Do Aneurysms Cause Headaches and When to Worry

Most brain aneurysms never cause headaches at all. Roughly 3% of the general population harbors an intracranial aneurysm, and the vast majority of these are small, unruptured, and completely silent.¹ When an aneurysm does produce head pain, the character of that pain ranges from a mild, nagging ache to the most severe headache imaginable, and the distinction between those extremes is what separates a manageable finding from a life-threatening emergency.

Why Most Aneurysms Produce No Symptoms

An unruptured brain aneurysm is a bulge in the wall of an artery, usually at a point where the vessel branches. When these bulges are small, they sit quietly inside the skull without pressing on anything sensitive. One imaging study that screened over 2,000 people found aneurysms in about 3.8% of them, with an average size of just 3 millimeters, and every single one was asymptomatic.² That is smaller than a pencil eraser. At that scale, the aneurysm does not stretch pain-sensitive structures, and the person carrying it has no idea it exists.

Many aneurysms are discovered incidentally during brain scans ordered for unrelated reasons, such as dizziness, head trauma, or sinus problems. The discovery itself can be unsettling. A large meta-analysis found that people living with an untreated, unruptured aneurysm had measurably higher anxiety and lower quality of life compared to controls, even though their aneurysm was not actively causing problems.³ Simply knowing the aneurysm is there generates worry about rupture, which can itself produce tension headaches and a heightened awareness of every twinge in the head. This psychological layer makes it harder for both patients and doctors to tease apart whether a headache is caused by the aneurysm or by the stress of knowing about it.

When an Unruptured Aneurysm Does Cause Pain

A minority of unruptured aneurysms do produce headaches, and the location of the aneurysm on the arterial tree matters. Research has found that aneurysms on the internal carotid artery and the middle cerebral artery are more likely to cause head pain, possibly because they sit close to the dura mater, the pain-sensitive membrane lining the inside of the skull along the middle cranial fossa.⁴ About a third of patients in that study experienced headaches on the same side as their aneurysm, lending support to the idea that the aneurysm itself, rather than some coincidence, was behind the pain.

A second proposed mechanism involves the aneurysm mechanically irritating sensory nerve fibers at the base of the skull, which triggers a chain of chemical signaling that widens blood vessels across the brain and produces a more diffuse headache.⁴ This can mimic a migraine or tension headache, which is part of the reason aneurysm-related headaches are sometimes dismissed as routine.

Larger aneurysms can also press on nearby cranial nerves. The classic example is a posterior communicating artery aneurysm compressing the third cranial nerve, which causes a drooping eyelid, double vision, and a dilated pupil on one side. When headache appears alongside any of those neurological signs, the urgency of evaluation goes up considerably.

The Sentinel Headache

One of the most debated concepts in neurology is the sentinel headache, a sudden, unusually severe headache that occurs days or weeks before a full aneurysm rupture. The idea is that a small leak of blood from the aneurysm wall triggers intense pain as a kind of warning shot. Research confirms that sentinel headaches exist in patients with unruptured aneurysms and may signal a high risk of rupture.⁵

How common are they? Estimates vary wildly. A systematic review of well-designed studies found that between 10% and 43% of patients with an aneurysmal bleed reported a preceding sentinel headache, with the wide range partly reflecting differences in how aggressively doctors look for one and how reliably patients recall it after the fact.⁶ Control groups without aneurysms reported sentinel-like headaches only about 5% of the time, suggesting that most of these headaches are genuine warnings rather than coincidences.

The practical problem is that sentinel headaches, by definition, look a lot like other headaches. A prospective Dutch study tracked 148 patients who showed up to their general practitioners with sudden, severe headaches. A quarter of them turned out to have a subarachnoid hemorrhage. But among the 103 patients whose only symptom was the headache itself, 12% still proved to have a bleed.⁷ The remaining patients with no underlying cause found had no subsequent bleeds over a year of follow-up. That means a sudden, severe headache in an otherwise well person is most often benign, but it carries enough risk that it warrants urgent investigation.

Thunderclap Headache and the Moment to Call 911

The hallmark headache of a ruptured aneurysm is a thunderclap headache: excruciating pain that reaches its peak intensity within one minute.⁸ Patients classically describe it as the worst headache of their life, and the speed of onset is the critical distinguishing feature. Most headaches build over minutes to hours. A thunderclap headache slams into full force almost instantly.

