Repeatability of subjective refraction in myopic and keratoconic subjects: results of vector analysis.

This paper evaluates the repeatability of refraction in keratoconus patients and normal myopes, using representations of spherocylindrical power that are theoretically valid. Data consist of test-retest refraction data from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study and similar data from normal myopes. Dioptric powers are transformed to an orthogonal vector representation of dioptric power. The metric of change is the dioptric difference between test and retest. The median difference between test and retest in keratoconus patients is four to six times larger than in normal myopes. Refraction over a rigid contact lens on a keratoconic cornea improves repeatability but remains approximately twice as large as in normal myopes. The methods demonstrated here possess advantages over previous methods and provide a more valid comparison between test and retest and between different subject groups. The repeatability of refraction in keratoconus patients is substantially worse than in normal myopes.

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.

Clinical refraction in three-dimensional dioptric space revisited.

The traditional clinical designation of spherocylindrical power unambiguously specifies the refractive properties of a thin lens or refractive surface. This representation of dioptric power is not, however, optimum in mathematical terms, as is apparent when, for example, two spherocylindrical lens powers are added. Alternative systems have been described which are not subject to this same type of difficulty, and the essential feature of these other systems is that spherocylindrical power is defined in terms of a three-dimensional dioptric space in which the axes are usually orthogonal. The advantages of this orthogonality can be exploited in the practice of clinical refraction, provided lens powers in these three dimensions can be physically implemented. Systems using these characteristics have been introduced in the past, but the clinical community has not adopted them on a widespread basis. However, systems which take advantages of these features do have unique advantages relative to traditional clinical refraction procedures. These characteristics, and refractive procedures which exploit their advantages, are described.

Evaluating the value of low-vision services.

BACKGROUND: Low-vision care is a widely accepted and valued service provided by many optometrists. As in other areas of health care, evaluation of the outcome of low-vision care is increasingly necessary so it can be properly positioned in the health care delivery system. METHODS: This article reviews the literature relating to the prevalence of low vision, its impact on affected individuals, and how low-vision intervention affects those with visual impairments. This review considers the ways in which the impact of low-vision care has been evaluated. RESULTS: The existing literature demonstrates that low-vision intervention can be highly valued by low-vision patients and can have a significant impact on an individual's daily life and activities. Evaluating this impact is a significant challenge-particularly if the goal is to gauge the outcome of low vision care as broadly as possible. CONCLUSIONS: Evaluation of health-related quality of life is a desirable option for evaluation of outcomes, and the application of quality of life instruments to the visually impaired population is necessary. There remain unresolved issues of optometric research that need to be addressed.

Corneal topography and irregular astigmatism.

BACKGROUND: The term irregular astigmatism has been used to refer to a variety of types of optical irregularity, although there are no widely recognized quantitative definitions of irregular astigmatism. This paper proposes a particular approach to defining and quantifying irregular astigmatism, and relates this model to blur disc formation. METHODS: A model of irregular astigmatism is developed and illustrated using corneal topography data. This method quantifies the variation in dioptric power with meridian by expressing that profile as a series of sinusoidal components. The relation between these components of irregular astigmatism and defocus blur discs is described, as well as the relation between irregular astigmatism and other models used to quantify aberrations. RESULTS: Irregular astigmatism is expressed in diopters, and may predict blur of the retinal image in the same way that spherocylindrical defocus causes blur. CONCLUSIONS: This model of irregular astigmatism is an efficient, quantitative means of describing irregular astigmatism.

Spherocylindrical refractive errors and visual acuity.

BACKGROUND: Understanding the relation between refractive error and visual acuity is complicated if astigmatic blur is present. No models are in widespread use that allow the combination of spherical and astigmatic errors for the purpose of predicting visual performance. METHODS: Models for combining spherical and astigmatic errors are discussed, and predictions of these models using data from the literature are presented. RESULTS: Three models for combining astigmatic with spherical errors are shown to predict visual acuity performance in uncorrected myopic refractive errors. A dioptric vector addition model is shown to have advantages over other candidate models. CONCLUSIONS: It is possible to combine spherocylindrical refractive errors into a single value when the objective is to correlate these values with visual acuity performance.

Reading with low vision.

Most low vision patients express a desire to read standard print as easily and as quickly as possible. There are a multitude of visual factors that can interfere with reading, yet we have only an incomplete understanding of these factors. These include the relationship between acuity, magnification, and reading performance, the role of eye movements in low vision reading, and the effect of central scotomas on reading performance. We describe current research in each of these areas, and discuss their potential clinical implications.

Visual performance at video display terminals--effects of screen color and illuminant type.

A limited set of illuminants and phosphor colors are commonly used in video display terminal (VDT) working environments. This study attempts to identify any combinations of such conditions that influence performance on a visually demanding counting task. Experiments were performed to test whether the phosphor color, ambient lighting spectrum, or temporal frequency characteristics of the display and ambient lighting could alter performance. Under conditions where potentially contaminating variables such as reflectance level and screen glare patterns were equalized, no significant differences in performance were produced by the particular sets of stimulus conditions tested. The results suggest that displays and illuminant types that are in common use allow substantially equivalent visual performance.

Visual prostheses and visual rehabilitation in low vision research, assessment, and management.

