Chapter 7 Visual Acuity PDF

7.1 Visual acuity (high contrast/distance) 7.1.1 Distance acuity charts Visual acuity is the assessment of the fi nest spatial detail that the visual system can resolve. Over the past 140 years, a range of charts, the majority of which use optotype test targets, has been developed for the assessment...

7.1 Visual acuity (high contrast/distance) 7.1.1 Distance acuity charts Visual acuity is the assessment of the fi nest spatial detail that the visual system can resolve. Over the past 140 years, a range of charts, the majority of which use optotype test targets, has been developed for the assessment of visual acuity (Fig. 7.1). These charts have different characteristics and adhere to different design strategies (Table 7.1).1 129 CHAPTER Assessment of visual function A. Jonathan Jackson 7 Low vision assessment 130 Snellen acuity charts Historically, visual acuity has been assessed using the Snellen chart,2 which has been extremely successful in screening for common causes of visual impairment and the detection of uncorrected refractive error. The Snellen chart does, however, have inherent weaknesses in that it uses an irregular geometric progression from top to bottom, thus reducing its sensitivity in the upper (6/60 \[1.0 LogMAR\] to 6/24 \[0.6 LogMAR\]) range. Within the context of the low vision clinic, this is the region most necessary when assessing visually impaired patients. A secondary weakness is that the chart fails to deal with the phenomenon of 'crowding' or 'contour interaction', which results in single-letter identifi cation tasks being much easier to undertake than those involving multiple-letter presentations. Furthermore, the legibility ratings given to the wide range of letters used on the traditional Snellen chart differ greatly. This is a particular problem at the upper end of the chart \[1.0 LogMAR (6/60) to 0.6 LogMAR (6/24)\], where very few letters are presented. The impact of design features, including font styles and letter selection, has been reviewed extensively by Bennett,3 whose work informed the fi rst British Standards publication on test types in Figure 7.1 Selection of high-contrast visual acuity charts: a, Bailey--Lovie LogMAR; b, Early Treatment of Diabetic Retinopathy Study (ETDRS); c, modifi ed Snellen; d, Keeler A series; e, Sonksen Silver; f, Sheridan--Gardner. 7 Assessment of visual function 7 131 1968.5 Finally, the conventional method of recording acuity measures from the Snellen chart (6/6 part, 6/7.5+, etc.), when individual letters on any given line are missed, is insensitive and somewhat arbitrary. It is for these reasons that considerable work has been devoted, over the past 30 years, to developing visual acuity charts that are more appropriate, particularly in the fi eld of low vision. Table 7.1 Visual Acuity Chart Design Characteristics Snellen chart Bailey--Lovie chart Origins 1862 Herman Snellen2 1976 Ian Bailey & Jan Lovie4 Relevant BS 4274-1 (1968)5 BS 4274-1 (2003)6 standards BS 4274-1 (2003)6 Optotype Letter dimension: 5 × 4 Letter dimension: 5 × 4 characteristics grid grid Legibility rating: 0.92--1.10 Legibility rating: 0.90--1.1 Letter style: sans serif Letter style: sans serif Letters used: 10 Letters used: 12 Letters per row: 1 (6/60) Letters per row: 5 to 8 (6/5) Scoring Snellen fraction i.e. 6/12 LogMAR value i.e. 0.30 methods (or Snellen decimal 6/12 Letter scoring: 0.02 units = 0.5) per letter (i.e. 0.26 = Letter scoring: +/− 6/12+2) (i.e. 6/12+2) Progression Lines per chart Lines per chart: 14 Original: 7 Geometric progression Current: 8--10 Uses standardised letter Arithmetic progression and row spacing: multiplication factor × 1.2589 Recorded test 6 metres -- variations 6 metres (20 ft USA) -- distance expressed in Snellen recalibrate scale for denominator alternative distance Alternative Range of historical and Keeler A series,7 ETDRS,8 designs current charts using Sloan9 various font styles Currently available charts fall broadly into two categories: those based on the original Snellen2 premise and those designed according to the principles advocated by Bailey & Lovie.4 Low vision assessment 132 Keeler A series charts This chart was designed by Charles Keeler with the specifi c intention of creating a chart that would be particularly useful in the assessment of visual impairment.7 The A series charts, based on a logarithmic (constant ratio) scaling system, were essentially the precursors to all currently available LogMAR charts. They had 20 different series of letters, ranging from A1 (6/6 \[LogMAR 0.0\] equivalent) to A20 (1/60 \[LogMAR 1.9\] equivalent). Each line differed from its nearest neighbour, in size, by a factor of ×1.25. The charts were supplied with easy-to-use conversion tables that enabled low vision practitioners to determine the magnifi cation levels required to assist patients achieve a desired level of acuity. Near equivalents, also calibrated in A series format, could be used in a similar manner, as they were calibrated according to the dioptric power of the near addition required to achieve an improvement in near acuity at a reference working distance of 25 cm. Sloan distance acuity charts An American equivalent to the Keeler chart, the Sloan distance acuity chart was also designed with the needs of the visually impaired in mind.9 This chart, which uses the 'M' or metric series notation, has never achieved worldwide usage. The system is best known for its use in the assessment of near acuity. Letters of size 1M, which are about the same size as lower-case newsprint, subtend an angle of 5 minutes of arc when located 1 m from the eye. The system is linear so that 3M letters are exactly three times larger than the 1M letters. Test distances and letter sizes can be recorded in Snellen format with a 3M letter read at 20 cm recorded as 0.2/3M. Bailey--Lovie charts Fundamental to the success of the Bailey--Lovie chart in low vision practice is its logarithmic scale and the inclusion of equal numbers of similarly legible letters on each line of the chart.4 The gaps between letters, and indeed between lines, are determined by the size of the letters used on each line. On the scale chosen, any change