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The Development and Maintenance of Emmetropia

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The Development and Maintenance of Emmetropia The development and NICHOLAS PHELPS BROWN, JANE F. KORETZ, ANTHONY J. BRON maintenance of emmetropia Abstract despite the changes in all eye dimensions during the period of growth and the continuing The human eye is programmed to achieve growth of the lens of the eye throughout life. emmetropia in youth and to maintain Studies have been made of the ocular emmetropia with advancing years. This is components of refraction and how these change despite the changes in all eye dimensions with age and with refractive state. during the period of growth and the Developmental biologists have identified and continuing growth of the lens throughout life. studied the physiological mechanism of The process of emmetropisation in the child's emmetropisation, by which the mainly eye is indicated by a shift from the Gaussian ametropic neonatal eye is directed to develop distribution of refractive errors around a emmetropia. Sufficient information is now to hypermetropic mean value at birth to the non­ hand to give an overall view of the process of Gaussian leptokurtosis around an emmetropic achieving and maintaining emmetropia. mean value in the adult. Emmetropisation is the result of both passive and active processes. The passive process is that of proportional Refraction and its components enlargement of the eye in the child. The proportional enlargement of the eye reduces Steiger, 1 whose monograph was published in the power of the dioptric system in proportion 1913, was the first to show that the emmetropic to the increasing axial length. The power of eye might be constructed with a combination of the cornea is reduced by lengthening of the various corneal curvatures and axial lengths. He radius of curvature. The power of the lens is also found that the distribution of refractive reduced by lengthening radii of curvature and power of the cornea in the population studied the effectivity of the lens is reduced by followed the uSj:lal Gaussian curve with a range deepening of the anterior chamber. Ametropia extending from 39 to 48 D. From this Steiger results when these changes are not calculated the range of axial lengths that would proportional. The active mechanism involves be needed to produce emmetropia as extending the feedback of image focus information from from 21.5 to 25.5 mm. Steiger's calculations the retina and consequent adjustment of the necessarily ignored the lens power, for which he axial length. Defective image formation lacked measurements. interferes with this feedback and ametropia Steiger considered that refractive errors were then results. Heredity determines the tendency variants arising from the free association of the to certain globe proportions and environment different components of refraction, from which plays a part in influencing the action of active it would be expected that the distribution of N.P. Brown emmetropisation. The maintenance of refractive errors would follow the normal A.J. Bron emmetropia in the adult in spite of continuing Gaussian curve. Clinical Cataract Research Unit lens growth with increasing lens thickness The Gaussian distribution of axial lengths Nuffield Laboratory of and increasing lens curvature, which is known Ophthalmology that Steiger had postulated was confirmed by as the lens paradox, is due to the refractive Oxford, UK the measurements made by Tron2 and index changes balancing the effect of the J.F. Koretz Stenstrom.3 Tron, Stenstrom and Sorsby4--6 were Center for Biophysics and increased curvature. These changes may be later to present data from the measurement of Department of Biology due to the differences between nucleus and the three main components of refraction - Rensselaer Polytechnic cortex or to gradient changes within the cortex. corneal power, lens power and axial length - Institute Troy, New York, USA that confirm the Gaussian distribution of the Key words Emmetropia, Emmetropisation, Eye measurements of these components. Nicholas Phelps Brown, MD, growth, Lens growth, Lens paradox, Refractive FRCS, FRCOphth � Since the work of Sorsby4--6 it has been 69 components Harley Street accepted that the distribution of refractive London W1 N 1 DE, UK errors in the adult Western population does not Fax: +44 (0)171 487 3901 The human eye is genetically programmed to follow the usual Gaussian curve found for other Received: 11 June 1998 achieve emmetropia in youth and to maintain human parameters such as height. Sorsby Accepted in revised form: emmetropia with advancing years. This is showed that the graph for the distribution of 25 November 1998 Eye (1999) 13, 83-92 © 1999 Royal College of Ophthalmologists 83 500 the axial length gives rise to a concept that has been called the 'inflatable globe' as discussed by Koretz et al. 11 As the globe becomes bigger, so the cornea becomes proportionately flatter. This would offer some automatic compensation for globe length in the healthy eye and 400 ametropia is then explained by a non-proportionality of the globe. These concepts can be upheld today, but