The pathophysiological mechanisms of aneurysmal subarachnoid haemorrhage (aSAH) involve early brain injury (EBI) and delayed cerebral ischemia (DCI). Several mechanisms contribute to EBI pathogenesis, including cell death, inflammatory response, oxidative stress, excitotoxicity, microcirculatory dysfunction, microthrombosis and cortical spreading depolarization. All are suggested to be linked due to common pathogenic pathways and direct interaction. Despite advances in research of diagnostics and treatment strategies, brain injury remains the major cause of death and disability in SAH patients. There is no sufficient treatment of SAH and its devastating consequences known so far. Developing and improving diagnostic methods to monitor SAH patients and to evaluate efficacy of treatment strategies are essential in SAH research. These include neuroimaging, biomarkers, and other parameters such as invasive multimodal neuromonitoring and intraoperative electrophysiological monitoring. Cerebral vasospasm (CV) - mostly responsible for DCI - can be depicted on angiograms. Altogether, tremendous efforts have been taken to conquer the occurrence and sustainability of CV. The mortality of patients suffering aSAH rises up to 50% if the patients' condition is critical (Hunt\&Hess (HH) Grade 5, WFNS Grade 5, modified Fisher Grade 4). Reports of beneficial outcome in patients with pre-existing CSF shunting have been published. The hypothesis of early CSF reapplication to the bloodstream, in order to prevent CV seems to be positively approved by the mentioned reports. Nevertheless, no data could be found on the mechanisms of action in this phenomenon. To confirm the presence of interaction of the mechanisms of EBI and evaluate the application of cerebrospinal fluid (CSF), a pilot clinical trial was planned. Due to the lack of validated animal models for aSAH it is necessary to perform the trial first-in-human. A pilot (proof of concept) trial - is done through inclusion of 10 patients with severe aSAH (≥HH4). According to clinical guidelines, these patients receive external ventricular drainages in order to drain CSF and lower intracranial pressure. An interim analysis of data will be performed after inclusion and treatment of 5 patients. Blood-/CSF-sampling for further analysis will be collected before, during and after treatment according to the study protocol.