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.     

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.     

Understanding how simple magnifiers provide image enlargement

The simple magnifier is a positive power, usually single lens placed between the page and the eye to give an enlarged image. Its ‘magnification’ is typically expressed as its dioptric power divided by four but this presumes that the reader uses the magnifier in a specific way. Magnifiers rarely meet their rated specification so they often disappoint the expectations of both patients and eye‐care practitioners for vision improvement. In this paper, I describe the basic optics of the simple magnifier and a protocol for the provision of a suitable magnifier to meet general vision needs. Advice on the use of a magnifier should be given to patients at the time of prescribing, thus ensuring practical outcomes for vision improvement and success in meeting the patient's vision needs.     

Low vision correction for high myopia.

High myopia is favorable for low vision corrections because the vision has usually been poor since childhood and the person has been able to read at close range, producing magnification. For near vision, in addition to bringing objects close, magnification is produced by high-add bifocals, trifocals, strong near vision spectacles and nonspectacle magnifiers. Hand magnifiers held at a distance from the eye are used with the spectacle correction. This paper describes the Visolett, used without correction or near vision correction, as the preferred non-spectacle magnifier because the reading area is self-illuminated, it doubles the approach magnification, rests on the paper, is simple to use, produces binocular vision, and attracts the least attention. Combination method of magnification consists in the use of the Visolett in addition to a spectacle magnifier, which doubles the magnification.     

Near Vision Aids In Geriatric Patients With Reduced Visual Acuity

ABSTRACT The usefulness of near vision aids was studied in one hundred and forty-one patients in a geriatric hospital. The patients were over 65 years of age and had reduced vision. Where lens power exceeded +10DS spectacle lenses seldom worked well. Aspheric stand magnifiers were the most useful device in cases where greater magnification was needed. Elderly patients had difficulty in adapting to optical aids over 5X. Impaired cerebral function was the main reason for failure of the low vision aids of lower magnification.     

Rehabilitative approach in patients with ring scotoma

Objective

To investigate the rehabilitative approach in patients with ring scotoma. A central scotoma is characteristic for patients with age-related macular degeneration (AMD). Sometimes patients with AMD maintain a residual central vision area within the scotoma (ring scotoma).

Design

Prospective, nonrandomized case series.

Participants

Twenty-four patients with AMD.

Methods

A formal low-vision assessment was performed for all study patients. The assessment included best corrected visual acuity (BCVA), contrast sensitivity, reading speed, and microperimetry. All patients were provided a low-vision assessment to satisfy patients’ needs. Devices were prescribed accordingly.

Results

The BCVA found was 0.4 logMAR (SD 0.1). All had central and stable fixation. Residual central retinal area size and sensitivity measured 2.4° (SD 0.8) and 3.1 dB (SD 0.8), respectively. Twenty patients achieved better vision with optical magnification in the eye with ring scotoma. Mean reading speed achieved was 50.2 words/min (SD 20.9). A linear correlation was found for reading speed with both central area sensitivity (r² = 0.5, p < 0.05) and contrast sensitivity (r² = 0.3, p < 0.05).

Conclusions

In patients with AMD with ring scotoma, moderate amounts of magnification seem to provide satisfactory rehabilitation outcomes. Central retinal spared area sensitivity may predict reading speed outcomes, whereas residual central area size is likely to be useful in determining magnification.     

Low vision management of retinitis pigmentosa.

The low vision treatment of patients with retinitis pigmentosa (RP) has evolved through an understanding of the needs of the patient with extreme peripheral field constriction. Patient care involves four main areas: 1) best refraction and simple magnification, including the possibility of electronic type magnification for reading; 2) control of glare and determination of the proper illumination for reading and mobility; 3) use of field enhancement procedures for sighting and for mobility, including the understanding of cane travel and scanning techniques; 4) additional counseling and ancillary care. Through the methods described, patients who might have been poor prospects for low vision care 15 or 20 years ago, may now be able to function at much higher levels.     

