Photogrammetry of the optic disc in glaucoma and ocular ...

R
Photogrammetry of the optic disc in
glaucoma and ocular hypertension with
simultaneous stereo photography*
Chris A. Johnson, John L. Keltner, Marijane A. Krohn, and Gerald L. Portney
Stereophotogrammetric evaluations of the optic cup were performed for normal, ocular hypertensive,
and glaucomatous eyes. Average volume, area, and depth measurements were
progressively larger from normal to ocular hypertensive to glaucomatous eyes, although the
distributions of individual values exhibited considerable overlap among the three groups.
Similar results were obtained for volume, area, and depth asymmetry between each pair of
eyes. None of these measurements was able to distinguish accurately between normal and
glaucomatous optic cups. However, normal eyes showed a high correlation (r = +0.85) between
area and depth of the optic cup, whereas this area/depth relationship was reduced in
ocular hypertensives (r = +0.63) and completely broke down for glaucomatous eyes
(r = +0.04). Approximately 89% of the glaucomatous eyes and 47% of the ocular hypertensive
eyes were beyond the range of normal area/depth correlation values. These findings represent
an improvement over most previous attempts to quantitatively differentiate between normal
and glaucomatous eyes on the basis of optic disc measurements alone, and support the hypothesis
that optic disc damage usually precedes visual field loss in glaucoma. With further technical
refinements such as computer image processing, stereophotogrammetry of the optic cup may
become a valuable differential diagnostic technique for glaucoma.
Key words: glaucoma, ocular hypertension, stereophotogrammetry, optic disc
evaluation, glaucomatous optic disc cupping
ecent studies have shown that both ste-
1-16 16 20
reo and nonstereo " photogrammetric
From the Department of Ophthalmology (C. A. J.,
M. A. K., and G. L. P.) and the Departments of Ophthalmology,
Neurology, and Neurological Surgery
(J. L. K.), School of Medicine, University of California,
Davis.
Supported in part by National Eye Institute Research
grant EY-01841 (JLK) and National Eye Institute
Academic Investigator Award K07-EY00095 (CAJ).
Submitted for publication July 24, 1978.
Reprint requests: Chris A. Johnson, Ph.D., Department
of Ophthalmology, University of California School of
Medicine, Davis, Calif. 95616.
This paper is dedicated to the memory of Gerald L.
Portney, M.D., who died on May 23, 1977. The present
investigation represents a continuation of his work
on photogrammetric analysis of the optic disc as an
objective diagnostic procedure for glaucoma.
processing of fundus photographs can produce
accurate measurements of optic cup topography.
Since this technique provides an
objective, quantitative evaluation of the optic
nerve head, it represents a potentially important
diagnostic tool for glaucoma and other
optic nerve diseases.
Several investigators 1 " 3 ' 17 have reported
volume, area, and depth characteristics of the
optic cup in normal populations, based upon
photogrammetric analysis. Their results indicate
that in normal subjects, these optic cup
measurements are highly symmetrical between
the two eyes. Portney 2 ' 3 and Holm et
al. 17 found that the average volume, area, and
depth of the optic cup are larger in glaucomatous
eyes than in normal eyes. However,
the distributions of individual values exhibit
1252 0146-0404/79/121252 +12$01.20/0 © 1979 Assoc. for Res. in Vis. and Ophthal., Inc.

Volume 18
Number 12
Stereophotogrammetry of optic disc 1253
Fig. 1. Schematic representation of the revised static-kinetic visual field test procedure, consisting
of 2 kinetic isopters beyond 30° and static perimetry along 10 meridians within the
central 30°.
considerable overlap between the normal
and glaucoma populations, thereby limiting
the diagnostic efficacy of these measurements.
Comparison of optic cup volumes in
unilateral and bilateral glaucoma patients revealed
significant asymmetry between eyes,
in contrast to the results for normal subjects.
Portney 2 ' 3 concluded that optic cup volume
asymmetry between eyes was the most sensitive
photogrammetric measurement for differentiating
normal subjects from glaucoma
patients.
