Visual function assessment questionnaires.

With increased emphasis on functional outcomes in ophthalmology, third-party health care payers and research funding agencies have turned their attention to the development and use of visual function questionnaires. Since 1980, more than a dozen such self-report visual function questionnaires have been developed. All of these instruments include items that ask about specific daily activities; patients must respond with a rating that represents the level of difficulty that they experience with the activity described. This article reviews all of the known instruments, with special attention paid to their validity and reliability. Most validation studies have reported high response consistency across items and significant correlations of instrument scores with visual impairment measures. Only two studies have measured test-retest reliability. The developers of visual function questionnaires typically divide the items into several different subscales, suggesting that different variables are being measured. Although the items are very similar for the different instruments, there is little agreement among instruments on the definition of subscales. All instruments are scored as the average of the ordinal patient ratings across items for each subscale and/or for the total instrument. Measurement issues underlying the scoring of ordinal patient ratings are discussed. It is argued that unless the instruments can be converted to interval scales, the averaging of patient ratings does not yield true measurements. The three visual function questionnaires that were calibrated with a statistical item response model, which estimates interval scales, are reviewed. It is concluded that future research and development should devote additional attention to the measurement properties of functional assessment instruments.

Nonlinear contribution of eye velocity to motion perception.

The aim of this study was to test the hypothesis that an extra-retinal signal combines with retinal velocity in a linear manner as described by existing models to determine perceived velocity. To do so, we utilized a method that allowed the determination of the relative contributions of the retinal-velocity and the extra-retinal signals for the perception of stimulus velocity. We determined the velocity (speed and direction) of a stimulus viewed with stationary eyes that was perceptually the same as the velocity of the stimulus viewed with moving eyes. Eye movements were governed by the tracking (or pursuit) of a separate pursuit target. The velocity-matching data were unable to be fit with a model that linearly combined a retinal-velocity signal and an extra-retinal signal. A model that was successful in explaining the data was one that takes the difference between two simple saturating non-linear functions, g and f, each symmetric about the origin, but one having an interaction term. That is, the function g has two arguments: retinal velocity, R, and eye velocity, E. The only argument to f is retinal velocity, R. Each argument has a scaling parameter. A comparison of the goodness of fits between models demonstrated that the success of the model is the interaction term, i.e. the modification of the compensating eye velocity signal by the retinal velocity prior to combination.

Evaluation of the NEI visual functioning questionnaire as an interval measure of visual ability in low vision.

The National Eye Institute developed a visual functioning questionnaire (NEI-VFQ) designed to assess health-related quality of life of patients with visual impairments. The developers of the NEI-VFQ distributed the original 52 items into 13 different domains. The recommended method for scoring the NEI-VFQ is to linearly transform the sum of the ordinal ratings to each item within each domain to produce 13 scores. The major shortcoming of this scoring method is that sums of ordinal numbers do not necessarily generate valid measurement scales. However, Rasch models can be used to estimate interval measurement scales from ordinal responses to items. We administered 27 items from the 52-item NEI-VFQ to 341 patients with low vision. Rasch analysis was used to estimate the 'visual ability' required by each item for a particular response (item measures) and to estimate the 'visual ability' of each patient (person measures). The validity of the model was evaluated by examining the distributions of residuals for item and person measures. We observed that the 17 items we tested from the NEI-VFQ that require difficulty ratings produce a valid interval scale for low-vision patients. The estimated person measures of visual ability are linear with log MAR acuity. The ten items that require frequency or level of agreement ratings do not work together to produce a valid interval scale. Rather, these items appear to be confounded by other variables distributed in the patient sample (e.g. psychological state). The visual ability scale estimated from the 17 NEI-VFQ items is proportional to the visual ability scales estimated from two earlier studies that also elicited difficulty ratings from low-vision patients.

Perceived visual ability for independent mobility in persons with retinitis pigmentosa.

PURPOSE: To determine the distribution of perceived ability for independent mobility in people who are at various stages of retinitis pigmentosa (RP). METHODS: A questionnaire was developed to ask subjects to rate how difficult they found each of 35 mobility situations if they had no assistance. The scale was 1 (no difficulty) to 5 (extreme difficulty). In each of 127 subjects, the Rasch analysis, a latent trait analysis, was used to convert the ordinal difficulty ratings into interval measures of perceived visual ability for independent mobility. RESULTS: Content validity of the questionnaire was shown by good separation indexes (4.55 and 8.0) and high reliability scores (0.96 and 0.98) for the person and the item parameters. Construct validity was shown with model fit statistics. Criterion validity of the questionnaire was shown by good discrimination among mobility-related behavior such as "limit independent travel," "always ask for accompaniment," "use a mobility aid," and "have a fear of falling." The mobility situation shown to require the least visual ability was "moving about in the home"; the situation requiring the most was "walking at night." Bivariate regression analysis determined that for every decade of disease progression, perceived visual ability for mobility decreased by approximately 0.5 logit, which was slightly less than 10% of the total range in the study sample. A linear combination of the visual function measures, log minimum angle of resolution, log contrast sensitivity, and log retinal area accounted for 57% of the variability in the person measure. CONCLUSIONS: The patient-based assessment, developed to determine difficulty across a range of mobility situations, is a valid way to measure perceived ability for independent mobility. This latent trait varies systematically with the progression of RP and with visual function measures.

