Colour blindness Health Dictionary

The statutory de?nition – for the purposes of registration as a blind person under the National Assistance Act 1948 – is that the person is ‘so blind as to be unable to perform any work for which eyesight is essential’. Generally this is vision worse than 6/60 in the better eye, or with better acuity than this but where ‘the ?eld of vision is markedly contracted in the greater part of its extent’. Partial sight has no statutory de?nition, but there are o?cial guidelines for registering a person as partially sighted: generally these are a corrected visual acuity of 3/ 60 or less in the better eye with some contraction of the peripheral ?eld, or better with gross ?eld defects. In the UK more than 100,000 people are registered as legally blind and some 50,000 as partially sighted. The World Health Organisation has estimated that there are over 40 million binocularly blind people in the world. The causes of blindness vary with age and degree of development of the country. In western society the commonest causes are glaucoma, diabetic retinopathy, other retinal diseases and senile cataract. (See also VISION.)

Any blind person, or his or her relatives, can obtain help and advice from the Royal National Institute for the Blind (www.rnib.org.uk).

Night blindness An inability to see in the dark. It can be associated with retinitis pigmentosa or vitamin A de?ciency (see EYE, DISORDERS OF).... blindness

See under BLINDNESS.... night blindness

Damage caused to the cornea of an unprotected EYE by the re?ection of the sun’s rays from snow. ULTRAVIOLET RAYS (UVR) are the damaging agent and people going out in snow and sunlight should wear protective goggles. The condition is painful but resolves if the eyes are covered with pads for a day or two. Prolonged exposure may seriously damage the cornea and impair vision.... snow blindness

Alexia: a condition in which, as the result of disease in the brain, a person becomes unable to associate their proper meanings with words, although he or she may be quite able to spell the letters.

Word deafness is an associated condition in which, although hearing remains perfect, the patient has lost the power of referring the names heard to the articles they denote. (See also DYSPHASIA.)... word blindness

A condition in which the patient sees better in a dim light or by night than in daylight. It is only found in conditions in which the light is very glaring, as in the desert and on snow, and is relieved by resting the retina (see EYE) – for example, by wearing coloured glasses for a time.... day blindness

See onchocerciasis.... river blindness

Inability to see at night or in imperfect light due to a deficiency of visual purple (rhodopsin) in the rods at the back of the eye due to low level Vitamin A. Night myopia usually affects people during twilight. “One in five people are not fit to drive at night.” May occur in glaucoma and other eye disorders. Other causes: old age, free radical damage.

Alfalfa tea freely.

Of value: Kelp, Irish Moss, Iceland Moss.

Diet. Vitamin A foods, carrots, bilberries, Cod Liver oil.

Supplements. Vitamin A, Beta-carotene. C (2g), E (400iu). B-complex, B2, Niacin, Zinc. ... eyes  - night blindness

The ability to see different parts of the colour spectrum. Light perceived by the human eye consists of electromagnetic radiation (energy waves) with a spectrum of different wavelengths

between about 400 and 700 nanometres. Different wavelengths produce sensations of violet, indigo, blue, green, yellow, orange, and red when they fall on the retina and stimulate nerve signals, which are processed in the brain.

As light falls on the retina, it strikes light-sensitive cells called rods and cones. The rods can detect all visible light, but only the cones can distinguish colour. There are 3 types of cones: red-sensitive, blue-sensitive, and green-sensitive. Each responds more strongly to a particular part of the light spectrum. Because the cones are most concentrated in a central area of the retina called the fovea, colour vision is most accurate for objects viewed directly and is poor at the edges of vision. When light hits a cone, it causes the cone to emit an electrical signal, which passes to the brain via the optic nerve. Colour perception requires a minimum level of light, below which everything is seen as shades of grey. (See also colour vision deficiency; eye; perception; vision.)... colour vision

see Doppler ultrasound.... colour flow ultrasound imaging

an unusual phenomenon in newborn babies characterized by transient red colour changes to half of the body, well demarcated at the midline. It is seen usually 2–5 days after birth and can last from 30 seconds to 20 minutes before fading away. It may recur when the infant is placed on his or her side as the intensity of the erythema appears to be gravity-dependent.... harlequin colour change

Any abnormality in colour vision that causes difficulty distinguishing between certain colours. Total absence of colour vision (monochromatism) is rare. The most common types of colour vision deficiency are reduced discrimination of red and green. Most cases of red and green colour vision deficiency are caused by defects in the light-sensitive cells in the retina. These defects are usually inherited, although occasionally defects are caused by retinal or optic nerve diseases or injury. The inherited defects tend to be sex-linked (see genetic disorders), which means that the majority of sufferers are male. A person with a severe green deficiency has difficulty distinguishing oranges, greens, browns, and pale reds. In severe red deficiency, all reds appear dull. A much rarer deficiency in which blue cannot be distinguished may be inherited or may be due to degeneration of the retina or optic nerve.... colour vision deficiency