Additional symptoms frequently accompany the headache when an aneurysm has ruptured: vomiting, stiff neck, confusion, sensitivity to light, and sometimes loss of consciousness.⁹ But the headache can occur in isolation, with no other neurological signs at all, and this is precisely the scenario that leads to missed diagnoses.

A thunderclap headache does not always mean an aneurysm has burst. Other serious causes include cervical artery dissection, cerebral venous sinus thrombosis, and reversible cerebral vasoconstriction syndrome, a condition where arteries in the brain temporarily spasm and narrow.¹⁰ In one series, reversible vasoconstriction was found in about 39% of patients presenting with recurrent thunderclap headaches.¹¹ Even when brain imaging and a spinal tap come back normal, the underlying cause can still be dangerous.¹² The bottom line is that any headache reaching maximal intensity within seconds to a minute needs emergency evaluation regardless of whether other symptoms are present.

How Often Ruptured Aneurysms Get Missed

Misdiagnosis of a ruptured aneurysm is distressingly common, especially when the patient looks well at first. In one multi-center study, a quarter of patients with a confirmed symptomatic aneurysm were initially sent home with the wrong diagnosis. Among those who appeared neurologically intact on arrival, the misdiagnosis rate was 38%.¹³ Nearly half of those misdiagnosed patients went on to deteriorate or rebleed before getting proper treatment, and their odds of a good outcome dropped from 91% to 53% compared to those diagnosed correctly the first time.

The most common wrong diagnosis was migraine or tension headache. The most common error was simply not ordering a CT scan.¹⁴ This pattern persists in more recent data as well. In a study of over 700 patients treated for acute subarachnoid hemorrhage, about 14% had been initially misdiagnosed, with enormous variation in the time between first symptoms and neurosurgical admission.¹⁵

The lesson here is not that every headache needs a brain scan. It is that a sudden, severe headache that is unlike anything you have experienced before deserves to be treated seriously by whoever evaluates you, and that treatment starts with imaging.

How Doctors Rule Out a Bleed

The first-line test is a non-contrast CT scan of the head. When performed within six hours of headache onset, CT is extraordinarily sensitive. A large prospective study found 100% sensitivity in the first six hours, identifying every single one of 121 patients with subarachnoid hemorrhage.¹⁶ A systematic review pooling multiple studies confirmed that sensitivity within six hours sits above 99%.¹⁷ After six hours, sensitivity drops to roughly 86-90%, because the blood begins to be reabsorbed and becomes harder to spot.¹⁸

When the CT scan is negative but suspicion remains, a lumbar puncture (spinal tap) is the traditional next step. The doctor looks for blood cells or a yellowish discoloration called xanthochromia in the spinal fluid. One study found that combining the red blood cell count in spinal fluid with the color of the fluid supernatant reliably distinguished a true bleed from a needle-related artifact (a “traumatic tap”).¹⁹ Among patients who had a negative CT but positive lumbar puncture findings, catheter angiography found a ruptured aneurysm in about 8% of those with xanthochromia.²⁰ That is a small percentage, but when the stakes are catastrophic brain injury or death, even a small yield justifies the test.

If an aneurysm is suspected but has not ruptured, CT angiography and MR angiography are the workhorses. A meta-analysis found that both techniques have pooled sensitivities in the mid-80% range for detecting aneurysms, with no statistically significant difference between them.²¹ Using both together pushes sensitivity above 91% and specificity near 89%.²² Catheter-based digital subtraction angiography remains the gold standard for the most difficult cases, but the non-invasive scans catch most aneurysms reliably.

The Ottawa SAH Rule and When Testing Is Warranted

Emergency physicians face a constant tension: thunderclap headaches are relatively rare, ordinary headaches are extremely common, and scanning everyone with a headache would overwhelm hospitals. Clinical decision rules help thread that needle. The Ottawa Subarachnoid Hemorrhage Rule was developed to identify which patients with sudden, severe headache need further investigation. In its validation study, the rule achieved 100% sensitivity for subarachnoid hemorrhage, meaning it caught every single case.²³ A subsequent implementation study confirmed the same 100% sensitivity when applied in practice, and its use was associated with fewer unnecessary tests and hospital admissions overall.²⁴

The rule applies to patients who are awake, neurologically intact, and have a non-traumatic headache that reached peak intensity within an hour. It flags patients as needing investigation if they meet any of several criteria, including age 40 or older, neck stiffness or pain, loss of consciousness, onset during exertion, or a history of witnessed thunderclap headache onset. The specificity is low, around 14%, meaning it sends a lot of people for testing who turn out to be fine.²⁵ But in a condition where missing the diagnosis can be fatal, high sensitivity at the cost of some over-testing is the right trade-off.

