The defect in berry aneurysm is
Multiple aneurysms are observed in about 20-30% of cases of berry aneurysm. Such aneurysms are congenital in that defect in aerial wall may be present from bih, but actual aneurysm develops over years; thus, rupture is most likely to occur in middle-aged adults. Refer Robbins page no 1270
Degeneration of internal elastic lamina
Degeneration of media
Deposition of mucoid material in media
Low grade inflammation of vessel wall
Nervous system