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Results for “Refraction, Ocular”

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At least 127 records · Page 7Linked to original sources

PubMed2005

Refractive error, ocular biometry, and lens opalescence in an adult population: the Los Angeles Latino Eye Study.

PURPOSE: To characterize age- and gender-related differences in refractive error, ocular biometry, and lens opalescence (NOP) in a population-based sample of adult Latinos. Also assessed were the determinants of age-related refractive differences. METHODS: Participants in the Los Angeles Latino Eye Study (LALES), a population-based study of Latinos aged 40 years and more, underwent an ophthalmic examination, including ultrasonic measurements of axial length (AL), vitreous chamber depth (VCD), anterior chamber depth (ACD), lens thickness (LT), and noncycloplegic automated and subjective refraction. Corneal curvature/power (CP) was measured using an autorefractor. NOP was graded at the slit lamp by an ophthalmologist using the Lens Opacity Classification System II. Age- and gender-related differences were calculated. Multiple regression models were used to identify the determinants of age-related refractive differences. RESULTS: Of the 6357 LALES participants, 5588 phakic individuals with biometric data were included in this analysis. Older individuals had shallower ACDs, thicker lenses, more NOP, and more hyperopia compared to younger individuals (P < 0.001). There was no age-related difference in AL (P > or = 0.05). Women had significantly shorter AL, shallower ACD and VCD, than did men (P < or = 0.01). The strongest determinants of refractive error were AL (primarily VCD) and CP. NOP was a small but significant determinant of refractive error in older individuals. CONCLUSIONS: Age- and gender-related differences in ocular biometric, refractive error, and NOP measurements are present in adult Latinos. While the relative contribution of NOP in determining refractive error is small, it is greater in older persons compared to younger individuals.

Adult↗

PubMed1986

The rapid change of corneal curvature in the neonatal period and infancy.

We measured the radii of corneal curvature of premature and mature newborns, using a rearranged automated keratometer and carried out a consecutive study in 18 of the newborns. The mean keratometry value in diopters (D) for seven premature babies was 49.50 D; that for 11 mature babies 47.00 D. The change of corneal curvature over time was observed in eight newborns (16 eyes). It showed rapid change from 49.01 to 45.98 D during 2 to 4 weeks of life and became very slow after 8 weeks of life (44.60 D). It would appear that the change of corneal curvature does not keep pace with the increase of axial length after 12 weeks of life, and constant ocular refractive status probably requires substantial changes in lenticular power.

Aging↗

PubMed1979

Lamellar keratoplasty in keratoconus.

The authors present the results of 11 keratoplasties in cases of keratoconus, comparing the visual acuity and the ocular refraction before and after the operation (ophtalmometry, objective and subjective refractometry). The statistical data indicate a meaningful flattening of the corneal curvature (12.84 dpt average) and a mean reduction of the myopia for 8.227 dpt. In fact, the basic regular astigmatism of the cornea had not been altered. The mean visual acuity was 0.68 (with an improvement of 0.495). No further damage of acuity had been observed. After 10 to 69 months a medical checkup revealed transparent grafts in five cases; four presented small maculae; one, thin folds of Descemet's membrane. Based on the authors' experience and on the literature, the paper recommends lamellar keratoplasty in cases of keratoconus, the surgeon must choose the appropriate moment in the course of the development of this disease.

Adult↗

PubMed2001

Statistical analysis when dealing with astigmatism: assessment of different spherocylindrical notations.

Ophthalmic epidemiological studies frequently deal with ocular refractive errors, which are commonly expressed in the form sphere/cylinder x axis. However, this representation has been shown not to be the most suitable one for performing statistical analysis. Although alternative analytical and graphic methods to represent this kind of data have been developed, these formalisms have often gone unnoticed by researchers, despite their usefulness and versatility. Besides, there has been no discussion of how each of them fits in with a particular type of study. In this paper, several mathematical representations of dioptric power are revisited in a comprehensive way. The aim is to encourage researchers in ophthalmology and optometry to use these formalisms in their epidemiological studies, thus profiting from their exactitude and simplicity. Consequently, the emphasis is not on complicated mathematical derivations but on how to use these representations. Their potential and suitability in different applications is analyzed in detail. In addition, some examples are presented to illustrate the mathematical methods considered.

