Nyctalopia

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**Nyctalopia**
*Classifications and external resources*
ICD-10	H53.6
ICD-9	368.6

Nyctalopia (Greek for "night blindness") is a condition making it difficult or impossible to see in the dark. 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).

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 congenital stationary night blindness, from birth the rods either do not work at all, or work very little, but as the name implies, sufferers do not get worse.

Another cause of night blindness is a deficiency of retinol, or vitamin A, found in fish oils, liver and dairy products. In the Second World War, pilots flying night missions were encouraged to eat plenty of carrots, which contain carotenoids and can be converted into retinol. This was also done to help hide the secret of radar from the general public. ^{[citation needed]}

The opposite problem, known as hemeralopia, is much rarer.

The outer area of the retina is made up of more rods than cones. The rod cells are the cells that enable us to see in poor illumination. This is the reason why loss of side 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.

[edit] Historical usage

Historically, nyctalopia, also known as moon blink, was a temporary night blindness believed to be caused by sleeping in moonlight in the tropics.<ref>The Sailor's Word-Book, Admiral W.H. Smyth, p. 483; Conway Maritime Press, UK, 1991. ISBN 0-85177-972-7</ref>