Age-related decreases in the prevalence of myopia: longitudinal change or cohort effect?

PURPOSE: The prevalence of myopia shows a decline with age in cross-sectional studies. This pattern may represent an increase in the prevalence of myopia in younger generations, possibly through increased exposure to near work, or an intrinsic age-related decline in myopia prevalence. Data were analyzed from published studies to determine which of these two alternatives better explains the data: a cohort effect of changing prevalence by decade or a longitudinal effect of changing prevalence as a function of age. METHODS: Prevalence data were taken from three studies conducted in the late 1980s and compared with those obtained indirectly from a national survey conducted in the early 1970s. The prevalence of myopia was then plotted as a function of age and year of birth. RESULTS: The pattern of change in the prevalence of myopia as a function of age was consistent across all studies when data were scaled relative to the prevalence of myopia at age-range midpoints from 44.5 to 49.5 years. The pattern of change was not consistent as a function of year of birth. When the data were scaled relative to the prevalence of myopia among those with years of birth from 1940 to 1942 and plotted by year of birth, results from the early 1970s were offset from those of later studies by approximately 18 years. CONCLUSIONS: The decline in the prevalence of myopia in older adults between the early 1970s and the late 1980s can be better explained by age than by year of birth. The prevalence of myopia appears to decrease because of an intrinsic age-related decrease in the amount of an individual's myopia rather than because of a cohort effect of increasing prevalence over time. The hypothesis that increasing environmental exposures to near work in recent decades have changed the prevalence of myopia is not supported by this analysis.

Naturally occurring vitreous chamber-based myopia in the Labrador retriever.

PURPOSE: To investigate whether myopia is present in a breed of domestic dog, the Labrador retriever, and how the ocular components are related to refractive error in this breed. METHODS: Cycloplegic refractive error was measured in 75 Labrador retrievers by retinoscopy. Corneal and crystalline lens radii of curvature were measured in the right eyes of 57 of these dogs using a video-based keratophakometer, with axial ocular dimensions measured using A-scan ultrasonography. RESULTS: Of the 75 dogs tested, 11 (14.7%) were myopic by at least -0.50 D in one eye, and 6 (8.0%) were myopic in both eyes (full range of refractive errors, +3.50 D to -5.00 D). Of the 57 dogs with ocular component measurements, seven (12.3%) were myopic by at least -0.50 D in the right eye. There was a significant negative correlation between refractive error and vitreous chamber depth (Spearman r = -0.42; P < 0.001). Myopic eyes had an elongated vitreous chamber depth (10.87+/-0.34 mm for myopic dogs, 10.02+/-0.40 mm for nonmyopic dogs; P < 0.0001, Kruskal-Wallis test). There was also a significant quadratic association between lens thickness and vitreous chamber depth (P < 0.005; R2 = 0. 11), indicating that thinner lenses occurred at both shorter and longer vitreous chamber depths. CONCLUSIONS: Myopia in the Labrador retriever is analogous to human myopia in that it is caused by an elongated vitreous chamber. Thinner crystalline lenses found at longer vitreous chamber depths may be analogous to lens thinning documented in human ocular development. The Labrador retriever warrants investigation as a potential model of myopia that is naturally occurring rather than experimentally induced.

The repeatability of measurement of the ocular components.

Studies of the ocular components of refraction typically neglect issues of repeatability of measurement methods or analyze method comparison/repeatability data inappropriately using correlation. The authors have examined the repeatability of refractive error measures (retinoscopy, subjective refraction, and Canon R-1 autorefraction, noncycloplegic and cycloplegic), axial dimension measures (Allergan-Humphrey A-scan ultrasound), and corneoscopy (keratometry and KERA photokeratoscopy), and the agreement between different refractive error and corneal measurement methods on 40 pre-presbyopic normal adults. The authors plotted the difference versus the mean of two different measurement occasions (repeatability), or two different methods (agreement), to determine the bias (mean of the differences relative to zero) and 95% limits of agreement of each technique. The most reliable measure of refractive error was autorefraction with cycloplegia, with 95% limits of agreement of +/- 0.32 diopters. Cycloplegic autorefraction had no statistically significant bias compared to cycloplegic subjective refraction. Cycloplegic retinoscopy was the least reliable refractive error measure, with repeat measures on two separate occasions extending over 95% limits of agreement of +/- 0.95 D. Anterior chamber depth was reliable to +/- 0.29 mm, lens thickness to +/- 0.20 mm, and vitreous chamber depth to +/- 0.37 mm. Corneal curvature measures show keratometry to be more reliable (to +/- 0.87 D) than photokeratoscopy (+/- 2.02 D) with a statistically significant bias (paired t-test, P less than 0.0001) of 0.57 D flatter for photokeratoscopy.

