Hyperopia and Presbyopia - part 2 docx

19Basic Optics of Hyperopia and Presbyopia Figure 2 Graphic representation of the decline in accommodative amplitude with age (2). accommodate diminishes (Fig. 2); thus they lose their ability to see clearly at any distance, while older myopes still retain at least a portion of their ability to see clearly at some distance. C. ACCOMMODATION FOR NEAR VISION The closer an object is to the cornea, the greater the divergence of light entering the eye and the greater the need for more plus power to make the near object conjugate with the retina. In youth, accommodation allows viewing at a variety of distances from infinity to very near targets. As a person ages, however, the accommodative ability decreases, and the near point moves away from the eye. Because uncorrected hyperopes often use a portion of their accommodative ability to correct their refractive error for distance, the near point is located farther from the eye; therefore hyperopes often experience near vision problems at an earlier age than myopes or emmetropes. It should be noted that some myopes may not experience any near vision problems in the uncorrected state if their refractive error maintains a clear image within a comfortable working distance that is neither too close nor too far from their eyes. It is important to appreciate that there is a limited and diminishing amount of accom- modation available at any given age and that the amount available depends in part on whether accommodation is being used to correct for a hyperopic error. This amount of accommodation in play is specified by the amplitude of accommodation, which is defined as the vergence difference between the far point and the near point. The relationship between age and accommodative amplitude was established by Donders (1) and later refined by Duane (2), who presented what has since become the classic representation of accommodative amplitude as a function of age (Fig. 2). Duane’s data show that accommo- dation begins to decrease in early adulthood, well before the decline is noticed during the performance of near vision tasks, such as reading. For adolescents, accommodative amplitude is approximately 14 D, which corresponds to a near point of approximately 7 20 Smolek and Klyce cm for an emmetrope. By age 45, this accommodative amplitude drops, due to changes in the accommodative apparatus controlling the crystalline lens power, to about4Dand results in at best a 25-cm near point distance for that same emmetrope. Normal reading distance is considered to be around 15 in. or 37 cm, which is still within the range of a person in his or her mid- to late forties. However, it must be remembered that a continuous and excessive need to accommodate can be tiring and uncomfortable, so the decline in accommodative amplitude will be noticed by many subjects who are only in their mid- forties and who still have a fair amount of accommodative amplitude in reserve. If the eye has insufficient accommodative amplitude, which normally occurs with advancing age and requires a plus lens addition for comfortable near vision, the condition is called presbyopia. There are no specific values that define the absolute onset of presbyopia, because its effects are dependent on a number of factors including the refractive error, age, amplitude of accommodation, and the near vision tasks and lifestyle of a particular patient. Because using accommodation to correct for distance vision is often tiring in itself, the hyperope will be more likely to complain of tired eyes, eyestrain, and diplopia, and may do so at an earlier age. Children do not normally experience vision problems from mild amounts of hyperopia because their accommodative reserve is large. However, those with moderate to high levels of hyperopia may experience visual problems ranging from mild eyestrain and headaches after near work to more severe problems such as strabismus and amblyopia (3). Some of these complaints are associated specifi- cally with the ability of the two eyes to fuse images binocularly, because the accommoda- tive process is neurologically tied to the convergence of the eyes. There is a clinical distinction made between accommodative amplitude, which is the optical difference between the near and far point measured in diopters, and the range of accommodation, which is the linear difference between the far point and the near point in terms of physical distance. In the uncorrected myope, the far point may be located very close to the eye. The myope’s range of accommodation is thus very limited, whereas prepresbyopic low hyperopes may have a range that allows vision to infinity, just as in emmetropia (Fig. 3). D. MANIFEST VERSUS LATENT HYPEROPIA The refractive state of the eye is measured at rest with respect to the far point, but achieving a