Night blindness

Nyctalopia
Effect of night-blindness. Left: normal night vision. Right: night-blind.
Classification and external resources
Specialty	ophthalmology
ICD-10	H53.6
ICD-9-CM	368.6
DiseasesDB	32761
MedlinePlus	003039
MeSH	D009755
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Nyctalopia /ˌnɪktəlˈoʊpiə/ (from Greek νύκτ-, nykt- "night"; ἀλαός, alaos "blind, not seeing", and ὄψ, ops "eye"), also called night-blindness, is a condition making it difficult or impossible to see in relatively low light. It is a symptom of several eye diseases. Night blindness may exist from birth, or be caused by injury or malnutrition (for example, a lack of vitamin A). It can be described as insufficient adaptation to darkness.

The most common cause of nyctalopia is retinitis pigmentosa, a disorder in which the rod cells in the retina gradually lose their ability to respond to the light. Patients suffering from this genetic condition have progressive nyctalopia and eventually their daytime vision may also be affected. In X-linked congenital stationary night blindness, from birth the rods either do not work at all, or work very little, but the condition doesn't get worse. Another cause of night blindness is a deficiency of retinol, or vitamin A, found in fish oils, liver and dairy products.

The opposite problem, the inability to see in bright light, is known as hemeralopia and is much rarer.

Since the outer area of the retina is made up of more rods than cones, loss of peripheral vision often results in night blindness. Individuals suffering from night blindness not only see poorly at night, but also require extra time for their eyes to adjust from brightly lit areas to dim ones. Contrast vision may also be greatly reduced.

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Wikipedia