Unruptured Intracranial Aneurysm: PHASES in Practice, Treat vs Watch, and Surveillance
Three percent of adults are walking around with one. The MRI found it; now the patient has read about ruptures at 2 a.m. The honest framework: size, location, history — and the risk factors you can actually change.
Most incidental aneurysms never rupture — the job is to find the minority that will, and stop scaring the rest.
- →Roughly 3% of adults harbor an unruptured intracranial aneurysm; most are small, anterior-circulation, and found by accident. Overall rupture rates are low — but far from uniform.
- →The natural-history anchors: in ISUIA and the Japanese UCAS cohort, risk climbed steeply with size, with posterior-circulation and posterior-communicating location, and with daughter-sac morphology; small anterior lesions ruptured rarely.
- →PHASES bundles the predictors — Population, Hypertension, Age, Size, Earlier SAH, Site — into a 5-year rupture estimate that makes the clinic conversation concrete instead of vibes-based.
- →Treat-versus-watch weighs that estimate against repair risk, life expectancy, and the patient's psychology; guidelines favor treatment consideration for larger, growing, symptomatic, posterior, or previously-ruptured-patient aneurysms — and surveillance for most small incidental ones.
- →Whatever is chosen: stop the smoking, control the blood pressure — the two modifiable rupture factors — and screen first-degree relatives only in the right families (≥2 affected, or aneurysm plus polycystic kidney disease).
The referral is almost always the same sentence: "Incidental 4 mm aneurysm on MRA done for headaches — please advise." By the time the patient arrives they have read three forum threads and one obituary, and the actual medical question — what is this lesion's rupture risk, and does intervening beat watching? — has been buried under a week of 2 a.m. searching.
The service this clinic visit performs is arithmetic plus honesty, in that order.
The natural history: what ISUIA and UCAS actually showed
Two cohorts anchor every number in this conversation. ISUIA followed thousands of unruptured aneurysms and found rupture risk stratified sharply by size and territory: small (<7 mm) anterior-circulation aneurysms in patients without prior SAH ruptured rarely, while risk rose with each size tier and with posterior-circulation or posterior-communicating location.1 The Japanese UCAS cohort — 6,697 aneurysms — put the overall rupture rate near 1% per year, again concentrated in larger lesions, posterior-communicating and anterior-communicating sites, and aneurysms with a daughter sac.2
Both cohorts carry the same honest caveats: managed patients had their scariest lesions repaired (selection bias trims the observed risk), and Japanese and Finnish populations run higher baseline risk — which is exactly why the P in PHASES exists.
PHASES: the estimate you can say out loud
PHASES pools six cohorts into a score built from Population, Hypertension, Age ≥70, Size (the dominant driver: points stack steeply from 7 mm up), Earlier SAH from another aneurysm, and Site (posterior-communicating and posterior circulation highest), returning a 5-year rupture probability that runs from under 1% at low scores to well into double digits at high ones.3 Its value in clinic is less precision than shared language: "your particular aneurysm's five-year risk is around 1–2%" is a sentence a patient can weigh against a repair risk — and against their own sleep. Its limits deserve equal honesty: it does not capture growth, irregular morphology beyond site, family history, or smoking — several of which push real decisions.
The patient heard "brain aneurysm" and priced it at catastrophe. The chart's job is to reprice it at its actual number.
Treat or watch
The AHA/ASA guideline frames the decision as risk-versus-risk, favoring treatment consideration when the lesion side of the scale is loaded — larger size, documented growth, symptomatic aneurysms (a new third-nerve palsy from a posterior-communicating aneurysm is an urgent version of this conversation), posterior location, prior SAH from another aneurysm, and younger patients with more years at risk — and surveillance for the small, stable, incidental majority.4 Repair itself is a neurovascular-team decision between endovascular therapy (coiling and its device descendants — generally lower upfront morbidity, some retreatment) and microsurgical clipping (more durable, more invasive), chosen by anatomy, age, and local expertise more than by ideology.
Surveillance is an active plan, not a shrug: periodic MRA/CTA (commonly at 6–12 months, then spaced out once stability is shown), with growth treated as a rupture-risk signal that reopens the treatment conversation. And every plan carries the same two prescriptions, because they are the modifiable rupture factors the scores undercount: smoking cessation and blood-pressure control.4 If the aneurysm was found during a stroke workup, those overlap entirely with the secondary-prevention work already underway.
Questions the visit must answer before it ends
- "What if it bursts?" — honest but proportionate: rupture is subarachnoid hemorrhage, a true emergency (its severity graded by scales like Hunt and Hess) — and the entire point of today's math is that this patient's yearly probability is what it is, usually small.
