5 Aneurysms

10.1055/b-0036-141869

5 Aneurysms

Introduction

By definition, an aneurysm is an abnormal dilatation, typically saccular or fusiform in shape, of an artery. Intracranial aneurysms are thought to result from hemodynamic stress, abnormal remodeling, and inflammation. Saccular aneurysms are generally found at arterial branch points, although many are not clearly associated with branch vessels. Multiple lobes and daughter sacs (“Murphy’s tit”) are common in ruptured aneurysms. Rupture is typically at the apex. Although variable percentages in terms of sites of occurrence are published in the scientific literature, for unruptured aneurysms without subarachnoid hemorrhage, MCA, cavernous carotid ( Fig. 5.1 ), and distal internal carotid are the most common (about 20% each), followed by PCOM and ACOM (including ACA) aneurysms (about 10% each). Posterior circulation aneurysms are least common, with about 5% vertebrobasilar/PCA and 5% basilar tip. The prevalence of aneurysms in the general population, without subarachnoid hemorrhage, is about 3%. The prevalence is higher in patients with atherosclerosis and also increases with age.

**Fig. 5.1** Partially thrombosed, large cavernous carotid aneurysm. A round mass lesion is noted on the unenhanced CT within the right cavernous sinus. Postcontrast, there is enhancement of the anterior portion of the mass, with the imaging findings suggestive of an aneurysm of the cavernous portion of the distal internal carotid artery, with thrombosis of the posterior portion of the aneurysm. The CTA confirms this diagnosis, with the calcification of the wall of the aneurysm best depicted on this exam. On the TOF MRA, the patent portion of the aneurysm is poorly depicted, which is common with large aneurysms (due to flow dynamics). The patent portion is well visualized, however, on the additional axial contrastenhanced T1weighted image and contrastenhanced TOF exam. VRT, with the view from anteriorly presented, shows the aneurysm and its continuity with the internal carotid artery well; however, inspection of additional rotations would be necessary to define the surface of the patent portion of the aneurysm.

Multiple aneurysms are found in about 20% of all aneurysm cases ( Fig. 5.2 ). Risk factors for multiple aneurysms include smoking and hypertension (which are felt to be risk factors as well for development of a single, isolated aneurysm and subarachnoid hemorrhage). Infundibula, conical dilatations at an artery origin, are benign incidental findings not to be confused with an aneurysm. Intracranially, an infundibulum is most common at the PCOM origin.

**Fig. 5.2** Small unruptured aneurysms of the middle cerebral and basilar arteries. This case points to the importance of close inspection of high-resolution, thin section axial T2weighted scans for detection of small aneurysms, and the critical role that TOF MRA plays both for detection and in depiction of brain aneurysms. The latter scan technique has markedly improved in recent years with routine imaging at 3 T and attention to setup (spatial resolution). In this instance, although the right MCA aneurysm is easily visualized (*black arrow*), as a flow void, on the axial screening T2weighted exam, that involving the distal basilar artery (*white arrow*) is more subtle. Both are well depicted on the VRT images from the TOF MRA exam. The MCA bifurcation/trifurcation aneurysm arises at the origin of the inferior trunk, has a broad base, and incorporates the artery, measuring 7 mm in diameter. The second aneurysm arises from the distal basilar artery (but not its tip), has a diameter of 5.5 mm, is also sessile in shape, and incorporates the origin of the left superior cerebellar artery. Frontal views from the DSA exam are presented for comparison, with both aneurysms subsequently coiled.

There are several medical conditions well known to be associated with aneurysms. The two most important are polycystic kidney disease and a familial disposition. Estimates of prevalence of intracranial aneurysms in autosomal dominant polycystic kidney disease (ADPKD) range widely (up to 40%). The risk of aneurysm rupture with subarachnoid hemorrhage appears to be higher than in the general population, with presentation at a younger age. Twenty-five percent of patients with ADPKD and an aneurysm develop a second aneurysm within 15 years. Screening by noninvasive imaging is reasonable in ADPKD patients with a known aneurysm or prior subarachnoid hemorrhage or in patients who have a familial history.

In considering familial aneurysms, specifically when at least two first-degree relatives are affected, assessments of prevalence range widely, with 10% likely a reasonable estimate. There is a predilection for the middle cerebral artery, as well as for multiple aneurysms and subarachnoid hemorrhage at a younger age. Screening of such patients, if pursued, should be by noninvasive imaging. Other less common conditions with an increased incidence of intracranial aneurysms include Ehlers-Danlos syndrome type IV, α₁-antitrypsin deficiency, fibromuscular dysplasia, and in association with an arteriovenous malformation.

The natural history of unruptured intracranial aneurysms is controversial. The overall risk of rupture is likely 1 to 2% per year. The rate of rupture appears to be lower for small anterior circulation aneurysms. Larger aneurysms are at greater risk for rupture. However, if an aneurysm ruptures (with subarachnoid hemorrhage), the mortality rate is very high, greater than 50%.

