OTM I Lecture: Visual Acuity
Dr. Julie A. Tyler
Using the patient’s visual acuity, accompanied with other testing, is necessary to determine the appropriate spectacle or contact lens prescription for the individual as well as give the physician insight into the health and integrity of the patient’s eye.
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Visual Acuity is the "resolving power" of the eye; that is, the smallest degree to which an individual can see a defined image. The measurement of visual acuity is dependent on a number of factors. Included in these factors is the type of task a patient is asked to perform. In 1955, measurement of visual acuity was classified by Weymouth into three main categories or tasks: minimum distinguishable (minimum visible), minimum separable, and minimum cognizable (or legible).
The task here is to see something as being distinguishable from the background; essentially what is the smallest "speck" the individual could pick out as an independent image.
For example:
Which is the smallest dot you can see?
.
. . . . . . . . . . .Resolution of this type (minimum distinguishable) varies depending on whether the target is a point (as above) or a line. For a point the expected resolution is 10 to 35 seconds of arc whereas a line may vary from .44 to 10 seconds depending on the width.
This represents the ability to determine if a group of targets are separate and distinct. When a grating is used (contrast sensitivity charts, interferometer, preferential looking) to measure acuity it is an example of this kind of acuity. A grating is a set of lines placed at various widths apart in order to determine if the patient can distinguish direction or separation of the lines.
For example:
When do you see 2 distinct lines?
II II II II II II II II II
When a grating is used, Borish reports that a minimum difference of 52 to 64 seconds of arc is required for minimum separation detection.
Another "type" of Mimimum separable acuity is called Vernier acuity. Vernier acuity is the ability to distinguish between two lines placed end-to-end with one laterally displaced. This is more sensitive (2 to 12 seconds of arc).
* The World Record is in Guiness for < 1 second of arc!Vernier acuity example
– when are they lined up?:I I I I I I
I I
I I I IThe task with this kind of measure is to be able to separate and distinguish forms. Generally, letters and numbers are used to perform this task but pictures/symbols are also used to measure minimum cognizable visual acuity. "True minimum legible", according to Davidson, is acuity in which complex patterns such as letters or numbers are used as the test image. "Form sense" is testing in which a simple form (the letter C, a wheel, the letter E) depends on orientation identification to measure acuity.
For example:
A
P E O T F T Z V E C
In 1862 Snellen introduced a system for measuring visual acuity that has become the fundamental method (the "gold standard") for measuring the visual acuity (resolving power/minimum cognizable) of the eye.
The Snellen test is a form of minimum legible visual acuity expressed in a fraction. The Snellen fraction is recorded in this way:
Testing distance
Distance at which the optotype (letter) would
subtend 5 minutes of arc at the retina
Snellen acuity is based on 20ft (6m) being equivalent to optical infinity. Optical infinity is the distance at which no accommodation (focusing) is being used by the patient to clear that distance and all distances further. Snellen acuity will be discussed in more detail later.
Figure 1 is an example of a Snellen letter and a standard Snellen acuity chart.
This measurement of visual acuity uses "form sense" minimum cognizable testing by using a letter "C" in different orientations and different sizes. Therefore, for patients that are pre-literate or illiterate, the doctor does not ask the patient to identify which letter or number is present but rather in which direction the opening of the letter C is oriented. Although this does provide letters of various size, a major drawback is that it is easier for the patient to guess which direction the opening is (1 in 4 chance) versus which letter is present (1 in 26).
Figure 2 shows an example of a Snellen letter & a Landholt "C" (or ring) next to side-by-side Snellen, Landholt & Tumbling E charts.
