Visual Acuity and Contrast Sensitivity

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Chapter 11 Visual Acuity and Contrast Sensitivity

Visual acuity tests

Chart design

Optotypes

Most familiar acuity tests require the subject to identify letters arrayed in rows of decreasing size. The so-called Snellen acuity test is the prime example, although Snellen acuity now usually refers to a way of reporting test results rather than to any particular type of chart. To facilitate testing of young children and people unfamiliar with the Latin alphabet, other optotype tests based on the tumbling E, Landolt C, numerals, or simple pictures of familiar objects are also used. Visual acuity can also be assessed with grating patterns, but grating acuity often overestimates Snellen acuity in patients with age-related maculopathy,3 typically by a factor of 2 or more.

The NAS-NRC recommends that the Landolt C test be used as the standard by which other acuity tests are compared. The Landolt C is a broken circle in which the width of the break and the stroke width are both equal to one-fifth the height of the C. The ring is shown with the break at one of four locations, and the subject responds by saying “right,” “down,” “up,” or “left,” or by pointing in the appropriate direction.

There are several advantages to the Landolt C test, including equal difficulty of all targets (unlike letters that vary in degree of difficulty), sensitivity to astigmatic refractive error, and suitability for use with illiterate subjects. However, the Landolt C test is not widely used because it has a guessing rate of 25%, so an alternative is to standardize another optotype set by comparing acuities obtained with it to Landolt C test acuities. The Sloan letters,4 a set of 10 upper-case sans serif letters, are the most popular substitute. An acuity chart based on Sloan letters was developed for the Early Treatment Diabetic Retinopathy Study (ETDRS)5 and is illustrated in Fig. 11.1. The original ETDRS chart has been replaced by the 2000 series revised ETDRS chart that more accurately equates the difficulty of letters on all lines. The ETDRS charts are the most widely used acuity charts for clinical research.

image

Fig. 11.1 The Early Treatment Diabetic Retinopathy Study (ETDRS) acuity chart.

(Reproduced with permission from Ferris FL III, Kassof PA, Bresnick GH, et al. New visual acuity charts for clinical research. Am J Ophthalmol 1982;94:91–96.)

Testing procedure

Luminance and contrast

Whatever the test distance, the chart must be adequately illuminated and of high contrast. Illumination standards vary from 100 cd/m2 in the USA to 300 cd/m2 in Germany. Increasing chart luminance improves visual acuity in normal subjects, but reaches a plateau at about 200 cd/m2.12 Various types of visual dysfunction can change the effects of luminance on acuity. For example, patients with retinitis pigmentosa may show a decrease in acuity at higher luminance, whereas patients with age-related macular degeneration often continue to improve at luminances well above the normal plateau.13 A luminance standard of 100 cd/m2 can be justified because it represents good room illumination for ordinary reading material. Furthermore, most of the currently proposed standards would yield the same acuity scores, plus or minus one letter (assuming five letters per line and a 0.1 log unit size progression) in normal subjects.

The relationship of visual acuity to letter contrast follows a square-root law.14 For example, decreasing contrast by a factor of 2 would decrease acuity by roughly a factor of 1.4. The NAS-NRC recommends that letter contrast be at least 0.85. Transilluminated, projection, and reflective charts (wall charts) can all meet these standards, but some transilluminated charts are deficient in luminance, and some projection systems lack sufficient luminance and contrast. Accurate calibration requires a spot photometer, for which procedures are described in the NAS-NRC document.1

Scoring

Until recently, visual acuity tests were usually scored line by line with the patient being given credit for a line when a criterion number of letters were identified correctly. The NAS-NRC recommends that at least two-thirds of the letters on a line be correctly identified to qualify for passing. Allowing a small proportion of errors improves test reliability.17 For tests that follow the recommended format of an equal number of letters on each line and a constant progression of letter sizes, it is preferable to give partial credit for each letter correctly identified. This is commonly done by counting the number of letters read correctly on the entire chart and converting this to an acuity score by means of a simple formula that values each letter as L/N, where L = difference in acuity between adjacent lines and N = number of letters per line. So for a chart with five letters per line and a 0.1 logMAR (see below) progression from line to line (such as the standard ETDRS chart) each correct letter is worth 0.1/5 = 0.02 logMAR. Although differences between scoring methods are usually small, it has been shown2,6,18 that letter-by-letter scoring is more reproducible than line-by-line scoring.

The most familiar method for reporting visual acuities is the Snellen fraction. The numerator of the Snellen fraction indicates the test distance, and the denominator indicates the relative size of the letter, usually in terms of the distance at which the stroke width would subtend a visual angle of 1 minute. Thus “20/40” indicates that the actual test distance was 20 feet and that the strokes of the letters would subtend 1 minute of arc at 40 feet.

To simplify comparison of acuities measured at different distances, the minimum angle of resolution (MAR) should be used. The MAR is the visual angle corresponding to stroke width in minutes of arc and is equal to the reciprocal of the Snellen fraction. Visual acuities are frequently converted to log10 and reported as “logMAR.” For example, 20/20 acuity corresponds to a MAR of 1 minute of arc, or a logMAR of 0, and 20/100 acuity corresponds to a MAR of 5 minutes of arc, or a logMAR of 0.7 (as does an acuity of 2/10 or 6/30). The MAR and logMAR scales increase with worsening acuity. One often sees visual acuities reported as the decimal equivalent of the Snellen fraction. However acuities are more normally distributed when converted to logMAR rather than decimal values. Moreover, decimals give the false impression that acuity scores can be equated with overall loss of visual function. For example, an acuity of 0.1 (the decimal equivalent of 20/200) may misleadingly suggest that the patient has retained 10% of residual visual function.

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