of three lines represents a doubling, or alternatively halving, of letter size. Visual acuity is scored as 0.1 LogMAR for each row and 0.02 LogMAR for each letter named correctly. The main advantage of the chart is that it, and its near vision equivalent, greatly simplify the process of calculating the estimated magnifi cation required by a patient interested in reading text of a specifi ed size. As LogMAR defi nes a visual angle, not the size of the letters, the score must be adjusted for the distance of the chart. LogMAR scores decrease with improved acuity (the opposite to decimal acuity). The most recent (2003) British Standard on visual acuity test types6 incorporates LogMAR notation and a modifi ed range of optotypes (Fig. 7.2). Figure 7.2 The Bailey--Lovie LogMAR chart, designed for use at 6 metres. Figures in the left margin of the chart and lower left insert indicate letter size in Snellen metric and Snellen feet. Figures in the right margin and lower right insert indicate letter size in LogMAR and VAR ratings. The lower scale lines indicate how LogMAR and VAR scores should be adjusted when testing is carried out at different distances. Low vision assessment 134 Waterloo charts Similar in design and concept to the Bailey--Lovie charts, the letters on these Canadian charts are oriented such that letters of equal size are placed in columns rather than rows.10 Patients are advised to read across the top line (row) until they reach a point where mistakes are made, whereupon vertical checks are made to determine the exact acuity. An additional feature of the Waterloo chart is the inclusion of interactive surround bars, which ensure that letters at the start and fi nish of each line are as diffi cult to read as those within the lines. Ferris LogMAR charts The most widely used of the LogMAR charts is the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart, an American chart designed by Ferris, Kassoff, Bresnick and Bailey,8 which uses Sloan optotypes. The designers recommend that one chart is used during the course of refraction and that the other two charts are used (one each) when determining the optimal acuities of the right and left eye. Results are recorded in conventional format, although the chart is designed for use at 4 m. Essentially, ETDRSin the actual letters used. Symbol charts As the prevalence of visual impairment is greatest in population subgroups with severe learning disabilities, it is important for the specialist low vision practitioner to have knowledge of, and access to, a selection of symbol and optotype matching charts. Although many of these charts have been developed for paediatric use, they are equally useful when assessing the visual status of those with learning disability. The system designed by Lea Hyvarinen, a Finnish ophthalmologist, includes LogMAR-based alpha numeric and picture symbol charts, matching symbols, single symbol books and crowded symbol books.11 Symbol matching is undertaken in much the same way as when using Sheridan--Gardner letter matching cards and Kay picture cards (Fig. 7.3). Computer-generated charts The potential to assess visual acuity using electronic technology is both exciting and carries some important advantages. Not only Figure 7.3 Selection of symbol and single-letter charts used in the assessment of visual acuity in children and adults with learning disabilities: Kay pictures, illiterate E, Sheridan--Gardner single-letter cards, Sonksen Silver crowded letter cards and Fuchs symbols. Low vision assessment 136 can optotype sequences be varied and randomised, thus eliminating the possibility of target memorisation, but the accuracy of measurements can be enhanced by presenting greater numbers of targets of any given size. Target luminance, contrast, spacing, exposure time and so forth can all be adjusted. Until recently, test chart design was limited by pixellation: to achieve reasonable shape fi delity, letters need to be at least 10 pixels in height. If a line of 6/3 (−0.3 LogMAR) letters were 100 pixels long, then a line of 6/60 (1.0 LogMAR) letters would need to be 2200 pixels long. Vertical restraints are less signifi cant as scrolling is a viable presentation strategy. Affordable display technology is now approaching an appropriate level of sophistication, and a useful development is the Test Chart 2000 system (Fig. 7.4 \[Plate 10\]).13 7.1.2 Distance acuity specifi cations Visual acuity measurements can be expressed in a number of ways, the most universally accepted of which is the Snellen fraction notation in which the numerator 'd' is the test distance and the denominator 'D' is the distance at which just resolvable letters must be placed so as to subtend an angle of 5 minutes of arc at the eye (d/D or 6/60). In the USA, measurements are expressed in feet as opposed to metres (6/60 = 20/200 \[LogMAR 1.0\]). The main benefi t of expressing acuities in this manner is that there can be no confusion over the testing distance at which acuities are recorded. The decimal notation, as used throughout most of Europe, is obtained simply by dividing the numerator by the denominator (6/12 = 0.5 \[LogMAR 0.3\]). Specifi cation of acuity in this way is, however, regrettable as results can easily be confused with LogMAR results, which are unrelated. Alternative methods of specifying acuity are according to the minimal angle of resolution (MAR), which is obtained by inverting the Snellen fraction and expressing the result in minutes of arc (6/24 → 24/6 → 4 min of arc). LogMAR is simply the logarithim to the base of 10 of the 'MAR' (6/6 = MAR 1 = LogMAR 0). Alternative methods of expressing LogMAR acuity measures are as visual acuity ratings14 or visual effi ciency ratio values.15 The benefi ts of using LogMAR scales and acuity charts have already been identifi ed. A comparison of the various acuity measurements is given in Table 7.2. In cases where visual acuities are extremely poor, it has become common practice within the UK to use the term counting fi ngers (CF) as an indicator of poor vision. This should be discouraged as fi nger width, distance from the patient, fi nger separation and target--surround contrast vary greatly. Those with