PubMed Health⌕ Search

Biomedical subjects

T Grosvenor

Publications and source records attributed to T Grosvenor.

At least 19 recordsLinked to original sources
PubMed1995

Mechanisms of emmetropization in the aging eye.

Refraction and its components were measured on 96 subjects consisting of 48 Young Adults (19 to 31 years old) and 48 Mature Adults (49 to 61 years old) having refractive errors in the range from +2.50 to -2.50 D. Subjects for the two groups were matched on the basis of sex and refractive error. A quota sampling technique was used to obtain subjects for myopic, emmetropic, and hyperopic refractive-error categories. An autorefractor was used to measure refractive error, an autokeratometer was used to measure corneal radius of curvature, and an A-scan ultrasonometer was used to determine the axial dimensions. Crystalline lens radii were determined by submitting Purkinje image data to a ray-tracing program, with the assumption that the index of refraction of the ocular media is constant throughout life. By comparing the results for the two groups of subjects, it was found that there were no significant age-related differences in mean corneal radius of curvature or in mean axial length. However, the following significant age-related differences were found: the Mature Adult eyes had shallower anterior chambers, thicker lenses, shorter vitreous chambers, steeper anterior and posterior lens surfaces (shorter radii of curvature), more highly powered lenses, and higher ocular refracting power than the Young Adult eyes. It was concluded that a decrease in the gradient-index of the lens occurs with increasing age, acting as an emmetropizing mechanism by compensating for the steepening of both the front and back surfaces of the lens.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1995

Stability of the Stiles-Crawford function in a unilateral amblyopic subject over a 38-year period: a case study.

The relative luminous efficiency curves for light entering different parts of the entrance pupil [Stiles-Crawford functions (SCFs)] of subject TG, whose right eye is amblyopic, were determined earlier for both the normal and the amblyopic eye. The left eye was found to have a normal SCF, varying from the center to the edge of the entrance pupil by an entire log unit. However, the SCF for the right eye was found to be abnormal, showing peak sensitivity far toward the temporal edge of the pupil and varying from center to edge by only one-third of a log unit. From these data it was inferred that the photoreceptors were in poor alignment. In October, 1993 the subject experienced a retinal detachment in the left (nonamblyopic) eye, requiring cryoretinopexy, drainage of subretinal fluid, and a scleral buckle. Postoperatively, visual acuity recovered to 6/9 to 6/7.5 (20/30 to 20/25), but vision was poor when illumination or contrast was low. In order to determine whether the decrement in visual performance was the result of misalignment of the photoreceptors, the subject's SCFs were determined 9 months postoperatively for both eyes, using a procedure similar to that used earlier. The relative luminous efficiency curves for both eyes were found to be surprisingly similar to the corresponding curves found 38 years earlier. It was concluded that any decrease in visual function resulting from the retinal detachment and repair was not caused by misalignment of the photoreceptors. We infer that the mechanisms resulting in the exceptionally high degree of photoreceptor alignment in the left eye and the disturbed alignment in the right eye are still present.

Aged↗
PubMed1991

Comparison of refractive components in youth-onset and early adult-onset myopia.

Refraction and its components were measured for 79 young adults, 29 of whom were youth-onset myopes, 26 were early adult-onset myopes, and 24 were emmetropes. By submitting the data to analysis of variance, it was found that: (1) corneal power was significantly greater for both groups of myopes than for the emmetropes; (2) lens power and thickness were not significantly different for the three groups of subjects; (3) anterior chamber depth was significantly greater for the youth-onset myopes than for the emmetropes; and (4) vitreous chamber depth and axial length were both significantly greater for youth-onset myopes than for early adult-onset myopes, and significantly greater for young adult-onset myopes than for emmetropes. When youth-onset myopes and early adult-onset myopes were matched on the basis of spherical equivalent refraction, no significant differences were found between the two groups for any of the components of refraction. It was concluded that the differences between youth-onset myopes and early adult-onset myopes found in the analysis of variance occurred only as a result of the fact that the youth-onset myopes had been myopic for longer periods of time and therefore were considerably more myopic.

Adolescent↗
PubMed1991

Rigid gas-permeable contact lenses for myopia control: effects of discontinuation of lens wear.

Conventionally fitted Paraperm O2plus contact lenses were worn for 44 months by 23 myopic children, who discontinued lens wear for 2.5 months and then resumed lens wear with Fluoroperm 30 lenses for a period of 8 months. Mean changes in myopia were: (1) an increase of 0.76 D during the initial 44 months of lens wear, (2) a further increase of 0.27 D during the 2.5 months when lenses were not worn, and (3) a decrease of 0.02 D during the 8-month period of Fluoroperm 30 lens wear. These results show that the effect of rigid gas-permeable lenses on myopia progression is diminished if lens wear is discontinued; however, the mean increase in myopia for these children was significantly less than would have been expected if glasses had been worn for the entire 54.5-month period. The results after discontinuation and then resumption of lens wear show that the effect of contact lenses in controlling the progression of myopia could not be accounted for entirely on the basis of corneal flattening as measured by the keratometer, therefore reinforcing the conclusion that corneal flattening due to rigid lens wear takes place primarily at the corneal apex, rather than in the zone of the cornea measured by the keratometer.

