glaucoma

Visual Fields Still Dictate Clinical Decisions

Here are five steps for dealing with the most dreaded — and revealing — glaucoma test.

IAN BEN GADDIE, O.D., F.A.A.O.
Louisville, Ky.

If you were to survey your technical staff as well as your glaucoma patients as to which test for glaucoma is the most dreaded, you would most likely have a unanimous vote for visual field testing. When it comes to the doctor side of interpreting visual fields, we often share in the frustration of our staff and patients. Despite the explosion of technology in glaucoma through the past decade, visual field progression still remains a constant trigger for changing course aggressively in glaucoma management.

Infrequently, the evidence of progression is obvious in glaucoma. Usually, the disease progresses slowly, and the practitioner has to make an interpretation as to whether a particular change in a visual field is actually glaucoma progressing. The anxiety of making a call on a particular visual field is not uncommon, so I will outline the steps that I take when reading visual fields. I will also discuss a new software enhancement that allows for verification of progression and rate of progression projections.

Evaluating the visual field

There are essentially five main points to consider when evaluating a visual field.

1. Is the field reliable, and was the correct eye tested?

2. Have you reviewed the probability plots (mean deviation, pattern standard deviation glaucoma hemifield test)?

3. If a defect is present, are the points that are depressed robust enough to qualify as a true glaucomatous defect? Are the test results, which show the defect, repeatable?

4. If a defect is present and qualifies as a true glaucomatous loss, does it correlate clinically to the optic nerve and Retinal Nerve Fiber Layer (RNFL) status?

5. If you are reading a follow-up visual field, is progression present?

Is the field reliable?

This is a fairly straightforward series of checks. First, look at the fixation losses (FL). If losses exceed 20%, you should use caution, and anything over 33% fixation losses should be considered unreliable unless the field technician can verify that the patient maintained adequate fixation and gaze.

Next, I look at the false positive (FP) rate. This is the rate at which a patient responds positively when no stimulus is presented. This rate should be as low as possible — ideally below 20%. As the number of false-positive responses increases, pseudo improvement of the visual field can occur. Finally, take a look at the false negative (FN) rate. This metric represents the percentage of time that a patient failed to positively respond to a stimulus that should otherwise be seen based on previous responses. For FNs, the goal is to have less than 25% loss. Also, as the field becomes significantly compromised, expect to see a higher rate of FN (see figure 1).

Figure 1: Low false positive and false negative rates indicate a reliable visual field test.

Review the probability plots

The mean deviation (MD) is the most common probability measure to evaluate the overall "hill of vision" or overall sensitivity. This can be misleading because glaucoma often causes more focal hemifield damage to the optic nerve and RNFL. For example, a patient with focal hemifield damage of 10 dB would only have an approximate 5 dB deficit on the MD, as the MD represents the average of both the damaged and healthier hemifields.

The next probability plot to consider is the pattern standard deviation (PSD), which is a measure of focal damage. A higher PSD is found in more focal damage. As the disease progresses to affect both hemispheres, the PSD can improve since the damage is becoming more diffuse and less focal. This makes the PSD excellent for identifying early glaucoma damage. In fact, the OHTS study identified PSD as one of the more important risk factors for converting from ocular hypertension to glaucoma.

Glaucoma is clearly an asymmetric disease, especially in the early to moderate stages. The glaucoma hemifield tells (GHT) compares symmetry between the superior and inferior fields. In addition, to raise the sensitivity of this technique, examine clusters of mirrored points (see figures 2a and 2b).

Figures 2a and 2b: The glaucoma hemifield test uses values from the pattern deviation map grouped in clusters.

What qualifies as a significant visual field defect?

This is one of the most intimidating aspects of evaluating the visual field because you have to ask yourself, "is this glaucoma?" Suffice it to say, no consensus exists in this area, and multiple scoring systems can be applied to the evaluation of a field test. At a minimum, while looking at the standard deviation plot, you need a cluster of three non-edge points depressed below the 5% probability level in an area known to suspect glaucoma. The points should be contiguous with each other, keeping in mind that points residing on the edge of the field may be suggestive of damage but shouldn't be used to make a definitive diagnosis. Most importantly, the field should be repeatable. The OHTS authors reported a high rate of improvement (or improvement to the point that the criteria for a glaucomatous field were no longer met) on repeat testing. By the same token, suggestive loss is sometimes confirmed on repeat testing (see figure 3).

Figure 3: Results of a visual field test. For reliability, results must be repeatable.

Does the visual field defect make clinical sense?

Although it seems like an intuitive question, it's important to remember that just like imaging and intraocular pressure (IOP), the visual field is but one piece of the glaucoma puzzle. If the results don't provide congruence with other findings, consider that the outlying test may be artifact or noncontributory.

Early in the disease continuum, it's not unusual to see results that are contradictory from your structural assessment (see figures 4a and 4b).

Figures 4a and 4b: Early in the disease continuum, it is not unusual to see results from structural assessment tests (left) that contradict visual field test results. However, this case demonstrates an excellent structure-function relationship in the left eye.

Is this patient progressing or stable?

Beyond diagnosis, it's crucial to be able to ascertain a particular patient's rate, or velocity, of progression. The rate of progression is important both for staging of the disease and when making treatment-related adjustments. At the intersection of staging of glaucoma and vision preservation is life expectancy. If we knew how long a patient were to live, we could tailor therapy to preserve vision until that patient dies. Consider the patient that develops glaucoma at age 78. Our target IOP is going to be 25% to 30% reduction, vs. a 47 year-old patient with myopia and migraine disease. With the increased life expectancy of the younger patient and other risk factors for progression, a much larger percentage of reduction in IOP must be achieved and maintained. Furthermore, the rate of progression has been shown to be strongly predictive of subsequent damage.

Zeiss Humphrey perimeters recently adapted a Visual Field Index for the calculation of glaucoma rate of progression. The concept is based on the work of Bengtsson and Heijl who developed a Glaucoma Progression Index (GPI). The GPI is based on trends from the pattern deviation plots and is represented as a percent of normal visual field remaining relative to the individual's age. This is represented commercially as the VFI on the Humphrey field unit. It appears that upwards of six fields may be needed to establish the rate of progression.

Progression has been the Holy Grail of clinical glaucoma research. The ability to identify the rate of progression and pick up the "quick progressors" early is needed. Expect to see more work in this area, as we attempt to further elucidate the relationship between structural loss and functional loss in glaucoma. OM

Dr. Gaddie is in private practice in Louisville, Ky. He is vice president of the Optometric Glaucoma Society and vice president of the Kentucky Optometric Association. E-mail him at ibgaddie@bellsouth.net.