A video technique for phakometry of the human crystalline lens.

Accurate measurement of the dioptric power of the human crystalline lens is important in any study of ocular development and refractive error. Previous studies have used comparison ophthalmophakometry, measuring crystalline lens curvature and power from still flash photographs of Purkinje images I, III, and IV. This report presents a video-based technique for in vivo measurement of crystalline lens power suitable for use with children. The repeatability and validity of this video system were evaluated in comparison to a still photograph system through the measurement of anterior and posterior lens curvatures of 40 normal adults. The video system's repeatability (95% limits of agreement) was +/- 0.52 diopters for the anterior lens power, +/- 0.73 D for the posterior lens power, and +/- 0.88 D for the total lens power. The repeatability of the still flash photography system was +/- 0.78 D for the anterior, +/- 1.43 D for the posterior, and +/- 1.84 D for the lens power as a whole. An indirect method of calculating lens power using other ocular component measures gave a repeatability of +/- 1.78 D. The validity of the video system was improved by having the camera and the light source closer to the optic axis of the eye. The bias induced by having a more off-axis configuration (40 degrees separation between camera and light source) was a +0.10 D overestimation of surface power for the anterior and +0.73 D for the posterior lens surface power compared to a more coaxial arrangement (20 degrees between camera and light source). The use of video phakometry improved the repeatability and the validity of lens curvature measures relative to still flash photograph comparison ophthalmophakometry and to an indirect method of calculating lens power. This was achieved through the system's ability to analyze multiple frames, the use of a collimated light source, and the placement of the light source for the Purkinje images closer to the eye's optic axis.

Peripheral refraction and ocular shape in children.

PURPOSE: To evaluate the relation between ocular shape and refractive error in children. METHODS: Ocular shape was assessed by measuring relative peripheral refractive error (the difference between the spherical equivalent cycloplegic autorefraction 30 degrees in the nasal visual field and in primary gaze) for the right eye of 822 children aged 5 to 14 years participating in the Orinda Longitudinal Study of Myopia in 1995. Axial ocular dimensions were measured by A-scan ultrasonography, crystalline lens radii of curvature by videophakometry, and corneal power by videokeratography. RESULTS: Myopic children had greater relative hyperopia in the periphery (+0.80 +/- 1.29 D), indicating a prolate ocular shape (longer axial length than equatorial diameter), compared with relative peripheral myopia and an oblate shape (broader equatorial diameter than axial length) for emmetropes (-0.41 +/- 0.75 D) and hyperopes (-1.09 +/- 1.02 D). Relative peripheral hyperopia was associated with myopic ocular component characteristics: deeper anterior and vitreous chambers, flatter crystalline lenses that were smaller in volume, and steeper corneas. Lens thickness had a more complex association. Relative peripheral hyperopia was associated with thinner lenses between refractive error groups but changed in sign to become associated with thicker lenses when analyzed within each refractive error group. Receiver operator characteristics analysis of the ocular components indicated that vitreous chamber depth was the most important ocular component for characterizing the myopic eye, but that peripheral refraction made a significant independent contribution. CONCLUSIONS: The eyes of myopic children were both elongated and distorted into a prolate shape. Thinner crystalline lenses were associated with more hyperopic relative peripheral refractions across refractive error groups, but failure of the lens to thin may account for the association between thicker lenses and more hyperopic relative peripheral refractions within a given refractive group. Increased ciliary-choroidal tension is proposed as a potential cause of ocular distortion in myopic eyes.

