V. INTRACRANIAL ANEURYSMS (abnormal localized dilatations in arteries)

A. Saccular aneurysms (berry or congenital aneurysms; most common type)

1. Etiology: 2 theories proposed


2. Characteristics

a. Prevalence: Aneurysms occur in about 1-5% of the population (most of these are asymptomatic).

b. Structure: The walls are composed of adventitia and intima only; the muscular and elastic coats are absent.

c. Most common sites: In adults, aneurysms most commonly occur at bifurcations of vessels at the base of the brain, especially in the anterior circulation (i.e. bifurcations of the internal carotid, middle cerebral, anterior cerebral and anterior communicating arteries). In children, saccular aneurysms are rarely seen, but occur most often in the posterior circulation.

d. Sequelae: Rupture is the most common sequel, with subarachnoid hemorrhage occurring most often. Intracerebral hemorrhage or rupture of the subarachnoid hemorrhage into the subdural space may also occur. In addition, aneurysm enlargement may exert local pressure on adjacent structures.

3. Aneurysm Rupture (rupture of a saccular aneurysm is the most common cause of spontaneous subarachnoid hemorrhage)

a. Incidence: The age group 20-50 is most often affected but a ruptured aneurysm may occur at any age.

b. Clinical course: Rapidly developing headache, followed by unconsciousness; consciousness may return soon. CAT scan, arteriography, and lumbar puncture are helpful in diagnosis.

c. Prognosis: One-third of patients die of initial rupture, one-third die of a subsequent rupture (recurrence is common within 2 weeks).

This coronal section shows subarachnoid hemorrhage, with blood in one lateral ventricle and the third ventricle. The base of the brain is severely involved as the site of a ruptured aneurysm.

The base of the brain shows subarachnoid hemorrhage due to a ruptured aneurysm; the right temporal tip has been removed to show the middle cerebral


This coronal section shows hydrocephalus of the frontal horns of the lateral ventricles. The hydrocephalus is secondary due to subarachnoid hemorrhage and blockage of CSF flow in the subarachnoid space, producing nonobstructive hydrocephalus.



B. Arteriosclerotic (atherosclerotic) Aneurysms (rare, secondary to atherosclerosis)

Arteriosclerotic aneurysms are uncommon; when found, patients are usually in the older age group. The aneurysm is usually spindle-shaped (fusiform) and located on the basilar artery. Thrombosis can occur due to arterial occlusion, and a pontine infarct may result.


C. Mycotic Aneurysms(refers to growth of any kind of microorganism in vessel wall; less than 1% of aneurysms)

Infection of a major artery that significantly weakens its walls is called a mycotic aneurysm. One major cause is septic emboli, usually from an infected cardiac valve. Thrombosis and rupture and possible complications.
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