This document discusses the evaluation of the optic disk and the utility of high-tech devices in assessing glaucoma. It details various parameters of optic disk evaluation like size, shape, neuroretinal rim size and shape, and cup-to-disk ratios. It emphasizes the importance of considering these parameters and using high-tech devices for objective assessment when subjective tests and ophthalmoscopic observations are equivocal. A thorough evaluation of the optic disk and retinal nerve fiber layer along with appropriate use of high-tech devices can help reduce under-diagnosis and over-diagnosis of glaucoma.

1. ISSUE H I G H L I G H T
Optic disk evaluation and utility of high-tech
devices in the assessment of glaucoma
Sherry J. Bass, O.D. and Jerome Sherman, O.D.
SUNY State College of Optometry and The Eye Institute and Laser Center. New York, New York
Background: Every clinician has at one time or another exam-ined
a patient who was misdiagnosed as having glaucoma
or whose diagnosis of glaucoma was missed. Although glau-coma
can exist with normal intraocular pressures, clinicians
often rely on the presence of visual-field defects and the
degree of optic disk cupping to direct care. However, assess-ment
of cupping is but one small part of optic disk evalua-tion
in glaucoma, and other features of the optic nerve head
and retinal nerve fiber layer must be closely inspected to help
diagnose borderline cases. In addition, glaucoma can exist
without visual-field loss. High-tech devices offer an added
dimension in the objective assessment of structure when
subjective tests of function and/or ophthalmoscopic obser-vations
are equivocal.
Methods: This article details the various parameters of optic disk
and retinal nerve fiber layer evaluation and their significance
in the assessment of glaucoma. In addition, the role of four
high-tech devices is evaluated for their utility in the assess-ment
and progression of glaucomatous damage.
Conclusions: When one attempts to classify a patient as hav-ing
glaucoma, the degree of cupping and the presence or
absence of visual f~eldlo ss can be misleading. Prior to defin-itive
diagnosis, a thorough evaluation of the optic disk and
retinal nerve fiber layer, and appropriate use of high-tech
devices, should help reduce the under-diagnosis and over-diagnosis
of this disease.
T he determination of whether or not a patient has glau-coma
and when to institute treatment is a diagnostic
challenge for many clinicians. A patient with the "Big
ThreeM-i.e., elevated intraocular pressures (IOPs), glauco-matous
optic disk cupping, and glaucomatous visual-field
loss-should be treated. The challenge presents when deter-mining
if a patient who has only one or two of these signs
should be treated, and persists when a patient is being
treated for many years who does not, in fact, have glaucoma
at all. Cupping of the optic nerve head and high intraocu-lar
pressure have always been classic hallmarks of glaucoma
and are frequently used to make the diagnosis of this dis-ease.
The presence of an elevated intraocular pressure, how-ever,
is no longer considered an absolute harbinger of
glaucoma. Glaucoma exists in patients with normal
intraocular pressures, and glaucoma never develops in the
majority of ocular hypertensives. l However, suspicious cup
size, shape, and depth continue to concern the eye care prac-titioner.
Visual-field loss is another feature that can be misleading.
Erroneous interpretation of visual-field loss can result in the
Key Words: Assessment, evaluation, glaucoma, high.tech misdiagnosis of glaucoma, while the absence of field loss
devices, optic disk, testing can falsely convince a practitioner that the patient does not
have glaucoma.
Clinicians have witnessed the birth of high-tech diagnostic
procedures that were developed to quantify structural
aspects of the optic nerve, retina, and retinal nerve fiber
layer (RNFL). Since most of these devices have normative
Bass SJ and Sherman J, optic disk evaluation and of databases, they can help classify the patient on the basis of
high-tech devices in the assessment of glaucoma. Optometry ~fati~ticaanla lysis of ~0mpari~0ton th~e ir normative data-
2004:75:277-96. base. Some of these techniques have the potential to aid cli-
VOLUME 75lNUMBER 5lMAY 2004 OPTOMETRY

2. ISSUE H I G H L I G H T
[iulre 1 A, A larye optic disk IS usually associated with a large cup; 13, a small optlc dlsk a usually associated with a small cup, or no cup at all
nicians in the assessment of an abnormality at an
initial visit, while others are best suited for the
determination of change over time. In addition,
these tests-which are objective-have added a
whole new dimension to glaucoma detection by
removing the variability factors associated with
subjective visual-field testing. And since structural
damage may precede functional loss in glaucoma,
instruments that can objectively and quantita-tively
assess structural damage now play an inte-gral
role in the early assessment of glaucoma.
Evaluation of the optic nerve head
Glaucoma affects the structure of the optic nerve
head in some way and at some point in the course
of the disease. There are several intrapapillary and
peripapillary features of the optic nerve head exam-ination,
in addition to cupping, whose association
with optic nerve damage has been suggested, war-ranting
close ophthalmoscopic inspection for
signs of the glaucomatous disease pro~ess.~
Optic disk size
Determination of optic disk size is important
because a large normal disk is nearly always asso-ciated
with a large cup (see Figure 1, A). The only
apparent exception to this rule is the rare con-genital
disk anomaly, Morning Glory syndrome,
in which a large disk is encountered with no cup
because of the presence of fibroglial tissue in the
center of the disk. In contrast, a small normal disk
nearly always has a small cup (see Figure 1, B).
Although a 0.5 CID ratio in a patient with a large
disk is probably normal, a 0.5 CID ratio in a
patient with a small disk is most likely glauco-matous.
One could argue, therefore, that a CID
ratio is meaningless unless the approximate size
of the disk is also considered. The size of the optic
disk varies between and within individuals. A
summary of 23 published studies2 established the
range of values for optic disk area between 1.70
mm2 to 3.34 mm2. A disk can be grossly esti-mated
as large or small using the 5-degree spot
size of a direct ophthalmoscope. The illumination
from this spot size should just about cover a disk
of normal size. In patients with large disks, the
light would cover only part of the disk, whereas
in small disks the light covers the disk plus reti-nal
areas outside the disk.
There are certain ethnic, gender, and ametropic
associations with disk size. Disk size is generally
smallest in whites, larger in Mexicans, and largest
in people of African d e s ~ e n ti;t ~al so tends to be
larger in males than in fern ale^.^ Although there
is no ametropic association with disk size
between -5.00 and + 5.00 D, large disks are asso-ciated
with high myopia and small disks are asso-ciated
with high hyperopia.
Large disks have a greater neuro-retinal rim area,3
more optic nerve fibers, and less nerve fiber
crowding than small disks.4 Small disks are asso-ciated
with non-arteritic anterior ischemic optic
neuropathy (AION),5d isk d r ~ s e na,n~d non-dis-tinct
disk borders (pseudopapilledema).7 But does
disk size determine susceptibility to glaucoma?
Since blacks have large optic disks and a greater
likelihood for development of glaucoma when
278
OPTOMETRY VOLUME 75lNUMBER 5lMAY 2004

