Cerebral Aneurysms In Patients Research Articles

Relationship between characteristic geometry and risk of rupture in cerebral saccular aneurysm

To explore the relationship between the characteristic geometry and the risk of rupture in cerebral saccular aneurysm. The clinical features of 473 cerebral saccular aneurysm patients with complete materials of cerebral angiography, 426 with ruptured aneurysm and 47 with unruptured aneurysm, were analyzed retrospectively. The relation of depth, neck width, and aspect ratio (AR) of aneurysms to the risk of rupture were analyzed. The mean depth, neck width, and the AR of the patients with ruptured aneurysm were (5.9+/-3.0) mm, (3.0+/-1.2) mm, and (2.1+/-0.9), all significantly lower than those of the unruptured aneurysm patients [(8.0+/-4.2) mm, (4.8+/-2.4) mm, and (1.7+/-0.6) respectively, all P<0.01]. AR is not correlated with the location of aneurysms, but reflects the geometrical and hemodynamic features of cerebral saccular aneurysms very well. High AR may be useful in predicting imminent aneurysmal rupture. But by now, no definitive AR level is accepted in discriminating between unruptured and ruptured aneurysms.

The comparison of complications on the endovascular and surgical treatment in elderly cerebral aneurysm patients.

The aim of this study was to compare intraoperative and postoperative complications and clinical outcome of endovascular coiling (EVT) with neurosurgical clipping (NST) under general anesthesia in the cerebral aneurysm patients older than 60 years. We retrospectively reviewed the charts, operative reports of patients who underwent EVT or NST at our hospital between January 2006 and August 2008. A total of 181 patients (EVT = 78, NST = 103) were included in this study. The rate of intraoperative event was higher in EVT than in NST but postoperative complication and Glasgow outcome scale (GOS) at 6 months did not show statically significance in both groups. Preoperative aneurysm rupture, age and the World Federation of Neurological Surgeons grade (WFNS) were the influencing factors for outcome in both groups. Anesthetic agents, body temperature and vasoactive drugs were significantly different between the two groups but the effects of these on the outcome of patients were insignificant. In EVT and NST, the variables related to the postoperative complications were preoperative aneurysm rupture, age and WFNS. When the elderly patients get these procedures, more close care should be considered postoperatively.

Symptomatic Cerebral Vasospasm after Surgical Ligation of Unruptured Aneurysms

Background Cerebral artery vasospasm accounts for the majority of delayed neurological deficits in ruptured aneurysm patients. We report two cases and review the literature" of patients who developed symptomatic vasospasm after treatment for unruptured cerebral artery aneurysms with clip ligation. Pre- and post-operative imaging and studies revealed absence of subarachnoid or focal hemorrhage. Case Description In a series of 104 consecutive cerebral artery aneurysm patients that underwent uncomplicated ligation without intra-operative rupture, two patients developed delayed neurologic deficits due to severe cerebral vasospasm. Both patients had no stigmata of rupture and were treated electively. Post-operative transcranial dopplers and angiography facilitated the early recognition of vasospasm. Permanent neurologic injury was prevented with the use of hypertensive, hemodilution and hyperdynamic (HHH) therapy along with endovascular treatment, intra-arterial papaverine and angioplasty. Conclusion After uncomplicated treatment of unruptured intracranial aneurysms, the cerebral vasculature may proceed to severe vasospasm by an unrecognized mechanism. This can be reversed with institution of HHH and endovascular therapy.

Ventral Intramedullary Cervical Spinal Cord AVM

Background Cerebral artery vasospasm accounts for the majority of delayed neurological deficits in ruptured aneurysm patients. We report two cases and review the literature" of patients who developed symptomatic vasospasm after treatment for unruptured cerebral artery aneurysms with clip ligation. Pre- and post-operative imaging and studies revealed absence of subarachnoid or focal hemorrhage. Case Description In a series of 104 consecutive cerebral artery aneurysm patients that underwent uncomplicated ligation without intra-operative rupture, two patients developed delayed neurologic deficits due to severe cerebral vasospasm. Both patients had no stigmata of rupture and were treated electively. Post-operative transcranial dopplers and angiography facilitated the early recognition of vasospasm. Permanent neurologic injury was prevented with the use of hypertensive, hemodilution and hyperdynamic (HHH) therapy along with endovascular treatment, intra-arterial papaverine and angioplasty. Conclusion After uncomplicated treatment of unruptured intracranial aneurysms, the cerebral vasculature may proceed to severe vasospasm by an unrecognized mechanism. This can be reversed with institution of HHH and endovascular therapy.

