Is it time to confine Snellen charts to the annals of history?

PURPOSE: A paper published by the author in 1988 in this journal provided some important findings about the lack of precision of visual acuity (VA) measures made with commonly used Snellen charts and the advantages of using letter charts designed using the principles proposed by Bailey and Lovie in 1976. That 1988 paper has been cited a number of times since, mostly supporting the findings. The purpose of this review is to examine the changes that have occurred in VA measurement in research and clinical practice since that earlier study. RECENT FINDINGS: While precise measures of VA using Bailey-Lovie or ETDRS charts are now commonly used in major, multi-centre research studies, it is disappointing to see that many research papers still report VA measured with Snellen charts and even use Snellen fractions, invalidly converted to logMAR notation, in parametric analyses of VA. Many studies have examined the test-retest variability (TRV) of VA measures in groups and individuals, but it is difficult to determine if clinicians or researchers determine patients' individual TRVs to more accurately detect real changes in VA for each individual. SUMMARY: This paper summarises the findings of the 1988 study: (1) Snellen charts and VA notations are not appropriate for accurate clinical and research measures of VA; (2) Charts employing the Bailey-Lovie design principles should be used to provide precise measures of VA. (3) Test-retest variability should be used to determine the limit for detecting significant change in VA. This author suggests that it is time for Snellen charts, Snellen fractions and decimal notation to be confined to the teaching of the history of VA measurement. A request is also made to stop the use of the redundant term 'best corrected' VA (BCVA). Recommended procedures are given for precise measures of VA and accurately monitoring changes in VA in clinical practice and research.

Visual acuity testing. From the laboratory to the clinic.

The need for precision in visual acuity assessment for low vision research led to the design of the Bailey-Lovie letter chart. This paper describes the decisions behind the design principles used and how the logarithmic progression of sizes led to the development of the logMAR designation of visual acuity and the improved sensitivity gained from letter-by-letter scoring. While the principles have since been adopted by most major clinical research studies and for use in most low vision clinics, use of charts of this design and application of letter-by-letter scoring are also important for the accurate assessment of visual acuity in any clinical setting. We discuss the test protocols that should be applied to visual acuity testing and the use of other tests for assessing profound low vision when the limits of visual acuity measurement by letter charts are reached.

Inferior field loss increases rate of falls in older adults with glaucoma.

PURPOSE: To examine the visual predictors of falls and injurious falls among older adults with glaucoma. METHODS: Prospective falls data were collected for 71 community-dwelling adults with primary open-angle glaucoma (mean age, 73.9 ± 5.7 years) for 1 year using monthly falls diaries. Baseline assessment of central visual function included high-contrast visual acuity and Pelli-Robson contrast sensitivity. Binocular integrated visual fields were derived from monocular Humphrey Field Analyzer plots. Rate ratios (RR) for falls and injurious falls with 95% confidence intervals (CIs) were based on negative binomial regression models. RESULTS: During the 1-year follow-up, 31 (44%) participants experienced at least one fall and 22 (31%) experienced falls that resulted in an injury. Greater visual impairment was associated with increased falls rate, independent of age and gender. In a multivariate model, more extensive field loss in the inferior region was associated with higher rate of falls (RR, 1.57; 95% CI, 1.06 to 2.32) and falls with injury (RR, 1.80; 95% CI, 1.12 to 2.98), adjusted for all other vision measures and potential confounding factors. Visual acuity, contrast sensitivity, and superior field loss were not associated with the rate of falls; topical beta-blocker use was also not associated with increased falls risk. CONCLUSIONS: Falls are common among older adults with glaucoma and occur more frequently in those with greater visual impairment, particularly in the inferior field region. This finding highlights the importance of the inferior visual field region in falls risk and assists in identifying older adults with glaucoma at risk of future falls, for whom potential interventions should be targeted.

Inferior visual field reductions are associated with poorer functional status among older adults with glaucoma.

