AUTOMATED PERIMETRY
Every few weeks I receive yet another advertisement for
the newest in automated perimeters. The mailman brings an
elegantly printed brochure which shows a handsome ivory
colored perimeter with an electronic plotter to display the
fields. Its "intelligence" resides in a microprocessor
which has been programmed according to the specifications of
unnamed experts to select successive stimuli and correlate
them with the patient's signalled responses. The machine in
the brochure appears to be plotting the visual field of an
attractive young woman sitting in front of it, who has
placed her head in the chin rest and is gazing into the open
hemisphere. Nearby, contemplating the scene with overt
satisfaction stands a middle aged man in a white coat with
whom, if I understand the copywriters correctly, I am to
identify myself. For a modest investment of between twenty
and ninety thousand dollars, I can take his place.
The brochure explains the sources of his satisfaction.
The estimation of the visual field has hitherto been a
bottleneck in his work. It made his days long and sometimes
disagreeable. It reduced the number of operations he found
time to do. It prevented him from spending time with his
family. Since he acquired the automated perimeter all that
has changed. The drudgery of examining thirty patients a
day has dissipated. In a few hours time, an aide with
nothing more than a high school diploma has learned to
operate the perimeter. Almost every day scotomata that
would previously have been overlooked are picked up. There
is the subtle, and sometimes not so subtle implication that
only by availing myself of its preprogrammed assistance can
I fulfill my professional obligations to my patients.
"Better patient care" was the salesman's promise, and the
brochure purports to illustrate how the promise can come
true. Then too, the practical side of things is not
forgotten. The perimeter has in effect created a new class
of services rendered, justified a new set of charges,
augmented the cash flow, and increased the depreciation
allowances permitted on Form 1040. No wonder the
perimeter's new owner looks content. A few days later, the
salesman stops by my office. "Better patient care," he
promises, and a higher net income. The salesman offers me
his pen to sign the purchase order. But just as I am about
to do so, second thoughts crowd into my mind. At this
difficult juncture of trying to decide whether or not
automated perimetry is worth the not insubstantial
investment, I review what I have learned in all the years of
measuring visual fields by hand. I try to pinpoint the
clinical decisions that hinge on changes in the field. I
try to assess the limitations of manual techniques and to
determine, if I can, whether automated perimetry will
actually overcome them.
The root problem of measuring the visual field is the
discrepancy between the objective characteristics of retinal
sensitivity and the patient's subjective experience of what
he sees. It is a simple psycho-physiological fact that one
can see objects at which one is not looking, that, in other
words, one possesses peripheral vision in the technical
sense. It is a fact, however, of which the patient is
initially unaware. The rapid effortless unconscious motions
of the eyes facilitate the illusion that the acuity is the
same at all angles and that the visual field is everywhere
equally sharp. One is unaware that the eyes are virtually
always in motion, that the area of acute vision is
constantly being repositioned, and that the visual scene
that he perceives is actually a composite of numerous
individual and identifiable visual impressions. The naive
and natural response to a visual stimulus in the peripheral
field, as all of us who have examined the visual fields of
our patients know only too well, is to turn the eyes to
fixate the object that has attracted ones attention. Thus
there is a continuing functional integration of peripheral
and central vision, and it is not at all surprising that
some of our patients have difficulty distinguishing between
the two.
When the examiner plots a patient's visual field for
the first time, he is in fact a teacher. His initial task
is to demonstrate to the patient that, while fixating on a
central target, one can "see" in the periphery. Next the
patient must be taught to respond consistently to peripheral
stimuli. Finally the patient must learn to recognize
scotomata. The identification of a strong peripheral
stimulus is easy, and many patients understand immediately
what is expected of them. But the determination of just
where a small target disappears into, and the precise point
at which it reappears from the depth of a scotoma are
judgments at times extremely difficult, and not even the
most experienced and intelligent of patients can be certain.
It is obvious and yet too readily forgotten that the
sensitivity in the peripheral visual field determined in
this manner is a function not only of the integrity of the
retina and the optic nerve but also of the clarity of the
media and of the steadiness of fixation. More than that, it
is also a function of attention and discrimination, of the
ability to coordinate and to perform simultaneously two
separate visual tasks. The identification of the peripheral
stimulus is a skill dependent in part on the patient's
alertness and intelligence; it is a skill which improves
with practice.
