Contrast Sensitivity Testing
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This is Mark Wilkinson from the University of Iowa Department of Ophthalmology & Visual Sciences. In this presentation I will discuss contrast and contrast sensitivity testing.
This slide shows the various components of vision that contribute to a person’s overall level of visual functioning. Visual acuity alone is important, but glare, color vision, fixation, stereopsis and visual field all play a factor in how well our visual system functions. That said, the single most important factor in how a person functions visually is directly related to their contrast sensitivity.
Visual acuity testing allows for the quantification of the degree of high contrast vision loss. Visual acuity testing also monitors stability or progression of disease, as well as the change in the patient’s visual abilities as treatment and rehabilitation progress. Visual acuity testing also allows assessment of eccentric viewing postures, scanning abilities and motivation. That said, visual acuity testing does not tell us about the individual’s quality of vision. It only speaks to the patient’s quantity of vision.
Let’s talk about visual quality versus visual quantity.
Visual quantity is determined by the patient’s visual acuity and visual field findings.
Visual quality describes how the person functions in vision-related activities. Visual quality gives the clinician a better understanding of how the patient is able to use their vision for activities such as reading, mobility, employment and activities of daily living.
This slide shows the value of higher contrast for safer mobility.
Here is another example of the benefits of higher contrast.
Contrast sensitivity testing is similar to hearing testing.
With a hearing test, different frequencies and decibel levels are tested to determine the full hearing abilities of the individual.
In a similar way, contrast sensitivity testing evaluates vision over a range of spatial frequencies and contrast levels.
By evaluating vision at different frequencies and contrast levels, contrast sensitivity testing provides a better assessment of visual function or quality of vision, than visual acuity testing alone does. As I mentioned earlier, visual acuity testing is a quantitative not a qualitative measurement of visual functioning, as contrast sensitivity testing is.
Contrast sensitivity testing helps to predict illumination, contrast & magnification needs. Contrast sensitivity testing also helps in predicting success with optical devices.
Here are some examples of sine wave grating contrast sensitivity testing charts.
The sine wave charts provide the clinician with information about how the patient is able to see for low, medium and high contrast targets. Remember that visual acuity testing is a high contrast test.
You see on this slide that our contrast sensitivity curve, particularly in the mid and higher spatial frequencies, decreases as we age secondary to ocular media and retinal changes that naturally occur over time.
Here you see a Pelli–Robson contrast chart. This is the contrast chart that has been used for most clinical research studies. The Pelli-Robson chart presents letters as triplets. Each triplet fades by 0.08 log units. Norms for different levels of contrast loss are Normal, when the patient can read 6-7 lines (12-14 triplets). Moderate contrast loss is based on the patient only being able to read 4-5 lines. Severe loss is when the patient can only read 2-3 lines. And, profound contrast loss is when the patient can only read 1 line or less.
An alternative to the Pelli-Robson chart is the Mars Letter Contrast Sensitivity Test.
The Mars Test has a set of 3 near charts. Each letter fades by 0.04 log units, allowing for a more precise measurement of contrast loss than is possible with the Pelli-Robson chart. Based on how many letters the patient is able to see, they are given a log contrast score.
Unlike the Pelli-Robson test, which uses a 1-meter testing distance. The Mars test uses a 50-centimeter testing distance.
Norms for different levels of contrast loss include profound loss at less than 0.48 log contrast, severe at 0.52-1.00, moderate 1.04-1.48, normal for individuals older than 60 years is 1.52-1.76 and normal for individuals age 60 and younger is 1.72-1.92 log contrast.
Contrast sensitivity testing results assist the clinician in determining the patient’s magnification needs, their ability to use optical magnification devices and the patient’s illumination needs. As we know, illumination is critically important for optimum visual performance.
Contrast sensitivity testing can also tell the clinician about the patient’s dominant eye and their overall quality of vision. Finally, contrast sensitivity testing helps clinicians to monitor changes in visual functioning over time.
In general, if contrast sensitivity function is down, the patient will need more magnification. They will also need more illumination when they have reduced contrast sensitivity.
When an individual has reduced contrast, their functional abilities can be improved by doing such things as using contrasting placemats on the table as you see on this slide.
Finally, we must remember that poor contrast may prevent success with optical magnification devices, and lead to other functional problems.
Reduced contrast can result in an increased risk for falls. This is especially true for older adults with poorer contrast from media opacities and age-related eye diseases. Also, individuals with reduced contrast will have severe problems with reading low contrast text, like what is found with colored letters on colored paper. Individuals with reduced contrast often require electronic magnification. Electronic magnification allows the user to read with enhanced or reversed contrast.
With contrast loss, the world appears hazy and washed out. This can lead to difficulties with driving at night or in the rain; a problem that affects all of us, to some degree or another as we age. There can also be problems with judging distances, walking down steps, recognizing faces, reading instructions on medicine containers and navigating in unfamiliar environments.
Now you know there is more to how a person functions with their vision that what they are able to see on a high contrast eye chart. Keep this in mind when your patient complains about their vision in the real world, yet still has 20/20 vision in your exam room.