We compared the accuracy of the present method with that which
could be obtained from the standard set of shape parameters calculated
by the HRT operating software. The DFA formula of Mikelberg et
al.
21 which is incorporated into the software (ver. 2.01)
gave a sensitivity of 49% and a specificity of 98%. Adjusting the
classification threshold to give more equal values yielded a
sensitivity of 77% and a specificity of 77%. A more fair comparison
is to subject the data to a new DFA. Thirteen of the parameters
(
ag [disc area] was excluded because it is derived from
mr [mean radius]) were entered into a forward-stepping
DFA, using an F-to-enter of 4.0 and an F-to-remove of 3.0. The overall
accuracy was 84%, and the cross-validated accuracy was 83.5%. The six
parameters selected by the analysis were
abr (area below
reference),
mhc (mean height of contour),
mr (mean radius),
var (volume above reference),
vas (volume above surface), and
vbr (volume below reference).
We calculated the correlations, across both normal and glaucoma groups,
between the model parameters and the standard HRT parameters, and with
the visual field MD.
Table 7 shows the values for some selected HRT and model parameters. As might
be expected, the HRT measure of disc area (
ag) did not
correlate strongly with any of the model parameters, because the model
does not provide an explicit estimate of disc area. The highest
correlation (
r = 0.54) was with the fit of the
parabolic function (
f p). However,
there was also a strong correlation (
r = 0.52) with the
model’s measure of cup radius (
r 0)
which can be explained because cup area and disc area are known to be
strongly correlated in normal discs.
37 38 39 There was a
strong correlation (
r = 0.94) between the model’s
measure of cup depth (
z 500) and the
corresponding HRT measure (
mdg). The fit of the parabolic
surface (
f p) also correlated strongly
(
r = 0.94) with
mdg, because its value is
small when a cup is absent, and therefore its value largely reflects
cup depth. Both
z 500 and
r 0 showed weak correlations with HRT
parameters
hvc and
var, on which the shape of the
cup would be expected to have little effect.
The HRT parameter csm (cup shape measure) has been shown by
several studies to give good discrimination between groups, although
its structural interpretation seems obscure. The model parameter that
correlated most strongly with csm was temporal gradient
measure, g r T (r =
0.40), however the correlation with cup diameter
(r 0) was nearly as strong
(r = 0.37).
We calculated the correlation between each parameter and the visual
field MD. These values are also given in
Table 7 (second row from the
bottom). The parameter showing the highest correlation with MD was
horizontal image curvature (
c;
r = 0.55).
The HRT indices showing the strongest correlations were
abr (
r = −0.43),
var (
r =
0.48), and
mhc (
r = −0.41).
For comparison with previous studies, we subjected all the HRT and
model parameters to a receiver operating characteristic (ROC) analysis,
using the methods described in Iester et al.
27 Some of
these values are given in the bottom row of
Table 7 . The parameter
showing the highest area under the curve (a measure of discrimination
between two groups) was the horizontal curvature measure (
c,
area = 0.93). The best HRT measure was
mhc (area =
0.91);
csm did less well, with an area = 0.77.