Comprehensive Stroke Center Quality Metrics

Recent advances in stroke treatment created a need for a consensus statement by industry experts detailing elements of quality stroke care. In 2005, the brain attack coalition published recommendations outlining elements constituting the highest level of stroke care, the comprehensive stroke center. Unlike primary level stroke care, comprehensive center recommendations have not resulted in creation of a corresponding national certification process-largely owing to difficulties in establishing quality metrics. The authors proposed 13 comprehensive quality measures and assessed them at a tertiary referral, teaching hospital in Houston, Texas. Proposed metrics were derived from the 2005 brain attack coalition's comprehensive center guidelines. Outcomes measures included morbidity and mortality rates for stroke, cerebral aneurysm, carotid endarerectomy, and stent patients. Process measures included timeliness of brain imaging interpretation, timeliness and consideration of antiplatelet treatment regimes for carotid stent patients, and documentation of National Institutes of Health Stroke Score. Metrics were defined by international classification of disease codes with accompanying inclusion and exclusion criteria. Internal quality was benchmarked against Primary Stroke Center, research literature, and University Health Consortium rates. Baseline data revealed significant opportunities for improvement in the categories of imaging interpretation timeliness and National Institutes of Health Stroke Score documentation. All other measures fell within expected ranges. Assessment of quality outcomes is the basis for disease-specific certification. Institutions that claim comprehensive capabilities must demonstrate high standards of performance on uniform validated quality metrics. The authors demonstrate the feasibility of operationalizing the metrics outlined in Brain Attack Coalition's comprehensive stroke center recommendations.

Ruptured Cerebral Aneurysm Patients Treated by Clipping or Coiling: Comparison of Long-term Neuropsychological and Personality Outcomes

A prospective study was conducted to compare the neuropsychological outcome of surgical versus endovascular treatment in patients with cerebral aneurysms. From April 2001 to 2005, 211 patients with ruptured cerebral aneurysms were treated at the senior author's institution. Of these 211 patients, 75 that were able and willing to undergo neuropsychological assessment 1 year after treatment of their aneurysm were enrolled in the study. Thirty-five patients were treated surgically and 40 by endovascular therapy. Standardized neuropsychological and personality tests were employed to assess cognitive and personality functions. One neurosurgical team using the same treatment protocols treated all patients. The two groups of patients did not differ significantly with respect to age, gender, concurrent diseases, Hunt and Hess grade, Fisher grade, frequency of complications, vasospasms and hydrocephalus development. No differences in performance on neuropsychological and cognitive tests (AVLT, TMT and WAIS-III) and personality variables and mood scales (TCI, BDI and SMS) were found one year postoperatively. If a full IQ as defined by WAIS-III and 1SD below the mean is considered as the main measure of cognitive deficits, 5.4% of the sample suffered from cognitive deficits. There were no differences between clipped and coiled patients (t=0.03; p=0.97). The differences in the neuropsychological assessment of patients treated by either coiling or clipping were small and non-significant. Given the small number of patients in the study, however, we suggest the need for further research with a larger sample size and the use of a randomized design before drawing any firm conclusions.

Retreatment of Ruptured Cerebral Aneurysms in Patients Randomized by Coiling or Clipping in the International Subarachnoid Aneurysm Trial (ISAT)

