摘要:

An attempt is made to gain a greater understanding of lens binding during overnight wear. To do this, a mathematical model is used to investigate the motion of a rigid gas permeable (RGP) lens normal to the eye surface, when the eye is closed. Graphs of the displacement of the lens as a function of time are computed for different lens designs. The rate of apical tear film thinning is found to depend upon the central fitting relation, with lenses that are fitted flatter giving the highest rate of thinning. The model also suggests the relevance of other factors such as eyelid muscle tension and tear-layer composition during sleep to the clinical phenomenon of lens binding.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Patient preferences and ocular responses were compared between rigid and soft contact lenses by randomly fitting 32 neophyte subjects with a rigid lens in 1 eye and a soft lens in the contralateral eye. Twentyseven of 32 subjects completed the 3-month study and 16 subjects were willing to continue for an additional 3-month extension. Subjects preferred the comfort and handling of the soft lens but preferred the vision provided by the rigid lens and initially its ease of maintenance. There was also a marked preference for the soft lens when all aspects of lens wear were compared. Objectively, the rigid lenses were responsible for more ocular changes than the soft lenses. Palpebral aperture sizes of the rigid gas permeable (RGP) wearing eyes decreased significantly (0.5 mm; p<0.05) compared to the soft lens wearing eyes. The incidence of corneal staining was significantly greater in the rigid lens wearing eye (50% RGP vs. 22% soft) but limbal injection was greater in the soft lens wearing eye (18% soft vs. 6% RGP). Refractive sphere, cylinder, and corneal astigmatism decreased in the rigid lens wearing eye after 3 months. This daily wear clinical trial has shown a marked subjective preference for wearing soft lenses with fewer short-term ocular effects.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

Although previous investigations have reported that changes in background luminance, stimulus contrast, and dioptric blur can each affect visual acuity independently, it has not been shown how these three variables interact to influence visual acuity. This is a particularly important issue if one is interested in predicting how individuals with different refractive characteristics will be able to perform acuity-based tasks in degraded visual environments with low background lighting and contrast levels. To investigate these relations, we conducted a series of experiments in which measurements of visual acuity were obtained for four subjects using Landolt C targets of varying contrast at several background luminances for levels of blur between 0 and 8 diopters (D). Our results show that visual acuity is significantly affected by all three factors, and that their effects are essentially additive. At all luminance and contrast levels, the reduction in visual acuity is greatest for dioptric blur up to 2.0 D, with a more gradual reduction in visual acuity for dioptric blur of greater than 2.0 D. At all blur and luminance levels, visual acuity decreases gradually for contrast levels down to 20%, and decreases sharply for lower contrast levels. Over the range of background luminances we tested (75.0 to 0.075 cd/m2), visual acuity decreases linearly with reductions in luminance. The additive effects of dioptric blur, contrast, and luminance provide a basis for predicting visual acuity-related task performance for individuals in different visual environments. For example, an individual with 6/6 (20/20) visual acuity under high luminance- high contrast co/ditions will fall to 6/18 (20/60) acuity for low luminance conditions and 6/30 (20/100) for low luminance-low contrast conditions. Similarly, an individual with an uncorrected visual acuity of 6/30 (20/100) under optimal conditions will fall to approximately 6/120 (20/400) under low luminance conditions and 6/240 (20/800) under low luminance-low contrast conditions.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

PurposeA study was conducted to identify variables that were different in children who were emmetropic and became myopic and in children who remained emmetropic.MethodsA cohort of initially emmetropic children had eye and vision examinations at 6-month intervals for a period of 3 years. Ocular optical components were measured by keratometry and ultrasonography. Crystalline lens power was calculated using Bennett's formulas.ResultsThere was a tendency for keratometer power of both principal meridians to be greater in the became-myopic group than in the remained-emmetropic group. Axial length to corneal radius (AL/CR) ratio was significantly greater in the became-myopic group than in the remained-emmetropic group. The variable with the highest sensitivity and specificity was the AL/CR ratio using the horizontal meridian corneal radius.ConclusionsGreater corneal powers and greater AL/CR ratios are risk factors for youth onset myopia.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

