A weakness in a blood vessel in the brain that balloons and fills with blood.

Most intracranial aneurysms occur between the underside of the brain and the base of the skull. The aneurysm can leak or rupture, causing life-threatening bleeding.
An unruptured aneurysm usually causes no symptoms. A key symptom of a ruptured aneurysm is a sudden, severe headache.
Treatments for an unruptured aneurysm include medication to control blood pressure and procedures to prevent a future rupture. Emergency medical care is required for a ruptured aneurysm.