The OCT, FDT and PERG examinations of 52 patients, consisted of 28 women and 24 men aged between 44 and 76
years (mean 56
years), were analyzed. All patients had an intraocular pressure (IOP) greater than 21
mmHg (mean 23.96
mmHg) and with no other ocular (e.g., cataracts or other opacities) or systemic diseases. The patients all had normal visual acuity (VA), a normal optic disk (in particular, with no sign of diffuse thinning or focal narrowing or notching of the neuroretinal rim, hemorrhage, cupping or visible or progressive changes in the fiber layer on ophthalmoscopic examination with a +78 diopter lens) and mean defect (MD) and pattern standard deviation (PSD) perimetric indices of less than 1.5
1.1 and 0.65
0.4, respectively). All patients were recruited from the Glaucoma Service of the S. Orsola-Malpighi Hospital of Bologna.
The control group consisted of 55 subjects: 28 males and 27 females aged between 42 and 75
years (mean 54.8
years). The healthy controls had no ongoing eye or systemic disorders nor such a history, an IOP of less than 21
mmHg, a normal optic nerve and normal visual field indices.
A normal visual field was defined by the absence of each of these responses: a cluster of 3 points lower than P
5% or a cluster of 2 points lower than P
1% on a pattern deviation plot, or PSD with P
Only one eye per subject, both in the OH and the control groups, was randomly chosen if both eyes were eligible for the study.
The study was approved by the Local Ethics Committee of the S. Orsola-Malpighi Hospital, Bologna. An oral informed consent was obtained from all the patients.
All patients underwent an ophthalmologic examination including visual acuity, applanation tonometry IOP assessment, corneal radius curvature measurement with automated keratometry (RK, Canon Inc., Tokyo, Japan), corneal thickness evaluation with a Tomey SP3000 pachymeter (Tomey Corp., Nagoya, Japan), biomicroscopy of the anterior and posterior segment with automatic measurement of the cup/disc (C/D) area ratio of the optic nerve head with an Stratus OCT 3 (Zeiss-Humphrey, Dublin, CA, USA). SAP was also performed with a Humphrey Field Analyzer-30.2, using the full-threshold program (Zeiss-Humphrey, Dublin, CA, USA). Three visual field tests were performed for each OH and healthy control individual, and only the results of the third test were assessed. All subjects underwent assessment of the RFNL with the Stratus OCT 3 (Zeiss-Humphrey, Dublin, CA), FDT perimetry with the FDT Visual Field Instrument (Welch-Allin FDT, Skaneatsles Falls, NY, USA and CarlZeiss, Meditec Inc., Dublin, CA, USA), and PERG with the RetimaxPlus system (CSO Instruments, Florence, Italy). For FDT, only the results of the third test were assessed, and the FDT and SAP were reviewed separately by two investigators who were blinded to any clinical data.
The OCT technique makes it possible to analyze the retinal structures and obtain in vivo tomographic sections illustrating the histological retinal structure. This instrument use low coherence interferometry principles, which separate the retinal microstructures by measuring the echo delay of the light reflected and retrodiffused by these structures. The OCT 3 makes it possible to obtain scans with an axial resolution of 10
μm and a transverse resolution of 20
μm. The instrument projects onto the retina a beam of light generated by a superluminescent diode, with a infrared wavelength (820
nm). The system detects, processes and stores the retinal delay patterns and displays and stores the selected scans so that they can be subsequently analyzed. Each eye was dilated with 1% tropicamide before recording the images, and internal fixation was chosen because it provides better reproducibility than external fixation.
This instrument was used to directly measure the thickness of the peripapillary RNFL using the RNFL Thickness Averaging program, in which three consecutive, circular scans are performed, each one 3.4
mm in diameter and centered on the optic nerve. The thickness of the fibers was defined as the number of pixels obtained between the anterior and posterior RNFL projection. The values obtained with each scan are displayed in graphical form, resembling a clock face divided into four quadrants, representing the superior, nasal, inferior and temporal sections of the RNFL expressed in microns.
FDT perimetry is a new technique designed for the rapid and effective identification of visual field impairment due to glaucoma. The FDT stimulus consists of a bar grid with a low-frequency spatial sinusoidal profile (0.25 cycles/degree) subjected to a sinusoidal temporal commutation at a frequency of 25
Hz. FDT is based on the principle of the frequency-doubling illusion, in which the subject perceives twice the number of bars actually presented [18
]. The cells that present a nonlinear response to the contrast in the test image, which are therefore responsible for this illusion, are a subgroup of M cells [19
FDT was performed using the full-threshold program N-30. With this test, target stimuli consisted of individual sinusoidal gratings, 10 degrees square at 0.25 cycles/degree, alternately flashing at 25
MHz. Targets were in one of the 19 areas within the central 30 degrees of the visual field. For each visual field, we evaluated the mean defect (MD) and the pattern standard deviation (PSD).
For PERG recording, we followed the International Society for Clinical Electrophysiology of Vision (ISCEV) standard guidelines [20
The patient sat in a chair at a distance of 114
cm from the television screen. The generated potential was measured with skin electrodes. The ground electrode was placed on the right ear lobe, and the interelectrode resistance was less than 3
kW. The PERG stimulus was a black-and-white checkerboard with a contrast of 99% at 1.6 cycles/degree, four reversal/s and a mean luminance of 110
. The monitor screen subtended a visual angle of 12.5°. The refraction of all subjects was corrected for the viewing distance. No mydriatic or miotic drugs were used. The transient PERG response was characterized by three subsequent peaks that, in normal subjects, are indicated on the basis of polarity and latency: N35, P50 and N95. The P50 amplitude was measured from the trough of N35 to the peak of P50. In some patients, the N35 was poorly defined; in these cases, N35 was replaced with the average between time zero and the onset of P50.
All statistical analyses were performed using the FASTAT Version 2 software package (Systat Inc., Evanston, Illinois). Unpaired student’s t
-test was used taking p
0.05 as significant. The Chi-square test was used for categorical data. The Receiver Operating Characteristic (ROC) curve analysis was performed to determine the diagnostic sensitivities and specificities of OCT, FDT, and PERG. To compare ROC curve areas, Delong test was used.