The effect of relative distance enlargement on visual acuity in the visually impaired

Background : Prescribing magnification is typically based on distance or near visual acuity. This presumes a constant minimum angle of visual resolution with working distance and therefore enlargement of an object moved to a shorter working distance (relative distance enlargement). This study examines this premise in a visually impaired population. Methods : Distance letter visual acuity was measured prospectively for 380 low vision patients (distance visual acuity between 0.3 and 2.1 logMAR) over the age of 57 years, along with near word visual acuity at an appropriate distance for near lens additions from +4 D to +20 D. Demographic information, the disease causing low vision, contrast sensitivity, visual field and psychological status were also recorded. Results : Distance letter acuity was significantly related to (r = 0.84) but on average 0.1 ' 0.2 logMAR better (1 ' 2 lines on a logMAR chart) than near word acuity at 25 cm with a +4 D lens addition. In 39.8 per cent of patients, near word acuity was more than 0.1 logMAR worse than distance letter acuity. In 11.0 per cent of subjects, near visual acuity was more than 0.1 logMAR better than distance letter acuity. The group with near word acuity worse than distance letter acuity also had lower contrast sensitivity. The group with near word acuity better than distance letter aculty was less likely to have age‐related macular degeneration. Smaller print size could be read by reducing working distance (achieved by using higher near lens additions) in 86.1 per cent, although not by as much as predicted by geometric progression in 14.5 per cent. Discussion : Although distance letter and near word acuity are highly related, they are on average 1 logMAR line different and this varies significantly between individuals. Near word acuity did not increase linearly with relative distance enlargement in approximately one in seven visually impaired, suggesting that the measurement of visual resolution over a range of working distances will assist appropriate prescribing of magnification aids.     

Improvement of reading speed after providing of low vision aids in patients with age‐related macular degeneration

Purpose: Age‐related macular degeneration (AMD) is the most common cause of severe visual impairment, including loss of reading ability, among elderly persons in developed countries. The aim of the present study was to evaluate reading ability before and after providing of appropriate low vision aids. Methods: Five hundred and thirty patients with different stages of AMD (age 82 ± 8 years) were included in this retrospective study. All patients underwent a standardized ophthalmological examination including evaluation of magnification requirement and careful providing of low vision aids. Before and after the provision of low vision aids, reading speed [words per minute (wpm)] was evaluated using standardized reading texts. Results: For the whole group, the average best‐corrected distance visual acuity of the better eye was 0.18 ± 0.15, with 69% of patients having visual acuity of 0.1 (20/200) or better. The mean magnification requirement was 7.4 ± 6.3‐fold (range 2–25). Visual rehabilitation was achieved with optical visual aids in 58% of patients, whereas 42% of patients needed electronically closed‐circuit TV systems. Mean reading speed was 20 ± 33 wpm before and increased significantly to 72 ± 35 (p < 0.0001) after the provision of low vision aids for the whole group. Between patients with visual acuity < 0.1 and patients with visual acuity of 0.1 or better, there are highly significant differences in reading speed before (0.4 ± 3.8 versus 20 ± 28 wpm, p ≤ 0.0001) and after providing of visual aids (40 ± 13 versus 84 ± 30 wpm, p ≤ 0.0001). Patients with severe visual impairment (visual acuity ≤ 0.1) showed significantly lower improvement of reading speed compared to patients with visual acuity of 0.1 or better following rehabilitation (p ≤ 0.0001). Before providing of low vision aids, only 16% of patients were able to read; in contrast, reading ability was achieved in 94% of patients after the provision of low vision aids for the whole group. Conclusion: Our results indicate the great value of low vision rehabilitation through adequate providing of vision aids for the improvement of reading ability, with a highly significant increase of reading speed without training of eccentric viewing in patients with retained central fixation. The prompt implementation of low vision aids in patients with macular degeneration will help them to maintain and regain their reading ability, which can lead to an increase in independence, communication, mental agility and quality of life.     

Crowded task performance in visually impaired children: magnifier versus large print

Background

This study compares the influence of two different types of magnification (magnifier versus large print) on crowded near vision task performance.

Methods

Fifty-eight visually impaired children aged 4–8 years participated. Participants were divided in two groups, matched on age and near visual acuity (NVA): [1] the magnifier group (4–6 year olds [n?=?13] and 7–8 year olds [n?=?19]), and [2] the large print group (4–6 year olds [n?=?12] and 7–8 year olds [n?=?14]). At baseline, single and crowded Landolt C acuity were measured at 40 cm without magnification. Crowded near vision was measured again with magnification. A 90 mm diameter dome magnifier was chosen to avoid measuring the confounding effect of navigational skills. The magnifier provided 1.7× magnification and the large print provided 1.8× magnification. Performance measures: [1] NVA without magnification at 40 cm, [2] near vision with magnification, and [3] response time. Working distance was monitored.