Because the long-term security of endovascular treatments remains uncertain, a follow-up study of the patients treated in the International Subarachnoid Aneurysm Trial was performed to compare the frequency, timing, and consequences of aneurysm recurrence. Patient data were reclassified by actual treatment performed. Aneurysm and patient characteristics, including occlusion grades, time and type of retreatment, and clinical outcomes, were compared. The relationship between these variables and late retreatment as a surrogate for recurrence was analyzed by means of the Cox proportional hazards model. Retreatment was performed in 191 of 1096 (17.4%) patients after primary endovascular coiling (EVT) and in 39 of 1012 patients (3.8%) after neurosurgical clipping. After EVT, 97 (8.8%) patients were retreated early and 94 (9.0%) late, 7 (0.6%) after rebleeding and 87 (8.3%) without. The mean time to late retreatment was 20.7 months. After neurosurgical clipping, 30 (2.9%) patients were retreated early and 9 (0.85%) late, 3 (0.3%) after rebleeding and 6 (0.6%) without. The mean time to late retreatment was 5.7 months. The hazard ratio (HR) for retreatment after EVT was 6.9 (95% CI=3.4 to 14.1) after adjustment for age (P=0.001, HR=0.97, 95% CI=0.95 to 0.98), lumen size (P=0.006, HR=1.1, 95% CI=1.03 to 1.18), and incomplete occlusion (P<0.001, HR=7.6, 95% CI=3.3 to 17.5). Late retreatment was 6.9 times more likely after EVT. Younger age, larger lumen size, and incomplete occlusion were risk factors for late retreatment after EVT. After neurosurgical clipping, retreatments were earlier; whereas EVT retreatments continued to be performed throughout the follow-up period. Short-term follow-up imaging is therefore insufficient to detect recurrences after EVT.

Anesthetic Management of Embolization for a Cerebral Aneurysm in Patient with Portal-systemic Encephalopathy - A case report -

The clinical syndrome of hyperammonemic encephalopathy is often encountered in the context of decompensated liver disease. Although it is rare in patients without hepatic disease, non-hepatic causes cannot be excluded. Anesthesiologists should be careful in choosing the anesthetic agent and perioperative management for hyperammonemic patients in order to avoid acute hyperammonemia and encephalopathy. We report successful general anesthesia during GDC (Guglielmi detachable coil) embolization for a large unruptured aneurysm in the right distal internal carotid artery in a female patient with hyperammonemic encephalopathy that was caused by a portal-systemic shunt. (Korean J Anesthesiol 2007; 53: 419～22)

高齢者破裂脳動脈りゅう治療成績向上の因子

The outcome for patients with ruptured cerebral aneurysms is improving due to various adjunct treatments, but the prognosis in elderly patients is still poorer than that in younger patients. To evaluate whether surgical results of ruptured cerebral aneurysms were improved in the elderly, we analyzed the outcome of ruptured cerebral aneurysms in patients over 70 years old between 1989 and 2003. Forty-four patients and 66 patients were treated with neck-clipping between 1989 and 1993 (Group A) and between 1999 and 2003 (Group B), respectively. Fifteen patients (Group C) underwent endovascular embolization with Guglielmi detachable coils from 1999 through 2003. The outcome was favorable in 36% of Group A and 52% of Group B, respectively (P=0.15). Factors for unfavorable outcome, e.g., vasospasm, surgical, and general complications, occurred equally in Group A and B. In Group C, 67% of the patients between Grade I and III in Hunt and Hess grading showed favorable outcomes. Surgical results of ruptured cerebral aneurysms tend to improve in elderly patients, but the factors contributing to the outcome have not changed. It is supposed that the better outcome results from development of surgical technique, neurointensive care and rehabilitation. Endovascular treatment can be employed more aggressively in elderly patients because of its safety and efficacy.

Commentary on the Article of A. Raabe, J. Beck, V. Seifert: Technique and Image Quality of Intraoperative Indocyanine Green Angiography During Aneurysm Surgery Using Surgical Microscope Integrated Near-Infrared Video Technology

An incompletely treated aneurysm may re-grow and lead to recurrent symptoms of hemorrhage or mass effect. The rebleeding rate of aneurysms with residual necks is between 3.5 % and 28 %; in the very long-term follow-up in extremely complex aneurysms operated on by Drake and Peerless the rebleeding rate was 10 %. Aneurysm re-growth has been reported to occur in 3.5 to 15 % of patients. The high rebleeding rate in the long term, i. e. 10-20 years, emphasizes the importance of perfect clipping (and nowadays also of complete occlusion by endovascular means). But also aneurysms with perfect clip placement may rupture or re-grow with mass effect and this was also our experience over the long term in Finland, a country with the possibility of carrying out long-term follow-ups and a country with a large number of patients with cerebral aneurysms (The Kuopio and Helsinki total experience is > 11 000 patients, close to 500 new cerebral aneurysm patients annually are seen in these two centers).