Astigmatism↗

PubMed1984

Complete ocular component analysis by vergence contribution.

Traditionally, the components of ocular refraction are analyzed in surface powers and their linear separations in the eye. More recently it has been suggested that the refractive effects of these components are better studied in terms of their vergence contributions. This study presents an improved method of vergence analysis which takes into account the refractive status of the eye. Results show a significant interaction of all components and indicate that their relative contributions are dependent on refractive status.

Cornea↗

PubMed1994

Early signs of myopia in Chinese schoolchildren.

We looked for the earliest signs of the onset of myopia in 194 emmetropic (-0.25 to +0.75 D) eyes by following them for 1 to more than 2 years. Visual acuity examination, cycloplegic retinoscopy, and A-scan ultrasonography were performed at intervals of 6 to 12 months. Of the 194 eyes 64 became myopic during the observation time. Our results show that during the progression from emmetropia to myopia the change in visual acuity is not a good indicator of the onset of myopia. When visual acuity decreased to below 6/6 (20/20) cycloplegic retinoscopy showed that most of the 64 eyes had become truly myopic and A-scan ultrasonography revealed a significant increase in vitreous chamber depth and axial length as compared to the non-myopic eyes. In the early stage of myopia development the anteroposterior diameter of the eye elongates. However, changes in anterior chamber depth and lens thickness did not differ between the myopic and non-myopic group. Our results suggest that routine ocular refraction should be conducted in addition to the school vision screening. For those who have visual acuity 6/6 (20/20) and ametropia of -0.25 D or more minus we would like to be able to prevent or at least retard the development of myopia.

Child↗

PubMed1990

Intraocular pressure in school myopia.

The relationship between intraocular pressure and ocular refraction and axial length were studied in a follow-up of myopic children and in a cross-section sample of school children. In the follow-up group intraocular pressure significantly decreased with age. Mean intraocular pressure at the mean age of 10.9 years was 17.4 mmHg, and three years later 16.1 mmHg. Intraocular pressure above 20 mmHg was recorded in 7.6% of total measurements in the follow-up group and in 8.7% of cases in the cross-section sample. There was a significant correlation between mean intraocular pressure and the spherical equivalent both at the beginning and at the end of the study in the follow-up group and in the cross-section sample among the boys only. There was also a significant positive correlation between intraocular pressure and axial length at the end of the follow-up among the boys but not among the girls.

Adolescent↗

PubMed1993

On the Stiles-Crawford effect and ocular imagery.

To study the modifying effect on spherical aberration of the retinal directional sensitivity (the Stiles-Crawford effect), ocular imagery was simulated by computer ray-tracing in a phakic and a pseudophakic model eye. The Stiles-Crawford effect was found to reduce the distance between the paraxial focus and the effective least blur focus. The modifying effect on spherical aberration was found to be insignificant for pupil sizes below 4 mm, and to increase in magnitude above this size. For an 8 mm pupil, the effective focus was found to move 0.3 mm and 0.2 mm toward the paraxial focus for the phakic and the pseudophakic eye, respectively. This corresponded to a shift of 1.0 D and 0.6 D in the spectacle plane, respectively. The modifying effect of the retinal directional sensitivity should be taken into account in the theory of ocular refraction and in objective keratometry and refractometry.

Cataract↗

PubMed2003

Myopia, genetics, and ambient lighting at night in a UK sample.

BACKGROUND: It has been reported that exposure to artificial lighting at night during the first 2 years of life was very strongly associated with subsequent myopia development. METHODS: The strength of this association was tested in a UK sample for the first time. The study population comprised 122 university students. RESULTS: Myopia occurred with approximately equal frequency in those who slept with and without light exposure at night. In contrast, two largely genetic factors, parental myopia and race, were both significantly associated with myopia development, as has been found previously. CONCLUSION: This study provides further support for the view that night-time light exposure during infancy is not a major risk factor for myopia development in most population groups. In a subset of this cohort for which spectacle prescriptions were available for both parents (49 trios), the heritability of ocular refraction was estimated to be 0.31.

Adult↗

PubMed2000

Scleral remodeling during the development of and recovery from axial myopia in the tree shrew.

