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[page 38] [Eye Reports 2011; 1:e12]

Optic disc parameters
in manifest and suspected
glaucoma
Peter Wanger,1 Lucian Vancea,2
Lene Martin1,3
1Department of Clinical Neuroscience,
Ophthalmology & Vision, Karolinska
Institutet, Stockholm; 2Eye Clinic,
Sundsvalls hospital, Sundsvall; 3Academy
of Health and Welfare, Mälardalen
University, Eskilstuna, Sweden

Abstract

Structure and function measurements are
important in glaucoma management. Digital
fundus photography has become a standard
procedure and the Heidelberg Retina
Tomograph (HRT), commonly used by glauco-
ma specialists, provides a glaucoma probabili-
ty score (GPS). The visual field index (VFI) is
a novel statistic, aiming to facilitate follow-up
of glaucoma patients. The aim of this study
was to compare the results from the digital
analysis of fundus photographs with HRT
measurements including GPS and VFI in
patients with ocular hypertension, suspect
glaucoma or glaucoma, and if possible define
an optic disc index, useful in glaucoma diagno-
sis. Fifty-eight consecutive patients from a
glaucoma service were included. Optic disc
parameters (disc and cup areas) were meas-
ured on digital fundus photographs, using a
semi-automatic method, and compared with
the GPS from the HRT and the VFI from stan-
dard automated perimetry. A significant rela-
tionship was observed between the GPS group
classification (normal, borderline, or abnor-
mal) and VFI classification (normal or abnor-
mal), both when the GPS borderline group was
regarded as normal (P = 0.0038 Fisher test)
and as abnormal (P=0.0179, kappa = 0.33). No
significant relationship was observed between
VFI and optic disc parameters. The three-
dimensional information in the GPS appears to
be more related to visual function, as meas-
ured by VFI, than the planimetric measures of
the optic disc.

Introduction

Digital fundus cameras are a standard part
of the ophthalmologic equipment, commonly
used in screening for retinal disorders, such as
age-related macular degeneration.1 Evaluation
of the optic disc parameters on fundus photo-
graphs is clinically relevant in glaucoma diag-

nosis and follow-up2 and several algorithms
have been evaluated for planimetric calcula-
tion of optic nerve head parameters.3 Optic
disc analysis can also be automatically per-
formed with confocal scanning laser ophthal-
moscopy4 using the Heidelberg Retinal
Tomography, HRT (Heidelberg Engineering
GmbH, Heidelberg, Germany). 

Recently, we developed a program which
was designed to provide clinically relevant
measures of optic disc parameters with a min-
imum of user input.5 The program can be used
directly on the acquired images, with the
examined subject still available for re-exami-
nation. 

In topographic analyses of the optic nerve
the effect of disc size has to be taken into
account.6 When calculating linear or area cup-
to-disc ratios, small discs may be classified
normal despite the presence of glaucoma and,
vice versa, large discs may be falsely labelled
as glaucomatous.7 Recently, a scoring system
for evaluation of the optic disc in glaucoma
was presented by the RAND study group8,
which relied on estimation of both the cup-to-
disc ratio and the disc size in order to over-
come this drawback.

The aim of this study was to compare the
results from the digital analysis of fundus pho-
tographs with HRT measurements including
glaucoma probability score (GPS) and visual
field index (VFI) in patients with ocular hyper-
tension, suspect glaucoma, or glaucoma, and,
if possible, define an optic disc index, useful in
glaucoma diagnosis. 

Materials and Methods

Subjects
The patients were recruited from a glauco-

ma service at a regional hospital. Inclusion cri-
teria were as follows: primary open angle glau-
coma (POAG), defined as intraocular pressure
(IOP) >21 mmHg at two or more occasions
AND either retinal nerve fiber layer (RNFL)
defect or visual field (VF) defect, without other
explanation, or both; suspected glaucoma,
defined as either IOP >21 mmHg at two or
more occasions OR either retinal RNFL defect
or VF defect, without other explanation; ocular
hypertension (OHT), defined as IOP >21
mmHg at two or more occasions with normal
VF and no RNFL defect; and, normal tension
glaucoma (NTG), defined as IOP <22 mmHg at
two or more occasions AND either RNFL defect
or VF defect, without other explanation, or
both. Exclusion criteria were other disorders
(such as optic nerve hypoplasia) that could
influence the eye or vision and subjects with
unreliable visual field results, defined accord-
ing to the manufacturer’s manual.

