Clinical Optometric Procedures: Visual Acuity

Clinical Optometric Procedures: Visual Acuity

CHAPTER 3 – VISUAL ACUITY

• This chapter includes a review of the following:
• Classification of visual acuity
• Notations of visual acuity
• Clinical testing of visual acuity
• Distance visual acuity measurement
• Near visual acuity measurement

CLASSIFICATION OF VISUAL ACUITY (VA)

Visual acuity is defined as the “spatial resolving capacity of the visual system” (Benjamin, 2006) and refers to the sharpness of vision or the patient’s ability to recognize a minimum size target. Visual acuity is customarily abbreviated as 'VA'. The measurement of VA should be performed at every patient’s (Px) visit on completion of a case history. It is one of the most informative tests conducted. Visual acuity provides information on;

• Refractive status of the eye
• Indication of macula function
• Indication of neural integrity

One can compare visual status of an eye and/or between one eye and another to see if the visual acuity is comparable and similar or different. VA allows us to compare the sharpness of vision between the 2 eyes, the eyes of the same Px at different times, or between the eyes of different Px’s. It provides a scale by which we can communicate our assessment of a Px’s visual ability to other optometrists.

• Figure 3.1 A Visual acuity chart International Centre for Eyecare Education

Tests of Visual Resolution

According to Benjamin (2006), there are a variety of tests of visual performance that measure some aspect of the limits of the visual system’s ability to discern detail or to recognize detailed targets.

• These include:

1. Minimum detectable resolution

Minimum detectable resolution refers to the threshold of an individual’s visual system to detect the presence of a spot or a line stimulus against its background. This assessment of visual performance does not require the discrimination of target detail but rather requires the individual to perceive the presence or absence of an aspect of the stimuli presented (Fig.3.2). Positive contrast is depicted by a bright stimulus (point or line) against a dark background (Fig. 3.2 a) while negative contrast is observed when a dark point or line is presented against a bright background (Fig. 3.2 b). Minimum detectable resolution is usually of little clinical application, except in nightdriving vision and is not considered to be a routine examination technique.

• Figure 3.2 (a) positive contrast; (b) negative contrast

2. Minimum resolvable

Minimum resolvable visual performance involves the resolution of detail. Clinical evaluation of VA is based upon this type of visual performance. Most of our tests of VA are recognition tests that involve the measurement of the smallest symbols, shapes, letters that can be correctly identified/resolved by the Px.

This type of acuity can be separated into 2 forms:

a. Form sense: (Landolt rings, Tumbling E’s), in which a simple form is used and some task, such as orientation, is used to determine the acuity (Fig 3.3 a and b).

Landolt C’s/Rings: These are circles with a gap (Fig. 3.3 a). The gap is orientated in four directions, viz. up, down, right or left. In some cases, charts also include oblique positions of the gaps in addition to cardinal positions.

Tumbling E’s: This test involves the presentation of an “E” in different orientations (cardinal and/or oblique) at every acuity level (Fig. 3.3 b). The Px is instructed to identify the direction in which the limbs of the E are pointing. This test, like the Landolt C test is useful in measuring the VA in Pxs who are not familiar with letters of the alphabet such as toddlers.

b. True minimum legible: In which complex patterns such as letters or numerals are used as the test stimulus (Fig. 3.3 c). These are referred to as letter optotypes (targets) (Fig. 3.3 c). These were originally designed by Snellen using sans-serif letters and later were changed to Sloan letters since the serifs did create some confusion. International Centre for Eyecare Education Figure 3.3 (a) Landolt C’s; (b) Tumbling E’s; (c) Letter optotypes 3. Minimum separable / Vernier acuity The assessment of vernier acuity involves the individual’s ability to detect that a group of points or lines are separate and distinct, e.g. detecting a break in a line. This type of acuity is usually used to measure the resolution capacity of the visual system. Targets are usually presented as either lines or gratings of equal separation from each other. The Px has the task of determining the minimum separation between line targets that allows them to distinguish the lines from each other. For grating targets, the Px has to determine the finest grating that can be distinguished from a uniform field of uniform luminance.

4. Stereoacuity

Stereoacuity involves the use of both eyes. It represents the ability of the Px to resolve slight differences in distance of target objects when looking at special stereoscopes. Stereoacuity is measured in seconds of arc (using the disparometer).

