Introductory Chapter: Loss of Vision

1. Introduction

Today, it is estimated that nearly one billion people face preventable or untreated visual impairment. The impact of vision loss affects all age groups but particularly the elderly population. Among this cohort, reduced visual acuity has a multi-factorial etiology, including refractive error, cataracts, glaucoma, age-related macular degeneration, and diabetic retinopathy. Statistical projections indicate that the upcoming decades will witness a twofold rise in the number of adults with age-related visual impairments, a trend mostly attributed to the aging global population. Moreover, the surge in chronic diseases like diabetes significantly contributes to an increasing visually impaired population.

In younger individuals, however, congenital cataracts and premature retinopathy stand out as the main culprits of early vision loss. Left undiagnosed, these conditions can result in reduced eyesight or even total blindness, subsequently impacting the psychomotor development and education of the affected children.

Efficient prevention and intervention are vital to mitigate the consequences of visual impairment. Immediate action is crucial to reducing the lasting socioeconomic effects of visual loss, which includes a lower quality of life due to reduced productivity and workforce engagement, to list a few examples.

2. Terminology

Specific terminology has evolved to better define eligibility for disability benefits and rehabilitation training of individuals affected by vision loss. As per the World Health Organization’s (WHO) International Classification of Diseases definition (ICD-11), low vision is defined by a visual acuity (VA) of ≤6/18 in the better eye or a visual field (VF) of <10° after medical treatments or with visual aids. Total blindness is indicated by no light perception (NLP), affecting around 15% of those with eye disorders. Legal blindness, as defined by the United States Social Security Administration (SSA), refers to a best-corrected visual acuity (BCVA) of ≤20/200 or VF of ≤20° in the better eye. Visual impairment, on the other hand, is a term describing decreased visual function, which affects the ability to perform normal activities. Rather than using visual acuity or visual field thresholds, visual impairment is defined focusing only on visual function [1, 2, 3, 4, 5].

Based on visual acuity and/or visual field of the better seeing eye, visual impairment has been classified into categories proposed by the WHO and ICD (Table 1).

Visual impairment grade	Best corrected visual acuity (BCVA)	Visual field range (spatial extent)
Normal	20/10–20/25
Near normal	20/30–20/60
Moderate	20/70–20/160
Severe	20–200 – 20/400	11–20°
Profound	20/500–20/1000	6–10°
Near total	Counting fingers – Light perception	≤ 5°
Total	No light perception	No light perception

Table 1.

Visual impairment classification and grading.

3. Etiology

Visual impairment arises from a diverse set of conditions, involving genetic, congenital, and acquired factors. As highlighted in The WHO World Report of Vision, cataracts, uncorrected refractive errors, age-related macular degeneration, glaucoma, diabetic retinopathy, and trachoma represent the primary contributors to global blindness.

4. Epidemiology

Globally, in accordance with the WHO, over 2.2 billion individuals experience distant or near visual impairment. Among this population, approximately one billion people suffer from visual impairment that could have been prevented or requires therapeutic attention. The primary factors contributing to distance vision impairment or blindness include cataracts (94 million), refractive error (88.4 million), age-related macular degeneration (8 million), glaucoma (7.7 million), and diabetic retinopathy (3.9 million) [6, 7].

In terms of near vision impairment, the predominant issue is presbyopia, impacting a staggering 826 million individuals [8, 9].

The prevalence of blindness is disproportionately higher in developing countries, primarily due to rapid population growth, limited access to ophthalmologic services, cost constraints, and lower education levels. The latter may explain why females are more susceptible to acquiring visual impairments than males [10, 11]. Notably, nearly 90% of blind individuals are indigenous to developing nations.

Moreover, roughly 82% of blind individuals are over the age of 50. Although the adult and senior population is at risk for many debilitating ocular morbidities, childhood blindness is also a significant economic burden, affecting an estimated 1.4 million blind children under 15 years old. Given the projected global population growth to 9.7 billion by 2050, alongside the inevitability of population aging, addressing vision impairment takes first place on the global health agenda [12].

Interestingly, the causes of preventable blindness over the last three decades have changed. The shift has been one away from infectious diseases like trachoma and onchocerciasis toward non-communicable eye diseases (NCED), such as glaucoma, diabetic retinopathy, and age-related macular degeneration (AMD). This shift can be attributed to changing lifestyles and healthcare trends in middle- and low-income nations [7, 8, 13, 14].

5. Presentation and evaluation

Vision loss can manifest gradually or suddenly, leading to a diverse set of possible visual outcomes, including central or peripheral vision loss, overall blurriness, reduced contrast sensitivity, color vision issues, night blindness, glare, or photophobia. The extent of these effects depends on the underlying cause of the vision loss [15].

