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IS FILTERING SURGERY REQUIRED ?
It is a general rule for the treatment of glaucoma that
when the pressure rises to such levels as to endanger the
disc, one institutes medical therapy, beginning with timolol
or epinephrine and adding miotics if necessary, until the
pressure is reduced to safe levels. If topical agents are
still inadequate, one supplements them with a systemic
carbonic anhydrase inhibitor. The next step is laser
trabeculoplasty. If, notwithstanding all these therapeutic
efforts, the excavation of the disc or the loss of visual
field continues to progress, one must then decide whether or
not to try to lower the pressure by means of intraocular
surgery. In the phakic eye one usually prefers a filtering
procedure. If the eye is aphakic, cyclodialysis or
cyclocryotherapy are considered.
It appears then that the treatment of the glaucomatous
eye is a series of decisions made from month to month and
from year to year, taking into account the intraocular
pressure, modified as it may have been by treatment, and
such changes in the optic disc or in the field of vision as
may have occurred prior to or inspite of therapy. The
making of such decisions is one of the most important tasks
of the physician who undertakes to treat glaucoma, and
understanding these decisions in their logical, emotional
and social perspectives is essential to the optimal
treatment of the disease; and yet, although these choices
are forever being made, the decision itself is seldom if
ever subjected to critical scrutiny. The spectrum of such
decisions is broad. Some are simple to the point of being
trivial, such as whether to use pilocarpine every four hours
or four times a day; some are complex, such as whether to
risk precipitating angle closure with yet stronger
concentrations of a miotic. But no decision is more
troublesome or more perplexing than to determine at any
given time whether or not an eye should be subjected to
intraocular surgery in order to lower the pressure. To
simplify the exposition, I shall refer only to filtering
procedures, although what I say applies, mutatis mutandis,
to cyclodialysis and cyclocryotherapy as well. These
procedures will be considered in greater detail in a
subsequent issue of the Glaucoma Letter.
The decision for filtering surgery is often presented
as a dogmatic prescription. You must not permit the
patient's visual field to deteriorate without having offered
him the benefits of filtration surgery. The rule is so
clear and unequivocal that even a medical student will
confidently make the decision, and the resident physician at
the beginning of his training asserts with confidence what
must be done. But the experienced surgeon, paradoxically,
implements the rule not without some trepidation. The
implementation is difficult because it constitutes a choice
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between alternatives neither of which is satisfactory. If
the operation is not performed, there is a strong
possibility that the patient will go blind from the disease,
but performance of the operation by no means assures that
the sight will be saved. Indeed, there is always the
possibility that the operation will leave the patient worse
off than he would have been without it, for filtering
surgery entails a risk of complications that rival in
seriousness the threat posed by the disease that was to be
alleviated. That is why, for every patient whose glaucoma
remains uncontrolled, the advisability of filtering surgery
must be carefully reexamined. This is an issue to which
uncounted discussions have been devoted, and which,
nonetheless, arises anew, its vexatiousness undiminished,
whenever one is confronted with a patient who seems to be
unresponsive to medical treatment. Since the rule is so
simple, why is it so difficult to apply ?
One reason why the rule is difficult to apply is that
it is a simplification, and, strictly construed, the
simplification is incorrect. The decision becomes far less
perplexing when one understands the logical insufficiency of
the rule. The teaching is that every patient with open
angle glaucoma who, after laser trabeculoplasty and on
maximum medical therapy, shows progressing glaucomatous
damage, either by an increase in the size of the excavation
or by enlargement of the field defect, should have filtering
surgery. A corollary to this teaching is the dictum that no
patient should go blind from open angle glaucoma unless
filtering surgery has been tried at least once. These
statements require critical reexamination in the light of
the following facts. The glaucomatous eye is subject also
to other pathology. In a certain proportion of patients
vascular disease will destroy vision before glaucoma takes
its toll. Although we think of glaucoma as an irreversible
and progressive disorder, in a certain small percentage, the
pressure will improve spontaneously, without apparent cause.
Most important, assuming that progression of glaucomatous
damage is correctly identified, this does not necessarily
mean that the patient will go blind from glaucoma. If the
pressure is within or near the statistically normal range,
loss of field may be very slow, may take place over the span
of many years, and the patient may die long before he
becomes blind, especially if his other eye is relatively
good. It is also very important to keep in mind that not
every patient who has the operation benefits from it. Some
patients who have successful filtering surgery continue to
lose vision nonetheless; some patients develop a filtering
scar which lowers the pressure insufficiently or not at all;
perhaps as many as two percent of patients develop
intraocular infection through the filtering bleb and lose
useful vision. Many patients develop cataract after
filtering procedure. It is true that the cataract may then
be extracted, but that operation in turn entails a
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substantial risk of losing the filtering bleb.
