blindness partial or complete loss of sight. Blindness may be caused by injury, by lesions of the brain or optic nerve, by disease of the cornea or retina, by pathological changes originating in systemic disorders (e.g., diabetes ) and by cataract , glaucoma , or retinal detachment. Blindness caused by infectious diseases, such as trachoma , and by dietary deficiencies is common in underdeveloped countries where medical care is inadequate. River blindness, caused by a parasitic worm transmitted by black flies, results in severe itching and disfiguring lesions. Infection of the eye area can destroy vision. An estimated 18 million people in Africa, Latin America, South America, and Yemen are infected with the parasite; 1 million of those infected are expected to become blind or severely impaired. Until recently, pesticides have been used to eradicate the flies. Two new drugs, ivermectin and amocarzine, have proved effective when used together. Most infectious diseases of the eye can be prevented or cured.

A major cause of congenital blindness in the United States, ophthalmia neonatorum, which is caused by gonorrhea organisms in the maternal birth canal, is now prevented by placing silver nitrate solution in all newborn infants' eyes. Retinitis pigmentosis, a hereditary and degenerative eye disease, affects 100,000 people in the United States. An early sign is night blindness which progresses to total blindness. Color blindness , an hereditary problem, is an inability to distinguish colors, most commonly red and green. Snow blindness is a temporary condition resulting from a burn of the cornea caused by the reflection of sunlight on snow. Night blindness results from a deficiency of vitamin A. See eye .

blindness Surprisingly, blindness rarely means total absence of light perception. Most definitions of blindness are based on measurement of visual acuity (the ability to read letters at a certain distance) and assessment of the ability of the person to carry out tasks needing vision. In the UK, the National Assistance Act 1948 states that a person can be certified as blind if they are ‘so blind that they cannot do any work for which eyesight is essential’. This rather circular definition refers to ‘any work’ and not just the person's normal job or one for which he has been specially trained.

Visual acuity is usually tested by asking the patient to read letters of various sizes on a chart viewed from a distance of 6 m or 20 feet (the Snellen method). Acuity is expressed as a fraction, the number on top referring to the distance at which a normal person can read a particular size of letter and the lower number the distance at which the subject being tested can read that size of letter. Hence ‘normal’ visual acuity is 6/6 (European) or 20/20 (American). A person should be certified blind if the visual acuity (while wearing corrective glasses) is 3/60 or below (when a letter that can be recognized from 60 metres by a normal person can be identified only from 3 metres or closer). A person should also be certified blind if their acuity is between 3/60 and 6/60 but they have completely lost the peripheral part of their visual field, hence restricting their vision to the central part of the field. Indeed, if the more useful lower part of the visual field is lost then someone with better than 6/60 acuity can be certified blind.

There is no legal definition of partial sight in the UK, but a person can be certified as partially sighted if they are ‘substantially and permanently handicapped by defective vision caused by congenital defect or illness or injury’. All certification must be done by a consultant ophthalmologist. The help from Social Services should be the same for both legally blind and partially sighted groups but Social Security benefits and tax concessions differ.

Definitions of blindness are not the same around the world and the vast majority depend on measured visual acuity with no allowance for any functional deficits. Consequently comparison of the incidence of blindness world-wide is inexact. The World Health Organisation has proposed categories of visual impairment but these have not yet been widely adopted.

The common causes of blindness vary in different countries according to the general levels of economic and physical health. The high rate of blindness in developing countries is mainly due to malnutrition and infectious diseases, coupled with the scarcity of medical care. Moorfields Eye Hospital was founded in London in 1805 to treat the ‘Egyptian ophthalmia’, a mixture of trachoma and purulent ophthalmitis brought back by British troops from Aboukir after their withdrawal from Egypt in 1803. The disease quickly spread throughout the country when the disbanded soldiers returned to their homes, taking the infection with them. Nowadays the condition is treatable with tetracycline eye ointment and tetracycline taken orally.

Causes of blindness

Lack of vitamin A has a direct effect on the eye, causing clouding and softening of the cornea (keratomalacia), but also increases the risk and severity of infections, so that measles can be a blinding or even fatal disease in children who are deficient in vitamin A. Night-blindness due to lack of vitamin A may occur in famines, and cure of this condition by eating liver, which is rich in vitamin A, has been known for over 3000 years.

