A woman in her forties sat down in my office last month holding a folded radiology report like it was a biopsy result. She had been having migraines for twenty years. A new doctor ordered an MRI, the MRA sequence picked up a 4 mm aneurysm on her left middle cerebral artery, and someone at the front desk had told her the neurosurgeon could see her in six weeks. She had not slept properly since. Her first question was not about the aneurysm. It was: “So this is what’s been causing my headaches all along?”
Almost certainly not. An unruptured aneurysm that size, sitting quietly on a vessel, does not produce headache. Roughly three in a hundred adults walk around with one and never know. What changed that morning was not her brain — it was the fact that someone finally looked. The aneurysm is a separate problem from the migraines, it usually needs watching rather than fixing, and the two questions should be answered separately. Here is how I work through it with patients.
Why a small aneurysm doesn’t hurt
Pain in the head comes from structures that carry pain fibres — the dura, the large vessel walls, the trigeminal system, the muscles and the scalp. Brain tissue itself has no pain receptors. A small outpouching on an artery deep inside the skull is not stretching or compressing anything. There is no mechanism by which it should hurt, and the epidemiology agrees: when researchers have compared headache frequency in people who have incidental aneurysms with people who don’t, the rates come out roughly the same. Headache is common. Aneurysms are common. They meet on the same scan often enough that the coincidence feels like a cause.
This is the same trap I see with other incidental findings. I have written about it with white matter spots on MRI and with low-lying cerebellar tonsils called a Chiari malformation. The pattern is identical: a scan ordered for headache finds something unrelated, the something becomes the diagnosis, and the actual headache disorder — usually migraine — stops being treated while everyone worries about the picture.
One more thing worth knowing before you panic about the report itself. MRA is a good screening test but it over-calls. A small funnel-shaped widening where the posterior communicating artery leaves the carotid — an infundibulum — is a normal variant that gets labelled “possible 2 mm aneurysm” all the time. Motion and flow artefact create phantom bulges. Before anyone talks about surgery, the finding should be confirmed on CT angiography or, occasionally, catheter angiography. I have had several patients whose “aneurysm” simply evaporated on the better study.
The three patterns that are not reassuring
Everything above applies to an aneurysm found by accident in someone whose headaches have been stable for years. There are three situations where the aneurysm genuinely is the story, and all three are urgent.
A thunderclap headache. Pain that goes from nothing to maximum in under a minute — the worst headache of your life, arriving like a switch being thrown. That can be a rupture, or a small warning leak in the days before one. This is an emergency department visit, immediately, not a phone call to your neurologist. I’ve covered what happens next in detail in this piece on thunderclap headache.
A new third nerve palsy. A drooping eyelid, double vision, and a pupil on one side that is larger than the other and sluggish to light — especially with pain behind that eye. An aneurysm at the junction of the carotid and posterior communicating artery sits right on that nerve, and when it starts pressing, it is often because it is enlarging. This combination is a neurosurgical emergency even though nothing has bled yet.
A genuinely different headache. Not worse migraines — a headache with a new character, new location, or a new pattern of waking you at night, in someone with a known aneurysm. Large aneurysms, the ones over about 15 mm, can produce mass effect and local pain. Small ones essentially never do.
How rupture risk is actually calculated
“Aneurysm” and “ticking time bomb” are welded together in most people’s minds, and that is the single most useful thing to take apart. Rupture risk is not one number. It depends heavily on size, on which vessel the aneurysm sits on, and on you.
The large international natural-history study of unruptured aneurysms found that small anterior-circulation aneurysms under 7 mm, in people who had never had a subarachnoid haemorrhage, had a five-year rupture rate close to zero. Move the same size of aneurysm to the posterior circulation — the basilar and vertebral system — or to the posterior communicating artery, and the risk climbs substantially. Size matters steeply too: risk rises sharply above roughly 7 mm and again above 12 mm.
