Sudden darkening of the eyes, if temporary, may be associated with a hypoglycemic reaction or vertigo, or upright hypotension. The onset in both eyes lasts for a few seconds and may be related to optic nerve edema. If it lasts more than a few minutes, it may be related to a transient ischemic attack, or inadequate blood supply to the vertebral basilar artery. If it lasts less than an hour, it may be related to a migraine. It may be an aura of central retinal vein obstruction, and it may also be ischemic optic neuropathy, cerebral ischemic syndrome, glaucoma, or central nervous system pathology. Persistent vision loss that does not return to normal after more than one day may be retinal artery or vein obstruction, vitreous hemorrhage, retinal detachment, optic neuritis, etc.