NOTATIONS OF VISUAL ACUITY

• VA expresses the angular size of the smallest target that can just be resolved by the Px.
• There are various different ways in which they can be specified, namely:
• 1. Snellen Fraction
• 2. Decimal Notation
• 3. Minimum angle of resolution
• 4. Logarithm of minimum angle of resolution
• 5. Visual acuity rating
• 6. Visual efficiency
• 1. Snellen Fraction

Snellen principle

The Snellen test is a form of minimum legible visual task (i.e. involving the resolution of detail). Helmholtz found that for 2 objects to be distinguished as separate, they must be separated by a minimum angle of resolution of one minute of arc at the nodal point of the eye. If the images of 2 distant bright objects fall on a single cone receptor in the retina, the observer will only be aware of a single light stimulus. International Centre for Eyecare Education If the stimuli are moved further apart so that their images form on 2 adjacent cone receptors, the observer will still be aware of only a single light stimulus. If the objects are moved still further apart so that one unstimulated cone receptor lies between the 2 stimulated cone receptors (Fig. 3.4), the observer will be aware of 2 light stimuli. This threshold of the Px’s ability to resolve detail is represented by the diameter of one foveal cone which is 0.002mm (2 microns).

Figure 3.4 (a) Diagram depicting separation of cones to resolve 2 objects as separate Figure 3.4a shows a diagrammatic representation of points A and B which represent 2 visual stimuli that are presented to the eye, N the nodal point of the eye, P the principle plane of the eye, a and b the retinal images of the visual stimuli A and B respectively.

When considering the angular subtense at the nodal point of the eye, it was found that this threshold was approximately one minute of arc. This implies that targets must be separated by at least 1' of arc in order for detail to be discernible.

Figure 3.4 (b) Angular subtense to perceive 2 separate visual stimuli In the construction of an ‘E’ on a letter chart, the thickness of the limbs and the spaces between them each needs to subtend an angle of ‘1 minute of arc’ at the nodal point of the eye for the eye to detect the separation between the various parts of the letter E. These gaps in the letter provide identity clues to the patient. Using the principle of gap separation and angular subtense of elements of letters presented to a patient, a test chart consisting of about 10 lines of letters of different sizes of letters was constructed. The overall height of all of these letters subtended 5 minutes of arc in total, with the detail size or limb width each subtending 1 minute of arc (Fig. 3.5). International Centre for Eyecare Education Figure 3.5 Construction of a Snellen E The overall height of a letter subtending 5 minutes at 6m was measured at 8.73mm. Snellen thus concluded that a test letter 8.7mm (Fig. 3.6) high presented at a distance of 6m would just be resolvable by an unaided, healthy, 'normal' eye, giving the patient a visual acuity of 6/6.

Figure 3.6 Height of a Snellen ’E’

If we designed a chart to measure the VA of a Px using letters which are 8.7mm high, we could measure the VA on anyone. For those with normal acuity, this task would be easy. However, for those Pxs who are either myopic or hyperopic, this task would be difficult at a 6m distance. The Px would need to move closer to the chart. The assessment of his VA would entail discerning the 8,7mm letter at 3metres, or at 2metres, etc. This would however be a cumbersome manner in which to measure the VA of a patient.

It follows that if a Px is able to discern an 8.7mm letter at 6 metres, then he will be able to discern a 17.4mm letter at 12 metres. So we could thus design a VA chart with letters in multiples of 8.7mm size changes. For our Px who could not see the 8.7mm letters at 6m, we could simply present letters of larger sizes at a distance of 6m and we can then determine his VA relative to a Px’s 'normal' VA.

E.g.: If the Px was able to discern the 26.1mm letter at 6 metres, then we could denote the VA of this Px as being the same as a normal sighted Px at 18 metres. The VA would be recorded as 6/18.

The Snellen fraction

The Snellen fraction is an expression of the angular size of optotypes by specifying the test distance and the height of the letters. In this fraction, the number indicating the height of the letters is actually the distance at which the letter height that is discernible subtends 5 minutes of arc. For example, a 6m letter is a letter of a specific height that subtends 5 minutes of arc at 6 meters.

• The Snellen fraction is denoted as:
• Letter size and test distance
• The range of letter sizes (Fig. 3.7) on most charts from top to bottom is as follows:-
• 6/60; 6/36; 6/24; 6/18; 6/12; 6/9; 6/7.5; 6/6; 6/5.

