gms | German Medical Science

Objective: Despite advances in neurointensive care and surgical/ endovascular aneurysm occlusion techniques subarachnoid hemorrhage (SAH) remains a disease with high mortality and morbidity. Experimental and clinical studies indicate that pathological changes in the early phase (72 hours) after SAH, summarized as early brain injury (EBI), determine the extent of brain damage and, thus, neurological outcome. A main characteristic of EBI is microcirculatory dysfunction. Potential mechanisms for this include microvascular constriction (microvasospasms) and impaired CO₂-reactivity of cerebral microvessels. Both pathophysiological phenomenona have been linked to nitric oxide (NO) depletion after SAH. Endothelial NO-synthase (eNOS) – the main source of cerebrovascular under physiological conditions - is dysfunctional early after SAH but it is unclear how eNOS function/ dysfunction is linked with early brain injury and posthemorrhagic brain damage. In the current study we evaluated parameters of EBI in mice lacking eNOS and respective wildtype controls.

Methods: SAH was induced in male eNOS^-/- mice and wildtype (WT) mice using the MCA-perforation model under continuous control of ICP, CBF, and MAP. Three hours after SAH, the cerebral microcirculation was directly visualized using in vivo-2-photon-microscopy (IV-2PM). CO₂ vessel reactivity was directly studied via IV-2PM before, during, and after induction of hypercapnia (7.5%); exogenous NO was supplied by inhalation (50 ppm) before CO_{2 challenge}.

Results: eNOS^-/- mice had significantly increased pathology after SAH with more severe intracranial hypertension and hypoperfusion starting immediately after induction of SAH (p< 0.05 vs. control until 30 min post SAH), significantly more re-bleedings (eNOS^-/- 4.2/h, WT 0.7/h, p< 0.05) which translated into massively increased post-SAH mortality: while 50% (10/20) of eNOS^-/- animals did not survive until IVM 3 hours post SAH no control animal (0/10) died. The cerebral microcirculation was significantly impaired 3 hours after SAH, there was significant microarteriolar narrowing and formation of microvasospasm; the extent of microcirculatory dysfunction was comparable in both groups. Exogenous NO increased CBF in WT animals compared to baseline but not in eNOS^-/- mice. While CO₂-reactivity is abolished after SAH in the MCA perforation model, hypercapnia led to significant vasodilatation and CBF increase in WT and eNOS transgenic mice after NO application via inhalation; the effect was more pronounced in WT animals.

Conclusion: eNOS^-/- mice have significantly more severe SAH and, subsequently, posthemorrhagic mortality and brain damage. Early microcirculatory changes were more pronounced in transgenic animals. These findings indicate a key role for eNOS produced NO in the pathophysiology of early brain injury after SAH. Further evaluation of the mechanisms is in planning.