PURPOSE: To examine the relationship between visual impairment and functional status in a community-dwelling sample of older adults with glaucoma. METHODS: This study included 74 community-dwelling older adults with open-angle glaucoma (aged 74 ± 6 years). Assessment of central vision included high-contrast visual acuity and Pelli-Robson contrast sensitivity. Binocular integrated visual fields were derived from merged monocular Humphrey Field Analyser visual field plots. Functional status outcome measures included physical performance tests (6-min walk test, timed up and go test and lower limb strength), a physical activity questionnaire (Physical Activity Scale for the Elderly) and an overall functional status score. Correlation and linear regression analyses, adjusting for age and gender, examined the association between visual impairment and functional status outcomes. RESULTS: Greater levels of visual impairment were significantly associated with lower levels of functional status among community-dwelling older adults with glaucoma, independent of age and gender. Specifically, lower levels of visual function were associated with slower timed up and go performance, weaker lower limb strength, lower self-reported physical activity, and lower overall functional status scores. Of the components of vision examined, the inferior visual field and contrast factors were the strongest predictors of these functional outcomes, whereas the superior visual field factor was not related to functional status. CONCLUSIONS: Greater visual impairment, particularly in the inferior visual field and loss of contrast sensitivity, was associated with poorer functional status among older adults with glaucoma. The findings of this study highlight the potential links between visual impairment and the onset of functional decline. Interventions which promote physical activity among older adults with glaucoma may assist in preventing functional decline, frailty and falls, and improve overall health and well-being.

Visual field size criteria for mobility rehabilitation referral.

PURPOSE: To investigate evidence-based visual field size criteria for referral of low-vision (LV) patients for mobility rehabilitation. METHODS: One hundred and nine participants with LV and 41 age-matched participants with normal sight (NS) were recruited. The LV group was heterogeneous with diverse causes of visual impairment. We measured binocular kinetic visual fields with the Humphrey Field Analyzer and mobility performance on an obstacle-rich, indoor course. Mobility was assessed as percent preferred walking speed (PPWS) and number of obstacle-contact errors. The weighted kappa coefficient of association (κr) was used to discriminate LV participants with both unsafe and inefficient mobility from those with adequate mobility on the basis of their visual field size for the full sample and for subgroups according to type of visual field loss and whether or not the participants had previously received orientation and mobility training. RESULTS: LV participants with both PPWS <38% and errors >6 on our course were classified as having inadequate (inefficient and unsafe) mobility compared with NS participants. Mobility appeared to be first compromised when the visual field was less than about 1.2 steradians (sr; solid angle of a circular visual field of about 70° diameter). Visual fields <0.23 and 0.63 sr (31 to 52° diameter) discriminated patients with at-risk mobility for the full sample and across the two subgroups. A visual field of 0.05 sr (15° diameter) discriminated those with critical mobility. CONCLUSIONS: Our study suggests that: practitioners should be alert to potential mobility difficulties when the visual field is less than about 1.2 sr (70° diameter); assessment for mobility rehabilitation may be warranted when the visual field is constricted to about 0.23 to 0.63 sr (31 to 52° diameter) depending on the nature of their visual field loss and previous history (at risk); and mobility rehabilitation should be conducted before the visual field is constricted to 0.05 sr (15° diameter; critical).

Does a line guide improve reading performance with stand magnifiers?

