REFRACTIVE ERRORS
Normal eye is Emmetropic, i.e. parallel rays of light are adequately refracted by ocular media, i.e. lens to focus exactly on retina, without accommodation efforts.
Ametropia denotes altered location of image formation due to refractive errors and includes: (a) hypermetropia, (b) myopia, (c) astigmatism, and (d) anisometropia.
Refractive errors may be tested in children—(a) Objectively by retinoscopy—focusing a beam of light on retina, through various lens powers in front of eye, and (b) Subjectively by asking an older child to read a Snellen's chart, first without lenses and than through various lens powers to identify the lens with clearest image.Retinoscopy is essential in children lt;3 years and preferable in older children. Refractive errors in a suspected case should be tested both, before and after instillation of cycloplegic agents, to exclude the effect of accommodation.
Hypermetropia is optically characterized by image formation behind the retina, due to: (a) decreased orbital size, (b) inadequate corneal refraction, e.g. in flattened cornea, (c) inadequate lenticular refraction due to reduced refractive power or posterior dislocation of lens.
Children are physiologically hypermetropic till 5-6 years of age due to smaller orbit (~3 diopters at birth). Unlike hypermetropic adults, hypermetropic children have difficulty to see both distant as well as near objects and tend to use excessive accommodation to increase lens thickness and decrease its focal length to ensure image formation at retina.
Consequently, eyestrain, headache, squinting while reading and eye rubbing is common. Severe cases may develop strabismus due to excessive use of accommodation and deprivation amblyopia due to cortical suppression of image from the affected eye.
Management aims to bring forward the visual image on retina without accommodative efforts, using convex lenses of suitable strength. As hypermetropia tends to
improve with age due to increasing orbital size, lenses of lesser strength are required gradually and sometimes, it is even possible in eliminate the needs for glasses.
Myopia (Near-sightedness) is optically characterized by image formation before the retina, due to: (a) larger orbital size, e.g. in glaucoma, (b) increased corneal refraction, e.g. in keratoconus, and (c) increased lenticular refraction due to anterior dislocation of lens. It is uncommon in early childhood (except preterms) and with increasing incidence and severity in school age. High myopia is usually hereditary with strong family history.
Earliest indicator of myopia is tendency to read by keeping the books close to eyes, frowning and squinting. Amblyopia is uncommon due to later onset of error.
Management of myopia involves correction of refractive error by concave lenses of suitable strength. Severe myopia tends to increase with age, sometimes complicated by retinal detachment.
Astigmatism is optically characterized by variable refractory powers in various meridians, leading to distortion of images. Most cases are caused by irregular curvature of cornea or lens.
While mild astigmatism is common and often asymptomatic, more severe cases use excessive accommodation to create a pin-hole effect for better image, leading to eyestrain, headache, frowning or squinting.
Correction of astigmatism requires use of cylindrical lenses, as early as possible to prevent amblyopia.
Anisometropia denotes significant differences in refractive states of two eyes, leading to disuse of more affected side and consequent amblyopia.
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