I also have had this for about 20 years. My ophthalmologist said they are like the aura of a migraine without the migraine. It lasts 15-20 minutes. Mine I call the "sparklies" because I see a sparkly crescent shape that if I'm reading, part of a sentence (a few words ) are missing. It only concerned me the first time. I know it will pass and I can go on with whatever work I'm doing (except reading😊).