This is a disorder of the pigmented epithelium of the retina, which results in progressive retinal degeneration. The retina is responsible for detecting and processing light, and turning it into signals that travel to the brain to be processed into vision. As the retina progressively degenerates, vision is progressively lost leading to blindness. Generally an initial night blindness, with a slow deterioration of vision until the dog is completely blind.  The age at which the dog becomes fully blind can vary, and affected eyes are not painful, unless complicated by a secondary problem, such as cataract or uveitis (inflammation due to a leaking cataract).