acuities of less than 0.5/60 \[LogMAR 2.0\] can be classifi ed as having hand movements (HM) vision, if indeed movement can be detected. Where movement cannot be detected, vision should be classifi ed as either perception of light (PL) with or without directional sensitivity, or as no perception of light (NPL). Less than 5% of the visually impaired population will fall into the latter category. Practical advice If working in a clinical environment where a combination of acuity charts is routinely used by a combination of practitioners, errors in comparing data can be avoided if, alongside every acuity measurement, note is made of test distance and chart type.7.2 Visual acuity (low contrast) Contrast sensitivity, despite the fact that its relevance in detecting ocular disease has been apparent since the 1960s,18 has until relatively recently been perceived by many in routine optometric practice as too diffi cult to assess within a conventional clinical environment. This perception is unacceptable, as the vast majority of our visual interaction with the world involves resolving low contrast detail. Table 7.3 illustrates the variation in contrast, inherent in a range of everyday tasks undertaken by adults and children. 7.2.1 Low contrast acuity charts The contrast detection threshold, as determined using sine wave gratings, is of course the reciprocal of the contrast sensitivity function (CSF). When considering the CSF and how it is affected by the disease process, it is important to remember that conventional high contrast optotype acuity is represented along the x-axis Table 7.3 Contrast Threshold Required to Achieve Fluent Text Reading and 'Spot' or 'Survival' Reading When Undertaking a Range of Visual Tasks Involving Subject Matter with Different Contrast Ratings Visual task Task Contrast threshold (%) contrast Fluent reading Survival reading DSE display 100 The score attributed to the patient is the logarithm of the contrast sensitivity of the last group of three letters, of which at least two were read correctly. The chart, when used at 1 metre, is designed to assess contrast sensitivity at the peak of the contrast sensitivity function curve, and is probably best placed to correlate with daily living activities in which reduced contrast sensitivity is likely to cause disability (mobility, face recognition, reading).21 Bailey--Lovie low contrast charts Bailey--Lovie charts are commonly available on white plastic panels that have a high contrast chart (black letters) on one side and a low contrast chart with grey letters on the other. The design of the high and low contrast charts is the same. The contrast of the low contrast chart is 10% Michelson (18% Weber). The difference between the high and low contrast visual acuities recorded on the two charts provides a measure of the slope of the contrast sensitivity function (CSF) as it approaches the individual's high spatial frequency cut-off. Even for patients who have signifi cantly reduced contrast sensitivity, this high frequency section of the CSF is approximately linear.22 Symbol charts The Lea Test system, referred to in the section on Symbol charts above, includes a set of low contrast charts with symbols of 10%, 5%, 2.5% and 1.25%. The symbols are exactly the same as those used on the high contrast charts, with the result that the same set of matching cards and symbols can be used when testing young children and those with a learning disability.11 More recently, paediatric contrast sensitivity screening cards have been developed. Those developed by both Hyvernan (the Hiding Heidi set) and Bailey (Mr Happy Faces) use smiling faces to construct tests that can measure low contrast acuity down to contrast levels of 1.25% and 0.25% respectively (Fig. 7.7). Practical advice When assessing optimal contrast sensitivity using optotype-based letter charts, participants must be given additional time to recognise the letters as temporal summation is required to achieve results approaching threshold. Low vision assessment 144 Edge detection tests Rectangular luminance profi les (edges) are thought to relate well to real-world objects as they consist of a range of spatial frequencies detected by the most sensitive spatial channels, the peak of the contrast sensitivity curve. The Melbourne edge test (MET) is a compact chart consisting of three main parts: a hand-held portable lightbox, a printed transparent acetate, and a response key card. Each circle is divided by a luminance edge so that a contrast differential is established between the two halves. The observer must identify the orientation of the edge (0°, 45°, 90° or 135° alternative forced choice) with successive circles decreasing in contrast.24 Sinusoidal grating tests These tests, when produced as printed cards, are generally used for screening purposes. Only by using expensive computer-generated gratings can a comprehensive plot illustrating the full relationship between spatial frequency and contrast sensitivity be obtained. This is important when determining the specifi c impact of cataract, amblyopia and other forms of ocular pathology on visual function. Figure 7.7 Happy Faces paediatric contrast sensitivity symbol charts. (Reproduced with permission from Harvey & Gilmartin 2004.23) 7 Assessment of visual function 7 145 The computer-generated test is, however, less useful within the low vision environment, where the practitioner generally wishes to use low contrast testing facilities to determine the relationship between specifi c aspects of disability and reduced visual functions. Hess, who has reported extensively on computer-generated contrast sensitivity measures, advises caution when interpreting the results of contrast sensitivity testing on patients with low vision, as results may be erroneously affected by scotoma site and depth.25 One grating-type test that is available as hard copy is the Arden test,26 which uses plates in which the contrast of the grating increases as one moves down the plate. The task for the patient is to identify, as the plate is gradually withdrawn from its envelope, the point at which the gratings fi rst become visible. The Vistech (VCTS) chart uses an alternative approach involving the presentation of fi ve rows of circular targets on which gratings have been superimposed. There are thus targets of fi ve different spatial frequencies, each of which is presented at nine contrast levels. The observer in this case has to identify the orientation of gratings on each line.27 Reeves et al28 have expressed concern about the repeatability of these tests. The Cambridge gratings use a square wave design of fi xed spatial frequency (four cycles per degree) and differing contrast (between 5% and 0.14%). 