3. ISSUE H I G H L l G H l
Fiuun 2 The normal neuroretinal rim. thickest inferiorly, then superiorly, then nasally, and th~nnest
temporally.
compared to whitest8 one might assume that a
larger disk size is associated with greater glau-coma
susceptibility. Non-highly-myopic patients
with normal tension glaucoma (NTG) have been
reported to have larger optic disk size than
patients with primary open-angle glaucoma
(POAG).9H owever, in POAG and secondary open-angle
glaucoma (S0AG)-e.g., pigmentary glau-coma-
the disk size is n ~ r m a l . ' ~ , ~ '
What about small disks? Small disks are associ-ated
with factors that affect perfusion of the optic
disk (as evidenced by the greater association with
non-arteritic AION5) and axoplasmic flow in the
axons of the ganglion cells (as evidenced by the
association with disk drusen6). In addition, the
nerve fibers are more crowded in small disks,
leaving them more susceptible to mechanical
damage in glaucoma.12 Furthermore, small disks
may have a smaller reserve because of a smaller
number of nerve fibers than larger disks3
Although these factors would support a greater
susceptibility of glaucoma in small optic disks,
some studies have demonstrated that glaucoma-tous
optic disk atrophy is independent of disk
~ i z e . ' ~D,i's~h size is therefore not definitive in deter-mination
of susceptibility to development of glau-coma.
Disk size is important, however, when
judging the degree of cupping and making the
determination as to whether a particular cup size
is likely physiological, as opposed to pathological,
for a certain disk size.
Optic disk shape
The normal shape of the optic
disk is oval, having a vertical
diameter that is about 10%
greater than the horizontal diam-eter.
Significant corneal astig-matism
and amblyopia associated
with increased corneal astigma-tism
correlate significantly with
an abnormal optic disk shape.15
Before suspecting glaucoma in a
patient with a vertically elon-gated
disk, a significant amount
of against-the-rule astigmatism
should be ruled out. Studies have
suggested that susceptibility to
glaucoma is not dependent on
optic disk shape.16 The shape of
the optic disk in highly myopic
eyes (> 12D), however, is more
oval, elongated, and oblique than
in other eyes. This suggests that
the myopic stretching in these eyes pulls more on
the optic disk in some directions than in others,
and that the vertical elongation may be the rea-son
high myopia is a risk factor for the develop-ment
of glaucoma.
Neuro-retinal rim size and shape
The size of the neuro-retinal rim is indicative of
the anatomic reserve capacity of the optic disk
and correlates well to the size of the disk: the
larger the disk, the greater the rim area and optic
nerve fiber count.3 Anatomically, axons from gan-glion
cells close to the optic disk are positioned
more centrally within the optic disk, whereas
axons from peripheral retina lie at the optic nerve
head margins. The shape of the neuro-retinal rim
usually follows the "ISNT" rule: It is broadest
inferiorly, followed by the superior, nasal, and
temporal regions, where it is the narrowed7 (see
Figure 2). In glaucoma, these regions are affected
differently at various stages of the disease. Early
glaucoma usually affects the inferotemporal
and superotemporal rim disk regions. As glau-coma
advances to the moderate stage, the great-est
amount of rim loss occurs in the temporal
horizontal region. In advanced glaucoma, only the
nasal sector of the disk rim remains.18 Ophthal-moscopic
observation of rim loss most often
occurs in glaucomatous optic neuropathy, while
it may not necessarily occur in other forms of
optic neuropathy. Therefore, careful examination
2 79
V O L U M E 7 5 l N U M B E R 5 l M A Y 2 0 0 4 OPTOMETRY

4. ISSUE HIGHLIGHT
of the size and shape of the neuro-retinal rim tis-sue
over time may be a prime way to differenti-ate
glaucomatous optic nerve disease from other
etiologies. Some of the high-tech instruments that
quantify changes in rim area over time are useful
in this regard.
Neuro-retinal rim pallor
Pallor of the neuro-retinal rim is more often a
sign of non-glaucomatous optic neuropathy
than glaucomatous optic neuropathy.lg The
overall disk pallor seen in advanced glaucoma is
due to a more excavated cup rather than pallor
of the neuro-retinal rim. It is more common to
see a healthy pink rim-albeit very thin-in mod-erately
advanced glaucoma, than a pale one.
Overall, a nerve head with a pale rim needs to
be investigated for other etiologies, such as
ischemic optic neuropathy, intracranial masses,
and hereditary and congenital optic nerve dis-eases.
In addition, the pseudophakic eye can
exhibit "pseudo-pallor" of the neuro-retinal rim.
One major exception to this rule is the eye with
angle closure glaucoma; following a large and
rapid rise in IOP, the disk often appears pale,
without dramatic cupping.20 Cupping of the optic
nerve head, along with neuro-retinal rim pallor,
may also be non-glaucomatous and has been
reported in posterior ischemic optic neuropathy21
and giant cell arteriti~.~~
Cupldisk ratio in relation to disk size
Of all disk parameters, cupldisk ratio appears to
be used most frequently in assessment of glau-comatous
risk. This one feature may label a
patient a glaucoma suspect, despite normal
intraocular pressures and the fact that all other
aspects of the disk are normal.
Cup-to-disk ratios vary widely, even in the nor-mal
population, and can range from 0.0 to
0.9.17 Therefore, it is possible for a large CID
ratio in a large optic disk to be physiological.
It is important to assess both the horizontal
and vertical cupldisk ratios, since the normal
optic disk may have a slightly greater hori-zontal
cup than vertical cup. In glaucoma, the
vertical cup increases faster than the hori-zontal
cup due to loss of the neuro-retinal rim
superiorly and inferiorly. Therefore, a cup that
is vertically elongated is more likely indicative
of glaucoma.
The over-diagnosis and under-diagnosis of
glaucoma can be minimized if the practitioner
understands the relationship of CID ratio to disk
size. Large disks are generally associated with
large ClDs and small disks are generally asso-ciated
with small ClDs. While a large CID ratio
in a large disk is expected, it is unusual to see
any significant degree of cupping in a small
optic disk. An average or large CID ratio in a
small disk is particularly suspicious in the pres-ence
of vertical elongation of the cup. In fact,
most small optic disks have either no cup or a
cup size that is 10% to 20% the size of the disk.
Early glaucoma in a small disk may be missed,
because a small degree of increase in cupping
in a cup that is small to begin with can easily
go unnoticed-especially when intraocular
pressures are normal.
In these cases, it is crucial that other available
means of optic nerve head assessment be used.
For example, small glaucomatous disks with small
cups have been shown to demonstrate abnor-malities
in the peripapillary region,23 primarily as
a thinning or decrease in visibility of the retinal
nerve fiber layer. This can be appreciated as an
increase in the clarity of the retinal vessels near
the disk. Other telltale signs include diffuse andlor
focal attenuations in the retinal arteri~lesa~n~d t~~
peripapillary chorioretinal atr~phy.~~,~~
The large cup (or "macrocup") may be seen in
patients with large optic nerve heads, or
macrodisks. The patient with a primary or con-genital
large disk or macrodisk will probably have
a macrocup, which is physiological and not glau-c~
matousA.~ft er the first year of life, these disks
are constant and the cupping does not ~hange.~
The secondary or acquired macrocup does
undergo change, as seen in glaucoma (due to
decreasing neuro-retinal rim tissue) and in high
myopia (in which myopic stretching enlarges the
disk and thereby the cup).
The inconclusiveness of CID ratio, as a diag-nostic
clue, has multiple implications in this
regard: a large disk with a large cup and a nor-mal
neuro-retinal rim should not summarily be
labeled as glaucomatous. Conversely, patients in
whom early glaucomatous changes develop in
the presence of a macrocup may be deceptively
perceived as having advanced glaucomatous cup
excavation. However, visual fields in these cases
280
OPTOMETRY VOLUME 75lNUMBER 5IMAY 2004

5. ISSUE HIGHLIGHT
[iNUr3e O ptic disk (Drance) hemorrhages in glaucoma are ephemeral and may
would probably exhibit early defects and not
advanced defects.
Optic cup shape and depth
When evaluating cup shape and/or cup depth
ophthalmoscopically, one must remember that
the border between the optic cup and optic disk
is based on contour, not color, and is best assessed
stereoscopically. This is especially useful in eyes
with shallow cups, as seen in myopic disks.
While the optic disk has a shape that is oriented
vertically, the optic cup shape is horizontally oval,
with a horizontal diameter about 7% to 10%
longer than the vertical.18 This discrepancy
explains the shape of the neuro-retinal rim,
which is thickest inferiorly and superiorly and
thinnest nasally and temporally. A change in the
optic cup shape from horizontal to vertical is the
result of loss to the superior and inferior neuro-retinal
disk rim seen in early glaucoma. Some of
the high-tech devices place a high emphasis on
CUP shape (contour) in the differentiation of nor-mal
vs. abnormal cupping.
In addition to evaluation of cup shape in detec-tion
of glaucoma, practitioners also evaluate cup
depth, which depends on the cup area. Generally,
the larger the optic cup, the greater its depth.28
Therefore, congenitally large disks (primary
macrodisks) are not only associ-ated
with large cups, but also
with deep cups. The exception to
this is the large disk found in the
high myopic eye with glaucoma;
many of these cups remain very
shallow, like their non-glauco-matous
counterpart^.^ Cup depth
also has some association with
the type of glaucoma; eyes with
traumatic glaucoma with angle
recession and juvenile onset
POAG reportedly have the deep-est
~ ~ p s . ~ ~ , ~ ~
Optic disk hemorrhages
Optic disk hemorrhages are
detected in less than 8% of
patients with glaucoma31 (see Fig-ure
3) and have a greater associ-ation
with NTG than other types
be missed (redarrow). of g l a ~ c o m a .O~p~tic, ~di~sk hem-orrhages
may also be caused by
other etiologies, such as vascular
occlusive disease, Valsalva maneuvers, and optic
disk drusen. In our clinical experience, optic disk
hemorrhages in glaucoma-previously termed
"Drance hemorrhagesn-are ephemeral, and
hence only a fraction of their true number are
ever observed.
Since optic disk hemorrhages occur in a small
percentage of patients with glaucoma and can
be associated with other etiologies, the presence
of a disk hemorrhage alone is not a sufficient
indicator of glaucoma, and has a low sensitiv-ity
for the diagnosis of this disease. The pres-ence
of an optic disk hemorrhage, along with
other glaucomatous risk factors, however,
should be viewed as an additional risk factor.
Optic disk hemorrhage can also be perceived as
a sign of progression in established cases of
glaucoma.
Peripapillary chorioretinal atrophy
Since the early 19001s, many investigators have
reported observable peripapillary atrophy in glau-coma
and have correlated the degree of atrophy
with the degree of glaucomatous disk damage34
and field loss.35 The role of this feature as an indi-cator
of glaucomatous damage to the optic nerve
head, however, remains controversial. The atro-phy
is basically divided into a central beta zone
VOLUME 75INUMBER 5/MAY 2004 OPTOMETRY

6. ISSUE H I G H L I G H T
and peripheral alpha zone36 (see
Figure 4). Irregularities in pig-mentation
and chorioretinal
thinning characterize the outer
alpha zone while visible sclera,
retinal pigment epithelium (RPE)
atrophy, and visibility of the
choroidal vessels characterize
the inner beta zone. While the
alpha zone is present in almost
all normal eyes, peripapillary
atrophy of the beta zone is more
likely to be indicative of glau-coma,
occurring in less than 20%
of normal patients.36 Peripapil-lary
atrophy occurs more fre-quently
in myopic disks and
tilted disks, but this atrophy is
generally localized temporal,
superotemporal, and inferotem-poral
to the optic nerve head,
m
and rarely in the nasal peripap- illary region. These zones are Fiuure 4 veln occlusion.
reportedly larger36 in glaucoma-tous
Peripapillary atrophy in a glaucomatous disk in a patient with disk collaterals from an old
disks and can encircle the
entire disk (the so-called "halo gl~ucomatosus"i)n Glaucoma causes nerve fiber layer damage via
end-stage disease. Because these zones can exist localized defects, diffuse loss, or a combination
in normal eyes, this feature, too, has a low dif- of the two. The localized defects occur in about
ferential specificity. However, an enlargement in 20% of glaucoma patients38 and manifest as
these two zones over time does sometimes dif- wedge-shaped dark areas that are widest further
ferentiate glaucomatous from non-glaucomatous from the disk, coming to an apex at the edge of
disease. the disk (see Figure 5). Notches in the neuro-reti-nal
rim tissue, optic disk hemorrhage (the cor-
Retinal nerve fiber layer evaluation responding RNFL defect appears 6 to 8 weeks
after the onset of the hemorrhage), and/or peri-
The RNFL contains bundles of all the retinal gan- papillary atrophy may appear in the same sector dion axons gathered Mueller as the RNFL defect. Such defects have been
processes. It is usually visible ophthalmoscopi- shown to be o p ~ t ~ a ~mo s c o p ~vicsiabl~e ~ayft er
cally, but its presence can be better detected with 50% of the RNFL is lost.39 ~h~ other type of loss,
the use of red-free filters and red-free photo- diffuse loss, results in a greater visibility and clar-graphs.
In addition, some modern high-tech ity of the large retinal vessels and is more diffi-instruments
have been designed cult to detect than focal loss. Since studies have
assess this aspect of the retina, via confocal scan- demonstrated that the degree of RNFL loss tor-laser
tOmOg- relates well with the degree of optic disk damage
raphy, and scanning laser polarimetry. The due to glaucoma,40~t4h1e RNFL must be carefully
RNFL is thickest and most visible inferotempo- evaluated to avoid the over-diagnosis of glaucoma
ral the disk, the in eyes with a large cup/large disk and the under-ral,
nasal superior, and nasal inferior sector.37 diagnosis of glaucoma in eyes with a small
Defects in the RNFL can occur in glaucoma, but cup/small disk.
can also be associated with other diseases that
cause optic neuropathy (e.g., disk drusen, toxo-plasmosis
retinochoroiditis, optic pits, optic neu- Optic disk drusen
ritis, compressive optic neuropathy, demyelinating Virtually ignored by both clinicians and researchers,
disease, etc.). the relationship between disk drusen and glaucoma
OPTOMETRY V O L U M E 7 5 l N U M B E R 5 l M A Y 2004

7. ISSUE HIGHLIGHT
m[iuure8 A wedgeshaped r e t i n h v e f lber layer (RGdLe)fe ct i n g h u c o m is an example of locallzed RNFL loss.
nerve fiber layer loss. The arcuate
scotoma and nasal step often found
in eyes with disk drusen are essen-tially
identical to the field deficits
found in glaucoma.4z Although disk
drusen do not necessarily prevent
the development of cupping, they
make the interpretation more dif-ficult."
Patients with disk drusen
often experience nerve fiber layer
loss, greatly reducing the redun-dancy
of the RNFL. Patients with
disk drusen in whom glaucoma
develops are, therefore, at greater
risk of frank field defects earlier in
the course of their disease process.
Optic nerve drusen or disk drusen
are comprised of hyaline bodies
with a mucolsrotein matrix nf
.- I in approximately 2% of all eyes and
generally progress throughout life.
In our clinical experience, buried
disk drusen in young children are
often considered the most common
cause of blurred disk borders I (pseudopapilledema). Since disk
drusen have an autosomal domi- Fiuure 6 A- Top: Dlsk with optic nerve head drusen and no observable cup; Lower left B-scan ultra-sonography
reveals an elevat~ona bove the disk (arrowl, and Lower right the elevation
nant inheritance pattern and sur-perslsts
at a lower sensitivity setting-a pattern typical of calcified optlc nerve drusen. face as age increases45 (see Figure 6),
VOLUME 75lNUMBER 5lMAY 2004 OPTOMETRY

8. ISSUE HIGHLIGHT
examination of the child's parents often reveals frank
disk drusen. Large surface disk drusen, which appear
calcified ophthalmoscopically, are remarkably easy
to document with B-mode ophthalmic ultrasonog-raphy.
This procedure may confii the diagnosis
because ultrasound reflections from calcified drusen
(like bone) persist at low sensitivity levels (see Figure
6). Although disk drusen often appear to be calcified,
not all disk drusen are calcified; thus, not all can be
confirmed with ultrasonography. Disk drusen that are
not calcified may also be more difficult to observe
directly. Subtle or even occult (hidden) disk drusen
are generally not diagnosed, but their presence often
prevents observable cupping from being observed.
This is to be expected when one considers that disk
drusen sitting in an optic cup are like ice cubes fdl-ing
up a glass. It is virtually impossible to determine
the depth of a glass when something solid is filling
it up. Likewise, instruments that measure disk topog-raphy-
and the naked eye observing stereo disk pho-tographs-
will also be unable to detennine the depth
of the optic cup when solid masses such as drusen
"fill it up."
The presence of disk drusen is the single most
common cause of glaucoma without cupping.46
Disk drusen alone (or disk drusen along with
glaucoma) often results in nerve fiber layer loss,
as demonstrated by visual fields and/or objective
instruments that measure RNFL thickness with
a paradoxically normal-or even super-normal-disk
topography (as demonstrated by objective
instruments that measure optic disk topography).
The combination of an abnormal nerve fiber layer
(via visual fields and/or objective measurement
of RNFL thickness) and a small cup (via stereo
disk photographs and/or normal optic disk
topography) strongly suggests the presence of disk
drusen, which can be obvious, subtle, or occult.
Summary of optic newe head evaluation
The degree of optic disk cupping is not necessarily
indicative of the presence of glaucoma. When any
patient with equivocal cupping in the presence
or absence of demonstrated field loss is assessed,
the most significant optic disk features to closely
evaluate include, but are not limited to:
a. shape of the neuro-retinal rim
b. relationship of cupldisk ratio to optic disk
size
c. optic cup shape and depth
d. decreased visibility of the RNFL and local-ized
RNFL defects
e. presence of disk drusen
284
Standard of care vs. state-of-the-art in glaucoma diagnosis
The decision faced by many clinicians regarding
whether to commence treatment can be difficult,
since the size of the cup is not always diagnos-tic.
Likewise, the presence or absence of elevated
intraocular pressures is not always diagnostic.
Optic nerve head and peripapillary features
require careful evaluation, although at times oph-thalmoscopic
clues are lacking. Certainly, the deci-sion
becomes easier as the patient is followed over
time, when the clinician has had an opportunity
to make observations of changes in the cupping
and decreases in the rim tissue. Diagnosis is also
less equivocal when field defects begin to
appear and when IOPs demonstrate an increas-ing
trend. However, when observing a patient for
the first few visits, treatment decisions may not
be obvious. While many clinicians realize this,
cases of under- and over-diagnosis of this dis-order
are encountered frequently. While some
clinicians prefer to err on the side of over-treating-
assuming they are less likely to expe-rience
litigious consequences in the future-there
are financial considerations, as well as quality of
life issues, for some patients-especially those
who experience side effects from medications
they may not need. Furthermore, while results of
the Ocular Hypertensive Treatment Study
(OHTS) have helped clinicians reconsider treat-ing
certain ocular hypertensive patients, nor-motensive
glaucoma patients continue to be
under-diagnosed until a significant increase in
cupping flags the clinician's attention.
Standard of care testing in glaucoma
Standard of care testing-namely, static automated
perimetry-continues to be the mainstay of deter-mining
functional loss in glaucomatous disease.
Its value in following disease progression is
unquestionable. However, as an aid in the
detection of early glaucoma, perimetry falls short
in some instances. This is especially true of the
patient whose field loss is inconsistent from visit
to visit, whose field loss falls outside of the cen-tral
30 degrees, whose field loss is not considered
"glaucomatous," and/or who fails to demonstrate
field loss despite the presence of other risk fac-tors
(elevated IOP, strong family history, suspi-cious
cupping, and RNFL loss).
Manufacturers of automated perimeters have
attempted to improve the ability of the practi-tioner
to detect glaucoma earlier. The develop-
-
OPTOMETRY VOLUME 75lNUMBER 5lMAY 2004

9. ISSUE HIGHLIGHT
ment of short wavelength automated perimetry
(SWAP) and frequency doubling technology
(FDT) have opened new avenues for early diag-nosis.
SWAP is based on the hypothesis that there
is less redundancy in the color vision pathway
than the luminance pathway. Therefore, a blue
stimulus against a yellow background is testing
a narrower population of cells and should theo-retically
be affected first in early disease. The high
sensitivity of SWAP in early detection of glaucoma
has been reconfirmed in a study of 500 eyes of
250 ocular hypertensive patients.47 FDT taps into
the motion detectors in the visual pathway, which
are believed to be affected in early glaucoma. The
potential for FDT to be used as a screening
method in glaucoma detection has been docu-mented.
48
Perimetry remains the single, most important
functional test to identify not only glaucoma, but
also retinal, optic nerve, and visual pathway dis-orders.
Unlike perimetry, tests that determine
structural integrity of the disk and RNFL are not
likely to alert the practitioner to suspect intracra-nial
tumors and other brain disorders that
affect the temporal, parietal, and occipital lobes.
Although perimetry has widespread applications,
at least four factors limit its clinical utility:
1. Redundancy of retinal ganglion cells
which result in normal visual fields in many
cases until half of the ganglion cells and their
axons are lost.
2. Speed; visual-field testing can never furnish
more than one bit of information per second
because visual-field testing is limited by the
patient's speed in pressing the button.
3. Subjectivity of perimetry.
4. Incomplete testing of the visual field, in
that not all retinal points are tested.
State-of-the-art the role of high-tech devices
Over the past several years, there has been an
explosion of new technologies that are particularly
well-suited for the diagnosis and management of
glaucoma. Collectively, these devices were
designed to provide objective evidence of abnor-malities
in disk topography and/or nerve fiber
layer thinning, and retinal thickness. Designed for
in-office use by the clinician, they vary in their
user-accessibility, ease of interpretation, and
affordability. A primary, common goal of these
technologies is the early detection of structural
abnormalities that precede functional loss
(i.e. ,normal visual fields). All the high-tech
devices have added a new and exciting dimension
to glaucoma detection. They are objective as
opposed to subjective, they are fast (65,000 bits
of information are acquired in less than one sec-ond
with several of the high-tech devices), and
hundreds of thousands of data points are col-lected,
in contrast to the limited number of points
in visual-field testing. However, the practitioner
must recognize the limitations of some of these
instruments, as well as the importance of obtain-ing
good quality images, and use all clinical data
available when interpreting the findings.
What do these devices offer and how accurate are
they? What information do they add to diagno-sis
and management of glaucoma and other optic
neuropathies? Does this information justify the
cost? What are the shortcomings?
HRT 11
Some practitioners claim that a good set of stereo
disk photographs is all that an astute clinician
needs to detect a glaucomatous disk. However,
evaluation and interpretation of disk photographs
is subject to inter-clinician ~ariabilityI.t ~is~ p~ar~- ~
ticularly difficult to quantify topographical
attributes of the optic nerve head from stereo pho-tographs.
A clinician's only quantitative meas-urement
when evaluating an optic nerve head is
an estimate of the C/D ratio to the nearest 5%,
with N f 5% inter-observational accuracy. An
accurate quantification of topographical features
offers the ophthalmic clinician a means of detect-ing
slow and subtle change.
The HRT I1 (Heidelberg Retinal Tomograph-Hei-delberg
Engineering, Inc., Carlsbad, California) is
a commercially available confocal scanning laser
ophthalmoscope that uses a 670-nm red-diode
laser source to provide real-time three-dimen-sional
images and measurements of optic disk
topography and RNFL. This is accomplished with-out
pupil dilation or ocular contact. Three
series of scans are captured in quick succession
and then averaged. Image quality is assessed by
the standard deviation or variability between the
three scans. Stereometric parameters of disk
topography are measured in detail. The "top five"
parameters (as reported by Heidelberg Engi-neering)
are: rim area, rim volume, cup shape
measure, height variation contour, and RNFL
thickness. Mean RNFL "thickness" is measured
at the disk border using an artificial reference
VOLUME 75lNUMBER 5lMAY 2004 OPTOMETRY

10. ISSUE H I G H L I G H T
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Figure 7 HRT II quantifies various parameters of disk topography (on the bottom left) and compares these values against the normative database in the
analysis (on the right). Areas in red represent the cup; green represents the neuroretinal rim. The superior temporal neuroretinal rim is flagged as
borderline (yellow exclamation point) and the superior, nasal, and inferior temporal neuroretinal rim are flagged as outside normal limits (red ic).
plane located 50 microns beneath the retinal sur- is used to differentiate optic nerve head cup from
face of the papillomacular bundle (at the temporal neuro-retinal rim. Of the top five parameters, cup
aspect of the disk border). The reference plane shape measure has been shown to have the best
286
OPTOMETRY VOLUME 75lNUMBER 51MAY 2004

11. ISSUE HIGHLIGHT
diagnostic pre~ision.S~ix~ e qual sectors of the
neuro-retinal rim are evaluated and compared
(corrected for age) against the instrument's nor-mative
database, which consists of approximately
200 whites. Sectors are assigned a green check
mark if the RNFL and rim are within the pre-dicted
range; a yellow exclamation mark if they
are borderline; and a red "x" if they are outside
normal limits. This six-sector evaluation uses
Mooresfields Regression Analysis, a linear regres-sion
analysis that has been demonstrated to have
a sensitivity of 93.5% and specificity of 88.4%.52
All the parameters are globally analyzed and the
patient's overall topographic results are catego-rized
as "normal," "borderline," or "outside nor-mal
limits" (see Figure 7).
This technology has some analytical shortcom-ings:
without providing for racial variation in the
normative database, Hispanics and blacks-whose
disks and C/Ds tend to be larger than those
of whites-tend to be flagged as "outside normal
limits," even if they are physiologically sound.
Furthermore, small to minimal cupping is con-sistently
classified as "normal," requiring that the
clinician carefully consider other findings (e.g.,
myopia, blurred disk borders, etc.), so as not to
miss the glaucomatous disk with small cups.
This technology may prove to be most useful in
helping the clinician determine the progression of
cupping and other topographical changes over
time, rather than flagging an abnormal disk at an
initial visit. A progression change probability
analysis and trend analysis are used to monitor
any change over time (see Figure 8).
GDxVCC
The GDxVCC (Laser Diagnostics Technology, Inc.,
San Diego, California) is a scanning laser
polarimeter that indirectly measures the thickness
of the RNFL using a near-infrared laser. GDx
makes use of the principle of birefringence of the
RNFL as an indirect indicator of its thickness.
Because the RNFL is a lamellar structure, con-sisting
of parallel layers, it will regularly and pre-dictably
slow (retard) the speed at which
polarized light passes through it. The thicker the
RNFL, the greater is the retardation or birefrin-gence.
RNFL thickness is analyzed over a "meas-urement
circle," with an inner radius 1.2 mm
from the center of the disk, extending outward
20 degrees. Since the disk is never assessed, this
instrument is completely independent of any disk
parameters.
The device performs a scan in 0.7 seconds and
the analysis takes about 1 minute per eye. Nei-ther
pupil dilation nor corneal contact is required.
Image quality is assessed on a scale from 1 to 10,
with 10 being the best. The printout contains
parameter summary measures, which include the
TSNIT (Temporal, Superior, Nasal, Inferior,
Temporal) average of RNFL thickness and inter-eye
symmetry. These and other parameters are
statistically compared to normative data, corrected
for age and race. The technology's normative
database consists of 600 patients (70% whites and
Hispanics, 12% Asians, and 18% blacks).
Abnormal data are depicted in red on the print-out,
along with the probability values depicting
the degree of statistical significance. In addition,
all the data are globally summated in the form of
a Nerve Fiber Indicator (NF1)-formerly called
"The Number" in an earlier version of this instru-ment.
The NFI, which will be anywhere from 1
to 100, is a mathematical construct based on an
algorithm that was trained to differentiate normal
subjects from patients with glaucoma, using a
combination of summary data and raw data
points. Generally, a number less than 30 is con-sidered
to be normal, numbers between 31 and
50 are considered highly suspicious of RNFL loss,
and numbers above 50 are indicative of signifi-cant
RNFL loss. The numbers assigned to all of
these parameters are also valuable for the deter-mination
of change over time. All measurements
are independent of disk topography. An additional
feature of the most recent version of the
GDxVCC includes a deviation plot, which
depicts the statistical significance of abnormal
points in the RNFL measured 0.6 degrees apart
superior and inferior to the optic nerve (see Fig-ure
9). The instrument also incorporates a pro-gram
to analyze progression over time. Areas in
the thickness map that are depicted in blue over
time are indicative of statistically significant RNFL
loss (see Figure 10).
A shortcoming of the first iteration of this tech-nology
was that other lamellar structures in the
eye (e.g., the cornea) affected these measure-ments-
some to a greater degree than others.
Since the cornea is a primary contributor of
birefringence in the eye, a corneal compensator
had to be incorporated to cancel out the
VOLUME 75/NUMBER 5/MAY 2004 OPTOMETRY

12. ISSUE HIGHLIGHT
?
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- Alre 8 Progression analysis on the HUT11 The area an red denotes stat~stcallys ~g n f~c alnots s an the anferaor neuroret~narll m over tame (redarrowj
corneal birefringence from the total measure- cel birerefringence from all corneas). A vari-ments.
The original compensator was "fixed," able compensator (GDxVCC) has replaced the
(i.e., it did not change position and did not can- fixed compensator. It initially measures each
288
OPTOMETRY VOLUME 75lNUMBER 5lMAY 2004

13. ISSUE H I G H L I G H T
. . . . . . . . .
'. .. '. .. '. '. .. Fiber Analysis : ":: . '. -,:
wmfucu--
. .
Figure 8 The GDxVCC display The thickness maps (red arrow) reveal retinal nerve fiber layer (RNFL) thinning in both eyes-greater left eye.
The deviat~onp lot (yellowarrow) denotes statistically significant RNFL attenuation in both eyes-greater left eye. The TSNlT (Temporal.
Superior, Nasal. Inferior, Temporal) curve (blue arrow) is reduced in both eyes-greater in left eye. The Nerve Fiber Indicator is 48 right eye
and 77 left eve, both abnormal.
individual patient's cornea and adjusts the istic (ROC) curve of 0.95 for the NFI, with a
compensator accordingly to cancel out the sensitivity of 90% and a specificity of 91%.53
corneal birerefringence from that eye. Data In our clinical experience, reproducibility of
accumulated so far on 390 normals and 253 measurements in both normal and glaucoma
patients with glaucoma have demonstrated an patients appears to be within 2 to 4 microns for
area under the Receiver Operator Character- all parameters.
289
V O L U M E 7 5 l N U M B E R 5/MAY 2004 OPTOMETRY

14. ISSUE H I G H L I G H T
Fijure 10 Progression analysis on the GDxVCC Areas of progressive retinal nerve fiber layer loss over a 3Xyear period in this patient are depicted in
blue (yellow arrow).
Stratus OCllM (OCT 3) posed axial A scans that approximate a cross-sec-
Optical Coherence Tomography /Stratus OCT, iion of the retina and opiic nerve head. Images
Carl Zeiss Meditec Inc., Dublin, California) uses are obtained through an undilated pupil with no
a superluminescent diode laser light that is scat- corneal contact. The quality of resolution
tered, reflected, and absorbed by retinal tissue. allows the differentiation of 7 to 8 retinal layers
The resultant image is up of many juxta- in this latest clinically available version of the
290
OPTOMETRY V O L U M E 7 5 l N U M B E R 5 l M A Y 2004

15. ISSUE H I G H L I G H T
. . . . . . .
Fiuure 11 A, Stratus OCT ONH Topography: On the left, the display demonstrates a cross-section through the disk (6 to 12 o'clock) and parameters of
disk topography in this patient. On the upper right, the cup is depicted in green and the disk is depicted in red Below these displays are
quantified disk parameters. B, Stratus OCT RNFL Thickness Analysis: On the left side, average RNFL thickness around the optic nerve head,
in a circular area 3.4 mm in diameter, is displayed in a TSNlT curve (Temporal, Superior, Nasal, Inferior, Temporal) for each eye. On the right
side, average thickness is displayed in 12 o'clock-hour sectors and in four quadrants for both eyes. The black curve represents the patient's RNFL
thickness, which is below the normative database in this case. Areas depicted in green are within 95% of the normative database for that age
group, areas in yellow in between 1 % and 5% of the normative database, and any areas in red lie below 1% of the normative database.
2 9 1
VOLUME 75lNUMBER 5 l M A Y 2 0 0 4 OPTOMETRY

