Optic disc morphometry was performed by means of the HRT by a trained
operator (TS). Details of the instrument and its reproducibility have
been published.
18 28 29 30 31 32 33 34 35 36 A topography image was taken as a
series of 32 two-dimensional transverse optic section images, acquired
in approximately 1.6 seconds, each consisting of 256 ×2 56 pixels
(65,536 picture elements). In this study three 20° images were
obtained for each eye, and the mean image of the three scans was used
for the optic disc measurements, according to Weinreb et
al.
30 The scanning depth of each topographic image series
ranged from 0.5 to 4.0 mm in 0.5-mm increments, depending on the
individual degree of disc swelling. Each mean topography image was
automatically corrected for horizontal and vertical
tilt.
37 HRT software (ver. 2.01; Heidelberg Engineering)
calculated the stereometric parameters of the optic nerve head.
Magnification errors were automatically corrected by using patients’
keratometry readings.
37 A contour line was marked by the
computer-generated circle outside the disc edema, according to a
published procedure.
17 18 34 Specifically, this circle was
located on healthy, nonedematous retina, to avoid height measurement
error by having a reference surface as flat as possible (the HRT curved
surface
38 ). For this purpose its mean radius was fixed to
2.5 mm for each eye—that is, along the extreme periphery of the image.
The overall three-dimensional parameters, volume above surface
(VAS), effective mean height (EMH), and maximum height in contour
(MxHC), were automatically measured relative to the curved surface. EMH
represents the mean height of all parts within the contour line that
are located above the curved surface. MxHC is defined as the mean
height of the 5% pixels with the highest height values within the
contour line.
37 Another axial boundary, the current
standard reference plane,
18 19 38 was used by the HRT
software to determine the volume above reference (VAR).
37 Volumetric parameters were also evaluated for the following predefined
HRT disc sectors: superior (S, 90°), nasal (N, 90°), inferior (I,
90°), and temporal (T, 90°). Optic disc sector analysis was
performed to evaluate the presence of a relationship between regional
disc edema and functional loss at corresponding disc and field sectors.