totally unaccommodated state can be problematic, especially in the uncorrected hyperope who uses accommodation to self-correct for distance vision. Consequently, refractions are separated into two basic types—manifest and latent refractions—which can give different refraction values for the same eye. A manifest refraction is the obvious, nonhidden part of the refraction that is based on the elimination of any natural stimulus to accommodate. Generally this is best accomplished by providing additional positive vergence of a known amount to the incoming light to the extent that the eye is made artificially myopic. The process is referred to as fogging. The far point thus moves to a finite distance in front of the eye, which in itself is beneficial with respect to interacting with and measuring the location of the far point. Of course, once the myopia-shifted far point is measured, the added vergence power is subtracted to provide the true far point location. While fogging a patient removes the manifest portion of the total accommodation that may be in play, it does not necessarily remove the latent or hidden portion of accommo- dation that may still exist. Latent accommodation is that part which cannot be relaxed due to excessive, spastic tonicity of the ciliary apparatus controlling accommodation. Self- 21Basic Optics of Hyperopia and Presbyopia A B C Figure 3 Example of the possible range of accommodation for three refractive states at three different ages. R is the far point and P is the near point. The dark line refers to the theoretical region in which unaided clear vision is possible. (A) In emmetropia, objects at optical infinity can be seen at any age. (B) In myopia, objects seen clearly are always located a finite distance in front of the eye, but objects at optical infinity cannot be seen clearly. (C) In hyperopia, objects at optical infinity can usually be seen clearly in youth and middle age; by the time late presbyopia occurs, however, no objects can be seen clearly at any distance unless the hyperopic error is corrected. 22 Smolek and Klyce correcting hyperopes tend to be prone to accommodative excess because they are constantly demanding additional plus power from the lens for both near and far tasks, and this effort builds up a constant level of spastic tonicity in the ciliary muscle. Therefore a cycloplegic drug is used to completely relax the spastic tonicity of the ciliary muscle, after which a refraction is performed to determine the full latent refractive error. Typically, the latent accommodation may account for approximately 1 D of total accommodation, so the differ- ence between manifest and latent refractions may be clinically significant. E. MAGNIFICATION AND VISUAL ACUITY A refractive error can be fully corrected and image blur eliminated, but the retinal image may be smaller or larger than it would be in the uncorrected state; therefore the ability to resolve details in the image may be harder or easier to accomplish. Suppose we have a hyperope with a ם5 D correction in a spectacle plane 1.2 cm from the cornea. The apparent image size will be reduced by 6% if the correction is moved to the corneal plane, as in the case of laser refractive surgery or contact lens wear (Table 1). If the spectacle correction is increased to ם10 D, the amount of minification for a corneal plane correction likewise doubles to 12%. The general rule of thumb is that spectacle magnification in percent equals the power of the spectacle lens in diopters multiplied by the distance between the spectacle plane and the cornea in centimeters. Because we are considering an image projected from the eye in order to assess the apparent visual angle change experienced by the subject, distances are considered positive when measured from the cornea to the spectacle plane and negative when moving from the spectacle plane back to the cornea. Thus, moving a correction from the cornea to a spectacle plane in the hyperope causes magnification of the retinal image, and the further the spectacle plane is from the eye, the greater the change in the magnification. However, when the correction is moved from the spectacle plane back to the cornea, the retinal image becomes physically smaller in the hyperope. Therefore, Snellen letters subtend a relatively smaller angle in the visual field and appear smaller to the patient and harder to distinguish. The opposite relationship holds true for the myope; moving the correction from the spectacle plane to the cornea causes Snellen letters to appear slightly larger to the myope corrected by refractive surgery or a contact lens. Applegate and Howland calculated the effects of magnification on Snellen visual acuity and, as expected, showed that the effective change in acuity was nonlinear and greater for myopes than for hyperopes (4). Whereas myopes had