Intracranial aneurysms are common vascular diseases, found in 1-3% of the population1,2. It constitutes of a localized abnormal dilatation of a cerebral artery. Over time, the blood flow within these arteries continuously pounds against the wall, especially at the point where the vessel divides. The resultant wear and tear lead to the thinning of the wall, followed by gradual dilation of the weakened segment.
Circle of Willis
The most common location of the cerebral aneurysms is a circle of arterial connections at the base of the brain, called the circle of Willis. The purpose of this arterial circle is to provide an alternative supply to certain areas of the brain if the primary vessel supplying that area got occluded. The point where the vessels divides and communicates, is highly exposed to the wear and tear due to the turbulent blood flow, leading to a higher risk of aneurysm formation.
Aneurysm Types
Aneurysms can be divided into three types based on their shape:
- Saccular aneurysms “Berry aneurysms” (see figure): These are the most common type of aneurysms, forming around 80-90% of all intracranial aneurysms. The berry-like dilation usually occurs at sites where a certain blood vessel leaves the parent artery, or at the side of bifurcation of the parent artery. They are also the most common cause of subarachnoid hemorrhage following their rupture.
Figure : Saccular aneurysm on the left Middle Cerebral Artery
- Fusiform aneurysms: (see figure) These aneurysms occur as a result of circumferential dilation of the vessel, due to a weakened segment of the wall. They have lower risk of rupture compared to saccular aneurysms, but they also expose patient to embolic strokes.
Figure: Fusiform aneurysm at the pericallosal artery.There is a concurrent saccular aneurysm at the callosomarginal artery
- Dissecting aneurysm (see figure). A dissecting aneurysm is caused by a tear in the inner arterial wall (Intima), which may result in a outpouching on one side of the artery wall and an obstruction of blood flow through the artery. Dissecting aneurysms may occur spontaneously or secondary to trauma. Dissecting aneurysm might put patient at risk for embolic stroke as well as rupture depending on type. They are classified in 4 types. Grade 1 describes mild intimal injury or irregular intima. Grade II entails dissection with a raised intimal flap, intramural hematoma with luminal narrowing >25%, and/or intraluminal thrombosis. Grade III involves a pseudoaneurysm. Vessel occlusion or thrombosis is grade IV. Finally, grade V – the most severe – is vessel transection.
Figure : Dissecting PICA-aneurysm. A: Preoperative Angiogram. B: Intraoperative image. C: Intraoperative image after wrap-clipping.
References
- Vlak MH, Algra A, Brandenburg R, Rinkel GJ. (2011). Prevalence of unruptured intracranial aneurysms, with emphasis on sex, age, comorbidity, country, and time period: a systemtatic review and meta-analysis. Lancet Neurology. 10(7): 626-636.
- Rinkel GJE, Djibuti M, Algra A, van Gijn J. (1998). Prevalence and risk of rupture of intracranial aneurysms: a systematic review. Stroke. 29: 251-256.