7.3 Practical relevance of contrast sensitivity When working in the low vision clinic, contrast sensitivity testing can be carried out more easily using optotype-based tests, which are familiar to the patient. In many cases, the dramatic difference experienced when switching from a high contrast Bailey--Lovie chart to the 10% equivalent helps the patient gain greater understanding of the nature of their visual impairment and why certain adaptive strategies should be implemented to maximise vision. Patients with glaucoma and medial haze, for example, often describe their view of the world as 'faded', 'grey' or 'washed out' to a degree that is out of proportion with their level of disability, as detected using high contrast optotypes. These patients are often relieved to be shown a clinical test that equates results to their experience. 7.4 Distance testing strategies Within the context of the low vision clinic, it is essential that accurate and repeatable measures of visual acuity and, when indicated, low Low vision assessment 146 contrast acuity or contrast sensitivity are obtained. This information is fundamental not only to the process of determining the range of low vision aids that will be demonstrated to the patient, but also in advising the visually impaired patient on rehabilitation strategies that may be used when tackling a range of far and intermediate distance tasks. Room lighting should be standardised and the test routine outlined as described in Chapter 6. Most importantly, patients must be given time to discriminate optotype detail, and positive feedback from the practitioner usually encourages an optimal result. 7.5 Visual acuity (near/reading) At the outset it must be stressed that, although the term 'near acuity' is often quoted when describing the visual performance of patients tested using conventional Faculty of Ophthalmologyapproved near vision charts, the measurement is not the near equivalent of distance acuity, irrespective of whether distance acuity has been recorded using Snellen or LogMAR-type charts. The reasons for this are threefold. First, distance acuity testing generally utilises upper-case single optotypes that, in the case of LogMAR charts, are separated both horizontally and vertically by distances equal in size to the optotypes. Virtually all near charts use either lower-case unrelated words or continuous text produced using a range of letter and word layout spacings. Second, distance acuity testing requires the patient either to name letters or to match letters or symbols, whereas at near patients are required to make sense of the letters and pronounce what they see as a recognised word, a task that requires higher-order cortical processes. Third, this process is even more complicated as the task of 'reading' involves not only pronouncing individual words, but also altering delivery to refl ect meaning and context. The process of comprehending what has been read will, of course, also be infl uenced by reading speed, which is infl uenced by eye movement control. It is for this reason that the authors advocate the term 'reading acuity', which is an entirely appropriate function to measure, as assistance with reading is, indeed, the stated goal of the majority of patients attending low vision clinics.29 7.5.1 Reading performance Many recent research studies have investigated the relationship between reading acuity/performance/speed and other quantifi - 7 Assessment of visual function 7 147 able aspects of visual function (distance visual acuity/contrast sensitivity/scotoma size and density), and subjected the outcome values to fairly complex multivariable analysis.30 The results have, however, simply confi rmed that it is diffi cult to accurately correlate reading ability with any single measure of visual function and, if one wishes to know how well one can read either with or without the help of a low vision aid, one must at the very least assess reading speed and possibly comprehension. A clinical interpretation of many of these data can be drawn from work published by authorities in the fi eld, including Whittaker & Lovie-Kitchin17,31 and Rumney.16 These individuals stress the importance of 'fl uency'; they refer to 'high fl uency', which we need to reach in order to read as a leisure pastime, as 160 words per minute. When 'survival' reading or 'spot' reading, a speed of only approximately 40 words per minute is needed. Whittaker & Lovie-Kitchin have shown that, when we approach our resolution threshold, we practise survival reading. To improve the reading ability and thus increase comfort and satisfaction, print size and/or contrast have to be increased well above those required for survival reading. The increases required have been deemed to be the acuity and contrast reserves (Table 7.4). Data presented by Whittaker & Lovie-Kitchin31 indicate that, to achieve optimal reading performance, text has to be 6 times larger and 30 times greater in contrast than text presented at threshold size and contrast levels. The patient with low vision hoping to regain reading fl uency, as opposed to optimal performance, musbe presented with text that is 3 times larger and 10 times higher in contrast than that detected at threshold. Whereas acuity reserves can be maximised through the provision of optimal low vision aids, contrast reserves, if they are to be maximised, may require specialist lighting or the use of electronic vision enhancement devices. The impact of target or letter size on reading speed is best illustrated graphically, where it can be seen that over a very wide range of print sizes the normally sighted pre-presbyopic individual achieves