a positive effect of gaining more letters of visual acuity, hyperopes lost letters of acuity. For example, a ם5 Table 1 Magnification Effect of Moving a Correction from the Spectacle Plane to the Cornea Spectacle Spectacle plane Spectacle Loss of letters for Snellen power (D) distance (cm) magnification (%) distance visual acuity ϩ2 Ϫ1.2 Ϫ2.4 ϳ1 ϩ2 Ϫ1.5 Ϫ3.0 ϳ1 ϩ5 Ϫ1.2 Ϫ6.0 ϳ2 ϩ5 Ϫ1.5 Ϫ7.5 ϳ2 ϩ10 Ϫ1.2 Ϫ12.0 ϳ3 23Basic Optics of Hyperopia and Presbyopia D hyperope wearing glasses who has successful refractive surgery is expected to lose two to four letters of acuity as a result of moving the correction to the cornea, depending on the exact distance of the spectacle plane from the cornea (Table 1). F. HYPEROPIA AND BIOMETRIC CHANGES DURING LIFE Based on spherical equivalent data obtained during cycloplegic refractions, the average eye is hyperopic through most of life (Fig. 4). The average refraction is approximately ם2.25 D at birth and reaches a hyperopic peak around 8 years of age, after which the refraction becomes increasingly less hyperopic during adolescence and comes close to being emmetropic during early adulthood (5). In the Beaver Dam Eye Study of adults, hyperopia was more prevalent than myopia in age-matched subjects (49 vs 26.2%, respec- tively, p ס 0.0001) (6). Hyperopia increases in later adulthood from 22.1% between ages 43 and 54 to 68.5% at age 75 and above; however, Slataper noted that the refraction tends to drift back toward myopia with very advanced age (5). The hyperopic shift for older adults between the ages of 45 and 65 has been attributed to reductions in the axial length of the eye and changes in the focal power of the lens (7). The cause of the myopic drift in advanced age may be attributed to a shrinking radius of curvature of the cornea, which leads to a higher corneal power (8). This effect occurs predominantly in females (9). Passive growth of the eye during childhood tends to be a correlated, uniform expan- sion of ocular dimensions (7,10). By “correlated” we mean that as eye growth causes the retina to recede from the optical elements of the eye, we also see changes in the lens and cornea that ideally allow emmetropia to be achieved if the eye is hyperopic or retained if the eye is already emmetropic. Furthermore, it must be remembered that as axial length increases, there is a reduction in the vergence power required to focus an image on the Figure 4 Graph based on Slataper’s data (5) of average refractive error during life. Note that the error tends to be hyperopic throughout life and relatively stable from young adulthood to middle age. N ס 34,570 eyes assessed by cycloplegic refractions. 24 Smolek and Klyce retina. During childhood, corneal power decreases by about 2 D because the radius of curvature of the cornea increases as part of the expansive growth of the corneoscleral shell (11). In addition, the anterior chamber depth decreases, which reduces the effective power of the lens, and the lens itself decreases in power as the radius of curvature of the front and back surfaces increases by up to 1 mm (11). Sorsby noted lens power to be on average 23 D at age 3 and only 20 D at age 14 (12). The lens also thins from an average of 3.6 mm at age 6 to about 3.4 mm at age 10, after which thinning essentially halts (11). The overall lens thinning can be attributed to a compression of the nucleus, even though the cortex grows and thickens at this time. There appears to be an active growth mechanism that uses feedback from the blur of the retinal image to make corrective growth changes to the ocular component dimensions (7,10). A defect in an active growth feedback pathway might be responsible for a runaway increase in axial length, which is often seen with myopia; but the active growth mechanism does not adequately explain hyperopic error. Hyperopia seems more likely to be a failure of the passive growth mechanism, such that the eye retains slightly immature globe dimen- sions into adulthood. Hyperopic eyes tend to be smaller in all dimensions (not just in axial length) compared to corresponding age-matched emmetropic eyes. Using high-resolution magnetic resonance imaging to measure dimensions in the major axes of the eye, Cheng and coworkers found that, on average, the hyperopic eye is consistently smaller overall than the mean emmetropic eye and significantly smaller than the mean myopic eye (Fig. 5) (13). Strang et al. used biometric data from 53 human subjects with refractive errors of up to ם10 D and found that there was a strong correlation between the mean hyperopic Figure 5 Data based on the findings of Cheng et al. (13) of eye size relative to refractive error. Error bars indicate standard deviations. The general trend is that myopic eyes are larger and hyperopic eyes smaller than eyes with no refractive error. The differences in globe dimension between hyperopic and myopic eyes are significant. 