Between arachnoid and pia mater.
May be traumatic or non-traumatic in origin.
Most common causes:
-
- Aneurysmal/vascular malformation (esp. sacular or “berry” aneurysms) → sudden, un-attenuated increase in intravascular pressure leads to rupture.
- Carotid artery dissection.
- Skull base fracture (can lead to internal carotid aneurism).
- Amyloid angiopathy.
- Vasculitis.
- Cerebral vasospasm, often 2/2 extravasation of blood from cerebral contusion.
- May experience “sentinel bleed” days, weeks, or months beforehand: Headache, nuchal rigidity, n/v.
- Actual hemorrhage event is often described as a “thunderclap headache” or “worst headache of [the patient’s] life.”
- Rapid onset.
- Reaches maximal intensity within minutes.
- Nuchal rigidity, photophobia, n/v also common.
- Varies.
- May be life-threatening.
- High likelihood of complication by cerebral vasospasm → delayed presentation may occur.
CT
- Hyperdense material seen filling the subarachnoid space, especially around the Circle of Willis (2/2 berry aneurysms) or Sylvian fissure.
- Modified Fisher scale = categorizes hemorrhage by amount of blood present on non-contrast CT. Correlates to risk of ensuing vasospasm.
MRI
- More sensitive to the presence of blood in the subarachnoid space, especially during first 12 hours of bleed.
- Hyperintensity in subarachnoid space on FLAIR.
Gross (autopsy) findings
- Blood located between arachnoid and pia mater.
- Often settles at base of brain.
- Determining source of blood can help differentiate specific etiology.