The purpose of the present investigation
was to refine existing photogrammetric methods
of evaluating the optic cup and thereby to
increase the diagnostic utility of this technique
for glaucoma. To enhance the early detection
of glaucomatous damage, particular
attention was directed toward patients at
high risk (elevated intraocular pressure with
no demonstrable visual field loss) and pa-
tients with early glaucomatous visual field
defects.
Materials and methods
All participants in the study received a complete
eye examination, including refraction, Goldmann
applanarion tonometry, ophthalmoscopy,
biomicroscopy, stereo disc photography, and kinetic
and static perimetry. Patients were excluded
if good quality stereo disc photographs could not
be obtained or if they were unable to perform reliably
on visual field testing. One hundred sixtyfour
eyes of 84 patients were selected for photogrammetric
evaluation according to the following
criteria: (1) average intraocular pressures less than
20 mm Hg with normal optic discs and visual fields
(normals, N = 40 eyes), (2) average intraocular
pressures greater than 25 mm Hg with normal visual
fields (ocular hypertensives, N = 106 eyes),
(3) early glaucomatous optic disc cupping with
early visual field defects (glaucoma, N = 18 eyes).
Following this initial classification, all stereo

1254 Johnson et al.
-700 -MO -500 -HOD -300 -200 -100
-400 -300 -200 -100 0 100 200 300 100 500 600
T RXrS FOB 90 DEGREE PLRNf
Invest. Ophthalmol. Visual Sci.
December 1979
Fig. 2. Contour map and cross-sectional plots of a representative glaucomatous optic disc.

Volume 18
Nutnlxr 12 Stereophotogrammetry of optic disc 1255
disc photographs were manually processed by a
photogrammetric engineer according to standard
techniques. 21 The photogrammetry data were
converted to digital form and analyzed by computer
to determine depth, area, volume, and
other pertinent geometric measurements of the
optic cup.
Visual field testing. A combination of extensive
static and kinetic perimetry was used to evaluate
visual fields. Kinetic perimetry (with either the
Goldmann or Tubingen perimeters) consisted of at
least 2 isopters beyond 30° radius, 3 isopters
within 30° radius, and numerous spot checks between
isopters within the central 40° radius, as
previously described by Portney and Krohn. 22
Static perimetry (with the Tubingen perimeter)
consisted of threshold determinations along the
45°, 135°, 225°, and 315° meridians in 1° intervals
out to 20° radius and 2° intervals between 20° and
30° radius. Additional static perimetry was performed
along both radial (meridian) and circular
paths which intersected the length and width of
visual field defects plotted by kinetic perimetry.
Approximately two thirds of the eyes were evaluated
with this procedure, which required about 1
hr per eye. The remaining one third of the eyes
were tested according to the method described
below.
In view of recent findings 22 which show that
static perimetry is more sensitive and reliable than
kinetic perimetry for detection of early glaucomatous
visual field defects, 42 eyes were tested with
the revised procedure illustrated in Fig. 1 (shown
for a right eye). This visual field examination consisted
of at least 2 isopters beyond 30° radius (kinetic
testing) and static perimetry along four
meridians (45°, 135°, 225°, 315°) in 2° intervals and
six intermediate meridians in 2.5° intervals across
the central 30° radius of the visual field. Additional
circular and radial (meridian) static perimetry was
performed when it was necessary to further define
specific portions of the visual field. All determinations
were conducted on the Tubingen perimeter
and required approximately 45 to 50 min per eye.
This revised procedure thus provided the dual advantages
of greater time efficiency and more effective
detection of early glaucomatous visual field
defects.
Specific criteria were established to define the
presence or absence of visual field defects, based
upon extensive previous experience and existing
guidelines developed by other investigators. 23 ' 24
According to our standards, areas of visual field
loss had to be at least (1) 5° by 5° in size and 0.5 log
unit of luminance (apostilbs) deep or (2) 3° by 3° in
60-
50-
.10 .30 .50 .70 .90 1.10
OPTIC CUP VOLUME (mm 3 )
NORMAL n = 40 eyes
OCULAR HYPERTENSION
n = 106 eyes
GLAUCOMA n= 18 eyes
Fig. 3. Frequency distributions of optic cup volume
for each of the three patient groups.
size and 0.7 log unit of luminance (apostilbs) deep.
