Susan J. Leat

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.

Outcome of low vision aid provision: the effectiveness of a low vision clinic.

Purpose. Although there is an increasing need for primary low vision (LV) care, few studies have considered the success rates of optometric LV rehabilitation. We considered the objective success and perceived benefit obtained by 57 elderly LV patients. Method. Tests of reading speed and questionnaires were administered in the patients home after initial and follow-up visits to a LV clinic. Additional information was taken from the patients clinic record. Results. Benefits from attending the clinic were reported by 89.5% of patients and 81% of patients were regularly using low vision aids (LVAs). There was a discrepancy between ability to read 1M print in the clinic (75% of patients) and the reported ability to read regular-sized print at home (35%). Perceived benefit from visiting the clinic was strongly associated with the ability to perform daily living tasks and to read 2M print. There was some association between perceived benefit and frequency of using the LVAs, but not with duration of use. Conclusion. The results encourage a change in emphasis during LV assessments from sustained reading to the ability to perform daily living activities

THE VALIDITY OF CURRENT CLINICAL TESTS OF CONTRAST SENSITIVITY AND THEIR ABILITY TO PREDICT READING SPEED IN LOW VISION

Purpose: Contrast sensitivity (CS) testing using chart tests of CS is becoming increasingly common in low vision assessment. Yet we know little about the validity of these charts, i.e. which region of the spatial frequency spectrum is being measured. In this study we aimed to determine the validity of currently available CS charts by comparison against oscilloscope-based CS. We also determined their relative ability to predict reading speed.Methods: CS was measured with five commercially available charts and the contrast sensitivity function was determined with sinusoidal gratings presented on a Joyce screen using a two-alternative forced choice staircase technique in 36 observers with low vision and 3 with normal vision. Reading rate was also measured with the subject reading with his or her own optical low vision aid.Results: The results show that the Pelli-Robson chart and the Cambridge gratings are good measures of medium to low spatial frequencies, as would be predicted from their design, while the Regan and UW charts correlated with medium to high frequencies. The Vistech chart was a good predictor of CS at each spatial frequency.Conclusions: The best chart test of CS depends on which region of the CS curve is of interest. All the charts were good predictors of reading rate.

Reduced accommodation in children with cerebral palsy

Accommodation in 43 subjects with cerebral palsy was measured objectively using a dynamic retinoscopy technique, which has already been shown to be reliable and repeatable. The subjects ages ranged from 3 to 35 years. Of these, 42% were found to have an accommodative response pattern which was different from the normal control group for his/her age. Nearly 29% had an estimated amplitude of accommodation of 4 D or less. The presence of reduced accommodation was found to be associated with reduced visual acuity, but was not associated with cognitive or communication ability, refractive error or age. The prevalence of other ocular disorders in this group is also high. These findings have developmental and educational implications.

The technique, validity and clinical use of the sweep VEP

Vision scientists have concentrated on studying two visual functions when it comes to assessing the sensory visual development in human: visual acuity and contrast sensitivity. The methods used to measure these visual functions can be either behavioral or electrophysiological. A relatively new technique for measuring the visual acuity and contrast sensitivity electrophysiologically is the sweep visual evoked potential (sVEP). This paper is a review of the literature on the sVEP technique: stimulus parameters, threshold determination, validity and reliability of sVEP are discussed. Different studies using the sVEP to study the development of visual acuity, contrast sensitivity, and vernier acuity are presented. Studies have demonstrated that the sVEP is a potentially important tool for assessing visual acuity and contrast sensitivity in non‐verbal individuals with disorders affecting their visual system.

Dyslexia: a review of two theories

Optometrists will frequently see patients, who may have a diagnosis or a suspected diagnosis of dyslexia (specific reading disorder) and will need to manage and counsel such patients. There are many propounded theories on the cause(s) of dyslexia. Although most professionals in this area consider that dyslexia is chiefly a linguistic disorder, the possibility of a visual component is contentious. This article is a selective review of two commonly discussed theories that suggest a visual component in dyslexia; the magnocellular deficit theory and Meares‐Irlen syndrome.

