The problem you are probably referring to is called an occular migraine or visual migraine. It can occur before the surgery or after (or both), it affects only some people, and it affects some people who have no known valve issues at all. It just seems to be more prevalent in people with valve issues. They're annoying, and initially scary, but not painful or permanent.
It is most often not accompanied by a migraine headache. A session usually consists of either a "half-sheet effect" or a blob effect of partial blockage to the vision of one, or occassionally both eyes. The effect is often similar to the blobby area you may see after you accidently look directly into the sun for a moment. There may be some sparkling or shimering effects, like pieces of broken stained glass, or an effect called "fortifications," which looks like an aerial view of a walled fortress (with sparkles). The effect lasts from a couple of minutes to (rarely) twenty minutes or so.
Complete blockage of the vision of one or both eyes may be a TIA or stroke. That is an ER-required event. If you're not on warfarin (Coumadin) and aren't allergic to it, taking an aspirin on the way to the ER shouldn't hurt, even if it turns out not to be a TIA in the end.
It's best the first time it happens to take a visit to an ophthalmologist, to ensure that you aren't having a retinal tear. Although quite a few here have then made the effort follow up with neurologists, I've not seen anyone come back with a satisfying result from the time and money spent for that effort. A few got possible names for it: none claimed to have a treatment that I can recall. The phenomenon tends to fade over time with most people.
I used to get them frequently, both before and after my surgeries, but they hardly ever show up now.
Best wishes,