PURPOSE: When reading with a stand magnifier (SM), navigation along each line of text and retracing back to the correct position at the beginning of the next line has been suggested as a major difficulty for people with low vision. In this study, we evaluated the immediate impact of using a simple and inexpensive line guide on navigation performance. METHODS: Twenty-nine participants with age-related macular degeneration read short passages of text using their habitual SM with and without a temporary line guide attached. Magnifier movements were recorded using a 3 SPACE Isotrak system. Reading time, magnifier movement strategies, navigation times, and navigation errors were determined. A short questionnaire was used to quantify participants' perceived difficulties with page navigation and their preference for reading with or without the line guide. RESULTS: For some participants, the line guide improved the control of the vertical positioning of the SM when reading along a line (p = 0.01), but it increased the number of corrective vertical movements at the end of the retrace (p = 0.001). There was a small but significant decrease (about 6 wpm) in reading speed and increase in navigation times (p < 0.05) when using the line guide; however, 48% participants indicated a preference for reading with it attached to their SM. There was a trend (p = 0.08) for those who preferred the line guide to report greater habitual difficulties with SM manipulation. CONCLUSIONS: After only minimal instruction in how to use the line guide, forward navigation control improved, but the design of the guide made retracing the SM to the start of the next line more difficult resulting in slower reading speeds. Nevertheless, 48% of participants expressed a preference for having the line guide attached to their SM. Improvements to the design of the line guide and strategies that may improve retrace navigation performance are suggested.

Visual function, visual attention, and mobility performance in low vision.

PURPOSE: The aim of this study was to determine if useful field of view (UFV) measures help to predict aspects of orientation and mobility in people with visual impairment. The UFV is a composite measure of visual attention, ability to detect objects in the presence of clutter and basic visual functions such as visual field loss and contrast sensitivity. METHODS: Thirty-five participants aged 20 to 80 years with low vision due to a variety of visual disorders took part. Mobility around a partly indoor and exterior real-life mobility course was measured, together with UFV and clinical measures of contrast sensitivity (CS), visual fields, and visual acuity. Two series of models were considered; series 1 using the UFV scores as measured and series 2 using the UFV scores corrected for visual field loss (only counting errors in areas of intact visual field). RESULTS: UFV was found to be an important predictor of some aspects of mobility performance. Mobility errors were best predicted by uncorrected UFV (R = 0.38), although CS was also a good predictor. Walking speed and preferred walking speed (PWS) were best predicted by uncorrected UFV and age (R = 0.575 and 0.573, respectively). The visual detection distance and visual identification distances were best predicted by clinical vision measures, such as contrast sensitivity, visual fields, and central vision function. The percent PWS was not predicted by any of the measures we used. None of these models was improved by the addition of the corrected UFV scores. CONCLUSIONS: These results indicate that attention and the presence of distractors, as well as visual function and age, are important factors in orientation and mobility performance, in particular mobility errors, walking speed and PWS.

Visual impairment and postural sway among older adults with glaucoma.

PURPOSE: To investigate the effect of visual impairment on postural sway among older adults with open-angle glaucoma. METHODS: This study included 54 community-dwelling participants with open-angle glaucoma, aged 65 and older. Binocular visual field loss was estimated from merged monocular Humphrey Field Analyzer visual field results and retinal nerve fiber layer (RNFL) thickness was obtained from the Stratus Optical Coherence Tomographer. Postural sway was measured under four conditions: eyes open and closed, on a firm, and a foam surface. Data were collected for additional vision measures (visual acuity and contrast sensitivity), physical performance measures (self-reported physical activity levels and 6-min walk test), and demographic measures (age, gender, body mass index, and medical history). Multivariate linear regressions, adjusting for confounding factors, were performed to determine the association between visual loss and postural sway. RESULTS: Participants with greater binocular visual field loss or thinner RNFL thickness showed increased postural sway, both on firm and foam surfaces, independent of age, gender, body mass index, and physical performance levels. These visual loss measures were significant predictors of postural sway, explaining almost 20% of its variance on the foam surface. Furthermore, participants with greater inferior hemifield visual field loss showed increased postural sway on the foam surface. Increasing glaucomatous visual impairment was accompanied by a steady decrease of the visual contribution to postural control. CONCLUSION: Among older adults with glaucoma, greater visual field loss or thinner RNFL thickness is associated with reduced postural stability. This postural instability may be a contributing factor in the increased risk of falls among older adults with glaucoma.

Reading with optical magnifiers: page navigation strategies and difficulties.

