CONTRAST SENSITIVITY AND GLAUCOMA

Contrast sensitivity (CS) is very useful in two areas of glaucoma evaluation. Data now shows that prior to treatment, glaucoma patients exhibit abnormal contrast sensitivity and this abnormality can be used to assess the presence or progression of the disease. Following treatment, 60 to 70% of the patients show clinically significant improvements in CS. Contrast sensitivity can be used to evaluate the patient response to the initiation of therapy or to a change in therapy.

Contrast Sensitivity Aids in Establishing the Presence of Glaucomatous Vision Loss Prior to Treatment:
Clinical use has isolated three primary criteria which can be used to determine whether a glaucoma suspect has abnormal contrast sensitivity prior to treatment. Example graphs of these criteria are shown below. All three criteria are based on the patient being tested with best correction such that any vision loss detected by contrast sensitivity is not due to refractive error.

Below the Normal Range
If a patient has good acuity, 20/30 or better, then he or she should have good contrast sensitivity. One hallmark of early glaucoma is that patients fall outside the normal range (patch 3 or below in either Row B or Row C), even when acuity remains normal. An example is shown on the left. It is important to keep in mind that this criteria can be confounded by the presence of other eye diseases, such as cataracts, diabetes or macular degeneration. If a glaucoma suspect presents with other eye diseases, then this criteria must be used with caution.

Asymmetry in Contrast Sensitivity
A difference between eyes or asymmetry in CS is an early indicator of disease progression in glaucoma, particularly if the asymmetry occurs in Rows B and C. The example below shows a typical patient with asymmetry in contrast sensitivity, no field loss and equivocal appearance of the optic nerve head. A difference between eyes of 2 contrast levels or more is considered a significant asymmetry.

Notch in Contrast Sensitivity
Clinical use has demonstrated the presence of a notch in contrast sensitivity in approximately 30 to 50% of untreated glaucoma patients. This notch most often appears in Row C, but also has been documented to occur in Row B. An example is shown below. The notch criteria is perhaps the most powerful indicator of early disease because other eye diseases such as cataracts, diabetes and macular degeneration have not been shown to cause a similar pattern in contrast sensitivity.

Typically, a glaucoma patient will present with a contrast deficit which matches more than one criteria. It is not unusual for a patient to present with all three; CS below the normal range, asymmetry between eyes and a notch. When more than one criteria is present for the patient, the probability is higher that the patient has progressing disease.

Retest the Patient if a Deficit is Found:
All psychophysical tests, such as perimetry and threshold contrast sensitivity, suffer inaccuracies from patient fatigue and learning. If a patient presents with a CS deficit, that patient should be immediately retested on CS to insure the presence of the deficit. Also, the refractive status of the patient should be evaluated to insure that the loss in vision is not the result of inadequate optical correction.

Contrast Sensitivity Improvement and the Treatment of Glaucoma:
Dr. George Spaeth suggested over a decade ago that improvement in visual function may be an important criteria for establishing the efficacy of glaucoma therapy. New data indicates that the improvements in CS following treatment may aid in providing such information.

George Spaeth, M.D.

"The documentation of change in visual field or optic disc (worse or better) is the most certain criterion of control. Unless improvement is noted to accompany lowering of intraocular pressure, adequacy of control cannot, with certainty, be assured."

Trans Ophthalmol Soc UK 1985

The following example shows the improvement in vision following initiation of glaucoma therapy.

Research and clinical data shows that 60 to 70% of glaucoma patients demonstrate reliable improvements in contrast sensitivity within 1 to 2 months of starting medical treatment (Pomerance and Evans. IOVS August 1994). This percentage may be higher in patients who undergo surgical treatment of glaucoma, but the data is still unclear in this area. These improvements are predictable and can be used clinically as a criterion to determine the efficacy of therapy.

Change in Treatment:
Contrast sensitivity can also be used to determine if a change in treatment is beneficial. This example shows the improvement in CS following a change in treatment, from a beta blocker to a prostaglandin analogue. (This example is a specific clinical case for one patient and is not intended to imply in any way that treatment with a prostaglandin analogue provides for better contrast sensitivity than treatment with a beta blocking agent. Other specific clinical case reports will appear in future updates of our website showing the opposite case. Current research is ongoing to aid in delineating which types of patients exhibit more improvement with each type of medication.)

