Types of Aneurysms, Etiologies and Risk Factors
There are three main types of cerebral aneurysms, each with its own pathogenesis.
Saccular (“berry”) aneurysms exploit a design feature of the cerebral vasculature. Because the cerebral vasculature possesses less smooth muscle and a thinner adventitia, these vessels are naturally weaker and more susceptible to aneurysmal formation given the right hemodynamic stresses and conditions1. In some people, there are gaps in the muscular layer (called “medial gap”), rendering these individuals particularly vulnerable.
Due to the increased turbulence at bifurcation points within the Circle of Willis, as well as the “elbows” of bending vessels, aneurysms are most common at these locations1,2. Most saccular aneurysms occur in the anterior circulation, with anterior communicating, posterior communicating and MCA bifurcation being the most common2. Posterior circulation aneurysms such as basilar tip aneurysms and PICA aneurysms are less common but do occur2.
Fusiform aneurysms are found in older individuals and result of severe form of athrosclerosis. They may be long and tortuous, calcified and with no identifiable neck. These aneurysms may lead to thrombosis and brainstem or cranial nerve compression2. They occur most commonly in the vertebrobasilar system1.
Dissecting aneurysms arise due to trauma or vasculopathy. They commonly occur in the extra cranial portions of the internal carotid or vertebral arteries. Intracranial dissecting aneurysms do occur and are usually due to traumatic injury.
For the present module, we will only concern ourselves with the first and third types of aneurysm, the saccular or “berry” aneurysm, and the fusiform aneurysm.