All patients had been examined at least

twice at the glaucoma service and were sched-
uled for follow-up. One eye from each of the 58
patients was randomly selected for analysis
(Tables 1 and 2). All but 16 of the subjects had
refraction within ± 3D (all refraction is
expressed in spherical equivalent). One sub-
ject had a hyperopia of +3.75D. The remaining
subjects had moderate myopia (-3.5D to -6D;
n=7) or high myopia (-6.75D to -14.75D; n=8). 

Methods
All subjects underwent a standard clinical

examination and visual field examination,
using the Humphrey Visual Field Analyser
(HFA), 24-2 Threshold test, Sita Fast (Carl Zeiss
Meditec, Dublin, California, USA). Digital
images of the optic nerve head (ONH) were
obtained using the Heidelberg Retinal
Tomography 3 (HRT3) (Heidelberg Engineering
GmbH) and the Zeiss VISUPAC and FF 450plus
telecentric fundus camera system (Carl Zeiss
Meditec). Measurements of optic disc parame-
ters were performed using the GPS calculations
in the HRT. GPS makes a global assessment of
the ONH by comparing the measured shape of
the optic disc and the surrounding retina to a
model for normal and glaucomatous optic
nerves. The output of the program is a number
between 0.00 and 1.00, which is classified as
within normal limits (below 0.28), borderline
(0.28 to 0.64) or outside normal limits (above
0.64).9 The quality of the images is evaluated by
the HRT software and classified as excellent,
very good, good, acceptable, poor or not usable
(manufacturer’s manual). Only recordings with
quality grade acceptable or better were used.
The global GPS measure and the cup area (CA)
measure were used. 

The Retinal Size Tool (RST) was used for
measurements on the digital fundus photo-
graphs.5 RST is a computer program developed
in-house, that can be used on any digital fun-

Eye Reports 2011; volume 1:e12

Correspondence: Lene Martin, Academy of Health
and Welfare, Mälardalen University, PO Box 325,
SE-631 05 Eskilstuna, Sweden.
Tel. +46.161.532.02 - E-mail: lene.martin@mdh.se

Key words: digital fundus photography, optic disc,
heidelberg retina tomograph, glaucoma probabil-
ity score, visual field index.

Received for publication: 13 April 2011.
Accepted for publication: 18 October 2011.

This work is licensed under a Creative Commons
Attribution NonCommercial 3.0 License (CC BY-
NC 3.0).

©Copyright P. Wanger et al., 2011
Licensee PAGEPress, Italy
Eye Reports 2011; 1:e12
doi:10.4081/eye.2011.e12

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[Eye Reports 2011; 1:e12] [page 39]

dus photograph where both the macula and the
ONH are visible. Optic disc parameters, i.e.
optic disc and cup, are assumed to be ellipti-
cal,10 and the user marks the endpoints of their
long and short diameters by mouse clicks. In
order to compensate for differences in magni-
fication due to camera and eye optics, the mac-
ula-optic disc centre distance is used as refer-
ence measure10-12 when converting pixel units
to metric distance.5 This measure is reported
to be quite constant among adults.13 The inter-
operator agreement has been shown to be very
good, both regarding disc and cup area (r=0.92
and 0.93, respectively).5 In the current study
the cup area (CA), disc area (DA), and the quo-
tient cup/disc area (CADA) were evaluated.

The VFI14 in the HFA was used for compari-
son with the structural measures. An HFA VFI
>98% was regarded as a normal (Bengtsson,
personal communication).

The study was performed according to the
Helsinki declaration and approved by the local
ethical committee. Written informed consent was
obtained from all patients prior to enrolment.

Statistics
For comparisons and correlations, the

Fisher test, the Kruskall-Wallis non-parametric
ANOVA-test and the Spearman Correlation test
were used. Binary logistic regression was used
in an attempt to define a diagnostic index
based on the measurement on the fundus pho-
tographs.

Results

Optic disc parameters in
Heidelberg Retina Tomograph 3 and
Retinal Size Tool

Table 3 show the disc parameters in all sub-
jects measured both with the RST (CA, DA,
CADA) and the HRT GPS (CA). There was a
strong correlation in the total group (r2=0.63;
P<0.0001) between the CA measurements per-
formed by the GPS and the RST, and no differ-
ence in median value (0.46 in both measures).
The CA’s were significantly smaller in the high
myopes, measured both with RST (median 0.33;
P<0.0001) and GPS (median 0.32; P=0.004).