HIV and cerebral aneurysms

Patients infected with the human immunodeficiency virus (HIV) or suffering from acquired immunodeficiency syndrome (AIDS) are now surviving for longer periods of time secondary to improvements in medical management. As the classical causes of morbidity and mortality in this patient population have come under better control, new complications are becoming more prevalent. In our clinical practice, there appears to have been a rise in the number of patients with HIV infection and AIDS who have experienced aneurysmal subarachnoid hemorrhage (SAH). In this paper, we review the available literature regarding cerebral aneurysms in patients infected with HIV and/or suffering from AIDS.

Management of unruptured cerebral aneurysms in patients with polycystic kidney disease

Background Autosomal dominant polycystic kidney disease (PKD) is a hereditary disorder characterized by bilateral multiple renal cysts and early onset chronic renal failure. PKD patients tend to suffer their subarachnoid hemorrhage at a younger age. Unruptured aneurysms in PKD patients are not always innocuous, and proactive treatment has been indicated for these lesions. However, the management of PKD patients undergoing unruptured cerebral aneurysm surgery has been documented on only a few occasions. The purpose of this study was to better define the management of unruptured cerebral aneurysms in patients with PKD. Methods We present a retrospective review of the management of unruptured cerebral aneurysms in 16 patients with PKD. Eight patients were maintained through chronic hemodialysis whereas the remaining 8 patients did not require hemodialysis, at the time of treatment of their cerebral aneurysms. The mean follow-up period was 24 months. Results In the nonhemodialysis patients prophylactic hemodialysis was routinely performed after cerebral angiography to prevent deterioration of the pre-existing renal dysfunction. Microsurgical clipping of the aneurysm was performed in 15 patients (7 nonhemodialysis and 8 hemodialysis patients) and intravascular coil embolization was performed in 1 nonhemodialysis patient. One nonhemodialysis patient who underwent microsurgical clipping required a temporary hemodialysis after surgery, but the patient was not shifted to chronic hemodialysis. No patients developed postprocedural complications, and each showed an excellent recovery. Conclusion PKD patients with unruptured cerebral aneurysms can be safely treated with an appropriate treatment strategy including the use of prophylactic hemodialysis.

Commentary

BACKGROUND Autosomal dominant polycystic kidney disease (PKD) is a hereditary disorder characterized by bilateral multiple renal cysts and early onset chronic renal failure. PKD patients tend to suffer their subarachnoid hemorrhage at a younger age. Unruptured aneurysms in PKD patients are not always innocuous, and proactive treatment has been indicated for these lesions. However, the management of PKD patients undergoing unruptured cerebral aneurysm surgery has been documented on only a few occasions. The purpose of this study was to better define the management of unruptured cerebral aneurysms in patients with PKD. METHODS We present a retrospective review of the management of unruptured cerebral aneurysms in 16 patients with PKD. Eight patients were maintained through chronic hemodialysis whereas the remaining 8 patients did not require hemodialysis, at the time of treatment of their cerebral aneurysms. The mean follow-up period was 24 months. RESULTS In the nonhemodialysis patients prophylactic hemodialysis was routinely performed after cerebral angiography to prevent deterioration of the pre-existing renal dysfunction. Microsurgical clipping of the aneurysm was performed in 15 patients (7 nonhemodialysis and 8 hemodialysis patients) and intravascular coil embolization was performed in 1 nonhemodialysis patient. One nonhemodialysis patient who underwent microsurgical clipping required a temporary hemodialysis after surgery, but the patient was not shifted to chronic hemodialysis. No patients developed postprocedural complications, and each showed an excellent recovery. CONCLUSION PKD patients with unruptured cerebral aneurysms can be safely treated with an appropriate treatment strategy including the use of prophylactic hemodialysis.

Co-existence of unruptured cerebral aneurysms in patients with hypertensive intracerebral hemorrhage

