Vision is lost instantly when blood stops flowing through the arteries which feed the retina (the light-sensing surface in the back of the eye). Depending on the size of artery and the amount of retina deprived of blood supply, visual loss varies all the way from a small blind spot to total blindness in the affected eye.
The usual cause of this sudden but painless blindness is a small blood clot wedging in a branch of the retinal artery that is already narrowed by atherosclerosis. This dams the artery and stops its blood flow. People sometimes experience this type of blindness only for a few minutes until the clot breaks up or drifts away from the narrowed part of the artery. Vision, of course, returns only if blood flow is restored soon enough — about one and three-fourths hours is usually the limit.
Now, according to the Journal of the American Medical Association, people can help themselves when this sudden blindness strikes by gently massaging the affected eye with on-and-off pressure applied over the closed upper eyelid about once every 15 seconds. The pressure should be firm but not enough to hurt.
Even though results cannot be guaranteed, this procedure, which is analogous to cardiopulmonary resuscitation, may disperse the clot and is surely worth trying. Recent eye surgery, of course, would contraindicate it. Also, when the afflicted person has diabetes, or a feeling of tightness, pain or any other hint of bleeding inside the eye, this first aid treatment must not be used.