Board Thread:Metal Gear Solid V/@comment-98.183.25.16-20150918215627/@comment-25214160-20150927193736

They are most likely cataracts. Kaz's mother contracted an STD during her time as a prostitute, which could very well have been syphilis. Prenatal syphilis can inflame the cornea, among other things, and while it can be treated with antibiotics to cure it, but congenital defects are permanent. If not that, exposure to radiation also can cause one to initially develop cataracts at an early age. They develop slowly, of course, and it would also explain why he is constantly wearing sunglasses because one of the ways to slow down development is to wear them due to UV light exposure making it worse.

This being said, he has always had cataracts, wore sunglasses to slow down their development, and by the time his eyes visibly change in appearance, he is almost completely blind. As for how and why they became opaque so quickly, that is anyone's guess.