Cataracts are caused when the proteins that make up the transparent substance of the lens of the eye begin to break down. This turns then from transparent to a whitish color, causing the cataract
, which can obstruct vision. In other words, a cataract is not caused by pigmentation shift, and the iris and the lens are quite distinct parts of the eye. One possible explanation of why your grandmother's eyes appear to be loosing pigment is due to a condition called arcus senilis. This a gray-white arc that becomes visible at the edge of the cornea (right at the outer edge of the irus) and is due to normal aging of this junction. It does not generally obstruct vision. As always the diagnosis and the management of your grandmother's particular condition will require a physical examination by her personal physician. She should set up an office visit with her primary care doctor
if she has concerns.