Sudden vision loss
A loss of vision may or may not be accompanied by pain or redness, any sudden loss of vision is serious and requires immediate medical intervention.
If the vision loss is in one eye, it may have happened some time ago and simply not noticed until the good eye is covered, but, until it is diagnosed, treat it as an emergency.
Uncontrolled diabetes and sudden vision loss
Sudden vision loss is treated as an emergency because, fast action and treatment can sometimes (not always), save the sight and even at times a life!
As the glucose levels rise and fall in uncontrolled diabetes, so to a person's vision can fluctuate. This can mean that a person might wake up and not be in blackness, but not be able to see as well as yesterday even with their current glasses. Until the diabetes is controlled this can continue.
Treatment of sudden vision loss
Once the cause of the vision loss is established in many cases, swift action, followed by ophthalmic surgery or treatment can save some or all of the sight.