Adolescent↗
PubMed1990

Is the relation between keratometric astigmatism and refractive astigmatism linear?

Javal's rule, which states that the total astigmatism of the eye--or refractive astigmatism--is equal to 1.25 (keratometric astigmatism) - 0.50 x 90, implies a linear relation between the two entities. Data published by Grosvenor, Quintero, and Perrigin (Am J Optom Physiol Opt 1988;65:292-7) show that for eyes having no more than 2.50 D of keratometric astigmatism, the slope of the regression line for refractive astigmatism plotted against keratometric astigmatism is less than 1.00, rather than the 1.25 required by Javal's rule. When data compiled for eyes having greater amounts of keratometric astigmatism are plotted in the same manner, the slope of the regression line is greater than 1.00, suggesting that the relation between keratometric and refractive astigmatism may be non-linear. However when data for these highly astigmatic eyes are plotted in combination with data published by Grosvenor, Quintero, and Perrigin, they are adequately characterized by a straight-line relation.

Adolescent↗
PubMed1990

Silicone-acrylate contact lenses for myopia control: 3-year results.

One hundred myopic children between the ages of 8 and 13 years were fitted with Paraperm O2plus silicone-acrylate contact lenses. After 3 years of lens wear, the mean increase in myopia for the 56 subjects remaining in the study was 0.48 D (+/- 0.70) D as compared with 1.53 (+/- 0.81) D for a group of spectacle-wearing myopes matched for initial age and initial refractive error. The mean change in corneal refracting power for the contact lens wearers was a decrease (corneal flattening) of 0.37 (+/- 0.32) D. Assuming that little or no corneal change would have occurred in the absence of the contact lenses, we may conclude that corneal flattening (as measured by the keratometer) accounts for less than half of the effect of contact lenses in controlling myopia progression. A possible explanation for this disparity is that although the keratometer provides a valid measurement of corneal refracting power for a "normal" cornea, it fails to provide a valid measurement for a cornea that has been flattened by wearing a contact lens.

Acrylates↗
PubMed1989

Use of silicone-acrylate contact lenses for the control of myopia: results after two years of lens wear.

Although a number of reports have been published concerning the use of polymethyl methacrylate (PMMA) contact lenses for the control of myopia, there have been no reports of the use of gas permeable contact lenses for this purpose. In the study reported here, 100 myopic children between the ages of 8 and 13 years were fitted with Paraperm O2 plus silicone-acrylate contact lenses to be worn for a period of 3 years. Lenses were fitted by the alignment method, most lenses having diameters from 8.5 to 9.0 mm. At the end of 2 years 60 subjects remained in the study, 53 of whom were wearing their lenses on a regular basis and the other 7 were irregular wearers. Mean increases in myopia during the 2-year period were found to be 0.28 D for the subjects who wore their lenses regularly and 0.93 D for the irregular wearers, compared to 0.80 D for a group of 31 age-matched single vision spectacle lens wearers. Mean corneal refracting power was found to decrease (the cornea flattened) 0.33 D for the regular wearers as compared to an increase of 0.14 D for the irregular wearers and a decrease of 0.13 D for the spectacle wearers. Mean changes in axial length were an increase of 0.1 mm for the regular wearers, an increase of 0.4 mm for the irregular wearers, and an increase of 0.6 mm for the spectacle wearers.(ABSTRACT TRUNCATED AT 250 WORDS)

Acrylates↗
PubMed1989

Myopia: what can we do about it clinically?

Methods that have been used by vision practitioners for the control of myopia include visual training, biofeedback training, undercorrection, overcorrection, the use of bifocal lenses, the use of contact lenses, the instillation of atropine, and refractive surgery. With some exceptions the use of these methods has achieved only limited success. The lack of success with the less-invasive methods--which are based on the supposition that myopia is caused by accommodation--may be due to the fact that they are used for eyes that are already myopic and therefore have already undergone axial elongation and scleral stretching. If it were possible to predict which children were at risk for the development of myopia, vision practitioners would be able to institute procedures for the control of myopia when only a minimum of scleral stretching has occurred. Risk factors that warrant investigation include the axial length/corneal radius ratio and the resting state of accommodation.

Accommodation, Ocular↗
PubMed1988

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.

Adolescent↗
PubMed1988

High axial length/corneal radius ratio as a risk factor in the development of myopia.

