Optimal Refractive Correction Research Articles

A Randomized Controlled Trial of Unilateral Strabismic and Mixed Amblyopia Using Occlusion Dose Monitors to Record Compliance

To investigate compliance with patching therapy and the dose-effect relationship in occlusion therapy in amblyopia by recording the effective patching time using occlusion dose monitors (ODMs). Fifty-two children with strabismic or mixed amblyopia (Snellen equivalent, 6/12-6/48) were given optimal refractive correction and randomly allocated for 12 weeks into three treatment groups: group 1, no patching; group 2, prescribed patching for 3 hours; and group 3, prescribed patching for 6 hours. The effective time of occlusion was monitored with ODMs continuously. Visual acuity (VA) was measured every 3 weeks with LogMAR (logarithm of the minimum angle of resolution) Crowded Tests. In the 3- and 6-hour groups, mean (SD) compliance was 57.5% (30.8%) and 41.2% (30.9%), respectively, and mean effective patching time per day was 1 hour 43 minutes (55 minutes) and 2 hours 33 minutes (1 hour 52 minutes), respectively. The mean (SD) improvement in logMAR VA of amblyopic eyes was 0.24 (0.17), 0.29 (0.14), and 0.34 (0.19) in groups 1, 2, and 3, respectively. There was no significant difference in compliance with the prescribed patching between the 3- and 6-hour groups. VA outcomes in the 3- and 6-hour groups were not significantly better than 0-hour patching. However, the VA of patients with eyes effectively patched for more than 3 hours improved significantly. A dose-effect relationship was observed. Age at treatment did not influence the visual outcome. Poor compliance with prescribed occlusion explains discrepancies in previous studies. No differences in the effect between the different prescribed patching periods were found. The dose-effect relationship observed should encourage development of methods such as educational intervention to improve visual outcome by increasing effective patching time.

Method of Determining a Patient's Subjective Refraction Based on Objective Measurement

To develop a method for predicting the manifest refraction from objective measurement using wavefront sensing. We tested the hypothesis that, when the eye has an optimal refractive correction, the refractive power of the eye is in the most stable state, with the least fluctuation in its total optical aberration and hence least amount of standard deviation in the wavefront error. Nine patients were enrolled in this study. All measurements were performed using a grating-based wavefront instrument manufactured by Ophthonix, Inc. The test procedure involved adding defocus power binocularly in the line of sight of the patient. Five added diopter powers were used: plano (no add), +1.00 D, +0.50 D, -0.50 D, and -1.00 D. The eyes were undilated, and the measurement was taken at a single added defocus power each day over 5 days. The patients had a binocular view and were asked to focus on a picture at 20 feet. Manifest refraction was used as the reference point (the +/- added powers are relative to this "plano" power). For each patient, the standard deviation (STDEV) of the total root mean square values of the wavefront measurement were used as a measure of vision fluctuation. Spherical equivalent manifest refraction was tested against the added defocus as a measure of the method's predictability. Graphic plots of standard deviation of the total wavefront error against the added diopter power showed that the minima of the vision fluctuation from the data sets were within -0.10 and +0.30 D (95% limit of agreement). The mean difference between the manifest refraction and the most stable point of vision was 0.14 D. The preliminary results of 10 eyes show promise that using an objective method of monitoring vision fluctuation may produce good agreement with the manifest refraction. The method should be further validated using a smaller step of added defocus and involving a larger number of test subjects representing a sample of the general population.

What are the visual benefits of eyelid squinting?

We propose that eyelid squint can reduce the impact of several conditions known to cause eyestrain such as uncorrected refractive error, accommodative dysfunction, presbyopia, and glare. Clinicians commonly accept that squint improves visual acuity (VA) in the presence of refractive error, and even though the benefit of eyelid squint in bright light seems self-evident, data are not available to support either benefit. The purpose of this study was to measure the effect of eyelid squint on VA in the presence of refractive blur and on visual field sensitivity. Nineteen subjects with optimal refractive correction were tested, with and without squinting, as follows: (1) distance VA with induced refractive errors of +0.50, +1.00, +2.00, +3.00, +1.00 -1.00 x 90, and +1.00 -1.00 x 180; (2) near VA with induced refractive errors of -1.00 x 90 and -1.00 x 180; and (3) central and peripheral threshold visual fields with a Humphrey Field Analyzer. Differences in visual acuity between squinting and nonsquinting were tested for significance with repeated-measures analysis of variance, and differences in visual fields were tested using mixed model analysis of variance with repeated measures. Eyelid squint significantly improved (p < 00.016) distance VA measurements for 1.00 to 3.00 D of induced myopia. The squint-induced VA improvement increased from 0.06 logarithm of the minimum angle of resolution for 1.00 D to 0.24 logarithm of the minimum angle of resolution for 3.00 D of myopia. Eyelid squint also significantly reduced visual field sensitivity below the horizontal meridian by an average of 1.4 dB, by 1.6 dB on the horizontal meridian, and with increasing reductions in the vertical field up to 11.6 dB for measurements 40 degrees above fixation. This pattern of decreased superior field sensitivity would decrease visual discomfort from overhead glare. The results provide empirical support that eyelid squint improves visual acuity for subjects with refractive error and reduces glare in the superior visual field.