Primary hypertensive intracerebral hemorrhage (PICH) is caused by a rupture of a small endartery, and diagnosis is made either by computed tomography (CT) or magnetic resonance imaging (MRI). Vascular abnormalities are not always evaluated in detail. In this study, we aimed to clarify the incidence of co-existing vascular abnormalities, especially unruptured cerebral aneurysms by reviewing selective intraarterial digital subtraction angiography (DSA) images in PICH patients. The cases of 169 hypertensive PICH patients who underwent selective intra-arterial DSA were reviewed. In all cases, CT or MRI showed no abnormality other than PICH, such as subarachnoid hemorrhage, component of arterio-venous malformation or cerebral tumor. The main reason for performing DSA was to exclude other causes of intracerebral hemorrhage such as ruptured cerebral aneurysm or small arterio-venous malformation prior to surgical removal of the hematoma. There were 33 patients with vascular abnormalities: unruptured cerebral aneurysm (n = 24, 14.2%), major vessels occlusion or stenosis (n= 8, 4.7%), and dural arterio-venous fistula (n = 1). Unruptured cerebral aneurysms were found in 9.4% of men and 20.5% of women. Vascular abnormalities co-existing with PICH are not rare, suggesting the necessity for angiographic evaluation. Special attention should be given to female PICH patients who have a high incidence of having an unruptured cerebral aneurysm.

Early Endovascular Treatment of Ruptured Cerebral Aneurysms in Patients in Very Poor Neurological Condition

Early Endovascular Treatment of Ruptured Cerebral Aneurysms in Patients in Very Poor Neurological Condition

Genesis of cerebral aneurysms--an update.

To elucidate the molecular pathogenesis of diseases has become a crucial step in the development of new treatment strategies. Although the pathogenesis of cerebral aneurysms has been studied intensively, it is poorly understood. Endogenous factors like elevated arterial blood pressure, the special anatomy of the Circle of Willis or the effect of haemodynamic factors, particularly originating at vessel bifurcation, are all known to be involved in the growth and rupture of an aneurysm. There is an ongoing discussion as to whether these factors also contribute to the very early steps of pathogenesis. Arteriosclerosis and secondary inflammatory reactions are thought to be elementary preconditions. Exogenous factors like cigarette smoking, heavy alcohol consumption or certain medications known to help generate arteriosclerosis and elevated blood pressure have also been found to be related to the occurrence of cerebral aneurysms. Furthermore, there has been a long-lasting debate on whether aneurysms might develop as a result of an inborn genetic defect. First-degree relatives of patients with cerebral aneurysms have a higher risk of having an aneurysm. In addition, the elevated prevalence of cerebral aneurysms in patients suffering from various inherited diseases points to a genetic background in the development of an aneurysm. Recent advances in molecular biology provide evidence that genetic variants of different candidate genes are associated with the occurrence of cerebral aneurysms. The aim of this review is to expose the current status of these various hypotheses and their contribution to the pathogenesis of cerebral aneurysms in order to provide a basis for future investigations in this field.

Incidental asymptomatic cerebral aneurysms in patients with extracranial cerebrovascular disease: is this a case against carotid endarterectomy without arteriography?

As less arteriography is performed before carotid surgery, concern arises about missing occult cerebral aneurysms and possible adverse outcomes. A study was conducted by the divisions of vascular surgery and neurosurgery of Northwestern University Medical School to evaluate the frequency of incidental cerebral aneurysms and outcomes of patients with extracranial cerebrovascular disease and asymptomatic cerebral aneurysms. From October 1995, through March 1997, 200 patients underwent intracranial and extracranial cerebrovascular angiography for evaluation of extracranial disease. Demographic data, symptoms, data of vascular lesions, surgical treatment and outcomes of stroke and death were recorded prospectively. Two patients (1%) had asymptomatic cerebral aneurysms found on angiography. Six more patients were referred with a known asymptomatic cerebral aneurysm with extracranial disease during this same period. Of these eight patients, five underwent extracranial vascular reconstruction surgery and seven received treatment for their aneurysms. There were two stroke complications, both occurred after treatment of a basilar artery aneurysm. One of these patients died. No aneurysms ruptured following 203 extracranial revascularizations during this same period. On the basis of the low prevalence of diagnosing coincidental cerebral aneurysms during work-up of extracranial disease, as well as the lack of evidence that carotid surgery predisposes to aneurysm rupture in these patients in both our study and the literature review, it is concluded that coexisting extracranial disease and asymptomatic cerebral aneurysms do not pose a case against carotid surgery without routine arteriography. However, arteriography should be considered in selected groups of patients where the yield of intracranial aneurysms is high; these include patients with a familial history of cerebral aneurysms, autosomal dominant polycystic kidney disease, extracranial internal carotid artery medial fibrodysplasia, Takayasu's arteritis, alpha1-antitripsin deficiency and atypical clinical presentations, including headache.

