Let me list the major disease that I have personally seen that someone has had with 20/20 vision in both eyes and needed treatment immediately, diabetic neovascularization, Glaucoma, Stage 4 hypertensive Retinopathy, Retinal holes, Retinal tears, Retinal detachment, pseudotumor cerebri, Branch retinal vein occlusion, Branch retinal artery occlusion, and fun one colon cancer, I know it very weird. These are only the ones that I have seen personally. We all say that the eyes are the windows to the soul but it also big for determining body health. There are a lot of diseases that show up first in the eye. The thing is that the retina is huge but only one small spot sees 20/20 in the eye, the fovea part of the macula. If that area is not affected then most likely you won't notice the problem. For most of the things I listed, they had to go to ER, or a Retinal specialist immediately. The others had Primary Care providers in the next couple of days. As you can tell a couple of them have to deal with diabetes and high blood pressure. Others we are worried about is a stroke, to high pressure in the skull affecting the brain, normally unknown cause. The last one is something that needs to be treated and could go blind in hours not days. A lot of these people I have seen had no to very minimal symptoms.
The last thing I want to say is if you had LASIK, PRK, SMILE, ICL, and/or RK you are still at increased risk of disease corresponding to how near-sided (myopic) you were before surgery. The table below shows the most common disorders with increased risk due to nearsightedness which are Cataracts, Glaucoma, Retinal Detachment, and Myopic Macular Degeneration. You may notice two of the four may not affect vision when they 1st occur and will only affect vision after it gets severe and that can be permanent damage.
There is a reason why we say you should have an eye exam every year!