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Epidemiology of Intracranial Aneurysm and Subarachnoid Hemorrhage.

Abstract: Intracranial aneurysmal hemorrhage is a common but devastating condition associated with significant morbidity and mortality. Epidemiologic studies have identified risk factors associated with this condition. Genetic factors involve family history and the presence of certain heritable connective tissue disorders such as Ehlers-Danlos syndrome, Marfan's syndrome, neurofibromatosis, and polycystic kidney disease. Acquired factors include traumatic brain injury, sepsis, smoking, and hypertension. Management of these patients consists of prevention, patient screening, and prophylactic aneurysm repair.

Each year in the United States there are 25,000 new cases of subarachnoid hemorrhage (SAH) secondary to rupture of an intracranial aneurysm. The reported incidence of SAH varies, but the presence of intracranial aneurysm noted on autopsy ranges from 2% to 5%. Once ruptured, the mortality from aneurysmal SAH is high, with an overall death rate of 16 per 100,000 population.[1,2]

Although the exact mechanism of intracranial aneurysm development is not clearly understood, there is evidence that both acquired and genetic factors play a role. Genetic factors consist of heredity as well as several genetically transmitted disease states. Acquired factors include traumatic brain injury, sepsis, cigarette smoking, and hypertension. This review provides neuroscience nurses with an understanding of the epidemiology and risk factors for intracranial aneurysm and the associated prevention measures.

Genetic Factors

Family History

A history of familial intracranial aneurysms is gaining appreciation as a considerable risk factor for intracranial aneurysm formation and SAH. Studies have demonstrated a two- to sevenfold increased risk for intracranial aneurysm in first-degree relatives compared to a control population.[1,11,12] Clearly, there is a strong relationship between genetics, disease transmission, and family heredity and the presence of intracranial aneurysm and SAH.

Approximately 5% of all cases of intracranial aneurysm are associated with heritable connective tissue disorders, the most important being Ehlers-Danlos syndrome type IV, Marfan's syndrome, neurofibromatosis type 1, and polycystic kidney disease.[12] These diseases manifest with vascular wall defects due to various etiologies; hence, patients with these diseases have a higher incidence of intracranial aneurysms than the average population.[2]

Ehlers-Danlos Type IV

Ehlers-Danlos syndromes are a heterogeneous group of disorders characterized by joint hypermobility, hyperelastic or fragile skin, easy bruising, and abnormal scarring. Of the nine established types, Ehlers-Danlos type IV is the least common and most lethal with a prevalence of 1 in 50,000-500,000 persons. However, the true frequency of patients with both aneurysms and Ehlers-Danlos syndrome is not known because of difficulty in diagnosing the disease. Ehlers-Danlos type 1V results from a deficiency of type III collagen, which constitutes the major component of distensible tissues such as arteries and veins. The typical presentation of this syndrome is rather vague and nonspecific; fragile skin and mild joint hypermobility are the most common findings. Spontaneous rupture, dissection, or aneurysm formation in large and medium-sized arteries characterizes the vascular complications. The association between intracranial aneurysms and Ehlers-Danlos type IV is well established.[12]

Marfan's Syndrome

The link between Marfan's syndrome and intracranial aneurysm is less clear In one small study; two out of seven autopsies of Marfan's patients demonstrated the presence of intracranial aneurysms. Elongation of the long bones and abnormalities of the eyes and the cardiovascular system typify Marfan's syndrome. The vascular pathology produces fragmentation of the elastic fibers in the media that can lead to aneurysm formation. Mutations in the gene encoding fibrillin-1 (FBN-1), a glycoprotein that is one of the major components of microfibrils, causes Marfan's syndrome. Approximately 1 in 10,000-20,000 people are affected by this disorder. Intracranial aneurysms associated with Marfan's syndrome can be saccular, fusiform, or dissecting and are usually found in the proximal intracranial carotid artery.[12]

Neurofibromatosis Type 1

Neurofibromatosis type 1 is a progressive congenital condition that affects 1 in every 3,000-5,000 individuals. It is characterized by numerous neurofibromas of the nerve cells and skin and, in some cases, by developmental anomalies of the muscles, bones, and viscera. The vascular complications of neurofibromatosis include stenosis, vessel rupture, and aneurysm or fistula formation in large and medium-sized arteries.[12]

Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is one of the more common inherited disorders affecting 1 in 400-1,000 people. Patients with ADPKD generally present with enlarged kidneys and the formation of cysts, not only in the kidneys but also in organs such as the liver, pancreas, and spleen. The formation of such cysts begins with a genetic mutation that causes hyperprolific cell growth, fluid secretion, and extracellular-matrix composition.[4,7] Intracranial arachnoid cysts and inguinal hernias are common findings as well.[12] Hypertension, a serious complication of ADPKD, is found in about 75% of patients with the disease. This increase in blood pressure serves as an additional risk factor for aneurysm rupture and SAH in patients with ADPKD.

