Delayed cerebral vasospasm and nitric oxide: review, new hypothesis, and proposed treatment

Despite years of research, delayed cerebral vasospasm remains the feared complication of a ruptured intracranial aneurysm. Worldwide effort has led to many promising experimental treatments that reverse or prevent cerebral vasospasm but none were confirmed to be effective in clinical trials. There are several sources for this failure: (1) the pathophysiology of delayed cerebral vasospasm remains poorly understood, (2) many experimental models of subarachnoid hemorrhage (SAH) do not mimic the actual clinical entity, and (3) many researchers erroneously extrapolate the data of peripheral and cerebral vascular physiological responses to the post-SAH situation. Thus, to explain the uniqueness of vasospasm and to address nitric oxide (NO) involvement in delayed vasospasm development, the following issues are addressed in this paper: (1) pathophysiological mechanisms of vasospasm, (2) NO-related contribution to its development. In addition, (3) a two-stage hypothesis of pathogenesis of delayed cerebral vasospasm is presented developed in the Vascular Laboratory of Surgical Neurology Branch of the National Institute of Neurological Disorders and Stroke using a primate model of SAH. According to this hypothesis, initially (Phase I) NO-releasing neurons are destroyed by oxyhemoglobin (oxyHb) leading to diminished availability of NO in the vessel wall and constriction of the vessels (Phase I). Increased shear stress evoked by narrowing of the arterial lumen should stimulate endothelial nitric oxide synthase (eNOS). But further metabolism of hemoglobin to bilirubin oxidized fragments (BOXes) increases asymmetric dimethylarginine (ADMA), an endogenous inhibitor of eNOS, in the vicinity of the artery further decreasing of NO availability and sustaining vasospasm (Phase II). In Phase III, the resolution of vasospasm, elimination of BOXes increases NO production by eNOS resulting in recovery of dilatory activity of endothelium. This hypothesis suggests that the key treatment to prevent delayed cerebral vasospasm should be focused on preventing oxyHb neurotoxicity, inhibiting BOX production, and exogenous NO delivery.