V. INTRACRANIAL ANEURYSMS (abnormal localized
dilatations in arteries)
A. Saccular aneurysms (berry or congenital aneurysms; most common
- 1. Etiology: 2 theories proposed
- Congenital defect in wall of artery leading to development of aneurysm
- Development and enlargement of aneurysm dependent on presence of hypertension
- 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
- 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
- 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|
- d. Complications:
- (1)Localized arterial vasospasm due to blood and its breakdown products
in the subarachnoid space
- (2)Hydrocephalus from subarachnoid hemorrhage and reactive leptomeningitis
||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
B. Arteriosclerotic (atherosclerotic) Aneurysms (rare, secondary to
- 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.