PURPOSE: Recent investigations have suggested that scleral thinning in mammalian eyes with axial myopia is a consequence of the loss of scleral tissue, rather than the redistribution of existing tissue as the eye enlarges. The present study investigated whether further changes in the distribution and metabolism of scleral tissue occur during the process of recovery from axial myopia. Scleral glycosaminoglycan (GAG) synthesis and content as well as scleral dry weight changes were monitored as indicators of remodeling in myopic and recovering tree shrew sclerae. METHODS: Myopia was induced in tree shrews by monocularly depriving them of pattern vision. Some animals then had the occluder removed and were allowed to recover from the induced myopia for periods of 1, 3, 5, 7, and 9 days. Newly synthesized GAGs were radiolabeled in vivo with [(35)S]sulfate. Sulfate incorporation and total GAG content in the sclera was measured through selective precipitation of GAGs from proteinase K digests with alcian blue dye. Dry weights of the sclerae were also determined. Changes in ocular refraction and eye size were monitored using retinoscopy, keratometry, and ultrasonography. RESULTS: Eyes developing myopia showed a significant reduction in scleral GAG synthesis, particularly in the region of the posterior pole (-36% +/- 7%) compared with contralateral control eyes. Scleral dry weight was also significantly reduced in these eyes (-3.7% +/- 1.2%). In recovering eyes, significant changes in GAG synthesis were apparent after 24 hours of recovery. After 3 days of recovery, significantly elevated levels of GAG synthesis were found (+79% +/- 15%), returning to contralateral control eye values after 9 days of recovery. Interocular differences in scleral dry weight were shown to follow a similar pattern to that observed for GAG synthesis. CONCLUSIONS: Active remodeling, resulting in either the loss or replacement of scleral tissue and not passive redistribution of scleral tissue, is associated with changes in eye size during both myopia development and recovery. Regulatory changes in scleral metabolism can be rapidly evoked by a change in visual conditions and the direction of regulation is related to the direction of change in eye size.

Animals↗

PubMed1999

[Investigation of the diplopia after intraocular lens implantation].

OBJECTIVE: To investigate the pathogenesis, mechanism and prognosis of diplopia after cataract extraction with intraocular lens implantation (IOL). METHODS: Besides routine ocular examinations, refraction, ocular position, ocular movement, fusional function and image of diplopia were examined on all the patients. Forced duction was examined on partial patients. RESULTS: Among 24 cases with diplopia after IOL implantation, there were 19 cases of binocular diplopia and 5 cases of monocular diplopia. Of the binocular diplopia, there were 17 cases of strabismic diplopia and 2 cases of diplopia due to central fusional impairment. Of the cases with monocular diplopia, there were 4 cases resulted from operative complication and one case with congenital iridocoloboma. CONCLUSIONS: The pathogenesis of strabismic diplopia resulted from dysfunction of ocular movement is unknown. Most of the patients can obtain binocular vision by early active treatment. Monocular diplopia is partly resulted from operative complication.

Adult↗

PubMed1997

[Keratometry in dogs: two automatic hand-held keratometers compared].

In human medicine keratometric examinations are an essential tool for the determination of ocular refraction preceeding cataract operations. In dogs the use of manual keratometric methods is not feasible due to the lack of active cooperation. It is therefore necessary to apply autokeratometers which are also utilized in human pediatric patients. In this investigations we compared two portable automated keratometers in respect to their applicability in dogs. 40 dogs, comprising eight clinic beagles and 32 patient-dogs of various breeds were examined awake and under anesthesia using both hand-held keratometers. The horizontal and vertical corneal radius was measured consecutively by one investigator in all dogs and assessed statistically (standard deviation, analysis of regression and Student t-test). Both instruments yielded comparable results in all dogs. Furthermore, they were both tolerated well as they ensure a both rapid and minimal stress-inducing method of keratometry in dogs.

Anesthesia, General↗

PubMed2002

Baseline refractive and ocular component measures of children enrolled in the correction of myopia evaluation trial (COMET).