Most units now fold this into a scoring tool called PHASES, which weighs population and ethnicity, hypertension, age, aneurysm size, earlier subarachnoid haemorrhage, and site. It is not a verdict — it is a way of putting a number on the conversation so that the risk of leaving the aneurysm alone can be compared honestly with the risk of treating it. If your surgeon has not shown you both numbers, ask for them.
Two personal factors shift the estimate independently of the anatomy, and both are worth taking seriously: smoking and uncontrolled blood pressure. Stopping smoking and getting blood pressure properly treated are the two interventions available to you that measurably lower rupture risk with no procedural danger attached. On the blood pressure point, I should add the obvious caveat — treating hypertension here is about the aneurysm, not the headaches. As I’ve argued elsewhere, moderately raised blood pressure is rarely the cause of headache.
Why “just fix it” is not automatic
Patients often expect me to be relieved when a neurosurgeon offers to treat. Sometimes I am. Often the honest answer is more complicated, because securing an aneurysm — by open clipping, or by endovascular coiling or a flow-diverting stent — carries its own risk of stroke or other complication, typically in the low single-digit percent range depending on the aneurysm and the operator. For a 4 mm middle cerebral artery aneurysm in a 45-year-old, that procedural risk taken today can exceed the cumulative rupture risk over the next decade. For a 9 mm basilar tip aneurysm in the same person, the arithmetic flips.
So the reasonable options are three, not one: treat it, watch it with repeat imaging at intervals — often at six to twelve months initially, then less often if it is stable — or, in an elderly patient with a tiny aneurysm and other serious illness, simply leave it. Growth on surveillance is the finding that changes the plan. Also worth asking: if two or more of your first-degree relatives have had a subarachnoid haemorrhage, or you have polycystic kidney disease, screening for the rest of the family becomes a real discussion.
Can you still treat the migraines?
This is where I see the most avoidable harm. A patient gets an incidental aneurysm diagnosis and a well-meaning clinician tells them to stop everything — no triptans, nothing that touches blood vessels — and they spend the next two years in untreated migraine while waiting for surveillance scans.
There is no evidence that triptans precipitate aneurysm rupture. Their genuine cardiovascular restrictions relate to coronary and cerebrovascular occlusive disease and uncontrolled hypertension — not to an unruptured aneurysm sitting stable on a scan. In practice I usually continue triptans in these patients, with blood pressure properly controlled. Where someone remains uneasy, or where hypertension is a real problem, the gepants and the CGRP monoclonal antibodies are non-vasoconstrictive and sidestep the question entirely. What I will not do is leave a migraine untreated out of a theoretical worry, because untreated frequent migraine reliably becomes more frequent migraine.
The larger point is the one I keep making: the scan found something, but the headache still needs a diagnosis of its own. If yours has never really been explained, it is worth reading what tends to get missed when headaches don’t improve, and what a scan can and cannot rule out. An aneurysm on the film does not close the case. It opens a second one.
Next steps
- Know the warning signs. Download the free headache red-flag checklist — the symptoms that mean a headache needs urgent assessment rather than a wait-and-see.
- Get a second opinion on the headache itself. If your headaches have never been properly diagnosed, or you’ve been told to stop treatment because of an incidental finding, you can talk to a specialist.
Dr. Tae-Ho Guak, MD, is a board-certified neurologist with 25 years of clinical practice and over 100,000 patients seen.
Disclaimer: This article is for general educational purposes and does not constitute medical advice, diagnosis, or treatment. It cannot account for your individual circumstances. Decisions about an unruptured aneurysm, and about any headache medication, should be made with your own neurologist or neurosurgeon. If you experience a sudden severe headache, a new drooping eyelid with double vision, or any other red-flag symptom, seek emergency care immediately.
From the textbook. Chapter 41, “Vascular Headaches,” of my clinical textbook Headache Medicine covers this topic in full for doctors, with the evidence and decision pathways behind it. About the book →