WATCHING FOR ANGLE CLOSURE
One of the great triumphs of modern ophthalmology is
the identification of mechanical angle closure as the cause
of severe glaucoma. There are various contributing factors
responsible for this disease, geometric narrowness of the
angle, abnormal lens size, anterior displacement of ciliary
processes, increase in vitreous volume, but the most
important, because it can be remedied surgically, is
pupillary block of the aqueous humor as it flows from the
posterior into the anterior chamber whence it is drained
through the trabecular meshwork into the canal of Schlemm.
The interplay between these various factors will be the
subject of a separate discussion. Of interest at this time
is a consideration of the techniques that are used for
evaluating the potentially troublesome angle.
The association between shallow anterior chambers and
closable angles is well known. Angle closure in a truly
deep chamber with an absolutely flat iris plane is extremely
rare if not indeed unheard of. Angle closure in the very
shallow chamber with a convex iris is a common occurrence.
Between these two extremes there is a large proportion of
anterior chambers varying from moderately deep to moderately
shallow where the risk of angle closure cannot be predicted
from the chamber depth.
It is possible if one directs a flashlight beam across
the anterior chamber to obtain some estimate of its depth
and to obtain some notion of the relative convexity of the
iris. The flashlight test is of some value if it reveals
the anterior chamber to be quite deep and the iris plane to
be absolutely flat. This test can be very misleading,
however, in all but the deepest of chambers, and we advise
resorting to it only in the unusual circumstance when
neither a slit lamp nor gonioscopy equipment is available.
With the slit lamp one can get a more accurate picture of
the configuration of the chamber periphery, and it is this
rather than the axial depth of the chamber which one really
wants to know about. If one projects the planes of the
inner surface of the cornea and of the peripheral iris to
the line in which they meet, one obtains a reasonable
approximation to the width of the angle. One must now
arrange in ones mind all the anterior chambers which one
examines into a series ordered according to the apparent
acuteness of the angle. One may now select for gonioscopy
those patients who are more likely to have closeable angles.
What proportion of all patients are gonioscoped will depend
on the age distribution of the patient population, on the
examiner's temperament and on his schedule. I think the
physician should use whatever method of gonioscopy seems
most satisfactory to him, taking into consideration not only
the completeness with which the angle is visualized, but
also the ease and speed with which this is accomplished.
The Koeppe gonioscopy lens used together with the hand-held
binocular microscope seems very satisfactory, and any
physician who finds the other gonioscopy lenses the least
bit awkward, should certainly give the Koeppe lens an
extended try. The angle is inspected with the gonioscope,
and for each hour of the clock, the examiner estimates its
width. I have found the following classification useful:
0 - closed
1 - slit-like, probably closed
2 - slit-like, probably open
3 - very narrow, risk of spontaneous closure
4 - narrow, risk of mydriatic closure
5 - narrower than average, negligible risk of closure
6 - of normal or greater than normal width
8 - pathologically recessed
9 - not visible on gonioscopy
This classification seems useful to me because it addresses
the decisive issue: How likely is it that any given section
of the angle will close? A simple means to develop ones
judgment in this important field is to gonioscope every
angle with varying pupil size. If the examining room is
dimly lit and the Barkan light is directed as far away form
the pupil as is consistent with reasonable illumination of
the angle, one will observe the pupil in semi-dilation.
When a bright overhead light is turned on, the pupil
contracts, and one can watch the angle become wider. It
also helps if one takes every opportunity to gonioscope
angles following mydriasis and/or cycloplegia. The
classification outlined above or one similar to it, may be
used to describe the angle in its various meridians.
Frequently there is much variation in angle width from one
sector of the angle to the next. The risk of angle closure
in any given eye is a composite of such variations. In
general, if the meshwork is otherwise healthy, at least one
third the angle must be closed before any significant
elevation of pressure occurs, and unless at least two thirds
of the angle is closeable, an attack of angle closure
glaucoma is unlikely.
