«Colenbrander - Basic Low Vision Exam THE BASIC LOW VISION EXAM August Colenbrander, MD, San Francisco This chapter is based (with some modifications) ...»
Colenbrander - Basic Low Vision Exam
THE BASIC LOW VISION EXAM
August Colenbrander, MD, San Francisco
This chapter is based (with some modifications) on a chapter that appeared in the (1994)
Vision Rehabilitation issue of the Ophthalmology Clinics of North America, 7-2: 151-162
Before we embark on any intervention we should know our starting points. This holds true for
Low Vision Rehabilitation as it holds for traditional medical interventions. When patients arrive for a Low Vision evaluation, they often ask why they should have another eye exam. "My doctor just dilated me and took lots of pictures". We explain that their regular ophthalmologist is concerned with the causes of their eye condition and with the treatment thereof, our service is aimed at dealing with the consequences of the eye condition. A dilated exam and dazzling indirect ophthalmoscopy are essential for determining proper treatment, but they prevent us from assessing the patient's undilated and undazzled reading performance.
Several aspects that represent subtle but important differences between a Low Vision evaluation and a traditional eye examination deserve mention.
In a traditional examination the patient is seated for the examiner's convenience and for easy access to the slitlamp, phoropter and ophthalmoscope. For the Low Vision evaluation the arrangement should be for the patient's viewing and reading convenience. An adjustable hospital bed table placed in front of the exam chair facilitates this. For many patients who have a need to search around their scotomata, an old fashioned trial frame with full diameter wire-rim lenses is more appropriate than a phoropter.
To indicate that macular function is below normal, vague statements such as 'count fingers' may be sufficient. To accurately predict the amount of magnification needed, more accurate measurements are necessary than are possible with a standard projector chart. Since most people do not spend their days in a semi-dark room looking at brightly projected images, visual acuity measurement on a printed chart in a lighted room is a more appropriate predictor of daily performance than is the use of a projector chart. We will discuss the details later in this paper.
In the prevalent model of medical and surgical treatment, the doctor has the active role and generally is the decision maker; the patient is asked for compliance and (informed) consent. In rehabilitation the roles are reversed: the doctor provides guidance and shows the patient options; the patient has the active role and must work at re-learning certain skills (whether that is to walk with crutches or to read with a magnifier) and must decide which of several options is most appropriate for a particular living situation. The doctor cannot select and 'prescribe' low vision aids in the way he/she prescribes glasses or medications; the doctor can only recommend solutions, the patient must choose which one to use. This is one of the reasons why the use of a loaner system is so successful, since it allows the patient to verify that what seemed to work best in the clinician's office is also the best solution at home, at work or at school. Failure to recognize this role reversal from the 'medical model' to the 'rehabilitation model', whether it is on the part of the doctor, the patient or both, may lead to failure of successful rehabilitation. Sometimes the role reversal is more readily accepted if the Low Vision adviser is an associate or consultant, rather than the primary ophthalmologist, since this avoids the need for repetitive role reversals between the medical and the rehabilitative model.
Colenbrander - Basic Low Vision Exam Yet it is important that the primary ophthalmologist presents the referral as a natural extension and completion of the medical and surgical treatment, rather than as an admission of failure.
VISUAL ACUITY MEASUREMENTVisual acuity notations are used so often that the underlying mathematical meaning is often forgotten. An eye is said to have a visual acuity of one half of normal if that eye can just recognize standard symbols that are twice as large or twice as close as those just recognized by a standard eye. If the symbols need to be ten times larger (or ten times closer), visual acuity is said to be one tenth of normal. Thus, visual acuity provides a direct estimate of the amount of magnification required to perform standard tasks. To determine that a macula is abnormal it does not make much difference whether the visual acuity is measured as 20/200 or 20/400; the latter measurement, however, indicates a need for twice as much magnification as the former.
The following pages will describe a simple, systematic approach for the accurate determination of visual acuity in low vision patients using a 1-meter test distance for letter chart acuity and a variable test distance for reading tests.
LETTER CHART TESTINGDistance visual acuity measurement is traditionally carried out at 6 meters (20 feet). For routine eye examinations this measurement is used to answer one of two questions. Is the visual system normal or abnormal, and can the refractive correction be improved? For both purposes the range of normal and near-normal acuity is most important. Our regular charts reflect this. In the 20/15 - 20/20 - 20/25 - 20/30 - 20/40 - 20/50 - 20/60 - 20/80 - 20/100 range three steps equal a change in the visual angle by factor of two. In the low vision range, accuracy on most charts drops significantly. For 20/100 - 20/200 - 20/400 each single step represents a factor of two. The common notations of 'Count Fingers' and 'Hand Movement' are even less accurate.
For low vision, more accurate measurements are critical. The measurement range can be extended by moving the test charts normally used at 20 ft. (6 m) to a closer distance. Many low vision practitioners do this by moving from 20 ft. to 10 ft. (3 m), thus gaining a factor of two. We recommend bringing the chart to 1 meter, thus gaining a factor of six. We use a printed chart with external illumination, since a printed chart in a lighted room is a more appropriate predictor of everyday function, than is a projector chart in a dark or semi-dark room.
Using a 1-meter viewing distance also brings us in line with the metric system and provides for much easier calculations than does the use of feet and inches. Since we use the metric system to measure the viewing distance, we should also use the metric letter size unit. Snellen used the letter 'D' for this unit; Louise Sloan introduced the term 'M-unit' to prevent confusion with the symbol 'D' for diopters. Standard acuity (1.0, 20/20) is defined as the ability to just recognize a standard letter (1M-unit) at a standard distance (1 meter) or a letter with the same visual angle (x M-units at x meters).
