Subarachnoid hemorrhage

Subarachnoid hemorrhage is bleeding into the cerebrospinal-fluid-filled space between the arachnoid and pia membranes. It can rapidly raise intracranial pressure and is a medical emergency.

Cerebral aneurysm: A localized abnormal dilation of a brain artery, often caused by weakening of its wall. Rupture of a cerebral aneurysm is a major cause of spontaneous subarachnoid hemorrhage.
Thunderclap headache: A headache that reaches maximum intensity within seconds to a minute and may signal a dangerous vascular event. This abrupt presentation is a classic warning symptom, though it has other possible causes.
Meninges: The three protective membranes surrounding the brain and spinal cord: dura mater, arachnoid mater, and pia mater. The hemorrhage is defined by its position among these membranes.
Aneurysmal subarachnoid hemorrhage: Subarachnoid bleeding caused by rupture of an intracranial aneurysm. This is the best-known spontaneous form and requires treatment of the ruptured aneurysm.
Cerebral arteriovenous malformation: An abnormal direct connection between arteries and veins in the brain, bypassing the capillary network. Ruptured malformations can release blood into the subarachnoid space.
Computed tomography of the head: An imaging examination that uses X-rays to produce cross-sectional images of the head. Noncontrast head CT is commonly the first test for suspected acute hemorrhage.
Subarachnoid space: The space between the arachnoid and pia membranes that contains cerebrospinal fluid and major blood vessels. This is the precise compartment into which the blood escapes.
Perimesencephalic subarachnoid hemorrhage: A usually nonaneurysmal hemorrhage centered around the midbrain, with a characteristic CT distribution. Its typical course and recurrence risk differ from aneurysmal hemorrhage.
Cerebrospinal fluid: Clear fluid that surrounds the brain and spinal cord and circulates through the ventricles and subarachnoid space. Blood mixes with this fluid and can obstruct its circulation or absorption.
Lumbar puncture: A procedure that samples cerebrospinal fluid by inserting a needle into the lower spinal canal. It may help detect bleeding when imaging is negative but suspicion remains.

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