One of the most important things an ophthalmologist needs to know about a
glaucoma patient is how fast the disease is progressing. Determining
that requires multiple visual field exams over a period of time. But
while rapid progression is usually detectable with only a few exams,
smaller rates of change require a greater number of data points and
usually a longer period of time to confirm—and the reality is that most
ophthalmologists perform a limited number of visual field exams on most
patients. That’s largely true for practical reasons, of course,
including reimbursement concerns. But it raises a clinically
significant question: Can a limited number of visual field exams
reliably detect slower rates of change—rates that are still high enough
to threaten a patient’s vision?
In recent years our team at Dalhousie University and collaborators have
been conducting studies to determine the number of visual fields
required in a given time period to determine different rates of
progression with statistical validity. During the past five or 10
years, some data has been published regarding the frequency of optic
disc examinations using photography or other, newer technologies.
Generally, this data indicates that most practices conduct these
examinations less frequently than is recommended by organizations such
as the American or European Glaucoma Societies. However, we found very
little data on the frequency of visual field exams, so we began looking
into this. Not surprisingly, the frequency of visual field exams
appears to vary tremendously from jurisdiction to jurisdiction and
practice to practice.
This observation led us to attempt to determine how many examinations
would be required in order to detect different rates of visual field
progression. Obviously this is a complex question that can’t be
definitely answered for a given patient without making a number of
assumptions, but we felt that the question was important enough that it
was worth attempting to answer.
In most cases, if the patient has a long life expectancy, you’d want to
know if his rate of progression is causing a loss of 2 dB per year or
worse—what we would consider a “fast progresser.” (Even a loss of 1 dB
per year in some patients could ultimately be catastrophic.)
The reality is that the accuracy of any decision you make regarding
progression in a patient will be directly proportional to the amount of
data you have. If you have five years worth of data and you’ve got two
to three good-quality visual fields per year, you’ve got a pretty good
idea of what’s happening to that patient. If you’ve done three tests
over a period of five years, you may not have any idea what’s going on.
How Many Visual Fields?
Given these concerns, we wanted to provide some practical guidelines
regarding the number of visual field exams that are necessary to
accurately detect a given rate of progression. So, we did a series of
analyses to come up with the number of examinations that would be
required to detect different levels of progression.
What our statistical analyses found is that six visual field exams have
to be performed during the first two years in order to detect a rate of
progression of about 2 dB per year—sufficient to threaten a patient’s
vision over time, but not easily determined with a smaller number of
exams during the same amount of time. Again, a really fast rate of
progression can be measured with fewer tests; but a rate of 2 dB per
year (or slower) cannot reliably be determined without doing six tests
in two years’ time.
For example, suppose a new glaucoma patient comes to see you. The first
visual field you perform will give you a pretty good idea of whether
the visual field is damaged. That visual field should be repeated to
ensure the accuracy of your baseline. If those two exams indicate the
presence of damage, then it becomes important To determine whether or
not the individual is a fast progresser. (This is true even if you
begin treatment to lower the intraocular pressure; some individuals will
continue to progress quickly despite being treated.) If that patient
is then followed with a total of six visual fields over the two years,
the ophthalmologist can get a good idea of how fast that patient is
progressing. Once that rate of change has been determined, the need for
more serious treatment and monitoring can be decided in light of the
patient’s visual field damage-vs.-age plot. (Note: Commercially
available software such as the Humphrey Progression Analysis system can
work well for the analysis; the issue is having a sufficient number of
visual field tests for the analysis to pick up the progression.)
In our published paper1
we have a series of tables that tells you how
many visual fields you need to do to measure different rates of change.
The ophthalmologist can apply these paradigms to detect these different
rates of change.
Our study does have some limitations. For example, we used mean
deviation as the key value, and we’re well aware that this is an
imperfect metric. If a patient has a paracentral visual field loss at
two or three points but the rest of the visual field is close to normal,
changes in those few points would have a minimal impact on the mean
deviation. Others factors such as the presence of a cataract or an
inattentive patient can also affect the mean deviation. But it’s an
index most of us are fairly comfortable using to describe the magnitude
of visual field damage in a given patient, so we chose to use it in our
The reality is, going from one visual field to two visual fields per
year gives you a dramatic information gain. Going from two to three
tests per year also gives you a significant information gain. Beyond
that, the clinical value begins to taper off. So the ideal frequency
should be between two and three visual fields per year, depending on how
great a rate of progression you feel a given patient can tolerate
without risking visual disability over the course of his or her
Dealing with Clinical Obstacles
Not surprisingly, the first thing people say when I recommend six visual
fields in the first two years, is: “I can’t possibly do that.” In
response, I’d like to make several points:
• Doing six exams in the first two years isn’t a big change from what
many ophthalmologists are already doing.
If a glaucoma patient comes to
you for the first time, you’d do one visual field, and you’d probably
want to do another to establish a reasonable baseline. You might have
to do a third one to rule out any potential learning effects. Then you
might want to do a visual field every six months after that. You’re
already approaching six visual fields in the first two years. If you’ve
done five, the information you gain by adding a sixth visual field,
statistically speaking, is tremendous—enough to ensure that you’ll
detect a rate of visual field loss of 2 dB per year.
• Although reimbursement currently will not cover this many visual
fields in some practices, this can be used as a powerful argument for
getting those kinds of limitations changed.
Many ophthalmologists tell
me that they can’t get reimbursed for more than one, or perhaps two,
visual fields per year. I would argue that fighting to get this changed
is a worthy cause.