Figure 3.7 Progression of letter size on a Snellen Chart The letter 6/60 is ten times larger than the 6/6 letter, i.e., 87mm. If the Px cannot see this letter, then the examiner should either move the chart closer to the Px or the Px closer to the chart. The chart is moved to a distance of 3 m and the procedure repeated. Following the inability of the Px to resolve the letters at 3m, the chart is moved to a distance of 1 m. If the Px is unable to read the letters on the chart even at the closest distance prescribed for a particular chart, then one may follow the sequence below for recording the VA.

• NB: The Snellen test assumes 6 meters to be equivalent to optical infinity.

i.e. there is no clinically significant stimulus to accommodation

VISUAL ACUITY =

NUMERATOR

DENOMINATOR ൌ Test DistanceDistance at which Letter subtends 5' of ArcInternational Centre for Eyecare Education It is important to note that VA charts are designed for used at a distance of 6 meters which represents optical infinity. When the testing distance is decreased in cases when the patient displays poor VA, then there has to be an accompanying adjustment incorporated into the trial frame for the closer than infinity testing distance. For example, when the viewing distance is 4 meters, this distance is 1/4th the distance from infinity (0.25DS). The VA must either be taken with this correction in place or the subjective finding must be adjusted accordingly.

Projectors and paper charts that present the Snellen letters the right way around are known as direct charts.

Those which present the letters reversed are known as indirect charts. These require the use of a mirror.

In the United States, the test distances are expressed in feet and are referred to as the Imperial notation of the Snellen acuity, while in many other countries it is expressed in meters (metric notation). Irrespective, these are interchangeable and one may adopt the notation that best suits him or her. e.g. 20/20 = 6/6.

• 2. Decimal Notation
• This notation reduces the Snellen fraction to a decimal quantity.
• For example, 20/20 (6/6) = 1.0 decimal notation,

20/200 (6/60) = 0.1 decimal notation.

This notation is generally used in European countries. One draw back of using this type of notation is that while it reduces the VA to a single number, it does not specify the distance at which the test was conducted.

3. Minimum Angle of Resolution (MAR) The MAR is typically expressed in minutes of arc and is equal to the reciprocal of the decimal acuity value or the Snellen fraction,

For example, 20/20 (6/6) = 1.0MAR,

20/40 = 2MAR, 20/100 = 5MAR.

This notation reflects the angular size of the critical detail with a just-resolvable target.

4. Logarithm of Minimum Angle of Resolution (LogMAR) This notation was introduced by Bailey and Lovie in 1976. It is merely a common logarithm of the MAR, For example, 20/20 = 1.0 MAR  LogMAR = log 10 (1.0) = 0.0.

• For 20/200 = MAR = 10,  LogMAR = log 10 (10) = 1.0.
• Finger Counting (FC) – record the farthest distance fingers are seen
• Hand movement (HM) – record the farthest distance fingers are seen
• Light projection (ability to detect a penlight at different quadrants and distance)
• Light perception (LP = ability to respond to a penlight). A failure of this task is recorded as NLP

= no light perception. International Centre for Eyecare Education It must be noted that when the VA score becomes better than 20/20, then the LogMAR becomes a negative value.

These charts are gradually becoming the favored charts for VA determination because of a consistent progression of size difference between the lines on the chart, each line on the chart has only 5 letters, each letter on the chart can be assigned a value of 0.02 on the LogMAR scale. Research studies tend to use this notation to record visual acuities since it provides a more precise measurement of visual acuity.

• 5. Visual Acuity Rating (VAR)
• This notation is derived from the LogMAR values. VAR = 100 – 50 (LogMAR).

For example, 20/20 has a VAR of 100, while a VA of 20/200 has a VAR of 50.

The VAR is greater than 100 when the VA is better than 20/20. When comparing the LogMAR chart and the VAR, one would notice that the VAR changes by 5 for each increment of LogMAR.

6. Visual efficiency (VE)

This VA notation/scale was introduced for use in quantifying visual loss for legal and compensation purposes.

VE is assumed to be 1.0 or 100% when the VA is 20/20 and an arbitrary benchmark of 20% (0.2) was adopted for a VA of 20/200 or 6/60.

