Visual Acuity Testing
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This is Mark Wilkinson from the University of Iowa Department of Ophthalmology & Visual Sciences. In this presentation I will discuss how to measure and record both distance and near visual acuity.
We will be discussing what visual acuity is. How visual acuity is used.
The various classifications of visual acuity. Types of visual acuity testing. Chart design and the assessment and recording of distance & near acuity.
Visual acuity is a clinical test of the spatial resolving capacity of the visual system. Visual acuity testing determines the ability of the eye to see fine detail.
The Snellen visual acuity notation was developed by Hermann Snellen in 1862, during the time of the Civil War. Snellen notation is the ratio of the test distance to the distance at which the critical details of the smallest optotype seen, subtend 1 minute of visual angle. With this in mind, the standard E used on a Snellen eye chart subtends 5 minutes of visual angle, with 1 minute for each of the 3 horizontal arms of the E and 1 minute for each of the 2 open spaces between these horizontal arms.
Visual acuity testing allows for the quantification of the degree of high contrast vision loss. Visual acuity testing also monitors stability or progression of disease as well as the change in the patient’s visual abilities as treatment and rehabilitation progress. Visual acuity testing also allows assessment of eccentric viewing postures, scanning abilities and motivation. Visual acuity testing does not tell us about the individual’s quality of vision. It only speaks to the patient’s quantity of vision.
We will discuss quality versus quantity of vision in a few minutes.
Accurately measuring visual acuity is important for a number of reasons. First, it allows the clinician to determine best-corrected acuity with refraction.
Near visual acuity testing allows the clinician to estimate the dioptric power of optical devices necessary for reading regular size print.
Visual acuity testing also allows the clinician to verify eligibility for tasks such as driving and also to verify eligibility as “legally blind”.
Inaccurate acuity testing underestimates the ability of the patient, which is why accurately measuring visual acuity is so important.
Visual acuity testing has traditionally has been used as the primary indicator of the magnitude of functional impairment due to vision loss.
This is because visual acuity is a strong predictor of self-reported vision-related quality of life.
However, it is important for the clinician to know that an individual’s disability level cannot be confidently predicted from his or her visual acuity alone.
For any arbitrary cutoff point, there will be substantial numbers of people with better vision who will have more difficulty than expected when performing a given task. And, there will be a similar number with poorer vision, who will have less difficulty than expected for a given task.
In 1966 the World Health Organization (WHO) reported that there were 65 different definitions of blindness used around the world. At that time, the WHO proposed a uniform definition of blindness as best-corrected vision of less than 20/400 in the better eye.
In the US, best-corrected vision of 20/200 or worse in the better eye was the standard until February 2007.
Currently, the Social Security Administration’s definition of legal blindness in the United States is “remaining vision in the better eye after best correction is less than 20/100 or contraction of the peripheral visual fields in the better eye (A) to 10 degrees or less from the point of fixation; or (B) so the widest diameter subtends an angle no greater than 20 degrees using a Goldmann III4e target.”
With a Humphrey perimeter, the clinician should use the Social Security Administration (SSA) Kinetic test. The SSA Kinetic uses a Goldmann III4e equivalent target with a kinetic testing pattern.
It is important to understand what this definition means. The definition of legal blindness means that a person cannot be legally blind in one eye, unless they only have one eye. Additionally, the person cannot be legally blind without their glasses. In both these cases, the definition of legal blindness is not being followed. Remember, the definition states “best-corrected vision in the better eye.”
Here you see the different classifications of vision loss based on WHO and International Council of Ophthalmology standards.
Normal 20/25 or better
Mild vision loss 20/32 - 20/63
Moderate vision loss 20/80 - 20/160
Severe vision loss 20/200-20/400
Profound vision loss 20/500-20/1000
Near blindness < 20/1000
It is important to remember that distance visual acuity does not tell the clinician what the person’s functional vision is. To understand how the person is able to use their vision functionally, the clinician needs to know the patient’s near acuity, contrast sensitivity, how glare affects them and the results of Amsler Grid testing. Additionally, is there central, para-central or peripheral visual field loss in one or both eyes? Also, is their color vision affected. Finally, for me, along with the results of contrast sensitivity testing, I want to know what the patient’s response to optical magnification is. If their contrast is down and they do poorly with optical magnification, I know that their quality of vision is less than their distance acuity would indicate.
Let’s talk about visual function versus functional vision.
Visual function describes how the eye functions. It tells us the patient’s quantity of vision, based on their visual acuity and visual field findings.
Functional vision describes how the person functions in vision-related activities. This tells us about the patient’s quality of vision and gives the clinician a better understanding of how the patient is able to use their vision for activities such as reading, mobility, employment and activities of daily living. Functional vision helps the clinician to understand how the patient’s vision affects their quality of life. This is because functional vision takes into account factors such as loss of contrast sensitivity, photophobia, and/or ocular motor problems.
Here you see a variety of different visual acuity testing charts.
When testing visual acuity, the clinician should test both distance and near acuity. For near acuity testing, the clinician can use single objects; numbers, words or letters as well as continuous text print. When doing near acuity testing, always document the working distance that the near testing was done at, if different than a standard 16-inch or 40 centimeter working distance.
Ideally, “counts fingers” should not be used as a distance acuity measurement. This is because if the patient can see fingers, they can read the larger numbers or letters on a low vision eye chart. That said, if counts fingers is used, note at what distance. For example, counts fingers at 2’ or CF at 1m.
If the patient has only hand motion vision, note in which quadrants and at what distance?
If the patient has only light perception with projection vision, note in which quadrants and at what distance?
Finally, note if the patient can only perceive light on versus off.