We present a detailed review of the medications used in the USA, Canada, and the United Kingdom for the prevention of migraine and the potential ocular adverse effects associated with the use of these medications. Those drugs that are administered for the purpose of reducing the frequency or severity of migraine attacks are classified according to whether they act on the cerebral vasculature primarily at serotonin (5-HT2) receptors (e.g., methysergide, cyproheptadine, and pizotyline), beta adrenergic (primarily beta-2) receptors (e.g., propranolol and timolol), via central nervous system (CNS) adrenergic (alpha-2) receptors (e.g., clonidine), or calcium channels (e.g., flunarizine). The roles and mechanisms of action of tricyclic antidepressants (e.g., amitriptyline) and nonsteroidal anti-inflammatory drugs (NSAIDs) in the prophylactic management of migraine are also discussed, along with possible pharmacogenetic differences in the kinetics of action of some of these drugs. The general indications, contraindications, and potential ocular and systemic adverse effects of each class of drugs is reviewed and presented along with the references to original literature on these effects.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

PurposeOrthokeratology procedures suffer from lack of predictability in the response of individuals. To identify factors contributing to this, we have retrospectively studied a range of ocular parameters in patients with varying outcomes from orthokeratology lens wear.Experimental Design.Three groups were studied: an experimental group (9 subjects wearing Contex OK-3 design orthokeratology contact lenses), and 2 control groups [10 rigid gas permeable (RGP) contact lens wearers and 10 non-contact lens wearers]. Three categories were identified among the orthokeratology group: those responding well, moderately, or poorly to orthokeratology lens wear. Measurements included subjective refraction, intraocular distances, corneal thickness, ocular rigidity, and epithelial fragility.Results.When comparing the three orthokeratology categories, there was no significant difference for central and peripheral epithelial fragility and ocular rigidity. There was also no significant difference for any of the biometric characteristics measured. The prefitting spherical equivalent power was found to be significantly different between categories of responders (p = 0.0228), with the poor responders having the highest initial level of myopia. None of the measured characteristics differed significantly among the orthokeratology group and the two control groups.Conclusions.In this pilot study, the successof orthokeratology lens wear was not related to ocularbiomechanical or biometric attributes, but it was related to prefitting refractive error.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

This report compares sunlight UV-A and UV-B fluxes in the Northeastern United States that reach the crystalline lens with thresholds that cause lens damage. The fluxes of UV-A and UV-B radiation that reach earth to penetrate the cornea and to reach the lens were calculated. Ten hours of continuous UV-A exposure or 23 min of UV-B would exceed the rabbit cornea threshold for photokeratitis. The lens threshold would be reached by 26 h of UV-A or 245 h of UV-B continuous exposure. The sequence of UV-induced damage follows: (1) UV-B photokeratitis; (2) UV-A photokeratitis; (3) UV-A lens damage; and (4) UV-B lens damage.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

The Developmental Eye Movement (DEM) test is a clinical oculomotor test of a visual-verbal format which has been normalized and whose reliability and validity have been shown elsewhere in an English speaking population. In this paper, the authors provide normative data for a Spanish speaking population from ages 6 to 11 years. Also, a comparison of results for both languages is provided. The DEM test appears to be highly efficient and not linguistically related for all subjects involved in the study. Results in that age bracket are clinically the same. For age 6 years, although results are not similar (Spanish subjects are significantly slower), the ratio is similar in both groups. The DEM test appears to be reliable; its results could be used in any population regardless of the language.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

A new method of quantifying the dynamics of the human dark adaptation function is presented. This method is based on nonlinear regression analysis and allows the derivation of a number of clinically useful indices of dark adaptation ability. The analysis method is applied to literature data describing the effect of ageing on dark adaptation and to clinical data from a retinitis pigmentosa patient. Ageing was seen to decrease the rate of adaptation in the cone portion of the dark adaptation function, and to have little effect on the rate of adaptation in the rod portion of the dark adaptation function. One case of retinitis pigmentosa was associated with increased rod and cone thresholds but little change in the rate of adaptation of the cone portion of the dark adaptation function. It is suggested that this analysis method may offer advantages in quantifying changes in dark adaptation dynamics.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID

摘要:

The stability of the precorneal tear film was measured noninvasively in normal subjects from four diverse ethnic groups (Chinese, Africans, Indians, and Caucasians) all resident in the west of Scotland. Statistically significant differences were found to exist among the groups. The respective mean (±SD) tear stabilities were: 9.8 s (3.9), 11.8 s (5.9), 16.4 s (6.9), and 19.9 s (8.3). Subjects were selected from an age range of 17 to 45 years.

ISSN:1040-5488    

出版商:OVID    

年代:1995

数据来源: OVID