Longitudinal evidence of crystalline lens thinning in children.

PURPOSE: Most earlier studies indicated that the eye's crystalline lens grows continually throughout life, but cross-sectional results of crystalline lens thinning during childhood have been reported. The authors investigated crystalline lens thickness in childhood using cross-sectional and longitudinal data. METHODS: The Orinda Longitudinal Study of Myopia is a community-based study of normal eye growth and myopia development in school-age children. During a 1-to 3-year period, A-scan ultrasonographic lens thickness measurements of 869 children 6 through 14 years of age were analyzed. RESULTS: On average, between the ages of 6 and 10 years, the crystalline lens thins in its axial dimension by almost 0.2 mm. This thinning can be depicted by a cubic model. In this sample, the children with myopia had thinner crystalline lenses than the children with emmetropia of the same age. CONCLUSIONS: This article provides the first longitudinal evidence that the crystalline lens thins during the period of coordinated ocular growth between the ages of 6 and 10 years. Further, it shows that lens thickness is associated with refractive error. Thinner crystalline lenses in children with myopia may result from one of two underlying mechanisms: Either the crystalline lens exhausts its ability to compensate for axial elongation after undergoing accelerated lens thinning before the onset of myopia, or the crystalline lens in the myopic eye may be thinner throughout childhood, during which it thins at a rate consistent with other refractive errors. If mechanical forces link eye growth to crystalline lens compensation, more complex, visually guided feedback loops may not be needed to explain the normal eye growth that results in emmetropization.

AC/A ratio, age, and refractive error in children.

PURPOSE: To examine how the response AC/A ratio (the amount of accommodative convergence per unit of accommodative response) varies as a function of refractive error and age, to determine whether it is a risk factor for the onset of myopia, and to examine the relation between ocular structural features and the AC/A ratio. METHODS: Accommodation was stimulated by a letter target presented in a Badal system at 0.00, 2.25, and 4.37 D to 828 children aged 6 through 14 years in 1996. Of these, 726 had no myopia in 1996 and were available for examination the following year. Accommodative response and cycloplegic refractive error were measured by autorefraction and convergence by monitoring the relative movement of Purkinje images I and IV. Lens radii of curvature were measured by video phakometry, corneal radius of curvature by topography, and ocular axial dimensions by A-scan ultrasonography. RESULTS: Adjusted for age, the response AC/A ratio was highest in myopes (6.39 delta/D), intermediate in emmetropes (3.94 delta/D), and lowest in hyperopes (3.40 delta/D; P < 0.0001; two-way analysis of variance [ANOVA]). The stimulus AC/A ratio did not vary with refractive error. Adjusted for refractive error, the response AC/A ratio did not change as a function of age. In non-myopic children, having a response AC/A ratio of 5.84 delta/D or more elevated the risk of development of myopia within 1 year by 22.5 times (95% CI = 7.12-71.1). In a subsample of children without myopia who had refractive errors less than +0.75 D, having a response AC/A ratio of 5.84 delta/D or more elevated the risk of development of myopia within 1 year by 3.21 times (95% CI = 1.14-9.07). The AC/A ratio was associated with all measured ocular features except lens spherical volume. Only the negative correlations with refractive error and the shape of the crystalline lens (Gullstrand lens power) were significant in a multiple regression model (adjusted R2 = 0.16). CONCLUSIONS: An elevated response AC/A ratio was associated with myopia and was an important risk factor for its rapid onset. The association between higher AC/A ratios and flatter crystalline lens shapes, as well as other reported features of accommodation in myopia, may be explained by "pseudocycloplegia," which the authors define as tension on the crystalline lens that increases the level of effort needed to accommodate. Accommodative deficits in myopia may be the functional consequences of the underlying anatomy of the enlarged eye.

The effect of cycloplegia on measurement of the ocular components.

