A brain aneurysm (AN-yoo-riz-um) — also known as a cerebral aneurysm or intracranial aneurysm — is a bulge or ballooning in a blood vessel in the brain. An aneurysm often looks like a berry hanging on a stem.

Experts think brain aneurysms form and grow because blood flowing through the blood vessel puts pressure on a weak area of the vessel wall. This can increase the size of the brain aneurysm. If the brain aneurysm leaks or ruptures, it causes bleeding in the brain, known as a hemorrhagic stroke.

Most often, a ruptured brain aneurysm occurs in the space between the brain and the thin tissues covering the brain. This type of hemorrhagic stroke is called a subarachnoid hemorrhage.

Brain aneurysms are common. But most brain aneurysms aren't serious, especially if they're small. Most brain aneurysms don't rupture. They usually don't cause symptoms or cause health problems. In many cases, brain aneurysms are found during tests for other conditions.

However, a ruptured aneurysm quickly becomes life-threatening and requires medical treatment right away.

If a brain aneurysm hasn't ruptured, treatment may be appropriate in some cases. Treatment of an unruptured brain aneurysm may prevent a rupture in the future. Talk with your health care provider to make sure you understand the best options for your specific needs.


  • Saccular aneurysm, also known as a berry aneurysm. This type of aneurysm looks like a berry hanging from a vine. It's a round, blood-filled sac that protrudes from the main artery or one of its branches. It usually forms on arteries at the base of the brain. A berry aneurysm is the most common type of aneurysm.
  • Fusiform aneurysm. This type of aneurysm causes bulging on all sides of the artery.
  • Mycotic aneurysm. This type of aneurysm is caused by an infection. When an infection affects the arteries in the brain, it can weaken the artery wall. This can cause an aneurysm to form.


Most brain aneurysms that haven't ruptured don't cause symptoms. This is especially true if they're small. Brain aneurysms may be found during imaging tests that are done for other conditions.

However, a ruptured aneurysm is a very serious condition, typically causing a severe headache. And if an unruptured aneurysm presses against brain tissue or nerves, it may cause pain and other symptoms.

Ruptured aneurysm

A sudden, severe headache is the key symptom of a ruptured aneurysm. This headache is often described by people as the worst headache they've ever experienced.

In addition to a severe headache, symptoms of a ruptured aneurysm can include:

  • Nausea and vomiting
  • Stiff neck
  • Blurred or double vision
  • Sensitivity to light
  • Seizure
  • A drooping eyelid
  • Loss of consciousness
  • Confusion

'Leaking' aneurysm

In some cases, an aneurysm may leak a slight amount of blood. When this happens, a more severe rupture often follows. Leaks may happen days or weeks before a rupture.

Leaking brain aneurysm symptoms may include:

  • A sudden, extremely severe headache that may last several days and up to two weeks.

Unruptured aneurysm

An unruptured brain aneurysm may not have any symptoms, especially if it's small. However, a larger unruptured aneurysm may press on brain tissues and nerves.

Symptoms of an unruptured brain aneurysm may include:

  • Pain above and behind one eye.
  • A dilated pupil.
  • A change in vision or double vision.
  • Numbness of one side of the face.

When to see a doctor

Seek immediate medical attention if you develop a:

  • Sudden, extremely severe headache

If you're with someone who complains of a sudden, severe headache or who loses consciousness or has a seizure, call 911 or your local emergency number.

Mayo Clinic Minute: What is an aneurysm?

Vivien Williams: An aneurysm is an abnormal bulge or ballooning in the wall of a blood vessel.

Bernard Bendok, M.D., Neurosurgery, Mayo Clinic: A portion of these patients will go on to have a rupture. And the challenge with rupture is that it's unpredictable.

Vivien Williams: Dr. Bernard Bendok says a ruptured aneurysm is a medical emergency that can cause life-threatening bleeding in the brain.

Dr. Bendok: The typical presentation is somebody who has the worst headache of their life.

Vivien Williams: Fast treatment is essential. It includes open surgery or less-invasive options, such as sealing the ruptured artery from within the blood vessel with metal coils and/or stents.

Dr. Bendok says 1 to 2 percent of the population have aneurysms and only a small percentage of that group will experience a rupture. People who have a family history of aneurysms, have polycystic kidney disease, connective tissue disease, and people who smoke are at increased risk of rupture and should consider screening. If a rupture happens, fast treatment can save lives.

For the Mayo Clinic News Network, I'm Vivien Williams.

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Brain aneurysms are caused by thinning artery walls. Aneurysms often form at forks or branches in arteries because those areas of the vessels are weaker. Although aneurysms can appear anywhere in the brain, they're most common in arteries at the base of the brain.