Another cause of night-blindness is pigmentary degeneration of the retina (retinitis pigmentosa) which, combined with partial loss of the visual field, eventually contracting down to ‘tunnel vision’, can be most disabling. This condition is mainly inherited as an autosomal recessive condition (showing itself only when both parents carry the mutant gene), but other forms occur. A high proportion of the population of the Atlantic island Tristan da Cunha was recently discovered to be affected when they were evacuated because of volcanic activity. The disorder is progressive and untreatable.

Trachoma, an infectious disease, affects some 500 million people world-wide, of whom 7 million are blind and 10 million visually impaired. The infectious agents are bacteria known as Chlamydia.

River blindness (onchocerciasis) is the next commonest infection, where microfilarial parasites, spread by black flies, which breed in the tropical, sub-Saharan belt across the whole of Africa and at similar latitudes in Mexico, Brazil, and Ecuador, invade the retina and the supporting, vascularized middle layer of the eyeball, the choroid. Treatment was revolutionized in 1987 when ivermectin, already used in veterinary medicine, was registered for human therapy.

From 1976 the total number of people registered blind in Britain has risen, but this rise is limited to those over 75 years old. Fifty per cent of all 75–85-year-olds registered with impaired vision in this country suffer from age-related macular degeneration (ARMD). Cataracts are now second as a cause of blindness, at around 40%, but these are essentially treatable by surgery except in those cases where extraction of the cataract reveals underlying, untreatable ARMD.

Damage to the retina caused by glaucoma (increased pressure in the eyeball) and by diabetes (diabetic retinopathy) make up almost all the remaining causes of blindness. Glaucoma is insidious in onset: acuity in the central visual field is not seriously affected and a diagnosis may not be made until much of the peripheral retina has been destroyed. Diabetic retinopathy is most prevalent and severe in long-standing insulin-dependent diabetes. This emphasizes the importance of striving for optimal diabetic control. Routine screening checks for both glaucoma and diabetic retinopathy are essential, but manpower and economic considerations have led to much of this work being transferred to orthoptists and optometrists. Retinal detachment (separation of the retina from the pigment epithelium behind it) is a rarer cause of blindness.

There is a long history of visual upsets from staring directly at the sun. The high energy optically concentrated at the central part of the retina for only seconds can produce prolonged after-images and even permanent loss of central vision. This is an occupational hazard for astronomers, and for members of the public who sun-gaze in a misguided attempt to strengthen their eyes or when under the influence of hallucinogenic drugs. There is a particular hazard during solar eclipses because the reduced total amount of light makes it easier to hold fixation on the sun, but the intensity on the remaining illuminated part of the retina is just as high (and just as damaging) as when there is no eclipse: hence the term ‘eclipse blindness’.

Possibilities for treatment

Given the immense social importance of vision, there is intense effort to develop new treatments for blinding conditions. These are focusing not only on the conventional approach of developing new vaccines to prevent infection and new drugs to treat specific conditions, but also on more innovative approaches. For instance, attempts have been made to implant an array of electrodes over the surface of the visual cortex, coupled to a video camera or an optical letter reader, in the hope of bypassing the eye and providing visual sensation by direct stimulation of the cortex. Unfortunately, such stimulation produces only the sensation of tiny pin-points of light, which appear to move with movements of the eyes. A more promising approach is the implantation of a thin sheet of light-sensitive electrodes into the retina, to take the place of degenerated receptors and provide direct stimulation to the fibres of the optic nerve.

Cortical blindness

Damage to the visual cortex in the occipital lobe of the cerebral hemispheres can also cause blindness — cortical blindness. When fixation is maintained on a point in space, a particular region of the visual field is blind (a ‘scotoma’) whether either eye is open, or both (because the cortex receives signals from corresponding regions of the two retinae). Cortical blindness can occur, for example, after a stroke affecting the posterior cerebral artery, which supplies blood to that part of the brain. If extensive damage occurs in one hemisphere, the opposite side of the visual field becomes blind (hemianopia). Often, a small region around the fixation point is spared. This ‘macular sparing’ is thought to be due to the fact that so much of the visual cortex is devoted to the central part of the retina that some part of this region has a high chance of surviving. Interestingly, even when the occipital visual cortex is bilaterally destroyed, resulting in total blindness with no light perception, the patient does not feel enveloped in darkness: rather, the outside world simply does not exist visually (as for the world behind our heads). This contrasts with blindness resulting from retinal damage (for instance from total bilateral retinal detachment), when the patient complains of being in complete darkness. Indeed, the cortically blind patients are subjectively unaware of their disability — blind to their blindness.

