Day blindness (hemeralopia) is a symptom in which vision is impaired in daytime (photopic conditions), whereas night vision (scotopic conditions) is preserved.


The visual system consists of two components that are relatively independent of one another. One component is responsible for vision in daytime, the photopic system, which is based upon the cone photoreceptors. The other component is responsible for vision under dark conditions, the scotopic system, which is based upon the rod photoreceptors. There is a slight overlap between the two systems during dim background illumination (mesopic conditions), but in general, rods cannot replace cones for daytime vision and cones cannot replace rods for night vision.

Difficulties in day vision, ranging from slight photophobia to day blindness, can result from any condition in which cone photoreceptor function is diffusely impaired, or from conditions in which too much light enters.


Hemeralopia is known to occur in several ocular conditions. Cone dystrophy and achromatopsia, affecting the cones in the retina, and the anti-epileptic drug trimethadione are typical causes. Adie's pupil, which fails to constrict in response to light; aniridia, which is absence of the iris; and albinism, where the iris is defectively pigmented, may also cause this. Central cataracts, due to the lens clouding, disperses the light before it can reach the retina and is a common cause of hemeralopia and photoaversion in the elderly. Cancer-associated retinopathy (CAR), seen when certain cancers incite the production of deleterious antibodies against retinal components, may cause hemeralopia.

Another known cause is a rare genetic condition called Cohen syndrome (Pepper syndrome). Cohen syndrome is mostly characterized by obesity, mental retardation and craniofacial dysmorphism due to genetic mutation at locus 8q22–23. Rarely, it may have ocular complications such as hemeralopia, pigmentary chorioretinitis, optic atrophy or retinal/iris coloboma, having a serious effect on the person's vision.

Yet another cause of hemeralopia is uni- or bilateral postchiasmatic brain injury.This may also cause concomitant nyctalopia.


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