Hemeralopia or day blindness is the inability to see clearly in bright light and is the exact opposite of nyctalopia (night blindness).
The inability to see clearly in low light is called heliophobia.
It can be described as insufficient adaptation to bright light.
In hemeralopia, daytime vision gets worse, characterized by avoidance of light rather than photophobia which is typical of inflammations of the eye.
Nighttime vision largely remains unchanged due to the use of rods as opposed to cones used during the day, which are affected by hemeralopia and in turn degrade the daytime optical response.
Many patients feel they see better at dusk than in daytime.
Hemeralopia is known to occur in several ocular conditions:
Cone dystrophy and achromatopsia, affecting the cones in the retina.
Anti-epileptic drug trimethadione.
Adie’s pupil, which fails to constrict in response to light;
Aniridia, which is absence of the iris.
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, seen when certain cancers incite the production of deleterious antibodies against retinal components, may cause hemeralopia.
Cohen syndrome ( Pepper 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.
People with hemeralopia may benefit from sunglasses.