You are not logged in.

 




The Aging Eye: Problems That Affect Acuity and Contrast Sensitivity

Theodore Grosvenor, OD, PhD, FAAO
5673 Correll Dr.
Ferndale WA 98248

 

Contents

 

Age-Related Changes in Sphrical Ametropia

Due to a gradual decrease in the power of the crystalline lens, emmetropic or hyperopic eyes tend to change in the hyperopic direction. This is called age-related hyperopia. Others (particularly those eyes with low myopia) may change toward emmetropia. In myopic eyes, the decrease in lens power may be counteracted by a continued increase in axial length, with the result that some myopic eyes become more myopic after age 45 or 50, some change very little.

There is also a tendency for some eyes - whether hyperopic, emmetropic, or myopic - to change in the myopic direction due to the development of nuclear cataracts.

As a result of a review of patient records in his optometric practice in Ojai, California, Monroe Hirsch (5) reported age-related changes in refraction of patients beyond the age of 45. He found a definite hyperopic trend in hyperopes and emmetropes, but not in myopes. Prevalences of hyperopia, emmetropia, and myopia at ages 45 to 49 years and ages 75 and over were:

In a retrospective longitudinal study that Peter Skeates and I conducted using data on patients he had examined in his optometric practice in a suburb of Auckland, New Zealand, we reported changes in refraction per decade for 100 patients who were hyperopic, 100 patients who were emmetropic, and 100 patients who were myopic at the age of 40 years (6).

Of 100 patients who were hyperopic (+1.00 D or more) at age 40:

Of 100 patients who were emmetropic (+0.87 D to -0.37 D) at age 40:

Of 100 patients who were myopic (-0.50 D or more) at age 40:

We concluded that when a hyperopic shift occurred, it was due to an increase in the index of refraction of the lens; but when a myopic shift occurred, it was most likely due to incipient (pre-clinical) cataracts. But for a myopic eye, a likely cause of a myopic shift was continued axial elongation.

 

Age-Related Changes in Astigmatism

Keratometric data routinely show that beyond the age of 40 to 45 years, there is a strong tendency for astigmatism to change in the against-the-rule direction. It has been suggested that when with-the-rule astigmatism is present, it is because the stiff upper tarsal plate causes pressure on the horizontal meridian of the cornea. With increasing age, this pressure gradually decreases, resulting in a change toward against-the-rule astigmatism. This common observation has been confirmed in studies reported by Hirsch (7), Lyle (8), and other investigators.

In the retrospective study of patients examined by Peter Skeates in New Zealand, we found that age-related changes in astigmatism tended to differ for myopic eyes as compared to hyperopic and emmetropic eyes:

 

Age-Related Changes in Retinal Illumination

In the last chapter of his book, Weale discussed age-related changes in what we would now call retinal illumination. (1) Referring to the results of a study he had published two years earlier, he commented:

"It can be shown that, as a result of miosis and lenticular yellowing, the 60-year-old retina receives approximately one-third of the amount of light which reaches the 20-year-old retina. It does not matter very much whether the eye is light or dark- adapted."

 

Figure 8. The relative amounts of light reaching the light-adapted (photopic) or dark-adapted (scotopic) retina at various ages between 20 and 60 years. Weale (1)

 

Weale’s study on retinal illumination (1) apparently didn’t include subjects who had undergone cataract surgery: anyone who has had the current ‘no-stitch’ lens extraction by phacoemulsification - followed by intraocular lens implantation - has found that everything appears to be much brighter, particularly in the violet and blue end of the spectrum. However, prior to the decade of the 1980s, patients were usually not referred for lens extraction until the cataract was ‘ripe’ - which would seldom occur before age 60.

 

Age-Related Changes in Visual Acuity and Contrast Sensitivity

Although the majority of people retain good high-contrast visual acuity into their sixties or seventies, low-contrast visual acuity and acuity in the presence of glare tend to show deficits at much earlier ages.

How Do We Define Contrast?

Contrast may be defined as the ratio of the difference between the maximum and minimum luminance (L) of a test stimulus, divided by the sum of the maximum and minimum luminance. To express contrast in terms of percentage, the result is multiplied by 100.

Percentage contrast = 100 [L(max) - L(min)] / [L (max) + L(min)]

For black print (the minimum luminance) on a white background (the maximum luminance), the contrast would be close to 100 percent; but for visual tasks such as viewing an airplane in a cloudy sky or recognizing a human face, the contrast may be close to zero.