It follows that the class of patients who show
progressive nerve damage on maximum medical treatment must
be divided into two subsets, those, presumably constituting
the larger of the two subsets, who will benefit from
filtering surgery, and those, fewer, but still significant
in number, who will be worse off as the result of surgery.
While these subsets cannot be rigorously defined, it is not
difficult to identify patients, for example, who despite
some deterioration of the optic nerve are nonetheless not in
imminent danger of functionally significant vision loss.
There is also a group of patients less likely to benefit
from the operation, those, for example, who lose field at a
relatively low pressure, those who are aphakic, or those who
have inflammatory glaucoma, those who are youthful, or those
who have had previous surgery. For each patient, moreover,
for whom the operation is less likely to be beneficial, it
is more likely to do harm. It is true that among patients
who are better off without filtration surgery some will
ultimately go blind from the disease, but their vision will
be preserved longer and the loss of vision will be less
injurious to them if it comes about as consequence of the
disease rather than as the result of surgical intervention.
It is indeed true in some instances where a patient goes
blind from glaucoma that his physician procrastinated beyond
the optimal time for filtering surgery. But it is also true
that, in other instances where a patient goes blind from
glaucoma, operation would not have prevented, but would only
have accelerated the onset of blindness. If one scrutinizes
the records of patients who have gone blind after filtering
surgery one cannot escape the conclusion that for a large
proportion of them not only did the operation fail to
preserve vision, but in fact accelerated its loss. In the
contemporary atmosphere of optimistic aggressive surgical
intervention the traditional teaching that the physician's
primary duty is to do no harm deserves at least passing
consideration.
Thus the decision for filtering surgery cannot properly
be made on the simple premise that the patient is losing
field or disc substance in the face of maximum tolerated
medical therapy, but must hinge on the determination whether
the advantages of the operation to a given patient exceed
its disadvantages, in technical terms, whether the economies
of the procedure outweigh the diseconomies. And this is by
far a more difficult decision. Since one cannot with
certainty predict the effect of the operation, good or bad,
for any given patient, one must take a probabilistic,
statistical approach and try to answer the question whether
it is more likely to help than to harm. Admittedly, stated
in this manner, the rule is far more complex, but if it is
less simple, it is also more valid. The cost benefit
analysis of filtering surgery is obscure, and the estimates
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that one ventures to make must always be fitted closely to
the particular circumstances of each case. It is an
elementary fact, however, which is often overlooked, that if
one wants to preserve the vision of the greatest number of
patients for the greatest length of time one must
systematically withhold the operation from those patients,
when one is able to identify them, for whom the risk is
highest in proportion to the potential benefit. Let me give
two examples. The patient whose pressure is 40 mm Hg. and
whose field is closing in rapidly and who has had a
successful filtering procedure in the fellow eye is very
likely to benefit from the operation and should receive it
promptly. On the other hand, the patient who loses field
slowly with a tension of 16 mm Hg. and whose fellow eye
developed flat chamber, cataract, peripheral anterior
synechias, and a scarred filtering bleb following surgery is
likely to be better off without the operation. In this
case, the second eye after filtering surgery is relatively
likely to suffer the same complications as the first,
thereby precipitously depriving the patient of his remaining
vision. Then too, even if the surgery is uncomplicated,
there is only a limited likelihood of attaining a tension
much lower than 16. Without surgery, this patient might
indeed ultimately go blind from glaucoma, but he would
almost certainly have months, if not years of useful sight.
These examples are illustrative of principles. Patients'
problems are seldom so clear cut. But the analyses that
persuade us to operate on the first and to withold surgery
from the second would be applicable to all patients
considered for filtering surgery, if only we would
systematically collect the necessary data on the successes
and failures of our therapies.
These considerations incidentally shed light also on
the origin of the conventional rule. The conventional rule
is premised on the illusion that filtering surgery is always
beneficial. For only if it were always beneficial would it
be rational to offer it to every patient who was losing
vision from glaucoma. Nor is it difficult to identify the
sources of the optimism that has fashioned the theory. It
arises on the one hand from the surgeon's determination to
help his patient. It arises also from the patient's
inextinguishable conviction that this operation will save
his sight. Only under that condition is it possible for him
to submit to it. And no matter how insistently the lawyer
urges them both to acknowledge the risks and hazards of
surgery, no matter how often the elaborate consent form is
signed and countersigned, I have rarely seen a patient go to
the operating room who was not secretly convinced that the
impending operation would save his sight. But so far as the
surgeon is concerned, one would be hard put to say, if one
were the patient, whether one should prefer him to begin the
operation with a detached acknowledgement of the possibility
of failure or with a passionate belief in its success.
