Subarachnoid Hemorrhage - Causes

Causes

In 85% of cases of spontaneous SAH, the cause is rupture of a cerebral aneurysm—a weakness in the wall of one of the arteries in the brain that becomes enlarged. They tend to be located in the circle of Willis and its branches. While most cases of SAH are due to bleeding from small aneurysms, larger aneurysms (which are less common) are more likely to rupture.

In 15–20% 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 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.

Subarachnoid blood can be detected on CT scanning in as many as 60% of people with traumatic brain injury. Traumatic SAH (tSAH) usually occurs near the site of a skull fracture or intracerebral contusion. It usually happens in the setting of other forms of traumatic brain injury and has been linked with a poorer prognosis. It is unclear, however, if this is a direct result of the SAH or whether the presence of subarachnoid blood is simply an indicator of severity of the head injury and the prognosis is determined by other associated mechanisms.