PURPOSE: The purpose of this study was to examine the effect of cycloplegic agent on the measurement of refractive error and the ocular components. METHODS: We compared two commonly used topical cycloplegic agents, 1% tropicamide and 1% cyclopentolate, for their effect on the measurement of refractive error (by Canon R-1 autorefraction), accommodative response (by Canon R-1 autorefraction and by the conventional, subjective "pushup" method), crystalline lens power (by video phakometry and by calculation), and axial ocular dimensions (by A-scan ultrasonography) in 20 emmetropic to moderately hyperopic children. RESULTS: Comparison of refractive error at each drug's reported time of maximum cycloplegia (30 minutes for tropicamide and 60 minutes for cyclopentolate) showed that distance autorefraction in the vertical meridian differed by +0.20 +/- 0.30 diopters (D) (P = 0.008). The average difference was +0.07 +/- 0.10 mm for anterior chamber depth (P = 0.004), -0.03 +/- 0.05 mm for crystalline lens thickness (P = 0.025), -0.65 +/- 0.69 D for phakometrically measured crystalline lens power (P < 0.001), +0.03 +/- 1.55 D for calculated crystalline lens power (P = 0.94), and -0.09 +/- 0.19 mm for vitreous chamber depth (P = 0.062, all paired t tests; positive signs denote greater values with cyclopentolate). Residual accommodation was 0.47 and 0.67 D greater with tropicamide when measured by autorefraction and the pushup method (P = 0.013 and 0.08 respectively, paired t test). All significant differences were consistently in the direction of poorer cycloplegia with tropicamide. CONCLUSIONS: Although tropicamide, as expected, showed poorer cycloplegia compared to cyclopentolate, the degree of difference appeared to be small, with minimal effect on the measurement of distance refractive error and the ocular optical components.

Ocular predictors of the onset of juvenile myopia.

PURPOSE: The purpose of this study was to identify reliable predictors of the onset of juvenile myopia. METHODS: The data from 554 children enrolled in the Orinda Longitudinal Study of Myopia (OLSM) as nonmyopes with baseline data from the third grade were evaluated to develop a predictive profile for later onset of juvenile myopia. Myopia was defined as at least -0.75 D of myopia in the vertical and horizontal meridians of the right eye as measured by cycloplegic autorefraction (n = 45 children). Chosen predictors were refractive error and the ocular components: corneal power, Gullstrand crystalline lens power, and axial length. Sensitivity and specificity were calculated. Receiver operating characteristic (ROC) curves were generated to evaluate and compare these predictors singly and combined. RESULTS: Refractive error, axial length, Gullstrand lens and pod corneal power were all significant predictive factors for the onset of juvenile myopia. The best single predictor of future myopia onset in the right eye was the right eye's cycloplegic autorefraction spherical refractive error value (mean sphere across 10 readings) at baseline. For a cut point of less than +0.75 D hyperopia in the third grade, sensitivity was 86.7% and specificity was 73.3%. The area under the ROC curve for this mean sphere was 0.880. Producing a logistic model combining mean sphere, corneal power, Gullstrand lens power, and axial length results in a slight improvement in predictive ability (area under the ROC curve = 0.893). CONCLUSIONS: Onset of juvenile myopia can be predicted with moderate accuracy using the mean cycloplegic, spherical refractive error in the third grade. Measurement of other ocular components at this age improves predictive ability, albeit incrementally. Further improvements in the prediction of myopia onset will require the use of longitudinal data in addition to one-time measurement of refractive error and the ocular components.

Optical and structural development of the crystalline lens in childhood.

PURPOSE: To document the development of key optical and structural parameters of the crystalline lens throughout childhood and examine possible mechanisms by which lens power remains coordinated with the growth of the eye to maintain emmetropia. METHODS: Using cycloplegic autorefraction, video-based phakometry, and ultrasonography, the authors measured refractive error and crystalline lens parameters in 994 children in the first through eighth grades, who participated in the Orinda Longitudinal Study of Myopia, between one and five times from 1989 through 1993. Polynomial growth curves were fit to the data by maximum likelihood estimation. The average annual rates of change in each parameter from each subject's longitudinal data were also estimated. RESULTS: The lens radii of curvature flattened throughout childhood, yet decreases in lens equivalent power stopped after 10 years of age. This indicates that the refractive index of the lens increased during later childhood. Lens thinning in early childhood also ceased after 10 years of age. The spherical volume of the lens showed no appreciable net increase, but the axial length of the eye continued to grow throughout childhood. The prevalence of myopia in our data increased sharply at age 10 years, reaching 21.3% by the age of 14 years. CONCLUSIONS: Concurrent thinning and flattening of the crystalline lens imply that the lens is mechanically stretched by the equatorial growth of the eye during childhood. Changes in the patterns of lens development near the age of 10 years, concurrent with the onset of myopia, suggest that forces arise which interfere with equatorial growth. Such forces might diminish the decreases in lens power and amplify axial elongation to promote myopia.