Although a small saccular aneurysm may be visualized on a conventional MR or CT scan, 3D time of flight (TOF) MR angiography (MRA) and CT angiography (CTA) are specifically employed for detection and delineation ( Fig. 5.3 ). CTA is the modality of choice in the acute presentation with subarachnoid hemorrhage, while 3D TOF MRA is often used for detection and evaluation of asymptomatic aneurysms, as well as for correlation and further definition of lesions in the acute setting and on followup. Modern scanners easily detect aneurysms as small as 2 mm in diameter. Treatment of intracranial brain aneurysms that have bled, or are deemed to present a significant risk to the patient because of potential bleeding in the future, is by either surgical clipping or endovascular occlusion. Surgery is much less common today, although not all aneurysms can be treated by an endovascular approach.

**Fig. 5.3** Small carotid terminus aneurysm. Small aneurysms can be difficult to see on conventional planar MR images, further emphasizing the importance of TOF MRA. Sagittal and coronal thin section 3D FLAIR images reveal a small, low SI, round lesion (*black arrows*, corresponding to a flow void) just superior and contiguous to the terminus of the left internal carotid artery. On contrastenhanced T1weighted images, small aneurysms may either enhance or remain as a flow void, due to specific selection of imaging technique. On fast spin echo imaging, high-velocity flow generally is low signal intensity, while on 3D gradient echo–based techniques such as MP-RAGE, the vessels are enhanced. Coronal images with both techniques, postcontrast, are illustrated in the upper right hand quadrant of the figure. A coronal thick MIP TOF MRA depicts well this small carotid terminus aneurysm (*white arrow*), also visualizing an additional MCA bifurcation aneurysm (*small white asterisk*). Twenty percent of intracranial aneurysms are multiple, thus mandating careful image inspection for a second (or third) aneurysm in all patients. The VRT image from DSA depicts well both aneurysms. Frontal projections from a left internal carotid artery injection are also presented, depicting the carotid terminus aneurysm prior to (*large black asterisk*) and following endovascular coiling.

Aneurysm Treatment

Asymptomatic patients with untreated intradural aneurysms are followed in some centers by annual CTA or MRA. Growth or new symptoms, such as headaches or cranial nerve palsies, raise concern in regard to impending rupture. Cessation of smoking and appropriate management of hypertension are felt to be important. Treatment of unruptured aneurysms, if desired or indicated, is currently performed either by surgery or by endovascular means.

Surgical

Surgical treatment was considered in the past to be the gold standard for treatment ( Fig. 5.4 ); however, complication rates are high. In surgical treatment of unruptured aneurysms, the mortality rate is about 3%, with permanent morbidity seen in up to 20%. Risk factors for surgery in this group of patients include age, size of the aneurysm (> 12 mm), and location in the posterior circulation. In surgical treatment of ruptured aneurysms, the mortality rate is about 15%, with substantial, permanent morbidity in an additional 15%.

**Fig. 5.4** Ophthalmic artery aneurysm. On an axial T2weighted scan, an oval flow void (*arrow*) is seen in the vicinity of the ophthalmic artery. A thick section axial MIP of the TOF MRA confirms the lesion to be an aneurysm (*arrow*). The targeted VRT (from the TOF exam) demonstrates well the saccular, multilobulated character of this aneurysm, with the origin of the ophthalmic artery not incorporated into the sac (not shown well on the presented image). This aneurysm was surgically clipped, with the CT and MR (T2) following surgery presented. The surgical clip is depicted clearly, with little artifact on each exam, due both to improvements in CT technology and the use of metal alloys with decreased artifact (on both MR and CT). The changes in the anterior low frontal lobe are unrelated, due to prior trauma.

In surgical series, the frequency of a residual aneurysm is 4 to 8%. Catheter angiography, which can be performed intraoperatively, is necessary to confirm complete occlusion of the aneurysm and preservation of associated vessels. There is an increased rate of rupture after clipping when a portion of the aneurysm remains, and enlargement of the residual portion of an aneurysm following clipping has been documented. Recurrence has also been shown in 1% of aneurysms after complete surgical obliteration, the latter confirmed by postoperative angiography ( Fig. 5.5 ).

**Fig. 5.5** Recurrent MCA aneurysm following surgical clipping. On the unenhanced CT, there is a question of a focal abnormal high-density lesion adjacent and just lateral to the surgical clip, placed 13 years earlier for a proximal MCA aneurysm. Postcontrast, a round 16-mm-diameter, enhancing lesion is noted, consistent with a recurrent aneurysm. No other aneurysms were noted on CT. A second aneurysm is identified on DSA, a 4-mm multilobulated aneurysm of the left posterior communicating artery. The latter is best depicted (*arrow*) on the volume-rendered projection. Both aneurysms were occluded with platinum microcoils, with DSA presented both prior to and following coiling.