Using a letter chart at one meter has significant advantages:
• It extends the measurable range down to 1/50 (20/1000) if a chart with 50M letters (20/200 at 5 m) is used and makes more letters per line available at each level. Table 1 demonstrates how a standard chart at 20 ft. covers mainly the normal and near-normal ranges; bringing the chart to 10 ft. includes the moderate low vision range (The single 'big E' at 20/200 is a memory test rather than a visual acuity test); bringing the chart to 1 meter extends accurate measurement to the ranges of severe and profound low vision. (see Table 1)
Colenbrander - Basic Low Vision Exam
• It is easy to maintain the exact testing distance by attaching a one-meter long string to the chart that can be extended to the patient's forehead.
• The chart is within arm's reach of the examiner, who does not have to get up to point to the chart.
• One meter is 1 diopter from optical infinity; thus, a refraction carried out at this distance simply needs 1 diopter subtracted for distance correction; conversely, a 1 diopter trial lens placed over the distance glasses will focus the patient for 1 meter testing. This can be done with a trial lens in a Halberg clip. A convenient alternative is to place a wide pair of +1D drugstore reading glasses over the patient's own correction.
• A simple metric Snellen fraction 1/... results from testing at one meter. The denominator of this fraction represents the theoretical number of diopters of add necessary for reading of 1M print. This is known as Kestenbaum's rule. Multiplying both numerator and denominator of the 1/... Snellen fraction by 20 will give the familiar 20/... notation.
• Do not push the patient too far for marginal performance, nor help by pointing to each individual letter. Determine the level at which patients can perform with reasonable comfort using their own fixation ability.
Colenbrander - Basic Low Vision Exam
• For occasional use and portability, a plastic chart with a 1-meter string attached to an occluder is useful. Ask the patient to hold the occluder to one eye and the chart will automatically be at 1 meter. For more frequent use it is helpful to place the chart on a movable easel. Again, use a 1-meter string so that the distance of the easel can be adjusted when the patient leans forward.
• In principle any letter chart can be used. However, charts with a consistent geometric progression of letter sizes and with letter size notation in M-units are preferred. Table 2 provides conversions if a chart without M-unit notation is used. A special chart with a 1 m cord and occluder attached and with an extra wide range of letter sizes from 50 M (20/1000 at 1m) to 1M (20/20 at 1m) is now commercially available .
Older charts and recent charts often carry metric letter size notations. When using a 20 ft. chart without such notations, the above conversions can be used for a chart calibrated for use at 20ft.
When in doubt, measure the height of the letters in mm. Remember that 1 M = 1.45 mm or 7 M = 1 cm. For a convenient clinical approximation 1 M = 1/16" may be used.
Jaeger numbers refer to the catalogue numbers in the print shop where Jaeger selected his reading tests in 1854. They have no mathematical meaning and are implemented inconsistently on different cards.
Point sizes are used in printing. Within one type style they are proportional to the letter size. However, actual letter sizes may vary for characters of different styles.
M-units are the units used in Snellen's visual acuity formula. They apply for distance charts and reading cards. Characters with the same M-unit designation have the same recognizability.
Colenbrander - Basic Low Vision Exam
READING PERFORMANCEWhen the purpose of measurements at different distances is to determine accommodative range and to prescribe for presbyopia, it is proper to distinguish between 'distance' and 'near' vision. For rehabilitation purposes the distinction between 'letter acuity' and 'reading acuity' is more relevant because reading difficulties constitute the most frequent complaint of low vision patients. Letter recognition requires only a very limited visual area; word recognition requires a larger area and fluent reading requires the availability of additional areas to the right of fixation to guide successive saccades. Letter acuity may be measured at far distances (4, 5, 6 m, 20 ft, most useful for patients with normal vision), at an intermediate distance (1 m, most useful for low vision) or at near distances (pocket screener, not recommended); reading acuity can also be measured at far distances (bill boards) or near (continuous text reading card).
Traditionally, reading vision is most often tested at a fixed distance, with some practitioners using 40 cm (16", 2.5D add), some 14" (35 cm, 2.87D add), some 3 D (33.3 cm, 13"). For low vision patients these reading distances may be too far away. An additional problem often is that the patient's current correction is not optimal for the standard test distance. Therefore, a method is needed that allows simple calculations for various short reading distances.
Such a method will be described.
As with distance acuity, two values are needed to calculate visual acuity: letter size and reading distance. Many practitioners are in the habit of recording just the letter size read (e.g.: 'J7') without specifying the reading distance. This is a bad habit in general, but for Low Vision work such notations are completely meaningless. The ability to read news print at 5" is very different from the ability to do so at 15" or 20".
Another problem is the use of 'Jaeger numbers' to describe letter size. Jaeger numbers refer to the catalogue numbers in the print shop where Jaeger selected his reading tests in 1854. They have no mathematical meaning. Worse, they are inconsistently used as one can easily determine by comparing charts from different manufacturers.
If both distance and letter size are recorded properly, the same visual acuity value should be calculated for all distances. The problem is that the most prevalent measures: inches and J numbers make such calculations impossible. The solution, again, is to adopt metric measurements. Recording the reading distance in cm and the letter size in M-units gives us the information we want and makes calculations possible. A remaining problem, however, is that the traditional notation of the Snellen fraction still leaves us with the need for division of two numbers and that the reading distance in meters becomes a fraction-within-a-fraction.
Since most people are better at mental multiplication than at mental division, could a multiplication be used instead? Indeed, this can be done. Since many optical formulas require the reciprocal of a distance, Monoyer (1872) suggested using the term 'diopter' to indicate the reciprocal of a distance in meters. We are thoroughly familiar with the use of diopters in connection with lenses (lens power expressed as the reciprocal of the focal distance), but tend to forget that diopters can be used in any formula where the reciprocal of a distance is needed.