You are not likely to see many projector charts around any more. This is a good thing because these charts had many problems that made accurate visual acuity testing more difficult. Those problems included poor contrast, variable illumination, too few letters per line and too few lines at low acuity levels. Additionally, these charts could not be moved closer to the patient when the patient had less than 20/400 acuity. As the result of these problems, this type of acuity chart routinely underestimated the patient’s real acuity, particularly for patients who were visually impaired. This often resulted in a depressing situation for patients with poor acuity. “I can only read the big E.” or “I cannot even read the big E”.
This is an example of a LogMAR designed chart. LogMAR stands for Log of Minimum Angle of Resolution. This is a metric designed chart.
The design of this chart allows for a logarithmic or proportional change in letter size and spacing. This results in the visual angle on the chart doubling every three lines. Also, the chart is set up to have the same number of letters on each line. Finally, the letters used are equally legibility and each line is equally difficult to read.
The Early Treatment of Diabetic Retinopathy Study (ETDRS) Chart uses a LogMAR design. The ETDRS chart is the chart used most often in clinical research. It can be rear or front illuminated.
There are 3 different charts to allow for testing of each eye and both eyes together on different charts. Each chart uses 10 Sloan letters in a Sans-serif font.
The charts are movable, allowing for testing at 1, 2 or 4 meters. 4 meters is the standard testing distance used with LogMAR charts. The different testing distances provide for a larger acuity range, from 4/2.5 = 20/10 to 1/40 = 20/800.
Here is an example of a Feinbloom acuity chart. The Feinbloom chart is moveable, allowing for a larger acuity testing range, from 20/10 to 1/700 (20/14,000). This is a very useful chart for testing patients with lower acuity levels.
The problem with this chart is there are a variable number of optotypes per line, ranging from 1 letter as you see here, to 9 letters at higher acuity levels. Typically, when testing with a Feinbloom chart, the clinician will begin testing at 5 or 10 feet.
When recording visual acuity, you must know you testing distance, if different than 20 feet or 6 meters for a Snellen acuity chart or 4 meters, which is 13.14 feet, for a LogMAR chart.
With a LogMAR chart, to measure metric visual acuity, the clinician will measure the testing distance in meters. The clinician will next note the letter size read in M units.
The visual acuity is then recorded as Test distance (m)/Letter size read (M units). For example, if the patient could read the 32 line at 1 meter, their acuity would be recorded as 1/32. 1/32 is equal to a Snellen acuity of 20/640.
You can use Snellen equivalent numbers with a LogMAR chart. To do this, you will again measure the test distance in meters. Next convert the metric distance to a foot equivalent. Because the standard testing distance for a LogMAR chart is 4 meter, this is equivalent to a 20 foot Snellen testing distance for purposes of these charts. 2 meters is equivalent to 10 feet and 1 meter is equivalent to 5 feet.
Next, note the letter size read in Snellen equivalent. The Snellen acuity = Test distance (feet)/Letter size read (Snellen equivalent). For example, if the patient could read the 160 line at 1 meter, a 5’ equivalent, their acuity would be recorded as 5/160. 5/160 is equal to a Snellen acuity of 20/640.
When using the Feinbloom Chart, the clinician will again note the testing distance and the size of the last numbers read. For example, if the patient could read the 180 letters at 2 feet, their acuity would be recorded as 2/180. 2/180 is equivalent to a Snellen acuity of 20/1800.
The use of a Feinbloom chart eliminates the need for the use of counts fingers acuity.
There is great value for the clinician to observe their patients during acuity testing. This allows the clinician to note the patient’s speed and accuracy when reading the eye chart. The clinician can also observe the patient’s head and eye position, including their eccentric viewing or null point position. The clinician will also be able to note consistent errors that would indicate the location of a scotoma. Missing one side of the chart may indicate your patient has a peripheral field defect.
Finally, visual acuity testing is typically done by first testing the right eye, followed by the left eye and finally both eyes together.
Now we will discuss near visual acuity testing.
It is important to know that when doing near acuity testing, reduced Snellen acuity is only accurate at a fixed testing distance, which is 40 cm for most near acuity cards.
You should also know that Jaeger numbers have no numerical meaning. Jaeger numbers refer to item numbers in a printing catalogue in Vienna in the 1850s. The International Council of Ophthalmology has stated that the lack of external definition of Jaeger numbers makes them extremely variable. With this in mind, Jaeger numbers should not be used for near vision testing.
The preferred method for near acuity testing uses the M-unit. The M-unit is the only letter size unit that is well defined. A 1M letter subtends 5 minutes of arc at 1 meter. For reference purposes, a 1M-sized letter is equivalent in size to newsprint. 2M is equivalent in size to standard, 18 point large print and 0.5M is equivalent in size to print half the size of newsprint.
When measuring near visual acuity, the patient will use their reading correction. Have the patient hold the reading card at their normal reading distance. The clinician should note errors related to scotomas and visual field loss. Near acuity is recorded as M units @ the testing distance. For example 1.25M @ 40cm (16”)
When doing near acuity testing with a Game Card, there are a number of classic “Hits and Misses”. For example, the patient may say Hair instead of Chair, Let instead of Left, Bake instead of Baker. These misses may be a better indicator of scotomas than Amsler grid testing in some patients.
It is important to recognize that reading single words is a much easier task that reading continuous text. Because our patients want to be able to read magazines, newspapers and their mail, it is important to check their continuous text reading abilities, not just their ability to read single words or letters.
Now you know how to accurately test distance and near visual acuity. Remember, inaccurate acuity testing underestimates the patient’s abilities and prevents the clinician from accurately knowing the progression of disease and benefits of therapy.