These values are generally beyond the range of
response variability in normal subjects. Visual
field defects were also verified on at least two
static profiles from adjacent meridians, or a combination
of one meridian and one circular static
profile. Additional testing with the' Fieldmaster
automated perimeter was conducted as a confirmatory
procedure. 25 Patients with questionable or
borderline results were tested a second time.
To minimize the influence of blur on perimetric
determinations within the central 30° radius, patients
were given an appropriate refractive correction
for distance plus an added near correction
for age. Accuracy of this correction was checked
at the perimeter bowl by subjective refraction
techniques. 26 Fixation was carefully monitored
throughout each testing session, and response
variability was determined at several visual field
locations to ensure that threshold variations were
less than 0.4 log unit of luminance (apostilbs) during
perimetric testing.
Classification of optic discs. Optic discs were
classified according to whether they appeared to
be normal or exhibited early glaucomatous damage.
Portney's cone-cylinder-hemisphere category
system 2 " 6 (based upon Elschnigs types I to IV
categories) was used to classify normal optic discs.
Judgments of glaucomatous damage to the optic

1256 Johnson et al.
.4 .8 1.2 1.6 2.0 2.4
OPTIC CUP ORIFICE AREA (mm 2 )
NORMAL n = 40eyes
OCULAR HYPERTENSION
n= 106 eyes
GLAUCOMA n= 18 eyes
Fig. 4. Frequency distributions of optic cup area
for each of the three patient groups.
disc were based upon Kronfeld's criteria of central
deep atrophy, mottled or "moth-eaten" appearance
of the base of the optic cup, upward or
downward extension, marginal excavation and
narrow nerve rims. 2 " 6 ' 27> 28 Other indicators of
glaucomatous optic cupping 29 " 36 (e.g., vertical
ovality of the cup, notching of the nerve fiber rim,
asymmetry of the cup between eyes, pallor of the
optic cup) were also employed to distinguish between
normal and glaucomatous optic cups. As
with the evaluation of visual field results, conservative
criteria were used for classifying normal vs.
glaucomatous optic cups. The investigators agreed
upon nearly all optic disc classifications, and repeated
evaluations at periodic intervals showed a
high degree of consistency.
Stereophotography and photogrammetric analysis.
Simultaneous stereo disc photographs were
obtained with the Donaldson stereo fundus camera.
37 Since Kodak photomicrography film (ASA
16) produced fundus photographs which were underexposed
for most eyes, it became necessary to
use Kodachrome 25 film (ASA 25). Frisen 38 has
reported that the resolution characteristics of
Kodak photomicrography film and Kodachrome 25
are approximately equivalent for luminance conditions
similar to those employed in this study.
2.8
.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0
OPTIC CUP DEPTH (mm)
Invest. Ophthalmol. Visual Sci.
December 1979
NORMAL n = 40 eyes
OCULAR HYPERTENSION
n = IO6 eyes
GLAUCOMA n= 18 eyes
Fig. 5. Frequency distributions of optic cup depth
for each of the three patient groups.
Preliminary investigations revealed that photogrammetric
measurements performed on both
types of film produced comparable results. 39
Several of the optical specifications provided
with our Donaldson stereo fundus camera were in
error. The stereo base (reported as 5.75 mm in the
specifications provided with the camera) was
found to be 2.87 mm by dividing the center-tocenter
distance of the photographic plate by the
magnification of the camera's objective lens (2x).
In addition the 3.25X and 4.25X magnification
settings were calculated to be 2.69X and 3.22X,
respectively. This was determined by photographing
a model eye containing a calibrated line grating
and calculating the ratio of image size to object
size. With these revised values, photogrammetric
measurements of Donaldson stereo photographs
were consistent with similar determinations performed
on Zeiss stereo photographs (with the
Allen rotary prism) of the same optic disc.
Each pair of stereo disc photographs was processed
by a photogrammetric engineer using a
Wild A-10 photogrammetric plotting instrument
(magnification, 6.55X; stereo base, 200 mm).
