1.Definition of subarachnoid hemorrhage (SAH): The surface of the human brain is covered with three layers of membranes, from inside to outside, the soft meninges, the arachnoid membrane and the dura mater in that order. The cavity between the arachnoid and soft meninges is called the subarachnoid space, which is normally filled with colorless and transparent cerebrospinal fluid. When a cerebral blood vessel ruptures, blood flows into the subarachnoid space, which is called subarachnoid hemorrhage (SAH). 2. Causes of SAH: SAH is classified into traumatic and spontaneous according to the cause. Traumatic, as the name implies, is related to head trauma, so it will not be discussed here. The most common cause of spontaneous SAH is the rupture of intracranial aneurysm, accounting for about 75%; followed by cerebrovascular malformation, accounting for about 5-10%; other cerebrovascular diseases including cerebral atherosclerosis, smog disease, etc. can also cause SAH; there are about 10% of patients with unknown causes. 3.Clinical manifestations of SAH: Patients show severe headache, neck straightening, self-perception of “tear-like” or “electric shock-like” headache, often accompanied by nausea, vomiting, severe convulsions, unconsciousness, and even respiratory and cardiac arrest. About 10-15% of patients die before reaching the hospital. The cranial CT shows high density in the subarachnoid space, which can be diagnosed; when the amount of bleeding is small, CT may not be diagnostic, and sometimes lumbar puncture is needed to confirm the diagnosis. 4.Treatment of SAH: The treatment of SAH is a comprehensive treatment, including symptomatic treatment and etiological treatment. Symptomatic treatment includes dehydration drugs to lower cranial pressure, hemostasis, sedation, pain relief, prevention and control of cerebrovascular spasm, nutritional support and functional rehabilitation. Etiological treatment is the most important, i.e., to understand what causes SAH first, and then give targeted treatment. The cause of SAH can be determined by non-invasive diagnostic methods such as MRA (magnetic resonance angiography) and CTA (tomography angiography), but the gold standard for confirming the cause is still the invasive digital subtraction cerebral angiography, the so-called DSA examination. This test requires a femoral artery puncture cannula to deliver a catheter into the aortic arch and then up into the cerebral artery, through which a contrast agent is injected into the cerebral artery so that the vascular images of the brain are clearly and dynamically displayed on the X-ray fluoroscopy screen, and a negative film can be developed by intercepting the static image. Through DSA examination, doctors can clarify the nature, location, morphology and severity of the underlying cerebrovascular lesion causing SAH, and use it to formulate the next treatment plan. If the SAH is caused by a ruptured intracranial aneurysm, emergency surgery is required to “destroy” the aneurysm. Why is emergency treatment necessary? According to statistics, 20% of patients with ruptured aneurysm SAH will have rebleeding within 2 weeks after the first bleeding, and the residual death rate is as high as 60-80% within one year, so we describe aneurysm as an “untimely bomb” in the brain, and removing this untimely bomb as soon as possible can effectively prevent it from Removal of this untimely bomb can effectively prevent it from “re-exploding”. There are two surgical approaches for aneurysms: craniotomy and interventional embolization, and the specific approach requires evidence-based medicine and physician-patient communication. If SAH is caused by rupture of cerebrovascular malformation, the cerebrovascular malformation can be treated surgically after the hemorrhage is absorbed and the condition is stabilized. Because the chance of rebleeding in the near future after bleeding from a cerebrovascular malformation is not very high, the rebleeding rate is 4-18% within 1 year, so surgery can be performed electively without the need for emergency surgery as in the case of aneurysm. Cerebrovascular malformations of the same surgical methods of craniotomy and interventional embolization, small vascular malformations less than 4cm in diameter can also choose gamma knife treatment – a kind of gamma ray stereotactic radiation therapy. Large vascular malformations are difficult to be cured by interventional embolization at one time, and often require multiple embolizations or a combination of gamma knife treatment. If SAH is caused by smog, surgical treatment such as temporal muscle patching and vascular bypass can be considered later. Smoggy disease was first discovered by the Japanese and is also known as Moyamoya disease, a cerebrovascular disease with unexplained progressive occlusion of the cerebral arteries, so named because it appears on cerebral angiography as a smog-like compensatory proliferation of capillaries at the base of the skull. It does not fall under the scope of interventional treatment. Finally, it is important to emphasize ruptured aneurysm SAH, which is the third most common cerebrovascular accident after cerebral infarction and hypertensive cerebral hemorrhage. The process of ruptured aneurysm bleeding is very brief and transient. The rupture of an aneurysm causes a rapid increase in brain pressure, and the pressure gradient inside and outside the aneurysm immediately reaches equilibrium, so that blood no longer flows outward and a thrombus soon forms at the rupture site, which provides an opportunity to treat the survivor as soon as possible. However, this thrombus is very unstable and will dissolve in a short period of time, causing rebleeding of the aneurysm, which will increase the mortality rate exponentially. Therefore, once aneurysmal SAH is clearly identified, surgical treatment should be performed as early as possible.