High-Contrast Visual Acuity

The experience of vision care practitioners is that until about age 65, very close to 100 percent of their patients can achieve 20/20 to 20/30 (6/6 to 6/9) visual acuity in one or both eyes with conventional glasses or contact lenses. This clinical experience has been reinforced by a study reported by Klein, et al., (10) involving 4,926 residents of Beaver Dam, Wisconsin between the ages of 43 and 86 years. All subjects who volunteered for the study were included regardless of any signs of ocular disease.

Klein, et al., reported that more than 99% of their subjects between ages 46 and 64 years had corrected visual acuity of 20/20 (6/6), reducing to 95% for patients between ages 65 and 74, and 77% between ages 75 and 86 years.

 

Figure 9. Percentages of subjects with low vision (20/40 to 20/160) (6/12 to 6/48), or blindness (20/200 [6/60] or worse), reported in the Beaver Dam Study (10).

 

Less than 1% of the subjects had visual acuity of 20/40 to 20/160 (6/12 to 6/48) (low vision) between ages 46 and 55 years, increasing to 21% at age 75 or older; while only 2% were ‘legally blind’ at age 75 and older.

Low-Contrast Visual Acuity

A growing body of research demonstrates that macular degeneration, cataracts, and other age-related ocular diseases can cause significant losses in contrast sensitivity for certain spatial frequencies. Sekular, et al., (11) compared contrast sensitivity for 70 subjects in their 60s, 70s, and 80s who were free of ocular disease, and 33 subjects in their 20s and 30s, also free of ocular disease. Their results indicated that although subjects of all ages had equal sensitivity for the lowest spatial frequencies (0.5 and 1.0 cycles per degree), there was a decrease in sensitivity for the higher spatial frequencies for each decade of life.

 

Figure 10. Age-related changes in contrast sensitivity. Sekular, et al. (11).

 

Noting that the average retinal illuminance of the 60 year-old eye was estimated by Weale (1) as only about one third of that of the 20 year-old eye - due to the smaller pupil and increased density of the lens – Owsley, et al., (12) tested contrast sensitivity on a group of 20 year-old subjects who wore a neutral density filter that decreased contrast sensitivity by a factor of 3. As shown in Figure 11, contrast sensitivity for these subjects through the filter was found to decrease for higher spatial frequencies, but not to the extent of the decrease found for older subjects.

 

Figure 11. Change in contrast for twenty-year-old subjects wearing a neutral density filter, compared with subjects in their 60s not wearing a filter. Owsley, et al. (12).

 

Owsley, et al., concluded that the decrease in the light reaching the retina for three older subjects, as found by Weale, might have been due to additional neural rather than optical factors.

 

Testing Contrast Sensitivity

In contrast sensitivity testing, the patient is presented with repetitive stimuli in the form of vertically-oriented gratings at various contrast levels. They may be square-wave gratings or sine-wave gratings. The gratings are designed so that the average luminance - half the sum of the luminance of the dark and light bars - is constant for all gratings.

The spacing between the outer edges of any two bars in a grating is the spatial frequency, which is analogous to the width of a stroke and a gap on a visual acuity chart. For example, a spatial frequency of 30 cycles per degree (30 bars and 30 gaps per degree) would indicate a stroke or gap width of one minute of arc, and would therefore be the equivalent of 20/20 (6/6) visual acuity. The first contrast sensitivity tests to be developed, which were used mainly for research, consisted of electronically-generated gratings.

The Arden Plate test.

Introduced in 1978 by C.S. Arden (13), it was one of the first contrast sensitivity tests designed for clinical use. The test is in the form of a 6-page booklet, with each page (Figure 12) displaying several sine-wave gratings of varying contrast and spatial frequency. Each grating is oriented vertically, with the contrast varying from the top to the bottom. For each grating, the examiner or the patient gradually moves a card (which masks the grating) downward over the page until the point is reached at which the grating is seen.

At that point, the examiner records the contrast from a scale provided with the grating. After a practice trial, the procedure is repeated for each of the six plates.

 

Figure 12. The Arden Plate test. Gratings (a) and (b) differ in spatial frequency, and gratings (b) and (c) differ in contrast. Arden (13).

 

The Vistech chart.

Developed in 1984 by Ginsberg (14), the Vistech chart is made up of 6 rows of 3-inch diameter sine wave gratings. Each row consists of a sample grating and various test gratings at a given spatial frequency but differing in contrast. Spatial frequencies utilized - from the top row to the bottom row - are 1, 2, 4, 8, and 16 cycles per degree. Each grating is oriented in one of 3 directions: vertical, slanted 15 degrees to the left, or slanted 15 degrees to the right.