• In some cases, it is expressed as:

Log (VE%) = 3.0777 – 0.0777 (MAR)

VE is given by the formula: VE = 0.2 (MAR – 1)/9International Centre for Eyecare Education Table 3.1 Conversion table for distance visual acuity notations:

• (Source: Borish’s Clinical Refraction by WJ Benjamin)
• LogMAR MAR Decimal Snellen equivalent
• Imperial notation (at 20ft)
• Metric
• (at 6M) VE %
• Overall
• letter size
• (in mm)
• VAR
• ‐0.30 0.5 3.00 20/10 6/3 109.4 115
• ‐0.2 0.63 1.60 20/13.5 6/4 106.8 110
• ‐0.1 0.8 1.25 20/16 6/5 103.6 6.96 105
• 0.00 1.00 1.00 20/20 6/6 100.0 8.70 100
• 0.10 1.25 0.80 20/25 6/7.5 95.6 10.88 95
• 0.20 1.60 0.63 20/32 6/9 89.8 13.05 90
• 0.30 3.0 0.50 20/40 6/12 83.6 17.04 85
• 0.40 3.5 0.40 20/50 6/15 76.5 21.75 80
• 0.50 3.2 0.32 20/63 6/18 67.5 26.10 75
• 0.60 4.0 0.25 20/80 6/24 58.5 34.80 70
• 0.70 5.0 0.20 20/100 6/30 48.9 65
• 0.8 6.3 0.160 20/125 6/38 38.8 60
• 0.9 8.0 0.125 20/160 6/48 28.6 55
• 1.00 10.0 0.100 20/200 6/60 20.0 50
• 1.10 13.5 0.080 20/250 6/75 13.8 45
• 1.20 16 0.063 20/320 6/95 6.8 40
• 1.30 20 0.050 20/400 6/120 3.3 35

1.40 25 0.040 20/500 6/150 1.4 30

CLINICAL TESTING OF VISUAL ACUITY

Visual Acuity Chart Design

1. Snellen Chart

The original Snellen design of chart comprised a single large letter at the top of the chart and smaller letters below. The number of optotypes increased progressively as the size of the letters became smaller. There have been several deviations from the original design which include letter design and selection, spacing between letters, size progressions and the number of letters at each size progression, however the standard Snellen chart still remains a chart with a single letter at the top of the chart followed by an increase in the number of letters as the letter size becomes smaller.

2. Bailey-Lovie Design

The Bailey-Lovie recognized some of the inherent flaws in the Snellen design and developed a set of principles that make the task of VA measurement the same at each size level. In order to achieve this form of standardization, the VA task required the following:

• A logarithmic size progression, i.e. a constant ratio from one letter size to the next.
• The same number of letters at each size progression.International Centre for Eyecare Education
• The spacing between the letters and rows must be proportional to the letter size.
• There must be equal or similar legibility for each optotype at each size level.

This chart used a VA scoring in LogMAR units. This scoring allowed for equal additional credit for each optotype that was read correctly. These charts are available in various forms, namely, landolt C’s, Tumbling E’s, letter charts and number charts. They have also been constructed in various language variations as well. While the Snellen VA chart is still more widely used, the LogMAR chart is gaining more popularity based on the aforementioned criteria and there is a shift toward it becoming the standard chart for VA measurements.

Chart Formats

• There may be various types of chart formats that VA charts are present in, namely:

1. Printed charts

These may be printed on opaque card or plastic. They require direct illumination. Some charts may be printed on translucent material that may be used with back illumination or may be mounted on a box and would require internal illumination. Printed charts are usually used at 6 meters (or 20 feet) and acuities are recorded in Snellen notation. In some cases if the room size does not permit or the Px has low vision, the chart may be presented at an alternative distance as long as there is an exact measurement of the testing distance in order to record the VA.

While 6 meters is the most commonly used test distance, 4 meters has been recommended by Hofstetter (1973) while making a dioptric allowance of 0.25D to the refractive correction since the chart is placed closer than optical infinity. It is suggested that this distance facilitates a comparison with near vision measurements in which 40 cm is the standard testing distance.

2. Projector charts

Projector chart image angular sizes are independent of observation distance when the projector lens and the Px’s eye are equally distant from the projection screen. The placement of the projector (Fig. 3.8) is performed carefully and the VA is recorded in whatever appropriate notation is adopted by the chart.

• Figure 3.8 The projector

3. Charts on display screens

Computer generated displays are gradually being used in practice. They bear some inherent advantages such as being able to vary optotypes, change letter sequences and vary stimulus parameters such as contrast, spacing, and presentation time to Pxs. International Centre for Eyecare Education

DISTANCE VISUAL ACUITY MEASUREMENT

• Procedure

1. This test should be conducted under adequate illumination conditions.

2. One should ensure that there are proper instructions given to the Px. Let the Px know that you are going to assess “how well they can see”.

