The Nepal longitudinal study: predicting myopia from the rate of increase in vitreous chamber depth.

Biometric data on 897 Tibetan children living in Kathmandu, Nepal were collected over the period 1992 to 2000 with regular visits every 2 years. Measurements included cycloplegic autorefraction, A-scan ultrasonography, and video phakometry. Children who had not been studied at least once at age 12 years or older were not included in these analyses. The other subjects were divided into two groups; myopic if the refractive error was ever <-0.50 D, and emmetropic/hyperopic if the refractive error was never <-0.50 D, the nonmyopic group. Using all children who had been examined with four or five observations over time, the change of vitreous chamber depth with age by group was determined using a mixed-model regression method. The increase in vitreous length was 0.070 mm/year for the emmetropic group and 0.165 mm/year for the myopic group, with the differences apparent before the onset of myopia. An independent group of 59 children in whom there were two vitreous chamber depth measures before the age of 12 years and one measure taken after 12 years of age were used to assess the rate of increase in vitreous chamber depth as a predictor of myopia. Two other methods were examined using the independent group; the ratio of axial length to corneal radius of curvature and refractive error at age 10 years. Predictors based on rate of increase in vitreous chamber depth and axial length/corneal radius of curvature had sensitivities of 75% and 45%, respectively, and refractive error at age 10 years as a predictor for those who will not become myopic had a sensitivity of 88%.

Gradient refractive index of the crystalline lens of the Black Oreo Dory (Allocyttus Niger): comparison of magnetic resonance imaging (MRI) and laser ray-trace methods.

The gradient refractive index of the crystalline lens in the Black Oreo Dory (Allocyttus Niger) was determined using two methods; an optimisation program based on finite ray-tracing and the path of laser beams through the lens, and magnetic resonance imaging (MRI) and the linear relationship between refractive index and nuclear transverse relaxation rates. The methods showed good agreement in the cortical zone of the lens, but the lack of free water in the core of the lens made MRI measurement impossible in this region. The laser-optimisation method gave mean values of 1.368 and 1.543 for the surface and core refractive indices respectively, with a radial distribution for the gradient refractive index given by n(r)=1.543-0.121r2-0.033r4-0.021r6.

The spherical aberration of the crystalline lens of the human eye.

The in vivo spherical aberration of the lenses of 26 subjects was estimated from the measured total aberration of the eye and that predicted from the measured shape of the anterior corneal surface. Since it was only possible to estimate the aberration contribution from the posterior corneal surface, its value led to an uncertainty in the final aberration level of the lens. For all the subjects and for a wide range of possible aberration levels at the posterior corneal surface, the spherical aberration of the relaxed lens was found to be negative.

Prevalence of myopia in Sherpa and Tibetan children in Nepal.

BACKGROUND: Tibetan and Sherpa children living in Nepal share a common ancestry in Tibet and areas to the north of Nepal, but it is evident that these people experience widely contrasting educational and environmental conditions. The purpose of this study was to compare the prevalence of myopia in children with similar genetic backgrounds but who are exposed to contrasting environments. METHODS: Unaided vision and refractive error was measured in 555 Tibetan children in Kathmandu and 270 Sherpa children in the Solu Khumbu region of Nepal. RESULTS: There were marked differences in vision and the prevalence of myopia in the two groups. Ninety-two percent of the Sherpa children had Snellen vision of 20/22 (0.89) or better compared with 70% of the Tibetan children. The range of refractive errors was -6.50 to +7.00 D for the Tibetan children and -1.00 to +3.50 D for the Sherpa children. The Sherpa children had a prevalence of myopia of 2.9% compared with 21.7% for the Tibetan children. CONCLUSIONS: The prevalence of myopia in Sherpa children is low and their rural lifestyle appears to be relatively unstressed. Tibetan children have a higher prevalence of myopia and more rigorous schooling. We did not establish a causal relationship between myopia and the type of schooling, or the environment in general, but we did demonstrate that a simple, rural lifestyle is at least compatible with a virtual absence of myopia.

Radius of curvature of the posterior surface of the cornea.