Nineteen patients out of 58 (33%) had nor-
mal optic discs according to the GPS program;
12 were judged as borderline, and 27 as abnor-
mal (example discs shown in Figure 1). In the 8
patients with high myopia, 7 were classified as
normal and 1 as borderline. Table 4 shows the
disc parameters, measured by RST, in the three
GPS groups. A significant difference was found
in CA and CADA between the normal group and
both the borderline and the abnormal group and
in DA between the normal and the abnormal
group (Table 5). CADA correlated significantly,

but weakly with GPS (r2=0.19, P<0.001).

Optic disc parameters and humphrey
visual field analyzer visual field index

Eighteen patients out of 58 (31%) had nor-

mal visual fields according to the VFI, and 40
had subnormal VFI values. In the 8 patients
with high myopia, 5 had normal VFI values.
Table 6 shows the relationship between HFA
VFI and disc parameters in all patients. There

Article

Table 3. Median values for disc parameters in relation to the degree of myopia.

All (n=58) High myopes (n=8) Non high myopes (n=50)

Median RST DA (mm2) 2.17 (1.19-3.23) 1.64 (1.42-2.14) 2.22 (1.19-3.23)
Median RST CA (mm2) 0.46 (0.12-1.89) 0.33 (0.12-0.41) 0.56 (0.12-1.89)
Median RST CADA (mm2) 0.11 (0.04-0.20) 0.1 (0.05-0.19) 0.11 (0.04-0.20)
Median HRT GPS CA (mm2) 0.46 (0.09-1.11) 0.32 (0.15-0.30) 0.48 (0.09- 1.11)
RST, retinal size tool; DA, disc area; CA, cup area; CADA, cup/disc area quotient; HRT, Heidelberg retinal tomography; GPS, glaucoma probabil-
ity score. Values in parentheses represent the range of values.

Table 4. Median values for retinal size tool disc parameters and humphrey visual field
analyzer visual field index in the three glaucoma probability score classification groups.

GPS normal GPS borderline GPS abnormal

Number of subjects 19 12 27
Median RST DA (mm2) 1.67 (1.19-3.02) 2.15 (1.31-2.68) 2.32 (1.38-3.23)
Median RST CA (mm2) 0.31 (0.12-0.65) 0.59 (0.13-1.04) 0.63 (0.17-1.89)
Median RST CADA (mm2) 0.16 (0.07-0.32) 0.28 (0.1-0.41) 0.28 (0.08-0.59)
Median HFA VFI (%) 99 (65-100) 97.5 (80-100) 97 (65-99)
RST, retinal size tool; GPS, glaucoma probability score; DA, disc area; CA, cup area; CADA, cup/disc area quotient; HFA, humphrey visual field
analyzer; VFI, visual field index. Values in parentheses represent the range of values.

Table 2. Diagnoses in the studied group. 

OHT POAG NTG Glaucoma Glaucoma Suspect
secondary to secondary to glaucoma

pseudoexfoliation pigmentary
dispersion

Number of subjects 7 13 10 15 3 10
Suspected glaucoma consisted of POAG, NTG, and pseudoexfoliative glaucoma.

Table 1. Age, gender and refraction (spherical equivalent).

Median age (range) 64 (30-85)
Gender (female/male) 31/27
Median refraction (range) -0.63 (-14.5 to +3.75)
Refraction expressed as spherical equivalent.

Table 5. ANOVA P-values of comparisons between GPS classifications for cup area, disc
area, and cup/disc area quotient.

CA DA CADA
GPS classification
Normal vs borderline <0.05 >0.05 <0.05 
Normal vs abnormal <0.001 <0.001 <0.01 
Borderline vs abnormal >0.05 >0.05 >0.05 
CA, cup area; DA, disc area; CADA, cup/disc area quotient; GPS, glaucoma probability score; VFI, visual field index.

Table 6. Relationship between humphrey visual field analyzer visual field index and
median disc parameters.

