Nyctalopia
Other names: Night blindness
Effect of night-blindness. Left: normal night vision. Right: night-blind.
Pronunciation	Nyctalopia:/ˌnɪktəˈloʊpiə/

Nyctalopia, also callednight-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 byinjury or malnutrition (for example,vitamin A deficiency). It can be described as insufficientadaptation to darkness.

The most common cause of nyctalopia isretinitis pigmentosa, a disorder in which therod cells in theretina 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. InX-linked congenital stationary night blindness, from birth the rods either do not work at all, or work very little, but the condition does not get worse.

Another cause of night blindness is a deficiency ofretinol, or vitamin A₁, found in fish oils, liver and dairy products.

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

Since the outer area of the retina is made up of more rods than cones, loss ofperipheral 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.

Rods contain a receptor-protein calledrhodopsin. When light falls on rhodopsin, it undergoes a series of conformational changes ultimately generating electrical signals which are carried to the brain via theoptic nerve. In the absence of light, rhodopsin is regenerated. The body synthesizes rhodopsin from vitamin A, which is why a deficiency in vitamin A causes poor night vision.

Refractive "vision correction" surgery (especiallyPRK with the complication of "haze") may rarely cause a reduction in best night-time acuity due to the impairment ofcontrast sensitivity function (CSF) which is induced by intraocular light-scatter resulting from surgical intervention in the natural structural integrity of the cornea.^[1]

Causes

• Retinitis pigmentosa
• Retinal detachment
• Certain medications, such asphenothiazines^[2]
• Oguchi disease
• Pathologicalmyopia
• Cataract (peripheral cortical)
• Refractive surgery (LASIK,photorefractive keratectomy,radial keratotomy)
• Sorsby's Fundus Dystrophy (Macular degeneration)
• Vitamin A deficiency; often viaxerophthalmia
• Choroideremia
• Glaucoma
• Visual snow

Historical usage

Aulus Cornelius Celsus, writing ca. 30 AD, described night blindness and recommended an effective dietary supplement,liver being a good source of vitamin A: "There is besides a weakness of the eyes, owing to which people see well enough indeed in the daytime but not at all at night; in women whose menstruation is regular this does not happen. But success sufferers should anoint their eyeballs with the stuff dripping from a liver whilst roasting, preferably of a he-goat, or failing that of a she-goat; and as well they should eat some of the liver itself."

Historically, nyctalopia, also known asmoonblink, was a temporary night blindness believed to be caused by sleeping in moonlight in the tropics.^[3]

In theFrench language,nyctalopie andhéméralopie have inverse meanings, the first naming the ability to see in the dark as well as in plain light, and the second the inability to do so. It is thought that this inversion fromLatin happened during the 2nd century AD,^[4] even though theAncient Greek νυκτάλωψ (nuktálōps) has been used in both senses.

Night blindness, sometimes referred to asgravel, was occasionally documented during theAmerican Civil War, particularly within theConfederate States of America. Likerheumatism, night blindness was viewed as a condition that could be easily faked or exaggerated, and initially, sufferers were regarded with some suspicion. Some soldiers who reported symptoms of nyctalopia also presented with symptoms ofscurvy, suggesting a fundamental cause of poor nutrition.^[5]

Epidemiology

Word origin

from Ancient Greek νύκτ- (núkt-) 'night', ἀλαός (alaós) 'blind, invisible', and ὄψ (óps) 'eye'.^[6]

Other animals

Congenital stationary night blindness is also anophthalmologic disorder inhorses withleopard spotting patterns, such as theAppaloosa. It is present at birth (congenital), not sex-linked, non-progressive and affects the animal's vision in conditions of low lighting.^[7] Congenital stationary night blindness (CSNB) is usually diagnosed based on the owner's observations, but some horses have visibly abnormal eyes: poorly aligned eyes (dorsomedial strabismus) or involuntary eye movement (nystagmus).^[7] In horses, CSNB has been linked with theleopard complex color pattern since the 1970s.^[8] A 2008 study theorizes that both CSNB and leopard complex spotting patterns are linked to theTRPM1 gene.^[9] The region on horse chromosome 1 to which theLp gene has now been localized also encodes a protein that channels calciumions, a key factor in the transmission of nerve impulses. This protein, found in the retina and the skin, exists in fractional percentages of the normal levels found in homozygousLp/Lp horses and so compromises the basic chemical reaction for nerve impulse transmission.^[7]

References

External links

• Pages with script errors
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• Blindness
• Visual disturbances and blindness