The radius of curvature of the posterior surface of the cornea was measured in the vertical meridian for 120 children aged 6 to 17 years, using a Purkinje image method. The mean radius of curvature (+/-SD) was 6.42 +/- 0.31 mm, with a range from 5.62 to 7.22 mm. The ratio of anterior to posterior corneal radius was 1.210 +/- 0.045 or 0.827 +/- 0.029 when expressed as a reciprocal. There was a significant correlation between the anterior and posterior corneal radius (y = 0.798x + 0.228, r2 = 0.45). The study provides data that may be applied to a four-surface schematic eye.

Changes in equivalent and gradient refractive index of the crystalline lens with accommodation.

The ocular dimensions and refraction of the eye were measured for accommodation stimulus levels of 0.0, 1.5, 3.5, 5.5, and 8.0 D for 11 subjects aged 18 to 28 years, mean 21.2 +/- 2.62 years using keratometry, autorefraction, A-Scan ultrasonography, and video phakometry techniques. The subjects had refractive errors in the range + 0.50 to -4.25 D, mean -1.88 +/- 1.64 D. With the maximum level of accommodation the anterior chamber depth decreased by 0.23 +/- 0.09 mm, the lens thickness increased by 0.28 +/- 0.09 mm, and no significant differences were recorded in axial length or vitreous chamber depth. The radius of curvature of the anterior surface of the crystalline lens decreased from 11.54 +/- 1.27 to 6.59 +/- 0.97 mm and the posterior surface from -6.67 +/- 0.97 to -5.30 +/- 0.4 mm. We determined the equivalent refractive index to be 1.4277 +/- 0.0011, with no significant differences at different levels of accommodation. When the crystalline lens was modeled as a gradient refractive index (GRIN) structure with elliptical iso-indicial lines, the mean surface refractive index of the lens was 1.3859 +/- 0.0009 for an assumed central refractive index of 1.406. The power of the anterior surface of the lens increased from 4.38 +/- 0.49 to 7.59 +/- 0.34 D, the posterior surface increased from 7.67 +/- 1.28 to 9.32 +/- 0.64 D, and the GRIN power increased from 9.70 +/- 1.31 to 13.74 +/- 0.77 D for the maximum accommodative stimulus of 8.00 D. On the basis of the model used, a substantial part of the increase in power of the crystalline lens with accommodation resulted from the change in refractive index distribution within the lens.

Changes in ocular dimensions and refraction with accommodation.

The purpose of this study was to measure the changes in ocular dimensions with accommodation, with particular reference to the radius of curvature of the posterior surface of the crystalline lens. The increase in power of the eye with accommodation is considered to arise primarily from a decrease in the radius of curvature of the anterior surface of the lens, with the role of the posterior surface somewhat unclear. We measured the axial dimensions (A-Scan ultrasonography), corneal radius of curvature (keratometry), refractive error (auto-refractor) and radii of curvature of the lenticular surfaces (video phakometry) for 11 subjects, mean age 21.2 +/- 2.6 years, for five levels of ocular accommodation up to 8.00 D. At maximum accommodation the mean changes were a decrease in anterior chamber depth of 0.24 mm, an increase in lens thickness of 0.28 mm, a decrease in radius of curvature of the anterior surface of the lens of 4.95 mm and 1.34 mm for the posterior surface. The corresponding increase in power of the lenticular surfaces for an equivalent refractive index of 1.422 for the lens was 5.53 D and 3.10 D for the anterior and posterior surfaces respectively. No significant changes were recorded in axial length or vitreous chamber depth. We conclude that when crystalline lens power is calculated on the basis of an equivalent refractive index, changes in the posterior surface of the lens contribute around one third of the increase in lens power associated with 8.00 D of ocular accommodation.

Calculation of the radii of curvature of the crystalline lens surfaces.

A new computing scheme was developed for calculating the radius of curvature of the anterior and posterior surfaces of the crystalline lens from the measured heights of the Purkinje images. The scheme can be applied to objects at any distance from the corneal vertex, for both a stationary object mounted independently of the camera and for a mobile object attached to the camera where the distance of the object to the corneal vertex will change as the camera is refocused from image PI to PIII and PIV. The method can also be used if different objects are employed to form each Purkinje image. The scheme also avoids the need to collimate objects in order to employ the equivalent mirror theorem or to calibrate phakometers with known spherical surfaces where exact relationships are unknown.

Determination of the radius of curvature of the anterior lens surface from the Purkinje images.