3. Use the occluder (cover) to cover the Px's LE and record the VA of the RE (Fig. 3.9a). One may just suggest that the Px form a cup with the left hand and place it over the left eye, ensuring that the fingers are not placed over the eye and that the Px is not peeking (Fig. 3.9b). One must also ensure that the Px does not apply too much of pressure on the eye because this can blur the vision to a certain extent.

• 4. Next, proceed to occlude the RE and record the VA of the LE.

5. Remove the occluder and record the VA of both eyes.

6. Don't correct any mistakes that the Px may make. If the Px reports that he can no longer see, urge him to try a few more letters by pointing out letters that you know are easier than others so that you get an accurate assessment of the limit of his VA. We conventionally record the VA of the right eye, then the left eye, followed by both eyes.

7. is important that one avoid memorization of the letters on the chart. This is best done if one knows which of the eyes is the poorer of the 2, and then this eye should be tested first. Another way to prevent memorization would be to allow the Px to read the letters first without the habitual Rx and then with.

8. One needs to ensure that the Px is not peering (sometimes referred to as squinting) in order to get better visual acuity. This action creates a pinhole effect thereby allowing the Px to resolve smaller letters than he/she ordinarily would without peering.

9. If the Px has an abnormal head posture, one needs to determine if it’s a posture that he assumes in order to see clearly, if not, instruct him to keep his head in the upright position. If it is a corrective head posture to obtain better vision (like in the case of Px’s who have media opacities that cause low vision), the practitioner must make note of the abnormal head posture.

10. Another method to prevent memorization is to have the Px read the line of letters in reverse.

• Figure 3.9 Occlusion of the eye when taking a VA
• a b International Centre for Eyecare Education

Recording of Distance Visual acuity

If the Px reads an entire line correctly and stops there, then you record that line as his VA, e.g. 6/6.

If the Px correctly reads more than half the number of letters on a line, then you score him as that line minus the number of letters that could not be read or were miscalled. e.g. The Px reads only the letters that are underlined below, then the VA measurement will be recorded as:

A B C D E (6/9) = 6/9 ‐2

or 6/9 ‐2/5

If the Px reads less than half the number of letters on a line, then you score him as the previous line plus the number of letters that he correctly identified in the last line.

• E.g.,

A B C D E

F G H I J

• 6/12

6/9 ൌ 6/12+2 or 6/12+2/5

Also essential in the recording of a Px’s VA is whether the VA is being taken with or without a prescription.

• VA is abbreviated without correction or with correction
• Recordings would thus look like this:
• e.g. Unaided VA (sometimes abbreviated UCVA = uncorrected VA or )

Projector chart

OD: 6/6 OS: 6/9 OU: 6/7.5

• Or

RE: 6/6 LE: 6/9 BE: 6/7.5

e.g. Best corrected VA [i.e. with Rx] (sometimes abbreviated BCVA = Best corrected VA or ) Projector chart

OD: 6/6 OS: 6/9 OU: 6/7.5

Or

RE: 6/6 LE: 6/9 BE: 6/7.5

• OD = Oculus Dexter RE = Right eye
• OS = Oculus Sinister LE = Left eye

OU = Oculi Uterque BE = both eyes

N.B.: Recording of LogMAR visual acuity measurements will be discussed in greater detail in the low vision module.

Pinhole Visual Acuity

The pinhole consists of either a single small hole or multiple small holes. The pinhole allows narrow bundle of light rays to enter the eye and prevents light scatter to provide an adequate image without the need for additional optical aids. This measurement is an essential part of the measurement of VA. It is probably the one measurement that gives the examiner some perspective regarding the goal of the refraction routine. It is the measurement that prevents the examiner from going down the futile road of trying to achieve 6/6 visual acuity when it is not possible. When the VA cannot be improved to 6/6, one can deduce that the visual impairment may not be entirely due to refractive error. The recommended diameter of the pinhole is about 2 mm. Smaller pinholes may reduce the best vision or VA achievable.

