AQP4 and brain edema: A study has shown significantly increased AQP4 expression in the artery after SAH, but no significant change in AQP4 expression was detected in the vein, which may indicate a significant increase in the inflow of CSF from the periarterial space, while the volume of ISF expands due to the constant expression of AQP4 in the vein to eventually lead to the deterioration of brain edema (Liu E. et al., 2020).