A new method for calculating the radius of curvature of the anterior surface of the crystalline lens from the measured heights of the Purkinje image PI and PIII is given. Equations are developed for determination of the radius of curvature of the equivalent mirror for three configurations commonly used in phakometry. The method can be applied to targets at any distance from the corneal vertex, for both a stationary target mounted independently of the camera, and for a mobile source attached to the camera where the distance of the target to the corneal vertex will change as the camera is refocused from image PI to PIII. The situation where only one recording is made of images PI and PIII, where the camera is focused on image PI, and the height of the defocused image of PIII is measured is also considered. The errors in calculating the anterior lens radius with the equivalent mirror method if no allowance is made for an object at a finite distance is examined. The new method described is an alternative to collimating targets to overcome the errors in phakometry that occur with targets at finite distances.

Ocular dimensions and refraction in Tibetan children.

A cross-sectional study of 404 Tibetan children (212 males, 192 females) aged 6 to 16 years was conducted in the Bouda region of Kathmandu, Napal in April, 1992. Examination procedures included retinoscopy (1% cyclopentolate HCl), keratometry A-scan ultrasonography, and video ophthalmophakometry. The mean refractive error was +1.11 D (SD: 0.56 D) at age 6 years decreasing to +0.63 D (SD: 0.34 D) at age 16 years with a prevalence of myopia in this group of 3.9%. Most children examined had low refractive errors, with 95.5% having errors in the range -0.50 to +1.50 D. Crystalline lens power decreased by 2.59 D, with an associated increase in its anterior radius of curvature of 1.98 mm and 0.49 mm in its posterior radius of curvature over the age range studied. Vitreous chamber depth increased by 0.69 mm, but no significant changes were recorded in anterior chamber depth, lens thickness, or corneal curvature. We conclude that the balance between the decrease in crystalline lens power and the increase in vitreous length is the major factor in maintaining the tendency to emmetropia in these children.

Change with age of the refractive index gradient of the human ocular lens.

PURPOSE: To study age-related changes in the refractive index distribution of the human ocular lens. METHODS: Biometric data collected on 48 eyes in subjects ranging in age from 19 to 31 years and 48 eyes in subjects ranging in age from 49 to 61 years allowed estimation of a single parameter related to the refractive index distribution of the crystalline lens. The authors selected a gradient index model of the lens characterized by a fixed index at the lens center, a somewhat lower fixed index at the surface, and a continuum of index values between center and surface depending on a single parameter, beta. This parameter was evaluated for each of the two age groups. RESULTS: The distributions of the gradient index parameter beta for the two age groups were found to be statistically well separated. On average, the older group was found to have an index gradient that was flatter near the lens center and steeper near the surface, implying a lower refractive power of about 2 D for representative lens surface curvatures. CONCLUSIONS: It has been observed that surface curvatures and thicknesses of the ocular lens increase with age, whereas other ocular dimensions apparently do not change, implying a trend toward myopia. This trend has not been observed. The authors' results are consistent with and strongly in support of the hypothesis that subtle index changes in the aging lens compensate to a large extent for changes in surface curvatures.

Effects of exposure to simulated altitudes on visual fields, contrast sensitivity, and dazzle recovery.

Previous studies have shown that visual performance may be impaired at altitudes above 12,000 ft when supplementary oxygen is not used. What is not known is whether there may be subtle changes in visual function at altitudes to which airline cabins are usually pressurized. In this study, we measured spatial and temporal contrast sensitivity, central visual field thresholds, and macular photostress recovery time in 12 male subjects at sea level and at simulated altitudes of 7000 ft and 12,000 ft. Our results showed insignificant changes in spatial and temporal contrast sensitivity, central visual field thresholds and dazzle recovery time with respect to simulated altitudes.

Crystalline lens power in myopia.

Growth of the eye shows a coordinated pattern whereby the reduction in refractive power of the cornea and crystalline lens tends to reduce the myopia that would otherwise result from the normal increase in axial length. There is some controversy as to whether the reduction in crystalline lens power is influenced by the refractive state of the eye, i.e., an active role in the emmetropization process, or is simply related to the changing lens dimension occurring with growth. We measured ocular dimensions and determined the crystalline lens powers in 19 myopes and 19 emmetropic subjects matched for age, gender, and ethnic origin. No significant difference was found in corneal radius of curvature for the two groups, but there was a significant difference (p < 0.05) in crystalline lens power of 2.30 D. These results suggest that greater compensation for axial elongation of the eye was afforded by the decrease in crystalline lens power than by corneal flattening.

