Vision is an essential sensory input from infancy throughout childhood that allows for normal physical, cognitive, educational, and social development and for adult occupational training. Vision is present at birth at approximately a 20/200 level. Visual acuity develops rapidly during the first year, with normal acuity reached by 9 to 12 months of age. Blindness or visual impairment can be assessed in terms of the level of visual function and by scoring functional vision related to quality-of-life achievements. Assessment of visual acuity is the most often used parameter for estimating vision. The assessment of visual acuity in young children is imprecise; therefore, it is necessary to define ranges of visual loss. Also, test results may improve with advancing age and development. Visual function has been categorized into 5 levels of performance, ranging from normal to visually impaired (Table 574-1).
TABLE 574-1VISUAL FUNCTION BASED ON LEVELS OF VISUAL ACUITY ||Download (.pdf) TABLE 574-1VISUAL FUNCTION BASED ON LEVELS OF VISUAL ACUITY
|Level of Vision or Impairment ||Best Corrected Visual Acuitya (Metric Equivalent) |
|Normal vision ||20/20 to 20/30 (6/9 or better) |
|Slight visual impairment (SVI) ||20/40 to 20/50 (6/12 to 6/15) |
|Visual impairment (VI) ||20/60 to 20/100 (6/18 to 6/30) |
|Severe visual impairment (SVI) ||20/200 to 20/400 (6/60 to 6/120) |
|Blind (BL) ||Worse than 20/400 (6/120) |
Unsteady eye movements may be evidence of poor vision since early in life. When related to an ocular etiology such as cataracts, retinal abnormalities, or optic nerve defects, pendular nystagmus is often present, which becomes a characteristic jerk nystagmus in lateral gaze. Jerk nystagmus is defined as a slow conjugate drift in one direction followed by a rapid corrective return to fixation. If the vision is very poor, the abnormal eye movements can become more random in many planes and are less sustained. These are sometimes referred to as “wandering eye movements.” Infantile sensory nystagmus may be seen with extreme vision loss in only 1 eye but, in this setting, is characteristically only jerk in type. In addition to infantile sensory nystagmus, other causes of nystagmus must be distinguished and include disturbances of motor origin. It is important to recognize the significance of early-onset nystagmus as a possible sign of early-onset vision loss.
EPIDEMIOLOGY AND CAUSES OF CHILDHOOD BLINDNESS
It has been estimated that there are 1.5 million blind children worldwide and that 90% live in developing countries. Of the estimated 500,000 children who become blind annually, 60% to 80% are estimated to die within 1 to 2 years. The reported prevalence rates of blindness and mortality in children under 5 years are directly related. Reported rates of blindness in England and Scandinavia are 0.25 per 1000 and 0.4 per 1000, respectively, versus 0.65 per 1000 and 0.63 per 1000 in India and Nepal, respectively. The ...