- "Can I exercise / fly / have coffee?" — normal life continues; no evidence-based prohibition list accompanies a small unruptured aneurysm. The warning that matters: a sudden worst-ever headache is an emergency-department event, full stop — kin to the other seconds-count presentations in what is a stroke.
- "Should my kids be scanned?" — family screening (MRA) is reserved for ≥2 affected first-degree relatives, or heritable associations like autosomal-dominant polycystic kidney disease; one sporadic aneurysm in one parent does not trigger cascade imaging.4
- Antithrombotics for other reasons — an unruptured aneurysm is generally not a contraindication to indicated antiplatelets or anticoagulation; the decision runs on its own merits (see antithrombotic selection) with the aneurysm managed in parallel.
- The psychology — name it. For many patients the aneurysm's chief harm is anxiety; a number, a plan, and a scheduled scan treat that better than reassurance ever does. Screen the ones who aren't sleeping (GAD-7).
The bottom line
An incidental aneurysm is a probability, not a prophecy. Size and site carry most of the information; PHASES turns them into a number the patient can actually use; and the decision weighs that number against repair risk and years at stake. Treat the ones that have earned it, watch the majority on a real schedule, extinguish the cigarettes and the hypertension in everyone — and send the patient home with the number, the plan, and permission to live their life.
Frequently asked questions.
How common are unruptured brain aneurysms?
Around 3% of adults harbor one, most small and in the anterior circulation, and most found incidentally on imaging done for other reasons. The overwhelming majority never rupture.
What is the risk of a brain aneurysm rupturing?
On the order of 1% per year overall in the large Japanese UCAS cohort — but highly uneven. Risk rises with size (especially above 7 mm), posterior-communicating or posterior-circulation location, irregular shape with a daughter sac, hypertension, and a prior subarachnoid hemorrhage from another aneurysm. Small anterior-circulation aneurysms in patients without prior SAH ruptured rarely in both ISUIA and UCAS.
What is the PHASES score?
A prediction score built from six pooled cohorts — Population, Hypertension, Age, Size, Earlier SAH, Site — that estimates an individual aneurysm's 5-year rupture probability. It anchors the treat-versus-watch conversation, though it does not capture growth, family history, smoking, or fine morphology.
Should an unruptured aneurysm be treated or watched?
It depends on the balance between estimated rupture risk and repair risk. Treatment is considered for larger, growing, symptomatic, or posterior-location aneurysms and in patients with prior SAH or long life expectancy; surveillance imaging suits most small, stable, incidental aneurysms. Repair, when chosen, is an anatomy-driven choice between endovascular treatment and surgical clipping made by a neurovascular team.
Who should be screened for brain aneurysms?
Screening with MRA is generally reserved for people with two or more affected first-degree relatives, or with heritable conditions such as autosomal-dominant polycystic kidney disease. A single sporadic aneurysm in one family member does not by itself trigger screening of children and siblings.
Can someone with an unruptured aneurysm exercise and live normally?
Yes — normal activity, travel, and caffeine are not restricted on current evidence. The two interventions with real payoff are smoking cessation and blood-pressure control. A sudden, worst-ever headache is the one symptom that means emergency department, immediately.
References.
- Wiebers DO, Whisnant JP, Huston J 3rd, et al; International Study of Unruptured Intracranial Aneurysms Investigators. Unruptured intracranial aneurysms: natural history, clinical outcome, and risks of surgical and endovascular treatment. Lancet. 2003;362(9378):103-110. PubMed
- Morita A, Kirino T, Hashi K, et al; UCAS Japan Investigators. The natural course of unruptured cerebral aneurysms in a Japanese cohort. N Engl J Med. 2012;366(26):2474-2482. PubMed
- Greving JP, Wermer MJ, Brown RD Jr, et al. Development of the PHASES score for prediction of risk of rupture of intracranial aneurysms: a pooled analysis of six prospective cohort studies. Lancet Neurol. 2014;13(1):59-66. PubMed
- Thompson BG, Brown RD Jr, Amin-Hanjani S, et al. Guidelines for the Management of Patients With Unruptured Intracranial Aneurysms: A Guideline for Healthcare Professionals From the American Heart Association/American Stroke Association. Stroke. 2015;46(8):2368-2400. PubMed
Related guides
Keep building the picture.
- Hunt and Hess scale The grading scale for the rupture this visit exists to prevent.
- Brain AVMs The other vascular lesion priced by rupture math and team decisions.
- Fibromuscular dysplasia An arteriopathy whose screening survey exists to find these.
- GAD-7 anxiety screen For many patients the aneurysm's chief harm is the worry.
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