Rupture of aneurysm. Subarachnoid hemorrhage is due to the rupture of a saccular aneurys. The aneurysm are likened to the weakening of a tire wall, only they occur in the wall of the cerebral vessels. There is increased incidence of sah in patients with poorly controlled blood pressure and in congenital polycystic kidney disease. There are about 26, 000 sah/yr. With peak incidence between 35 and 65 yrs.