Behind your cornea is the iris, or the colourful part of your eye. The pupil is the black circular centre of your iris. The pupil determines how much light enters your iris by contracting and constricting in response to brightness. If your pupils dilate and stay dilated, or one pupil dilates while the other stays constricted, there may be a serious medical issue.
Depending on the extent of your head trauma, concussions range from mild to severe. It can take days to hours for symptoms from a concussion to appear. A serious sign of injury will include visual disturbances, such as dilated pupils, as indicated by the Mayo Clinic website. Pupils can also appear uneven, with one being larger than the other.
Many forms of illicit drugs that include narcotics, stimulants, hallucinogens and sedatives can cause your pupils to dilate. Drugs block the chemical messengers to your brain, which tell your eye muscles in the iris to relax. As a result, your pupils remain contracted.
Ophthalmologists occasionally use eye drops during eye examinations. A medication known as a mydriatic is used, according to Downtown's Vision Care. Mydriatics stimulate the iris dilator muscle, which causes your pupil to dilate. The purpose of eye dilation can help your ophthalmologist see your optic nerve, retina and eye in detail.
Small cerebral aneurysms usually don't produce symptoms. As an aneurysm grows, the aneurysm can press on nerves and brain tissue. Nerves within the eye can be affected. As noted by the Mayo Clinic, large unruptured aneurysms can cause dilated pupils.
When you die, your brain ceases to function. The brain can't send out signals to the muscles in your eyes. The pupils can no longer constrict and become fixed and dilated.