Scanning electron microscopy of saccular intracranial aneurysms.

Eight intracranial saccular aneurysms from 6 autopsies were studied with the scanning electron microscope and the cerebral arteries from control autopsies and rabbits were compared. The intima of the aneurysms had a more uneven and rugged surface than the surroundings. Only slight atherosclerotic changes were discovered in the immediate vicinity of the aneurysms. The windows of the internal elastic lamella were enlarged at the mouth of the aneurysms; the edge of the muscle layer was rounded and showed fibrosis. The structure of the walls of the aneurysms differed from that of the control arteries because they were composed of collagenous connective tissue. The adventitia of the aneurysms resembled that of control arteries. The findings are in accord with the assumption that saccular aneurysms develop at sites of developmental media defects. The internal elastic lamella over the area of the media defect shows primarily compensatory hypertrophy and later degeneration, extension and decay.