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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↗
PubMed1995

How predictable are the results of excimer laser photorefractive keratectomy? A review.

At the close of 1994, the AOA News reported that at least 14 companies were preparing to market equipment for excimer laser photorefractive keratectomy (PRK). More than a dozen PRK centers had been formed for the purpose of recruiting optometrists to co-manage PRK patients. Because the surgery is a "no-touch" computer-driven procedure whose duration is measured in seconds, the preoperative and postoperative care of PRK patients will assume major importance. Optometrists who will be asked to take part in the management of PRK patients must be able to counsel patients on matters such as the predictability of the procedure in terms of postoperative refractive error and visual acuity, as well as the possibility of unintended consequences such as difficulty in night driving. Information currently available, mainly as a result of studies conducted in other countries, shows that the results of PRK are highly predictable for preoperative myopia up to about -3.00 D and somewhat less predictable for myopia between -3.00 and -6.00 D, whereas for myopia greater than -6.00 D the probability of achieving a full correction decreases rapidly with increasing amounts of myopia. As compared to radial keratotomy (RK) in which the postoperative refractive error drifts relentlessly in the hyperopic direction, PRK brings about an initial hyperopic shift followed by regression leading to increasing myopia. Researchers disagree on the cause of the postoperative hyperopic shift and regression, and on the value of various methods of controlling regression including the use of wider and deeper ablation profiles and the postoperative use of corticosteroids and nonsteroid anti-inflammatory drugs. It is too early to determine whether the myopic creep in PRK will be as persistent as the hyperopic creep in RK, but it is likely that whereas presbyopic post-RK patients may have adequate distance vision but require corrective lenses for reading, presbyopic post-PRK patients may be sufficiently myopic to require lenses for distance vision but not for reading.

Adult↗
PubMed1994

Role of the axial length/corneal radius ratio in determining the refractive state of the eye.

The purpose of our study was to investigate the extent to which the ratio between axial length and corneal radius (the AL/CR ratio) determines the refractive state of the human eye. Subjects for the study were 194 young adults between the ages of 18 and 30 years, whose spherical equivalent refraction ranged from +7.91 to -13.32 D. For each subject, manifest objective refraction, corneal radii, and axial ocular distances were measured. Lens power was determined using a formula proposed by Bennett and Rabbetts. The AL/CR ratio was found to be approximately 3.00 for emmetropic eyes, ranging from 2.60 for the most highly hyperopic eye to 4.10 for the most highly myopic eye. The coefficient of determination of linear regression for the AL/CR ratio as a function of spherical equivalent refraction was 0.84, suggesting that 84% of the variance in refraction could be accounted for by variance of the AL/CR ratio. The results of the study suggest that for a given amount of ametropia, an eye having a relatively high AL/CR ratio would tend to have a low-powered lens (indicating that the lens had "emmetropized"), whereas an eye having a relatively low AL/CR ratio would tend to have a high-powered lens. We conclude that the AL/CR ratio can provide information concerning the extent to which the lens has emmetropized by reducing its power concurrent with axial elongation.

Adolescent↗
PubMed1993

Three-year changes in refraction and its components in youth-onset and early adult-onset myopia.

We compared refractive components, and their changes during a 3-year period, for 79 young adults of whom 29 were youth-onset myopes, 26 were early adult-onset myopes, and 24 were emmetropes. In the initial evaluation we found that mean corneal power was greater for both groups of myopes than for the emmetropes, whereas mean vitreous chamber depth and mean axial length were greater for the youth-onset myopes than for the other two refractive error groups. However, the differences between the two groups of myopes appear to be related to the fact that the mean amount of myopia was significantly greater for the youth-onset myopes than for the early adult-onset myopes. During the 3-year period, mean spherical equivalent refraction for subjects in all three refractive error groups changed in the direction of increasing myopia. For each of the three groups, the only refractive component changes having significant correlations with changes in refraction were vitreous chamber depth and axial length. We interpret these results as indicating that: (1) whether axial elongation occurs before, during, or after the completion of the normal growth period, the result is an eye whose cornea is significantly steeper, whose vitreous chamber depth and axial length are significantly greater, and whose lens differs little from that of an emmetropic eye of a person of the same age and (2) when myopia progresses with time, the progression is due to an increase in axial length that is not fully compensated by a decrease in lens power.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1993

Diagnosis and management of temporal arteritis: a review and case report.

Temporal arteritis is an insidious disease which, if not recognized and treated with high-dosage oral prednisone or intravenous prednisolone, can result in unilateral or even total blindness due to anterior ischemic optic neuropathy (AION) or closure of the central artery of the retina. Unfortunately, the symptoms and clinical signs of temporal arteritis mimic those of a number of other conditions including angle-closure glaucoma, hypertension, migraine, trigeminal neuralgia, temporomandibular joint syndrome, carotid artery occlusive disease, Foster-Kennedy syndrome, and nonarteritic AION. When a patient complains of a severe pain in the temporal region, along with scalp tenderness and a feeling of malaise or depression--with or without episodes of transient loss of vision--he or she should be referred for a diagnostic work-up which includes an erythrocyte sedimentation rate and a temporal artery biopsy. We present here a review of the recent literature concerning temporal arteritis, followed by a report of an unusual case in which high-dosage prednisone therapy was effective in relieving the patient's symptoms and lowering the sedimentation rate in spite of a negative temporal artery biopsy.

Administration, Oral↗
PubMed1993

Structural model for emmetropic and myopic eyes.

A structural model for the eye was tested using refractive component data for 42 emmetropic eyes (-0.50 to +1.50 D) and 42 eyes having from 5.00 to 7.00 D of myopia. Parameter estimates derived by applying the structural model to emmetropic eyes and to myopic eyes demonstrated that a common model described the two types of eyes, but that emmetropic eyes and myopic eyes could be considered to belong to statistically discrete populations. Corneal radius and vitreous chamber depth were the most important components determining refractive state.

Adolescent↗
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↗