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.

To prescribe or not to prescribe? Guidelines for spectacle prescribing in infants and children

This paper discusses the considerations for prescribing a refractive correction in infants and children up to and including school age, with reference to the current literature. The focus is on children who do not have other disorders, for example, binocular vision anomalies, such as strabismus, significant heterophoria or convergence excess. However, refractive amblyogenic factors are discussed, as is prescribing for refractive amblyopia. Based on this discussion, guidelines are proposed, which indicate when to prescribe spectacles and what amount of refractive error should be corrected. It may be argued that these are premature because there are many questions that remain unanswered and we do not have the quality of evidence that we would like; the clinician, however, must make decisions on whether and what to prescribe when examining a child. These guidelines are to aid clinicians in their current clinical decision making.This paper discusses the considerations for prescribing a refractive correction in infants and children up to and including school age, with reference to the current literature. The focus is on children who do not have other disorders, for example, binocular vision anomalies, such as strabismus, significant heterophoria or convergence excess. However, refractive amblyogenic factors are discussed, as is prescribing for refractive amblyopia. Based on this discussion, guidelines are proposed, which indicate when to prescribe spectacles and what amount of refractive error should be corrected. It may be argued that these are premature because there are many questions that remain unanswered and we do not have the quality of evidence that we would like; the clinician, however, must make decisions on whether and what to prescribe when examining a child. These guidelines are to aid clinicians in their current clinical decision making.

Clinical testing of contrast sensitivity in children: age-related norms and validity.

Purpose. The Hiding Heidi (HH) test and the LEA low-contrast symbols are two commercially available charts of contrast sensitivity (CS) for children. However, there are no published normal data and no indication of how CS measured by these charts relates to other measures of CS. In this study, normal age-related data for both tests are reported, and validity against the Pelli-Robson (PR) chart is measured. Methods. Eighty-eight normally sighted children were divided into four age groups: 1 to <2.5 years, 2.5 to <4 years, 4 to <6 years, and 6 to <8 years. An adult group with normal vision and with low vision also took part. CS was measured with the HH test, the LEA symbols at 1 m and 28 cm, and the PR chart, as the child’s ability permitted. Because there were obvious differences between the contrast levels of the PR chart and the nominal contrast for the children’s charts, each contrast level for the children’s tests was recalibrated. Results. The HH test and the LEA symbols at 28 cm and 1 m all showed a floor effect; that is, most children of all ages correctly responded to the lowest contrast. The median CS for the LEA symbols at 28 cm and 1 m was 2.22 log CS, which was 1.65 when recalibrated. There was a significant difference of PR CS between the 6- to <8-year-olds and adults (p < 0.001). Of the children’s charts, the LEA symbols at 28 cm, once recalibrated for contrast, had the best agreement with the PR chart. Conclusion. The LEA and HH charts cannot measure a true contrast threshold for children with normal vision because of the floor effect. The LEA symbols at 28 cm gave the most useful information, once recalibrated for contrast, and may be useful to predict performance of children with low vision, when CS is likely to be compromised.

Generic and customised digital image enhancement filters for the visually impaired

This study compares the effectiveness of various image enhancement filters for improving the perceived visibility of coloured digital natural images for people with visual impairment. Generic filters were compared with Pelis adaptive enhancement and adaptive thresholding and custom-devised filters based on each subjects contrast sensitivity loss. Subjects with low vision made within filter rankings followed by between filter ratings. In general, subjects preferred filters with lower gains. Unsharp masking resulted in a significant increase in perceived visibility for some image types (p < or = 0.05) while Pelis adaptive enhancement, edge enhancement and histogram equalization resulted in borderline improvements. Adaptive thresholding and the custom devised filter did not result in overall improvements in perceived visibility.