Consider the economics involved: In our province in Canada, we’re
reimbursed about $50 for doing a visual field. We have an estimated
300,000 potential glaucoma patients; doing six visual fields instead of
five for these individuals would cost the health-care system about $15
million total. Spending $15 million to create a dramatic improvement in
our ability to figure out which patients are going to end up with
visual disability, allowing us to tailor treatment in those patients to
prevent that outcome, is a small investment for a potentially great
health-care cost saving—the cost of rehabilitating patients with severe
visual field loss—in the long run. (Consider the amount being spent to
cover the cost of recently developed treatments for macular
degeneration.) So not doing this is a false economy.
• Not every patient needs the additional visual fields.
suggesting that we need to do this many visual fields on every patient.
Again, whether a patient needs to be checked for fast progression can
be decided based upon a group of factors, including age and initial
condition. And this wouldn’t apply to ocular hypertensives.
Furthermore, fast progressers probably only constitute about 20 percent
of the glaucoma population—maybe even less—and those patients are the
only ones you might want to test more frequently.
Ensuring Usable Test Results
Clearly, the clinical value of any number of visual field tests depends
on having good quality data. I would offer these suggestions to help
maintain the reliability of test results:
• Make sure you and your staff realize the value of visual field data.
Sometimes the staff— perhaps even the ophthalmologist— may not fully
appreciate that the perimetry examination provides crucial information
about the patient’s disease status and progression. The other tests
that we do—measuring eye pressure, looking at the optic disc, looking at
the nerve fiber layer—relate to the disease process, but they don’t
measure the functional consequences of the disease. A plethora of
studies tell us that patients’ ability to function well—how many times
they fall, how many car crashes they have, how many hip fractures they
have—are all related to visual field damage. If everyone on the staff
appreciates this fact, testing will be done more carefully, and even the
patient will pick up on the fact that the visual field exam is
important. (Ultimately, of course, this attitude has to start with the
• Mitigate patient fears.
While it’s important for patients to
understand the value of the visual field test, you don’t want them to
see it as a daunting task. For example, patients often get anxious
because they’re not seeing the stimulus, and that can reduce the quality
of the examination. It’s important for the technician to tell the
patient that about 50 percent of the time he won’t see the
stimulus—that’s intentional. It’s also important to let the patient
know that the technician will be there during the test; that the patient
can stop the test at any time to take a break or ask questions; if her
eyes get dry, she can have a moisturizing eye drop. This reduces
patient anxiety, and it can make a huge difference in how reliable the
test information is. If it takes an extra minute of the technician’s
time at the beginning of the exam to reinforce these things, our
experience has been that the extra minute is very worthwhile. You’ll
have far fewer unreliable examinations.
• Standardize the instructions.
It’s important that every patient
receives the same instructions. Having standardized instructions
ensures that every patient gets all the information necessary to take
the test properly and with minimum anxiety. (These instructions should
include the statements mentioned in the previous point.) Obviously, if a
patient has taken the visual field test for several years and you know
he produces reliable results, you don’t need to repeat the instructions
• Be careful about eliminating test data on the grounds of a learning
I don’t advocate eliminating visual fields from your data
automatically on the grounds of a possible learning effect. (Learning
effects usually occur when there’s an unexplainable improvement in the
visual field.) It’s true that most patients learn between the first and
second examination, but that’s not always the case. The removal of a
visual field from the analysis is a judgment call and can usually be
made when there are additional visual fields, which may indicate the
truer nature of the visual field loss.
• Don’t perform alternate types of visual field exams at the expense of
Ophthalmologists currently have access to a number of
alternate visual field testing strategies and instruments. I see these
tests as primarily valuable in a research environment today, but I’m
concerned that some clinicians may choose to perform them at the cost of
doing fewer standard automated perimetry exams. Again, a greater
number of standard exams translates to a greater ability to catch
progression at rates that are dangerous in the long run but difficult to
measure with only a handful of visual fields. In some situations,
using alternate perimetric tests may be necessary, but I would urge
clinicians to be conscious of the potential trade-off.
• To ensure data quality, don’t just depend on reliability indices.
These can be useful, but we encourage ophthalmologists not to trust them
automatically. They’re meant to be a guideline. For example, it’s
possible to have high fixation losses in a patient who is doing the test
perfectly well. There are also scenarios in which the fixation error
may appear within acceptable limits from the perimeter’s standpoint, but
a technician supervising the test might feel otherwise.
For that reason, I’d encourage ophthalmologists to get the technician to
write down subjectively whether or not the patient did the test well.
Was the patient attentive? Falling asleep? Particularly anxious that
day? All of these things will give you an idea of how reliable the test
is. Sometimes this kind of observation corroborates what the perimeter
is reporting, but often it doesn’t.
The important thing is to not assume test data is good or bad just
because the instrument says so. Dropping a legitimate result or
including a suspicious one may undercut your ability to catch
• Don’t mix the results of different test strategies.
For example, you
shouldn’t mix a SITA-standard strategy with a SITA-fast strategy, or a
10-2 strategy with a 24-2 strategy. If you have a Humphrey perimeter,
we recommend using the SITA-standard strategy for all testing. The time
savings you get going from SITA-standard to SITA-fast is not that
significant, but the information loss is.
Taking the Next Step
The work we reported in our published paper on this topic is just a
beginning. Our intention is to eventually generate concrete guidelines
for visual field testing that clinicians can use, based on each
patient’s baseline criteria. Eventually we’d like to create
user-friendly software that works in conjunction with perimeters to aid
in analysis of the data and projection of likely future progression. Of
course, we also hope to aid in the process of getting insurers to see
the preventive value of doing more visual fields for some patients, so
that those visual fiields are routinely reimbursed.
At the end of the day, we want to make sure that the clinician gathers
the most effective amount of useful data and uses that data sensibly to
make clinical decisions. If we’re successful, this should have a
significant impact on our ability to prevent glaucoma from leading to