One thing that happens if you are in an accident that stops the bike suddenly. Your legs get brushed up as you drift over the handlebars. This starts your body rotating and you have a really good chance of landing on your head.
I was wearing a helmet but my eyebrow knocked a chunk of concrete out of the curb. Got home and had a jagged line and debris in my field of vision.
Detached retina I assumed. Spent 6 hours over the last 2 days getting appointments and bicycling to them. A fair amount of that was waiting time in the doctor's and specialist offices. (Canadian health care, so fortunately it didn't cost a bundle, just time.)
Fortunately they couldn't see any damage, so it doesn't look like a detached retina, but I have to return in 6 weeks to make sure.
But I do have a couple of lines and all kinds of debris in my field of vision. Mostly it isn't too bad and I don't even notice it unless I'm in front of a computer screen, or a light background. Guess the brain can compensate and I'll get used to it.
Quite grateful it wasn't more serious.