Tonic accommodation, age, and refractive error in children.

PURPOSE: An association between tonic accommodation, the resting accommodative position of the eye in the absence of a visually compelling stimulus, and refractive error has been reported in adults and children. In general, myopes have the lowest (or least myopic) levels of tonic accommodation. The purpose in assessing tonic accommodation was to evaluate it as a predictor of onset of myopia. METHODS: Tonic accommodation was measured in children enrolled in the Orinda Longitudinal Study of Myopia using an infrared autorefractor (model R-1; Canon, Lake Success, NY) while children viewed an empty lit field or a dark field with a fixation spot projected in Maxwellian view. Children aged 6 to 15 years were measured from 1991 through 1994 (n = 714, 766, 771, and 790 during the 4 years, successively). Autorefraction provided refractive error and tonic accommodation data, and videophakometry measured crystalline lens curvatures. RESULTS: Comparison of the two methods for measuring tonic accommodation shows a significant effect of age across all years of testing, with the lit empty-field test condition yielding higher levels of tonic accommodation compared with the dark-field test condition in children aged 6 through 11 years. For data collected in 1994, mean (+/-SD) tonic accommodation values for the lit empty-field condition were significantly lower in myopes, intermediate in emmetropes, and highest in hyperopes (1.02 +/- 1.18 D, 1.92 +/- 1.59 D, and 2.25 +/- 1.78 D, respectively; Kruskal-Wallis test, P < 0.001; between-group testing shows each group is different from the other two). Age, refractive error, and Gullstrand lens power were significant terms in a multiple regression model of tonic accommodation (R2 = 0.18 for 1994 data). Lower levels of tonic accommodation for children entering the study in the first or third grades were not associated with an increased risk of the onset of myopia, whether measured in the lit empty-field test condition (relative risk = 0.90; 95% confidence interval = 0.75, 1.08), or the dark-field test condition (relative risk = 0.83; 95% confidence interval = 0.60, 1.14). CONCLUSIONS: This is the first study to document an association between age and tonic accommodation. The known association between tonic accommodation and refractive error was confirmed and it was shown that an ocular component, Gullstrand lens power, also contributed to the tonic accommodation level. There does not seem to be an increased risk of onset of juvenile myopia associated with tonic accommodation.

Repeatability of corneal topography: the "corneal field".

BACKGROUND: The purpose of this study was to determine the interoccasion repeatability of keratometry, photokeratography, and videokeratography and to describe the "corneal field," a scheme for explaining videokeratography results. METHODS: A single examiner obtained corneal curvature measurements with a keratometer, a photokeratoscope, and the TMS-1 in 29 adult patients on two occasions. RESULTS: Estimates for the repeatability of keratometry were +/- 0.49 and +/- 0.65 diopters (D) for the horizontal and vertical meridians respectively. The repeatability of photokeratography was +/- 0.90 and +/- 1.21 D. We presented a rational method for presenting numeric videokeratographic data by temporally and spatially averaging corneal curvature values and grouping them into 24 regions. The repeatability of videokeratography with the TMS-1 was +/- 0.50 D centrally, +/- 0.65 D paracentrally, and +/- 0.80 to +/- 1.00 D in the midperiphery. CONCLUSIONS: Repeatability of the photokeratoscope for central measurements is considerably worse than the keratometer. The repeatability of videokeratography is worse toward the periphery. Refractive surgeons and contact lens investigators need to be aware of these limitations so that true change can be distinguished from measurement error.

Repeatability and validity of astigmatism measurements.