This review is a highly selective and personal viewpoint regarding the past year's publications in low vision clinical practice and clinical research. This article reviews and provides references on selected aspects of low vision technology, reading, pediatric considerations, visual assessment for the multiply handicapped, and the psychosocial aspects of low vision and rehabilitation. We are impressed that the wealth of current publications underscores that this is an important, productive, and growing area of research and clinical activity.

Human electroretinogram responses to video displays, fluorescent lighting, and other high frequency sources.

Time-averaged human electroretinogram (ERG) responses were determined for several workplace visual stimuli which are temporally modulated at rates exceeding the perceptual critical fusion frequency (CFF). A clearly identifiable synchronous response was in evidence for a video display terminal (VDT) stimulus operating with a refresh rate as high as 76 Hz. A directly viewed fluorescent luminaire with controllable driving frequency elicited a synchronous response at rates as high as 145 Hz. In addition, an intense stimulus created by modulating the light from a slide projector produced responses at least as high as 162 Hz. The implications of these high-frequency responses are representing a potential basis for visual symptoms are discussed.

Clinical grading and the effects of scaling.

In clinical practice, there has been a need to grade the magnitude or the severity of the functions and qualities that are assessed in the examination. It is popular to use a four-step grading scale to categorize the severity of clinical findings. The authors discuss clinical grading scales and their influence on the clinician's ability to detect change. These principles have been applied to grades or measures derived from either objective measuring instruments, subjective tests, or techniques in which the clinician makes subjective judgments. A hypothetical data set was used to show the problems associated with using grading scales that are too coarse. The authors presented a mathematic model that helps to estimate the benefits of using use of a finer scale. Data were presented from two separate studies, one on visual acuity measurement and the other on grading nuclear opacity, to show the advantages of using finer scales to enhance the sensitivity of clinical measurement. High levels of concordance between independent observations indicated that the grading scale was too coarse and that these scales needlessly reduced the clinician's ability to detect change in the parameter being assessed. For moderate sensitivity, the size of the scale increments should not exceed one standard deviation of the discrepancy so that the concordance of paired comparisons would not exceed 37%. For fine clinical sensitivity, the size of the scale increments should not exceed one third of the standard deviation of the discrepancy, in which case the concordance of paired comparisons would not exceed 13%. The theory and evidence presented here could prompt re-evaluations of common methods of clinical grading.

Optical matrices of lenticular polyindicial schematic eyes.

The optical matrices for modified schematic eyes are derived. The primary modification is to replace the usual isoindicial or bi-indicial crystalline lens with a polyindicial lens so as to more closely approximate the living human lens. Further modifications are made to simulate the accommodated state. The use of optical matrices to solve an optometric problem is demonstrated.

Refractive changes induced by intraocular lens tilt and longitudinal displacement.

There is a tendency for the posterior chamber intraocular lens to tilt about the points of insertion. This is found clinically and in postmortem studies of eyes that have had intraocular lenses surgically implanted. One end is often located in the lens capsule and the other end is located in the ciliary sulcus. The possibility of anteroposterior displacement also exists. Using a modified Gullstrand schematic model eye, we have computed the amount of spherical and cylindrical errors that are induced due to the tilt and/or displacement of the intraocular lens. If a cylindrical error of approximately 90 degrees from the axis of insertion (lens plus haptic loops) and/or axis of tilt is detected, further careful investigation is warranted.

Funduscopic systems: magnification in ametropia and aphakia.

The magnification of the fundus image with various forms of funduscopy in ametropia and aphakia is poorly understood. This paper investigates these relationships. Direct and indirect ophthalmoscopy and biomicroscopic funduscopy is included. The Gullstrand Simplified Schematic Eye is modified by changing either the axial length, the power of the cornea, or the index of refraction of the lens to produce ametropia. Aphakia is produced by considering these same eyes with the crystalline lens removed. The magnification of these various eye/funduscope systems is presented, along with general principles illustrated by the figures.

Visual acuity and chart luminance.

It is desirable to standardize the conditions under which visual acuity is measured because of its importance in determining whether subjects meet occupational vision standards and as an indicator of the extent or stability of pathological conditions. The chart luminance is one parameter which needs to be standardized, and the effect of the luminance level upon acuity measurement is a critical factor in this determination. These data are measured for a subject population using Landolt rings and British letters. Over a "normal" photopic range of 40 to 600 cd/m2 the relation between the logarithm of the minimum angle of resolution (log MAR)) and log L (luminance) can be approximated by a straight line. A doubling of the luminance level within this range improves the acuity measurement by approximately one letter on a five-letter row. Landolt ring acuities are less affected by luminance than letter acuities. The application of these results to establishing a standardized luminance level and tolerance range is discussed.

Funduscopic systems: a comparison of magnification.

The magnification properties of several funduscopy techniques are compared. These techniques include direct and indirect ophthalmoscopy, and slitlamp funduscopy with accessory lenses, such as the Hruby lens and fundus contact lens. Two new approaches are taken in the determination of funduscopic magnification. As a standard, the magnification of direct ophthalmoscopy is assumed to be 1.0X. The magnification provided by other methods is compared to this to yield relative magnification.

Bioptic low vision aid: a simple approach.

This report describes the design of a variable focus telescopic system for a low vision patient who wanted to be able to see at numerous fixed working distances. A simple and effective means of mounting the telescope onto an existing spectacle lens is described. The discussion presents an analysis of the optical principles used and a formula for determining the equivalent power of a nearpoint telescope.