An unexplained dilation of the pupils. Under normal conditions, a pupil is supposed to dilate only in conditions of low light when it expands to let more light into the eye. An expanding aneurysm could well put pressure on different nerves that lead from the brain, and this, of course, includes the optic nerve. If you or someone you know suffers an unexplained dilation of the pupils, it must be considered as requiring immediate emergency care, as this symptom is a certain sign of serious issues that can affect the brain, such as, as tumors, or an aneurysm, or stroke.