Robert W. Massof

Morphologic Changes in the Lamina Cribrosa Correlated with Neural Loss in Open-Angle Glaucoma

We divided 25 glaucomatous human eyes into three groups representing mild (Group 1, seven eyes), moderate (Group 2, 11 eyes), and severe (Group 3, six eyes) optic nerve damage, based on visual field testing or remaining number of optic nerve fibers. The optic nerve head of each eye was examined by scanning electron microscopy. Compression of the successive lamina cribrosa sheets was the earliest detected abnormality, occurring in some eyes before the detection of visual field loss. Backward bowing of the entire lamina cribrosa was a later change and involved its upper and lower poles more than the mid-nerve head. The diameter of the scleral opening at the level of Bruchs membrane did not enlarge in these adult glaucomatous eyes. Mechanical compression of the nerve head occurred early enough to be considered a primary pathogenetic event in glaucomatous damage.

Vector model for normal and dichromatic color vision

The inclusion of cone mechanisms in a slightly revised version of an earlier model allows accounts of phenomena that involve receptor effects as well as dichromatic color vision. Intensity-dependent parameters that stimulate the adaptation of receptors and opponent and nonopponent mechanisms are varied to predict a wide range of data for both normals and dichromats, including: (i) color matching; (ii) the approximate apparent hue and saturation of the spectrum; (iii) foveal spectral sensitivities obtained by flicker photometry and by detection in the dark and under conditions of achromatic or chromatic adaptation; (iv) heterochromatic additivity failures in the dark-adapted and chromatically adapted eye; (v) approximate differences between brightness and luminance; and, (vi) color and wavelength discrimination under varying adaptation conditions.

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.

The measurement of vision disability

The American Medical Association’s (AMA) visual efficiency scale, a vision disability metric based on visual impairment measurements, was adopted in 1925. That scale was based on a 30-year history of theoretical models in vision economics, a misinterpretation of Snellen notation for visual acuity, and an erroneous application of Weber’s psychophysical law. The AMA visual efficiency scale survived uncontested for 75 years. In 2001, the AMA adopted a new vision disability scale based on logarithmic transformations of visual acuity and visual field diameter. Like the earlier visual efficiency scale, the new scale is theoretical—it is not supported by any data that speak to the relationship between vision disability and visual impairments. Attempts to measure vision disability date to the early 1980s with the development of self-assessment visual function rating scale questionnaires. Nearly all of the questionnaires developed over the last 20 years use Likert scales, but use them incorrectly. The development of a vision disability metric based on Likert scaling parallels the historical development of other forms of measurement. A tutorial review of psychometrics—classical test theory, item response theory, and Rasch analysis—shows how vision disability measurement scales can be estimated from Likert-type visual function rating scales. We conclude that preliminary data relating measures of vision disability to measures of visual acuity and visual fields support the new AMA vision disability scale.

Two forms of autosomal dominant primary retinitis pigmentosa.

Two types of autosomal dominant retinitis pigmentosa (RP) are identified on the basis of perimetric measures of rod sensitivity relative to cone sensitivity. Type 1 dominant RP patients are characterized by an early diffuse loss of rod sensitivity with a later loss of cone sensitivity and by childhood onset of nightblindness. Type 2 dominant RP patients are characterized by a regionalized and combined loss of rod and cone sensitivity with adulthood onset of nightblindness. Comparisons of losses in the photopic and scotopic electroretinogram amplitudes corroborate the psychophysical results. Clinical findings are similar for the two dominant RP subtypes, however, there are differences in natural history.

The activity inventory: An adaptive visual function questionnaire

Purpose. The Activity Inventory (AI) is an adaptive visual function questionnaire that consists of 459 Tasks nested under 50 Goals that in turn are nested under three Objectives. Visually impaired patients are asked to rate the importance of each Goal, the difficulty of Goals that have at least some importance, and the difficulty of Tasks that serve Goals that have both some importance and some difficulty. Consequently, each patient responds to an individually tailored set of questions that provides both a functional history and the data needed to estimate the patients visual ability. The purpose of the present article is to test the hypothesis that all combinations of items in the AI, and by extension all visual function questionnaires, measure the same visual ability variable. Methods. The AI was administered to 1880 consecutively-recruited low vision patients before their first visit to the low vision rehabilitation service. Of this group, 407 were also administered two other visual function questionnaires randomly chosen from among the Activities of Daily Living Scale (ADVS), National Eye Institute Visual Functioning Questionnaire (NEI VFQ), 14-item Visual Functioning Index (VF-14), and Visual Activities Questionnaire (VAQ). Rasch analyses were performed on the responses to each VFQ, on all responses to the AI, and on responses to various subsets of items from the AI. Results. The pattern of fit statistics for AI item and person measures suggested that the estimated visual ability variable is not unidimensional. Reading-related and other items requiring high visual resolution had smaller residual errors than expected and mobility-related items had larger residual errors than expected. The pattern of person measure residual errors could not be explained by the disorder diagnosis. When items were grouped into subsets representing four visual function domains (reading, mobility, visual motor, visual information), and separate person measures were estimated for each domain as well as for all items combined, visual ability was observed to be equivalent to the first principal component and accounted for 79% of the variance. However, confirmatory factor analysis showed that visual ability is a composite variable with at least two factors: one upon which mobility loads most heavily and the other upon which reading loads most heavily. These two factors can account for the pattern of residual errors. High product moment and intraclass correlations were observed when comparing different subsets of items within the AI and when comparing different VFQs. Conclusions. Visual ability is a composite variable with two factors; one most heavily influences reading function and the other most heavily influences mobility function. Subsets of items within the AI and different VFQs all measure the same visual ability 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.