These considerations suggest that it might be useful to
distinguish between retinal sensitivity on the one hand and
the visual field on the other. The visual field has been
described as an island of vision rising from a sea of
darkness. But it is the spatial representation of retinal
sensitivity which so appears as an island with a peak at the
fovea, and with a shoreline that corresponds to the boundary
of the sensory retina. This interpretation of retinal
sensitivity is a model constructed from experimental data
whose usefulness requires no re-emphasis. Confusion arises
when one forgets that this model does not correspond to the
patient's experience. What the patient sees is something
quite different. He sees, by and large, what he expects to
see. That is why he has difficulty in recognizing, not to
speak of defining, a scotoma in the paracentral or
peripheral visual field. It is true that in the trained
experimental subject the isopters of the visual field and
the sensitivity of the retina coincide. But in a large
proportion of our patients the sensitivity of the retina, be
it constant or otherwise, cannot be clinically calibrated,
for it is obscured by unsteady fixation, wandering
attention, intervening fatigue, and the intercurrence of
both external and internal distractions. The visual field,
therefore, that we are trying to plot is from a
psychological perspective, an idealization. No wonder we
have difficulties in getting consistent results.
The examination of retinal sensitivity is an
idealization also from a purely geometrical point of view.
It would require, allowing five seconds for each
measurement, more than 316 24 hour days of continuous field
testing to determine the sensitivity to four different
intensities of stimulus of each square millimeter of the
visual field at a distance of 330 millimeters. No
perimetric technique, however automated it might be, can
measure the visual sensitivity of but a small fraction of
the retina. Thus to map the visual fields completely is out
of the question. No visual field examination will do
anything more than to sample the sensitivity of the retina
at selected points. Which points are sampled, how
frequently, and with how many different stimuli, these are
matters of optimizing a cost-benefit ratio. The value of
increased precision and completeness must be balanced
against increasing expense. An exhaustive examination is
precluded, however, not only by financial cost, but by the
fatigue which attends prolonged examination and which makes
the responses less and less reliable, until, after a
certain point, valid initial data become obscured by error.
Automated perimetry is promoted with two
distinguishable promises which are customarily lumped
together, but which deserve separate consideration. The
first of these promises is that when automated perimetry is
substituted for manual procedures, there will be a net
saving, accruing to the vendor or possibly to the purchaser
of the health care in question. If an examiner is very busy
with other tasks, so that his time is relatively valuable,
then indeed the automated perimeter, if it can be operated
reliably by a less skilled technician, may pay its way.
However when the examiner, as is sometimes the case, is not
fully occupied, his time is less valuable, and then manual
examination of the visual field, requiring as it does
minimal capital investment, becomes economically preferable.
In any event, it would seem wise to defer so large a
commitment of funds until a practice is firmly established
and the cost-effectivenss of the machine can be confidently
predicted.
The second issue is whether or not automated perimetry
is inherently more reliable or sensitive than manual
techniques. The major problem about field testing with
conventional methods is that even in the best of hands they
frequently produce inconsistent results. It is a common
experience that if a field is repeated several times, a
different result may be obtained on each occasion. Thus the
real size of the field defect remains in doubt, and even
more important, if the examinations in question have been
separated by any substantial interval of time, one cannot
tell whether the progression of field loss is apparent or
real. As a result, one has accustomed oneself, however
unsatisfactory it may be, to disregard small changes in the
visual field, and to defer taking action until progression
of field loss has been established beyond reasonable doubt.
The precise nature of the limitations of field testing
is not generally recognized. An excessively mechanical
interpretation of the visual function presupposes that the
visual field coincides with retinal sensitivity and that
given a cooperative and reasonably intelligent patient, a
competent examiner should be able in every instance to plot
a consistent and reliable field. Thus the difficulty of the
task is denied and, ironically, its performance is then
delegated to a technician or perhaps to a junior resident.
Under these circumstances, the shortcomings of an unreliable
field test are blamed on the subordinate, with the
implication that the chief, had he not been busy with more
important matters, might have done better. Apprised of this
state of affairs, the electronic engineer comes forward and
offers to help us by building what on first thought might
appear to be the ideal examiner, a machine that can provide
a reproduceable stimulus without getting tired or bored or
impatient.
Can automated perimetry circumvent the limitations of
field testing? It offers the great advantage of an
absolutely reproduceable stimulus. Yet variation in the
stimulus is relatively unimportant among the factors that
account for the variability and inaccuracy of the visual
field. Of far greater importance is the inconsistency of
the patient's response which is often apparent even when, in
the course of a single examination, the stimulus is
evidently reproduceable. The basic assumption that at any
given location in the visual field the patient's response to
a stimulus should correspond to the sensitivity of the
retina is valid only for a trained experimental subject.
The average patient who has glaucoma usually lacks the
qualities that make someone a good subject for
psychophysical testing. Much of the variation that obscures
the significance of clinical field examination almost
certainly derives from fluctuations in attention and
awareness to which the average patient is prone. When, as
is often the case in elderly patients, the optical media are
cloudy, the reliability of visual field measurements
decreases even more.
