The effect of prism adaptation on the response AC/A ratio.

Vergence adaptation, also known as prism adaptation, is a phenomenon in which a patient's heterophoria changes after prolonged viewing through prism. The effect of prism adaptation on the accommodation-convergence relationship, quantified by the AC/A ratio, is not known. Previous studies of AC/A ratio stability and alterability have used only stimulus AC/A ratio calculations, or have measured accommodative responses to only one or two stimuli. The ideal study of AC/A ratio stability and alterability would measure accommodative responses to several accommodative stimuli, and use these along with vergence responses to calculate response AC/A ratios, rather than stimulus AC/A ratios. In addition, the gradient method should be used to avoid any effect of proximal vergence resulting from changes in target distance. This paper describes a project which investigated the effect of vergence (prism) adaptation on the gradient response AC/A ratio, using accommodative responses measured for five different accommodative stimuli. The response AC/A ratio did not significantly change following a period of adaptation to base-in prism for six of the eight subjects in this study. In addition, the response AC/A ratio did not significantly change following a period of adaptation to base-out prism for six of the eight subjects.

Relationship of accommodative response and nearpoint phoria in a sample of myopic children.

Esophoria has been associated with onset and progression of myopia in children. The induction of myopia by optical defocus shown by animal models suggests that a high lag of accommodation during near work may contribute to myopia in children. This paper examines the relationship of nearpoint phoria and accommodative response in a sample of children with myopia. Accommodative response was measured under binocular conditions with the Canon Autoref R-1 autorefractor with a 40 cm viewing distance. Phorias were measured with the von Graefe method using a 40 cm test distance. In the statistical analysis exophoria was scored as a negative number and esophoria was scored as a positive number. The coefficient of correlation of accommodative response with phoria was -0.32 (n = 73; p<0.01), thus showing an association of a more positive (more convergent) near phoria with lower accommodative response. The correlation coefficient increased to -0.39 when an exponential function was used. When only esophores were considered, the correlation coefficient was -0.59 (n = 44; p<0.001). Lower accommodative response (higher lag of accommodation) was associated with greater esophoria.

Inter-examiner repeatability of heterophoria tests.

Relatively few studies have reported on the repeatability of heterophoria (phoria) measurement procedures. The purpose of this study was to assess the inter-examiner repeatability of seven clinical tests of phoria measurement, using correlational and mean difference analyses. Two experienced optometrists performed each of the 7 phoria tests on 72 healthy adult subjects. Subjects ranged in age from 22 to 40 years, had visual acuity correctable to 20/20 in each eye, had no strabismus or amblyopia, and had normal eye health. The seven phoria tests used were: (1) the estimated cover test, (2) the prism-neutralized objective cover test, (3) the prism-neutralized cover test with subjective reporting of target movement, (4) the von Graefe phorometry with continuous target presentation, (5) the von Graefe phorometry with flashed target presentation, (6) the Thorington method, and (7) the modified Thorington method. All tests were performed in the same way by both examiners in random sequence, using standard clinical procedures. Inter-examiner correlations were determined for each test, as well as the mean and standard deviation of the differences between each examiner measurement (in prism diopters) on each subject. The modified Thorington method had the smallest mean difference and standard deviation of all tests and, thus, may be considered the most repeatable. This test also had the highest inter-examiner correlation. The prism-neutralized cover test with subjective reporting of target movement had the second highest correlation and the second lowest 95% limits of agreement, but had the largest mean difference of all tests. von Graefe phorometry methods were the least repeatable of the phoria tests.

Relation of childhood myopia progression rates to time of year.

BACKGROUND: One potential method for assessment of the effect of near work on childhood myopia progression is to compare progression rates overtime spans that have differing near-point demands. METHODS: Myopia progression rates were calculated for a 6-month period during the school year (called school rate in this article) and for a 6-month period that included the 3 months of summer vacation (called summer rate). Data used for analysis were right eye spherical equivalents from manifest subjective refractions of 27 children in a longitudinal study at Northeastern State University in Oklahoma, and 15 children in a longitudinal study at Indiana University. All subjects wore single-vision spectacle lenses. RESULTS: The mean summer rate for the 42 subjects was -0.39 diopters per year (D/yr) (SD = 0.46). The mean school rate was -0.72 D/yr (SD = 0.57). The rates during the two periods were significantly different by two-tailed paired t-test (p = 0.006). CONCLUSIONS: The differing amount of nearpoint activity during the school year and the summer is a potential explanation for the different rates of myopia progression during those two time periods.

Reliability of and comparisons among three variations of the alternating cover test.

One of the most commonly used methods for measuring heterophoria is the alternating cover test. Differences in specific procedures and observational criteria used by examiners may result in different measurements of heterophoria on the same patient. This study evaluated the inter-examiner reliability of three different cover test techniques: the estimated cover test, the prism neutralized objective cover test, and the prism neutralized subjective cover test. Two examiners performed each technique on each of 72 subjects. Reliability was assessed using correlational methods and mean difference calculations. Although there were some inter-examiner differences for the different techniques, all of these differences were within previously determined minimum detectable eye movement ranges. Therefore, when used by experienced clinicians, each of these techniques appears to be a reliable method of heterophoria determination, and their results are comparable.

Ocular components measured by keratometry, phakometry, and ultrasonography in emmetropic and myopic optometry students.

Ocular components were measured by keratometry, phakometry, and ultrasonography on 176 young adults. Refractive error was measured by retinoscopy. Mean vitreous depth was greater and the cornea was steeper in myopes than in emmetropes. There were no significant differences between myopes and emmetropes in mean anterior chamber depth, crystalline lens thickness, anterior crystalline lens radius, posterior crystalline lens radius, or crystalline lens power. Coefficients of correlation were calculated for each of the ocular components with refractive error for the entire group of 176 subjects. The components that showed statistically significant correlations with refractive error were vitreous depth and anterior corneal radius. Eyes with greater vitreous depths tended to have flatter anterior corneal surfaces. The slope of the principal axis relating these two variables was significantly different in emmetropes and myopes. Eyes with greater vitreous depths tended to have lesser crystalline lens power. Vitreous depth showed a statistically significant correlation with posterior lens radius, but not with anterior lens radius.

Reliability of and comparisons among methods of measuring dissociated phoria.

Many methods of heterophoria measurement are available clinically. This paper reviews several studies which have examined the reliability of phoria measurements, and have compared various tests of phoria measurement. Different methods of data analysis make comparison of studies difficult. Two studies indicated 95% limits of agreement of 2 to 4 delta for reliability of modified Thorington, von Graefe, and Maddox rod phoria tests. Studies using correlational analysis to compare different measurement methods have found a high degree of association of the results of these methods. Studies using statistical methods assessing the agreement of test results found a high level of agreement among some tests and a low level of agreement among some tests. Some of the various phoria measurement methods differ in the technique used for dissociation, in the ability to control accommodation adequately, in the level of proximal convergence induced, or in the method by which the phoria is quantified. These differences can result in different phoria measurements on the same patient.