Actor Tom Sizemore, best known for his role in Saving Private Ryan, suffered a brain aneurysm last weekend and was rushed to the hospital where he remains in critical condition in intensive care. Sizemore’s spokesperson, Charles Lago, told CNN that the 61-year-old actor was in a “wait and see situation”.

According to Insider, a brain aneurysm ruptures every 18 minutes in the U.S. and can be fatal 50% of the time. A brain aneurysm is a bulge or ballooning in a blood vessel caused by a weakness in the vessel wall. The U.K.’s National Health Service (NHS) says that the condition rarely causes symptoms until it bursts, but an unruptured aneurysm can cause a change in vision or double vision, pain above or around the eye, headaches, loss of balance, difficulty speaking and difficulty concentrating.

Most aneurysms don’t rupture or cause symptoms so may go undetected, or may be spotted during a test for another condition. If an aneurysm ruptures, the sufferer will experience an agonizing headache, as if someone or something hit them on the dead, the NHS said. According to the Brain Aneurysm Foundation, an estimated 6.7 million people in the U.S. have an unruptured brain aneurysm. That’s one in 50 people. About 30,000 people in America suffer a rupture annually and women are at a higher risk than men to experience this life-threatening condition.

A ruptured vessel can lead to a subarachnoid hemorrhage, or bleeding in the brain which can cause brain damage, epilepsy, and depression. Around 50% of ruptured brain aneurysms are fatal, says the NHS.

If an aneurysm is detected, preventative treatment can be given to those at high risk of having their aneurysm burst. Doctors can perform surgery in which a surgical clip or endovascular coil are used to seal off the aneurysm to help prevent a future rupture. To reduce the risk of brain aneurysm, the NHS recommends avoiding activities that can damage delicate blood vessels. These include smoking, eating a high-fat diet, not monitoring blood pressure and being overweight, says Insider.

Seek immediate medical help if you suffer a sudden, severe headache ─ the most common aneurysm symptom. In addition, a ruptured aneurysm may cause nausea and vomiting, stiff neck, blurred or double vision, or a drooping eyelid.