating arteries. Only approximately 12% of unruptured aneurysms are located in the posterior circulation (Figure 13-14).²⁶

♦ Aneurysms most frequently present with rupture in the fifth through seventh decades,²⁸ but they are found in individuals of all ages.

♦ They are considered to be acquired lesions. In fact, de novo formation in previously normal vessel segments is not uncommon. This phenomenon can be seen in up to 32% of patients with previously treated aneurysms.²⁹

♦ Aneurysms typically occur at vessel branch points, at locations with abrupt change of flow direction, and in patients with underlying collagen vascular disorders, such as Ehlers-Danlos Type IV syndrome and polycystic kidney disease, fibromuscular dysplasia, and aortic coarctation.

♦ Smoking and arterial hypertension have been associated with aneurysm formation and rupture.^30,³¹

♦ Family history is a risk factor for the occurrence of intracranial aneurysms. Twenty percent of patients with aneurysms have a family history of aneurysm rupture. Although the incidence of aneurysms is highest in families with two affected first-degree family members,³² the cost-effectiveness of screening family members of patients after treatment of an aneurysm is disputed.³³

♦ Multiple intracranial aneurysms also increase the risk of rupture.

♦ The risk of aneurysm formation is also increased in vessels with increased flow demand such as those proximal to an AVM or contralateral to occluded or hypoplastic vessels.

♦ A subgroup of patients harbor symmetric aneurysms, most commonly “mirror” middle cerebral artery (MCA) aneurysms or bilateral cavernous internal carotid artery (ICA) aneurysms. It is not known why similar focal vessel segments are affected in these patients.

♦ Pathological analysis of resected aneurysms have documented structural abnormalities of the internal elastic lamina.³⁴

Figure 13-14 Proportions of ruptured versus unruptured aneurysms at the University of Miami.²⁷

Types of Aneurysms

♦ Aneurysms are typically classified as saccular and nonsaccular (or fusiform). The latter group is further subdivided into true fusiform, dolichoectatic, and transitional forms.³⁵

♦ “Dissecting” aneurysms are somewhat less well defined. Often fusiform aneurysms of the intracranial vertebral arteries and posterior inferior cerebellar arteries (PICA) are classified under this label on the basis of appearance.

Nonsaccular Aneurysms

Fusiform Aneurysms

♦ By convention, the term fusiform indicates dilatation of the entire vessel to at least 1.5 times normal diameter without a discernible neck.

♦ Fusiform dilatation can occur simultaneously in all proximal intracranial cerebral vessels (dolichoectasia), but usually only one vessel segment is prominently affected, most commonly the cavernous and supraclinoid segments of the ICA and the vertebrobasilar system.

♦ The annual rate of rupture of these fusiform aneurysms is high (2.3%/year),³⁵ reflecting the often large diameter of the lesions as well as their aggressive biological behavior.

♦ Presentation with hemorrhage heralds poor prognosis.

♦ Ischemia may also be associated with increased risk of rupture (Figure 13-15).

♦ Aneurysms in the M1 segment and basilar artery often manifest with ischemic stroke.

♦ Posterior circulation aneurysms may also cause cranial neuropathy because of mass effect from brainstem compression (see Figures 13-19 and 13-20).³⁵

♦ The mortality of enlarging lesions is 5.7 times higher than that of stable lesions, especially for lesions larger the 15 mm, in patients with compressive symptoms, and in fusiform and transitional aneurysms.³⁶

♦ The shape and location of fusiform aneurysms involving small, penetrating branches often renders both surgical and endovascular treatment unfeasible (Figure 13-16).

♦ New endovascular approaches with “flow-directing” devices, such as the “pipeline” device, remain to be validated.³⁷

♦ Parent vessel ligation was initially the only treatment option for aneurysms–at the expense of a morbidity of 40% and mortality of 13%,³⁸ in particular, due to ischemia (Figure 13-17). In patients with fusiform aneurysms, particularly those involving segments harboring penetrating arteries such as the middle cerebral M1 segment or basilar trunk, parent vessel occlusion may still be the only treatment option. ICA occlusion (see Figure 13-29) or occlusion of both vertebral arteries or the proximal basilar artery (Figures 13-18 to 13-20), with or without a bypass, may be performed. In a large series by Drake³⁹ in which fusiform basilar trunk aneurysms comprised roughly 25% of the vascular lesions treated, good outcomes were documented in 67% of cases with partial vessel ligation.

