In 1998, McKeever was a promising 19-year-old skier when he was diagnosed with Stargardt’s disease, a type of juvenile macular degeneration that gradually leads to blindness. Twelve years later, McKeever only has 10 percent of his vision, and that tiny fraction is in his peripheral vision.
Rather than rolling over when he lost his sight, though, McKeever got back on his skis.
Read about how McKeever does it at mental_floss. Link