25Basic Optics of Hyperopia and Presbyopia error and the axial length of the globe (r 2 ס 0.611, p ס 0.0001) (14). There was also a weak but significant correlation between mean corneal radius and mean refractive error (r 2 ס 0.128, p ס 0.009). Grosvenor also found that hyperopic eyes were smaller and tended to have flatter corneas than emmetropic eyes (15). G. OPTICS OF THE CRYSTALLINE LENS The lens has an average index of refraction that higher than the index of corneal stroma (1.427 vs. 1.376) (16). However, the contribution of the lens to the total power of the eye is about half that of the anterior corneal surface, because the lens is surrounded by fluid with an index near 1.336, whereas the cornea is exposed to air with an index of 1.0, which greatly increases its refractivity. While a single index of refraction of the lens is useful for simple calculations, in reality, the lens cannot be defined by a single value. Mapping the gradient index of the lens has proved difficult. Simple models using concentric shells of varying index gradients do not yield accurate ray-tracing results, and the models do not agree with refractive index measurements made by tissue probes (17). It is interesting to find that significant levels of transient hyperopia have been attributed entirely to changes in the refractive index of the lens. Saito and coworkers noted hyperopia peaking between 1 to 2 weeks after abrupt decreases in plasma glucose and attributed this effect to water influx into the lens (18). Okamoto et al. also noted hyperopia after treatment for hyperglyce- mia and found no changes in lens thickness or anterior chamber depth, thus implicating a change entirely due to the refractive index of the lens (19). Although the lens is the primary component associated with accommodation for near vision, the contribution of depth of focus of the eye should not be discounted, particularly in presbyopic eyes. Brighter viewing conditions or the use of miotics that constrict the pupil increase the depth of focus and help to extend the effective range of accommodation. H. OPTICAL ABERRATIONS The shape of the gradient index profile across the lens as well as shape changes due to accommodation alter not only effective power but also the spherical aberration of the eye (20). By accommodating to approximately3D(a33-cm viewing distance), the negative spherical aberration of the lens corrects for much of the positive spherical aberration induced by the cornea (21). Further accommodation tends to give the eye an overall negative spherical aberration, but the exact amount varies among individuals (22). In general, near accommodation tends to increase the monochromatic wavefront aberrations of the eye (23). Fourth-order aberrations can either increase or decrease with increasing accommodation, but higher-order aberrations tend to increase (22). It has been suggested that there is no correlation between the change in aberration during accommodation and the total amount of aberration for the relaxed eye (22). It can be concluded that any clarity of vision provided by refractive surgery must diminish by a measurable extent with accommodation, but certainly more work needs to be done to ascertain the significance of aberration change on visual performance. REFERENCES 1. Donders FC. On the Anomalies of Accommodation and Refraction of the Eye. London, 1864. 2. Duane A. Normal values of accommodation at all ages. JAMA 1912; 59:1010–1013. 26 Smolek and Klyce 3. Moore B, Lyons SA, Walline J. A clinical review of hyperopia in young children. The Hyper- opic Infants’ Study Group, THIS Group. J Am Optom Assoc 1999; 70:215–224. 4. Applegate RA, Howland HC. Magnification and visual acuity in refractive surgery. Arch Ophthalmol 1993; 111:1335–1342. 5. Slataper FJ. Age norms of refraction and vision. Arch Ophthalmol 1950; 43:466–481. 6. Wang Q, Klein BEK, Klein R, Moss SE. Refractive status in the Beaver Dam Eye Study. Invest Ophthalmol Vis Sci 1994; 35:4344–4347. 7. Brown NP, Koretz JF, Bron AJ. The development and maintenance of emmetropia. Eye 1999; 13:83–92. 8. Hayashi K, Hayashi H, Hayashi F. Topographic analysis of the changes in corneal shape due to aging. Cornea 1995; 14:527–532. 9. Goto T, Klyce SD, Zheng X, Maeda N, Kuroda T, Ide C. Gender and age related differences in corneal topography. Cornea 2001; 20:270–276. 10. van Alphen GWHM. On emmetropia and ametropia. Ophthalmologica Suppl 1961; 142:1–92. 11. Zadnik K, Mutti DO, Fusaro RE, Adams AJ. Longitudinal evidence of crystalline lens thinning in children. Invest Ophthalmol Vis Sci 1995; 36:182–187. 12. Sorsby A, Benjamin B, Sheridan M. Refraction and Its Components During the Growth of the Eye from the Age of Three. MRC special report series no. 301. London: Her Majesty’s Stationery Office, 1961. 