BACKGROUND: As more patients inquire about refractive surgical procedures, the measurement of astigmatism prior to surgery becomes more important in assessing refractive outcome. Knowledge of the repeatability of the astigmatism measurement allows one to distinguish a true change in cylinder power from measurement error. METHODS: Forty adults with structurally normal eyes and refractive errors were evaluated for the repeatability of astigmatic refractive error measures. Noncycloplegic and cycloplegic measurements of refractive astigmatism were made by retinoscopy, subjective refraction, and autorefraction. All measures were made at 2 visits within 2 weeks by the same examiner. Difference versus mean plots and the 95% limits of agreement of each technique determined the repeatability of a measurement and the agreement between the methods of measurement. RESULTS: The most reliable measure of astigmatic refractive error was cycloplegic autorefraction, with 95% limits of agreement of +/- 0.28 D, followed by noncycloplegic autorefraction (+/- 0.35 D) and cycloplegic subjective refraction (+/- 0.44 D). Noncycloplegic retinoscopy was the least reliable astigmatic refractive error measure, with interoccasion 95% limits of agreement of +/- 1.02 D. The most repeatable measurement of cylinder axis was cycloplegic autorefraction; none of the measurements differed by 10 degrees or more. The least repeatable measurement was noncycloplegic retinoscopy; 40% of the measurements differed by 10 degrees or more. CONCLUSION: For studies seeking to measure changes in astigmatism in normal eyes, cycloplegic autorefraction is the method of choice.

Quantifying corneal toricity from videokeratography with Fourier analysis.

BACKGROUND: Fourier analysis can be used to quantify corneal toricity from videokeratography data. This study measures the repeatability of Fourier-derived toricity values in normal adult corneas. METHODS: The Topographic Modeling System (TMS) was used to model the corneas of 29 subjects on two occasions, and Fourier analysis applied to the data. Repeatability of Fourier-derived toricity values between sessions was examined, and 95% limits of agreement were established. RESULTS: The 95% limits of agreement between sessions for the Fourier method were -0.02 +/- 0.16 diopters (D) and -0.5 +/- 5.7 degrees for toricity amount and axis, respectively. CONCLUSIONS: The Fourier method yields highly repeatable toricity values and thus provides a sensitive means of detecting longitudinal toricity change in normal corneas.

How applicable are animal myopia models to human juvenile onset myopia?

Investigations into the plasticity of eye growth and refractive error development have significantly expanded our knowledge of animal models of myopia in the last 15 yr. The applicability of this information is as yet undetermined, but hopefully this information will be useful in learning more about human myopia. This paper presents a critical review of the animal myopia literature as those data relate to the human condition. Differences between the chicken, tree shrew, and primate animal models of myopia are outlined, and the various experimental paradigms used to investigate refractive error development and ocular growth in the chicken are compared. Specific arguments against the application of animal models of myopia to the etiology of human juvenile onset myopia include the following: (1) there is no deprivation of form vision in the environment of the school-aged child as severe as that required to induce myopia in animals; (2) the sensitive period for deprivation myopia in animals appears to be too early to account for human juvenile onset myopia; and (3) studies in the chicken using spectacle lenses to create dioptric blur involve a choroidal thickness modulation that has no human analog. Ultimately, the results of investigations into the cellular and biochemical modulation of eye growth in animals may be the most relevant to human myopia.

The utility of three predictors of childhood myopia: a Bayesian analysis.

Any treatment to prevent the onset of juvenile myopia will require predictive tests in order to determine which children should receive treatment. Three risk factors for myopia were evaluated for their ability to predict myopia: (a) refraction at school entry; (b) refraction in infancy; and (c) parental history of myopia. Bayes' theorem was used to estimate these conditional probabilities. Refraction at school entry had twice the power to predict myopia (probability of juvenile myopia given the child is near emmetropia at school entry = 0.53) compared to either infant refraction (0.21-0.28) or parental myopia (0.20-0.25). While a history of any parent having myopia had the highest test sensitivity (probability of a positive family history of myopia given juvenile myopia in the child = 0.90) and refraction at school entry the highest test specificity (probability of more hyperopia than +0.50 D at school entry given no juvenile myopia = 0.91), none of these three factors had high values for both sensitivity and specificity. Further work is required to develop a battery of tests which could predict the onset of juvenile myopia with both adequate sensitivity and specificity.