Several studies support the existence of a relationship between intracranial aneurysm and ADPKD. Estimates of the frequency of aneurysms in ADPKD patients range from 10% to 41%.[5] In a retrospective study of 86 ADPKD patients, 20% were found to have died from cerebral complications. Seventy percent of these deaths were due to hypertensive intracerebral hemorrhage, 17.5% were secondary to cerebral infarction, and 11.5% resulted from rupture of a saccular aneurysm.[4] Intracranial aneurysms have been reported in one-fourth of ADPKD patients and are the primary cause of death in one-fifth, roughly 20%.[12]

Family history also plays a role in the incidence of intracranial aneurysms and SAH in patients with ADPKD. Between 18% and 22% of ADPKD patients with intracranial aneurysm have a family history positive for aneurysm.[4]

Acquired Factors

Acquired, or secondary, factors contribute to the incidence of intracranial aneurysm and SAH. Although not as strongly suggestive as genetics and heredity, acquired factors are the most important etiologies of intracranial aneurysm and SAH. Traumatic brain injury (TBI), hypertension, smoking, and sepsis are among the acquired factors thought to contribute to intracranial aneurysm formation and subsequent SAH.

Traumatic Brain Injury

Aneurysms associated with TBI represent less than 1% of all aneurysms.[10] Traumatic aneurysms are the result of hemorrhage and clot formation from a vessel tear or from dissection as a result of arterial wall injury. Although the incidence of aneurysm associated with TBI is small it must be considered as a risk factor in trauma patients for up to several months postinjury, particularly with severe injury to the mid and lower face and head.[10]


Septic aneurysms account for 2%-6% of aneurysms and result from emboli that transmit organisms that then adhere to the vessel wall, causing inflammation and necrosis. Septic aneurysms, most commonly associated with infections of the heart valves or pulmonary veins, carry a high risk for death whether treated medically or surgically.[14]

Smoking and Hypertension

Although the relative incidence of intracranial aneurysm and SAH from traumatic injury and sepsis are low, hypertension and smoking pose much greater threats. Smoking is a substantial but modifiable risk factor for intracranial aneurysm and SAH. The relative risk of spontaneous aneurysmal SAH in smokers is reported to be twice that of nonsmokers.[8,15] Cigarette smoking is also correlated with younger age of onset of SAH (by 5-10 years) and increased incidence of clinically confirmed vasospasm.[15] Not only is smoking associated with a heightened incidence of aneurysm and SAH, but it also contributes to the development of hypertension. There is a significant relationship between hypertension and intracranial aneurysm and SAH.[13] In one study, hypertension was reported to be 8.3 times more frequent among patients with SAH than among those in the control group.[3]

Miscellaneous Acquired Factors

Several other factors--including age, gender, alcohol use, seasonal variations, and atherosclerosis--are thought to contribute to increased frequency of intracranial aneurysm and SAH. Nakagawa et al. found that there was a rising occurrence of intracranial aneurysm associated with increasing age and the female gender.[9] Drinking 150 grams of alcohol or more per week also has been correlated with increased rates of SAH.[13] In several studies, atherosclerosis has been linked with intracranial aneurysm and SAH.[11] Interestingly, some studies have suggested a link between weather changes and increased incidence of aneurysmal rupture.[6] Further investigation is required to determine the mechanism of association between these risk factors and the development of intracranial aneurysm and SAH.


The potential benefits of minimizing the risks associated with acquired factors for intracranial aneurysm and SAH are obvious. Hypertension and smoking are both preventable risk factors. Controlling hypertension and eliminating smoking can reduce the incidence of aneurysm formation and rupture.[8]


Screening for intracranial aneurysm is gaining in popularity as a means of decreasing the progression to rupture and SAH. New technologies make this process more accurate and less invasive. Although the most specific diagnostic examination remains four-vessel arteriography, computed tomography (CT) scans (with or without angiography), magnetic resonance imaging (MRI), and magnetic resonance angiography (MRA) are also sensitive for detecting aneurysms. CT angiography and MRI/MRA not only are accurate, but also have the added benefit of being non-invasive. Persons with a strong family history of cerebral aneurysms or heritable connective tissue disorders, and those at high risk from acquired disorders, are candidates for screening.

Of these diagnostic studies, MRA is the most utilized because it does not require contrast material, carries essentially no risk, and is able to detect intracranial aneurysm as small as 2-3 millimeters.[9,11] Five millimeters is thought to be the critical size for intracranial aneurysm rupture.[9,11] However, due to the expense of MRA ($500-$1,000), there is currently only support for screening persons with significant demonstrated risk.