Cerebral aneurysms in patients with autosomal dominant polycystic kidney disease--to screen, to clip, to coil?

higher frequency in 27 patients with a definite or Subarachnoid haemorrhage (SAH) from ruptured suspected family history for aneurysms. In those two intracranial saccular aneurysms of the circle of Willis clinical studies, only one patient with an aneurysms is a devastating event with a mortality rate of over was younger than 30 years, i.e. 24 years. He had a 325% and an additional high risk of permanent disability mm aneurysm of the petrous segment of the internal [1–5]. The high incidence of intracranial aneurysms in carotid artery, which is associated with a very low risk patients with autosomal dominant polycystic kidney of bleeding. All of the detected aneurysms were smaller disease (ADPKD) has long been recognized. The than 7 mm in diameter. These findings were confirmed availability of non-invasive screening methods for by Ronkainen et al. [9]. asymptomatic aneurysms and the major advances in Based on the available literature, the following conmicrosurgical and endovascular techniques raise the clusions can be drawn: (i) the prevalence of aneurysms question of whether systematic screening and treatment in adult ADPKD patients is approximately 10%; of those patients is warranted. Because many variables (ii) the great majority of such aneurysms is small, less have to be taken into account in order to evaluate the than 10 mm, often less than 5 mm in diameter; natural risk as opposed to the risk of prophylactic (iii) familial clustering does occur; (iv) aneurysms are treatment, a clear consensus has not been reached and rarely present in patients younger than 30 years of age, decision processes vary among institutions. Based on and if so they are very small and are not detected by recent data we discuss the risks and benefits of screenMR angiography; (v) as compared to patients with ing and prophylactic treatment of asymptomatic intrasporadic aneurysms, no significant differences concranial aneurysms in patients with ADPKD. cerning number and location of aneurysms have been

Which unruptured cerebral aneurysms should be treated? A cost-utility analysis.

To determine which unruptured cerebral aneurysms should be treated considering the risks. benefits, and costs. Asymptomatic unruptured cerebral aneurysms are commonly treated by surgical clipping or endovascular coil embolization to prevent subarachnoid hemorrhage (SAH). We performed a cost-utility analysis comparing surgical clipping and endovascular coil embolization with no treatment for unruptured aneurysms. Eight clinical scenarios were defined based on aneurysm size, symptoms, and history of SAH from a different aneurysm. Health outcomes of a hypothetical cohort of 50-year-old women were modeled over the projected lifetime of the cohort. Costs were assessed from the societal perspective. We compared net quality-adjusted life years (QALYs) and cost per QALY of each therapy to no treatment. For an asymptomatic unruptured aneurysm less than 10 mm in diameter in patients with no history of SAH from a different aneurysm, both procedures resulted in a net loss in QALYs, and confidence intervals (CI) were not compatible with a benefit from treatment (clipping, loss of 1.6 QALY [95% CI 1.1 to 2.1]; coiling, loss of 0.6 QALY [95% CI 0.2 to 0.8]). For larger aneurysms (> or = 10 mm), those producing symptoms by compressing neighboring nerves and brain structures, or in patients with a history of SAH from a different aneurysm, treatment was cost-effective. Coiling appeared more effective and cost-effective than clipping but these differences depended on relatively uncertain model parameters. Treatment of small, asymptomatic, unruptured cerebral aneurysms in patients without a history of SAH worsens clinical outcomes, and thus is neither effective nor cost-effective. For aneurysms that are > or = 10 mm or symptomatic, or in patients with a history of SAH, treatment appears to be cost-effective.

Prevalence of cerebral aneurysms in patients with persistent primitive trigeminal artery.

The aim of this study was to determine the prevalence of cerebral saccular aneurysms in patients with persistent primitive trigeminal artery (PPTA). The prevalence of cerebral saccular aneurysms in patients with PPTA previously has been reported to be 14 to 32%, but this rate range is unreliable because it is based on collections of published case reports rather than a series of patients chosen in an unbiased manner. The authors retrospectively evaluated their own series of 34 patients with PPTA to determine the prevalence of cerebral aneurysms in this population. The prevalence of intracranial aneurysms in patients with PPTA was approximately 3% (95% confidence interval 0-9%). The prevalence of intracranial aneurysms in patients with PPTA is no greater than the prevalence of intracranial aneurysms in the general population.