Nothing that has been published suggests that automated
perimetry solves the problem of the inattentive or
unreliable patient or the problem of plotting a meaningful
field in the presence of media opacities. Indeed it seems
possible, and this is perhaps the greatest hazard of
automated perimetric techniques, that a technician who has
no other duty than to operate the machine, who is
indifferent to the patient's state of mind and who knows
nothing about his cataract or vitreous opacity, may
misinterpret an apparent loss of field as being glaucomatous
in origin, when in fact it is anxiety or inattention, lens
opacity or vitreous infiltrate that is responsible. For the
elderly, forgetful and confused patient, with perhaps poor
central fixation or cloudy media, with short memory and
defective hearing, automated perimetry will probably be even
less reliable than manual techniques. Indeed, its
uncritical use will give highly misleading results that
would have been immediately apparent to a human examiner
attempting to enlist the patients cooperation in the
classical tangent screen examination. It is a common
observation that the only truly satisfactory field
examinations are those that are obtained on patients who by
virtue of their intelligence and insight and by virtue also
of their concern about their problem have acquired many of
the characteristics of trained experimental subjects.
Indeed clinical field testing is successful largely to the
extent that the examiner is able to teach the patient to
remain alert and objective in reporting what he sees. Thus
field testing is a process of communication. One of the
examiner's most important tasks is to evaluate the patient's
understanding of the field testing procedure and interpret
the patient's replies accordingly. Clearly any elaborate
instrumentation is likely to interfere with this process.
One of the reasons why tangent screen examination is so
satisfactory is that close and reliable communication
between patient and examiner may be so readily established.
One of the disadvantages of automated perimetry is that it
interposes a complex electromechanical device between
patient and examiner which cannot but block the processes of
communication that are so important in the interpretation
and evaluation of the visual field examination.
Whatever scientific value the geometric model of the
visual field may have, it is the purpose of field testing in
clinical practice not to demonstrate retinal physiology but
to obtain specific information essential to determining the
treatment that the patient is to receive. We consider,
therefore, the actual function which the information derived
from the field examination fulfills in our diagnosis and
treatment of the glaucoma patient. (It may be noted
parenthetically that similar analyses are apposite in the
evaluation of potential or actual field defects in diseases
of the retina and of the nervous system, and that, mutatis
mutandis, the conclusions to which we come about the
glaucoma fields may well have relevance in those other areas
also.)
As a screening technique for glaucoma, perimetry is
unsatisfactory, because disease thus uncovered will be far
advanced and would better have been detected by tonometry
and ophthalmoscopy in an earlier stage. We do not wait for
field loss to occur before initiating treatment for
glaucoma. That decision is made on the basis of the
pressure and the appearance of the disc. Information about
the visual field is required primarily for one purpose: to
ascertain whether, given the prevailing intraocular
pressure, one may continue to treat the patient with
medication or whether filtering surgery is required. This
question arises in two distinguishable contexts. We
encounter it in the situation where the disc is badly cupped
and the tension remains significantly elevated on maximum
tolerated medical therapy and after laser trabeculoplasty.
Consider a patient with an excavated disc and a pressure
that can not be brought below, for example, 34 mm Hg. If
such a patient were found to have a nerve fiber bundle field
defect, one would recommend prompt filtering surgery, since
under those circumstances the likelihood of field loss
progressing to blindness would be very great. If, on the
other hand, no field loss were demonstrable, it would be
essential, no matter which technique of examination had been
utilized, to repeat the field examination at relatively
frequent intervals because of the rapidity with which large
amounts of field loss can occur under these conditions. The
second situation where the results of field examination are
decisive in determining the course of glaucoma treatment is
where, given preexisting field loss, the tension has been
brought to statistically normal levels but where a filtering
procedure offers the possibility of lowering the pressure
yet further. In this situation one would advise surgery
only if there were evidence of progression of field loss,
and it would be of the utmost importance to obtain precise
and reproduceable fields. In both situations we now content
ourselves with identifying gross and unmistakable
alterations, and we have adapted our clinical decision
making to the imperfections of our field testing techniques.
More reliable methods of estimating visual field loss would
enable us to proceed with greater confidence to the surgical
treatment of glaucoma. Nonetheless, it is important not to
forget that inaccuracy in the field examination is seldom
the limiting factor in glaucoma therapy: When field
examination is performed sufficiently frequently, its
inaccuracy is almost never the cause of therapeutic failure.
How much help, if any, automated perimetry will provide
us in monitoring the progression of glaucoma cannot be
predicted on the basis of available data. The literature
with which the manufacturers supply us is devoid of any
meaningful calibration of their machines. We need to know
how accurate the automated perimeters are, how reproduceable
their results. We need to know what proportion of the
population that has glaucoma lends itself to satisfactory
examination with the automated perimeter. It is the absence
of this information which makes one hesitate to place ones
order. If automated perimeters are reliable and accurate,
data to support their reliability and accuracy should be
forthcoming. Until then, we would do better to stay with
manual techniques.
* * * * *
Back
Next
Glaucoma Letter Index
Website Index
Copyright 2006, Ernst Jochen Meyer