While most cases are due to bleeding from small aneurysms, larger aneurysms (which are less common) are more likely to rupture. Aspirin also appears to increase the risk.In 15–20 percent of cases of spontaneous SAH, no aneurysm is detected on the first angiogram. About half of these are attributed to non-aneurysmal perimesencephalic hemorrhage, in which the blood is limited to the subarachnoid spaces around the midbrain (i.e. mesencephalon). In these, the origin of the blood is uncertain. The remainder are due to other disorders affecting the blood vessels (such as cerebral arteriovenous malformations), disorders of the blood vessels in the spinal cord, and bleeding into various tumors.Cocaine abuse and sickle cell anemia (usually in children) and, rarely, anticoagulant therapy, problems with blood clotting and pituitary apoplexy can also result in SAH. Dissection of the vertebral artery, usually caused by trauma, can lead to subarachnoid hemorrhage if the dissection involves the part of the vessel inside the skull. Pathophysiology
Cerebral vasospasm is one of the complications caused by subarachnoid hemorrhage. It usually happens from the third day after the aneurysm event, and reaches its peak on 5th to 7th day. There are several mechanisms proposed for this complication. Blood products released from subarachnoid hemorrhage stimulates the tyrosine kinase pathway causing the release of calcium ions from intracellular storage, resulting in smooth muscle contraction of cerebral arteries. Oxyhaemoglobin in cerebrospinal fluid (CSF) causes vasoconstriction by increasing free radicals, endothelin-1, prostaglandin and reducing the level of nitric oxide and prostacyclin.
Besides, the disturbances of autonomic nervous system innervating cerebral arteries is also thought to cause vasospasm. Diagnosis
As only 10 percent of people admitted to the emergency department with a thunderclap headache are having an SAH, other possible causes are usually considered simultaneously, such as meningitis, migraine, and cerebral venous sinus thrombosis. Intracerebral hemorrhage, in which bleeding occurs within the brain itself, is twice as common as SAH and is often misdiagnosed as the latter. It is not unusual for SAH to be initially misdiagnosed as a migraine or tension headache, which can lead to a delay in obtaining a CT scan. In a 2004 study, this occurred in 12 percent of all cases and was more likely in people who had smaller hemorrhages and no impairment in their mental status. The delay in diagnosis led to a worse outcome. In some people, the headache resolves by itself, and no other symptoms are present. This type of headache is referred to as "sentinel headache", because it is presumed to result from a small leak (a "warning leak") from an aneurysm. A sentinel headache still warrants investigations with CT scan and lumbar puncture, as further bleeding may occur in the subsequent three weeks.The initial steps for evaluating a person with a suspected subarachnoid hemorrhage are obtaining a medical history and performing a physical examination. The diagnosis cannot be made on clinical grounds alone and in general medical imaging and possibly a lumbar puncture is required to confirm or exclude bleeding.