Subarachnoid hemorrhage

What it means

The brain is wrapped in three thin protective layers. Between the middle and inner layers sits a narrow space filled with cerebrospinal fluid, which cushions the brain and carries small blood vessels along its surface. When one of these surface vessels tears or an aneurysm bursts, blood spills into this fluid-filled space. On a fresh CT scan this shows up as bright white lines tracing along the surface and grooves of the brain — a pattern radiologists recognise quickly.

Why it appears on a CT or MRI report

Reports describe where the blood is seen (which surface, which grooves, around the base of the brain), how much is present, and whether the fluid spaces inside the brain are getting larger from blocked drainage. The report often comments on whether the pattern looks more typical of a burst aneurysm — usually pooling near the base of the brain — or of trauma, which tends to leave thinner streaks over the surface. Further scans of the brain's arteries are often ordered to look for the source.

What it usually means

This finding is taken seriously because of where the blood sits and what tends to cause it. The most common non-traumatic cause is a burst aneurysm — a weak spot on an artery wall — which carries a risk of bleeding again in the first hours and days if not treated. In these cases the care team works urgently to find the aneurysm and seal it off, either through a procedure inside the blood vessel or with surgery. Traumatic causes from a head injury are also common and are often managed with close observation and supportive care. Other causes include bleeding disorders, blood-thinning medication, a vascular malformation, or, less often, no clear source. Possible complications include rising pressure inside the skull, blocked fluid drainage causing the ventricles to enlarge, and narrowing of nearby arteries days later — all of which the team watches for closely.

When to follow up

If this is a new finding on an emergency scan, the medical team is already mobilising. If you are reading the words on an outpatient report, treat it as needing same-day specialist input. The classic warning sign of a fresh bleed of this type is a sudden, extremely severe headache often described as "the worst headache of my life" or as if you were hit on the head — this needs emergency care immediately. Other red flags include sudden stiff neck, vomiting, sensitivity to light, brief loss of consciousness, confusion, or any new neurological symptoms.

A plain-language way to picture it

Imagine the brain as a soft fruit sitting inside a thin water-filled bag, and that whole bag sitting inside the skull. If a small vessel running across the surface of the fruit springs a leak, the leak does not push into the fruit itself — it spreads through the water in the bag, tracing along every dip and groove on the fruit's surface. That spreading pattern is what the radiologist sees, and it is why the location of the leak can take a careful look to pin down.