Page 12 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

CHAPTER 1
AMBLYOPIA
History
The term amblyopia was already used by Hippocrates to describe diminished acuity. The Greek word amblyopia means dimness or dullness of vision. Strabismus was known as a disorder of eye position and eye movement and was regarded as a symptom rather than a disease. In the Byzantine Empire strabismus was believed to be caused by a spastic state of the eye muscles. During the time of the crusades the first eye hospital, to treat patients with low vision, was opened in France by Saint Louis IX. Strabismus was believed to be caused by an unequal strength of the muscles or lack of concordance. George-Louis Leclerc, Comte de Buffon was the first to believe that strabismus was caused by disruption of binocular vision due to poor vision in one of the eyes. He also described that the weak eye could regain its strength by occluding the good eye.1
The basic mysteries regarding the aetiology of amblyopia were not to be resolved until the beginning of 1960 through the neurophysiological research performed by Hubel and Wiesel. They explored the receptive field properties of neurons in the visual cortex2,3 and demonstrated the existence of ocular dominance columns.4-6 They showed that there is an upper age limit for the development of amblyopia in animals. Subsequently they performed experiments on cats, where deprivation was induced by suturing the eyelids closed for various amounts of time and at different ages.2 Keech and Kutschke studied the upper age limit for development of amblyopia in humans and found that no subject developed amblyopia after age 6 years.7 Few cases of amblyopia will develop after screening, if vision screening is taken place in the late preschool period. If vision screening is performed early, a second screening session might be necessary in order to detect cases developing after the first vision screening.
Definitions and risk factors
Amblyopia has conventionally been described as reduced visual acuity despite optimal optical correction and without any signs of an organic cause. Clinically, amblyopia is often defined as a two-line difference or more in best-corrected visual acuity (VA) in LogMAR or Snellen lines between eyes in the presence of amblyopia risk factors, such as strabismus, anisometropia and/or visual axis obstruction. Currently amblyopia is defined as a form of cortical visual impairment which results from abnormal visual stimulation in the first few years of life. Most amblyopia arises as a result of ocular misalignment (strabismus), a difference in refractive errors between the two eyes (anisometropia), or a combination of both.8 Another cause of amblyopia is deprivation
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