Unless the angle is made rigid by peripheral anterior
synechias, its width may be expected to vary somewhat from
one examination to the other. If the angle is wide, such
variation is obviously insignificant. Such variation
depends not only on the size of the pupil but also on other
poorly observable factors such as vitreous volume, lens size
and position, and the rate of aqueous flow. Often these
fluctuations in angle width are so slight as to be of no
significance. In other eyes they are sufficiently large to
require to be taken into account in assessing the need for
further examination and for treatment. When the angle is
narrow this variation explains the sudden onset of angle
closure. Gonioscopy is repeated at intervals appropriate to
the observed width of the angle. Where inspite of an
axially shallow chamber gonioscopy shows the angle to be
widely open, repeating gonioscopy after two years, for
example, may be sufficient. On the other hand, if there
seems to be risk of spontaneous closure, gonioscopy every
three or four months will make the assessment of that risk
more reliable. If spontaneous closure seems improbable but
the angle is so narrow as to make closure with mydriatics a
possibility, then repeating gonioscopy at perhaps six
months' intervals seems reasonable.
The darkroom test is a useful procedure for evaluating
the patient whose angle is so narrow that spontaneous
closure seems an imminent risk. It is a simple procedure
which requires a minimum of the physician's time, and is
very informative, especially when a significant pressure
elevation is provoked. It is helpful to give the patient an
explanation of the purpose of the test. We tell them that
the pupillary dilation in the dark is sometimes sufficient
to precipitate a small attack of glaucoma which is likely to
be mild, and which, if it has to take place, had better
occur in the doctor's office. The patient should have a
chance to orient himself in the darkroom before the light is
turned off. He should be given a comfortable chair. If he
is elderly, he should not be left alone in the dark, lest he
become disoriented, but should bring a family member to keep
him company. Sometimes a patient likes to listen to a
transistor radio. We hand him a small dinner bell which he
may ring if he wants assistance. The tension is then
measured with the Perkins hand-held applanation tonometer,
and the eyes are covered with a black mask. Then the
lights are turned out, and the patient is reminded not to go
to sleep and not to keep his eyes open without blinking for
too long a time. One of the office assistants looks in on
the patient every 5 to 10 minutes to make sure that he is
comfortable. At the end of one hour, the mask is removed
and in dim illumination the tension is again measured with
the Perkins applanation tonometer. In general, the darkroom
test is considered negative if in the dark the tension does
not rise more than 7 mm Hg. If the test is positive, its
results are then correlated with the patient's history, with
the gonioscopic findings, and with the rest of the
examination, and the plan of treatment is reconsidered.
A negative darkroom test suggests that an attack of
angle closure glaucoma in the immediate future is relatively
unlikely. Conversely, a positive darkroom test indicates
the presence of angle closure glaucoma. However, it is
unwise to be guided by a set of rigid rules. One might be
lulled into a false security by a spuriously negative
darkroom test, or proceed unnecessarily with surgery that
however skilfully performed is never entirely without risk.
There is, moreover, no way of determining the validity of
any given test. The terms "false positive" and "false
negative" are statistical concepts that can only shed light
on the probability of a given fact, never on its certainty.
When, after the results of the darkroom test have been
evaluated in conjunction with other clinical data, it is
decided that peripheral iridectomy is not (yet) in order,
the patient is cautioned to be observant for possible
symptoms of angle closure glaucoma and is advised to return
for reexamination after an appropriate interval. Usually
this interval is three months, unless repeated gonioscopy
shows an angle much wider than previously recorded, in which
circumstance and interval of six or even twelve months may
be appropriate. On the other hand, if the darkroom test was
positive, and/or the angle was observed to be extremely
narrow, repeat examination as early as one month may be in
order. It has been customary, even if the patient is seen
more frequently, to repeat darkroom tests no oftener than
every six months, but there seems little justification for
such parsimony unless the patient objects to the procedure.
Given the availability of a suitable darkroom, the test is
quite economical of the ophthalmologist's time, and
repeating it relatively frequently provides valuable
information not only concerning the patient's clinical
status but also concerning the consistency and reliability
of the test. It goes without saying that in patient's whose
eyes are under the influence of mydriatic, cycloplegic, or
more likely miotic topical medication, the darkroom test
should be deferred until, the drops having been
discontinued, all their effects have dissipated. This might
be several weeks in the case of Phospholine iodide, and even
in the case of pilocarpine, several days. While the effect
of the drops is wearing off, the physiologic state of the
eye is in continuous change and the risk of spontaneous
angle closure may well be much higher than it was before
they were discontinued.
The detection of angle closure glaucoma in the presence
of medication that affects the pupil or the depth of the
anterior chamber is a problem that warrants more detailed
consideration. This and other aspects of the diagnosis and
treatment of angle closure glaucoma I will discuss later.
* * * * * * *
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Copyright 2006, Ernst Jochen Meyer