a relatively constant reading speed (Fig. 7.8). Only when print of N4 is being read at a working distance of 40 cm does reading speed begin to drop. Thereafter, performance drops rapidly, word reading becoming impossible at approximately N3 in an eye with a distance acuity equivalent to N2.32 In patients with low vision, the reading speed curve (as on the graph) assessed with a conventional near add moves to the right. Depending on the nature of the pathology, the slope may also be less severe and reading speed, even with large print, may never be optimal. With the introduction of a suitable low vision aid, the curve moves back to the left, but the best possible reading speed may never reach optimal levels because of handling limitations and restricted fi eld size. Furthermore, if used to read large print, the process may be complicated by restriction in the fi elds of view with the result that the right-hand side of the curve begins to rise. Figure 7.8 Reading speed as infl uenced by print size in an individual with normal vision (A) and a person with age-related macular degeneration (B). 300 100 30 10 0.03 0.1 0.3 1 Character size (degrees) 3 10 30 Reading speed (wpm) A = Normal vision B = Low vision A B 7 Assessment of visual function 7 149 7.5.2 Near (reading) acuity charts Historically, near acuities have been specifi ed in one of three forms: Jaeger, N Point and Snellen equivalent. More recent charts include the Keeler A series and Sloan M series charts, whereas those used in progressive low vision units include the Bailey-- Lovie word reading charts, the Pepper reading test charts and the MN read charts (Fig. 7.9). A comparison of acuity values specifi ed in the most common formats is shown in Table 7.5. Jaeger Familiar to most older practitioners, the origins of the Jaeger chart were in the printing houses of Vienna. Text is formulated from type of 20 different sizes, the size progressions of which have never been standardised. Many of the charts produced according to the Jaeger system also used highly variable word and letter spacings.33 Snellen equivalent system charts The scientifi c basis for the Snellen near system is identical to that of the distance acuity system in that each letter has been constructed such that, when held at a specifi ed distance, it will subtend an angle of 5 minutes of arc at the eye (see Section 7.1 above). Most near vision Snellen charts have been produced as one-seventeenth of the original chart and are designed to be used at 35 cm. The major problem with these charts is that when acuity values are expressed as Snellen equivalents the value holds true only when the chart is used at the specifi ed test distance. N point system charts The N point system has been incorporated into the series of charts approved for use in the UK by the Faculty of Ophthalmologists.34However, the letters do not extend to the end of the block, so the letters are approximately 1/44 of 1 cm (1/107 of an inch). The height of the lower-case letter 'h', as printed in N5 text, is thus 5/107 of an inch. Lower-case letters are smaller again by a factor of approximately 0.68. The typeface used is that designed in 1932 for The Times newspaper (Times roman). Although not easily comparable with LogMAR-based equivalent near charts, N point charts have the advantage of being familiar to virtually all UK practitioners; a doubling in point size represents a doubling in letter size and, hence, when viewed at an identical distance, a doubling in retinal image size. Point size is also used in computing to specify font dimensions. Keeler A series charts Designed to complement the similarly named distance charts (see Section 7.1), letter sizes have been calculated such that letters forming A1 text, when held at a working distance of 25 cm, will subtend 5 minutes of arc at the eye. Successive lines increase in size by a factor of ×1.25 or 0.1 Log units. Designed specifi cally to assist the low vision practitioner with the task of calculating expected magnifi cation requirements, the system was in many ways ahead of its time. The only real disadvantage concerned the layout and spacing of words, which varied considerably from top to bottom.7 Sloan M series charts Specifi ed in M notation and designed to complement Sloan distance charts (see Section 7.1), the M system was, until recently, used almost universally throughout the USA. The series of fi ve reading cards was designed specifi cally to assist with the calculation of the anticipated reading addition required to achieve any given reading task by patients with low vision. The recommended working distance at which to use these charts is 40 cm.35 Bailey--Lovie word reading charts Ranging in size from 1.6 to 0.0 LogMAR (N80 to N2, or M10 to M0.25, equivalents), these charts (26 × 20.6 cm) incorporate 17 lines of unrelated words and are designed for use at 25 and 40 cm. There are two words in each of the larger categories, increasing to six as one moves down the chart towards the 0.0 threshold level. The charts are ideal for measuring reading acuity, but are restricted in their ability to assess reading speed. Print sizes are also specifi ed in N point notation.32 Pepper visual skills for reading test (VSRT) Available in text sizes of N8 to N32 (M1--M4), these charts were designed to test reading speed and fl uency in patients with macular disease. The charts consist of 13 lines of text of identical size. The complexity of the reading task increases as one moves down the page, ranging from well spaced single-letter identifi cation at the top to complex unrelated longer words at the bottom. These charts are designed for use in low vision clinics where patients with reduced reading performance are to be given training exercises designed to increase reading speed.36 MN read charts The Minnesota low vision reading test is available in several forms, all of which are available in both conventional and reverse con 7 Assessment of visual function 7 153 trast. The original test was computer based and designed specifi - cally to assess on-screen reading speed using scrolled simple sentences.37 Printed versions, which achieved comparable results when used to assess reading