The equivalent refractive index of the crystalline lens in childhood.

Despite the importance of crystalline lens power in ocular development, schematic refractive index values used to calculate lens power have been validated for children. We measured refractive error and ocular component dimensions in 519 schoolchildren, calculating lens power using phakometrically measured lens radii and three different refractive index profiles: (1) Gullstrand-Emsley schematic indices [Gullstrand-Emsley lens power (GELP)]; (2) a 10-shell gradient index model [gradient index lens power (GILP)]; and (3) the equivalent refractive index (IND) needed to bring calculated and measured refractive error into agreement [calculated lens power (CLP)]. GELP was significantly lower than either GILP or CLP, indicating the Gullstrand-Emsley refractive index of 1.416 is too low for use in children. Variation in IND cannot be explained by measurement error alone. GILP and CLP also differed as a function of lens shape, with GILP greater than CLP at steeper external curvatures and less than CLP at flatter external curvatures. Variation in equatorial gradient index profile as a function of lens shape is proposed as an explanation for this bias. Equivalent index appears to be a useful tool for encompassing individual variation in lens gradient profiles as well as for assessing the relative role of lens surface curvature and refractive index changes during lens power development in childhood.

Retinoscopic measurement of the refractive state of the rat.

Using retinoscopy, we measured the refractive state of 96 eyes of three different strains of rats: albino Sprague-Dawley, Royal College of Surgeons (RCS) with and without inherited retinal dystrophy, and lean and obese varieties of Zucker rats. Contrary to previous reports, we do not find consistent high hyperopia in the rat, but rather refractions that range from near emmetropia (-0.12 D) to extreme hyperopia (+18.95 D). This range of refractive errors suggests a poorly developed emmetropization mechanism in the rat, and that individual refractions should be performed on animals utilized in experiments where refractive state is critical.

Factors influencing graft clarity.

Successful penetrating keratoplasty depends on both host- and donor-related factors. We compared the results of keratoplasty in a group with a wide range of donor ages to gauge the effect of donor age on graft success. We conducted a retrospective review of donor data, recipient age, preoperative diagnosis, and postoperative complications with respect to graft clarity at 24 months after surgery in 99 consecutive patients in an effort to determine the role of donor age in graft clarity. The only factors that we isolated that appeared to influence graft clarity at 24 months postoperatively were a preoperative diagnosis classified as inflammatory/traumatic and the occurrence of postoperative complications. There was no association between graft clarity and recipient age, donor age, death-to-preservation time, or preservation-to-surgery time.

Effect of optical defocus on visual acuity in dogs.

OBJECTIVE: To determine the effect of optical defocus (such as what develops in spontaneous myopia and subsequent to cataract extraction) on visual acuity in dogs. ANIMALS: 3 young adult male Beagles. PROCEDURE: The effect of optical defocus on visual acuity was determined by sweep visual evoked potential, using a within-subjects/repeated measures design in which each dog served as its own control. Dogs were positioned so that the eye being tested was 60 cm in front of the video display, and the target was centered on the area centralis. To create ametropia relative to the video screen, a series of concave and convex spherical lenses were placed 1 cm in front of the eye, and sweep visual evoked potential acuities were obtained. RESULTS: Maximal acuity was 7.0 to 9.5 cycles/degree. Defocusing by 2.0 diopters reduces Beagle grating acuity approximately 1 octave. Mimicking aphakia resulted in a marked depression of acuity to 0.7 cycles/degree or less. CONCLUSIONS: Even mild degrees of ametropia have appreciable impact on the resolving power of the canine visual system. CLINICAL RELEVANCE: Spontaneous myopia is encountered in dogs and may be associated with impaired visual performance attributable to a reduction in visual acuity. Previous reports indicate the possibility of myopia in dogs to have a heritable component. On the basis of our results, refractive correction of aphakia is advisable, and refractive screening of dogs with demanding visual tasks (eg, service dogs, field-trial Labrador Retrievers) is recommended.