Aneurysm & Nontraumatic SAH
- Nontraumatic subarachnoid hemorrhage (SAH) is blood in the cerebrospinal-fluid spaces, and the classic cause is a ruptured brain aneurysm — a blowout in an artery wall.
- The patient story is loud: a sudden, worst-ever "thunderclap" headache. Take it seriously even when the head CT looks normal.
- On non-contrast CT, hunt for white (blood) filling the normally black grooves and cisterns around the brain — especially the basal cisterns.
- A normal early CT does NOT clear the patient. If the suspicion is real, the next steps are a lumbar puncture and/or CT angiography to look for the aneurysm.
- The scary sequels are rebleeding, hydrocephalus, and delayed vasospasm — so finding the aneurysm and securing it is urgent.
Imagine the brain as a soft sponge floating in a thin moat of water. That water is cerebrospinal fluid (CSF), and the moat — the subarachnoid space — wraps around every groove and pools in little reservoirs at the skull base called cisterns. Now picture an artery wall ballooning out like a worn spot on a garden hose, then bursting under pressure. Blood floods the moat. That is nontraumatic subarachnoid hemorrhage, and it is one of the few times a headache is genuinely an emergency.
The story that should make your neck hairs stand up
The textbook line is the thunderclap headache: "the worst headache of my life," arriving in seconds like someone swung a bat. Patients often describe it more vividly than any radiologist could, which is convenient, because the history does half your work. Add neck stiffness, vomiting, a brief loss of consciousness, or a sudden collapse, and the alarm should be deafening.
Here is the trap, and it is a famous one: a meaningful chunk of these patients are walking, talking, and frustratingly well-appearing when they arrive. The headache may have been a smaller "sentinel" leak that settled down. The disease does not care how good the patient looks.
What blood looks like in the moat
On a non-contrast head CT, fresh blood is white (hyperdense). The spaces it fills are supposed to be black (CSF). So acute SAH shows up as white where there should be black — bright material smeared through the sulci and, classically, filling the star-shaped cluster of basal cisterns around the brainstem. Normal CSF makes a tidy black star there; SAH paints it white.
CT is excellent for this early on, but its sensitivity fades as the hours and days pass and the blood gets diluted and reabsorbed. Within the first several hours after a thunderclap onset, a good-quality scan catches the large majority of bleeds. Wait too long, and a real SAH can fade to a CT that looks reassuringly normal.
The "normal CT does not equal no SAH" rule
This is the single most important takeaway, so I will say it plainly.
A normal early head CT does not rule out subarachnoid hemorrhage. If the clinical story is convincing, the workup continues — typically a lumbar puncture looking for blood or its breakdown pigment (xanthochromia) in the CSF, and/or vessel imaging to find the culprit.
Think of it like smelling smoke in your house and being told the one smoke detector you checked is silent. You do not shrug and go back to bed. You keep looking, because the cost of missing a fire is catastrophic.
Finding the aneurysm
Once SAH is on the table, the next question is where is the leak? That usually means CT angiography (CTA) — a CT timed to a bolus of contrast so the arteries light up bright — to find the aneurysm, most commonly at the branch points of the circle of Willis at the skull base. Catheter angiography remains the detailed gold standard and is often part of treatment planning.
Not every nontraumatic SAH is aneurysmal. A subset shows blood pooled in front of the brainstem with no aneurysm found — a generally more benign pattern — and other causes exist too. But because a ruptured aneurysm is both common and dangerous, it is the assumption you must actively disprove, not casually dismiss.
Why the clock matters
Securing the aneurysm is urgent because of what blood in the moat does next:
| Complication | What goes wrong | Why it is feared |
|---|---|---|
| Rebleeding | The unsecured aneurysm bleeds again | Early, often deadly; the reason to treat fast |
| Hydrocephalus | Blood clogs CSF drainage pathways | Pressure builds, mental status drops |
| Vasospasm / delayed ischemia | Arteries clamp down days later | Can cause a delayed stroke |
That second row connects to hydrocephalus: blood gumming up the plumbing backs CSF up and swells the ventricles. The third row is a delayed ischemic stroke risk that classically peaks several days in — which is why these patients are watched closely well after the headache fades.
Do not confuse this with traumatic SAH or other intracranial hemorrhage. Trauma tends to lay blood over the convexities near an impact; aneurysmal SAH classically centers on the basal cisterns. And watch the floor of the scan — a thin layer of blood can hide along the tentorium or interpeduncular cistern and quietly masquerade as normal.
The one-sentence version
If a patient describes the worst headache of their life and the CT shows white in the black CSF spaces — or even if the CT is clean but the story is loud — treat it as a ruptured aneurysm until proven otherwise, because a fast, large bleed can also push the brain toward herniation, and there is no prize for being reassuring about a fire.