PURPOSE: To read efficiently with a simple hand or stand magnifier, people with visual impairment have to move (navigate) the device along each line (forward phase) and back to the correct position at the start of the next line (retrace phase). Page navigation difficulties have been implicated as limiting factors when reading with hand and stand magnifiers, but have not been objectively measured. METHODS: Magnifier movements were recorded using a 3SPACE Isotrak system for 43 participants with age-related macular degeneration (AMD) who read two short stories using their habitual hand or stand magnifier. Page navigation was quantified in terms of magnifier movements and navigation errors for the forward and retrace phases. Visual acuities and visual fields were measured, and magnifier usage and page navigation difficulties were surveyed. RESULTS: During the forward phase, participants primarily used either a straight (47%) or diagonal downward (46%) movement, whereas during the retrace phase, the majority (56%) used a downward movement. On average, forward navigation time was four times longer than retrace navigation time (p<0.001). The most common navigation error was incorrect positioning of the magnifier at the end of the retrace movement. Near word acuity correlated strongly with forward time (r=0.78), and moderately with retrace time (r=0.53) and forward errors (r=0.50). Vertical field of view correlated with retrace errors (r=-0.53). Participants' estimates of page navigation difficulties were not predictive of objective measures of performance. CONCLUSIONS: We quantified page navigation strategies and difficulties of people with AMD reading with magnifiers. Retrace, which presents the most common difficulty, is not well predicted by vision measures or magnifier characteristics; future studies should investigate the relationship between motor skills and navigation performance, and the impact of training or devices on reducing retrace navigation difficulties.

Measuring mobility performance: experience gained in designing a mobility course.

BACKGROUND: This paper reviews the most common methods of measuring and scoring orientation and mobility (O and M) and the effects of visual impairment on O and M. We discuss the difficulties inherent in designing a 'real-world' course to measure O and M and we describe the course that we finally used. METHODS: Thirty-five participants in two age groups, with low vision due to a variety of disorders, took part in mobility trials on the final version of the course. Aspects of visual function were measured. RESULTS: Factor analysis indicated that mobility errors, visual detection distance and visual identification distance were grouped with measures of visual acuity, contrast sensitivity and Humphrey visual field mean deviation, while preferred walking speed and walking speed were separately grouped. Humphrey pattern standard deviation did not group with any other measure and neither did percentage preferred walking speed. This study is in agreement with other studies that visual field and contrast sensitivity, sometimes with low contrast visual acuity, were the best clinical visual predictors of mobility performance. Based on our experiences we present a number of recommendations for designing courses for assessing mobility. CONCLUSIONS: For future studies, it would behove researchers to include a range of mobility measures, until further understanding is gained about how they are interrelated and contribute information on the relationship among mobility, vision and other individual factors.

Short-term in-office practice improves reading performance with stand magnifiers for people with AMD.

PURPOSE: People with low vision often use optical low vision aids to assist reading. There have been numerous training programs recommended to train people using magnifiers for reading. However, most of the programs are time consuming and labor intensive. In this study, we investigated the effects of home-based large print reading practice on reading performance when stand magnifiers (STM's) are first prescribed. METHODS: Thirty-two subjects with age-related macular degeneration (AMD) and with minimal experience in using magnifiers for reading were recruited. They were divided into three groups: control, practice 1 (P1), and practice 2 (P2). Before the prescription of STM's, all the subjects were given the same amount of in-office practice with the STM (weeks 0 to 2). In addition, in these 2 weeks, P1 and P2 subjects were given large print books to read daily at home. P2 subjects were required to read the large print books through a reduced field of view. The control group subjects received no additional reading practice. Reading rates with and without STM's on passages of text were assessed for all the subjects regularly for 20 weeks. RESULTS: There were no significant differences between the control, P1, and P2 groups in the increase in reading rate with STM (p = 0.29). At week 0, reading rate for small print with STM was significantly slower than reading rate on the equivalent-sized large print (p = 0.004); however, as time went on, reading rate with STM's increased significantly (p = 0.02). After 2 weeks of in-office magnifier practice and repeated measures of reading rate with STM, reading rate with STM had improved such that it was not significantly different from reading rate on large print (p = 0.11). CONCLUSION: Supervised, short-term, in-office practice with the magnifier was effective in improving magnifier reading performance to achieve maximum reading rate. Additional large print reading practice did not result in any greater improvement in reading rate than in-office magnifier practice alone.