Results

There was no difference in performance between the two types of magnification for the 4–6 year olds and the 7–8 year olds (p’s?=?.291 and .246, respectively). Average NVA in the 4–6 year old group was 0.95 logMAR without and 0.42 logMAR with magnification (p?<?.001). Average NVA in the 7–8 year was 0.71 logMAR without and 0.01 logMAR with magnification (p?<?.001). Stronger crowding effects predicted larger improvements of near vision with magnification (p?=?.021).

Conclusions

A magnifier is equally effective as large print in improving the performance of young children with a range of visual acuities on a crowded near vision task. Visually impaired children with stronger crowding effects showed larger improvements when working with magnification.     

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.     

Principles and designs of the iseikonic lenses for near vision.

PURPOSE: The purposes of this study were to find the problems of afocal iseikonic lenses as used for near vision and to elaborate the principles and designs of the ideal near iseikonic lenses. METHODS: By analyzing the image-object relationship and imaging processes, the formulae were derived to quantitatively and qualitatively describe two errors of afocal iseikonic lenses as used for near vision: 1) inequality of accommodative demands to both eyes and 2) the difference between the actual and the nominal magnification. Formulae were derived to calculate and design the ideal near iseikonic lenses. RESULTS: For iseikonic lenses now used for near vision, inequality of accommodative demands to both eyes and difference between the actual and nominal magnification increased as the magnification increased and the viewing distance decreased. Afocal iseikonic lenses have significant inequality of accommodative demands to both eyes with 3% magnification at a viewing distance of 25 cm or with 5% magnification at 40 cm. The ideal near iseikonic lenses were found to be flatter than the afocal ones. A series of the ideal near iseikonic lenses were designed with some practical considerations. The central thickness and the refractive index of the lenses affect magnification and lens shape. The thicker the lenses, the higher the magnification. The higher refractive index also gives higher magnification. CONCLUSIONS: Iseikonic lens design is much different for distance and near vision. When the iseikonic lenses are used at a distance other than the designated distance, they will cause blurred vision and artificial aniseikonia as a result of inequality of accommodative demands to both eyes. In research as well as clinical settings, it is necessary to apply the ideal near iseikonic lenses for near vision.     

Ophthalmoscopy and vitreoretinal surgery in patients with an ARRAY refractive multifocal intraocular lens implant

We developed a clinical strategy for dealing with situations in which ophthalmoscopic examination and vitreoretinal surgery are difficult in patients with an ARRAY refractive multifocal intraocular lens (IOL) implant. The ARRAY zonal-progressive IOL has a central 2.1-mm distance-vision zone for optically-unobstructed posterior pole observation. A concentric near-vision zone (+3.5-diopter add) surrounds this central zone. Optical ray-tracing is used to determine how a 2.1-mm pupil limitation restricts monocular and binocular retinal image size in head-mounted, slit-lamp, and operating microscope ophthalmoscopy. A 2.1-mm pupil decreases the retinal field of view of high magnification, narrow field lenses much more than that of wider-field, lower magnification lenses. This "worst-case" analysis suggests an ophthalmoscopic strategy, but is not strictly valid for the ARRAY lens because the near-vision zone surrounding its 2.1-mm central zone is not opaque. The near-vision zone contributes defocused information to the ophthalmoscopic image, diminishing its resolution and depth information. Wide-field, low magnification lenses are potentially less problematic than higher magnification lenses for examining and treating patients with an ARRAY IOL implant. This strategy is useful for panretinal photocoagulation or photodynamic therapy, but not for procedures requiring high magnification stereoscopic vision such as macular vitreoretinal surgery.     

Determining the power of a negative lens field expander

In 1984, Kozlowski, Mainster and Avila derived a formula for the power of the lens needed in the negative lens field expander. Their method is based on the principle of telescopic magnification. In this technical note, we use the principle of spectacle magnification and obtain the power of the same lens. Both methods are presented for comparison. Practitioners may choose either method in determining the power of the negative lens field expander for their low vision patients, but the spectacle magnification method has the advantages of simplicity and familiarity to ophthalmic practitioners.     