Obtaining accurate patient history, in addition to a comprehensive clinical assessment, is crucial to successful patient diagnosis and optimal therapy. An effective eye examination should consist of testing for visual acuity, visual field, extraocular muscle movement, pupil sensitivity, binocular vision, intraocular pressure, slit lamp analysis of the anterior segment, and a dilated fundus examination for posterior segment evaluation. However, formal visual field assessment, color vision and contrast sensitivity testing, optical coherence tomography, fluorescein angiography, visual evoked potential, electroretinography, electrooculography, and genetic testing might be additional diagnostic tools essential to accurately diagnose ocular diseases, monitor their progression, and determine patient-appropriate treatment strategies [16].

Those comprehensive eye exams are crucial to prevent, detect, treat, and manage conditions that can cause blindness. These exams should go beyond visual acuity assessments for basic lens prescriptions and include in-depth evaluations of ocular health.

6. Prognosis

The outcome of vision loss depends on the condition at hand, either remaining stable or worsening over time. For instance, age-related macular degeneration mostly affects central vision without leading to total blindness, whereas retinitis pigmentosa begins with peripheral vision loss, progresses to central vision loss, and can eventually result in complete blindness. Timely detection, accurate diagnosis, and appropriate treatment can prevent blindness caused by such conditions. For this reason, it is strongly advised for people of all ages to undergo a comprehensive eye exam every 1–2 years and become proactive in sight-preserving programs that may be offered by their healthcare providers. Also, there are many programs that provide free comprehensive vision and eye health assessments to infants within their first year of life as well as screening and prevention program for children to improve the public health infrastructure supporting the early detection of children’s vision problems [17, 18, 19, 20].

6.1 Achieving universal coverage for eye health and improving quality of life

Numerous global, national, and local organizations are actively promoting eye health awareness, improving healthcare accessibility, and tackling blindness. Extensive worldwide research programs aim to understand the causes of various eye conditions leading to permanent vision loss and develop effective protocols to better control and treat blinding conditions. The WHO plays a leading role in monitoring such trends, coordinating anti-blindness initiatives [21].

The impact of vision loss on one’s quality of life is undeniable. For this reason, vision rehabilitation offers solutions to the functional challenges encountered by individuals with poor vision, including effective use of their residual vision through visual aids and skills training. Moreover, ophthalmologists and optometrists with specialized training in vision rehabilitation have learned to evaluate the patient functional complaints and assess vision thereafter. Such rehabilitation services aim to enhance the quality of life for those with low vision, addressing physical, social, functional, and psychosocial aspects and improving participation in fulfilling basic patient functional needs [22, 23].

Unfortunately, global statistics indicate that only 5–10% of individuals requiring vision rehabilitation services have access to them. Various barriers, including inconsistent service delivery, lack of awareness, mobility issues, language barriers, and inadequate referrals, prevent patient access to better ocular healthcare. Low vision services in developing and developed countries, when compared, reveal distinct challenges [24, 25].

Developing nations face issues like information scarcity, lack of eye health professionals, and uneven service distribution [26], notably in the eastern Mediterranean region, where service availability varies from none to ≤10%. Financial constraints further limit services, often leading the healthcare systems dependent on non-governmental organizations for support [27].

In developed countries, incomplete services result from insufficient data on disease prevalence and causes of blindness and low vision. In high-income region, such as North America and Canada, ophthalmologists serve as the primary referral sources that direct patients to low vision service centers [28]. However, more than just poor vision is usually required to prompt such referrals. Therefore, even in developed countries, inadequate education in vision rehabilitation, high costs associated with misdiagnoses, and a lack of specialized services impact patient vision. A multidisciplinary approach is essential for complete patient assessment but is often not fully implemented. Ultimately, the need for specialized rehabilitation services remains crucial in both developing and developed nations [29].

On another note, data scarcity regarding rehabilitation coverage in developing countries highlights the importance of prioritizing data collection. The Universal Health Coverage study suggested prioritizing data collection in this field [30]. Most studies lack specific clinical diagnoses, and there is therefore a call for comprehensive and accurate data collection [30, 31].

To address the challenges in eye care, such as inequalities in the coverage and quality of prevention, treatment and rehabilitation services, and poor integration of eye-care services into health systems, WHO published World report on vision in October 2019 [32]. Endorsing this approach, The Lancet Global Health Commission on Global Eye Health [33] proposed ways for advancing health systems toward delivering high-quality integrated eye care within universal health by implementing strategies for the effective integration of eye health services between the primary, secondary, and tertiary levels to improve referral pathways, therefore ensuring recognition of those who need secondary care.

Low-vision rehabilitation services should be available to help patients maximize their residual vision, maintain their independence, and improve their quality of life. These services are provided by a multidisciplinary team of low-vision professionals including optometrists or ophthalmologists, low vision therapists, occupational therapists, orientation and mobility specialists, vocational rehabilitation specialists, rehabilitation teachers, social workers, and other rehabilitation professionals. It is important to recognize that blindness, regardless of the degree, carries significant morbidity and that addressing the challenges of vision loss necessitates a comprehensive interdisciplinary effort to achieve utmost effectiveness in managing and maximizing patient vision and well-being [29, 30, 31].