contained by layers of the normal arterial wall. They are often

irregularly shaped and generally arise on vessels distal to the

circle of Willis.

A PA develops when a completely disrupted blood vessel

hemorrhages. A paravascular hematoma forms and then

cavitates, establishing a communication with the parent vessel

wall. The wall of a PA thus consists only of organized clot. PAs

are much less common than either SAs or fusiform aneurysms.

PAs are acquired lesions caused by trauma, infection, or

inflammation ("mycotic" aneurysm), drug abuse, and

neoplasm ("oncotic" aneurysm).

Blood blister-like aneurysm (BBAs): BBAs are eccentric

hemispherical outpouchings that can develop at any location.

Most commonly found on the greater curvature of the

supraclinoid internal carotid artery, they are lined only by a

thin layer of adventitia. They are difficult to detect, difficult to

treat, and prone to rupture at smaller size and younger age

than typical SAs.

Fusiform aneurysms: Fusiform aneurysms can be

atherosclerotic (common) or nonatherosclerotic (rare). They

involve long, nonbranching vessel segments and are seen as

more focal circumferential outpouchings from an ectatic

vessel. Fusiform aneurysms are more common in the

vertebrobasilar (posterior) circulation.

Vertebrobasilar dolichoectasia: Fusiform enlargement or

ectasia, also called arteriectasis, is commonly seen in patients

with advanced atherosclerotic disease. Less commonly,

fusiform ectasias occur with collagen-vascular disorders and

non-atherosclerotic vascular disease arteriopathies.

Brain: Pathology-Based Diagnoses: Malformations,

Trauma, and Stroke

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