PURPOSE: To describe baseline refractive and ocular component measures in children with myopia enrolled in the Correction of Myopia Evaluation Trial (COMET). COMET is a multicenter, randomized clinical trial to evaluate whether progressive-addition lenses slow the progression of juvenile-onset myopia compared with single-vision lenses. METHODS: Four hundred sixty-nine children with myopia between -1.25 and -4.50 D spherical equivalent and without eye or systemic conditions known to affect refractive development were recruited from four geographically and ethnically diverse communities in the United States. Their ages were 6 to 11 years inclusive, and 52% were girls. The main outcome measure for the overall trial is progression of myopia determined by cycloplegic autorefraction after inducement of cycloplegia with 2 drops of 1% tropicamide. Axial length, the secondary outcome measure, was assessed by ultrasonography. The distance correction was determined by subjective methods before cycloplegia, with noncycloplegic autorefraction values as the starting point. RESULTS: Because data were similar in both eyes, they are reported for the right eye only. The mean spherical equivalent measured by cycloplegic autorefraction was -2.38 +/- 0.81 D. Young children had significantly less myopia than older children (P = 0.03), but the amount of myopia did not differ by gender or ethnicity. Mean axial dimensions were 4.0 +/- 0.2 mm (anterior chamber), 3.4 +/- 0.2 mm (lens), 16.8 +/- 0.7 mm (vitreous chamber), and 24.1 +/- 0.7 mm (axial length). Girls' eyes had significantly shorter axial length than boys' (P < 0.0001). Mean corneal radii were 7.73 +/- 0.25 mm (horizontal) and 7.59 +/- 0.24 mm (vertical). Ninety-five percent of the eyes had a ratio of axial length to corneal radius higher than 3.0. CONCLUSIONS: These baseline measures provide cross-sectional data on a large group of ethnically diverse children with myopia. Refractive and axial component dimensions are consistent with data in other studies showing that myopic eyes have longer vitreous chambers than emmetropic eyes. The measures reported herein will serve as a basis for examining changes that occur over a minimum of 3 years of follow-up of children enrolled in COMET.

Accommodation, Ocular↗

PubMed1991

[The measurement of visual refraction of pupils].

The ocular refraction was measured by applying 1% atropine eye-drops for 3 days in 5,458 eyes (male 2,944, female 2,514) of the pupils aged 7-17 in rural areas. The results were as follows: the majority of the pupils had hyperopia (male 84.71%, female 80.55%); the frequency of the measurements was not in normal distribution; the degree of far-sight began to recede and gradually become converted to myopias with advancing age but varying greatly between age-groups. The myopias began to appear at the age of 9 and increased rapidly at the age of 14 for girls and 15 for boys. From then on the rate of conversion continued to increase with schoolgirls more than schoolboys and, as a result, formed "the dangerous stage of myopia", the values of physiological visual refraction were shown as follows: pupils aged 7-9 + 2.00 D- + 2.50 D, aged 10-13 + 1.50 D- + 1.75 D, aged 14-17 + 1.00 D- + 1.25 D. The possibility of pathological hyperopia was greater if the pupils aged 7-9 and the value of visual refraction over +3.75 D, aged 10-13 had the value over +3.00 D and aged 14-17 had the value +2.50 D. The possibility of developing myopia was great in those pupils aged 7-9 whose value was under +1.00 D and in those aged 10-17 whose value was under +0.50 D. The deprived vision of the pupils should be routinely corrected by skiascopy.

Adolescent↗

PubMed2006

Changes in refraction and ocular dimensions after cataract surgery and primary intraocular lens implantation in infants.

PURPOSE: To study refraction and axial length changes after cataract extraction and primary intraocular lens (IOL) implantation in children younger than 1 year of age. SETTING: Two regional hospitals. METHODS: After determining the IOL power for emmetropia, 80% of the value was used to choose the IOL for implantation to counter anticipated myopic shift with age. The main outcome measures were changes in refraction and axial length 3 years after surgery. RESULTS: Thirty-four eyes of 20 children (mean age 6.7 months +/- 3.9 [SD]) were studied. Refraction in the immediate postoperative period was +4.53 +/- 1.45 diopters (D). Three years after surgery, the mean refraction was -2.49 +/- 3.08 D (P<.001). Twenty-two eyes (64.7%) had surgery during the first 6 months of life (group 1) and had a shorter axial length at surgery (mean 18.92 +/- 1.32 mm) compared with 12 eyes (35.3%) that received surgery between 7 and 12 months (group 2, mean 20.29 +/- 1.00 mm) (P = .007). However, the final axial length was greater in group 1 (mean 22.67 +/- 1.04 mm) than in group 2 (mean 21.23 +/- 0.26 mm) (P = .019). CONCLUSIONS: Primary IOL implantation is an option for children having cataract surgery in the first year of life. Significant myopic shifts occurred, and this seemed to be more pronounced in younger children. It appears that rethinking current strategies for IOL power calculation may be required to achieve more optimal refractive outcomes.