Results of refractive component measurements have shown that eyes of Melanesian schoolchildren living in Vanuatu, almost all of whom are emmetropic, have relatively short axial lengths and relatively flat corneas in comparison to emmetropic eyes of children examined in England. Evidence is presented to suggest that the relation between the axial length of the eye and corneal radius--the axial length/corneal radius (AL/CR) ratio--may be a useful predictor of future refractive state. Mean axial length/corneal radius ratios for 6- to 19-year-old Melanesian school children were found to be 2.88 for boys and 2.86 for girls, as compared to mean AL/CR ratios for 6- to 19-year-old British schoolchildren of 3.05 for boys and 3.04 for girls. It is proposed that an eye having a high AL/CR ratio is at risk for the development of myopia, and that such an eye has maintained its state of emmetropia by virtue of a compensatory flattening of the crystalline lens. In order to test the proposition that an increase in axial length occurs as a precursor to the development of myopia, a prospective longitudinal study of a group of 6- to 7-year-old schoolchildren is proposed.

Adolescent↗
PubMed1988

Myopia in Melanesian school children in Vanuatu.

Visual acuity, refraction, and the components of refraction were measured in 788 Melanesian children in the South Pacific island nation of Vanuatu. Of the 788 children, 97.2% were found to have uncorrected visual acuity of 6/6, 96.8% were found to have a spherical equivalent refraction between -0.25 and +1.00 D, 2.9% were found to have myopia of 0.50 or more, 0.25% were found to have hyperopia of 1.25 D or more, and 0.25% were found to have refractive astigmatism of 1.25 D or more. The low prevalence of ametropia may be considered to be due mainly to genetic factors; however, the possibility of environmental factors cannot be ruled out.

Child↗
PubMed1988

Predicting refractive astigmatism: a suggested simplification of Javal's rule.

In the period of almost 100 years since Javal proposed an empirically determined rule for the prediction of refractive astigmatism on the basis of corneal astigmatism, many authors have made suggestions for the modification of "Javal's Rule." These modifications, rather than being based on clinical data, have been based on concepts of mechanisms that can contribute to refractive astigmatism, but in most cases the modifications do nothing more than complicate the process of predicting refractive astigmatism, with the result that only Javal's original rule is widely used. In this paper, keratometric and refractive data for three groups of subjects are used to demonstrate that a simplified version of Javal's rule is more effective in predicting refractive astigmatism than is Javal's rule itself.

Adolescent↗
PubMed1987

A review and a suggested classification system for myopia on the basis of age-related prevalence and age of onset.

Systems for the classification of myopia are usually based on etiological dichotomies such as hereditary vs. environmental, physiological vs. pathological, structural vs. functional, or axial vs. refractive. The purpose of this paper is to propose a system for the classification of myopia based not on assumed etiological factors but on readily available and easily verifiable information, including age-related prevalence and age of onset. The proposed system classifies myopia into four categories: congenital, youth-onset, early adult-onset, and late adult-onset. Paradoxically, such a classification, being devoid of etiological assumptions, may help to make possible a better understanding of the etiology of the various categories of myopia.

Adolescent↗
PubMed1987

Houston Myopia Control Study: a randomized clinical trial. Part II. Final report by the patient care team.

In a randomized clinical trial designed to test the efficacy of bifocal lenses for the control of juvenile myopia, each of 207 children between the ages of 6 and 15 years wore single vision lenses, +1.00 D add bifocals, or +2.00 D add bifocals for a period of 3 years. For the 124 subjects who completed the study, the mean changes in refraction were found to be -0.34 D per year for subjects wearing single vision lenses, -0.36 D per year for those wearing +1.00 D add bifocals, and -0.34 D per year for those wearing +2.00 D add bifocals. These differences were not statistically significant. When subjects in all three treatment groups were combined, it was found that the rate of progression tended to be the most rapid for subjects who entered the study at an early age with a large amount of myopia, and tended to be the least rapid for subjects who entered the study at a later age with a small amount of myopia. It was also found that subjects having with-the-rule astigmatism progressed more slowly than those having no astigmatism or against-the-rule astigmatism.

Adolescent↗
PubMed1987

Reduction in axial length with age: an emmetropizing mechanism for the adult eye?

Mechanisms accounting for the fact that the eye tends to become and remain emmetropic have been proposed for the developing eye, but no such mechanisms have been proposed for the adult eye. In the study reported here, refractive component data published by Sorsby and his co-workers were reanalyzed in terms of the variations in these components with age. Included in the analysis are component data for one eye of each of 271 subjects from age 4 to age 70 years, whose ocular refraction was between plano and +2.00 D. The results of this analysis show that the axial length of the eye appears to decrease during the adult years of life, concurrently with a decrease in anterior chamber depth and an increase in the refractive power of both the cornea and the lens. It is proposed that a reduction in the axial length of the adult eye serves as an emmetropizing mechanism, occurring in harmony with the increase in the refracting power of the eye, which would otherwise cause the refraction of the eye to move in the myopic direction.

Adolescent↗