ABSTRACT

Due to the increasing age of the global population, rates of visual disability are increasing. Visual rehabilitation is an effective method for increasing quality of life among individuals with low vision or blindness due to unpreventable or untreatable causes. The goal of low vision rehabilitation is to produce people who are independent, have an economically viable profession or skill, and are able to enjoy their lives. The stages of modern low vision rehabilitation include the intake interview, assessment of residual visual functions, assessment of residual functional vision, interventions and recommendations, and vision rehabilitation therapies.

Intraduction

According to World Health Organization (WHO) data from 2010, there were an estimated 285 million people living with visual impairment worldwide. Of these, 39 million were reported as blind and 246 million as having low vision. The most common causes (80%) of these visual impairments are treatable conditions such as uncorrected refractive errors and cataract. These are followed by age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy. It has been reported that 65% of visually impaired and 82% of blind people are 50 years of age or older. Considering that the population is aging, this suggests that more people will be at risk in the future.¹

Definitions of low vision and blindness may vary between countries. According to the definition accepted in the USA, best corrected visual acuity less than or equal to 20/200 in the better eye or a visual field less than or equal to 20° in the better eye is considered legal blindness.² In the 2016 version of the International Classification of Disease (ICF)-10, visual impairment is classified in 5 categories based on presenting visual acuity. While older definitions were based on best corrected visual acuity of the better eye, the current definition is based on presenting visual acuity (with glasses if any, without glasses if not) in order to emphasize the burden of uncorrected refractive errors (Table 1). According to this, presenting visual acuity in the better eye equal to or better than 6/18 is defined as mild or no visual impairment; equal to or better than 6/60 and worse than 6/18 as moderate visual impairment (category 1); equal to or better than 3/60 and worse than 6/60 as severe visual impairment (category 2); and worse than 3/60 as blindness. Blindness is also separated into 3 categories: visual acuity worse than 3/60 (category 3), worse than 1/60 (or counting fingers at 1 meter) (category 4), and no light perception (category 5).³ According to this classification, those with moderate and severe visual impairment (visual acuity worse than 6/18 and equal to or better than 3/60) and those with a visual field less than or equal to 20° are defined as having “low vision” and require rehabilitation. Functionally, low vision can be regarded as a level of vision that prevents someone from performing their everyday activities. Having a presenting visual acuity worse than 3/60 and a corresponding visual field smaller than 10° is defined as blindness.³ Because this new definition also includes uncorrected refractive errors which were previously unaccounted for, the prevalence of blindness in various countries increases to 15% in all age groups and 25-30% among older adults. Studies have shown that the prevalence of low vision is up to 60% among older adults.^4,5,6,7,8

The prevalence and causes of blindness and low vision in different societies vary based on their level of development. According to WHO data, the prevalence of blindness is 7.3/1000 in Africa, 3.5/1000 in the USA, 8.5/1000 in the Eastern Mediterranean Region, 3.0/1000 in Europe, 6.9/1000 in Southeast Asia except India, and 5.3/1000 in the Western Pacific Region except China. Global data indicate there are 3 people with low vision for each blind person; in the USA and Europe, which have the lowest rates of blindness, the prevalence of low vision is 25.6 and 28.7 per 1000, respectively. This rate is 25.4/1000 in Africa and 32/1000 in Southeast Asia.^9,10

According to data from 2000, it was estimated that there were 937,000 (0.78%) blind and 2.4 million (1.98%) people with low vision over 40 years of age in the USA. Age-related macular degeneration (AMD) is the most common cause of blindness among Caucasians, accounting for 54.4% of cases. By 2020, the prevalence of blindness in the USA is predicted to increase by 70% to reach 1.6 million, and a similar increase is expected in the low vision population.¹¹

Globally, 42% of visual impairment is due to uncorrected refractive errors, while 33% is caused by cataract. Other major causes include glaucoma, diabetic retinopathy (DR), trachoma, AMD, and corneal opacities. The primary cause of blindness is cataract (51%) (Figures 1 and 2).¹ In North America and other developed countries, the main causes of vision loss are AMD, DR, and glaucoma. Other causes include herpes simplex keratitis, retinal detachment, retinal vascular diseases, and hereditary retinal degenerative diseases. In developing countries, the primary causes of vision loss are uncorrected refractive errors and cataract, followed by glaucoma, infectious diseases, injuries, and xerophthalmia.¹² In short, visual impairment in developed countries is a result of unpreventable and/or currently untreatable causes, whereas preventable (infectious, e.g. trachoma, or nutritional, e.g. vitamin A deficiency) and/or treatable (e.g. cataracts) causes still play a major role in developing countries. The fact that most of the diseases that cause blindness and low vision are preventable or treatable has prompted many organizations to take action, especially WHO. According to the VISION 2020 report from WHO, low vision prevention and rehabilitation are among the primary global goals.⁹

A person’s ability to perform important sight-based tasks is defined as “visual functioning”. Reduced visual functioning due to disorders of the eye or visual system results in low vision. In addition to visual acuity, visual functioning should be assessed using parameters such as visual field, contrast sensitivity, electrophysiological tests, adequacy of preferred retinal locus, color vision, binocularity, and stereopsis.

Low vision rehabilitation aims to increase quality of life by enabling patients to live independently, have a vocation or skill with which they can financially support themselves, and enjoy life. The stages of modern low vision rehabilitation include the intake, assessment of residual visual function, assessment of residual functional vision, interventions and recommendations, and vision rehabilitation therapy.¹³