First, a mea culpa.
I mentioned that no pattern in horses is associated with blindness or deafness. While researching something else I found out I was wrong. Congenital deafness is sometimes (but not always) associated with the splashed white coloring. Oops.
Horses can also become deaf with age (the same as any other animal), or as a result of arthritic changes in the skull, inner or middle ear infections, and certain drugs.
Modern testing for deafness includes using a BAER test (the same as is used for humans), but older school tests include shaking a grain bucket where the horse can't see it.
Deaf horses won't spook at loud noises, can't be trained to voice, and sometimes become startled by sudden arrivals in their field of vision. They can, however, be ridden mostly normally.