Prophylactic Repair

Surgical treatment of diagnosed but unruptured aneurysm is an option. Several studies have demonstrated that early aneurysm detection and appropriate invasive intervention decrease the incidence of a subsequent SAH and extend life span.[9,16] In one study of 400 healthy volunteers, 26 (6%) were found to have unruptured cerebral aneurysms. Aneurysm clipping was performed in 20 of these cases with only one minor complication--a postoperative complaint of decreased olfaction.[9] The authors concluded that "early detection and aggressive surgical treatment may improve the outcomes for patients harboring cerebral aneurysms by preventing the devastating effects of SAH."[9] A more recent study, however, reported cognitive deficits post-treatment and suggested only higher risk aneurysms be treated.[16]

Endovascular treatment, or aneurysm coiling by angiography, is a relatively non-invasive procedure compared to traditional aneurysm clipping. In this procedure the vessel is either obstructed at the aneurysm site with coils, or the parent artery is completely occluded. Endovascular treatment is effective and is associated with lower risks than intracranial surgery.[9] Stroke and aneurysmal rupture are potential consequences. The long-term outcome of these patients remains unknown.


Currently, high morbidity and mortality rates from aneurysmal subarachnoid hemorrhage can be minimized by implementing strategies for the prevention of acquired factors, screening populations at risk, and providing aggressive intervention. With the advent of new diagnostic technologies and surgical procedures, it is possible to prospectively identify and treat persons with aneurysms, thus preventing the devastating effects of rupture. Ongoing research is necessary to evaluate the current theories and management strategies.


[1.] Awasthi D: Screening for cerebral aneurysms. Retrieved October 30, 1999, from the World Wide Web:

[2.] Camarata PJ, Latchaw RE, Rufenacht DA, Heros RC: Intracranial aneurysms. Invest Radiol 1993; 28(4): 373-382.

[3.] Canhao O, Pinto AN, Ferro J, Ferro JM: Smoking and aneurysmal subarachnoid hemorrhage: A case control study. J Cardiovasc Risk 1994; 1(2): 155-158.

[4.] Chapman AB, Johnson AM, Gabow PA: Intracranial aneurysms in patients with autosomal dominant polycystic kidney disease: How to diagnose and who to screen. Am J Kidney Dis 1993; 22(4): 526-531.

[5.] Kaehny WD, Everson GT: Extrarenal manifestations of autosomal dominant polycystic kidney disease. Semin Nephrol 1991; 11(6): 661-670.

[6.] Landers AT, Narotam PK, Govender ST, van Dellen JR: The effect of changes in barometric pressure on risk of rupture of intracranial aneurysm. Br J Neurosurg 1997; 11(3): 191-195.

[7.] McCarthy S, McMullen M: Autosomal dominant polycystic kidney disease: Pathophysiology and treatment. ANNA J 1997; 24(1): 45-51.

[8.] Morris KM: Smoking and subarachnoid hemorrhage: A case control study. Br J Neurosurg 1992; 6(5): 429-432.

[9.] Nakagawa T, Hashi K: The incidence and treatment of asymptomatic, unruptured cerebral aneurysms. J Neurosurg 1994; 80(2): 217-223.

[10.] O'Brien D, O'Dell MW, Eversol A: Delayed traumatic cerebral aneurysm after brain injury. Arch Phys Med Rehabil 1997; 78(8): 883-885.

[11.] Rinkel GJ, Djibuti M, Algra A, van Gijn J: Prevalence and risk of rupture of intracranial aneurysms: A systematic review. Stroke 1997; 29: 252-256.

[12.] Schievink WI: Genetics of intracranial aneurysms. Neurosurgery 1997; 40(4): 651-663.

[13.] Teunissen LL, Rinkel GJ, Algra A, van Gijn J: Risk factors for subarachnoid hemorrhage: A systematic review. Stroke 1996; 27(3): 544-549.

[14.] Toole JF: Intracranial arterial aneurysms. Pages 470-669 in: Cerebrovascular Disorders, 4th ed. Raven Press, 1990.

[15.] Weir BK, Kongable GL, Kassell NF, et al: Cigarette smoking as a cause of aneurysmal subarachnoid hemorrhage and risk for vasospasm: A report of the Cooperative Aneurysm Study. J Neurosurg 1998; 89(3): 405-411.

[16.] Weibers DO: Unruptured intracranial aneurysms--risk of rupture and risks of surgical intervention: The International Study of Unruptured Intracranial Aneurysms Investigators. New Engl J Med 1998; 339: 1725-1733.

Question or comments about this article may be directed to: Maret Pfohman, BSN RN, Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, Mailcode 8Q, Portland, OR 97201. She is the clinical manager of the Trauma/Neuro ICU at Oregon Health Sciences University, Portland.

Laura M. Criddle, MS RN CCRN, is an emergency, trauma, and neuro clinical nurse specialist at Oregon Health Sciences University.
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Author:Pfohman, Maret; Criddle, Laura M.
Publication:Journal of Neuroscience Nursing
Geographic Code:1USA
Date:Feb 1, 2001
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