Optical analysis to predict outcomes after implantation of a double intraocular lens magnification device

PURPOSE: To analyze the effect of different optical parameters of the eye on the outcomes of implantation of a double intraocular lens (IOL) magnification device in patients with low vision. SETTING: Fundación Oftalmológica del Mediterráneo, Valencia, Spain. METHODS: The eye's refraction and magnification before and after surgery were evaluated using a theoretical eye model based on paraxial optics approximation. Four parameters on which refraction and magnification are dependent were evaluated: anterior chamber depth (ACD), axial length (AL), mean keratometry (Km), and the distance between the 2 implanted IOLs. The 4 variables were analyzed separately in the first stage. Next, different combinations of ACD, AL, and Km values were studied. The goal was to predict what type of eye (in terms of ACD, AL, and Km values) is suitable for successful implantation of the double-IOL magnification device. RESULTS: Axial length had a significant influence on eye magnification. The Km value mainly affected refraction. The ACD did not influence the success of the procedure. The distance between the 2 IOLs was a decisive parameter in the analysis. CONCLUSIONS: Analysis of optical and biometric eye parameters could guide selection of patients for implantation of the double-IOL magnification device. Eyes with higher AL and Km values (myopic eyes) are the most suitable for this procedure.     

Treatment of low vision in the progressive cone dystrophies

Twenty-three patients suffering from the progressive cone dystrophies (PCD) were evaluated retrospectively, after they had undergone treatment for low vision. All patients were able to read 1 M or smaller print with magnification, despite poor central acuity. Eighteen (78%) patients achieved their primary rehabilitation goal, and 86% of the low vision devices prescribed were used at follow-up. Photosensitivity was reported by a large number of patients. Seven patients indicated that alleviating this problem was their primary rehabilitation goal. Eight (35%) found sun filters to be helpful. Our successful experience with patients with these disorders suggests that examination for, and trial of, low vision devices is warranted.     

Improved reading performance using individualized compensation filters for observers with losses in central vision

By boosting the amplitudes of the intermediate spatial frequencies more than the amplitude of the lower spatial frequencies, reading performance improved significantly when observers with losses in central vision read words that were filtered. Words that were filtered using an image enhancement function based on an observer's losses in visual function relative to a normal observer (1) reduced the magnification (30-70% less magnification was needed) and (2) increased the reading rate (2-3 times), measured in words per minute. The greater the loss in central visual function, the more individualized compensation filters reduced the magnification needed for word recognition. Individualized compensation filters improved the clarity and visibility of words for low vision observers. This study also found that the shape of the enhancement function was important to determine the optimum compensation filter for improving reading performance. In addition, the individualized compensation filters can be implemented by inexpensive hardware, for example in a closed circuit television (CCTV), to provide a significantly more effective low vision aid for observers with losses in central vision to read text, than is provided using only magnification.     

Rehabilitation of the visually handicapped with advanced macular degeneration

Hundred-and-twenty patients with advanced macular degeneration (senile, disciform, and hereditary forms), which could not be helped in a satisfactory way by simple magnifying aids, were studied for an average of 5 years (a minimum of 3 years), during which they had access to a full range of low vision rehabilitation measures including advanced optical aids and educational methods for training in the use of these aids and in the utilization of residual vision. The mean age of the patients was 72 years. They were all in a mental condition enabling them to understand and follow instructions. Just over half the patients were obliged to learn from the outset how to utilize peripheral retina, i.e. through eccentric viewing, high magnification low vision aids and very short reading distances, a mean of 4.4 cm, which demand specially qualified educational methods for low vision training. (A second group of 198 patients with macular degeneration was handled by the optician alone because either macular degeneration was moderate and the patients could manage with simple optical aids, or the patients were in such a bad mental condition, obvious already from the referral documents, that they were unable to use sophisticated aids in spite of the fact that they would have needed them with regard to their poor vision.)Telescopes were the most frequent aid used for distance, having a mean magnification of 4.6 X. Among the aids for reading, the most common were increased near additions and hyperocular lenses, having a mean power of 22.6 dioptres (magnification 5.7 X). Aids were also furnished for intermediate distance and for spot use. The mean number of series of visits was 2.5 and the number of training sessions (1 h) per series of visits was 3.0.Mean visual acuity improved from 0.18 to 0.64 with aids and training on the first series of visits, and reached 0.53 by the last series. The number of patients able to read TV titles increased from 6.7% to 57.5% and was 49.2% after the last series of visits. The number of individuals able to read newspaper text increased from 0.8% to 92.5%. After the last series of visits, the number was still as high as 80.0%.The results show that the presented methods for rehabilitation of patients with macular degeneration through optical aids and educational methods for training in the utilization of these aids and of residual vision are outstandingly successful. The use of the individual's vision and his situation in life often improve dramatically. In addition, costs for other forms of social aid are reduced or even eliminated.