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.
Reduction of vision has significant consequences. Differentiating between different degrees of reduction of vision is important because the demands for medical, social, and rehabilitative interventions vary.
Reduction of vision has been defined in many different ways, resulting in multiple 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. “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 reduction of vision. The World Health Organization (WHO) and almost all population surveys now use the International Classification of Diseases, 10th Revision (ICD-10), of which the 2015 version categorizes reduction of vision according to the presenting (using whatever refractive correction the subject has) distant visual acuity of the better eye as mild or no visual impairment, moderate visual impairment, severe visual impairment, and blindness, but other categorizations are used (Table 20–1). (Confusingly, the term “visual impairment” may be used to encompass the moderate and severe visual impairment and blindness categories of ICD-10, ie, not including mild visual impairment. In this chapter, the term “vision loss” is used instead.) Maximum diameter of visual field of 20° or less in the better eye is also categorized as blindness by the ICD and as legal blindness in the United States. The AMA Guides to the Evaluation of Permanent Impairment provides a graduated categorization of visual field impairment, as it does for impairment of visual acuity. The differences between the various categorizations and the variable terminology emphasize the importance of knowing which definitions are used whenever statistics about reduction of vision are compared.
Table 20–1.Categorizations of Reduction of Vision |Favorite Table|Download (.pdf) Table 20–1. Categorizations of Reduction of Vision
|Snellen Visual Acuity ||LogMAR ||EDTRS Letters ||WHO (ICD-10 2015)1 ||AMA2 ||United States ||United Kingdom |
|20 ft ||6 m ||4 m ||3 m ||1 m ||Decimal |
|NLP ||Blindness ||Total blindness ||Legal blindness ||Severely sight impaired (blind) |
|20/2000 ||6/600 ||4/400 ||3/300 ||1/100 ||0.010 ||2.00 ||0 ||Near-blindness |
|20/1600 ||6/480 ||4/320 ||3/240 ||1/80 ||0.013 ||1.90 |
|20/1250 ||6/375 ||4/250 ||3/188 ||1/63 ||0.016 ||1.80 |
|20/1200 ||6/360 ||4/240 ||3/180 ||1/60 ||0.017 ||1.78 || |
|20/1000 ||6/300 ||4/200 ||3/150 ||1/50 ||0.020 ||1.70 |
|20/800 ||6/240 ||4/160 ||3/120 ||1/40 ||0.025 ||1.60 ||5 |
|20/640 ||6/192 ||4/128 ||3/96 ||1/32 ||0.031 ||1.50 ||10 |