A systems model for low vision rehabilitation. II. Measurement of vision disabilities.

BACKGROUND: This study presents and tests a method of measuring vision disabilities. Based on the model presented in the first paper of this series, cognitive and motor activities ("tasks") are organized in a hierarchy that identifies the tasks' behavioral goals (purposes) and the social objectives the goals serve. Two latent variables important to the definition of vision disability are identified: "value of living independently" and "visual ability for independent living." The basic principles of the Rasch measurement model are reviewed, and Rasch models are used to measure the two variables that define vision disability. The need for rehabilitation to meet each goal is represented by "rehabilitative demand," a mathematical function of the measurements made of the value and difficulty of achieving each goal independently. METHODS: Over 400 patients with low vision rated both the importance and difficulty of independently achieving each of 24 goals. Rasch analysis was used to derive interval measures of the social value of each goal, the value that individual patients placed on each goal relative to the derived social definition of independence, the visual ability required to achieve each goal without help, and the visual ability of individual patients to live independently. A rehabilitative demand function was ascertained from the judgments of 17 AAO Diplomates in Low Vision through triadic comparisons and multidimensional scaling. RESULTS: Self-care had the greatest "social value" for independence; performing music had the least. Recreational reading required the greatest "visual ability" to accomplish independently; self-care required the least. Rehabilitative demand was linear with value and nonmonotonic with difficulty. CONCLUSIONS: Rehabilitative demand, an algorithm for defining vision disability, incorporates interval measures of visual ability and the value of independent living estimated from patient-based assessments, a social scale of the value of activities relative to independent living, and a consensus opinion of low vision experts on the prioritization of the need for rehabilitation.

A systems model for low vision rehabilitation. I. Basic concepts.

The basic concepts and structure of a hierarchical systems model for low vision rehabilitation is developed and presented. The model is built on the assumption that the patient has a desired life state, which is not realized because of the visual impairment. Life state is defined as a set of activities that the individual wants or needs to perform. The activities that constitute life state are represented hierarchically in a system called the activity breakdown structure (ABS). The highest level of the ABS represents the objectives served by the activities (e.g., daily living, vocational, recreational, social, and educational objectives). Under each objective is a set of goals and serve that objective. Any goal could serve multiple objectives. The requirements for successfully achieving a goal may vary with the objective that goal serves. The lowest level of the ABS consists of tasks that must be performed successfully to achieve particular goals. Any task could serve multiple goals. Each goal and each task has a value to the patient (i.e., level of importance) and also is associated with a level of difficulty. The need for rehabilitation, which is captured in the formal construct rehabilitative demand, for each goal and its subsidiary tasks, scales with the value of the goal or task and with the difficulty in achieving the goal and/or performing the task. Successful rehabilitation will reduce rehabilitative demand by reducing the level of difficulty associated with the goal or task and/or by reducing the value assigned by the patient to the goal or task (as a result of counseling or of learning alternative strategies). Within the framework of the model, changes in rehabilitative demand define effectiveness of intervention.

Foveal cone involvement in retinitis pigmentosa progression assessed through flash-on-flash parameters.

PURPOSE: To compare psychophysical Naka-Rushton parameters in retinitis pigmentosa (RP) patients and healthy controls using a flash-on-flash increment threshold paradigm, and to measure changes of these parameters with RP progression. METHODS: Sixty-six RP patients and 10 normal subjects were tested, and their maximum response (Rmax), half-saturation intensity (sigma), and slope (n) parameters were estimated. RESULTS/CONCLUSIONS: Rmax in RP patients is decreased significantly with respect to the range in normal controls and continues to decrease (0.024 log units/yr) with disease progression. The distribution of sigma in RP patients differs from that in normal subjects, showing lower values in general, but no progression. Small differences in parameter distributions among genetic or pathophysiologic RP subcategorizations were found, but these do not fulfill stricter statistical criteria required for multiple comparisons. Measurement noise, inherent in the flash-on-flash paradigm, exert considerable influence on the quality of the data, as was demonstrated through repeated measures and a Monte Carlo simulation.

