1. Introduction 1.1 Amblyopia 1.1.1 Definition Amblyopia or „lazy eye‟ has conventionally been defined as “a unilateral or bilateral decrease of visual acuity caused by deprivation of pattern vision or abnormal binocular interaction, for which no cause can be detected by physical examination of the eye and which in some cases can be reversed by therapeutic measures” (1). Clinically, amblyopia is defined as a reduction in best-corrected visual acuity to less than 6/9 monocularly in Snellen optotype or as a two-line difference or more in best-corrected visual acuity between the eyes in LogMAR optotype. This compares with findings in normal subjects, in which the interocular difference in best-corrected visual acuity has been found to be less than two lines (0.2 LogMAR optotype) in both infants and adults (2). However, clinical definitions are debated with different studies using different inclusion criteria for the amblyopic subjects participated. 1.1.2 Prevalence Amblyopia is a significant cause of unilateral visual deficit in childhood and is still considered as one of the most common causes of persistent unilateral visual impairment in adulthood, including populations in which advanced medical care is offered. The prevalence of amblyopia detected in children is estimated between 0.2-5.4% (3-30, 30-35) and in adults between 0.35-3.6% (36-41). It is also classified among the major causes of unilateral visual loss in visually impaired children (13, 42-47) and adults (48-60), in parallel with refractive error, retinal lesions, cataract, corneal opacities and age-related macular degeneration. However, prevalence estimates of amblyopia are affected by the criteria of visual loss used to define amblyopia, the socio-economic properties of the population, the efficacy of the applied screening programmes for amblyopia and amblyogenic risk factors and the effectiveness of the prescribed treatment regimens (1, 24, 61-72). 1.1.3 Aetiology Amblyopia is a form of cerebral visual impairment, in the absence of an organic cause (73-76). It is considered to derive from the degradation of the retinal image associated with abnormal visual experience during the developmental period of the visual system in infancy and early childhood (73-76). Children with anisometropia, strabismus or any other condition causing a reduction in the clarity of the image in one or both eyes, thereby disrupting equal binocular vision, are at risk of developing amblyopia (74-76). Amblyopia is therefore classified according to the type of pathology underlying the abnormal binocular interaction and/or form vision deprivation as (1, 77): (i) Anisometropic, in which a difference in the refractive error between the two eyes represents a risk for developing amblyopia due to creation of dissimilar images; (ii) Strabismic, in which the confusion and diplopia caused by the misalignment of the visual axes of the two eyes can lead to binocular rivalry and suppression of input from the deviating eye at the level of the visual cortex; (iii) Mixed, if anisometropic and strabismic amblyopia co-exist and, (iv) Stimulus deprivation, if there is some obstruction to vision during the sensitive period of visual development (opacities in the media e.g. cataract or severe ptosis). The results of the adult population study of Attebo et al (39) indicated that, the predominant cause of amblyopia was anisometropia in 50%, followed by strabismus in 19%, mixed in 27% and visual deprivation in 4%.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:551877 |
| Date | January 2009 |
| Creators | Kanonidou, Evgenia |
| Contributors | Gottlob, Irene |
| Publisher | University of Leicester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/2381/9935 |
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