Contrast Sensitivity Testing as a Diagnostic Tool
More precise equipment now makes it possible to detect disease earlier, relatively inexpensively.
BY DAVID W. EVANS, PH.D., M.B.A.

In today's medical environment, early diagnosis and treatment of pathology continues to become more important for practice growth and management. Of course, early detection of progressing pathology leads to happier, healthier patients and better word-of-mouth referrals. In addition, it can move patient reimbursement levels from general examination-related coding to more specific disease-related coding, and it helps to protect the practice from ever-present legal risks.

Because of the cost of diagnostic equipment, it's been impossible for most practices to purchase every instrument that's available. However, thanks to advances in technology, some relatively inexpensive general screening tools can now provide great diagnostic power. These tools can help funnel patients into more specific care involving higher-cost equipment.

Changes in contrast sensitivity can signal a serious underlying vision problem.

In particular, instruments that measure contrast sensitivity (CS) and visual acuity have become more precise, and hence more diagnostic in nature. For example, visual acuity can now be tested using a standardized Early Treatment Diabetic Retinopathy Study (ETDRS) chart format, making this test a more reliable clinical tool.

Here, I'd like to discuss the use of the latest contrast sensitivity tests as general screening diagnostic tools, including how to administer the tests and interpret the results.

How It All Started

The first published report of the clinical use of contrast sensitivity to diagnose a visual problem didn't come from an ophthalmologist, but a neurologist. In the early 1970s, Ivan Bodis-Wolner, M.D., working at Columbia University, noted that many patients with head injuries resulting from car accidents complained of vague visual disturbances, even though they retained good acuity. After measuring contrast sensitivity in these patients, he discovered that it could diagnose visual problems that remained invisible to other forms of testing.

Although Dr. Bodis-Wolner used CS testing for a specific purpose, he foresaw and described the broad use of the test as a diagnostic tool. He stated the principle that if patients have good acuity, they should also have good contrast sensitivity. If you find a divergence between acuity and contrast sensitivity, a vision problem exists, meriting further evaluation and perhaps treatment.

Today, contrast sensitivity testing is becoming more widely perceived as an essential part of an eyecare professional's repertoire. In fact, the Food and Drug Administration now requires that new ophthalmic devices meet basic levels of contrast sensitivity performance in order to be approved.

New Testing Standards

To eliminate much of the variability previously associated with contrast sensitivity evaluation and ensure standardization, the FDA recently proposed standards for CS testing in clinical trials. Under the new standards, patients are tested with best correction using four spatial frequencies of sine-wave gratings (light and dark bar patterns) with a standardized light level of 85 cd/m². Testing is conducted before dilating the patient or measuring IOP, as both of these can reduce contrast sensitivity. (Routine clinical CS testing should follow a similar protocol.)

The patient receives a separate score for each spatial frequency, just as a patient who has taken a hearing test receives a score for each sound frequency. The scores provide the information necessary to aid in diagnosis. Typically the spatial frequencies are lettered from A to D, so the patient's chart shows a score for each letter, as in the following example. (The normal ranges provided below are for the VectorVision CSV-1000 Contrast Sensitivity Test.)

Sample patient contrast sensitivity scores:

OD: A 5 B 5 C 4 D 4
OS: A 6 B 5 C 5 D 4

Normal scores:

Up to age 60: A 4 - 6 B 5 - 7 C 4 - 6 D 3 - 5
Over age 60: A 4 - 6 B 4 - 6 C 3 - 5 D 2 - 4

Many contrast sensitivity systems can plot a patient's CS scores as a graph. However, graphing the data is time-consuming and unnecessary. It's not recommended except in cases where the graph is used as an educational tool.

Interpreting the Results

Once the patient's contrast sensitivity scores have been determined for the four spatial frequencies, three basic indicators suggest that a problem exists:

Scores outside the normal range for the patient's age group. Normal ranges should be posted in the patient record (as shown in the example above), or provided in a separate key. If the patient has good acuity (20/25 or better) and falls outside the normal range, further evaluation is needed. (Note: Contrast sensitivity is very sensitive to refractive error, so the first thing to check is whether the patient was properly refracted before the test was administered.)

