Search

Purpose. To investigate changes in corneal thickness and the radius of curvature of the posterior corneal surface after orthokeratology (OK) rigid lens wear. Methods. Nineteen young myopic subjects wore reverse-geometry OK lenses (BE/ABE, Ultravision Contact Lenses, Brisbane, Australia) every night for 1 month. Central and midperipheral corneal thickness (Allergan Humphrey ultrasound, Carl Zeiss Meditec, Dublin, CA), topography (EyeSys v.3.1, Houston, TX), subjective refraction, and posterior corneal radii (video photography of Purkinje images) were evaluated withi n2ho f waking, prelens wear, and on four occasions postlens wear during a 1-month period. A mixed-models approach was used to analyze the data. We modeled the changes in posterior corneal radius of curvature and corneal thickness in terms of the sagittal height of the anterior and posterior cornea using an ellipsoidal model for the corneal surfaces. Results. Refractive error reduced from 2.28 to 0.01 DS within 1 month. A significant thinning of the cornea was evident between 1 (p 0.03) and 2 weeks (p 0.0048) postlens wear. A significant increase in the anterior corneal radius of curvature was present at all time periods beyond 1 night (p < 0.001), and a significant posterior corneal flattening occurred centrally and midperipherally after 1 week (p 0.04 and p 0.013, respectively). Conclusions. These findings suggest that in addition to the significant topographic flattening of the anterior corneal surfaces, there is also a significant flattening of the posterior surface during the early adaptive stages of OK lens wear. (Optom Vis Sci 2004;81:421-426)

The Stiles-Crawford effect of the first kind (SCE-I) was measured on both emmetropic and myopic subjects at six different retinal locations. The results revealed a number of significant discrepancies in receptor alignment between the groups of different refractive errors. In myopic subjects, the receptors in the nasal retina (i.e. between the fovea and the optic nerve head) were found to be aligned nasally towards the optic nerve head, whereas the receptors in the temporal retina were aligned towards the centre of the exit pupil. In emmetropic subjects, the receptors across the retina were finely tuned towards the centre of the exit pupil. The magnitude of the receptor displacement in myopic subjects was found to be directly associated with the length of the eyeball.

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.

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.

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.

A glare score based on intra-ocular light scatter and a questionnaire-based subjective assessment of glare were determined for 18 patients who had undergone radial keratotomy (RK) two weeks to 27 months previously. Results were compared to those of 15 control subjects. T-tests showed no significant difference in glare experienced under photopic conditions by RK patients and control subjects. There was no systematic relationship between glare scores and the subjective assessment of glare, nor between glare scores or subjective assessment of glare and the time which had elapsed since surgery. However, a weak relationship (r2=0.36) was found between glare scores and absolute refractive error. It was concluded that RK did not produce significant levels of intra-ocular light scatter under photopic conditions (155 lux).

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.

PURPOSE. To identify differences in potential biometric markers for predicting refractive error in school children. METHODS. Biometric data on 895 Tibetan children, aged 6 to 18 years, residing in Katmandu, Nepal, were collected biennially from 1992 to 2000. Measurements included cycloplegic autorefraction, A-scan ultrasonography, and video phacometry. Only those children who had been studied at least once at age 12 years or more were included in the analysis. Subjects were divided into two groups: a myopia group if the refractive error was myopic by more than -0.50 D and a nonmyopia group if the refractive error was maximally myopic by -0.50 D, expressed as a spherical equivalent error in the left eye. RESULTS. Biometric measures that differed significantly with increasing age between the two refractive groups included: anterior chamber depth + 0.012 mm/year (p = 0.014), anterior lens radius of curvature + 0.073 mm/year (p = 0.001), lens power -0.059 D/year (p = 0.082), lens thickness -0.005 mm/year (p = 0.02), and vitreous chamber depth + 0.084 mm/year (p < 0.001). Corneal radii of curvature of the myopic group were steeper at all ages by 0.09 mm (p < 0.001), but the rate of change with age was equivalent across the refractive groups. CONCLUSIONS. Compared with those who remained nonmyopic, children who developed myopia had a crystalline lens that was initially thicker and steeper, and a vitreous chamber that was initially shorter. With age, children who became myopic developed greater lens thinning, greater flattening of the anterior lens surface radius, and a greater increase in vitreous chamber depth than their nonmyopic counterparts. Copyright © 2006 American Academy of Optometry.

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.