The task of the patient is to report the orientation of each grating in each row until the orientation cannot be determined. When the test is completed, the data are plotted and compared to a ‘normal’ contrast sensitivity curve. Two separate Vistech charts are available: the VCTS- 6500 for distance testing, and the VCTS-6000 for near testing. A projector slide, the VCTS-500S, has also been available.

 

Figure 13. The Vistech contrast sensitivity test. Ginsberg (14).

 

The Melbourne Edge Test.

Developed by Verbaken and Johnson (15) in 1986, this test was based on the idea that contrast sensitivity for a single edge appears to be a reliable indicator of the contrast sensitivity function peak. The test makes use of the boundary between light and dark backgrounds, rather than a grating.

Shown in Figure 14, the test is made up of 20 circular stimuli, 2.5 cm (1 inch) in diameter. Each of the circles, or disks, presents an edge that separates light and dark backgrounds with gradually reducing contrast. The identifying feature is the orientation of the edge. The patient is shown a key card that presents four circles, having horizontal, vertical, and obliquely oriented dividing lines; and the patient is asked to identify the orientation of each of the test edges.

 

Figure 14. The Melbourne Edge Test. Verbaken and Johnston (15).

 

In one study, the Melbourne Edge Test was administered to 497 consecutive clinical patients. Patients were split into 3 groups on the basis of visual acuity and media haze. It was concluded that edge contrast sensitivity provided diagnostic data that were not available from visual acuity testing alone (15).

Variable-Contrast Letter Charts

The possibility that some older patients, whose visual acuity as tested in the eye doctor’s office was 20/20 (6/6), might have difficulty in some ‘real world’ situations, was understood as long ago as the 1950s, when at least one ‘low contrast’ visual acuity chart was developed. But the majority of clinicians failed to understand the need for low-contrast acuity testing until widespread contrast sensitivity testing was done in several research laboratories. Once the ‘low contrast acuity’ problem gained acceptance, variable-contrast letter charts began to appear in the clinical setting.

The Mentor B-VAT II.

This instrument was one of the first low-contrast visual acuity tests to make use of the familiar Snellen letters. This test is described by the manufacturer as a high-contrast, high-resolution monitor providing nine different optotypes and over 50 functions with a built-in red-green test for refining refractions. Stimuli are presented randomly to prevent memorization.

The B-VAT II SG

(Figure 15) is designed not only for routine vision testing, but it also presents sinusoidal gratings at various levels of contrast and spatial frequencies. The test can be set at either 3 or 6 m (10 or 20 ft) and uses 20 contrast steps. For each presentation, an auditory tone is used to alert the patient. Gratings may be oriented vertically or 14 degrees clockwise or counterclockwise from vertical.

 

Figure 15. Mentor B-VAT II SG Video Acuity Tester.

 

In a study using 69 optometry students as subjects, Corwin, et al., (16) concluded that the B-VAT II SG system provides reliable contrast sensitivity data, which are consistent with norms from other systems when obtained under similar conditions.

The Peli-Robson Letter Chart.

In this chart, all letters are the same size, but contrast decreases from the top to the bottom. As described by Peli, et al., (17), the test is in the form of a printed cardboard chart, which presents 8 lines of letters consisting of 6 letters each. All of the letters subtend an angle of 0.5 degrees at a testing distance of 3 meters.

 

Figure 16. The Peli-Robson Letter Chart.

 

Because each Snellen letter consists of 3 strokes and 2 gaps (2.5 cycles) the spatial frequency of each letter is 1.25 cycles per degree; this is equivalent of 6/36 (20/120) visual acuity.

As for contrast, each line consists of 3 letters, all letters in a group having the same contrast. The contrast is highest for the first 3 letters on the top row, and lowest for the last 3 letters on the bottom row, decreasing for each successive group of 3 letters. The patient’s score is determined by the last group in which 2 of the 3 letters are read correctly.

Regan Low-contrast Letter Charts.

This test consists of 3 letter charts, having contrasts of 94%, 7%, and 3%. They are used at a distance of 3 m, and are designed on the Bailey-Lovie principle, with the exception that there are 8 letters, rather than 5, in each line. When using these charts, patients are instructed to start reading at the top and to continue reading until they can correctly identify none of the letters on a line. In most cases, only the 97% and 3% charts are used.

In discussing the use of these low-contrast charts, Regan (18) suggested that their chief role is in detecting early visual loss, especially in diabetes and glaucoma, in the hope of enabling timely management to prevent or delay further visual loss. He also discussed their role in the detection of Parkinson’s disease, central serous chorioretinopthy, and cataracts.

