Visual Impairment and its Rehabilitation: A Review

Visual impairment (VI) is a condition of reduced visual performance that cannot be remedied by refractive correction (spectacles or contact lenses), surgery or medical methods. Visual Impairment is a broader term that encompasses both Low vision and Blindness. The prevalence of visual impairment is estimated to be around 2.2 billion in the world as per World Health Organization (WHO). Causes of Visual Impairment differs significantly in different regions. Saying that, still cataract and refractive errors are main causes of visual impairment globally. Owing to its negative impact on the individual’s dailies, professional life, productive time, confidence, and self-respect, VI needs to be addressed timely and appropriately. Visual impairments do have profound consequences for the individual, however effective rehabilitation services can restore independence and therefore ameliorate these consequences and restore the individual’s quality of life.


Introduction
Impairment of any kind (sensorial or motor) is a bane for any human being. With visual impairment being one of the leading causes of sensorial impairment, it is necessary to acknowledge its prevalence, causes and methods to avoid them, if possible. Also, at the same time, it is important to understand that the term "visual impairment", "blindness" and "Low vision" are not synonymous; rather visual impairment includes low vision as well as blindness.
Simply, Visual impairment (VI) is a condition of reduced visual performance that cannot be remedied by refractive correction (spectacles or contact lenses), surgery or medical methods [1]. [2] and defines as:

Various definitions of visual impairment have been given by various
• low vision is defined as visual acuity of less than 6/18, but equal to or better than 3/60, or a corresponding visual field loss to less than 20 degrees in the better eye with best possible correction • blindness is defined as visual acuity of less than 3/60, or a corresponding visual field loss to less than 10 degrees in the better eye with best possible correction.
Since, the ICD definition of visual impairment is based on best corrected visual acuity (BCVA), which exclude uncorrected refractive error as a cause of visual impairment, leading to substantial underestimation of the total visual impairment burden by about 38%, Lalit Dandona [3] and colleagues debated a revision in these definitions and defined: • a person with low vision as one who has impairment of visual functioning even after treatment and/or refractive correction, and has a visual acuity of less than 6/18 to light perception, or a visual field of less than 10° from the point of fixation, but who uses, or is potentially able to use, vision for the planning and/or execution of a task [4].
assistance, such as Social Security disability benefits. Legal Blindness is defined as: Remaining vision in the better eye after best correction is 20/200 (6/60) or less OR contraction of the peripheral visual fields in the better eye (A) to 10 degrees or less from the point of fixation; or (B) so the widest diameter subtends an angle no greater than 20 degrees [7].
Some other definitions and types of blindness adopted by different countries and institutions are:

Avoidable Blindness
It is the blindness that could be either treated or prevented by known, cost effective means. It can either be preventable or curable.
It includes: • Cataract

Manifest Blindness
It restricts the accomplishment of tasks for daily living leading to impairment in mobility.
Vision less than 1/60 to just perception of light in the better eye with best correction is considered as manifest blindness

Absolute Blindness
It restricts the ability of even perceiving the light. E.g.: Optic atrophy.

Economic Blindness
It is the level of blindness that prevents an individual from earning his wages. Vision less than 6/60 to 3/60 in the better eye with best correction is considered as economic blindness

Social Blindness
It hampers an individual from socially interacting with the family and peer groups in a satisfactory manner. Vision less than 3/60 to 1/60 in the better eye with best correction is considered as social blindness.

Classification:
The classification of visual impairment varies worldwide [8].
Since ICD is considered the standard worldwide classification therefore, the ICD definitions of visual impairment are used most often for worldwide estimates of visual impairment. visual impairment (presenting visual acuity less than 6/18 but equal to or better than 6/60) and severe visual impairment (presenting visual acuity less than 6/60 but equal to or better than 3/60) [9].
Also, the previous classifications of visual impairment considered visual acuity as the only determining factor for categorizing visual impairment; loss of function due to hemianopia, loss of contrast sensitivity, photophobia, visual distortion, diplopia, or visual perceptual difficulties were not considered. These factors have been addressed on ICD-10 and ICD-11. Vision impairment results as an involvement of more than one factor which may include visual acuity, visual field, and contrast sensitivity. Therefore, these are the most clinically useful quantifiers of visual impairment [8].

Prevalence and Incidence:
According to World Health Organization, globally, at least 2.2 billion people have a vision impairment or blindness, of whom at least 1 billion have a vision impairment that could have been prevented or has yet to be addressed [10], among whom 1 billion people includes those with moderate or severe distance vision impairment or blindness [10] However, with the introduction of the global initiative to eliminate avoidable blindness (vision 2020: the right to sight), many agree that the initiative is in the right direction to reduce the prevalence of avoidable blindness [11]. Estimates of the number of visually impaired persons vary, depending upon the criteria used [12]. In 2002, it was estimated that with bestcorrected vision there were 161 million individuals with VI globally [13]. However, when the prevalence of uncorrected refractive error was included, this value increased substantially to 314 million individuals with VI [14]. This implies that an additional 153 million individuals were visually impaired from uncorrected refractive error alone [14]. This statistics stress upon the underestimation of total visually impaired population if uncorrected refractive error is to be looked. Thus, the importance of correction and management of refractive error is significantly high for the management of visual impairment [15].
An interesting fact is that, the majority of the visually impaired population are among the elderly and shows female preponderance. The incidence of visual impairment increases with age; more than two-thirds of persons with low vision are over the age of 65 [16] The global prevalence of blindness is greater in women than in men [17] [21].

Causes and Risk Factors:
Not only the ocular morbidities, but also the systemic health   The incidence of these conditions increases with age. So, increasing age is the single-most risk factor for visual impairment. The implications of childhood blindness and VI may be more significant because of greater life expectancies [10].

Rehabilitation:
It has been reported that proper management of individuals with VI can provide the same quality of life as that of normally sighted individuals [24]. Therefore, rehabilitation should be made accessible to every visually impaired person. A visually impaired person needs following types of rehabilitation:

Medical Rehabilitation:
This includes early identification of the visually impaired

2.
Non-optical type: Large print books, Type writers, Typo scopes, Special illumination devices, Talking books etc.

Educational Rehabilitation:
Educational rehabilitation includes education avenues provided to the visually impaired in the residential blind schools with the facility of Braille system of education. Louis Braille's invention of Braille alphabet system in 1832 provided a tremendous impetus to the education of visually impaired children throughout the world.
After gaining primary education from these schools, these children are placed in regular schools in the general education system. This

Conclusion
The burden of visual impairment is prevalent in every country,