Sudden loss of vision in one eye should be treated as a medical emergency, and the person should immediately be taken to the emergency room. This symptom can be a warning sign for a stroke, just like loss of function in an arm or leg on 1 side or sudden difficulty with speech can be a warning sign for a coming stroke. Caught early, the entire stroke may be prevented, where waiting could leave the person paralyzed.
Other possibilities include multiple sclerosis, retinal tears, a blood clot in the heart, and other conditions.
Call 911 or your local ambulance phone number and go to the emergency room — NOW!