Risk Factors That Make Rupture More Likely

Not every aneurysm carries the same risk. The factors that push an aneurysm toward rupture include characteristics of both the person and the aneurysm itself. A systematic review and meta-analysis identified smoking as one of the strongest modifiable predictors of aneurysm growth, roughly doubling the risk. Irregular aneurysm shape carried an even higher risk, and having multiple aneurysms was similarly associated with about twice the growth rate.²⁶

The relationship between smoking and rupture specifically has been studied in detail. Current smokers have roughly double the odds of presenting with a ruptured rather than unruptured aneurysm compared to never-smokers, and former smokers carry about 1.5 times the odds. Disturbingly, duration since quitting did not significantly reduce the risk, though smoking fewer cigarettes per day and for fewer total years was associated with better outcomes.²⁷ For someone who knows they have an unruptured aneurysm, quitting smoking is still the single most actionable thing they can do, but the data suggests the accumulated damage to vessel walls does not fully reverse.

Aneurysm shape also matters. Ruptured aneurysms tend to be more irregular and are exposed to more turbulent blood flow compared to unruptured ones, a pattern that holds across different arterial locations.²⁸ This is why follow-up imaging of known aneurysms pays close attention to changes in shape, not just changes in size.

Genetics and Who Should Be Screened

Brain aneurysms run in families more than most people realize. About one in five patients who suffer an aneurysmal subarachnoid hemorrhage have a family history of the condition.²⁹ Several heritable connective tissue disorders substantially increase risk, including autosomal dominant polycystic kidney disease and Ehlers-Danlos syndrome type IV. Genome-wide studies have also identified multiple common genetic variants that each contribute a small amount of risk, collectively painting a picture of aneurysm susceptibility as partly inherited even in people without a named genetic syndrome.³⁰

Current screening guidelines reflect this. Modeling studies suggest that people with two or more first-degree relatives who have suffered an aneurysmal subarachnoid hemorrhage are good candidates for preventive screening with brain MR angiography, as are patients with polycystic kidney disease. Some evidence supports screening even when only one first-degree relative has been affected.³¹ If you have that kind of family history, it is reasonable to ask your doctor whether screening makes sense for you, especially if you also smoke or have high blood pressure.

Headaches After Treatment

A question that often gets overlooked is what happens to headaches after an aneurysm has been treated, either by surgical clipping or endovascular coiling. For people whose unruptured aneurysm was causing headaches beforehand, treatment sometimes resolves the problem, but not always. A prospective study found that patients who had preexisting migraines, more severe headaches before treatment, higher anxiety, or who required stent-assisted coiling were less likely to see their headaches improve afterward.³² This suggests that in some patients, the aneurysm was not the sole driver of the headache in the first place, and treating it does not eliminate all sources of pain.

For survivors of a full subarachnoid hemorrhage, persistent headaches are a major burden. Studies have found that roughly 40-47% of people continue to experience significant headaches months to years after their bleed.³³ ³⁴ These post-hemorrhage headaches have a complex origin involving inflammation, changes to the brain’s pain-processing pathways, and the mechanical effects of surgery or coiling itself. They substantially reduce quality of life, and the treatment options for them remain limited, with concerns that certain pain medications could increase the risk of vasospasm or rebleeding.³⁵

The Psychological Weight of an Incidental Finding

Finding out you have an unruptured brain aneurysm that nobody plans to treat immediately is a psychologically strange place to be. The aneurysm is not doing anything harmful right now, but you know it is there, and you know what it could theoretically do. A large meta-analysis covering over 400,000 patients found that people with untreated unruptured aneurysms showed significantly elevated anxiety compared to people without aneurysms. Interestingly, the analysis did not find a statistically significant increase in clinical depression, suggesting the emotional toll is more about vigilance and worry than sustained low mood.³

A separate analysis found that every measured mental health diagnosis became more common after the discovery of an aneurysm, with depression and anxiety each rising by about 4.5 to 5 percentage points.³⁶ This matters for the headache question because anxiety itself is a potent headache trigger. Someone diagnosed with an incidental aneurysm who then develops new headaches may reasonably fear the aneurysm is growing or about to rupture, when the more likely explanation is stress-driven tension headache. Awareness of this cycle can help both the patient and their care team avoid unnecessary panic without dismissing legitimate warning signs.