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Nyctalopia (from Greek νύκτ-, nykt- "night"; and αλαός, alaos "blindness") 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 some time for their eyes to adjust from brightly lit areas to dim ones. Contrast vision may also be greatly reduced.
Refractive "vision correction" surgery is a widespread cause of nyctalopia due to the impairment of contrast sensitivity function (CSF) which is induced by intraocular light-scatter resulting from surgical intervention in the natural structural integrity of the cornea.
- congenital night blindness (Retinitis pigmentosa)
- Certain medications, such as phenothiazines
- oguchi disease
- pathological myopia
- peripheral cortical cataract
- refractive surgery (LASIK, photorefractive keratectomy, radial keratotomy)
- retinitis pigmentosa
- Sorsby's fundus dystrophy
- Vitamin A deficiency
Historical usage 
Aulus Cornelius Celsus, writing ca. 30 AD, described night blindness and recommended an effective dietary supplement: "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 as moonblink, was a temporary night blindness believed to be caused by sleeping in moonlight in the tropics.
Nyctalopia with animals 
Congenital stationary night blindness is also an ophthalmologic disorder in horses with leopard spotting patterns, such as the Appaloosa. It is present at birth (congenital), not sex-linked, non-progressive and affects the animal's vision in conditions of low lighting. 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). In horses, CSNB s has been linked with the leopard complex color pattern since the 1970s. A 2008 study theorizes that both CSNB and leopard complex spotting patterns are linked to the TRPM1 gene. The gene to which Lp has now been localized encodes a protein that channels calcium ions, a key factor in the transmission of nerve impulses. This protein, which is found in the retina and the skin, existed in fractional percentages of the normal levels in homozygous Lp/Lp horses.
- Laser in situ keratomileusis for myopia and the contrast sensitivity function Nadia-Marie Quesnel, John V Lovasik, Christian Ferremi, Martin Boileau, Catherine Ieraci Journal of Cataract & Refractive Surgery June 2004 (Vol. 30, Issue 6, Pages 1209-1218)
- Goldman, Lee. Goldman's Cecil Medicine (24th ed. ed.). Philadelphia: Elsevier Saunders. p. 2429. ISBN 1437727883.
- The Sailor's Word-Book, Admiral W.H. Smyth, p. 483; Conway Maritime Press, UK, 1991. ISBN 0-85177-972-7
- Bellone, Rebecca R; Brooks SA, Sandmeyer L, Murphy BA, Forsyth G, Archer S, Bailey E, Grahn B (August 2008). "Differential Gene Expression of TRPM1, the Potential Cause of Congenital Stationary Night Blindness and Coat Spotting Patterns (LP) in the Appaloosa Horse (Equus caballus)". Genetics (Genetics Society of America) 179 (4): 1861–1870. doi:10.1534/genetics.108.088807. PMC 2516064. PMID 18660533.
- Witzel CA, Joyce JR, Smith EL. Electroretinography of congenital night blindness in an Appaloosa filly. Journal of Equine Medicine and Surgery 1977; 1: 226–229.
- Oke, Stacey, DVM, MSc (August 31, 2008). "Shedding Light on Night Blindness in Appaloosas" (Registration required). The Horse. Retrieved 2009-02-07.