Nyctalopia

Nyctalopia (/ˌnɪktəˈloʊpiə/ from Greek νύκτ- (nykt-), meaning 'night', ἀλαός (alaos), meaning 'blind, not seeing', and ὄψ (ops), meaning '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, vitamin A deficiency). It can be described as insufficient adaptation to darkness. Nyctalopia (/ˌnɪktəˈloʊpiə/ from Greek νύκτ- (nykt-), meaning 'night', ἀλαός (alaos), meaning 'blind, not seeing', and ὄψ (ops), meaning '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, vitamin A deficiency). 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. Rods contain a receptor-protein called rhodopsin. When light falls on rhodopsin, it undergoes a series of conformational changes ultimately generating electrical signals which are carried to the brain via the optic 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 (especially PRK with the complication of 'haze') may rarely cause a reduction in best night-time acuity 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. 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. In the French language, nyctalopie and héméralopie 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 from Latin happened during the 2nd century AD, even though the ancient greek νυκτάλωψ (nuktálōps) has been used in both senses.