13. Cheng H-M, Singh OS, Kwong KK, Xiong J, Woods BT, Brady TJ. Shape of the myopic eye as seen with high-resolution magnetic resonance imaging. Optom Vis Sci 1992; 69:698–701. 14. Strang NC, Schmid KL, Carney LG. Hyperopia is predominantly axial in nature. Curr Eye Res 1998; 17:380–383. 15. Grosvenor T. High axial length/corneal radius ratio as a risk factor in the development of myopia. Am J Opt Physiol Opt 1988; 65:689–696. 16. Mutti DO, Zadnik K, Adams AJ. The equivalent refractive index of the crystalline lens in childhood. Vis Res 1995; 35:1565–1573. 17. Pierscionek BK. Refractive index contours in the human lens. Exp Eye Res 1997; 64:887–893. 18. Saito Y, Ohmi G, Kinoshita S, Nakamura Y, Ogawa K, Harino S, Okada M. Transient hyperopia with lens swelling at initial therapy in diabetes. Br J Ophthalmol 1993; 77:145–148. 19. Okamoto F, Sone H, Nonoyama T, Hommura S. Refractive changes in diabetic patients during intensive glycaemic control. Br J Ophthalmol 2000; 84:1097–1102. 20. Smith G, Pierscionek BK, Atchison DA. The optical modelling of the human lens. Ophthalmic Physiol Opt 1991; 11:359–369. 21. Koomen MJ, Tousey R, Scolnik R. The spherical aberration of the eye. J Opt Soc Am 1949; 39:370–376. 22. He JC, Burns SA, Marcos S. Monochromatic aberrations in the accommodated human eye. Vis Res 2000; 40:41–48. 23. He JC, Marcos S, Webb RH, Burns SA. Measurement of the wave-front aberration of the eye by a fast psychophysical procedure. J Opt Soc Am A Opt Image Sci Vis 1998; 15:2449–2456. 3 The Helmholtz Mechanism of Accommodation ADRIAN GLASSER College of Optometry, University of Houston, Houston, Texas, U.S.A. “There is no other portion of physiological optics where one finds so many differing and contradictory ideas as concerns the accommodation of the eye, where only . . . in the most recent time have we actually made observations where previously everything was left to the play of hypotheses.” H. Von Helmholtz (1909) A. INTRODUCTION In 1853 Hermann von Helmholtz described the mechanism of accommodation of the human eye. This was not the first description of how the human eye accommodates. Many descriptions of and much research on accommodation preceded the work of Helmholtz (1), yet the accommodative mechanism of the human eye is still generally referred to as the “classic Helmholtz accommodative mechanism.” Helmholtz succeeded where others had failed at providing a comprehensive and consistent explanation of how accommodation occurs. It was comprehensive in that he described the functions of all of the major elements of the accommodative apparatus, and it was consistent in that it required no significant modifications of what was known with certainty at the time regarding how accommodation occurs. B. THE ANATOMY OF THE ACCOMMODATIVE APPARATUS In order to understand how accommodation occurs, it is necessary to have a clear under- standing of the accommodative apparatus and the relationships of the accommodative structures to each other. While in recent years there has been some limited debate over 27 28 Glasser Figure 1 Hermann Ludwig Ferdinand von Helmholtz (b, 1821; d, 1894) was not the first to describe the accommodative mechanism of the human eye, but he provided the first comprehensive and most accurate description based on the experiments he had performed and on the work done by many preceding him. Helmholtz succeeded where others had failed at providing a consistent and harmonious description of how accommodation occurs. Although the description that Helmholtz provided was largely accurate, subsequent experimental studies have shown that some aspects of the accommodative mechanism are not as Helmholtz described. For example, Helmholtz believed that the posterior surface of the lens did not move with accommodation and that the iris played an important role in mediating the accommodative changes in the lens. the gross anatomy of the accommodative apparatus, in general there is a consensus, and has been for some time (2). The primary accommodative structures of the eye consist of the ciliary body, the ciliary muscle, the posterior and anterior zonular fibers, the lens capsule, and the lens substance. C. THE CILIARY MUSCLE The ciliary muscle consists of three subgroups of muscle fiber cells differentiated by their positions and orientations within the ciliary body (3). The muscle fibers group are (1) the longitudinal fibers, sometimes referred to as meridional fibers or Bru ¨ cke’s muscle (4); (2) the radial or reticular fibers; and (3) the equatorial or circular fibers. The longitudinal fibers are located most peripherally in the eye, just inside the sclera at the ciliary region. Inward of the longitudinal fibers and closer to the vitreous are the radial fibers, and inside these are the circular fibers, located most anteriorly in the ciliary body and closest to the lens (5). The ciliary muscle is located within the ciliary body, bounded externally by the [...]