Foveal cone involvement in retinitis pigmentosa progression assessed through psychophysical impulse response parameters.

PURPOSE: To compare psychophysical impulse response parameters in retinitis pigmentosa (RP) patients and healthy controls using a temporal contrast sensitivity threshold paradigm, and to measure changes in these parameters with RP progression. METHODS: Sixty-six RP patients and 10 healthy control subjects were tested, and the amplitude and timing parameters of the psychophysical impulse response function were computed through time-domain transformation under assumption of minimum-phase properties. RESULTS/CONCLUSIONS: The initial rise time of the impulse response, ta, was significantly lengthened in RP patients compared to controls, as was the fall time from peak to trough, tp-->t. The log peak-to-trough amplitude, log R, was significantly reduced. With disease progression, all impulse response parameters continued to move away from the normal range. Only minor distinctions according to RP pathophysiologic subtype or mode of inheritance were found, supporting the hypothesis of a common course of the secondary retinal degeneration across different RP subcategories.

Obstacles encountered in the development of the low vision enhancement system.

The Johns Hopkins Wilmer Eye Institute and the NASA Stennis Space Center are collaborating on the development of a new high technology low vision aid called the Low Vision Enhancement System (LVES). The LVES consists of a binocular head-mounted video display system, video cameras mounted on the head-mounted display, and real-time video image processing in a system package that is battery powered and portable. Through a phased development approach, several generations of the LVES can be made available to the patient in a timely fashion. This paper describes the LVES project with major emphasis on technical problems encountered or anticipated during the development process.

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.

Visual acuity loss in retinitis pigmentosa. Relationship to visual field loss.

We compared visual acuity with visual field radius in 235 patients with typical retinitis pigmentosa and no evidence of other visual acuity-limiting problems (such as cataract or foveal cystoid edema). Results show a strong relationship between visual acuity loss and proximity to the fovea of the visual field border (shortest distance from the foveal center to the border of the V-4-e isopter) for these patients. Ninety-six percent of patients with central visual field radii greater than 30 degrees have visual acuities of 20/40 or better; 32% of patients with central visual field radii smaller than 10 degrees have visual acuity of 20/40 or better.

Decision making with clinical diagnostic tests in terms of statistical decision theory and information theory.

The value of a diagnostic test for an individual patient is expressed in terms of the test's ability to determine whether the patient does or does not have a disease. For diagnostic tests with a continuous diagnostic variable, various decision thresholds can be selected to adjust the sensitivity and specificity of the diagnostic test to conform with the clinician's concerns (e.g., health costs, financial costs, and diagnostic information of the test). Three methods of statistical decision theory and information theory are presented for critically evaluating the value of a clinical diagnostic test. To provide an example of the application of the three methods we critically evaluate the value of using intraocular pressure (IOP) data from three populations: open angle glaucoma, open angle glaucoma suspect, and no open angle glaucoma. Based on these methods, IOP data are shown to provide a means of indicating the higher probability of the presence of open angle glaucoma (whereupon the clinician would need to conduct further diagnostic tests), but on average IOP data are shown to have very little value as a diagnostic variable.

Diminished foveal sensitivity may predict the development of advanced age-related macular degeneration.

Visual function testing was performed on one eye with drusen from each of 18 elderly patients in 1984. Eleven patients had advanced age-related macular degeneration (AMD) in the fellow eye, and seven had only bilateral drusen. These patients were all followed prospectively (median, 45 months), at which time one eye had a new vessel membrane, three eyes had pigment epithelial detachments, and one eye had geographic atrophy. Only two of these five eyes had AMD-related visual loss in the fellow eye in 1984. The degree of loss of foveal dark-adapted sensitivity in 1984 predicted which patients developed advanced AMD with 100% sensitivity and 92% specificity. The presence of high-risk drusen characteristics in 1984 predicted the development of advanced AMD with 100% sensitivity but only 55% specificity. For this small group of patients, foveal dark-adapted sensitivity loss was an excellent predictor of the subsequent development of advanced AMD in eyes with drusen. A prospective study of a large group of patients with drusen is being undertaken to validate this finding.

Selective loss of pattern discrimination in early glaucoma.