speed, have been produced by Ahn et al.38 The most well known versions have, however, been produced as acuity charts incorporating text ranging in size from 1.3 LogMAR through to −0.2 LogMAR when held at the recommended working distance of 40 cm. The reverse contrast facility is particularly useful when testing patients who fi nd refl ected glare from the white page uncomfortable or debilitating.39 PNAC chart The PNAC (practical near acuity chart) represents an attempt to standardise the number and diffi culty of words on a LogMAR chart and to allow a quick measurement of near visual acuity. It uses related three-, four- and fi ve-letter words on each line. In a comparison with the Bailey--Lovie near chart, which uses unrelated words, the use of related words was found not to affect the threshold near acuity measured.40 The chart can be read from the top downwards until the person can no longer resolve the words. The print size at which the reading speed slows, as well as the threshold for near vision, should be noted. 7.5.3 Near vision testing strategies As outlined in Chapter 6, it is imperative to undertake near vision assessment only after having completed an accurate refraction and having determined optimal distance acuities and, where appropriate, contrast sensitivity or low contrast acuity measurements. Near (reading) acuities should be recorded through the patient's existing reading correction and, thereafter, through a +4.00 near addition. In the UK, it is standard low vision practice to increase the strength of the addition in +4.00 steps until the patient can achieve the desired acuity. This can be done using any of the LogMARbased acuity cards referred to above. Once reading acuity has been determined monocularly, and under binocular conditions if the objective is to achieve binocularity, the process of evaluating reading speed and fl uency using Pepper, MN read cards or in-house equivalents can be commenced. After the assessment and the provision of appropriate low vision aids, non-optical aids and rehabilitation advice, consideration Low vision assessment 154 should be given to low vision training (see Chs 16--18), which may assist patients to read faster and more fl uently and to utilise their near vision potential, with or without a low vision aid, most effectively. 7.6 Visual fi elds In a conventional optometric setting (primary care), visual fi elds are generally assessed to detect the presence of early-onset disease (glaucoma). Within a secondary care environment, visual fi elds are more likely to be used to assess progression of the disease. The situation within the low vision clinic is entirely different in that assessment of visual fi elds is undertaken in order to determine the magnitude of loss and, thereafter, to equate functional loss with disability. The elderly patient who has experienced a stroke and developed a right-sided hemiparesis will thus have visual fi elds assessed to determine the presence and, if detected, the extent of right-sided hemianopic fi eld loss (Fig. 7.10). The presence of fi eld loss and potential neglect will have a profound impact on reading perforFigure 7.10 Humphreys 24/2 monocular fi elds (right and left) illustrating a dense right-sided homonymous hemianopia in a patient with right hemiparesis resulting from a left-sided cerebrovascular accident. 7 Assessment of visual function 7 155 mance and social awareness. Individuals with retinitis pigmentosa will similarly have visual fi elds assessed to correlate loss with the extent of mobility problems experienced by the patient. Those with inferior altitudinal loss may have fi elds quantifi ed in order to help explain mobility restrictions and to illustrate the need for safety strategies. Those with central fi eld loss resulting from macular disease will have characteristic impairments to reading fl uency. 7.6.1 Peripheral visual fi elds In the authors' opinion it is unhelpful, and in fact unfair, to assess the visual fi elds of those with profound visual impairment using complex full-fi eld threshold automated perimetry. This process, although providing information that may be useful in the process of rehabilitation, is time consuming and often distressing for the patient (see Fig. 7.10). The resulting fi eld plots, which are characterised by 'blackness' and 'loss', often overestimate the true extent of disability and underestimate the extent of useful vision. Screening programmes, of which there are many, generally run 6--10 dB above threshold and are, on the whole, more useful. Much more useful, however, are data that can be obtained from accurately and carefully performed confrontational visual fi elds assessment or, if available, the results of perimetry performed using a Tangent screen, Goldmann bowl perimeter or Arc perimeter. Crucial to obtaining repeatable and accurate results is the process of ensuring stable central fi xation. This can be facilitated through the use of an enlarged 'cross type' fi xation target. 7.6.2 Central visual fi elds Central perimetry, utilising automated perimeters, may be useful in determining the extent of scotoma size and depth in those with macular or parafovial lesions. This can be undertaken using, for example, Humphreys 10/2 type programmes. Alternatively, use of the Amsler chart to determine the subjective quality of the central fi eld and the signifi cance of metamorphopsia may be invaluable. There are, however, concerns regarding the reliability and repeatability of the Amsler chart, and patients often fi nd it diffi cult to be specifi c about areas of loss and distortion.41 The development of the scanning laser ophthalmoscope in the early 1990s paved the way for the coupling of central visual fi eld testing and fundus Low vision assessment 156 observation. Using this new technology, patients with central scotomas could be tested longitudinally in the knowledge that fi xation loss could be compensated for, thus enhancing the chances of obtaining accurate and recordable fi elds in patients with macular damage.42 More recently, the development of microperimeters such as the Nidek MP1 have made the technology more affordable and less dependent on technical expertise (Fig. 7.11 \[Plate 11\]).43 7.6.3 Rehabilitation advice and assistance Although less can be done to alleviate the problems resulting from fi eld loss than for the problems associated with a reduction in visual acuity, therapeutic options are available. Patients can be instructed in scanning and peripheral viewing techniques and, most importantly, made aware of the need to develop safe viewing strategies (see Ch. 17). Highly motivated patients may, however, benefi t from the prescription of reverse telescopes, Fresnel prisms or clip-on mirrors. Reverse telescopes, which are generally low powered Galilean telescopes (×2), can be hand held or spectacle mounted. Field expansion is gained at the expense of minifi cation. Fresnel prisms, usually 25Δ to 30Δ in strength, have been used in cases of hemianopic fi eld loss. These are mounted on the spectacle frame such that the prism is applied only to the half of the spectacle lens that is in the blind fi eld. As the user glances towards the prism, the viewer is able to see into the non-seeing fi eld. The base of the prism is always placed in the direction of non-seeing fi eld. After adaptation, prisms can be fi lled binocularly, although users are often bothered by a combination of aberrations and 'Jack in the box' image jump.44 Clip-on mirrors have also been advocated for treating fi eld loss resulting from hemianopia. In these cases, the mirrors, which are usually mounted on the frame of the spectacles, superimpose a reversed image of the picture falling on the non-seeing retina on to the seeing retina. The diffi culty for the user is in differentiating nasal from temporal images.45 One of the most important aspects of fi eld assessment, associated with low vision, concerns driving. Current UK legislation states that failure constitutes fi eld loss of 'at least 120° on the horizontal measured using a target equivalent to the white Goldmann III4e settings'. In addition, there should be 'no signifi cant defect in the binocular fi eld which encroaches within 20° of fi xation above or below the horizontal meridian'.46 Many of those with borderline 7 Assessment of visual function 7 157 B A Figure 7.11 (Plate 11) A,B, The Nidek MP1 microperimeter permits the examiner to position targets accurately, and is used to assess retinal sensitivity within, and adjacent to, retinal areas of specifi c interest. (Reproduced with kind permission of Nidek Technologies.) Low vision assessment 158 fi elds feel perfectly safe when driving, and the prospect of having a licence withdrawn, and the associated loss of independence, may induce distress or even anger. It is essential that practitioners advising on these issues record appropriate advice in the clinical records and inform the patient's general practitioner of the fi ndings. Bioptic telescopes are spectacle-mounted telescopes that are positioned to allow the user to interchange from viewing through the telescope to viewing through the spectacle lens.47 When the telescope is intended for viewing distant objects, it is usually mounted high in the spectacle lens. Typically the user locates the object of interest while viewing the world directly through the carrier lens. Then, in order to obtain the magnifi ed view through the telescope, the head is tilted forward so that the viewing axis of the telescope is directed towards the object of regard. Bioptic telescopes may be used for viewing objects at near; the telescope is typically mounted low in the spectacle lens and used in a manner similar to that of a bifocal lens. In the USA more than half of the states permit the use of bioptic telescopes for driving, with rules and limitations that vary from one state to another. In Europe, this issue is currently under review. Practical advice The purpose of undertaking visual fi eld assessment within the low vision clinic should be to explain the nature of disability described or noted by the patient. 7.7 Colour vision Optometrists generally equate defective colour vision with genetically determined red/green loss in males (protanopes and deuteranopes). Colour vision tests such as the Ishihara test are designed specifi cally to differentiate normal from abnormal, and anomalous from total. The colours of the targets chosen are such that they lie along dichromatic confusion lines. Colour vision loss associated with low vision, particularly when it is acquired and chronic, is different; the amount of loss, the extent of the defect and the nature of loss equate primarily with the damage caused to cone photoreceptors and their neural network (Table 7.6). One also hasan underlying congenital colour vision defect. For these reasons, there is little point in using pseudoisochromatic plate tests to evaluate the nature of colour vision loss in patients with low vision. Research conducted using simulated blur in normal subjects would tend to suggest that conventional test results become unreliable when acuities drop below 6/20 \[LogMAR 0.5\], and extremely diffi cult when acuities drop below 6/60 \[LogMAR 1.0\].48 It is generally assumed that colour matching tests, such as the Farnsworth 100 hue and D15 tests, are more appropriate for detecting and differentiating acquired colour vision defi cits. Here, too, size can pose problems, hence the development of the Jumbo D15 and PV16 tests, which can be used by those with acuities as low as 3/60 \[LogMAR 1.3\] (Fig. 7.12 \[Plate 12\]).49 As with conventional colour vision testing, all tests should be carried out under standard conditions when illuminated with northern daylight equivalent light sources. Importantly, defects, when detected, should not be classifi ed into the conventional congenital categories but should instead be categorised according to colour confusion categories (i.e. red/green or blue/yellow). Numerous tests for the detection and monitoring of eye disease list the frequently reported confusion pairs associated with various pathologies: cataracts -- blue/yellow; cone dystrophy -- red/green; Table 7.6 Classifi cation of Acquired and Hereditary Colour Vision Loss Hereditary Acquired Usually male (M : F ratio 10 : 1) Sex equality Usually red/green Often blue/yellow Predictable colour