Vision Impairment

Low vision, also known as partial sight, is characterized by a considerable decrease in visual ability that cannot be corrected to a normal level using standard glasses, contact lenses, medical treatments, or surgery (Silverstone et al., 2000). People with low vision encounter difficulties in performing visual tasks, even with corrective lenses, but improve their performance by employing compensatory strategies, assistive devices, and environmental adjustments (Corn and Koenig, 2004).

The term "low vision" encompasses a variety of conditions that fall between normal sight and blindness, ranging from mild to severe visual impairment, excluding total blindness. The term "low" indicates a vision level that is below normal, while "vision" differentiates it from blindness. This condition can notably hinder a person's capacity to learn or carry out daily activities, yet it permits some functional visual discrimination. The WHO defines low vision as having a visual acuity of less than 20/60 (0.3). The International Council of Ophthalmology has proposed a functional definition: a vision level that benefits from enhancements such as magnification, illumination, and contrast, along with vision substitution methods like talking books (Dutton and Bax, 2010; Colenbrander, 2002; WHO, 2016).

Blindness as defined by the dictionary refers to the total absence of vision, which is relatively rare. The World Health Organization (WHO), provides a numerical definition for blindness as having a visual acuity of less than 3/60 (20/400, 0.05). This means that a person with this level of visual impairment can see at 3 meters what a person with normal vision can see at 60 meters. The International Council of Ophthalmology (ICO), adds a functional perspective, defining blindness as a condition where individuals have no vision, or so little that they must primarily rely on vision substitution techniques like using cane, reading Braille or enhancing their hearing to navigate their environment (Dutton and Bax, 2010; Colenbrander, 2002; WHO, 2016). ​

Ocular Visual Impairment - OVI, refers to vision loss caused by disorders affecting the eyes or the optic nerve (Dutton and Bax, 2010). This can encompass a wide range of conditions that impair the ability of the eyes or the optic nerve to transmit visual information effectively to the brain. 

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Conditions that can lead to OVI include:

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• Cataract: Clouding of the lens, leading to decreased vision.

• Glaucoma: Increased intraocular pressure causing damage to the optic nerve.

• Macular Degeneration: Deterioration of the central portion of the retina, leading to central vision loss.

• Diabetic Retinopathy: Damage to the blood vessels in the retina due to diabetes.

• Optic Neuritis: Inflammation of the optic nerve, often associated with multiple sclerosis.

• Retinitis Pigmentosa: A group of generic disorders that affect the retina's ability to respond to light (Corn and Koenig, 2004; Silverstone et al., 2000). 

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Understanding the specific cause of OVI is crucial for determining appropriate treatment and management strategies to support those affected. ​

Cerebral visual impairment - CVI, refers to a vision impairment in children caused by damage or disorders affecting the visual pathways and centers in the brain, which include those responsible for visual perception, cognitive processing, and the guidance of movement (Hall Lueck and Dutton, 2015). This condition can arise from a range of neurological issues and requires personalized approaches to support affected individuals in their learning and daily activities.

Cortical vision loss is a condition that occurs due to damage or disease affecting one or both occipital lobes of the brain, which are responsible for processing visual information. The primary cause in adults is often a stroke or cerebrovascular accident that impacts these areas. Other contributing factors may include brain tumors or trauma to the occipital lobes (Corn and Koenig, 2004).

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Cortical or cerebral blindness refers to a condition which leads to significant vision loss or total lack of vision because of bilateral damage to the visual pathways behind the lateral geniculate bodies which often occurs in combination with damage to other parts of the brain that are responsible for vision (Hall Lueck and Dutton, 2015). 

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These conditions can lead to significant visual impairments, affecting the individual's ability to see even though their eyes may function normally.

The functional effects of various visual diseases or conditions can significantly impact daily life and overall well-being. Here's a brief overview of some common conditions and their functional implications:

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• Insufficient visual resolution and low central visual acuity: This refers to the lack of sharpness or clarity in vision, particularly in the central part of the visual field. Tasks like seeing small print or text clearly, driving, recognizing faces or performing detailed work can become difficult. 

• Inadequate field of vision: This means the visual field is reduced, leading to difficulty in seeing the full range of what's in front of you, often impacting peripheral vision. Tasks like driving, navigating through crowded spaces and judging distances can become difficult. 

• Reduced peak contrast sensitivity: This is the diminished ability of the visual system to detect the smallest differences in luminance or color contrast between an object and its background.  

• Blurred, hazy vision: A condition where vision is not clear, causing objects to appear out of focus or cloudy. 

• Achromatopsia: Total color blindness, a condition accompanied with extreme light sensitivity, nystagmus and low vision acuity. 

• Photophobia: Extreme sensitivity to light, causing discomfort or the need to squint or close the eyes in bright environments. Constant need for protective eyewear in bright conditions and avoidance of outdoor activities during daylight are some  implications. 

• Strabismus: Misalignment of the eyes leading to double vision and depth perception issues. 

• Amblyopia: Also known as "lazy eye", this condition results in reduced vision in one eye due to the brain and eye not working together properly. Tasks thar require precise hand-eye coordination can become difficult. 

• Nystagmus: Involuntary, rapid and repetitive movement of the eyes leading to shaky and blurry vision, and balance and coordination issues. 

• Central visual-field defects and central scotomas: Loss of vision in the central part of the visual field, often leading to difficulties in reading, recognizing faces and other tasks thar require detailed central vision. 

• Constricted fields or peripheral scotoma: Narrowing of the overall visual field or blind spots in the peripheral vision leading to challenges in detecting objects or movement outside the central vision. 

• Visual perceptual difficulties: Challenges in interpreting, understanding and processing visual information (Silverstone et al., 2000; Corn and Koenig, 2004). 

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Each of these conditions can have a unique impact on daily life but there are various adaptive strategies and techniques available to help manage these challenges. 

Vision Impairment can significantly impact Activities of Daily Living (ADLs), which are the basic tasks individuals need to manage daily life independently. Activities of daily living affected by vision loss include: ​​

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• Literacy

• Grooming and Health Care

• Meal Preparation

• Housekeeping 

• Financial management 

• Functional mobility 

• Shopping 

• Leisure and social activities (Silverstone et al., 2000).​

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Colenbrander, August (2002) Visual Standards, Aspects and Ranges of Vision Loss with emphasis on Population Surveys. Sydney: International Council of Ophthalmology. Available at: https://www.researchgate.net/publication/248343777_Visual_Standards_aspects_and_ranges_of_vision_loss_with_emphasis_on_population_surveys. ( Accessed: 11 January 2025).

Corn, A.L. and Koenig, A.J. (eds) (2004) Foundations of low vision: Clinical and functional perspectives. New York: American Foundation for the Blind, AFB Press. 

Dutton, G.N. and Bax, M. (eds) (2010) Visual Impairment in children due to damage to the brain. London: Mac Keith Press. 

Hall Lueck, A. and Dutton, G.N. (eds) (2015) Vision and the brain. Understanding cerebral visual impairment in children. New York: American Foundation for the blind, AFB Press.

Silverstone, B., Rosenthal, B. and Lang, M. A. (eds) (2000) The Lighthouse Handbook on Vision Impairment and Vision Rehabilitation. New York: Oxford University Press. 

World Health Organization (2016) International statistical classification of diseases and related health problems, 10th revision (ICD-10). Geneva: World Health Organization. 

Zihl, J. and Dutton, G.N. (2015) Cerebral visual impairment in children. Visuoperceptive and visuocognitive disorders. Wien: Springer Verlag. ​