Berry aneurysm most commonly occurs due to

Ref: Harrison 20th, P 2084As an aneurysm develops, it typically forms a neck with a dome. The length of the neck and the size of the dome vary greatly and are important factors in planning neurosurgical obliteration or endovascular embolization. The arterial internal elastic lamina disappears at the base of the neck. The media thins, and connective tissue replaces smooth- muscle cells. At the site of rupture (most often the dome), the wall thins, and the tear that allows bleeding is often <=0.5mm long. Aneurysm size and site are important in predicting risk of rupture. Those>7mm in diameter and those at the top of the basilar artery and at the origin of the posterior communicating artery are at greater risk