A cerebral aneurysm (also known as an intracranial or intracerebral aneurysm) is a weak or thin spot on a blood vessel in the brain that balloons out and fills with blood. The bulging aneurysm can put pressure on a nerve or surrounding brain tissue. It may also leak or rupture, spilling blood into the surrounding tissue (called a hemorrhage). Some cerebral aneurysms, particularly those that are very small, do not bleed or cause other problems. Cerebral aneurysms can occur anywhere in the brain, but most are located along a loop of arteries that run between the underside of the brain and the base of the skull. Cerebral aneurysms are pathologic focal dilatations of the cerebrovasculature that are prone to rupture. These vascular abnormalities are classified by presumed pathogenesis. Saccular, berry, or congenital aneurysms constitute 90% of all cerebral aneurysms and are located at the major branch points of large arteries. Dolichoectatic, fusiform, or arteriosclerotic aneurysms are elongated outpouchings of proximal arteries that account for 7% of all cerebral aneurysms. Infectious or mycotic aneurysms are situated peripherally and comprise 0.5% of all cerebral aneurysms. Other peripheral lesions include neoplastic aneurysms, rare sequelae of embolized tumor fragments, and traumatic aneurysms.
A common location of cerebral aneurysms is on the arteries at the base of the brain, known as the Circle of Willis. Approximately 85% of cerebral aneurysms develop in the anterior part of the Circle of Willis, and involve the internal carotid arteries and their major branches that supply the anterior and middle sections of the brain. The most common sites include the anterior communicating artery (30-35%), the bifurcation of the internal carotid and posterior communicating artery (30-35%), the bifurcation of the middle cerebral artery (20%), the bifurcation of the basilar artery, and the remaining posterior circulation arteries (5%).
Most cerebral aneurysms result from an inborn abnormality in an artery wall. Cerebral aneurysms are also more common in people with certain genetic diseases, such as connective tissue disorders and polycystic kidney disease, and certain circulatory disorders, such as arteriovenous malformations - congenital malformations in which a snarled tangle of arteries and veins in the brain disrupts blood flow.
The signs and symptoms of an unruptured cerebral aneurysm will partly depend on its size and rate of growth. For example, a small, unchanging aneurysm will generally produce no symptoms, whereas a larger aneurysm that is steadily growing may produce symptoms such as loss of feeling in the face or problems with the eyes. Immediately before an aneurysm ruptures, an individual may experience such symptoms as a sudden and unusually severe headache, nausea, vision impairment, vomiting, and loss of consciousness.
Most aneurysms go unnoticed until they rupture. In about 10 to 15 percent of cases, however, there are symptoms. Common warning signs include an enlarged pupil in one eye, a drooping eyelid, or pain above or behind the eye. Other symptoms include a headache in one specific part of the head, difficulty in walking, double vision, or numbness in the face.
Diagnosis of a ruptured cerebral aneurysm is commonly made by finding signs of subarachnoid hemorrhage on a CT scan (Computerized Tomography, sometimes called a CAT scan). The CT scan is a computerized test that rapidly X-rays the body in cross-sections, or slices, as the body is moved through a large, circular machine. If the CT scan is negative but a ruptured aneurysm is still suspected, a lumbar puncture is performed to detect blood in the cerebrospinal fluid (CSF) that surrounds the brain and spinal cord.
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