Eyes normally adjust well to changes in light by dilation or constriction of the pupil, which allows light to enter the eye. In some diseases, however, errors in development allow extra light to enter the eye or keep the eye from adapting to light changes. Light sensitivity, also known as photosensitivity or photophobia, is more than an annoyance; it can seriously impact your ability to function in daylight.
Albinism
Albinism is a hereditary absence of melanin, the pigment that gives skin, hair and eyes their color. Albinism affects 1 in 17,000 people. People with albinism are photosensitive because they don't have a normal amount of pigment in their eyes to block light. Albinism can cause many eye symptoms in addition to photosensitivity: nystagmus, or rapid back and forth eye movement; refraction errors, such as nearsightedness or farsightedness; or lazy eye. The retina often doesn't develop properly before birth, the University of North Carolina at Charlotte states. Specially tinted dark glasses help with photosensitivity
Anirida
Aniridia is a congenital absence of all of part of the iris, the colored part of the eye. People with aniridia are extremely light sensitive because the iris usually filters some light out of the eye. It can be inherited as an autosomal dominant or recessive disease, or it can occur spontaneously. The disorder is associated with a number of other eye abnormalities, such as cataracts, glaucoma, nystagmus, dislocation of the lens and problems with the macula and retina.
Achromatopsia
Vision is normally dependent on two types of photoreceptors in the retina: rods and cones. Cones are responsible for color vision and are concentrated in the macula; rods are responsible for monochromatic night vision and don't see color. Achromatopsia, an inherited disease that occurs in 1 in 33,000 people, is characterized by a lack of cones in the retina. This causes severe photophobia, because the rods are overwhelmed by bright light, according to Understanding Achromatopsia, published in "Vision Enhancement Journal" by Richard and Laura Windsor, Doctors of Optometry. Achromatopsia can be complete or incomplete; people with incomplete achromatopsia have some color vision and are able to see traffic lights well enough to drive. People with achromatopsia can't be cured of their disease, but dark-tinted glasses may help them function during the day