Properties of electroretinographic intensity-response functions in retinitis pigmentosa.

Dark-adapted electroretinogram (ERG) b-wave amplitudes and implicit times were recorded as a function of stimulus luminance for 15 retinitis pigmentosa (RP) patients and 15 normal subjects. B-wave amplitude as a function of log stimulus luminance was fit by non-linear regression with the Naka-Rushton equation, which has 3 independent parameters: The maximum response (Rmax), slope (n) and half-saturation constant (K). B-wave implicit-time as a function of log stimulus luminance was fit by linear regression. Compared to normal, the RP Rmax values were markedly reduced, suggesting response compression; the RP K values were elevated by an average of 0.76 log unit, suggesting relatively small losses in retinal sensitivity. There was no correspondence between Rmax and visual field area for the RP patients (coefficient of correlation = -0.02). All but 2 of the 15 RP patients had normal or shallower-than-normal implicit-time intensity-response functions, indicating that over most of the dynamic range of the ERG, the implicit-times were either normal or faster-than-normal. These results are discussed in terms of possible RP disease mechanisms.

A model of the prevalence and incidence of low vision and blindness among adults in the U.S.

Population-based vision screening studies of the prevalence rate of low vision and blindness in the U.S. are reviewed to evaluate the sources of disagreement among studies. The major reasons that studies disagree on prevalence rate estimates are differences in best-corrected visual acuity criteria for low vision and differences in the age range of the oldest age category. When corrections are made for these differences, the results of all prevalence rate studies, except the Mud Creek Valley Study, fit the same prevalence rate vs. age function. The greater prevalence rate of low vision and blindness for each age category that was observed in the Mud Creek Valley Study can be attributed to the higher prevalence rate of cataract associated with a paucity of health care services in the Mud Creek Valley population. The time-derivative of the prevalence rate vs. age function fit to the data provided an estimate of the annual incidence rate of low vision and blindness vs. age. The estimated annual incidence agreed with estimates from unpublished 8-year incidence data of the Baltimore Eye Survey. The incidence rate of low vision and blindness for Americans aged 40 to 60 years is higher among blacks than among whites. For Americans greater than age 60 years, the incidence rate for whites exceeds that for blacks. This observation probably reflects the different natural histories of glaucoma, a leading cause of low vision and blindness among black Americans, and age-related macular degeneration, a leading cause of low vision and blindness among white Americans. Using the age-dependent models of prevalence rate of low vision and blindness for white and black populations, an estimated 1.5 million Americans over age 45 years have a best-corrected visual acuity in the better eye that is ≤20/70. Based on the incidence rate estimates, approximately 240,000 new cases of low vision and blindness occur each year. With the aging of the U.S. population, that number is expected to double over the next 25 years.

Fear of Falling and Visual Field Loss from Glaucoma

OBJECTIVE To determine if visual field (VF) loss resulting from glaucoma is associated with greater fear of falling. DESIGN Prospective, observational study. PARTICIPANTS Fear of falling was compared between 83 glaucoma subjects with bilateral VF loss and 60 control subjects with good visual acuity and without significant VF loss recruited from patients followed up for suspicion of glaucoma. METHODS Participants completed the University of Illinois at Chicago Fear of Falling Questionnaire. The extent of fear of falling was assessed using Rasch analysis. MAIN OUTCOME MEASURES Subject ability to perform tasks without fear of falling was expressed in logits, with lower scores implying less ability and greater fear of falling. RESULTS Glaucoma subjects had greater VF loss than control subjects (median better-eye mean deviation [MD] of -8.0 decibels [dB] vs. +0.2 dB; P<0.001), but did not differ with regard to age, race, gender, employment status, the presence of other adults in the home, body mass index (BMI), grip strength, cognitive ability, mood, or comorbid illness (P ≥ 0.1 for all). In multivariate models, glaucoma subjects reported greater fear of falling as compared with controls (β = -1.20 logits; 95% confidence interval [CI], -1.87 to -0.53; P = 0.001), and fear of falling increased with greater VF loss severity (β = -0.52 logits per 5-dB decrement in the better eye VF MD; 95% CI, -0.72 to -0.33; P<0.001). Other variables predicting greater fear of falling included female gender (β = -0.55 logits; 95% CI, -1.03 to -0.06; P = 0.03), higher BMI (β = -0.07 logits per 1-unit increase in BMI; 95% CI, -0.13 to -0.01; P = 0.02), living with another adult (β = -1.16 logits; 95% CI, -0.34 to -1.99 logits; P = 0.006), and greater comorbid illness (β = -0.53 logits/1 additional illness; 95% CI, -0.74 to -0.32; P<0.001). CONCLUSIONS Bilateral VF loss resulting from glaucoma is associated with greater fear of falling, with an impact that exceeds numerous other risk factors. Given the physical and psychological repercussions associated with fear of falling, significant quality-of-life improvements may be achievable in patients with VF loss by screening for, and developing interventions to minimize, fear of falling.