16. ISSUE H I G H L I G H T
Olaucoma - Analysis
nation Date: 211031
Filure 12 A. The Retinal Thickness Analyzer (RTA) optic nerve head topography display. The cup is depicted in red and the neuroretinal rim in green
(upper halfl. Various optic nerve head parameters are noted in the lower half of the display. B (on facing page), The RTA measures retinal
thickness at the macula. Retinal thinning due to ganglion cell loss may be a sign of early glaucoma. Areas in blueon the deviation map
represent statistically significant areas of retinal thinning (blue arrow).
OCT, including the RNFL. The OCT was origi-nally
developed to detect structural abnormal-ities
in the macula, not readily apparent on
ophthalmoscopy, such as various stages of
macular holes, cysts, cystoid macular edema,
central serous choroidopathy, etc. An additional
feature of the Stratus OCT is the ability to obtain
quantitative measurements of optic disk topog-raphy
and RNFL thickness. A series of 4-mm
long radial line scans at 12 clock hours across
the disk allows topographic measurement of disk
parameters (see Figure 11, A). The instrument
objectively finds the margin of the disk, using
a signal from the end of the retinal pigment
epithelium. Disk parameters that are measured
include cup volume, disk, cup and rim area, and
OPTOMETRY V O L U M E 7 5 l N U M B E R 5 l M A Y 2004

17. ISSUE HIGHLIGHT
cupldisk ratios. To determine RNFL thickness,
a circular scan of 3.4-mm diameter is centered
on the optic nerve head and a peripapillary
cross-sectional image is obtained and displayed
as a TSNIT curve (see Figure 11, B).
'Itvo recent developments in this technology
include the establishment of a normative data-base
to establish reference values in a normal
population and software that allows the assess-ment
of image quality. The normative database
was obtained by evaluating the RNFL thickness
measurements in an ethnically wide variety of
410 subjects (60% white, 27% Hispanic, 8%
black, 3% Asian, 2% other), from which 328
qualified scans were obtained.54 Evaluating the
293
VOLUME 75lNUMBER 5IMAY 2004 OPTOMETRY

18. ISSUE H I G H L I G H T
signallnoise ratio after obtaining an image
assesses the image quality. Clinical experience
has demonstrated good reproducibility when the
RNFL is normal or mildly reduced. However, the
technology currently appears to have difficulty
when the RNFL is very thin, and an error mes-sage
is generated that the data is sub-optimal for
analysis.
The Retinal Thickness Analyzer
The Retinal Thickness Analyzer (RTA-Talia
Technology, Newe-Ilan, Israel) is a digitized laser
slit-lamp that makes use of a helium-neon laser
(543-nm wavelength) as a light source. The instru-ment
measures retinal thickness using a beam
splitter that splits the incoming light into two sep-arate
beams. One beam is reflected off the reti-nal
pigment epithelium (RPE) and the other
reflects off the internal limiting membrane
(ILM). The difference between these two reflec-tions
is a measurement of the retinal thickness at
that point. The RTA was originally developed for
the objective detection of retinal thickening, as
seen in clinically significant macular edema
(CSME)i n diabetic retin~pathyC.~u~rr ently, the
RTA has added software to create a topographi-cal
map of the optic nerve head. It maps out a
two-dimensional rimlcup area map with hori-zontal
and vertical cross-sectional graphs, a disk
area image, RNFL cross-section (TSNIT Curve),
and a three-dimensional topography map of the
optic nerve (see Figure 12). The RNFL thickness
is determined by measuring retinal thickness in
the macula, since the paracentral RNFL is
believed to be affected early in glaucoma. In a
study of 10 normal patients comparing the RTA
measurements with HRT I1 measurements, the
mean cup depth, mean RNFL thickness, and
cross-sectional area were significantly smaller on
the RTA than the HRT 11, but the reproducibility
was not significantly different between instru-m
e n t ~ . ~ ~
When using this technology, the pupil must be
a minimum of 6 mm, usually requiring dilation.
In addition, images may be difficult to obtain
in eyes with media opacities or in pseudophakia.
The RTA measures retinal thickness but does
not selectively evaluate the RNFL. Since it meas-ures
total retinal thickness, a diabetic patient
having macular CSME (resulting in retinal thick-ening)
and glaucoma (resulting in retinal thin-ning)
could conceivably end up with a normal
retinal thickness in the macula. Based on the
fact that 40% of the RNFL ganglion cell bodies
are in the central 20 degrees, the RTA is
designed to detect early glaucoma by evaluat-ing
retinal thickness around the macula.
Although there is a substantial population of
ganglion cell bodies in the macula, the RNFL in
this region is actually very thin, perhaps only
10% of its thickness around the optic nerve
head. Any detected loss in retinal thickness in
the macula is due, therefore, to loss of ganglion
cell bodies, not RNFL loss.
Although the RTA, like the HRT 11, can generate
a TSNIT curve around the disk, both are really
topographical curves and not true thickness
curves. In contrast, the GDxVCC and Stratus
OCTTM generate TSNIT curves around the optic
disk that are based on RNFL thickness. Moreover,
both latter technologies theoretically measure all
the RNFL and not just the RNFL in the macula.
Summary
The clinician is not challenged by the clear-cut
cases of glaucoma and non-glaucoma. But a dis-ease
whose presentation has such a broad over-lap
with the normal population necessarily
creates a confounding "gray" area into which glau-coma
suspects are lumped, until further evidence
either affirms or negates their status. The diag-nosis
of glaucoma is complicated in these equiv-ocal
cases. The careful assessment of various optic
nerve head features-apart from cupping-is help-ful
in the correct classification of such patients.
In cases in which optic nerve head assessment is
not decisive, the high-tech instruments available
today provide a valuable adjunct with the objec-tive
assessment of optic nerve head and RNFL
structure.
Disclaimer
Neither Dr. Sherry J. Bass nor Dr. Jerome Sherman has any financial or propri-etary
interest in any of the companies mentioned in this article.
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Corresponding author:
Sherry J. Bass, O.D.
SUNY State College of Optometry
33 West 42nd Street
New York, New York 10036
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