♦ Other surgical options include wrapping, complex clip reconstruction, and bypass. However, only anecdotal data are available regarding the long-term success of these treatment modalities (Figure 13-21).

Figure 13-15 A 58-year-old man with a longstanding history of smoking presented with a 3-year history of progressive brainstem dysfunction. Serial imaging documented a greater than 35% increase in the size of the basilar artery over 2 years. The patient had acute deterioration of balance and swallowing due to a right cerebellar stroke, as seen on the diffusion-weighted image (A). Computed tomography angiography (B) clearly depicts this massive fusiform basilar trunk aneurysm, and magnetic resonance imaging (axial T2 image) (C) outlines the extent of brainstem compression. The aneurysm was deemed untreatable, and the patient died 2 weeks later from fatal subarachnoid hemorrhage (D).

Figure 13-19 A 59-year-old woman with a previous brainstem stroke presented with progressive dysphagia. Fusiform dilatation of the basilar artery was disclosed by brain magnetic resonance imaging (T1-weighted sequence with contrast) (A and B) and digital substraction angiography (anteroposterior projection of vertebral artery injection) (C). The patient was treated with unilateral vertebral occlusion and then kept on aspirin and clopidogrel. While awaiting surgical bypass, she suffered a fatal subarachnoid hemorrhage as seen on computed tomography scans (D and E).

Figure 13-20 A 17-year-old man presented with progressive sixth nerve palsy. Magnetic resonance imaging outlined a large fusiform basilar trunk aneurysm with extensive mural thrombus (A). Patient tolerated bilateral vertebral balloon-test occlusion, followed by permanent coil occlusion of the vessels. Digital substraction angiography shows the left vertebral artery coil occlusion distal to PICA (B and C). Internal carotid angiogram documents retrograde basilar filling (D and E). The patient suffered a fatal subarachnoid hemorrhage 4 months after treatment.

Figure 13-16 An 83-year-old woman with progressive painful diplopia. Angiography documented an 18-mm fusiform in the left cavernous aneurysm; (A) digital substraction angiography (DSA), (B) three-dimensional DSA. The patient failed balloon-test occlusion and was treated conservatively. Of interest is the modest concomitant fusiform dilatation of the right cavernous internal carotid artery (C).

Figure 13-17 A 72-year-old woman presented with subarachnoid hemorrhage from rupture of a left paraclinoid carotid aneurysm seen on head computed tomography scan (A), digital substraction angiography (DSA) (B), and three-dimensional DSA (C). The patient initially tolerated balloon-test occlusion without deficits on physical examination or electroencephalographic changes. However, she later developed refractory hypotension due to a nosocomial infection, resulting in extensive left-hemispheric hypoperfusion, as shown by the large mismatch between diffusion-weighted imaging (D) and perfusion-weighted imaging (E), followed by a massive and fatal stroke 48 hours after embolization (F).

Figure 13-29 A 74-year-old woman presented with diplopia. Three-dimensional angiography (A) showed a wide-necked cavernous internal carotid artery aneurysm. The patient tolerated balloon-test occlusion and was treated successfully with partial coiling and parent vessel occlusion (B).

Figure 13-21 Patient with a fusiform middle cerebral artery (MCA) aneurysm and an associated stroke treated with internal carotid artery to MCA bypass. The M1 aneurysm with its penetrating arteries was perfused retrogradely to achieve a reduction in flow through the abnormal vessel segment. (A) Digital substraction angiography (DSA), (B) three-dimensional DSA, (C) intraoperative microscopic photograph, (D) photograph of the radial artery bypass, (E) DSA of the bypass after anastomosis with retrograde filling of the M1 segment.

Transitional Forms

♦ In transitional forms, dolichoectatic enlarged vessel segments transition into truly fusiform vessel segments. These appear to have a similar history to strictly fusiform aneurysm.

♦ Often fusiform aneurysms give rise to superimposed saccular aneurysms and involve the takeoff of major branches, complicating visualization and treatment (Figure 13-22).

♦ As seen in Figures 13-22 and 13-23, 3D reconstructions of CT angiography and rotational DSA allow optimal visualization of these complex structures and have become essential in treatment decision making.

♦ Of note, the term transitional aneurysm is also used for saccular aneurysms extending from the cavernous sinus to the intradural space.⁴⁰