We present an iterative tomographic algorithm to reconstruct refractive-index profiles for meridional planes of the lens of the spherical fish eye from measurements of deflection angles of refracted rays. Numerical simulations show that the algorithm allows accuracy up to the fourth decimal place, provided that the refractive index can be regarded as an analytical function of the radial coordinate and the experimental errors are neglected. An experimental demonstration is given by applying the algorithm to retrieve the refractive-index profile of a spherical fish lens. The method is conceptually simple and does not require matching of the index of the surrounding medium to that of the surface of the lens, and the related iterative algorithm rapidly converges.

Changes in the sagitta of the anterior corneal surface associated with a change in the corneal radius of curvature have been used to calculate the change in refractive error of the eye in two areas: the ablation depth for laser surgery, and the change in corneal thickness associated with orthokeratology lens wear. An approximate formula known as Munnerlyn's formula is commonly used to calculate the refractive error change from sagittal data. This article compares the change in refraction calculated using the approximate formulae with the change calculated from a formula based on an elliptical corneal section. The approximate formula underestimates the ablation depth for a given refractive change and overestimates the refractive change for a given change in corneal thickness, assuming a constant asphericity. When the corneal asphericity increases together with an increase in radius of curvature, a suggested mechanism in orthokeratology, the approximate formula underestimates the change in ocular refraction.

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

The Stiles-Crawford effect of the first kind (SCE-I) was measured on both post-photorefractive keratectomy (PRK) and anisometropic subjects at six different locations of retina. The eye that underwent the PRK procedure showed results that were typical of myopic eyes, although the vision in that eye has been maintained at 6/5 (or 20/15) since the operation. The anisometropic subject had one eye that was emmetropic and the other eye was myopic with a refractive error of -3.00 DS. The emmetropic eye showed the normal well-centered SCE-I functions across the retina, whereas the myopic eye of the same subject showed the nasal tilting of receptors in the nasal retina, which has been the typical finding among myopic subjects.

Tonic accommodation is known to be different in different refractive gr Myopes show a lower degree of tonic accommodation than do hyperopes and eminetrof is not known is whether the lower tonic accommodation in emrnetrope,s predisposes the i’D myopia. In this study, we initially measured the refraction and tonic accommodation of 210 children, aged 6 to 16 years. We identified the emmetropic children and refracted them again 2 years later. We found that myopic children had lower tonic accommodation than emmetropic and hyperopic children. For the emmetropic children, there was no clear relationship between tonic accommodation and subsequent refractive change 2 years later. We conclude that the level of tonic accommodation does not predict refractive change in the short term.

Background: Cyclopentolate continues to be the cycloplegic of choice for refracting young children, although many studies of ocular biometry promote the use of tropicamide. Methods: To clarify the role of drug type in biometric measurements, cycloplegia was induced in two disparate age groups using cyclopentolate and tropicamide on two separate occasions. Refraction, phakometry and A-scan ultrasonography measurements were made on two groups of Tibetan children resident in Nepal. Results: Cyclopentolate produced significantly more cycloplegia in the younger group, which was supported by phakometry measurements. However, in clinical terms, the difference between the measurements was not significant. Conclusion: We conclude that although cyclopentolate is more effective than tropicamide in relaxing accommodation in young children, the use of a local anaesthetic prior to instillation of tropicamide produces refractive data virtually equivalent to that of cyclopentolate, regardless of the age group measured. However, biometric measurements may be susceptible to greater error when near fixation targets are used during phakometry procedures.

We measured the ocular dimensions and refractive error in 400 Tibetan children using autorefraction, A-Scan ultrasonography, and video phakometry. Crystalline lens power decreased by 2.55 D with an associated increase in the anterior radius of curvature of 1.98 mm and an increase in the posterior radius of curvature of 0.49mm. When we modeled the crystalline lens as a gradient refractive index structure with isoindicial lines, the surface refractive index was 1.386 ± 0.001 for an assumed central refractive index of 1.406. When the refractive index of the lens was modeled in this way, the decrease in power of the lens with age was not only due to the decrease in power of the lens surfaces as calculated with a uniform equivalent refractive index, but was also due to changes in the refractive index distribution within the lens structure. The consequences of these changes are considered in terms of the possible emmetropizing mechanisms for these children.

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.