• Figure 3.10 The pinhole test (with a multiple pinhole)
• Procedure

1. The pinhole may be hand-held (Fig. 3.10) or placed in a trial frame.

2. When performing the test, the examiner must ensure complete occlusion of the fellow eye.

3. The examiner must instruct the Px about the aim of using the pinhole.

4. The Px is asked to look through the tiny hole in the lens. This is sometimes quite difficult to achieve. The Px is therefore encouraged to move his/her head or his/her eyes till he/she is able to find the hole in the lens.

The measurement of VA is the same as without the pinhole lens in place.

5. The test may be performed either at the beginning of the refraction when taking the VA, and if 6/9 or better is not achieved. It may be performed during the subjective refraction when the examiner becomes aware that normal vision cannot be achieved. The point at which this test is performed is sometimes an area of contention among eye care practitioners.

6. A single pinhole may decrease acuity in cases of media opacities. In these cases, the multiple pinhole is preferred.

Interpretation of results

• If the pinhole VA is better than the VA presenting during subjective refraction, then the examiner should be

aware that he might need to adjust the refractive findings to achieve the “goal VA” presented with the pinhole.

• If the VA stays the same, then prompting the Px to achieve better would be futile. The visual acuity may not

improve either due to reasons such as media opacities, optic nerve/macula diseases, strabismus, varying types of amblyopia, etc. International Centre for Eyecare Education

Common Errors When Taking Visual Acuities

1. Allowing the Px to decide their acuity and not prompting them to try further. Permitting Pxs to squint.

• 2. Permitting the Px to look around the occluder or through their fingers.
• 3. Using a dirty chart or one of low illumination.

4. Placing the chart at an incorrect working distance.

5. Not recording the result immediately and guessing the wrong result at the end of the examination.

6. Allowing the Px to use their hand to occlude their eyes but not ensuring that the palm of the Px’s hand is blocking the eye. This permits the Px to peek through the gap in the fingers.

• Factors Affecting the Measurement of Visual Acuity
• 1. Refractive error
• Myopia decreases the VA by approximately one line for every 0.25DS of refractive error.
• Astigmatism decreases the VA by 2 lines for every 0.50DC of refractive error. Oblique astigmatism has a

greater effect on VA than vertical astigmatism which in turn has a greater effect on VA than horizontal astigmatism.

• Hyperopia generally does not affect VA in young Px’s adversely because they can accommodate to

neutralize the error and they appear emmetropic. Hyperopia, in elderly Px’s on the other hand will decrease the VA by one line for every 0.25DS error present since no accommodation is present to neutralize the error.

2. Illumination And Pupil Size

• The brighter the room illumination, with a smaller pupil size will serve to increase the depth of focus effect of

the pupil. The blur circles are reduced and the adverse effect of refractive error is decreased and thus the VA is improved (similar to the effect of a pinhole).

• In a dark room, the pupil enlarges and spherical aberration increases. Thus there is an increase in the

myopia in most emmetropic and myopic Pxs.

• Room illumination should therefore be representative of the daily environment. There are some tests

requiring the use of lower illumination levels or situations in which the lighting should be adjusted for the Px, for example, some amblyopic Pxs present with better VA under mesopic conditions and low vision Pxs with specific pathological conditions leading to their VA decrease present with better VA when the lighting is not too bright, e.g. Pxs with albinism. These are areas that will be covered in greater detail in the specialist areas of clinical optometry.

3. Spacing between letters and lines (Crowding Phenomenon)

• The closer the spacing of the letters the more difficult it is to discern the letter. Single letter acuity is

therefore better than single line acuity. It is therefore advisable not to use cramped letter charts, especially if they are Tumbling E's or Landolt C's. This crowding phenomenon is especially important in the diagnosis of and prognosis for amblyopic Pxs.

4. Choice of letters / optotypes

• Targets used in the measurement of VA must follow several specific design requirements. These specially

designed test targets/letters are often referred to as optotypes. The variable legibility, readability, discernability, and difficulty of the different letters of the alphabet determines the choice of letters on a VA chart. The following letters have been recommended in the literature:

L T V U C O Y F P D Z N E R S G H B

It was also recommended that each of the letters above appear only once on the total chart and that not more than one of the following letters appears once on any line:

B – E – S

C – G – O

• F – P

D – O

Other letters e.g. 'I' was not included in chart design because it did not fit the 5 x 5 format and it was too easy to guess.

• Because different letters of the same subtense (angular size) vary in difficulty, it is often found that a Px will

be able to read only some letters on the smallest line that can just be distinguished. Letters that are similar in appearance are more difficult to distinguish than letters that vary greatly in their appearance.