Risk factors

Several factors can contribute to weakness in an artery wall. These factors may increase the risk of a brain aneurysm or aneurysm rupture.

Some of these risk factors develop over time. But some conditions present at birth can increase the risk of developing a brain aneurysm.

Risk factors include:

  • Older age. Brain aneurysms can occur at any age. However, they're more common in adults between ages 30 and 60.
  • Being female. Brain aneurysms are more common in women than in men.
  • Cigarette smoking. Smoking is a risk factor for brain aneurysms to form and for brain aneurysms to rupture.
  • High blood pressure. This condition can weaken arteries. Aneurysms are more likely to form and to rupture in weakened arteries.
  • Drug use, particularly using cocaine. Drug use raises blood pressure. If illicit drugs are used intravenously, it can lead to an infection. An infection can cause a mycotic aneurysm.
  • Heavy alcohol use. This also can increase blood pressure.
  • Inherited connective tissue disorders, such as Ehlers-Danlos syndrome. These disorders weaken blood vessels.
  • Polycystic kidney disease. This inherited disorder results in fluid-filled sacs in the kidneys. It also may increase blood pressure.
  • A narrow aorta, known as coarctation of the aorta. The aorta is the large blood vessel that delivers oxygen-rich blood from the heart to the body.
  • Brain arteriovenous malformation, known as AVM. In this condition, arteries and veins in the brain are tangled. This affects blood flow.
  • A family history of brain aneurysm. Your risk is higher if you have family members who have had a brain aneurysm. This is particularly true if two or more first-degree relatives — such as a parent, brother, sister or child — has had a brain aneurysm. If you have a family history, you can ask your health care provider about getting screened for a brain aneurysm.

Some types of aneurysms may occur after a head injury or from certain blood infections.

Risk factors for a ruptured aneurysm

There are some factors that make it more likely an aneurysm will rupture. They include:

  • Having a large aneurysm.
  • Having aneurysms in certain locations.
  • Smoking cigarettes.
  • Having untreated high blood pressure.


When a brain aneurysm ruptures, the bleeding usually lasts only a few seconds. However, the blood can cause direct damage to surrounding cells and can kill brain cells. It also increases pressure inside the skull.

If the pressure becomes too high, it may disrupt the blood and oxygen supply to the brain. Loss of consciousness or even death may occur.

Complications that can develop after the rupture of an aneurysm include:

  • Re-bleeding. An aneurysm that has ruptured or has leaked is at risk of bleeding again. Re-bleeding can cause further damage to brain cells.
  • Narrowed blood vessels in the brain. After a brain aneurysm ruptures, blood vessels in the brain may contract and narrow. This is known as vasospasm. Vasospasm can cause an ischemic stroke, in which there's limited blood flow to brain cells. This may cause additional cell damage and loss.
  • A buildup of fluid within the brain, known as hydrocephalus. Most often, a ruptured brain aneurysm occurs in the space between the brain and the thin tissues covering the brain. The blood can block the movement of fluid that surrounds the brain and spinal cord. As a result, an excess of fluid puts pressure on the brain and can damage tissues.
  • Change in sodium level. Bleeding in the brain can disrupt the balance of sodium in the blood. This may occur from damage to the hypothalamus, an area near the base of the brain. A drop in blood sodium levels can lead to swelling of brain cells and permanent damage.


In many cases, brain aneurysms can't be prevented. But there are some changes you can make to lower your risk. They include quitting smoking if you smoke. Also work with your health care provider to lower your blood pressure if it's high. Don't drink large amounts of alcohol or use drugs such as cocaine.