Endovascular

The complication rate with endovascular treatment of an unruptured brain aneurysm is approximately 10% ( Fig. 5.6 ). The rate of permanent complications is less than half this figure. Repeat hemorrhage is uncommon, seen in 3%.

**Fig. 5.6** Nonruptured aneurysm of the anterior choroidal artery. The patient presented with diplopia, anisocoria, and ptosis, due to compression of the oculomotor nerve on the right. A small aneurysm is noted projecting posteriorly from the internal carotid artery, seen as a flow void (*black arrow*) on the axial T2weighted FSE image. Postcontrast, there is thin, smooth circumferential enhancement (*white arrow*), a finding seen in aneurysms on high-resolution imaging. On DSA, the aneurysm (*asterisk*) is confirmed to lie at the origin of the anterior choroidal artery (just proximal to the terminus of the internal carotid artery), being directed laterally and posteriorly. It has a narrow base and a maximum diameter of 8 mm. The aneurysm was subsequently coiled, with preservation of the anterior choroidal artery. On followup MR exam, obtained 6 days after coiling, a small infarct is noted (*small white arrows*, a known complication of coiling being thromboembolic stroke), with hyperintensity on DWI, within the right parahippocampal gyrus.

The advent of detachable coils, led by the development of the Guglielmi detachable coil (GDC) conceived serendipitously in the early 1980s, enabled development of the field of aneurysm coiling as we know it today. With this system, until the coil is in satisfactory position, it remains attached to the pusher wire. Detachment is achieved by application of a low-amplitude electrical current, causing electrolysis of the connection between the coil and the wire. Numerous technical refinements have followed, with an array of shapes and sizes available. Augmented (bioactive) coils continue to attract interest, with the goal to promote thrombosis and fibrosis, thus reducing the likelihood of subsequent recanalization. Wide-neck aneurysms present an additional challenge, with one current approach being deployment of a thin wire mesh stent across the neck, within the parent vessel. Coils are then placed via a microcatheter that enters the aneurysm through the interstices of the stent, with the latter acting as a scaffold to hold the coils within the aneurysm. The relatively recent advent of flow-diverting stents, for example, the Pipeline Embolization device, may lead to a further paradigm shift in endovascular aneurysm treatment.

Complete occlusion of an aneurysm following coiling is reported in about 50%, with “near complete” in 90%. Recurrence after coiling (defined as recanalization sufficiently large to allow retreatment, either surgical or endovascular) is seen in about 20% of patients 1 to 2 years following initial treatment ( Fig. 5.7 ). Larger aneurysms (> 10 mm) and those with a wide neck are more prone to recurrence. Recurrences are also more common following suboptimal initial endovascular treatment. In terms of the relationship to the parent vessel, terminal aneurysms tend to recur more frequently than sidewall aneurysms. Due predominantly to the risk of recurrence, longterm followup/surveillance with MR or DSA is recommended ( Fig. 5.8 ).

**Fig. 5.7** Posterior communicating artery (PCOM) aneurysm recurrence following endovascular coiling. TOF MRA (axial MIP and source images) reveals a partially recanalized saccular aneurysm (*white arrow*), with a signal void in the coiled (nonpatent) portion of the aneurysm (*asterisk*). The subsequent DSA depicts well this recurrence (*black arrow*), with VRT demonstrating the complexity of the patent portion of the aneurysm, which incorporates the origin of the PCOM. Following re-embolization with platinum microcoils, complete occlusion was achieved, as demonstrated on both the followup TOF MRA and DSA. Aneurysm recurrence following endovascular coiling is seen in 20% of cases, whereas surgical series report aneurysm recurrence in only 1%.

**Fig. 5.8** Recanalization of a previously coiled ACOM aneurysm. The patient presented with acute subarachnoid blood on CT (not shown), consistent with a ruptured aneurysm. DSA revealed a complex, broadbased, lobulated 8-mm aneurysm of the ACOM (well visualized on the VRT image presented). This was embolized using microcoils without complication, with images shown from the DSA exam both prior to and immediately following occlusion. Eighteen months later, the patient presented for a followup MR. The axial T2weighted scan is unrevealing, as the signal void could be due to the coiling or represent a flow void. The AP projection from the MIP of the TOF MRA reveals a small area of recanalization (*asterisk*) at the base of the prior aneurysm. Close inspection of TOF images in all aneurysm MR followup exams is mandated.

Comparison of Surgery and Endovascular Treatment

Significantly lower rates of morbidity and mortality are found with coiling of unruptured intracranial aneurysms when compared to clipping. Decreased length of stay, hospital charges, and periprocedural complications also significantly favor coiling of unruptured aneurysms. With ruptured aneurysms, several studies have shown little difference between surgical and endovascular treatment, other than the greater recurrence rate following the latter. Other studies, and specifically the International Subarachnoid Aneurysm Trial (ISAT), have shown decreased mortality and significant morbidity in the endovascular group, together with a significantly reduced risk of vasospasm and ischemic neurologic deficits.

Only gold members can continue reading. Log In or Register to continue