The residual spherical refractive error was corrected
by the VERIS™ autorefractor, mounted on the
stimulation monitor. The alignment of the patient’s
pupil with the monitor optic and the fixation stability
are controlled by an attached infrared camera. Each
monocular recording lasts about 9 minutes
(exponent of the stimulation m-sequence = 13). To
make the process more comfortable for the patient,
the recording process was divided into eighteen 30-
second segments. Segments contaminated with
ocular movements were discarded and recorded
anew. The signals are amplified with a Grass
Neurodata Model 15ST amplification system (Grass
Telefactor, NH), with a 50,000 gain, filters with 10-
300 Hz bandwidth and a sampling interval of 0.83
milliseconds (1200 Hz).
Each participant was given a complete
ophthalmic exam, including general anamnesis, best-
corrected visual acuity, slit lamp biomicroscopy,
intraocular-pressure measurement using the
Goldmann applanation tonometer, gonioscopy,
dilated fundoscopic examination (90D lens), stereo
retinographs and a 24-2 SITA Humphrey automated
perimetry (Swedish Interactive Threshold
Algorithm. Carl Zeiss Meditec Inc.). A diagnosis of
open angle glaucoma was established where there
were at least two consecutive abnormal visual fields
in the Humphrey campimetry, (threshold test 24-2),
defined by: 1) a pattern standard deviation (PSD)
and/or corrected pattern standard deviation (CPSD)
below the 95% confidence interval; or 2) a
Glaucoma Hemifield Test outside the normal limits.
We define as abnormal an altitudinal hemifield in
the Humphrey visual field analysis giving three or
more contiguous sectors below the 95% confidence
interval, with at least one of them below the 99%
confidence interval. The visual field was dismissed
as unreliable if the rate of false positives, false
negatives or fixation losses was higher than 33%. A
control database was also established on the basis of
normal eye records established within the
longitudinal prospective study. All these normal eye
records had an intraocular pressure of 21 mmHg or
less (with no previous history of ocular
hypertension). An ophthalmic examination of the
optic papilla was also conducted to check that it fell
within the normal structural parameters.
The signals obtained from the 103 hexagons
were regrouped and averaged to build up a new 56-
sector map as shown in figure 1. The purpose of this
regrouping was to simplify the analysis and to
improve the signal-to-noise ratio. A 56-sector
topography was therefore chosen, similar to that
studied in automated campimetry, the clinical ¨gold-
standard¨ for evaluating the visual field. It should
also be noted here that sector 41 is the average of a
greater number of hexagons, since it is the area
containing the blind spot and, as such, more difficult
to analyse.
Two mfERG record databases were built up, one
containing healthy or control individuals and the
other glaucoma-affected individuals for study by
means of the Discrete Wavelet Transform (DWT).
Two other specific databases were also created to be
studied by means of an alternative technique,
Morphological Analysis, all made up by a complete
56-sector map as shown in figure 1.
Not all the sectors making up the map to be
analysed by the Wavelet Transform belonged to a
single patient; the map groups together 56 clearly
glaucoma-identified sectors from among the fifty
patients diagnosed with the same symptom.
Following a similar procedure, a sector map
comprising the control database was built up, this
time on the basis of healthy individuals.
As regards the databases used for the
morphological analysis, these were made up by two
15-record collections from the 56 sectors: the first
coming from 15 patients affected with early-stage
OAG and showing between 3 and 12 diseased
sectors, and the other built up from the 15 healthy
control subjects.
2.2 Study of Severe Lesions by Wavelet
Analysis
DWT was better than morphological analysis as a
mfERG-record analysis tool for detecting severe
retina lesions. Conversely, morphological analysis
was much more efficient for detecting early-stage
glaucoma by extracting certain markers present in
the records.
The great drawback of the Fourier transform-
based analysis is that the time information is
forfeited when the signal is transformed into the
frequency domain. The drawback is particularly
telling when the signal to be analysed is transitory in
nature or of finite duration, as in the case of mfERG
signals, whose frequency content changes over time.
The discrete wavelet transform (DWT) surmounts
this drawback by analysing the signal in different
frequencies with different resolutions, using regions
with windowing of different sizes and obtaining a
two-dimensional time-frequency function as a result.
Wavelet analysis uses finite-length, oscillating, zero-
mean wave forms, which tend to be irregular and
asymmetrical. These are the windowing functions
called mother wavelets. In principle there may be an
MULTIFOCAL ELECTRORETINOGRAPHY - Early Detection of Glaucoma based on Wavelets and Morphological
Analysis
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