are based on general experience, and the only randomized controlled trial directly comparing the different modalities was performed in relatively well people with small (less than 10 mm) aneurysms of the anterior cerebral artery and anterior communicating artery (together the "anterior circulation"), who constitute about 20 percent of all people with aneurysmal SAH. This trial, the International Subarachnoid Aneurysm Trial (ISAT), showed that in this group the likelihood of death or being dependent on others for activities of daily living was reduced (7.4 percent absolute risk reduction, 23.5 percent relative risk reduction) if endovascular coiling was used as opposed to surgery. The main drawback of coiling is the possibility that the aneurysm will recur; this risk is extremely small in the surgical approach. In ISAT, 8.3 percent needed further treatment in the longer term. Hence, people who have undergone coiling are typically followed up for many years afterwards with angiography or other measures to ensure recurrence of aneurysms is identified early. Other trials have also found a higher rate of recurrence necessitating further treatments. Vasospasm Vasospasm, in which the blood vessels constrict and thus restrict blood flow, is a serious complication of SAH. It can cause ischemic brain injury (referred to as "delayed ischemia") and permanent brain damage due to lack of oxygen in parts of the brain. It can be fatal if severe. Delayed ischemia is characterized by new neurological symptoms, and can be confirmed by transcranial doppler or cerebral angiography. About one third of people admitted with subarachnoid hemorrhage will have delayed ischemia, and half of those have permanent damage as a result. It is possible to screen for the development of vasospasm with transcranial Doppler every 24–48 hours. A blood flow velocity of more than 120 centimeters per second is suggestive of vasospasm. The pathogenesis of cerebral vasospasm following subarachnoid hemorrhage is attributed to the higher levels of endothelin 1, a potent vasoconstrictor, and the lower levels of endothelial NOS (eNOS), a potent vasodilator. Both of which are produced from a series of events that begin from the inflammatory reaction caused by the products released from erythrocytes' degradation. Following subarachnoid hemorrhage, different clotting factors and blood products are released into the surrounding perivascular spaces known as (Virchow-Robin spaces). The released clotting factors like; fibrinopeptides, thromboxane A2 and others lead to microthrombosis around near vessels that leads to extrinsic vasoconstriction of these vessels. Besides that extrinsic vasoconstriction, the