Prospective quantification of near work using the experience sampling method.

INTRODUCTION: The assessment of individuals' near work by survey methods is challenging. The feasibility of the Experience Sampling Method to quantify daily visual tasks was evaluated. METHODS: Twenty-one subjects were randomly paged five times per day for 6 days. When paged, the subjects dialed into a telephone survey to report the nature, duration, and working distance of their visual activity at the time of the page. RESULTS: The overall response rate was 81.3% (512/630 pages). The individual response rates varied from 6.7% to 100% (median, 86.7%). Responses were grouped into 11 categories/activities for ease of analysis. Of 506 responses, the most common activity category was "distance tasks," which included driving and walking (N = 92). Other common responses included computer use (N = 68), reading (N = 66), household tasks, e.g., cleaning, cooking, and showering (N = 64), and watching television (N = 61). Activities with a mean distance < or =26 inches (arm length) were combined as near-work responses and accounted for 54.3% (258/475) of all responses. CONCLUSION: A modification of the Experience Sampling Method can be used to obtain a "real-time" sampling of visual activities.

The repeatability of corneal thickness measures.

PURPOSE: To compare the repeatability of three measures of corneal thickness: Orbscan Slitscan pachymetry, ultrasound pachymetry, and optical pachymetry. METHODS: Twenty normal subjects were tested on three occasions. Two occurred on the same day and the third was on a different day at approximately the same time of day as one of the first two visits. Central corneal thickness of the right eye was measured with a Haag-Streit optical pachymeter, a Humphrey Model 855 ultrasound pachymeter, and the Orbscan system. Day-to-day and same-day repeatability was assessed by calculating the difference between the values from two visits and determining the mean difference, the SD, and the 95% limits of agreement (LoA) (LoA = mean +/- 1.96 SD). RESULTS: Mean (+/- SD) central corneal thickness as measured by each instrument was as follows: 539 +/- 33 microm for optical pachymetry, 542 +/- 33 microm for ultrasound pachymetry, and 596 +/- 40 microm for Orbscan pachymetry. For day-to-day comparisons, optical pachymetry showed the poorest repeatability with 95% LoA of -61 to +32 microm. Ultrasound pachymetry showed better repeatability with 95% LoA of -22 to +24 microm. The Orbscan showed the best repeatability centrally with 95% LoA of -10 to +17 microm. Peripheral Orbscan pachymetry was less repeatable than that measured centrally but still more repeatable than central optical pachymetry. Similar results were found with same-day comparisons. CONCLUSION: The Orbscan system is the most repeatable technique for measuring corneal thickness but shows a significant bias toward greater corneal thickness measures than both ultrasound and optical pachymetry.

Overnight orthokeratology.

PURPOSE: Orthokeratology is defined as the temporary reduction in myopia by the programmed application of rigid gas-permeable contact lenses. New reverse geometry contact lens designs and materials have led to a renewed interest in this field. The purpose of this study is to assess visual, refractive, topographic, and corneal thickness changes in subjects undergoing overnight orthokeratology. METHODS: Ten myopic subjects (mean age, 25.9+/-3.9 years) were recruited for a 60-day trial of overnight orthokeratology using reverse geometry rigid contact lenses. After commencing lens wear, subjects were examined on days 1, 7, 14, 30, and 60 at several times throughout the day. High- and low-contrast logarithm of the minimum angle of resolution (logMAR) visual acuity, monocular subjective refraction, autorefraction, autokeratometry, corneal topography, corneal thickness, and slit lamp examinations were performed at each session. RESULTS: Eight subjects completed the study. Both high- and low-contrast uncorrected visual acuity improved significantly by day 7. The mean change in uncorrected high contrast visual acuity at day 60 was -0.55+/-0.20 logMAR (mean at day 60, -0.03+/-0.16; Snellen equivalent, 20/19). The mean change in uncorrected low-contrast visual acuity at day 60 was -0.48+/-0.26 logMAR (mean at day 60, +0.22+/-0.23; Snellen equivalent, 20/33). The mean subjective refraction and autorefraction were significantly reduced from baseline at day 60 (mean change in subjective refraction, +1.83+/-1.23 D; mean change in autorefraction, +0.64+/-0.52 D). Corneal topography showed significant central flattening (mean change in apical radius, +0.20+/-0.9 mm; mean change in shape factor, -0.11+/-0.18 at day 60). The central cornea also showed significant thinning (mean change, -12+/-11 microm at day 60). All visual, refractive, and topographic outcomes were sustained over the course of an 8-h day. CONCLUSIONS: Overnight orthokeratology is an effective means of temporarily reducing myopia. The possible mechanism of corneal remodeling through central corneal thinning is discussed.

