What is a Subarachnoid Hemorrhage?

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  • Written By: Niki Foster
  • Edited By: C. Wilborn
  • Last Modified Date: 17 October 2019
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A subarachnoid hemorrhage is a type of stroke characterized by bleeding in the area between the arachnoid membrane and the pia mater, two membranes that cover the brain. The arachnoid membrane and pia mater are the two innermost meninges, membranes covering the structures of the central nervous system. The pia mater is delicate and adheres to the brain surface, while the arachnoid layer is made of spider-web-like connective tissue and helps cushion the central nervous system. Subarachnoid hemorrhage may be due to head trauma or a ruptured aneurysm in the brain.

The most common symptom of subarachnoid hemorrhage is a sudden, intense headache known as a thunderclap headache. Many patients experience no other symptoms. However, vomiting, delirium, neck stiffness, seizure, coma, and weakness on one side of the body may also accompany subarachnoid hemorrhage.

In severe cases, brain herniation may occur, resulting in the dilation of one pupil and failure of the pupil to contract in response to light. Intraocular or subhyaloid hemorrhage, bleeding into the eye or its surrounding membrane, respectively, also rarely occurs. Subarachnoid hemorrhage also triggers the release of adrenalin and other hormones, resulting in a spike in blood pressure and increased heartbeat. Cardiac arrhythmia, accumulated fluid in the lungs, and even cardiac arrest can follow.


Subarachnoid hemorrhage is most often caused by a ruptured cerebral aneurysm, a weakened area in a cerebral artery that becomes enlarged. Other possible causes include other blood vessel disorders, head injury, cocaine abuse, sickle cell anemia, and blood clotting disorders or anticoagulant medication. Subarachnoid hemorrhage is always an emergency and should be treated as soon as possible. Half of all cases are fatal and many people who survive suffer cognitive or neurological impairment.

Subarachnoid hemorrhage patients are stabilized as quickly as possible. Depending upon the severity of the condition, immediate surgery may be required to remove the blood and close off the bleeding site. Other patients are stabilized for a longer period of time and undergo a femoral angiogram to discover the source of the bleeding. The aneurysm may be treated with clipping or coiling. Clipping requires opening the skull and placing clips on the artery around the aneurysm, while coiling is performed by running a catheter through the arteries and placing platinum coils by the aneurysm, causing a blood clot that destroys the aneurysm.


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