5. Design of the letters

• The 'old-fashioned' printed capitals with serifs/hooks were used since they conformed to the 5 x 5 format.

These decorative features are discarded in modern charts since these serifs obscure the legibility of the letter.

6. Contrast

• Printed charts: maximum black/white contrast must be maintained. Charts should not be soiled as this

disrupts the contrast and the thus alters the VA.

• Internally illuminated charts: should be viewed with the room lights on. In a dark room the internal

illumination from the chart generates a contrast glare which degrades the legibility of the letters.

• Modern projector charts: should also be viewed with the room lights on. Older charts which utilize an

incandescent bulb do not provide enough light to be used with the room lights on and therefore it is recommended that these projectors be discarded.

7. Time

• One should not rush the Px as he may get the impression that you are in a hurry and he may not try to
• discern the smaller letters.

8. Chart luminance

• In most instances, VA is measured under photopic light conditions.International Centre for Eyecare Education
• Standardized chart luminance range is from 85 to 300 cd/m2. Because it is usually difficult to achieve a

specific luminance level with the various types of charts (printed, projector, display screens), 80 to 320 cd/m2 has been adopted as a clinical tolerance

NEAR VISUAL ACUITY MEASUREMENT

Background

Just as a Px has to have their VA assessed at distance, so too is it required for near. Just as the Px who has their VA for distance on a 20/20 letter subtend 5’ of arc, a 20/20 letter on a near chart subtends a visual angle at the eye of 5' of arc.

It has been noted that myopic Pxs without a correction, tend to have a better visual acuity at near than at far, while a hyperopic Pxs without correction have better acuity at far than at near. Usually, an individual who has 20/20 VA at distance tends to also have 20/20 VA at near (if not accommodative anomalies exist), until they reach their early to mid-forties, at which time presbyopia sets in. Their uncorrected near visual acuity decreases, creating the need for reading spectacles or bifocals.

Near VA is typically measured at distances within an arm’s length. A testing distance of 40cm is usually considered to be the standard. However, there are special cases in which this distance may be altered.

Notations for Near Visual Acuity

There are several different methods/notations that are used to specify the near VA of a Px. They include the following:

• The M notation
• This notation of “M units” was introduced by Sloan and Habel
• In this notation, the size of the letters is indicated by the distance (in meters) at which lowercase letters

subtend 5 minutes of arc at the retina. Thus a 1M letter would subtend 5 minutes of arc at a distance of 1meter and is of 1.45mm in height.

• Regular newsprint is usually about 1.0M in size.
• Near VA is usually recorded in this notation as 0.40/1.0M, indicating that the test distance was 40cm and the

near VA was 1.0M letter size, just as with standard recording of VA measurements.

• In cases where Pxs may bring in the print that they wish to view, one may be able to determine the visual

acuity of that print in M-notation by measuring the height of the letters and multiplying it by 0.7.

The Point system

• This notation was established by the printing industry to categorize letter sizes with 1 point being equivalent

to 1/72 of an inch. This size extends from the bottom of a descender on a letter like “j” to the top of an ascender like a letter “f”. Usually smaller letters like “x, a, e, etc” are half the size of a total height.

• Newsprint is usually 8 points in size. Therefore the letter “x” for example, is about 4 points.
• Usually an 8 point raring in a newspaper is equivalent to an M unit rating of 1.0M.
• According to Benjamin in Clinical Refraction (2006):

The Jaegar notation

• Jaegar was the first to introduce a reading card with paragraphs set in type of ascending size. He numbered

the type available so that the smallest became known as the Jaegar1, or J1 and the largest J20. The notation is given by the size of print preceded by the letter J. (J2 letter will be equivalent to a 0.6M letter @ 40cm)

• The near VA is recorded as the print size and the test distance, e.g. J6 @ 40cm.
• The lack of standardization of the letter sizes has made the Jaeger notation inappropriate for the
• measurement of near VA.

The N-notation / Meter system

• Initially the London Faculty of Ophthalmology decided to follow the example of their favorite newspaper

which used Times Roman type-face and they declared this the most suitable for reading-chart use.

• The size label of N4 for example indicated that a standard near test font was being used and that its size was

4 points.

• The near VA recorded with these charts was recorded in the N notation followed by the distance at which the

test was conducted, e.g. N4 @ 40cm.

• Print of N-notation can be converted to M-notation by using dividing the N-notation value by 8.