When damage is restricted to the primary visual cortex (not extending into the surrounding cortical areas) some patients are still able to detect certain forms of visual stimulation (especially moving objects and sudden changes in brightness) in the ‘blind’ part of the visual field. Amazingly, if the stimulus is not very intense or rapidly moving, they are often unaware of their residual visual capacity, but can reliably ‘guess’ whether, for instance, the stimulus has moved, and even in which direction. This bizarre dissociation of vision from consciousness is known as ‘blindsight’. Recent research even suggests that the facial expression of faces ‘seen’ in the blind part of the field can be recognized. Blindsight is not magic! Even when the primary visual cortex is damaged, information from the eyes still reaches parts of the midbrain and other visual parts of the cerebral cortex. These secondary pathways presumably mediate the impoverished visual performance.

If a stroke or injury leaves the primary visual cortex intact but destroys visual areas further forward in the occipital lobe of the cerebral hemispheres, remarkable disorders of visual perception, without frank blindness, can occur. These include the inability to see movement, even though stationary objects are quite normally perceived (akinetopsia), and a lack of perceived colour, despite normal perception of shape and movement (achromatopsia). These observations are entirely compatible with evidence from experiments in animals in which the activity of nerve cells has been recorded with microelectrodes, as well as with studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to detect activity in the normal human brain caused by different forms of visual stimulation. These experimental approaches have shown that the primary visual cortex is surrounded by a patchwork of other areas in which neurons are devoted to the analysis of one aspect or another of the visual image — motion in some areas, colour in others, etc.

Damage further forward, in the lower part of the temporal lobe, can precipitate even more curious failures of perceptual interpretation, generally known as visual agnosias (from the Greek for lack of knowledge). Not uncommon, especially after damage on the right side, is prosopagnosia — an inability to recognize faces, sometimes even of family members, although other aspects of object identification (even knowing that a face is a face) are intact. In extreme cases, the poor patient has great difficulty in recognizing a wide variety of everyday objects (until he or she touches them), even though all basic aspects of vision (acuity, colour vision, detection of movement, etc.) are unaffected.

Injury to the rear part of the corpus callosum — the great cable of millions of nerve fibres that links the two hemispheres — or to regions at the junction of the occipital and temporal lobes can cause specific disorders of visual integration (associational disturbances), such as word blindness (alexia).

Provision for the visually disabled

The reaction of the public to handicapped and disabled people remains capricious, and often prejudiced. The deaf have long been figures of fun: they are often ignored and easily retreat into solitude. However, the blind generally receive more sympathy, even admiration. Social Services for the blind unfortunately are not uniformly good throughout the UK. However, some national organizations such as the Royal National Institute for the Blind and Guide Dogs for the Blind give great help and provide funds for research into blindness as well. In 1835 Louis Braille introduced his system of raised writing, where projecting dots represent a letter or number and are interpreted by touch, but it took 30 years to gain acceptance. In this electronic age there are many devices which can make an enormous difference to the blind person's quality of life. One is a computer that reads out text audibly as it appears on screen. This can be set to speeds as fast as the subject can comprehend the speech. A braille printer and labelling machine help, for example, to identify foodstuffs in the kitchen or deep-freeze, or to catalogue a CD library. Microwave units can respond to and speak instructions and will defrost different foods correctly once they have been weighed. For contact with the outside world there are talking newspapers, which can be sent by compressed e-mail, or put on to the Internet. A CD-ROM of all British daily newspapers is available weekly. Never has so much been available for blind people who can afford it.

Peter Fells, and Colin Blakemore

Bibliography

Cullinon, T. R. (1987). The epidemiology of blindness. In Clinical ophthalmology, (ed. S. Miller), p. 571. Wright, Bristol.
Walsh, F. B. and and Hoyt, W. F. (1969). Clinical neuro-ophthalmology, Vol. 1, (3rd edn), pp. 87–120. Williams and Wilkins, Baltimore.

blindness (blynd-nis) n. the inability to see. For administrative purposes, the term covers both total blindness and certain degrees of visual impairment (see blind register). The commonest causes of blindness worldwide are trachoma, onchocerciasis, and vitamin A deficiency, and in Great Britain age-related macular degeneration, glaucoma, cataract, myopic retinal degeneration, and diabetic retinopathy. See also colour blindness, day blindness, night blindness, snow blindness.