 

Disability Glare

Sources of illumination that present no problems for most people can sometimes cause devastating effects on vision for patients who have conditions such as corneal edema, lens opacities, various forms of maculopathy, and dry-eye problems. The first test for disability glare - the Miller-Nadler Glare tester - has been available for more than 30 years, but has seen very little use. More recently, however, several new tests of disability glare have been introduced, many of which are also intended for the measurement of low-contrast visual acuity.

The Mentor Brightness Acuity Tester (BAT).

This test is designed to convert a visual acuity test or a contrast sensitivity test to a test for disability glare. It is an illuminated white hemisphere, 60 mm in diameter with a 12 mm central aperture. It is held over the patient’s eye, as the patient views a visual acuity chart through the aperture. The internal brightness of the hemisphere can be varied, therefore varying the amount of glare. An important advantage of this instrument is that it can be used with any visual acuity chart, and with many variable-contrast acuity charts including the Peli-Robson test and the Regan charts.

 

Figure 17. Mentor Brightness Acuity Tester (BAT).

 

The Berkeley Glare Test.

Developed by Bailey and Bullimore (19), this test is based on the Bailey-Lovie principle and takes the form of a single chart with a constant contrast of 10%.

As shown in Figure 18, the chart is in the form of a triangle and is surrounded by an opal Plexiglas background, which can serve as a glare source. When used without the glare source, the chart is front-illuminated, independent of the surrounding glare source.

When background illumination is desired, incandescent bulbs behind the Plexiglas screen can yield surround luminances of 30, 800, and 3,000 candelas per square meter. To determine the effects of glare on high-contrast visual acuity, the low-contrast chart can be replaced by the high-contrast Bailey-Lovie chart. The test is designed for use at a distance of 1 m. The eye being tested is corrected with the appropriate lens for this viewing distance (e.g., for an absolute presbyope this would be the distance correction with a +1.00 D add).

The method of specifying visual acuity is by means of a visual acuity reading (VAR) scale. This is a logarithmic scale that gives 5 points for each line of letters read (each line consists of 5 letters) and one point for each additional letter.

The Bailey-Lovie Acuity Chart shown in Figure 18, is a high-contrast visual acuity chart in which there is a size-progression ratio of 5 to 4 throughout the chart, i.e., the angular size of each row of letters is four-fifths that of the preceding row. Each row has the same number of letters; and a constant spacing is used between rows and between letters.

The chart is designed on a logarithmic basis, and visual acuity is designated in terms of the logarithm of the minimum angle of resolution (logMAR). For example, visual acuity of 6/60 (20/200) represents a minimum angle of resolution of 10 minutes of arc. Because the logarithm of 10 is 1, visual acuity of 6/60 (20/200) can be expressed as a logMAR of 1.0.

 

Figure 18. Left: The Bailey-Lovie acuity chart. Right: The Berkeley glare test.

 

It follows that an acuity of 6/6 (20/20), representing a minimum angle of resolution of 1 minute of arc, whose logarithm is 0.0, has a logMAR of 0.0. On the chart, there are 10 steps between 6/60 (20/200) or logMAR 1.0, and 6/6 (20/20) or logMAR 0.0. The chart is designed to be used at a distance of 6 m (20 ft), and if it is used at a shorter distance, a distance correction factor must be applied.

Contrast sensitivity and glare test results compared to patient symptoms

Visual acuity, contrast sensitivity, glare sensitivity, and other visual functions were compared for 50 older patients (68 to 87 years) and 20 middle-aged patients (40 to 60 years) by Rumsey (20). Visual acuity was tested monocularly at distance and near; monocular contrast sensitivity was tested with the Bailey-Lovie low contrast charts; and monocular glare sensitivity was assessed with the Mentor Brightness Acuity Tester at high, medium, and low illumination levels using at a 6 m viewing distance.

Mean values of distance visual acuity, contrast sensitivity, and glare sensitivity were all found to be significantly poorer for the older patients than for the middle-aged patients. Rumsey compared the results of the clinical tests to the complaints made by the patients in routine case histories, with the following results:

In discussing these results, Rumsey suggested that when taking case histories, using more specific, task-oriented questions might be more effective in identifying decrements in visual function.

Should we test low-contrast acuity and glare acuity on all older patients?

Many contrast sensitivity tests results bear no obvious relationship to those of conventional visual acuity charts based on Snellen letters. However, they serve an important purpose in enabling researchers and practitioners to evaluate vision in low-contrast situations. Several variable-contrast visual acuity tests are now available, in addition to tests for visual acuity measured in the presence of glare.

 

 

 

< < previous