Article

Add Contrast Sensitivity Testing for AMD

A look at how and why to acquire this information on visual function

As AMD impacts the quality of vision for each patient differently, it is imperative we use the latest diagnostic and management tools to care for these patients. Contrast sensitivity (CS), often overlooked in its utility to contribute clinically, adds valuable insight to the approach we take with AMD patients. This is due to the strong correlation of CS with real-world visual function, its potential to reveal damage before there are physical manifestations and its unique ability to detect responses from nutritional, anti-VEGF or other intervention.

Here, I discuss how and why optometrists should include CS as part of their AMD work-up. (See “The AMD Diagnostic Device Toolkit,” p.20).

HOW

Contrast is calculated by comparing the difference in luminance of a target to its background and can be expressed as a percentage ranging from 0% to 100%. As a target becomes progressively fainter, it will eventually no longer be visible to the observer, and a contrast sensitivity threshold (the reciprocal of contrast sensitivity) can be recorded.1 Letter CS can be tested with either a Pelli-Robson chart, or equivalent, or a validated electronic CS chart.

The Pelli-Robson chart. This is comprised of eight rows of equally sized letters with six letters per line, and is organized with equally visible letter triplets that gradually change from high to low contrast from the top to the bottom of the chart. The optometrist directs the patient to read the letters and notes the last group where the patient was able to read at least two of three letters correctly to determine the score. The score indicates the patient’s log contrast sensitivity, which can easily be converted to a CS threshold percentage to specify the minimum luminance difference of the target from the background required for the patient to recognize it. For example, if the patient’s score was logCS 2.0 or 1.0% CS threshold, this would represent a normal finding.2 (See below for additional information on interpreting test results.)

An electronic chart with a validated CS test. With this method, Sloan letters (shown to be equally legible) are displayed on a bright background. The optometrist should measure CS monocularly, having the non-dilated patient use her best distance prescription. Next, the O.D. turns off the lights in the room, isolates a 20/100 Sloan letter of 5% contrast on the properly calibrated electronic chart, and then decreases or increases the contrast level by one step as needed while allowing enough time for the letters to come into view. From there, the optometrist records the lowest CS threshold where the patient can correctly identify at least 2 of 3 random letters and then documents this for OD or OS. (The percentage is displayed on the screen.) (Example OD@20/100: 2.5%). These percentages are based on 0.1 logarithmic steps to match the steps used in VA charts like the Early Treatment Diabetic Retinopathy Study chart.3,4 This helps improve accuracy by creating equal changes in letter size from one step to the next and in sequential CS thresholds. A comparison can also be made to the visual impact that occurs with a change of VA, but it is essential to note that CS cannot be predicted from high contrast VA results.5

Severe impairment begins at a threshold of 10%.
Image courtesy of Dr. Mark W. Roark.

In general, when using a 20/100 letter size, average CS thresholds will range from 1.5% to 3.2%, de-pending on the patient’s age. CS thresholds of 4% define the onset of visual impairment for those age 60 and above, while severe impairment begins at a threshold of 10%.6

WHY DETERMINE CS?

The three reasons optometrists should measure CS:

1. VA doesn’t tell the whole story. Although high contrast VA measurement has long been the most used method for assessing visual performance and remains the only internationally recognized visual standard for use in clinical trials, measurement of CS captures the response of the visual system to the wide range of light intensities and levels of contrast encountered in daily activities.5 This is significant because research indicates that CS is closely related to real-world function and predicts the ability to identify faces, signs and objects — things that may be especially challenging to see due to AMD — whereas VA levels do not show this correlation.7

The bottom line is that measuring dark letters on a bright background alone simply does not provide enough visual data about the patient’s visual world, so a CS threshold at a 20/100 letter size should also be determined. With the high prevalence of low contrast images in the everyday world, quantifying CS in AMD patients is an essential step in providing appropriate care.5-6

Subretinal fluid in a patient whose CS dropped three steps.
Image courtesy of Dr. Mark W. Roark.

2. CS determination can be sensitive to changes in retinal health. In one AMD study, for example, increases in central drusen were correlated with decreasing CS results, yet all subjects maintained 20/20 VA.8 I saw this recently when a patient of mine with a history of non-advanced AMD returned for re-evaluation. While VA in the left eye showed a slight decline from 20/25 to 20/30, an initial clinical exam of the fundus appeared stable. However, when CS testing revealed a change in threshold from 3.2% to 6.3% in this eye, a drop of three steps, I performed an OCT of the macula and discovered subretinal fluid, likely secondary to an early neovascular membrane. The patient was referred promptly to a retinal specialist where anti-VEGF therapy was started.

Another important use of CS involves appreciating the response to intervention. Whether a patient needs anti-VEGF injections for treatment of wet AMD or has dry AMD and is using appropriate nutritional supplements, CS testing can show changes in visual performance not seen in VA results.

This outcome was demonstrated in a double-blind, placebo-controlled study of patients who had early or intermediate dry AMD treated with a modified AREDS 2 supplement. Over a 24-month period, VA remained stable while other measures of visual performance, including CS at a letter size of 20/100 or larger, showed significant improvement.9 Being able to monitor for improvements or setbacks in vision can help with patient management and encourage compliance.

3. CS determination can provide an understanding of the effects of a patient’s vision loss. When AMD leads to significant visual dysfunction, CS testing can give insight into the level of difficulty a patient is likely to encounter in activities of daily living.10 This will vary, of course, with the specific task.

For most older adults attempting to perform visually intensive activities, such as reading, dysfunction can be expected at a CS threshold of 5% or above, while disability affecting mobility will likely occur at a threshold above 10%.11-12 There is also a correlation between the increased likelihood of automobile accidents and a CS threshold of 6.3% or worse in one or both eyes.13 Optometrists can use this data to educate patients about helpful tools, such as low vision aids, and to counsel them appropriately regarding everyday activities.

AMD DIAGNOSTIC DEVICE TOOLKIT

In addition to CS diagnostic tools, the following, in alphabetical order, make up the AMD diagnostic device toolkit:

  • Dark adaptation testing. This allows for the identification of functional impairment that may precede the clinical appearance of drusen, thus leading to an earlier diagnosis of AMD. (See the "Focus" column on p.45.)
  • Electroretinography. This reveals and measures issues the retina has in processing light.
  • Fundus autofluorescence. This can identify metabolic dysfunction, leading to abnormal accumulation of lipofuscin in the retinal pigment epithelium, as well as areas of geographic atrophy.
  • Fundus photography. This is key in identifying and grading drusen and pigmentary disturbances, enhancing the ability to identify progression.
  • Microperimeter. This diagnostic device provides simultaneous data on visual function and anatomical pathology.
  • MPOD measuring device. Low macular pigment optical density (MPOD) has been shown to be associated with increased risk for AMD.
  • OCT angiography. This is now available as a non-invasive way to evaluate blood flow to identify ischemia or signs of choroidal neovascularization.
  • Spectral Domain (SD)-OCT. This can provide a high-resolution view of the individual retinal layers to look for subretinal fluid and differentiate between dry and wet AMD.

CAPTURE THE PATIENT’S ENTIRE VISION

Evidence shows traditional VA measurement is important, but simply not enough when assessing visual performance in AMD patients. Fortunately, CS can provide better insight into disease progression and impact on daily life. Additionally, proper measurement of visual performance in AMD may allow for earlier detection of the disease and enable essential patient understanding of the severity of this visual dysfunction. OM

REFERENCES

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