VFI normal (>98%) VFI abnormal (<=98%)

Number of subjects 18 40
Median DA (mm2) 2.01 (1.19-3.02) 2.21 (1.23-3.23)
Median CA (mm2) 0.43 (0.12-0.96) 0.5 (0.12-1.89)
Median CADA (mm2) 0.22 (0.07-0.4) 0.23 (0.07-0.59)
Median HRT GPS CA (mm2) 0.17 (0.02-0.7) 0.64 (0.08-0.92)*
VFI, visual field index; DA, disc area; CA, cup area; CADA, cup/disc area quotient; HRT, Heidelberg retinal tomography; GPS, glaucoma proba-
bility score; *P=0.001. Values in parentheses represent the range of values.

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[page 40] [Eye Reports 2011; 1:e12]

was a significant difference between VFI in the
GPS normal group compared with the GPS
abnormal group (<0.05) (Table 7). The corre-
lation between VFI and GPS score was weak
(r2=0.083, P=0.027). Regarding classification,
a significant relationship was observed
between the GPS group classification (normal,
borderline, or abnormal) and VFI classification
(normal or abnormal), both when the GPS bor-
derline group was regarded as normal and as
abnormal (Tables 7 and 8). There was no cor-
relation between CADA and VFI (r2=0.006, P =
0.56). A logistic regression equation based on
DA and CADA did not discriminate between
subjects with normal and abnormal VFI. 

Discussion

Optic disc parameters in
Heidelberg Retina Tomograph 3
and Retinal Size Tool and GPS 
classifications

In a previous study, comparing disc meas-
urements from RST and HRT3 in non-glauco-
matous subjects, no significant difference
between the measurements were found either
in DA or in CA.5 This observation was con-
firmed regarding CA in the current study of
glaucoma subjects, but could not be confirmed
regarding the DA, since the GPS program does
not report this measure. 

It is well known that disc size influences the
ability to classify the optic disc as normal or
glaucomatous.6,8,9 This influence is true also
for the HRT3 GPS classification.9 In the current
study, the largest discs, median disc area of
2.32 mm2, were found in the GPS abnormal
group, compared to median disc area of 2.15
mm2 in the GPS borderline group, and median
disc area of 1.67 mm2 in the GPS normal group.

For use in glaucoma management, the
RAND system8 defines a score for the linear
cup to disc ratio (CDR): score of 0 is CDR <
0.5, score 1 is CDR 0.5 to < 0.7, score 2 is CDR
0.7 to 0.9, and score 4 is CDR > 0.9, and the
RAND system adjusts this score by subtracting
one unit if the DA is ophthalmoscopically
judged to be large and adding one unit if the
DA is judged to be small. In the current study,
which included patients with no or low degree
of glaucomatous ON damage, binary logistic
regression showed no significant effects of
combining optic disc parameters for the iden-
tification of eyes with abnormal VFI. Thus, the
significant relationship between GPS and VFI
appears to depend on the three-dimensional
analysis of the neuro-retinal rim area per-
formed by the HRT. A weakness in the RST
method is that notching of the rim area is not
visible in the measurement values. However,
visual evaluation of the fundus photographs

revealed notching in one eye only. The concor-
dance between GPS and VFI in classification as
normal and abnormal was moderate. Alencar,
et al.15 reported that GPS values were predic-
tive of conversion in a population of patients
with suspected glaucoma. Thus, the combina-
tion of GPS and VFI data may provide an easily
analyzed basis for decision-making in glauco-
ma management.

Conclusions

A statistically significant relationship was
found between HRT GPS and HFA VFI, but not
between optic disc parameters, obtained by
planimetric measurements on digital fundus
photographs. The three-dimensional informa-
tion in the GPS appears to be more related to
visual function, as measured by VFI, than the
planimetric measures of the optic disc.

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Article

Figure 1. Examples of optic discs with dif-
ferent glaucoma probability score classifi-
cations and visual field index findings. (A)
Female born 1944, with OHT, GPS nor-
mal, RST CA/DA 0.19,VFI 99%; (B) Male
born 1938, with pseudoexfoliation, GPS
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Table 8. Relationship between glaucoma prob-
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GPS normal GPS borderline 
or abnormal

VFI >98 10 8
VFI <=98 9 31
GPS, glaucoma probability score; VFI, visual field index; P=0.0179
Fischer test; kappa = 0.33.

A

B

C

Table 7. Relationship between glaucoma prob-
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borderline group was regarded as normal.

GPS normal or GPS abnormal
borderline

VFI > 98 15 3
VFI ≤=98 16 24
GPS, glaucoma probability score; VFI, visual field index; P, 0.0038
Fisher test; kappa = 0.36.

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