do not explain the leptokurtic distribution of refractive errors around emmetropia. The excess of refractions near emmetropia (Fig. 1) was 300 considered to be possible by Sorsby because the multifactorial nature of the inheritance of the eye parameters could allow a number of combinations of ... , these parameters to produce emmetropia - a concept " \ 200 initiated by SteigerY The studies of monozygotic twins I by Sorsby and Leary,B Kimura14 and Minkovitz et al.,15 I , showed that there is a high correlation for refraction and , , for its components (corneal power, lens power and axial I length) in twins, which clearly indicates a hereditary I 100 I component to the refractive state. But since the time of I I Sorbsy, the existence of the excess refractions near I I I emmetropia has led to the intriguing idea that the growth I I of one or more of the components influencing refraction may be regulated to achieve emmetropia. The hereditary o basis for refraction may thus act via the control of the 10 emmetropisation process, which has been suggested by Norton and Siegwart.16 Fig. 1. The distribution of refractive errors. x-axis, refraction in Recent work by Koretz et alY (Table 1) supports the dioptres; y-axis, numbers of individuals. The dashed line represents a concepts of Sorsby and has given us a detailed insight normal Gaussian distribution. After Sorsby.4 into the relationship between the various eye parameters that may be assembled together to achieve emmetropia. refractive errors has a leptokurtic distribution, having an Koretz et al. found that the globe length correlates with excess of refractions near emmetropia (Fig. 1), although corneal refractive power in emmetropes, but only weakly he also confirmed that the distribution of other in ametropes. The anterior segment length correlates parameters, namely the power of the cornea, the depth of with the corneal refractive power in emmetropes, but not the anterior chamber, the power of the lens and the axial in ametropes. The anterior segment length correlates length of the eye, followed the common Gaussian with globe length in emmetropes, but again not in distribution. Emmetropia was shown to be due to a ametropes. These findings, as Koretz et al. have variety of combinations of corneal power, lens power commented, confirm Sorsby's concept of the ametropic and axial length. Sorsby confirmed the previously eye as one that has failed to achieve proportionality. suggested relationship between globe size and the power Conversely, the vitreous cavity length corresponds with of the cornea and of the lens in emmetropia. the globe length in ametropes, but only weakly in A problem with many of the early studies is in the emmetropes. This too is explained on the basis of lack of methodology by which the results were obtained, as critically evaluated by van Alphen7 in his monumental Table 1. Eye correlates in emmetropia and ametropia analysis of emmetropia and ametropia in 1961. While the Emmetropia Ametropia data for corneal refractive power and axial length have Refraction! globe length Correlated Correlated stood the test of time, anterior chamber depth, lens Refraction! anterior segment Correlated Not correlated thickness and lens curvature have been shown to be age­ length dependent. Refraction! corneal power Not correlated Not correlated The knowledge of ocular biometry in 1976 was Globe length! corneal power Strongly Weakly reviewed in the monograph by Delmarcelle et al.8 It had correlated correlated Globe length! vitreous cavity Weakly Correlated then been established by ultrasound measurement that length correlated the lens grew steadily in width through life and that this Anterior segment length! Not correlated Correlated was associated with the reduction in depth of the corneal power anterior chamber. Anterior segment lengthl Correlated Not correlated The inverse relationship between the power of the globe power Anterior segment length! Not correlated Correlated cornea and the axial length was confirmed in vitreous cavity length emmetropia by Francois and Goes9 and by Seiler et al.lO 11 The relationship between the power of the cornea and After Koretz et al. 84 proportionality of the globe in the ametrope. This lack of However, in more recent studies in which non­ proportionality is seen in the myope as a cycloplegic near retinoscopy has been used, the results disproportionately long vitreous cavity.17 show less hypermetropia. The study by Gwiazda et al.31 The power of the lens may have some relationship to indicates approximate emmetropia at birth, reaching a the axial length of the eye. The lens power is less in the hypermetropic maximum of about +0.75 D at the age of longer eye, and it had long been considered that the 5 years and thereafter declining to emmetropia by the power of the lens might be important in age of 12 years. Cycloplegic studies have shown that emmetropisation.18 Sorsby5 demonstrated a relationship hypermetropia declines steadily to a mean of about 1 D between lens power and globe length, which is by the age of 14 years,23,27,29 after which the change is confirmed by Garner et al.19 McBrien and Millodofo in slow.
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