Blindness and visual impairment | Types and causes

Blindness and visual impairment | Types and causes

Visual impairment and blindness have a significant impact on the socioeconomic development of individuals and societies. Their consequences are an important public health issue with greater impact in the developing countries, where 80% of the world blindness occurs

Blindness and visual impairment | Types and causes

Blindness is defined as visual acuity of less than 3/60 (0.05) or corresponding visual field loss (a field less than 10°) in the better eye with best possible correction. This corresponds to loss of walkabout vision. Low vision corresponds to visual acuity of less than 6/18 (0.3) but equal to or better than 3/60 (0.05) in the better eye with the best possible correction

Low Vision generally refers to a significant visual impairment, not limited to distance that measures between 20/70 and 20/160. Learners are generally unable to read standard print at a normal viewing distance, even with the aid of eyeglasses or contact lenses. They use a combination of vision and other senses to learn, often requires adaptations in lighting and accommodations such as low vision devices, large print, and sometimes braille.

Legally blind indicates that a learner has less than 20/200 vision in the better eye or a very limited field of vision (20 degrees at its widest point). These learners often requires adaptations in lighting and accommodations such as low vision devices, large print, audio, braille, or a combination of learning medium.

Totally blind generally indicates that the learner has little to no light perception. These learners often require accommodations such as audio and braille. Cortical Visual Impairment (CVI) is a neurological disorder, which results in unique visual responses to people, to educational materials, and to the environment. These learners exhibit characteristic behaviors associated with cortical visual impairment. Intervention, adaptations, and accommodations are designed along these identified behavioral characteristics.


Types of blindness

Curable blindness: That stage of blindness where the damage is reversible by prompt management e.g. cataract
Preventable blindness: The loss of blindness that could have been completely prevented by institution of effective preventive or prophylactic measures e.g. xerophthalmia, trachoma, and glaucoma
Avoidable blindness: The sum total of preventable or curable blindness is often referred to as avoidable blindness.

Causes of avoidable blindness

Cataract, a loss of the normal transparency of the crystalline lens due to an opacity, is responsible for half of all blindness worldwide, and is largely related to the ageing process (the older a person, the greater the chance of developing a cataract) all over the world. Therefore cataract is an universal cause of blindness. There is no prevention for cataract. Cataract surgery, including the implantation of an intra-ocular lens, may restore the visual acuity to near normal levels. The cost-effectiveness of cataract surgery has been demonstrated worldwide. A real restoration of the vision and a real improvement in the quality of life is obtained; furthermore adults can return to their work. That is why the volume of cataract surgery has increased worldwide

Trachoma is responsible for 6 million blinded people or 15% of world’s blindness. There are some 146 million cases of active disease due to infection with Chlamydia trachomatis. Repeated eye infections cause trichiasis and hence corneal blindness. Trachoma is common in areas of the world that are socio-economically deprived of basic needs in housing, health, water and sanitation. There is no medical treatment once the cornea has lost its transparency. Trachoma can be prevented trough the implementation of the ’’SAFE strategy’’: surgery for trichiasis, azithromycin (antibiotic treatment), facial cleanliness and environmental improvement (hygiene)

Glaucoma is responsible for 5.2 million blind people or 13.50 % of the total burden of world blindness and is therefore considered to be the third largest cause of blindness worldwide. According to Thylefors and Négrel research glaucoma is not a single disease but rather a group of disorders that have common features such as cupping and atrophy of the optic disc, characteristic visual field loss and often an increased intra-ocular pressure. Glaucoma is more common in populations of African of Asian heritage. There is no preventive treatment for glaucoma, but visual loss can be avoided if glaucoma is detected and treated at an early stage.

Childhood blindness

Childhood blindness is a priority area, considering the number of years the visual handicap plays a role, the high frequency of developmental anomalies and the fact that many of the conditions associated with blindness in children are also causes of child mortality. Each year, an estimated half a million children go blind, mostly in the poorest countries of Asia and Africa, of whom up to 60% die in childhood. Seventy-five per cent of these children suffer from Vitamin A deficiency, which causes night blindness, dry eye syndrome, an increased susceptibility to ocular infections and a higher risk of dying from infections due to an impaired immune system.

Vitamin A deficiency is considered to be the main cause of childhood blindness. There are still 78 countries that have children with vitamin A deficiency. Malnutrition is a contributing factor in half of all childhood deaths. Vitamin A deficiency can be corrected by supplementation of vitamins or by teaching the people to change their diet. Another way of dealing with childhood blindness is to take care that all children get the necessary vaccinations at the right time. The combination of measles vaccination and vitamin A supplementation has been one of the major success stories in reducing death from infectious diseases.

Onchocerciasis (River blindness)

Onchocerciasis is caused by Onchocerca volvulus, a parasitic worm that lives for up to 14 years in the human body. Each adult female worm produces millions of microfilariae that migrate throughout the body and give rise to a variety of symptoms: serious visual impairment due to an intense inflammatory reaction caused by the dead of the microfilaria in the eye; lesions of the skin; lymphadenitis and general debilitation.
The treatment of this disease is:

1. By eliminating the black flies through application of selected insecticides through aerial spraying of breeding sites in fast-flowing rivers.

2. By killing the larval worms with Ivermectin, a safe and effective medication (one dose a year is distributed free of charge to all those who need it)

Diabetes mellitus

Diabetes is a disease that in the long-term may lead to complications such as blindness, kidney failure and limb amputation, as well as to heart disease and stroke. After 15 years of diabetes, approximately 2% of people become blind while about 10% develop severe visual handicap. Loss of vision and blindness in persons with diabetes is the leading cause of blindness and visual disability in adults in economically developed societies, such as Belgium.


Ocular injuries

Ocular trauma is a common cause of visual loss and is known to be the most common cause of unilateral loss of vision. Each day eye injuries occur that cause suffering, medical costs and one or more days of restricted activity. In developing countries, the problem of injuries is more severe as there is often a delay before these eye injuries are handled in the proper way. Ocular trauma is the cause of bilateral blindness in more than one million people.

Ocular trauma can be prevented by risk appreciation and avoidance, widespread wearing of high-performance spectacles or protective goggles, appropriate occupational processing and shielding, extension of on-the-job safety to the home environment and the application of modern principles of surgical salvage

Leprosy is a chronic infection caused by Mycobacterium Leprae (Hansen – 1873) that mainly affects the skin, the peripheral nerves, the mucosa of the upper respiratory tract and also the eyes. It has been estimated that there are some 250 000 blind from the disease.

The WHO ensures that all leprosy patients have free and equal access to Multiple Drug-Therapy, the most modern treatment available that cures leprosy within twelve months.
It has been estimated that there are some 250 000 blind from the disease. The WHO ensures that all leprosy patients have free and equal access to Multiple Drug-Therapy, the most modern treatment available that cures leprosy within twelve months.



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