This measure of visual acuity is very similar to the Landholt C visual acuity testing as it relies on "form sense" for evaluation of minimum cognizable/minimum legible acuity. However, testing with the letter E involves a more detailed letter than Landholt’s C
(Snellen’s "perfect" letter is E as it has 5 distinct details measuring 1 minute of arc each -resulting in an image perfect for subtending 5 minutes of arc on the retina). This test is commonly used in pre-literate children and may be used in illiterate patients as well. Again, the "E" game is performed by having the patient identify the orientation of the "legs" of the letter but is limited by a one in four chance vs. one in twenty six with Snellen acuity.Figure 2 shows an example of a Tumbling E visual acuity chart.
Used for pre-literate or illiterate patients, especially in children as a measure of minimum cognizable acuity. Common images are displayed on the chart in black and white for the patient to identify. One example of this type of test is seen in Figure 3. With this test, small cards are placed in front of the patient with images of the apple, house, and umbrella. Then the patient is asked to look across the chart and point to the image they see on the small card. Thus, for non-verbal patients they can identify the image by matching. However, in this scenario the patient has only a 1 of 3 choice – increasing the occurrence for correct "guesses". Another picture chart uses various basic images (e.g.- cake, bird, etc) that even a child may be able to identify verbally.
Figure 3 shows an example of a symbol visual acuity chart.
B. Non wall/projection charts
Visual acuity assessment in children, especially non-verbal children, presents particular challenges. However, studies reveal that infants will view a perceived pattern stimulus over a non-pattern stimulus when presented at the same time. With preferential viewing techniques the child is shown two cards equally distant from the child. One card has a grating (a pattern), the other has the same overall luminance (non-pattern). If the child can distinguish the grating by "preferentially" looking at it, it is presumed the child can see it distinct and more interesting than the other target and thus has minimum separable acuity for that level.
Although Snellen visual acuity charts are the "gold standard" in measuring acuity, the do have some defects. Such defects include uneven number of letters on a line causing crowding and non-proportional spacing between lines and letters. Therefore, the Bailey-Lovie chart was developed. The Bailey-Lovie chart overcomes the specific problems mentioned above and may give a more accurate assessment of visual acuity (minimum cognizable), especially in a patient with decreased potential vision (low vision patients). This chart was designed for testing at 4 meters and covers the Snellen equivalents of 20/10 to 20/200 letters with ALL rows having 5 letters.
An example of the Bailey-Lovie Visual Acuity Chart is shown below:
Another challenge to measuring visual acuity occurs when patients have cataracts/media opacities and trying to determine the patient’s potential acuity if surgery were performed. The most widely used method is to project an image onto the retina. In the case of a potential acuity meter, a miniature Snellen chart is projected onto the retina:
The left shows the potential acuity meter mounted on a slit lamp. On the right a drawing provides an image of how this projects on the retina through a gap in the lens opacities ( the cataract).
One other instrument that is used to measure potential acuity is a hand held device called a laser interferometer. Although this instrument is often more successful in projecting through the cataract it uses gratings for targets; thus, the patient has a 1 in 4 chance of guessing correctly versus 1 in 26 for a PAM using letter targets.
C. Pinhole Testing
Pinhole Visual Acuity makes use of a small hole in an occluder to determine if the patient's visual acuity would improve with a refractive correction. The pinhole acts to "focus" and direct light straight onto the macula without scattered rays. By doing this it simulates a lens with refractive correction that would focus light for a patient. This test allows the tester to get an idea of the patient's potential acuity. It will improve the visual acuity of a patient with uncorrected refractive error and some "disease" such as cataract.
A pinhole visual acuity should be performed when the patient's visual acuity with their current prescription does not measure better than 20/30. To perform the test, keep the untested eye completely occluded and cover the eye being tested with the pinhole occluder over the top of the patient's habitual prescription. The patient should be able to see a chart through the small hole.
If the patient's visual acuity improves with a pinhole that should be noted and to what level it improves. Also, if improvement occurs refraction should be performed on the patient. If, however, the pinhole placed in front of the patient's pupil does NOT improve the visual acuity other things should be considered that may be causing the decreased visual acuity such as ocular disease.
