Brain tissue is fueled by a network of cerebral arteries. If the walls of one of these arteries weakens, it can swell, and an aneurysm can form.
A brain aneurysm can continue to grow until it bursts, causing blood to fill the areas inside or around the brain. While most aneurysms do not show symptoms, warning signs can include a droopy eyelid, double vision, pain above or behind the eye, a dilated pupil or numbness or weakness on one side of the face or body.
According to the U.S. National Heart, Lung, and Blood Institute, treatment depends on the size and location of the aneurysm, and on whether it is infected or not, or if it ruptures. If a brain aneurysm ruptures, symptoms may include a sudden headache, nausea, vomiting, a stiff neck, loss of consciousness and signs of a stroke. Any of these symptoms require urgent medical attention.
The Mayo Clinic lists a number of factors that contribute to weakening an artery wall: age, smoking, high blood pressure (hypertension), drug abuse and excessive alcohol consumption.
Related article: 4 ways to detect a brain aneurysm
The following video shows how a brain aneurysm occurs: