LOOKING AT THE DISC
I
Looking at the disc is one of the most perplexing, and
again one of the most interesting, and, when all is said and
done, one of the most satisfying activities of my daily
practice. If I had to name a single task that prevented the
examination of even the normal eye from becoming tiresome, I
would nominate the inspection of the optic disc. I have
found it an unending source of fascination and that in more
perspectives than one.
The optic disc is the intraocular vestige of the optic
stalk from which the eye developed in the embryo. It is the
anatomic and functional link between the retina and the
brain. It is the boundary between the low pressure tissue
environment and the sphere of elevated intraocular pressure
within the globe. In some instances it reveals a record in
physiological archeology as it were, of the pressure history
of the eye. Description of the optic disc puts to the test
ones powers of observation, ones judgment, ones good sense,
and ones intellectual integrity. It gives occasion for
examples of astute clinical judgment no less than for naive
unwitting parodies of reason. It provides the setting alike
for thoughtful, conscientious observation and for rash,
overly zealous pretentions to knowledge. To interpret the
changes which the disc appears to undergo is a challenge and
has on occasion proved a trap for the aspiring
ophthalmologic statistician or biophysicist. The obvious
fallacies of reason that it has inspired invite comparison
with those other disciplines, phrenology and iridology,
which systematically require of the observer that he
intoxicate himself with fantasy. And there are few
structures in the human body that have been more lavishly
bedecked with emperors' new clothes.
Given the ubiquity of ophthalmoscopes and the ever
increasing numbers of examiners, the profusion of medical
periodicals and meetings to communicate new discoveries, one
would think that the last word about the optic disc had long
ago been confided to the microphone at some national
meeting, or appeared on the glossy pages of a refereed
scientific publication. But I do not think this has been or
will be the case, if only because, at least for the
forseeable future, the results of disc examination will not
lend themselves to codification or to other mathematical
description, any more than will mathematical description
disclose the identity of an acquaintance whom we recognize
intuitively by looking at the features of his face, an
achievement of which the infant is capable long before its
grandfather has given up on the research project for
parametric description of the disc. The interpretation of
the disc of any given patient, therefore, remains a
subjective enterprise, a skill that is acquired with
practice, that cannot be forced, a power of observation and
judgment that increases with the years, until it becomes
attenuated and ultimately extinguished with the growth of
ones own lens opacities or the deterioration of his macula.
These essays, therefore, by no means purport to give the
last word on the interpretation of the disc, or to foreclose
the possibility of computer-aided image analysis or
photogrammetry adding significantly to our knowledge. All I
want to do is to report some of the ideas that come to my
mind as I look at discs day after day. It also helps, if
one wishes to entertain new ideas, to do a certain amount of
preliminary intellectual housecleaning, and to try to
dispose of various misleading suppositions that make life
difficult for the beginner.
As a first step in thinking about the disc, nothing has
proved so valuable as simple considerations concerning its
geometry and structure, facts that are so elementary as to
be almost beneath repetition, but facts that each observer
can confirm by looking through his ophthalmoscope and for
which he is not dependent on specialized but inaccessible
instrumentation. Think of the nerve fibers that converge
from all quadrants of the retina onto the optic disc, there
to turn backward through a defect in the sclera into the
sheaths of the optic nerve. The nerve fibers are
transparent, the very phenomenon of vision depends on that
fact, and therefore one can often, but not always, see the
rim of scleral tissue that is the edge of the foramen.
Because such a large proportion of the nerve fibers come
from the macula, there is a heaping up of the fibers at both
the upper and lower poles of the disc. The disc appears to
be covered with a glistening transparent membrane, which
seems continuous with the internal limiting membrane of the
retina. It is not uncommon to see hemorrhages beneath the
internal limiting membrane of the retina extend onto the
optic disc. In the center of the nerve head, at what was
once the origin of the vascular supply to the primary
hyaloid, there is said to be a defect in the internal
limiting membrane. The varying degrees of excavation that
are visible in healthy eyes have developed incident to the
atrophy of the primary hyaloid artery. Inasmuch as this
vessel atrophies to the point at which it branches from the
central retinal artery, the depth of the excavation in
healthy eyes is most likely determined by the location at
which vessels of the primary hyaloid originally branched
from those of the retina. The configuration of the cup is
in part the expression of genetic traits. Unusually deep or
broad physiologic cupping often runs in the family. It is
helpful, therefore, when confronted with a youthful patient
who has a large central excavation, to find that one of his
brothers or sisters or one of his parents has a disc of
similar appearance. However, the circumstance that an
excavation in an otherwise healthy eye is not corroborated
in other members of the family has only very limited
diagnostic import.
Two observations suggest that there may be much
variation in the depth of the lamina cribrosa, and much
variation, consequently, in the orientation of the nerve
fibers that constitute the substance of the optic disc. On
occasion one sees a patient with an unusually deep and broad
optic cup at the bottom of which one gets a glimpse of the
lamina cribrosa. It is also not uncommon to see patients
with discs excavated and atrophic from glaucoma, where the
cup is shallow and the lamina cribrosa is relatively
anterior. Thus where the lamina cribrosa is anterior, the
substance of the optic disc is thin and the orientation of
the nerve fibers is predominantly parallel to the plane of
the disc. On the other hand, where the lamina cribrosa is
posterior, the substance of the disc is thick and the
orientation of the nerve fibers is predominantly
perpendicular to the plane of the disc.
Finally, but not least in interest, there are the
central retinal vessels which divide on the surface of the
disc into superior and inferior branches. The vein is
usually collapsed as it descends into the nerve. Between
the ribbon-like vein that disappears from view and the
distended cylindrical vein that crosses the margin of the
optic disc, there is often visible a segment of vein with
spontaneous pulsation, a nice demonstration of nature's
automatic pressure-reducing valve which maintains pressure
in the retinal veins at a level greater than intraocular
pressure, and thereby prevents the intraocular pressure from
emptying the retinal vascular bed. It is worth noting that
the circulation at the upper and lower poles of the disc
drains into a more peripheral segment of vein which contains
blood under higher than intraocular pressure, while the
nasal and temporal poles of the disc drain into a more
proximal section of central vein of which one can say with
certainty, since it is collapsed, that it is under lower
than intraocular pressure. When the intraocular pressure
rises, the upper and lower poles of the disc, those areas
which show the earliest evidence of atrophy, drain into
segments of retinal vein whose pressure is maintained, by
the mechanism just described, at higher than intraocular
pressure, while the temporal and nasal poles of the disc,
those areas which are most resistant to pressure induced
atrophy, largely drain into more central segments of vein,
which are collapsed and contain blood whose reduced pressure
is unaffected by the rise in intraocular tension.
The potential effects of increased intraocular pressure
on the optic disc are multiple, and it is important to
distinguish between them. In the first place we must remind
ourselves that even in the healthy eye, with normal
intraocular pressure and without any excavation or atrophy
of the disc, there is a pressure gradient through the
tissues of the disc. If we could place a transducer on the
internal limiting membrane at the surface of the disc and
another transducer between the sclera and the nerve fibers
as they turn to enter the disc, we should find a pressure
gradient, albeit small, which would have the effect of
compressing the nerve fibers. It is this difference in
pressure which molds the disc and determines the contour of
its surface, a fact which becomes strikingly apparent when,
with increased intracranial pressure, the gradient is
reversed, the excavation disappears, and papilledema ensues.
Another effect of increased intraocular pressure which
concerns us is that which brings about the atrophy of the
individual nerve fibers. The most widely held theory
concerning the mechanism by which increased intraocular
pressure causes destruction of the nerve is the hypothesis
that increased intraocular pressure deprives the optic nerve
head of its circulation, which is derived from the short
posterior ciliary arteries and the Circle of Zinn, a supply
source of lower pressure than the retinal artery and
therefore more susceptible to impairment by pressure
elevation. A second hypothesis is that damage to the nerve
is simply mechanical in origin. It is thought that with
deepening of the cup there is increasing stretch of
individual nerve fibers which atrophy as a result of the
induced strain. It has also been suggested that nerve
fibers might be strangulated in the constricted interstices
of a lamina cribrosa that is ballooned posteriorly by
chronically elevated tension. We quickly dispose of this
notion by pointing out that where intersecting fibrous
strands of lamina cribrosa are not free to slide one upon
the other, axial distention of these fibrous strands
necessarily entails distention, not constriction, of the
interstices that they enclose. More to the point is the
possibility that it is not impairment of the arterial but of
the venous circulation, by a mechanism suggested above,
which produces atrophy of the nerve. This possibility would
seem to be corroborated by the circumstance that those areas
where the venous pressure is demonstrably highest are the
ones where cupping begins and where the initial field
defects in glaucoma are customarily observed.
Glaucomatous excavation necessarily entails
displacement of tissue. The nature of this displacement is
a fundamental issue of physics that requires consideration.
In the first place, it appears possible that as a result of
prolonged tension elevation, the lamina cribrosa might
itself become atrophic and that the resultant loss of
fibrous tissue might contribute to the volume of the cup.
Secondly, it is conceivable that the lamina cribrosa is
stretched posteriorly, and that this posterior displacement
adds to the volume of the excavation. Attenuation of the
lamina cribrosa from either cause would go far to explain
the apparently exponential relationship between intraocular
pressure and the rate of excavation that was commented upon
in previous issues of the Glaucoma Letter. How consistently
destruction of fibrous tissue and/or its posterior
displacement contribute to cup formation is a question that
we must leave to future observation and reflection. There
is no doubt about the atrophy of nerve fibers. So far as
the loss of nerve fiber substance is not compensated by
glial ingrowth, the configuration of the resulting cavity
will be affected by the orientation of the nerve fibers
destroyed. Where the lamina cribrosa is relatively far
posterior and the orientation of the nerve fibers is
predominantly perpendicular to the surface of the disc,
atrophy of the nerve fibers may be expected to bring about
primarily the peripheral enlargement of a preexisting
central excavation. As individual nerve fibers atrophy, the
intraocular pressure compacts the remaining fibers, the
sleeve of nerve tissue lining the posterior scleral foramen
becomes thinner, and the central excavation enlarges until
it finally occupies the entirety of the opening. In eyes,
however, where the orientation of the nerve fibers is
predominantly parallel to the surface of the disc, atrophy
of individual nerve fibers will lead to a diffuse depression
of the surface of the disc, causing it to assume somewhat
the shape of a saucer. In this circumstance the
glaucomatous excavation develops not by enlargement of a
pre-existing "physiological" cup, but by the progression of
concavity over the entire surface of the disc. Between
these two extremes there is, of course, a spectrum of
intermediate cases which combines some of the
characteristics of each.
In the light of these considerations, we may now
reflect once more on the task of inspecting the disc for
glaucomatous damage. The presence of a membrane of
undefined stiffness covering large areas of the nerve fibers
on the disc suggests at least the possibility that some
early and perhaps some not so early nerve fiber loss might
remain obscured under a uniform contour. The occasional
disc that has an overhanging rim of tissue, projecting like
a cornice over a bulbous central excavation, may possibly
sustain undetectable loss of nerve substance for long
periods of time. Similarly, when glaucoma develops in an
eye with a perfectly flat disc, one would also expect
considerable loss of nerve fibers to occur before the
resulting volume deficit became visible as saucerization.
On the other hand, if, as in infantile glaucoma, the
elevated intraocular pressure produced a posterior
displacement of the lamina, saucerization and even frank
excavation might become apparent before there had been any
significant loss of nerve tissue. It is important to
understand that in the flat disc the first evidence of rim
formation may be at the margin of the disc itself.
The identification of disc pathology is not always
easy. At each hour of the clock around the periphery of the
disc, one assesses its color, and one looks for excavation,
of which the apparent discontinuity of a vessel coursing
over the margin of the disc is perhaps the single most
reliable index. For the examiner who is not an accomplished
artist, the best way to record the appearance of the disc,
short of photography, is to describe what one sees in simple
English prose, as if one were explaining to an inexperienced
associate the significant findings for which he should look.
The evaluation of the disc in terms of a "cup/disc ratio" is
in general unsatisfactory. The only instance to which it is
applicable is that of an excavation with a circular margin,
everywhere clearly discernable, which is centered upon the
disc. Otherwise, a single estimate of the cup disc ratio,
derived from but a single meridian, as proposed by some
authors, clearly fails to do justice to the complexity of
the alterations that are visible on the surface of the
glaucomatous disc. In its early stages, the glaucomatous
excavation is seldom circular and is usually eccentric. It
almost always encroaches upon the vertical poles of the disc
before it encroaches upon the horizontal ones. More likely
than not, the excavation of advancing glaucoma will damage
the lower pole of the disc some months or years before it
reaches the upper pole, and will reach the nasal margin only
much later. Inferotemporally there is often a gradual
declivity, a saucer-shaped deformation of the disc which
produces a rim only when it is far advanced. In this
situation, the notion of a cup/disc ratio is meaningless,
and the requirement that it be estimated is an invitation to
fabrication.
Of particular interest is the description of incipient
glaucomatous cupping in a disc that was previously entirely
flat, where cupping progresses as a saucer-shaped deformity
which depresses the entire surface of the disc without
fashioning any rim of nerve tissue whose diameter one might
estimate. When finally an edge becomes apparent, it is a
rim of sclera, not of nerve fibers, and then the damage that
one would have liked to prevent with timely treatment of
glaucoma has been done. In this situation, the attempt to
assess the development of glaucomatous atrophy by
calculation of the cup/disc ratio will be of no help. It
may indeed make for a false sense of security and the
examiner who dismisses his patient as having "ocular
hypertension" with a pressure of 30 and a C/D ration equal
to 0, may be unpleasantly surprised when he encounters his
patient some time later to find a C/D ratio equal to .9 and
a substantial field defect.
Without disparaging the contributions which many
investigators have made to our understanding of disc
pathology, it seems fair to note that few authors who write
on the subject acknowledge with sufficient candor that in
many situations the condition of the disc cannot be
unambiguously interpreted. Then the examiner's duty is,
above all, to refrain from claims of precision which cannot
be sustained. In that situation, the experienced examiner
accepts the appearance of the disc as an unknown, and deals
with this unknown factor as he would with a dependent
variable in an algebraic equation. Under these
circumstances extraneous information, for example, the
presence or absence of a field defect, a history of elevated
intraocular pressure, the appearance of the disc in the
fellow eye, comparison with photographs on prior occasions,
and comparison with the appearance of the disc in siblings
or parents will all contribute to the elucidation of the
unknown.
* * * * *
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Copyright 2006, Ernst Jochen Meyer