D. When the others don’t work……………
When patients are not able to see a standard visual acuity chart due to ocular pathology, the next best measure of acuity is counting fingers. To perform this test the patient is asked to occlude one eye and looking straight ahead with the uncovered eye. Then the tester presents non-moving fingers in front of the patient at a specific distance and asks the patient to say how many fingers are seen. (In general, one, two or five fingers are displayed – three and four get confused too easily)
If the patient can see the correct number of fingers, the tester may move further away. If the patient cannot see the correct number of fingers the tester may move closer and test peripheral points.
Recorded: CF (counting fingers) at 2 feet OD,OS
If the patient cannot count non-moving fingers displayed by the tester at near, the tester then moves their hand in front of the patient’s eye (remember to test visual acuity monocularly first). If the patient can detect the hand motion the tester can move further away, if the patient cannot detect the hand motion the tester may move closer or test peripheral views.
Recorded: HM (hand motion) at 1 foot OD
If the patient cannot detect hand motion in an eye, the next test of acuity uses light. A trans-illuminator or pen light is placed in front of the patient’s non-covered eye and the light is shown into the patient’s eye. If the patient can detect the light then the tester moves the light to different positions (up, up/left, up/right. Right, left, etc) and asks the patient to state from which position the light is seen.
If the patient can tell from which direction the light is projected, the patient has light projection acuity in that eye. If the patient can see the light but cannot tell from which direction it originates then the patient has light perception without projection.
Recorded: Lproj (light projection) OD
LP (light perception) OS
5. No Light Perception-
If the patient is not able to detect light in an eye, that eye is considered totally blind. This is recorded as no light perception.
Recorded: NLP (no light perception) OD
(A discussion of calculating letter size and distance is also found in Figure 1)
Targets used to measure visual acuity by means of minimum cognizable acuity/minimum legible, need to follow specific design requirements for the test target resulting in OPTOTYPES. A typical Snellen optotype subtends 5 minutes of arc with the ability to have five "details" that subtend 1 minute of arc. Snellen chose this design because of the belief that an individual could not separate images (minimum separable acuity) unless the images were at least 1 minute of arc in size. And because the English alphabet has letters with a maximum of 5 details in one oriention, 5 minutes of arc was used for the Snellen optotype total size.
Thus, if a patient is able to resolve the "detail" of 1 minute of arc in an optotype that subtends 5 minutes of arc at 20 feet, that patient’s visual acuity would be 20/20.
If the patient is not able to resolve an optotype of that size, other size optotypes are calculated for different distances on the Snellen chart. For each size of letter, that letter is described by the distance at which it would subtend 5 minutes of arc. The size of each letter is determined using the tangent of the 5 minute angle at a given distance – therefore, a letter for 40 feet would be twice the size of the letter for 20 feet.
For example:
We can set up a right triangle and solve for X (representing the overall letter size that subtends 5 minutes of arc at a specific distance).
Let’s say that the distance of the person from the chart is 18.5 feet and the image is to subtend just 5 minutes of arc.
X
E_________ 5min of arc18.5 feet
Tan < = opposite length
Adjacent length
Tan 5’= X (tan 5’) 18.5ft = X
18.5 ft
(.00145) 18.5ft = X .0268ft = X
(.0268ft) (12inches) (2.54cm) (10mm) = 8.18 mm
(1 foot) (1inch) (1cm)
Thus, a letter that is 8.18mm subtends 5 minutes of arc at 18.5feet and therefore, a letter that is 8.18mm tall is called an 18.5 Foot-Letter
Example 2:
What is the size of a letter that subtends 5 minutes of arc at 20feet?
X
E__________5min of arc20feet
Tan 5’= X (tan 5’) 20ft = X
20 ft
(.00145) 20ft = X .029ft = X
(.029ft) (12inches) (2.54cm) (10mm) = 8.839mm
(1 foot) (1inch) (1cm)
**** Remember that when performing this equation we are taking the tan of 5 minutes of arc. If the calculator used can convert this directly then take the tan of 5'. However, if the