Comparison of three methods of evaluating glare.

A Humphrey Automatic Refractor Model 570 was used to measure the impairment of visual acuity for low contrast optotypes as a result of glare for normal subjects and for subjects with cataracts. This was compared with a direct measure of intraocular light scatter as measured by a compensating technique and with a subjective assessment of glare determined by a questionnaire. Only a weak correlation (r = 0.28) was found between the decrease in low contrast letter acuity and intraocular light scatter. The subjective assessment of glare sensitivity was not correlated with the decrease in low contrast letter acuity, but correlated to some extent (r = 0.55) with the measurement of intraocular light scatter.

Ocular dimensions and refractive power in Malay and Melanesian children.

A cross-sectional study of 753 Melanesian children in Vanuatu and 904 Malay children in Malaysia included measurement of refractive error and ocular dimensions. All children were between the ages of 6 and 17 years. The prevalence of myopia in Malay children was 4.3% at 7-8 years and 25.6% at 15-16 years with corresponding figures of 0.8% and 4.3% for Melanesian children. The range of refractive error was greater for Malay children at all ages. Mean refractive error for Malay children showed greater hypermetropia, together with a shorter axial length at 6 years, than Melanesian children, but at 17 years the situation reversed and Malay children had more myopia and longer axial lengths than their Melanesian counterparts.

Refraction and its components in Melanesian schoolchildren in Vanuatu.

Refraction and its components were measured on 788 Melanesian children and 39 children of other races, aged 6 through 19 years, in the South Pacific island nation of Vanuatu. Of the 788 Melanesian children, 766 (97.2%) were found to have uncorrected visual acuity of 6/6 or better, and 763 (96.8%) were found to have a spherical equivalent refraction between -0.25 and +1.00 D. Only 23 (2.9%) Melanesian children were found to have myopia greater than 0.25 D, only 2 (0.3%) were found to have hyperopia greater than 1.00 D, and only 2 (0.3%) were found to have refractive astigmatism greater than 1.00 D. Mean corneal refracting power was greater for girls than for boys, at all ages, but did not appear to change in any regular manner with age. Mean axial length and mean vitreous length were greater for boys than for girls, at all ages, each increasing approximately 1.0 mm from age 6 and 7 years to age 18 and 19 years. Although the low prevalence of ametropia may be considered to be due mainly to genetic factors, the possibility of environmental factors cannot be excluded.

The effect of a moderate level of hypoxia on human color vision.

This study reports the effect of a moderate level of hypoxia on human color discimination. We found a generalized loss of color vision affecting both red-green and blue-yellow discrimination at an altitude of 12,000 feet. Although the residual color discrimination at this altitude was within age-matched, sea-level norms, a statistically significant increase over sea level error scores was measured on the Farnsworth-Munsell 100-Hue test and the Pickford-Nicolson anomaloscope. An analysis of psychophysical and electrophysiological studies indicates that hypoxia acts by depressing retinal ganglion cell activity and that it can affect photopic visual processes as well as scotopic vision. We conclude that studies evaluating man's visual performance at altitude must consider post-receptoral processes.

Eye protection from airborne allergens.

The need for eye protection from hazards such as flying objects, particles, dust, noxious gases and chemicals is well established, and is part of normal safety procedures where these hazards exist. There have been recent reports that eye protection from airborne allergens may be an effective method for reducing the symptoms of allergic conjunctivitis and hay fever. This review examines the literature on these devices which appear to offer an alternative to conventional drug therapy.

Use of an automatic refraction device in a Third World country.

The vision welfare services in many Third World countries are compromised by the shortage of trained ophthalmic vision care workers. We investigated the feasibility of fulfilling part of the needs in this area by training local personnel in the use of automatic refraction devices. A nurse from the Vila Base Hospital in Vanuatu was given minimal training in the use of such an instrument (Nikon NR 2000), and given guidelines for the preparation of ophthalmic prescriptions. Sixty-nine subjects in the Village of Mélé in Vanuatu were seen by both the nurse and an optometrist, and the prescriptions issued by both workers were compared. The use of automatic refractors under those conditions holds promise for filling the needs in Third World countries, and suggestions are given for adapting these instruments for the particular conditions encountered during this study.