This chapter addresses vision loss as a worldwide health problem, providing information on causes, with data on prevalence, and measures to prevent it. All of the disorders that may cause vision loss are discussed more fully in other chapters.
Vision loss has significant consequences. Differentiating between different degrees of vision loss is important, because the demands for medical, social, and rehabilitative interventions vary.
Vision loss has been defined in many different ways, determined by the intended purpose but resulting in many terms that may not be consistent with one another. Whereas to the lay person it implies complete loss of vision, the term “blindness” is often used for individuals who have significant and useful residual vision, an extreme example being the use of the term “color blindness” for individuals with mild color vision deficiency. “Industrial blindness,” a term from the early 19th century, may be used to describe the impact on employability. “Automobile blindness” may be used to indicate that the individual does not meet the requirements for a driver's license. “Legal blindness” is used in the United States for those who meet various legal requirements for benefits.
An important challenge is categorizing the broad range of vision loss. The eighth revision of the International Classification of Diseases (ICD-8) recognizes only two categories of vision: Sighted and Blind. In the 1970s the International Council of Ophthalmology (ICO) worked with the World Health Organization (WHO) to define three major categories in the US adaptation of the ninth revision (ICD-9-CM): Normal Vision, Low Vision, and Blindness. ICD-9 and ICD-10 do not utilize the first category.
The ICD major categories are now used in almost all population surveys, with the WHO definitions (ICD-9/ICD-10) usually having been used for surveys aimed at the detection of eye disease. Low vision is defined as best-corrected visual acuity less than 20/70 (6/18, 0.3) but equal to or better than 20/400 (3/60, 0.05) and blindness is defined as visual acuity less than 20/400, or maximum diameter of visual field 20° or less in the better eye. To describe how people live their lives, the WHO has subsequently recommended categorization according to visual acuity with presenting correction, that is, using whatever correction the subject has. This definition acknowledges the importance of uncorrected (or under-corrected) refractive error as a cause of vision loss worldwide and almost doubles the number of people counted as having vision loss.
In the United States and Canada, legal blindness is defined as best-corrected visual acuity equal to or less than 20/200 (6/60, 0.1), which corresponds with the ICD-8 criteria, or maximum diameter of visual field less than 20° in the better eye. Low vision has been defined as best-corrected visual acuity worse than 20/40 (6/12, 0.5) but better than 20/200 (6/60, 0.1). Across Europe the visual acuity criterion for registration as blind (or certification as severely sight impaired) varies, ...