A brain aneurysm is an abnormal bulge or "ballooning" in the wall of an artery in the brain. They are sometimes called berry aneurysms because they are often the size of a small berry. Most brain aneurysms produce no symptoms until they become large, begin to leak blood, or burst.

If a brain aneurysm presses on nerves in your brain, it can cause signs and symptoms. These can include

• A droopy eyelid
• Double vision or other changes in vision
• Pain above or behind the eye
• A dilated pupil
• Numbness or weakness on one side of the face or body

Treatment depends on the size and location of the aneurysm, whether it is infected, and whether it has burst. 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.

NIH: National Heart, Lung, and Blood Institute