BACKGROUND: When the ageing crystalline lens is modelled on the basis of a constant equivalent lens, the changes in ocular dimensions would lead to an increase in power of the order of two dioptres. A comparable increase in myopia is usually not evident with increasing age and this inconsistency has been referred to as the lens paradox. It has been proposed that this paradox can be resolved if the refractive index is modelled as a gradient refractive index. The purpose of this paper was to study differences in the equivalent, gradient and surface refractive index of the crystalline lens between a young and old age group. METHODS: Biometric data was collected for 96 subjects: 48 young adults with an age range 19 to 31 years (mean 22.10 +/- 2.93 years) and 48 old adults with an age range 49 to 61 years (mean 53.88 +/- 3.88 years). The equivalent refractive index was determined for each subject by paraxial ray tracing and a merit function based on refractive error and Purkinje image height. The refractive index gradient was determined by modelling the crystalline lens as a bi-elliptical iso-indicial structure in a three-surface Gullstrand-Emsley schematic eye and a merit function based on the surface power, the gradient refractive index power and the equivalent power of the lens. The central refractive index of the lens was assumed to be 1.406. RESULTS: The differences between the groups included a decrease in the mean equivalent refractive index from 1.427 +/- 0.007 to 1.418 +/- 0.006, an increase in surface refractive index from 1.386 +/- 0.007 to 1.394 +/- 0.006 with a concurrent change in the gradient refractive index profile. The refractive index changes maintained a constant mean lens power in each group. CONCLUSIONS: The so-called 'lens paradox' whereby an increase in the power of the crystalline lens does not lead to an increase in myopia is resolved by a decrease in the equivalent refractive index of the lens or when modelled as a gradient refractive index structure, by an increase in the surface refractive index and an associated change in gradient for an assumed central refractive index of 1.406.

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.

In the first part of this paper we presented a tomographic method to reconstruct the refractive index profile of spherically symmetrical lenses. Here we perform the generalization to lenses that are rotationally symmetrical around the optical axis, as is the ideal human lens. Analysis of the accuracy and versatility of this method is carried out by performing numerical simulations in which different magnitudes of experimental errors and two extreme case scenarios for the likely shape of the refractive index distribution of the human lens are considered. Finally, experimental results for a porcine lens are shown. Conceptually simple and computationally swift, this method could prove to be a valuable tool for the accurate retrieval of the gradient index of a broad spectrum of rotationally symmetrical crystalline lenses.

The transmittance properties of 25 commercially available sunglass lenses measured with the Hoya ULT-2000 Universal Light Transmission Meter were compared with the equivalent requirements specified in Australian Standard AS1067-Sunglass Lenses and calculated from data obtained with a Cary 14 spectrophotometer. The validity of readings obtained with the ULT-2000 as a measure of the requirements of AS1067 is discussed and some possible sources of error in the routine use of the instrument are noted.

The front surface topography of spherical flexible contact lenses of varying back central optic radii was assessed with the auto-collimating photokeratoscope. The study indicates a correlation between corneal asphericity and the asphericity of the front surface of the contact lenses when on the eye.

(1979). Future Directions in Optometric Education. Clinical and Experimental Optometry: Vol. 62, No. 6, pp. 230-235.

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.

Kava is a drink produced from the plant Piper methysticum and used as a social and ceremonial beverage on many South Pacific islands. Visual functions were measured in one subject following the taking of this drink. A reduced near point of accommodation and convergence, an increase in pupil diameter and disturbance to the oculomotor balance were noted. No changes were recorded in visual or stereoacuity or in ocular refractive error.

The classical or Helmholtz theory of accommodation has formed the basis of optometric practice for many years. This theory maintains that the ciliary muscle receives only parasympathetic innervation and that the position of rest in the emmetropic eye lies at optical infinity. The suggestion of a dual sympathetic/parasympathetic innervation to the ciliary muscle also dates from the last century. Recent studies indicate that the “dark focus” or resting point of accommodation lies at some intermediate distance and that the refractive state in the absence of an adequate stimulus represents the tonus position of the ciliary muscle for minimal autonomic innervation. This paper considers the possible role of a sympathetic innervation to the ciliary muscle in the development and correction of refractive errors.

Proposed tolerances for the manufacture of the back surface of flexible contact lenses are discussed and the clinical significance of variation within these tolerances is examined. The validity and clinical relevance of existing methods for measuring flexible lenses is considered and alternatives suggested.

A sociological survey and visual screening of 599 university students at two campuses in New Zealand was conducted in 1980. This paper reports on the demographic and sociological aspects of the study which found that 33.2% of the sample wore spectacles or contact lenses and 27.1% had never had a visual examination. A relatively high proportion (64.6%) of the sample reported discomfort or eyestrain which they associated with their vision. Despite a relatively high proportion reporting visual problems, many had not sought professional advice. Other aspects covered in the survey include studying habits and environments, attitudes to visual problems and the costs of ophthalmic appliances.