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The conventional rule is premised on illusion also in
respect to the validity of the data on which it relies. The
theory assumes that one can determine conclusively that the
patient has received the maximum tolerated medical
treatment, that the disc excavation is increasing and or
that the field is shrinking. It assumes moreover that one
can determine reliably that shrinkage of field is not the
result of lens opacity, ischemic neuropathy or retinal
degeneration. And yet, considered closely, it is rare to
encounter a patient undergoing filtration surgery in whom
one or more of these assumptions are not open to serious
question. So far as deterioration of the disc is concerned,
comparison of descriptions or drawings from examination to
examination often reflects what the examiner thinks should
be happening rather than what is in fact occuring. Except
in the most intelligent and astute of patients, visual
fields are seldom strictly reproduceable. And even then,
the occurrence of cataract or of macular degeneration makes
the plotting of the field inaccurate. When a patient is
first examined, he assumes that the visual field is full and
tends to deny scotomata. As he becomes trained in the
procedure of the examination he learns to recognize
scotomata which previously he might have denied. As for the
examiner, when the pressure rises and he becomes concerned
about a possible loss of vision, it is only natural that he
will begin to search more assiduously for field defects; and
he may well find field loss that previously, when the
concern for the integrity of the visual field was less
acute, might have been overlooked. In general, a new
examiner is likely to discover a field defect previously
missed. Whenever there is a suggestion of progressive field
loss, one may indeed confirm the accuracy of the most recent
field by repeating it, but there is no way to go back three
or six months in time to confirm the earlier absence of a
scotoma.
The other important uncertainty concerns the question
whether or not the field loss that one observes is in fact
attributable to glaucoma or to some other disease. Arcuate
field defects, as is well known, may occur also from causes
other than glaucoma, most notably from vascular disease of
the optic nerve, and we ordinarily hesitate to attribute
them to glaucoma unless there is a concomitant excavation of
the optic disc. There is no reason, moreover, to assume
that the existence of a pathologic cup should immunize the
nerve against such vascular damage, and thus it is almost
certain that some patients who are subjected to filtering
procedures for the presumed progression of glaucoma have
unbeknownst to the physician sustained vascular accidents to
the nerve. The lower the intraocular pressure at which the
patient seems to be losing field, the more likely that such
loss is from causes other than glaucoma, and the less likely
that a reduction of pressure even to very low levels will
arrest the progression of field loss.
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Turning now to the decision itself, fashioned as it is
within the framework of patently inadequate theory,
dependent on manifestly unreliable facts, one begins to
understand the function of the highly prized process of
decision-making. To make a decision is to presume to be
able to reconcile the inexorable constraints of reality with
imperfect theory and inadequate facts. In any situation
where all the consequences of various possible courses of
action were known with certainty, problems of decision-
making would disappear. What was necessary would be
obvious. There would be no question as to what we should
do, no room for decision. Conversely, it is the
unavailability of data, the inability to look into the
future, that creates the need for decision as a counterfeit
of reality which substitutes our "judgment", our hunches,
our prejudices, our veiled self-interest, for those facts
which, if they were available, would determine conclusively
what we had to do. To make a decision is to insinuate ones
subjectivity into the conceptual representation of reality.
It is to fill in the blank areas on the map of the future
with ones intuition. The ophthalmologist who makes the
decision for - or against - filtering surgery in fact
compensates with the skill of his intuitive judgment for all
that the data conceal and for all that the theory denies.
The less precisely articulated the theory and the less
reliable the factual data underlying the decision, the
greater the skill and experience required to make it
satisfactorily. The converse also is true: the more
reliable the theory and the more accurate the data, the
easier it will be to arrive at a decision that turns out to
have been correct. This insight leads to two important
conclusions. In the first place it sheds light on how we
should proceed when faced with decisions that are difficult
to make. Much of this difficulty flows from the
unreliability of the data. The obvious remedy, and often
the only effort required, is to reexamine the records and to
reexamine the patient, and wherever feasible, to determine
for oneself by serial examinations
* * * * *
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Copyright 2006, Ernst Jochen Meyer