Small brain aneurysm found by chance on a scan

What the report means

Doctors call it an unruptured intracranial aneurysm: a widening at a branch point of a major brain blood vessel that has not burst. (ESO 2022) The ESO guideline says these are commonly found on scans done for unrelated reasons, such as headache, dizziness, an injury or a stroke workup, and that more are found as brain scans get better and more frequent. (ESO 2022) A report often gives the size in millimetres, the vessel it sits on, and its shape. These details, with your age and history, are what specialists use to judge the risk. (ESO 2022)

Words a report may use (simplified)
Report wording	What it describes
Unruptured intracranial aneurysm (UIA)	A brain aneurysm that has not burst (ESO 2022)
Size in mm	The widest measurement; size is one of the main risk factors for bursting (ESO 2022)
Location	Which brain artery it is on; location also affects risk (ESO 2022)
Irregular or lobulated	An uneven outline with small bulges; in a Japanese study of 5,720 people these were more likely to burst than regular ones (ESO 2022)
MRA or CTA	MR or CT scans of the blood vessels, used to find and follow aneurysms without a catheter test (ESO 2022)

How common they are, by age and sex

About 3 in 100 adults. The ESO guideline puts the share of adults with an unruptured brain aneurysm at up to 3%. (ESO 2022) A review of 68 studies estimated 3.2% in adults without other conditions, with a mean age of 50. (Vlak 2011)
Age. The same review found aneurysms were much rarer under age 30, with no clear difference by age after that. (Vlak 2011)
Sex. They are about 1.6 times as common in women, and about 2.2 times in groups with a mean age over 50. (Vlak 2011)(ESO 2022)
Higher in some groups. The chance is higher with polycystic kidney disease or a family history of aneurysm or a bleed from one. (Vlak 2011)(ESO 2022) See screenings for people with polycystic kidney disease.

How often small ones burst

The NHS says small aneurysms that cause no symptoms often do not get bigger and have a low risk of bursting. (NHS) In the ESO guideline:

Pooled cohort studies put the overall risk of bursting at about 0.8% a year, and a meta-analysis at about 3.4% over 5 years. (ESO 2022)
The 5-year risk ranged from 0.3% to over 15%, depending on age 70 or over, high blood pressure, a past bleed, size, location and region. (ESO 2022)
In two groups where nearly all aneurysms were 7 mm or less and were watched with scans, the yearly rate of bursting was 0.24% and 0.1%. (ESO 2022)
Growth on a later scan is linked to a higher risk. Factors linked to growth include a past bleed, location, age over 60, population, size and shape. (ESO 2022)

These are results from groups of people, not a forecast for one person.

What comes next

Specialist review. The ESO guideline suggests assessment by a team covering neurosurgery, interventional neuroradiology and neurology, at centres that see many people with these aneurysms. (ESO 2022)
Repeat scans. For an aneurysm that is watched, ESO recommends repeat imaging to look for growth or change in shape, with MRA or CTA, for as long as treatment remains an option. (ESO 2022) The NHS says you may just need regular follow-up scans to check whether it is getting bigger. (NHS)
How often. ESO says there is not enough evidence to set one interval for everyone. Timing should be based on the aneurysm, the person and their earlier scans, agreed in a shared decision. (ESO 2022)
Earlier scans. Comparing with an older scan helps show whether an aneurysm has changed. Bring dates of earlier brain scans if you have them.

Your doctor decides which follow-up fits your scan, your age and your history.

Who it applies to

Adults told that a brain MRI, CT or angiogram done for another reason shows a small aneurysm that has not burst. Other brain scan findings are covered in white matter changes seen by chance on a brain MRI and pituitary spot found by chance. For scans offered to people without symptoms, see full-body MRI screening.

When to get help now

MedlinePlus, brain aneurysm: (MedlinePlus)

"If a brain aneurysm bursts, symptoms can include a sudden, severe headache, nausea and vomiting, stiff neck, loss of consciousness, and signs of a stroke. Any of these symptoms requires immediate medical attention." In the US, call 911.

The NHS says to call 999 if you or someone else suddenly gets a headache that is extremely painful and does not go away, weakness in the face, weakness or numbness in one arm or one side of the body, speech problems, or blurred vision or loss of sight. Do not drive yourself to A&E. (NHS)(NHS)

The NHS says to ask for an urgent GP appointment or contact NHS 111 if you keep getting headaches, they do not go away or they get worse, or you have pain above or around your eye. (NHS)

Routine age-based screening

High blood pressure is one of the factors the ESO guideline links to aneurysms forming, growing and bursting, (ESO 2022) and the NHS lists it among the things that raise the chance of having one. (NHS) That is why routine blood pressure checks tie in.

Blood pressure. USPSTF recommends screening adults 18 and older, every year from 40 and every 3 to 5 years for adults 18 to 39 at lower risk with a past normal reading. (USPSTF 2021) See blood pressure screening, blood pressure readings and screenings if you have high blood pressure.
All screenings by age. See screenings by age and gender.

Where they differ:

Strength of the evidence. ESO says all its recommendations rest on very low quality evidence, as no completed randomised trials exist. (ESO 2022) The AHA/ASA 2015 guideline also covers natural history, screening, imaging and follow-up, but its full text is not openly available. (AHA/ASA 2015)
Repeat scan timing. ESO sets no fixed interval and says the gap may be longer for aneurysms stable over years, while noting there are no data to support that; (ESO 2022) the NHS describes regular follow-up scans without a set schedule. (NHS)
Age. Being 70 or over is linked to a higher chance of bursting, and over 60 to growth, (ESO 2022) yet the review found no clear difference in how common aneurysms are after age 30. (Vlak 2011)

Your doctor decides what your result means for you.

Questions people ask

How common is a brain aneurysm found by chance?

Fairly common. The ESO guideline says up to 3% of adults have an unruptured brain aneurysm, and that many are found on scans done for something else, such as headache, dizziness or an injury. A large review estimated 3.2% in adults with a mean age of 50. (ESO 2022) (Vlak 2011)

Will a small aneurysm burst?

Most do not. The NHS says small aneurysms often do not get bigger and have a low risk of bursting. In two groups of people with mostly small aneurysms (7 mm or less) who were watched with scans, the yearly rate of bursting was about 0.1% to 0.24%. Your doctor decides what your aneurysm means for you. (NHS) (ESO 2022)

Does age matter?

Yes, in two ways. A review found aneurysms are rare before 30 and about as common at every age after that. The ESO guideline lists age 70 or over among the factors linked to a higher chance of bursting, and age over 60 among the factors linked to growth. (Vlak 2011) (ESO 2022)

What happens after the scan?

The ESO guideline suggests review by a team of brain specialists (neurosurgery, interventional neuroradiology and neurology). For an aneurysm that is watched, it recommends repeat scans with MR angiography (MRA) or CT angiography (CTA), with timing based on the aneurysm, the person and a shared decision. (ESO 2022)

How often are repeat scans done?

There is no fixed interval. ESO says there is not enough evidence to set one for everyone; the timing depends on the aneurysm and the person, and the gap may be longer if it stays stable over years. The NHS says you may need regular follow-up scans to check whether it is getting bigger. (ESO 2022) (NHS)