Cataract↗

PubMed1987

Twin study on myopia.

In order to reassess the relative importance of genetic and environmental factors in the development of myopia in Chinese schoolchildren, 90 pairs of MZ and 36 pairs of like-sex DZ twins were enrolled for detailed ophthalmological examination about their ocular refractions. Corneal curvatures and axial lengths were also measured. With equivalent settings of the range for concordance, corneal curvatures showed higher concordance rate (84%) than ocular refractions (65%) and axial lengths (59%) in MZ twins. F-test on the intrapair variances between MZ and DZ twins revealed a significant hereditary role in determining the ocular refraction and its optical components. The degree of genetic determination was expressed by heritability indices, which were derived from intraclass correlation coefficients. The diversity of sample ages and refractions, while making the obtained data difficult to further explore the gene-environment interaction, led to the observation of more intrapair differences with age and myopic progression in MZ twins.

Adolescent↗

PubMed1993

Experimental myopia in a diurnal mammal (Sciurus carolinensis) with no accommodative ability.

1. We examined the functional morphology of the intra-ocular muscles of the grey squirrel using pharmacological and histological methods. Using sympathomimetic (phenylephrine) and parasympathomimetic (carbachol) agents, administered by transcorneal iontophoresis, the response of the iris sphincter and dilator muscles and the ciliary muscle were recorded. Measurement techniques included both streak retinoscopy and coincidence optometry for measurement of ocular refraction and high resolution ultrasonography to monitor changes in the intra-ocular component dimensions. 2. The grey squirrel was found not to possess a functional accommodative system. No change in ocular refraction or intra-ocular dimensions could be induced with 40% carbachol. Marked changes in pupil diameter occurred with topical application of both phenylephrine (dilation) and carbachol (constriction). Histological findings were in agreement with pharmacological findings in showing well developed iris sphincter and dilator muscles but only a poorly developed ciliary muscle. 3. Calculation of the depth of focus of the grey squirrel eye reveals that this could be sufficient to account for the behavioural observations of near viewing habits. 4. We then determined whether we could induce axial elongation of the vitreous chamber and a consequent myopia by monocular deprivation (MD) of pattern vision. 5. Monocular deprivation of pattern vision produced a significant experimental myopia due to axial elongation of the vitreous chamber in the deprived eye. 6. The results demonstrate that a functional accommodative system is not necessary to induce experimental myopia in the grey squirrel eye.

Accommodation, Ocular↗

PubMed2000

[A study on the effect of vitreous cavity length in primary ametropia].

OBJECTIVE: To study the nature of primary ametropia and disclose the actions of ocular refractive components, especially the length of vitreous cavity, on primary ametropia. METHODS: The refractive state, corneal refractive power (CP), anterior ocular segment length (ASL), vitreous length (VL) and ocular axial length (AL) were measured for 1 336 eyes, including 523 hyperopic eyes, 199 emmetropic eyes and 614 myopic eyes. The relationships of these elements were studied. RESULTS: CP had no effect on the refractive state. There was no significant difference between CP of these three refractive groups (hyperopia vs. emmetropia and emmetropia vs. myopia) (P > 0.05). The test of normality showed that the distribution of CP was normal in each refractive state. Case control study of paired sample revealed that ASL did not change regularly with the change of refraction, while VL changed progressively with the change of refraction and there was significant difference between each pair of groups (F = 204.29, P < 0.01). The study of the relationship with matching t test between AL and VL showed that the change amount of AL and VL was nearly the same, that was, the growth of AL was about the same as the growth of VL (P > 0.05). Analysis of correlation and regression showed that these two elements were highly correlated, the coefficient of correlation being 0.879. CONCLUSION: VL determines AL and finally determines refractive state. VL is the most important element in determining primary ametropia while the anterior eye segment has no significant contribution.

Adolescent↗