The development of the LV Prasad-Functional Vision Questionnaire: a measure of functional vision performance of visually impaired children.

PURPOSE: To develop a reliable and valid questionnaire (the LV Prasad-Functional Vision Questionnaire, LVP-FVQ) to assess self-reported functional vision problems of visually impaired school children. METHODS: The LVP-FVQ consisting of 19 items was administered verbally to 78 visually impaired Indian school children aged 8 to 18 years. Responses for each item were rated on a 5-point scale. A Rasch analysis of the ordinal difficulty ratings was used to estimate interval measures of perceived visual ability for functional vision performance. RESULTS: Content validity of the LVP-FVQ was shown by the good separation index (3.75) and high reliability scores (0.93) for the item parameters. Construct validity was shown with good model fit statistics. Criterion validity of the LVP-FVQ was shown by good discrimination among subjects who answered "seeing much worse" versus "as well as"; "seeing much worse" versus "as well as/a little worse" and "seeing much worse" versus "a little worse," compared with their normal-sighted friends. The task that required the least visual ability was "walking alone in the corridor at school"; the task that required the most was "reading a textbook at arm's length." The estimated person measures of visual ability were linear with logarithm of the minimum angle of resolution (logMAR) acuity and the binocular high contrast distance visual acuity accounted for 32.6% of the variability in the person measure. CONCLUSIONS: The LVP-FVQ is a reliable, valid, and simple questionnaire that can be used to measure functional vision in visually impaired children in developing countries such as India.

Vision and mobility performance of subjects with age-related macular degeneration.

PURPOSE: To investigate the effects of age-related macular degeneration (ARMD) on mobility performance and to identify the vision determinants of mobility in subjects with ARMD. METHODS: Walking speed and the number of obstacle contacts made on a 79-m indoor mobility course were measured in 21 subjects with ARMD and 11 age-matched subjects with normal vision. The mobility measures were transformed to percentage preferred walking speed and contacts score. The vision functions assessed included binocular visual acuity, contrast sensitivity, and visual field. RESULTS: In this study, subjects with ARMD did not walk significantly slower or make significantly more obstacle contacts on the mobility course than the normally sighted subjects of similar age. Between 29% and 35% of the variance in the ARMD mobility performance was accounted for by visual field and contrast sensitivity measures. The most significant predictor of mobility performance scored as percentage preferred walking speed was the size of a binocular central scotoma. CONCLUSION: As the size of a binocular central scotoma increases, mobility performance decreases.

Determining magnification for reading with low vision.

BACKGROUND: In the past, practitioners have used distance and/or near visual acuity (VA) to calculate required magnification for low vision aids. Magnification was usually under-estimated when compared with the final magnification prescribed. Recent studies have emphasised the importance of acuity reserve in determining the required magnification for optimum reading rate. Two different approaches have been proposed for the appropriate acuity reserve to use in calculating magnification. These are a fixed acuity reserve of 0.3 log unit or an individual determination of optimum acuity reserve. The aim of this study was to investigate the magnification and reading rates with low vision aids selected by the two methods. METHODS: Nineteen low vision subjects with age-related macular degeneration (AMD) who were experienced magnifier-users were recruited. Reading rates and near VA with low vision aids determined by the fixed and individual acuity reserve methods were compared with the same measures made with the subjects' own magnifiers. RESULTS: There were no significant differences in reading rate and near VA measured with low vision aids selected by either the fixed or individual acuity reserve methods or the subjects' own magnifiers. Reading rate with low vision aids was not significantly different from reading rate for large print with conventional near additions. Thus, for experienced users, magnifiers do not cause reduced reading rate. CONCLUSIONS: The fixed acuity reserve method is simple to apply as only near VA and print size of the target reading task are required. For the individual acuity reserve method, reading rates at different print sizes need to be measured. We recommend the use of a fixed acuity reserve (0.3 log unit) for the calculation of required magnification for low vision patients. If near VA or reading rate are not satisfactory with the magnification calculated by this method, individual assessment of required acuity reserve is necessary.