The left figure shows the contrast sensitivity (CS) for both eyes of a glaucoma patient who has been treated for 3 years with Betoptic S (IOP: OD/OS 17/20). Note that both eyes are below the normal range. The right figure shows the CS of the same patient after switching to three months of treatment with Xalatan (IOP: OD/OS 13/15). Note that both eyes have increased into the normal range. The appearance of the patient's superior/inferior arcuate scotoma OD and early inferior arcuate scotoma OS did not change with treatment (Data provided by Dr David Stewart).

Better Management of Glaucoma Therapy:
Clinical use shows that in many patients, treatment can be better managed if contrast sensitivity is added to the evaluation process (i.e. visual field, optic disc appearance and IOP). If a patient exhibits a CS deficit which matches one of the three pretreatment criteria discussed previously, then a goal of treatment should be to eliminate this CS deficit. For example, if the patient presents with a notch in Row C and an asymmetry between eyes, then a possible goal of treatment would be to extinguish these CS deficits. Additionally, a two contrast level improvement in CS is considered clinically significant. In cases where a complete reversal of the CS deficit(s) cannot be obtained, then a secondary goal of treatment would be to obtain a two contrast level improvement.

Patients should be evaluated with best correction. A change in correction between visits can cause a shift in contrast sensitivity that is unrelated to the disease state.

Normal Contrast Sensitivity May Indicate that the Patient is Under Therapeutic Control:
After a glaucoma patient has been properly treated, it is not unusual for that patient to exhibit normal levels of contrast sensitivity. If a treated patient has normal CS, then it is inappropriate to conclude that glaucoma has no impact on that patient's central vision, only that the impact is being successfully offset by treatment. These patients should be monitored to ascertain if any reductions in CS occur over time, potentially indicating a reduction in treatment efficacy or possibly a lack of patient compliance.

Compliance With Glaucoma Medication:
Contrast Sensitivity is commonly used by many clinicians to ascertain the level of patient compliance with medical treatment. Many times, patients discontinue use of the glaucoma treatment and only "drop their medication" on the morning before the visit to the eye doctor. The IOP measurement for these patients indicates proper compliance. However, if the patient has not been compliant, the CS will remain low. Four to 6 weeks of consistent medical treatment are normally required to fully affect the improvement in CS. The CS test can also be used to demonstrate to patients the impact of poor compliance.

What Causes the Improvement in Contrast Sensitivity?:
Many clinicians and several research reports have noted that the improvements in contrast sensitivity following glaucoma treatment are not correlated to change in IOP (Pomerance and Evans. IOVS Aug 1994; Harris et al. AJO Oct 1997). That is, in some patients a small change in IOP is associated with a very large change in contrast sensitivity, whereas in other patients, treatment which provides for a large change in IOP provides for a diminutive or no change in CS. Vigorous research is now ongoing to determine the cause of improvement in CS following glaucoma treatment. Several authors have suggested that changes in ocular circulation are related to the improvements in central vision.

The bar plots show the changes in blood flow velocity in the ophthalmic artery of normal tension glaucoma patients following six months of treatment with a calcium channel blocking agent (nifedipine). The 16 patients in the study have been broken into two groups; a contrast sensitivity responder group in which each patient demonstrated an improvement of two contrast levels or greater in contrast sensitivity (0.3 log units) and a contrast sensitivity nonresponder group in which no patient demonstrated an improvement in CS following treatment. Neither group showed a change in IOP.

Note that the CS responder group displayed on average an improvement in blood flow velocity while the nonresponder group showed a decrease in the velocity measures. This data suggests that contrast sensitivity may be useful for predicting which patients respond favorably to glaucoma treatment. (Graphs adapted from Harris et al. AJO Nov 1997).

These bar charts show the change in peak systolic velocity (PSV) and end diastolic velocity (EDV) in the ophthalmic artery of normal tension glaucoma patients following 6 months of treatment with oral nifedipine. The two groups are broken down by contrast sensitivity response. Each of the patients in the responder group showed a significant improvement in CS while the patients in the nonresponder group showed either no change or a decrease.

The left graphs show the change in contrast sensitivity of a patient from the CS responder group. Her contrast sensitivity improved dramatically from outside to inside the normal range. Her blood flow velocity in the ophthalmic artery as measured by color Doppler imaging increased nearly 70%. The right graph shows the change in contrast sensitivity of the worse patient in the CS nonresponder group. Her CS fell during treatment and the ophthalmic artery velocity also fell dramatically (> 90%). (Contrast Data From Evans et al. ARVO Abstract 1996).

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