March 07, 2023
  1. AskMayoExpert. Unruptured intracranial aneurysm (adult). Mayo Clinic; 2021.
  2. Kim B-S. Unruptured intracranial aneurysm: Screening, prevalence and risk factors. Neurointervention. 2021; doi:10.5469/neuroint.2021.00451.
  3. Cerebral aneurysms fact sheet. National Institute of Neurological Disorders and Stroke. https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Cerebral-Aneurysms-Fact-Sheet. Accessed Dec. 14, 2022.
  4. Cerebral aneurysm. American Association of Neurological Surgeons. http://www.aans.org/en/Patients/Neurosurgical-Conditions-and-Treatments/Cerebral-Aneurysm. Accessed Nov. 15, 2021.
  5. Jankovic J, et al., eds. Intracranial aneurysms and subarachnoid hemorrhage. In: Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Elsevier; 2022. https://www.clinicalkey.com. Accessed Nov. 15, 2021.
  6. Singer RJ, et al. Unruptured intracranial aneurysms. https://www.uptodate.com/contents/search. Accessed Dec. 1, 2022.
  7. Srinivasan J, et al., eds. Subarachnoid hemorrhage. In: Netter's Neurology. 3rd ed. Elsevier; 2020. https://www.clinicalkey.com. Accessed Nov. 15, 2021.
  8. Simon RP, et al. Headache and facial pain. In: Clinical Neurology. 10th ed. McGraw Hill; 2018. https://accessmedicine.mhmedical.com. Accessed Dec. 15, 2022.
  9. Rinkel GJ. Management of patients with unruptured intracranial aneurysms. Current Opinion in Neurology. 2019; doi:10.1097/WCO.0000000000000642.
  10. Chancellor B, et al. Flow diversion for intracranial aneurysm treatment: Trials involving flow diverters and long-term outcomes. Neurosurgery. 2020; doi:10.1093/neuros/nyz345.
  11. Daou BJ, et al. Clinical and experimental aspects of aneurysmal subarachnoid hemorrhage. CNS Neuroscience and Therapeutics. 2019; doi:10.1111/cns.13222.
  12. Rinaldo L, et al. Natural history of untreated unruptured intracranial aneurysms in the elderly. Journal of Neurosurgical Sciences. 2020; doi:10.23736/S0390-5616.16.03891-1.
  13. Dai D, et al. Histopathological findings following pipeline embolization in a human cerebral aneurysm at the basilar tip. Interventional Neuroradiology. 2016; doi:10.1177/1591019915622165.
  14. Thielen E, et al. Concomitant coiling reduces metalloproteinase levels in flow diverter-treated aneurysms but anti-inflammatory treatment has no effect. Journal of Neurointerventional Surgery. 2017; doi:10.1136/neurintsurg-2015-012207.
  15. Kallmes DF, et al. Aneurysm study of pipeline in an observational registry (ASPIRe). Interventional Neurology. 2016; doi:10.1159/000446503.
  16. Brinjiki W, et al. Treatment of ruptured complex and large/giant ruptured cerebral aneurysms by acute coiling followed by staged flow diversion. Journal of Neurosurgery. 2016; doi:10.3171/2015.6.JNS151038.
  17. Becske T, et al. Pipeline for uncoilable or failed aneurysms: Results from a multicenter clinical trial. Radiology. 2013; doi:10.1148/radiol.13120099.
  18. Wiebers DO, et al. Unruptured intracranial aneurysms: Natural history, clinical outcome and risks of surgical and endovascular treatment. 2003; doi:10.1016/s0140-6736(03)13860-3.
  19. Kerezoudis P, et al. Predictors of 30-day perioperative morbidity and mortality of unruptured intracranial aneurysm surgery. Clinical Neurology and Neurosurgery. 2016; doi:10.1016/j.clineuro.2016.07.027.
  20. Zhao B, et al. Stent-assisted coiling versus coiling alone of poor-grade ruptured intracranial aneurysms: A multicenter study. Journal of Neurointerventional Surgery. 2017; doi:10.1136/neurintsurg-2016-012259.
  21. Sorenson T, et al. Trials and tribulations: An evidence-based approach to aneurysm treatment. Journal of Neurosurgical Sciences. 2016; https://pubmed.ncbi.nlm.nih.gov/27102908/. Accessed Dec. 3, 2021.
  22. Brown RD, et al. Screening for brain aneurysm in the Familial Intracranial Aneurysm study: Frequency and predictors of lesion detection. Journal of Neurosurgery. 2008; doi:10.3171/JNS/2008/108/6/1132.
  23. Brown RD (expert opinion). Mayo Clinic. Dec. 27, 2022.
  24. Thompson BG, 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; doi:10.1161/STR.0000000000000070.
  25. Quality check. The Joint Commission. https://www.qualitycheck.org/search/?keyword=mayo clinic. Accessed Nov. 21, 2021.
  26. Caffes N, et al. Unruptured cerebral aneurysms in elderly patients: Key challenges and management. Annals of medicine. 2021; doi:10.1080/07853890.2021.1990393.
  27. Korhonen A, et al. Subarachnoid hemorrhage during pregnancy and puerperium: A population-based study. Stroke. 2022; doi:10.1161/STROKEAHA.122.039235.
  28. What you should know about cerebral aneurysms. American Stroke Association. https://www.stroke.org/en/about-stroke/types-of-stroke/hemorrhagic-strokes-bleeds/what-you-should-know-about-cerebral-aneurysms. Accessed Dec. 20, 2022.
  29. Brain aneurysm basics. Brain Aneurysm Foundation. https://www.bafound.org/about-brain-aneurysms/brain-aneurysm-basics/. Accessed Dec. 22, 2022.
  30. Surgical clipping. Brain Aneurysm Foundation. https://www.bafound.org/treatment/surgical-clipping/. Accessed Jan. 20, 2023.


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