The effect of refractive blur on postural stability.

The effect of refractive blur upon postural stability was investigated under three conditions: normal standing, standing with input from the somatosensory system disrupted and standing with input from the somatosensory and vestibular systems disrupted. Standing stability was assessed using the centre of pressure (COP) signal from force plate data in four young subjects (mean 23.9+/-3.1 years) and five repeated sets of measurements were taken. The subjects looked straight ahead at a horizontal and vertical square wave pattern of 2.5 cycles (degree)(-1). Under each of the three test conditions, standing stability was measured with the optimal refractive correction and under binocular blur levels of 0, + 1, + 2, + 4, and + 8 D and with eyes closed. In the normal standing condition, dioptric blur had only a mild effect on postural stability. However refractive blur produced large increases in postural instability when input from one or both of the other two sensory systems were disrupted. We hypothesized that dioptric blur would have an even great effect on postural stability if the visual target used was of higher spatial frequency. This was confirmed by repeated measurements on one subject using a target of 8 cycles (degree)(-1). The study highlights the possible importance of an optimal correction to postural stability, particular in situations (or people) where input from the somatosensory and/or vestibular systems are disrupted, and where the visual surrounds are of high spatial frequency.

HAS THE 18-MONTH OUTCOME FOR EXTREMELY LOW GESTATIONAL AGE (ELGA) INFANTS OF 23-25 WEEKS GESTATION IMPROVED? † 1614

Our purpose was to examine the neurodevelopmental outcome at 18 months corrected age of surviving ELGA infants of 23-25 weeks gestation born in 1991-3, and to compare the results with those from an earlier cohort born in 1983-9 (Synnes A et al J Pediatr 1994:125:952-60). ELGA infants delivered in British Columbia's tertiary perinatal center had their gestational age (GA) ascertained as accurately as possible by early ultrasound and dates using predetermined criteria. There were no survivors at <23 weeks in the two time periods. The results of multidisciplinary assessments performed at a corrected age of 18 months were reviewed for survivors of 23-25 weeks GA. Major impairment was defined as a Bayley MDI below -2 s.d., definite signs of cerebral palsy (CP), visual acuity in the better eye with optimal refractive correction worse than 20/200, or sensorineural hearing loss requiring amplification. Of 111 live births, 48 survived to 18 months corrected age and follow-up data was available in 35 (73%). 4 of 12 infants with major impairment, had multiple impairments. The recent cohort was smaller in size(3-year vs 7-year period) and the follow-up rate was lower (73% vs 93%), with a sampling bias toward the lower birthweight infants. Survival rate (to discharge) for 23-25 weeks GA has not improved (44% vs 44%) and the survivors had comparable incidence of major impairment at 18 months (34%; vs 36% resp.). Of children with impairments, multiple impairments were present in 33% (vs 38% in earlier cohort). As there were so few 23 week babies, the results were re-analyzed for infants 24-25 weeks GA. Survival (53% vs 50%) and impairment rate (32% vs 34%) remained not statistically different. There is no evidence in this survey that the introduction of recent innovations such as surfactant therapy has produced any improvement in outcomes for the smallest babies.Table

Visual acuity changes throughout adulthood in normal, healthy eyes: seeing beyond 6/6.

There are many useful longitudinal and cross-sectional epidemiological studies of how visual acuity (VA) deteriorates with age. However, few studies have evaluated age-related VA changes in normal, healthy eyes. Data from three previous studies that included VA measurements taken with logMAR charts from subjects with normal, healthy eyes were collated and additional data from 42 subjects were added. This provided VA results from 223 subjects (age range 18 to 80 years). Mean logMAR VA improved from -0.13 (Snellen equivalent 6/4.5) in 18- to 24-year-olds to -0.16 (Snellen 6/4(-1)) in 25- to 29-year-olds and then gradually became worse with age to a mean value of -0.02 (Snellen 6/6+1) for subjects over 75 years of age. The inadequacy of 6/6 as a norm value of VA is illustrated. The data show VA levels far superior to the data from the standard references of Pitts (1982) and Weale. (1975, 82) We suggest that this is because we used logMAR charts (providing nontruncated data), the subject's optimal refractive correction (rather than habitual) and strict exclusion criteria ensuring that only subjects with normal, healthy eyes were recruited for the studies.