The goal of this prospective multi-center observational study is to learn about the osmotic therapy in severe subarachnoid hemorrhage (SAH). The main question[s] it aims to answer are: • whether the two osmotic therapy, 20% mannitol and 10% hypertonic saline(HTS), under the same osmotic equivalent, has similar influence on the outcome of SAH patients? Participants will be given proper treatment according to local expert consensus, including the choice of osmotic medicine. Investigators just record and analyse the data, compare mannitol group and HTS group to see if the outcome of latter is not worse than the former.