Previous studies 6 have reported that the variability
of this procedure (using simultaneous stereo
photographs) is approximately 10%. Digital x, y,

Volume 18
Number 12
Table I. Optic cup volume, area, and depth measurements in normal,
ocular hypertensive, and glaucomatous eyes
Volume (mm 3 ):
Mean
S.D.
Range
Area (mm 2 ):
Mean
S.D.
Range
Depth (mm):
Mean
S.D.
Range
Normal
(N = 40 eyes)
0.13
0.13
0.001-0.53
0.65
0.37
0.01-1.33
0.34
0.20
0.01-0.79
and z coordinates were established for 400 to 700
data points on each optic disc (according to an arbitrary
"count" scale used by the photogrammetric
plotting device) and then transferred to punch
cards for subsequent computer processing. Horizontal
(x) and vertical (y) counts were converted to
millimeters according to the photogrammetric
plotter-to-camera and camera-to-eye magnification
ratios, and depth (z) counts (in arbitrary units)
were converted to millimeters by the following
equation:
AZ = R b,) • AP
where AZ is the change in depth (in mm); Mi is
the magnification of the Donaldson fundus camera
(2.69x or 3.22x); b! is the stereo base of the
Donaldson fundus camera (2.87 mm); M2 is the
magnification of the Wild A-10 photogrammetric
plotter (6.55X); b2 is the stereo base of the Wild
A-10 photogrammetric plotter (200 mm); R is the
ratio of the magnification and stereo base of the
Donaldson stereo fundus camera to the magnification
and stereo base of the Wild A-10 photogrammetric
plotter, i.e., R = (M, • b,)/(M2 • b2); fe is
the distance between the posterior principal point
of the eye and the fundus (17.2 mm); and AP is
the horizontal parallaxes measured on the photographs.
Volume, area, and depth measurements were
then calculated by the computer. A significant
problem in determining optic cup volume is specifying
the top of the optic cup. Since the rim of the
optic cup has considerable topographic variation
about its circumference, volume measurements
may show substantial variation as a result of different
criteria employed to establish the top of the
optic cup. Comparisons among different optic
Stereophotogrammetry of optic disc 1257
Ocular hypertensive
(N = 106 eyes)
0.20
0.16
0.007-0.74
0.98
0.47
0.16-2.10
0.43
0.19
0.11-0.92
Glaucoma
(N = 18 eyes)
0.41
0.24
0.14-1.15
1.58
0.52
0.70-2.61
0.55
0.14
0.36-0.88
cups exacerbate the problem because of marked
variation in the overall configuration of the cup
from one eye to another. In the present study, the
top of the optic cup was defined by the lowest
point on the rim of the cup orifice. This point was
determined by a computer algorithm which began
at the bottom of the optic cup (greatest depth, or
largest z value) and checked the x, y coordinates
around the cup for successively smaller depth values.
A "spill-over" at the lowest point of the optic
cup rim was thereby defined by an abrupt, extremely
large change in x, y coordinates at some
position along the cup orifice. This permitted a
standard, objective criterion to be applied to all
optic cups undergoing photogrammetric analysis.
An illustrative example of a contour map and
cross-sectional graphical plots derived from photogrammetric
processing of stereo disc photographs
is shown in Fig. 2.
Results
Figs. 3 to 5 present the frequency distributions
of volume, area, and depth of the
optic cup, respectively, for normal, ocular
hypertensive, and glaucomatous eyes. Each
of these optic cup measurements exhibited
differences in the distribution of values
among various patient groups. Small optic
cup volumes, areas, and depths occurred
more frequently in normals, followed by ocular
hypertensives. Patients with glaucomatous
optic disc cupping and visual field loss
showed a higher percentage of optic cups
with large volume, area, and depth values. A
chi-square analysis of these data revealed

1258 Johnson et al.
60
50-
40-
30-
20-
10-
0
60-1
50-
40-
30-
20-
10o
I ii
NORMAL (Bilateral)
n= 20 Patients
OCULAR HYPERTENSION
(Bilateral)
n= 45 Patients
50-
GLAUCOMA
(Unilateral or Bilateral)
40-
30-
20-
10-
0
n = 15 Patients
.10 .20 .30 40 .50
OPTIC CUP VOLUME ASYMMETRY BETWEEN EYES (mm 3 )
Fig. 6. Frequency distributions of volume asymmetry
between fellow eyes for each of the three
patient groups.
NORMAL (Bilateral)
n= 20 Patients
OCULAR HYPERTENSION
(Bilateral)
n = 45 Patients
GLAUCOMA
(Unilateral or Bilateral)
n=l5 Patients
.2 .4 .6 .8 1.0 1.2 1.4
OPTIC CUP AREA ASYMMETRY BETWEEN EYES (mm 2 )
Fig. 7. Frequency distributions for area asymmetry
between fellow eyes for each of the three
patient groups.
50-i
40-
30-
20-
10-
0
Invest. Ophthalmol. Visual Sci.
December 1979
NORMAL (Bilateral)
n = 20 Patients
l i i i r
OCULAR HYPERTENSION
(Bilateral)
n= 45 Patients
GLAUCOMA
(Unilateral or Bilateral)
n= 15 Patients
.10 .20 .30 .40 .50 .60
OPTIC CUP DEPTH ASYMMETRY BETWEEN EYES (mm)
Fig. 8. Frequency distributions for depth asymmetry
between fellow eyes for each of the three
patient groups.
statistically significant differences in the distribution
of volume, area, and depth of the
optic cup among the three patient groups,
(X 2 =115.9, p < 0.0001 for volume; x 2 =
158.0, p < 0.0001 for area; x 2 = 110.3, p <
0.0001 for depth). Despite these highly significant
differences, the extensive overlap
among the three groups greatly restricts the
differential diagnostic utility of these measurements
for individual patients. Means,
standard deviations, and ranges of optic cup
volume, area, and depth for the three patient
groups are presented in Table I.
The asymmetry of optic cup volume area,
and depth between eyes was determined for
patients with (1) bilaterally normal eyes
(N = 20 patients), (2) bilaterally ocular hypertensive
eyes (N = 45 patients), and (3)
early glaucomatous optic disc cupping and visual
field defects in either one or both eyes
(N = 15 patients). Frequency distributions
of volume, area, and depth asymmetry between
fellow eyes are presented in Figs. 6 to
8 for the three patient groups. Each of the
optic cup measurements in glaucoma patients
showed (on the average) greater asymmetry

Volume 18
Number 12
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NORMAL
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1260 Johnson et al.
2.8
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OCULAR HYPERTENSION 28.
Normal Optic Disc
n=70 eyes
r=+.65l (p

Volume 18
Number 12
means of accurately differentiating between
normal and glaucomatous optic cups for potential
clinical diagnostic use? To evaluate
this question, we determined the percentage
of ocular hypertensive and glaucomatous
eyes that exceeded the range of values found
in normal eyes for each of the seven optic
cup measures (volume, area, depth, volume
asymmetry, area asymmetry, depth asymmetry,
and the area/depth relationship). The
findings are presented in Table III.
Depth and depth asymmetry between fellow
eyes showed rather poor results, whereas
volume, area, and volume and area asymmetry
between fellow eyes produced somewhat
better separation between the three
groups. However, the area/depth relationship
revealed the most impressive findings;
89% of the glaucomatous eyes and 47% of the
ocular hypertensive eyes were beyond the
range of area/depth values for normal optic
cups. It should be noted that the two glaucomatous
optic cups (11%) that fell within the
range of normal area/depth values were from
the same individual, who happened to be a
high myope. Therefore our photogrammetric
magnification factors, which are based upon
Gullstrand's reduced schematic eye, 45 introduced
errors that may have appreciably underestimated
the overall dimensions of the
optic cup. Future evaluations that provide
corrections for large refractive errors might
produce even greater separation among normal,
ocular hypertensive, and glaucomatous
eyes.