Reading performance in children with low vision.

PURPOSE: While educators and optometrists are concerned with reading efficiency in children with low vision, in most low vision assessments, children's reading performance is assessed only by a brief evaluation of reading fluency. We examined the relationships between clinical vision measures and reading performance in children with low vision. METHODS: Subjects were 71 students with low vision, aged seven to 18 years. The vision and reading performance measures were: high contrast distance visual acuity, contrast sensitivity using both the Pelli-Robson and LH symbol charts, near text visual acuity and reading rates on a range of print sizes. RESULTS: Most children achieved maximum reading rate at print sizes between 2.5 and seven times larger than threshold print size. Maximum reading rate increased significantly with age and near visual acuity. There was no significant relationship between reading rate and contrast sensitivity. CONCLUSIONS: The majority of children with low vision can achieve near normal reading rates with magnification. Age is the strongest predictor of reading rate in children with low vision. They need print sizes well above threshold to achieve maximum reading rate. Routine contrast sensitivity testing of visually impaired students is not indicated for reading assessment.

Multi-disciplinary low vision care.

Case reports for three patients with low vision due to different ocular diseases are presented. In each case, a different rehabilitation professional was the main provider of the low vision care needed to meet the patient's goals. However, in all cases the patients benefited from the services of a range of professionals, agencies or groups for visually impaired people. The importance of defining patients' specific goals, referral to multi-disciplinary low vision services and regular reviews of vision and visual tasks is highlighted.

Preferred walking speed for assessment of mobility performance: sighted guide versus non-sighted guide techniques.

BACKGROUND: If visually impaired people had their sight fully restored, it is assumed that they would walk at their optimal speeds. Previous mobility studies have used preferred walking speed (PWS) to measure walking efficiency of visually impaired adults. Therefore, the actual speeds that visually impaired people adopt on a route can be expressed as a percentage of their preferred walking speed (PPWS). There have been two approaches used in previous studies for determining preferred walking speed. 1. the sighted guide technique (SG), which requires a subject to walk with a sighted guide and regard the latter as a perfect mobility aid, with the subject setting the preferred (optimal) walking pace 2. the non-sighted guide technique (NonSG), which requires a subject to walk alone along an unobstructed straight path for a certain distance, adopting the preferred (optimal) walking speed. There is some debate on which technique is the better. This study was conducted to determine if there is any difference between the two techniques for determining visually impaired subjects' preferred walking speeds. METHODS: Fourteen visually impaired adult subjects were recruited. PWS was determined by recording the time taken for a subject to walk an unobstructed, straight 20-metre corridor path using each technique. RESULTS: There was no significant difference in PWS using the sighted guide and non-guided techniques. CONCLUSIONS: Either the SG or NonSG techniques can be used to determine PWS for a heterogeneous group of visually impaired subjects.

Prescribing near magnification for low vision patients.

Most patients with low vision request help with reading. Despite improvements in the assessment of visual functions, determining the appropriate magnification for reading still often involves a trial and error approach. Recent research has shown that with accurate and systematic assessments of vision, the required magnification can be predicted but this magnification needs to be much higher than has been previously recommended. This paper presents a systematic approach to enable practitioners to determine the power of a near addition or simple magnifier needed to assist patients with mild or moderate low vision who may present seeking help for reading. Guidelines for appropriate referral to low vision services are also provided.