Visual performance after photorefractive keratectomy with a 6-mm ablation zone.

PURPOSE: To prospectively examine the effect of photorefractive keratectomy with a 6-mm ablation zone on best-spectacle-corrected visual performance. METHODS: A prospective study was conducted of 164 eyes of 164 patients with an average (+/-SD) of -4.02 +/- 1.74 diopters (range, -0.63 to -8.38 diopters spherical equivalent). Best-spectacle-corrected high-contrast and low-contrast visual acuity (18% Weber contrast) was measured with both natural and dilated pupils. Patients were tested preoperatively and at 3, 6, and 12 months after photorefractive keratectomy. Photorefractive keratectomy was performed with an argon fluoride excimer laser. Fifty-five eyes of 55 patients also underwent astigmatic keratotomy. RESULTS: Twelve months after photorefractive keratectomy, best-spectacle-corrected high-contrast visual acuity with natural pupils showed no significant change from preoperative values; mean (+/-SD) change was 0.004 +/- 0.10 logMAR (t = 0.45, P = .65). Best-spectacle-corrected low-contrast visual acuity with natural pupils was significantly reduced compared to baseline; mean (+/-SD) change was 0.04 +/- 0.13 logMAR (t = 3.3, P = .001). The low-contrast loss was larger (1.5 lines) with dilated pupils; mean (+/-SD) change was 0.13 +/- 0.15 logMAR (t = 9.31, P < .001). Greater losses in dilated low-contrast visual acuity were associated with concurrent astigmatic ketatotomy (t = 2.28, P = .025) and corneal haze of grade 1 or greater (t = 2.71, P = .005). CONCLUSIONS: Reductions in visual performance occur after photorefractive keratectomy with a 6-mm zone. These changes are greatest for low-contrast visual acuity with dilated pupils. Corneal haze and concurrent astigmatic keratotomy are associated with greater losses in low-contrast visual acuity. Best-spectacle-corrected low-contrast visual acuity is a sensitive measure for evaluating visual performance after refractive surgery.

Normal aging effects for frequency doubling technology perimetry.

PURPOSE: To determine the influence of normal aging on contrast sensitivity for frequency doubling technology (FDT) perimetry. METHODS: Contrast sensitivity measures were obtained for frequency-doubled stimuli (0.25 cycles per degree sinusoidal gratings undergoing 25 Hz counterphase flicker) at 17 target locations (4 per quadrant plus the central 5 degrees ) using a prototype of the Welch Allyn (Skaneateles, NY)/Humphrey Systems FDT perimeter (Humphrey Systems, Dublin, CA). A total of 407 normal subjects (761 eyes) between the ages of 15 and 85 years were tested. RESULTS: Between the ages of 15 and 60 years there was an approximately linear decrease in contrast sensitivity of 0.6 dB per decade. After the age of 70, there was a slightly greater sensitivity loss with age. There were no meaningful differences in sensitivity loss as a function of age for different visual field locations. A small but consistent reduction in contrast sensitivity (approximately 0.7 dB) was found at all visual field locations for the second eye tested that may be due to a central adaptation process. CONCLUSIONS: Normal aging effects for FDT perimetry are similar to those obtained for conventional automated perimetry, except that the FDT perimetry aging effects do not appear to be eccentricity dependent. These normative data provide a basis for establishing a statistical analysis procedure and probability plots for FDT perimetry.