There are at least two hypotheses that may
be proposed to account for the relationship
between glaucomatous damage and optic cup
characteristics. (1) Large optic cups may generally
be more susceptible to sustaining glaucomatous
damage. (2) Glaucomatous damage
to the optic disc produced changes in the size
and shape of the optic cup. The consistent,
graduated increases in optic cup measurements
across the three patient groups in this
study appear to support the second hypothesis.
In particular, the area/depth relationships
presented in Figs. 9 and 10 are highly
consistent with the second hypothesis and
Stereophotogrammetry of optic disc 1261
would be difficult to explain solely on the
basis of large optic cups. Many previous investigators
have reported that glaucomatous
damage to the optic cup usually precedes visual
field loss. 8 ' 40 ' 41 Our current photogrammetric
data generally support this interpretation.
The results of this study represent a significant
improvement over most previous attempts
to differentiate between normal and
glaucomatous eyes on the basis of optic disc
evaluation alone, 2 " 6 ' 17< 33j 40 - 41 and at the
same time to avoid the problems and variability
introduced by methodologic differences
in the measurement of cup/disc ratios.
42 Recent multivariate analysis studies by
Susanna and Drance 43 and Drance et al. 44
exhibit detection rates which are comparable
to our current photogrammetric results, although
many of the statistical evaluations
were performed on subjective clinical observations.
Stereophotogrammetry offers the
advantage of objective measurements of the
optic cup characteristics and may produce a
higher degree of uniformity and standardization
among different examiners. We feel that
both stereophotogrammetric evaluations of
the optic cup and multivariate statistical
analysis may be of potential value for clinical
diagnostic purposes in glaucoma.
We are indebted to Dr. V. Ralph Algazi and Mr.
James Stewart from the Department of Electrical Engineering,
University of California, Davis, for performing
the computer processing of photogrammetric data.
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32. Hitchings, R., and Spaeth, G.: The optic disc in
glaucoma. I. Classification, Br. J. Ophthalmol.
60:778, 1976.
33. Fishman, R.: Optic disc asymmetry: a sign of ocular
hypertension, Arch. Ophthalmol. 84:590, 1970.
34. Shaffer, R., and Heatherington, J.: The glaucomatous
disc in infants: a suggested hypothesis for disc
cupping, Trans. Am. Acad. Ophthalmol. Otolaryngol.
73:929, 1969.
35. Schwartz, B.: Cupping and pallor of the optic disc,
Arch. Ophthalmol. 89:272, 1973.
36. Schwartz, B., Reinstein, N., and Lieberman, D.:
Pallor of the optic disc, Arch. Ophthalmol. 89:278,
1973.
37. Donaldson, D.: A new camera for stereoscopic fundus
photography, Arch. Ophthalmol. 73:253, 1965.
38. Frisen, L.: Resolution at low contrast with a fundus
camera. Comparison of various photographic films,
INVEST. OPHTHALMOL. 12:865, 1973.
39. Krohn, M.A., Keltner, J.L., and Johnson, C.A.:
Comparison of photographic techniques and films
used in stereophotogrammetry of the optic disc.
Am. J. Ophthalmol. (in press).
40. Armaly, M.: The optic cup in the normal eye. I. Cup
width, depth, vessel displacement, ocular tension

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and outflow facility, Am. J. Ophthalmol. 68:401,
1969.
41. Armaly, M.: Optic cup in normal and glaucomatous 44.
eyes, INVEST. OPHTHALMOL. 9:425, 1970.
42. Schwartz, J.T.: Methodologic differences and measurement
of cup-disc ratio, Arch. Ophthalmol.
94:1101, 1976. 45.
43. Susanna, R., and Drance, S.M.: Use of discriminant
analysis. I. Prediction of visual field defects from
Information for authors
Stereophotogrammetry of optic disc 1263
features of the glaucoma disc, Arch. Ophthalmol.
96:1568, 1978.
Drance, S.M., Schulzer, M., Douglas, G.R., and
Sweeney, V.P.: Use of discriminant analysis. II.
Identification of persons with glaucomatous visual
field defects, Arch. Ophthalmol. 96:1571, 1978.
Helmholtz, H.: Treatise on Physiological Optics
(J. P. C. Southall, editor), New York, 1962, Dover
Publications, vol. I.
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