... Function, and Pathology New York: Marcel Dekker, 1985:1–60 24 Schachar RA Cause and treatment of presbyopia with a method for increasing the amplitude of accommodation Ann Ophthalmol 19 92; 24 :445–4 52 25 Schachar RA Pathophysiology of accommodation and presbyopia: understanding the clinical implications J Fla Med Assoc 1994; 81 :26 8 27 1 26 Strenk SA, Semmlow JL, Strenk LM, Munoz P, Gronlund-Jacob J,... infrared retro-illumination video photography and pixel unit measurements Trans Am Ophthalmol Soc 1997; 95 :26 1 26 7 42 Glasser A, Kaufman PL The mechanism of accommodation in primates Ophthalmology 1999; 106:863–8 72 43 Schachar RA, Anderson D The mechanism of ciliary muscle function Ann Ophthalmol 1995; 27 : 126 –1 32 44 Malecaze FJ, Gazagne CS, Tarroux MD, Gorrand J Scleral expansion bands for presbyopia. .. lenses (22 ) Rafferty (23 ) cites this study when stating that the lens undergoes an increase in diameter of 0. 02 mm/ year Rafferty (23 ) is the single source cited by Schachar to support the notion that the lens grows in equatorial diameter (24 ,25 ,40,51–53) Smith (22 ) recognized that his measurements of the diameter of the isolated lens do not reflect the diameter of the lens in the living eye Smith (22 )... implications Arch Ophthalmol 1994; 26 :36–38 10 McCulloch C The zonule of Zinn: its origin, course, and insertion, and its relation to neighboring structures Trans Am Ophthalmol Soc 1954; 52: 525 –585 11 Glasser A, Campbell MCW Presbyopia and the optical changes in the human crystalline lens with age Vision Res 1998; 38 :20 9 22 9 The Helmholtz Mechanism of Accommodation 45 12 Paterson CA, Delamere NA The Lens... Munoz P, Gronlund-Jacob J, DeMarco KJ Age-related changes in human ciliary muscle and lens: a magnetic resonance imaging study Invest Ophthalmol Vis Sci 1999; 40:11 62 1169 27 Keeney AH, Hagman RE, Fratello CJ Dictionary of Ophthalmic Optics Boston: ButterworthHeinemann, 1995:4 28 Tscherning M Physiologic Optics, 4th ed Philadelphia: The Keystone Press, 1 924 :1 92 22 8 29 Cramer A Het accommodatievermogen... Vis Sci 20 01; 78:411–416 20 Dubbelman M, Van Der Heijde GL The shape of the aging human lens: curvature, equivalent refractive index and the lens paradox Vision Res 20 01; 41:1867–1877 21 Brown N The change in lens curvature with age Exp Eye Res 1974; 19:175–183 22 Smith P Diseases of the crystalline lens and capsule: on the growth of the crystalline lens Trans Ophthalmol Soc U K 1883; 3:79–1 02 23 Rafferty... accommodation (6) The patients ranged in age from 20 to 34 years with a mean age of 26 years and a standard deviation of 5 years The patients had a correctable visual acuity of 20 /20 and accommodative mean amplitude of 9.5 D One drop of 1% tropicamide was placed in the right eye The pupil and the near point without correction were measured 25 min later using four-point print Ultrasound biomicroscopy (UBM)... Based upon his theory of accommodation and presbyopia, Schachar has developed a number of surgical techniques to expand the sclera, using bands or segments (22 ), in an effort to increase the effective working distance between the ciliary muscle and the equator of the crystalline lens Others have attempted to expand the sclera by incisions or laser treatment (15), and these cumulative studies have shown... demonstrating Schachar’s hypothesis of accommodation Ann Ophthalmol 1994; 26 :4–9 39 Schachar RA, Cudmore DP, Black TD A revolutionary variable focus lens Ann Ophthalmol 1996; 28 :11–18 40 Schachar RA Theoretical basis for the scleral expansion band procedure for surgical reversal of presbyopia (SRP) Ann Ophthalmol 20 00; 32: 271 27 8 41 Wilson RS Does the lens diameter increase or decrease during accommodation?... video mixer and computer subtraction techniques Over 20 ,000 images of each of the 12 subjects were compared Separate and different images of the same patient in the unaccommodated and the accommodated states were superimposed The cornea and sclera were used as positional references, which provided a reliable method to avoid errors that accompany misalignment and rotation, since the cornea and sclera . magnification (%) distance visual acuity 2 Ϫ1 .2 2. 4 ϳ1 2 Ϫ1.5 Ϫ3.0 ϳ1 ϩ5 Ϫ1 .2 Ϫ6.0 2 ϩ5 Ϫ1.5 Ϫ7.5 2 ϩ10 Ϫ1 .2 Ϫ 12. 0 ϳ3 23 Basic Optics of Hyperopia and Presbyopia D hyperope wearing glasses who. adults, hyperopia was more prevalent than myopia in age-matched subjects (49 vs 26 .2% , respec- tively, p ס 0.0001) (6). Hyperopia increases in later adulthood from 22 .1% between ages 43 and 54. T, Ide C. Gender and age related differences in corneal topography. Cornea 20 01; 20 :27 0 27 6. 10. van Alphen GWHM. On emmetropia and ametropia. Ophthalmologica Suppl 1961; 1 42: 1– 92. 11. Zadnik K,