The transmission properties of 15 pairs of allyl resin plastic (CR39) sunglass lenses and 21 pairs of glass sunglass lenses were calculated according to Australian Standard AS 1067--1971 Sunglass Lenses, and the results compared with the requirements of this standard. Language: en

Nine subjects wore fenestrated and unfenestrated soft contact lenses for 6-hr periods. The lenses worn in the experimental eye were fenestrated with four, 0.80-mm holes, located in the midperipheral region of the lens. THe corneal swelling was monitored with a biomicroscope and pachometer and the fenestrations significantly reduced the corneal swelling compared with the same lens worn prior to fenestration.

The selection of tinted ophthalmic lenses is discussed in terms of some of the factors which may not always be considered when deciding upon a particular tinted lens. The factors considered for the various tinted lenses available include the transmittance properties in the non-visible regions of the spectrum and the effects of tinted lenses on the performance of drivers of motor vehicles. Language: en

A team from the University of Auckland conducted an assessment of vision needs in Tonga in August 1981. A total of 667 people was examined with a mean age of 50.6 years. The vast majority (91.6%) had no spectacle correction for either distance or near. The onset of presbyopia appeared to occur at an earlier age than in European societies and the majority of spectacle prescriptions ordered (77.4%) were for near work. A high incidence of pterygium (23.2%) was detected by the team.

(1976). Aspects of Optometry in Canada. Clinical and Experimental Optometry: Vol. 59, No. 8, pp. 272-276.

A random sample of 2000 persons throughout New Zealand was questioned on the use of and attitudes toward vision care. One-half of the respondents had had a vision examination within the past 4 years. Of those that had been examined, 72% were seen by an optometrist and 22% were seen by an ophthalmologist. Fifty-two percent of the sample wore spectacles or contact lenses, and in the 25 to 34 year age group 12.5% wore contact lenses. The majority (66%) reported no visual problems, and of this group 42% would visit a medical practitioner, 41% an optometrist, and 11% an ophthalmologist if they were to develop a visual problem. At the last visual examination 28% were advised that they did not need a refractive correction, 22% were told their present aid was adequate, and 46% were prescribed new spectacles.

Many government departments both State and Federal, require certain standards of colour perception as part of the vision requirements for entry to their particular department. This paper reviews some of these requirements and is based on a survey conducted in 1975. Language: en

A comparison of the social composition of students in Optometry, Dentistry and Medicine at the University of Melbourne was obtained from questionalres given to first year students. Type of secondary schooling, parents, education and fathers, occupation were taken as measures of socio-economic grouping.

A brief discussion of some of the physical properties of ophthalmic materials is given together with a general introduction to the optics of ophthalmic lenses including the concepts of surface power, effectivity and lens power. The common errors of refraction and the methods of correction are outlined. Ophthalmic prisms and prismatic effects are discussed and the results of calculations of the prismatic effects of afocal lenses are considered with reference to Australian Standard 27–1967. Some of the more common optical problems associated with ophthalmic lenses are given, with particular attention given to the problems that may arise with afocal lenses.

The transmission properties of 54 standard ophthalmic tints are compared to the requirements of Australian Standard AS 1067–1971 Sunglass Lenses. Variations in luminous density for solid ophthalmic tints with back vertex power is considered In relation to this standard.

A modified clinical routine was used to screen 599 university students at two campuses in New Zealand; 207 at the University of Canterbury and 392 at the University of Auckland. In Auckland, 21.9% failed to meet the pass/fail criteria adopted for the survey, and in Christchurch 14.6% failed.

A team of 4 optometrists, 2 optometry students, an ophthalmologist and a general medical practitioner examined 977 school children and 685 adult villagers over a 3 week period. This paper reports our findings for the adults. Most common problem was blurred vision for distance (54.3%) or near (48.9%); sorelwatering eyes was the only other significant complaint. 88.9% achieved 6/9 VA with correction. Presbyopia had an earlier onset than in European populations. Cataract extraction and removal of pterygium were the most common surgical procedures.

The heating characteristics of the Bausch & Lomb Aseptron and AseptorR, the AUergan HydrothermR and a vacuum flask method for the disinfection of soft contact lenses were measured. All methods maintained a minimum temperature of 80oC for at least 10 minutes under the conditions of the test, although there was considerable variation between each method.

The onset of myopia typically occurs in childhood and increases during puberty. We studied the vision and refractive errors of 977 school children (ages 6 to 17 years) in 1983. Myopia in the group was extremely low; 97% had vision of 6/6, and 1.3% had myopia greater than 0.25D. Regression analysis revealed a change in mean refraction of -0.016D/year for males and -0.024D/year for females. There were no significant differences between males and females. These children engaged in about 8 hours of school work per day, and we conclude that genetic factors predominate over environmental factors in the determination of myopic refractive errors for this group.