Cerebral aneurysms are balloon-like dilatations along normal arteries that develop most frequently along a ring of interconnected arteries at the base of the brain, a structure known as the circle of Willis. The prevalence of cerebral aneurysms in the population is 5%, and about a third of the patients with aneurysms have more than one.
Types of Cerebral Aneurysms
Saccular aneurysms, the most common type of cerebral aneurysm, develop along weak spots in the arterial wall. About 85% of saccular cerebral aneurysms are located in the anterior circulation, such as the anterior communicating artery, posterior communicating artery, and the middle cerebral artery. Fifteen percent occur along the posterior circulation, such as the basilar apex, the vertebral artery, or the superior cerebellar artery.
Other types of aneurysms also exist. Dissecting aneurysms form from injuries to the innermost layers of the blood vessel, such as after traumatic injury or from the formation of atherosclerotic plaque. These subtypes are not associated with branch points of the arterial tree. Mycotic aneurysms are caused by infectious agents, often in association with subacute bacterial endocarditis. Multiple mycotic aneurysms, found along the distal (superficial) portions of the brain arteries, are common. Pseudoaneurysms, a dilatation of an artery, can form when abrupt, severe trauma injures an artery. They usually form in the regions where the falx or tentorium is near cerebral arteries.
The four types of aneurysms (A) saccular, (B) dissecting, (C) mycotic, and (D) pseudoaneurysm.
Most aneurysms occur sporadically. However, they can be associated with other medical conditions such as Marfan's syndrome, fibromuscular dysplasia, polycystic kidney disease, and Ehlers-Danlos syndrome. In rare instances, cerebral aneurysms are a heritable condition that runs in families. Current guidelines recommend screening tests for cerebral aneurysms if two or more first-degree relatives in one family are known to have harbored cerebral aneurysms.
Signs and Symptoms
||Three-dimensional angiography of a bilateral middle cerebral artery aneurysm.
The most common mode of presentation is rupture of the aneurysm, producing a serious condition known as subarachnoid hemorrhage (SAH). Invariably such hemorrhage is accompanied by a sudden, severe onset of headache known as a "thunderclap" headache. If located in areas where the enlarging lesion distorts adjacent brain structures, unruptured aneurysms can also produce focal neurological deficits, cranial nerve palsies, seizure disorders, meningismus, ocular hemorrhage, and coma. Computed tomography (CT) angiography, magnetic resonance (MR) angiography, lumbar puncture, and catheter-based angiography can be used to diagnose aneurysms.
Risks of Aneurysmal Rupture
The overall cumulative risk of aneurysmal rupture is about 1 to 2% per year. Each year aneurysms rupture in 10 to 20 per 100,000 persons. The size and location of an aneurysm can predict its risk of hemorrhage. For example, large aneurysms of the posterior fossa, such as a basilar artery aneurysm, are associated with a much higher risk of rupture annually than small aneurysms of the anterior circulation. The rates of mortality and permanent neurological morbidity associated with a ruptured aneurysm range from 20 to 50%. Patients who survive the initial rupture of an aneurysm typically require 2 to 3 weeks of monitoring in the intensive care unit. Depending of the severity of the hemorrhage, however, they may require weeks to months to recover. Because aneurysmal subarachnoid hemorrhage poses a substantial risk to patients, unruptured aneurysms frequently warrant treatment. Decisions for treatment depend on factors such as the size and location of the aneurysm, the presence of other aneurysms, cigarette smoking, and family history.
Several days after aneurysmal rupture and subarachnoid hemorrhage, independent of treatment for the aneurysm, a condition called vasospasm may develop. Vasospasm is a constrictive narrowing of cerebral arteries that can restrict adequate blood flow from reaching brain tissue. Vasospasm usually begins 3 to 5 days after an aneurysm ruptures and can last as long as 2 to 3 weeks. The effect on the patient can be mild, moderate, or severe depending on the degree of vasospasm and the patient's responsiveness to treatment. About 50 to 70% of patients with a ruptured aneurysm have some degree of vasospasm. It can be fatal in 7% of patients.
Vasospasm can be monitored by cerebral angiography, CT perfusion, transcranial Doppler ultrasonography, and careful neurological evaluation of the patient. Hyperdynamic therapy helps combat vasospasm. This treatment uses hypervolemia, or an increase in the volume of the circulating blood, and hemodilution (thinning of the blood) to improve the flow characteristics of the blood. At the same time, hypertension, a medically-induced increase in blood pressure, is used to enhance blood flow through the narrowed vasospastic arteries. Medications such as nimodipine, a calcium-channel blocker, also may be used to help reduce vasospasm. This medication is usually taken by mouth every 2 to 4 hours for 14 to 21 days after an aneurysm ruptures.
If a patient's vasospasm does not respond to hyperdynamic therapy, then endovascular techniques may be used. Microcatheters can be guided into the arteries of the circle of Willis to infuse vasodilating drugs. Focal areas of severe vasospasm can also be alleviated by inserting balloon microcatheters. The balloon is then dilated to open the narrowed arteries.
Microsurgery. Microsurgical treatment requires a craniotomy (an opening in the skull) to reach an aneurysm. The location of the craniotomy depends on the aneurysm. At Barrow Neurological Institute, we routinely use skull-base approaches to maximize exposure to aneurysms and to minimize brain retraction. During dissection, the operating microscope provides high-powered magnification and illumination along natural corridors of the brain that help the surgeon to reach the aneurysm. Titanium aneurysm clips may be used to clip the base of the aneurysm closed while blood flow is maintained through the normal arteries. More complex aneurysms may require additional microsurgical techniques, such as trapping the aneurysm and creating an arterial bypass around it.
Endovascular. Endovascular treatment involves using microcatheters to place metal coils into the lumen of the aneurysm from inside the arteries. Access to the arterial system is obtained through the common femoral artery in the groin. Using an iodine-based contrast dye and biplanar fluoroscopy to visualize instruments and arterial anatomy, catheters are carefully guided into the brain arteries. Once the microcatheter has been placed inside of the aneurysm, specialized platinum and coated coils are deposited into the lesion. These materials occlude the aneurysm, isolating it from the circulation so that it no longer fills with blood. Obliteration of the aneurysm in this fashion prevents it from rupturing.
Read more about Endovascular Neurosurgery at Barrow.
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