What is low vision? A re-evaluation of definitions.

PURPOSE: To re-evaluate definitions of low vision, visual impairment, and disability. METHODS: We review current definitions of legal blindness and low vision and how these definitions are variably based on disability or impairment. We argue for a definite distinction being made between criteria for visual impairment and visual disability, low vision being defined as the presence of a visual impairment that results in a disability. Visual impairment is defined according to population norms and a statistical cut-off is used. Visual disability is defined by consideration of the level of visual measures which result in measurable or reportable disability. We consider the evidence that contrast sensitivity should be a criterion for visual disability in addition to visual acuity and visual field. CONCLUSIONS: According to the current information, we define visual impairment as best monocular or binocular visual acuity <(worse than) 6/7.5, total horizontal visual field <146 degrees (Goldmann III-4e) or <109 degrees (III-3e), and contrast sensitivity <1.5 (PelliRobson); we define visual disability as best monocular or binocular visual acuity <6/12 or contrast sensitivity <1.05.

The repeatability of automated and clinician refraction.

PURPOSE: Auto-refractors are used as a starting point for clinicians' refractions and in studies of refractive error. We investigated the repeatability of the Hoya AR-570 and clinician refraction. METHODS: Eighty-six subjects, aged 11 to 60 years, were recruited by mailing inquiries to 500 randomly selected patients who had received recent examinations at the University of California Optometric Eye Center. Contact lens wearers, patients with best corrected visual acuity worse than 20/30 in either eye, and patients with a history of diabetes were excluded. Each subject was examined by two clinicians during one visit. The first clinician obtained five auto-refractor readings for each eye (which were later averaged), performed a balanced subjective refraction (with spherical masking lenses in the phoropter), and repeated the automated refractor measurements. This protocol was then repeated by the second clinician. Clinicians were randomized with regard to testing order and masked to automated refractor results, each other's refractions, and previous spectacle prescriptions. RESULTS: To quantify repeatability, we used mixed model analyses of variance to estimate the appropriate variance components while accounting for the correlation among, for example, repeated measurements of the same eye. Astigmatic data were analyzed by converting into Fourier form: two cross-cylinders at axis 0 degrees (J0) and axis 45 degrees (J45). For mean spherical equivalent, the average difference between five averaged automated refractor readings, taken by two different optometrists, was +0.02 D (95% limits of agreement = -0.36 to +0.40 D). The average difference between the two optometrists' subjective refractions was -0.12 D (95% limits of agreement = -0.90 to +0.65 D). The 95% limits of agreement for the automated refractor were about half those of the clinician for both astigmatic terms (J0 and J45) and for all comparisons. CONCLUSIONS: Automated refraction is more repeatable than subjective refraction and therefore more appropriate for studies of myopia progression.

Repeatability of visual acuity measurement.

PURPOSE: This study investigates features of visual acuity chart design and acuity testing scoring methods which affect the validity and repeatability of visual acuity measurements. METHODS: Visual acuity was measured using the Sloan and British Standard letter series, and Landolt rings. Identifiability of the different letters as a function of size was estimated, and expressed in the form of frequency-of-seeing curves. These functions were then used to simulate acuity measurements with a variety of chart designs and scoring criteria. RESULTS: Systematic relationships exist between chart design parameters and acuity score, and acuity score repeatability. In particular, an important feature of a chart, that largely determines the repeatability of visual acuity measurement, is the amount of size change attributed to each letter. The methods used to score visual acuity performance also affect repeatability. CONCLUSIONS: It is possible to evaluate acuity score validity and repeatability using the statistical principles discussed here.