There is a group of retinal degenerations, primarily genetic in origin such as retinitis pigmentosa, that destroy peripheral vision first and in general slowly progress toward the fovea. These diseases tend to produce tritanopia. This may reflect the possibility that blue-yellow color vision extends further into peripheral retina and therefore is more vulnerable to this disease process. It may also reflect the possibility that S cones are more susceptible to the degeneration than L and M cones.
There is a second class of diseases that tend to affect central vision and are due to defects in either the optic nerve or the macula area of the retina. These can be
associated with red-green color vision defects but they are always accompanied by a diminution in acuity. The vulnerability of red-green defects in these diseases may reflect the possibility that red-green color vision is more developed centrally than blue-yellow vision.
There is a class of autoimmune diseases that arise from cancers that express antigens, which are also expressed by photoreceptors. The immune system detects these antigens and reacts to them both on the cancer cells and on the photoreceptors. This class of diseases has been called cancer-related retinopathies. Some of these diseases are highly specific, involving only cones and not rods. Such subjects can lose all cone vision and consequently color vision. Recently, we found a subject who lost only L and M cones but not S cones or rods. These are examples of acquired achromatopsia.
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