According to WHO definition: “Cataract is clouding of the lens of the eye which impedes the passage of light. Although most cases of cataract are related to the aging process, occasionally children can be born with the condition, or a cataract may develop after eye injuries, inflammation, and some other eye diseases.”
Refractive errors (myopia, hypermetropia, astigmatism, presbyopia) result in an unfocussed image falling on the retina. Uncorrected refractive errors, which affect persons of all ages and ethnic groups, are the main cause of vision impairment. They may result in lost education and employment opportunities, lower productivity and impaired quality of life.
Trachoma,which is the commonest infectious cause of blindness, is caused by Chlamydia trachomatis. Children who have the active stages of the disease are the reservoir of infection, while blindness, which occurs after repeated episodes of infection, principally affects adults.
Low-vision services are aimed at people who have residual vision that can be used and enhanced by specific aids. Low vision is currently defined as ‘visual acuity of < 6/18 down to and including 3/60 in the better eye’, from all causes.
Onchocerciasis is caused by infection with the filarial parasite Onchocerca volvulus, which is transmitted by the blackfly species. The vast majority of the 37 million infected people live in West, Central and East Africa, with smaller foci in Latin America and Yemen.
Glaucoma is not a single disease entity but a group of conditions characterized by damage to the optic nerve (detected by pathological cupping of the optic disc) and loss of the field of vision. The two main types are primary open-angle glaucoma and primary angle-closure glaucoma.
Diabetic retinopathy is a well-recognised complication of diabetes mellitus. Well-conducted clinical trials have shown that good control of diabetes and hypertension signifi cantly reduces the risk for diabetic retinopathy, and there is evidence from studies spanning more than 30 years that treatment of established retinopathy can reduce the risk for visual loss by more than 90%.