The Reduced Snellen / Equivalent Snellen Notation

• The system utilizes the conventional Snellen distance VA chart which is photographically reduced such that a

6/6 letter subtends 5 minutes of arc at the retina at a test distance of 40cm.

• A print size of 1.0M presented at 40cm is usually labeled as 20/50 equivalent because the Snellen fraction of
• 20/50 is equivalent to 0.40/1.00.
• This is not usually a favored method of recording near VA.

N 8 / 8 = 1.0 M

• 1.0M units = 1.45mm ≈ 8 points (lower-case newspaper style) ≈ typical
• newsprint. International Centre for Eyecare Education

Purpose of Recording Near VA

The recording of near visual acuities is essential in selection of near fixation targets, determining the Px’s functional losses as a result of refractive error, pathology, etc. An important purpose of determining a near visual acuity is to determine an individual’s near vision correction, especially in the case of presbyopes.

Procedure

1. Provide adequate illumination for the near card (Fig. 3.11). The use of additional lighting is sometimes controversial as this may not be the habitual practice of the Px and by using additional lighting the task becomes even easier for the Px.

2. Position the near visual acuity chart at 40cm from the Px. In some cases the near chart may be held at whatever distance the Px is most comfortable at, however it is vital to note this test distance.

Figure 3.11 Near VA chart

3. Occlude the Px’s LE (or if known – the better eye) and proceed with recording the visual acuity of the RE.

4. Ask the Px to identify the smallest line/paragraph of letters that he/she is able to see clearly.

5. Occlude the RE and repeat the procedure for the LE.

6. Remove the occluder and repeat the procedure to obtain the binocular near visual acuity.

Recording Near VA

This depends on the type of near card that one may utilize to determine the near VA. Record the size of the print that the Px was able to see and the test distance.

• If one uses the Jaegar notation: OD: J3 @ 40cm OS: J6 @ 40cm OU: J3 @ 40cm
• In M-notation: OD: 0.40/1.0M OS: 0.40/3.0M OU: 0.40/0.8M
• Table 3.2 Near visual acuity equivalents at a working distance of 40cm
• Source: Clinical Procedures in Primary Eye Care (DB Elliot)
• N-notation Common usage M-notation Point notation J-notation
• 3 Medicine bottle labels 0.4 3 ‐‐
• 4 Stock market print 0.5 4 1
• 5 Footnotes 0.6 5 2
• 6 Telephone directories 0.8 6 3
• 8 Small column newsprint 1.0 8 5
• 10 Typewritten 1.2 9 7
• 13 Books age 9-12 years 1.6 12 10
• 16 Computer display (80 column) 3.0 14 ‐‐
• 20 Books age 7-8 years 3.5 18 12
• 24 Large print books 3.0 ‐‐ 14
• 32 Subheadings 4.0 24 15
• 65 Newspaper headlines 8.0 ‐‐ 16

Interpretation of Results

The near visual acuity measurement that we record is essential in telling us about the functional losses of the Px.

So, one has to note the deviation from the normal. Also note any asymmetry between the 2 eyes.

The visual acuity allows us to cater for the specific needs of the Px, e.g. If the Px needs to read the newspaper, then the near visual acuity that is required is 1M. So when the examiner is providing the near addition, he needs to make certain that the Px is able to view the 1M print. If the Px is working in the printing industry, then they would encounter print even smaller than 1M, the prescription of a near spectacle needs to take this into account.

While the overall procedure of taking a visual acuity will remain the same for most Pxs, there are some variations that may be employed in certain circumstances. For example when taking the VA of a Px with nystagmus, one would need to use a translucent occluder/fogging lens over the eye not being tested. In addition, in nystagmus Pxs finding the null point (point of least movement of the eyes), may be essential to obtain a visual acuity assessment. In cases of Amblyopia, single letter acuity tends to be better than line acuity and therefore each letter should be isolated. In addition, the age and literacy level of a Px will determine the type of chart being used.

BIBLIOGRAPHY

• 1. Benjamin WJ. Clinical Refraction. WB Saunders Company. 1998.

2. Elliot DB. Clinical Procedures in Primary Eye Care. Butterworth-Heinemann 2001.

3. Eskridge JB, Amos JF and Bartlett JD. Clinical Procedures in Optometry. JB Lippincott Company. 1991.International Centre for Eyecare Education Section A – PRELIMINARY EXAMINATION