confusion Unpredictable colour confusion characteristics characteristics Binocular Monocular and asymmetric Generally repeatable results Variable test--retest results Static Often progressive Often unaware of nature of loss Aware of changing colour perception Normal visual acuity, contrast Abnormal visual acuity, contrast sensitivity and visual fi elds sensitivity or visual fi elds Low vision assessment 160 glaucoma -- blue/yellow. There are, however, many exceptions to the rule. The often quoted Kollner's law, that outer retinal larger lesions give rise to blue/yellow type defects whereas inner retinal layer and optic nerve lesions give rise to red/green defects, is also a generalised oversimplifi cation.50 Figure 7.12 (Plate 12) Selection of colour vision tests: a, City colour vision test; b, Ishihara plates; c, Jumbo D15 (PV16) (buttons), which are of particular use when testing patients with a visual acuity of 6/20 \[LogMAR 0.5\] or less. Practical advice In considering the characteristics and impact of defective colour vision in patients with low vision, remember that an acquired defect may be superimposed on a pre-existing congenital colour vision disorder. As is the case with reduced contrast sensitivity, defective colour vision cannot be cured; however, advice on how to compensate for demonstrable loss can be invaluable, particularly in the educational and employment environments. Research results indicate that patients with low vision exhibit preferences for high contrast colour combinations irrespective of what the actual colours are.50 Colour and luminance contrast should be used to complement each other when advising on rehabilitative issues. 7 Assessment of visual function 7 161 7.8 Glare sensitivity Whereas photophobia, in its true sense, occurs as a result of pathology affecting the trigeminal nerve, glare is the result of excessive brightness within the visual fi eld.51 Glare is defi ned as either 'discomfort' or 'disability' glare, and is associated with asthenopia, headaches and 'squinting'. 7.8.1 Disability and discomfort glare Disability glare occurs when individuals, both normally sighted and visually impaired, are subjected to light levels that are higher than those to which they can comfortably adapt. The Chartered Institute of Building Services Engineers (CIBSE) recommendations are that, in order to avoid discomfort glare, the task : surround illumination ratio should not exceed a ratio of 3 : 1.52 Discomfort glare is more prevalent in certain individuals with visual impairment, including those with medial haze (cataract, corneal scarring, etc.) and those with albinism, achromatopsia and aniridia. Disability glare differs from discomfort glare in that it results in a reduction in retinal resolution. The glare source can result in a diffuse overall increase in retinal illumination, or it can be focal and discrete. When focused, it needs to be close to the line of fi xation for signifi cance to be achieved. The effect can be enhanced in the presence of medial haze and cataract, when it may be referred to as 'veiling glare'. The disability induced is particularly marked when the targets observed are low contrast in make-up. Disability glare may also occur when the light source involved is seen refl ecting off the working surface, in which case it is called 'refl ective glare'.53 7.8.2 Glare testing The essence of all glare sensitivity testing is to attempt to quantify the impact that the introduction of a glare source has on vision. In its simplest form, glare sensitivity can be assessed by introducing a glare source, such as a pen torch or light from an anglepoise lamp, into the visual fi eld close to the line of fi xation while the subject under investigation undertakes a visual task.54 Quantifi cation can be achieved if an acuity chart or low contrast letter chart Low vision assessment 162 is used as the principal target. Correlating the actual reduction in performance with the patient's subjective comments can prove useful. The most well known instrument designed specifi cally to assist with the quantifi cation of disability and discomfort glare is the brightness acuity tester (BAT) (Fig. 7.13). With this instrument, the subject views an acuity or low contrast chart through a 12-mm aperture hole in a 60-mm white hemispherical dome, which is held in close proximity to the eye. Acuity measurements can be recorded in the standard way when the rheostat is adjusted to one of three positions (12-, 100- and 400-foot Lamberts). The three settings correspond to bright overhead fl uorescent lighting, indirect sunlight on a cloudy day, and direct overhead sunlight.55 Other tests that have also been evaluated as glare disability tests include the Vistech MCI 8000 and the Miller--Nadler glare tester.56 7.8.3 Management of glare The principal approach to managing glare is to do everything possible to remove the source. If, for example, it is caused by light from a luminare adjacent to, or refl ecting off, the working plane, simply increase the angle of substence between the light source and the eye. Alternatively, one may try to reduce the brightness of the offending source by reducing the bulb strength or using an appropriate diffusing fi lter. If this is impractical or impossible, consideration should be given to fi lter lenses such as the NoIR sun shields, or to peaked caps and typoscope-type devices. Figure 7.13 The brightness acuity tester (BAT). 7 Assessment of visual function 7 163 7.9 Summary In this chapter, we have sought to review methods used to quantify the main visual functions, a defi cit of which may impact on levels of visual impairment. Specifi c attention has been paid to measurements of visual acuity and the variety of charts now available to those interested in the measurement of visual resolution. Attention has also been paid to the assessment of near acuity and the importance of assessing reading speed, ability and fl uency. Third, the importance of evaluating performance when undertaking low contrast tasks has been determined. Finally, issues surrounding the assessment of visual fi elds, colour vision and glare sensitivity in visually impaired patients have been covered.