This type of evaluation will detect diseases such as ARMD, diabetic retinopathy and glaucoma. (Hundreds of published papers support this. For two examples, see Arend et al., Contrast Sensitivity Loss is Coupled With Capillary Dropout in Patients with Diabetes. Invest Ophthalmol Vis Sci. 1997. 38(9); 1819-1824, or Pomerance G, Evans D. Test-Retest Reliability of the CSV-1000 Contrast Test and Its Relationship to Glaucoma Therapy. Invest Ophthalmol Vis Sci. 1994. 35(9); 3357-3361.) Once a problem has been detected, the cause of the loss in contrast sensitivity can be followed using more rigorous tests, such as angiography (in cases of diabetic retinopathy) or scanning laser ophthalmoscopy (in glaucoma).

Asymmetry between the eyes. If both eyes have similar visual acuity, both eyes should have similar contrast sensitivity. In early disease progression, one eye typically shows signs of vision loss before the other, which is often reflected in asymmetrical CS scores. (This asymmetry can occur even when the patient's scores still fall within the normal range in both eyes.) Asymmetry between the eyes occurs frequently in ARMD, diabetic retinopathy and glaucoma.

If you find a numerical difference of 2 or more between the eyes at the same frequency, and this occurs more than once in the scores, a vision problem is indicated. A problem is also indicated if you find a three-level difference at a single frequency. This patient is an example of the former:

Patient contrast sensitivity:

(Age 65, 20/20 acuity in both eyes)
OD: A 5 B 4 C 3 D 2
OS: A 6 B 5 C 5 D 4

Normal scores:

Up to age 60: A 4 - 6 B 5 - 7 C 4 - 6 D 3 - 5
Over age 60: A 4 - 6 B 4 - 6 C 3 - 5 D 2 - 4

Note that both eyes scored in the normal range for a person over the age of 60. However, when the right eye is compared with the left eye, the contrast sensitivity scores for rows C and D in the right eye are depressed by two contrast levels compared with the left eye. In most cases, this result is highly diagnostic of an advancing vision problem. (Again, check to be sure that the differences between the eyes are unrelated to spherical blur or astigmatism.)

Once asymmetry has been detected, other diagnostic tests can ascertain the full extent of the progressing problem.

Abnormally distributed contrast scores. Patient scores for either row C or row D (12 and 18 cycles/degree, respectively) should not be two or more levels higher than the score of the preceding row. In other words, if the patient scores two contrast levels higher on row C than on row B, or two levels higher on row D than on row C, it's a diagnostic indicator of a vision problem. In the case of the patient below, the right eye has produced a score in row D that's three contrast levels higher than row C.

Patient contrast sensitivity:

(Age 70, 20/20 OD, 20/25 OS)
OD: A 5 B 5 C 3 D 6
OS: A 6 B 5 C 5 D 4

Normal scores:

Up to age 60: A 4 - 6 B 5 - 7 C 4 - 6 D 3 - 5
Over age 60: A 4 - 6 B 4 - 6 C 3 - 5 D 2 - 4

This type of response is highly diagnostic for glaucoma. In fact, this kind of abnormally distributed scoring isn't associated with any other eye disease, and it can't be caused by residual refractive errors. (It's not unusual for the abnormal distribution in scores to resolve following the initiation of glaucoma treatment.)

The Contrast Sensitivity Advantage

Because contrast sensitivity testing is minimally expensive and easy to do, it's an ideal diagnostic tool. It lets you conduct a quick screening that aids in diagnosis and can justify further testing with more time-consuming and expensive test equipment. And, from a legal standpoint, incorporating contrast sensitivity testing into your practice demonstrates a proactive stance toward early diagnosis, and can provide additional documentation should a legal issue arise.

In today's challenging environment, it's a valuable tool that can benefit any practice.

Dr. Evans is a consultant faculty member at the University of Alabama, Birmingham, and a member of the FDA subcommittee on vision standards. He's been involved in contrast sensitivity research since 1978 and has written extensively on contrast sensitivity and its relationship to ocular disease and refractive surgery. He has a financial interest in the CSV-1000 standardized contrast sensitivity device marketed by VectorVision Inc.