A new perimetric pattern discrimination test was compared with conventional automated perimetry (Humphrey program 30-2 or Octopus program 32) in glaucoma patients, glaucoma suspects, and control subjects. The new test is based on the rationale that a greater percentage of retinal ganglion cells should be needed to detect a stimulus by its shape, or pattern, than by its brightness. The pattern discrimination stimulus was apatch of nonrandom dots embedded in a surrounding random dot field of the same average density. Pattern discrimination thresholds were measured by changing the degree of regularity, or coherence, of the stimulus dots. The fully coherent target was a static, 1-s duration, 20 x 20-dot checkerboard. Using a criterion-free relative operating characteristic analysis, we estimated the ability of both the pattern discrimination and conventional tests to distinguish the normal data distribution from the suspect and glaucoma distributions. The pattern discrimination test appeared to produce separations greater than conventional perimetry for glaucoma suspects and separations equivalent to conventional perimetry for glaucoma patients.

S cone pathway sensitivity in eyes with high risk and low risk drusen characteristics.

A method of measuring S cone pathway sensitivity which corrects internally for media absorption, flicker sensitivity variations, and diffuse losses in retinal sensitivity is presented. Foveal S cone pathway sensitivity was measured in elderly subjects who had no ophthalmoscopically detectable retinal abnormality other than the presence of drusen. S cone pathway sensitivity was decreased in the elderly group compared with a group of young normals. The elderly group was subdivided by drusen characteristics present in fundus photographs into a group felt to be at high risk of developing age-related macular degeneration and a group judged to be at low risk. There was a mean difference in age of 7 yr between the groups. There was a weak association between a lower S cone pathway sensitivity and the presence of high risk drusen characteristics not attributable to the age difference. Yellow flicker sensitivity better separated the high risk group from the low risk group, but this was attributable to the mean difference in age between the groups. The limited usefulness of the drusen risk classification in assessing the likelihood of developing severe age-related macular degeneration highlights the need for following a group of drusen patients prospectively to assess the true worth of S cone pathway sensitivity in this disease.

Retinal sensitivity over drusen and nondrusen areas. A study using fundus perimetry.

Elderly patients with drusen and good visual acuity have a decrease in dark-adapted retinal sensitivity in the central retina. We used the fundus camera stimulator to determine whether this sensitivity loss is caused directly by the presence of drusen. We measured retinal sensitivity over drusen and in drusen-free areas in eight patients with drusen and age-related macular degeneration (AMD). There was no significant difference in sensitivity between drusen and nondrusen areas in each patient. The sensitivity loss seen in patients with drusen thus seems to reflect a more diffuse disease of the retina and retinal pigment epithelium and is not a direct effect of drusen. More marked sensitivity losses are found in even small areas of more advanced AMD changes, suggesting that a large focal loss in retinal sensitivity may be an indicator of developing AMD changes.

Theoretical interpretation and derivation of flash-on-flash threshold parameters in visual system diseases.

Visual increment thresholds measured on backgrounds flashed on simultaneously with the test flash exhibit saturation rather than following Weber's law. These flash-on-flash thresholds have been modeled with saturating nonlinearities similar to those used to describe intensity-response functions of retinal neurons. Recently, the flash-on-flash technique has been used to assess the mechanisms of threshold elevations in visual system diseases. However, the results were interpreted in a qualitative way. This paper extends the clinical application of flash-on-flash threshold measures by providing a theoretically derived algorithm for estimating nonlinear model parameters from flash-on-flash threshold data. The performance of the algorithm is evaluated with Monte Carlo techniques, and the theoretical interpretations of the model parameters are tested experimentally. Published clinical flash-on-flash threshold data are reanalyzed using the algorithm. A previously unappreciated decrease in the half-saturation constant of retinitis pigmentosa patients was revealed by this analysis.

Relation of the normal-deviate vision receiver operating characteristic curve slope to d'.

Past studies have shown that vision receiver operating characteristic curves plotted on normal-deviate coordinates fit straight lines for which the inverse of the slope is proportional to the index of detectability d'. This Communication offers a theoretical interpretation of that empirical relationship.

Criterion-free parameter-free distribution-independent index of diagnostic test performance.

The area of the relative operating characteristic (ROC) curve is a criterion-free parameter-free distribution-independent index of the diagnostic performance of a test. The area of the ROC curve is equivalent to the probability of making the correct choice in a two-alternative forced-choice decision task. In this paper, we describe several methods for computing the area of the ROC curve from data distributions, we provide algorithms for computing confidence intervals on ROC curve areas and for performing statistical comparisons of ROC curve areas, and we show how the ROC curve areas can be used to evaluate the efficacy of different diagnostic tests or data reduction procedures. As an example of the application of the different techniques for computing the ROC curve area, we compare three distinctly different data reduction procedures applied to static visual field data from glaucoma patients, glaucoma suspects, and normal controls. Based on ROC curve areas, it is concluded that none of the three procedures is significantly superior to the others at discriminating glaucoma patients or glaucoma suspects from normals.