We conducted a retrospective review of patients who were being followed up in our glaucoma clinic between January 1, 2011, and November 28, 2016. Our study was approved by the local ethical committee and adhered to the tenets of the Declaration of Helsinki. Seventy-eight glaucoma patients were included in the study.
All participants were 18 years or older. Media opacities that could affect RNFL measurements, including corneal scars, posterior subcapsular cataracts, anterior segment dysgenesis, history of diabetic retinopathy, visual acuity worse than 20/40, and refractive error outside the − 6 D to + 6 D range, comprised the exclusion criteria. Patients with retinal, choroidal, or other optic nerve pathologies, demyelinating diseases and intracranial tumors affecting the VF were also excluded. Our subjects were diagnosed as primary open angle glaucoma or pseudoexfoliation glaucoma. They were all under topical anti glaucomatous treatment. No one of them were treated surgically. Diagnosis criteria for glaucoma were suspicious optic nerve head with borderline hemifield VT result or abnormal hemifield VF test with abnormal OCT with or without elevated intraocular pressure. An abnormal visual field result was defined as having a pattern standard deviation outside the 95% confidence limits or a glaucoma hemifield test (GHT) result outside the reference range on the Swedish interactive threshold algorithm (SITA Standard 24 − 2) of the Humphrey visual field analyzer (Carl Zeiss Meditech, Inc., Dublin, California, USA) [13, 14]. Suspicious disc means that the optic disc with cup/disc asymmetry or bilateral wide cups and thin rims [15].
Optical Coherence Tomography
OCT measurements were performed using Cirrus –HD 5000 OCT (Carl Zeiss Meditech, Dublin, California). The scan rate of the device is 26,000 A scans per second with an axial resolution of 5 µm. Optic nerve head analysis was performed for all patients. The software outputs discrete RNFL thicknesses for each clock-hour position for the right eyes and the opposite for the left eyes. A signal strength greater than or equal to 6/10 was accepted for the study. The images were obtained by the same technician. RNFL thicknesses criteria was database range at the 1% level (outside normal limits) [16]
VF Analysis
Tests were performed using the Humphrey 750i (Carl Zeiss Meditech, Dublin, California) Swedish Interactive Threshold Algorithm 24 − 2 test. The tests were grouped into early, moderate, and severe glaucoma defects as described by the Hodapp-Parish-Anderson classification. All tests were performed by the same technician.
Garway-Heath maps were created with OCT sectors and the VF pattern deviation map for each patient. Yanagisawa and colleagues [17] named the 12 OCT sectors as S, ST, TS, T, TI, IT, I, IN, NI, N, NS, and SN in the clockwise direction for the left eye. They matched these sectors in the VF and showed them the colored squares. By using this method, we gave numbers for each OCT sectors from 1 to 12 and matched them in the VF (Fig. 1).
We applied the same in the counterclockwise direction for the right eyes.
Statistical Analysis
First, we wanted to find any correlation between sector RNFL thickness and the number of absolute defects in the matched VF region. Thus, we used correlation analysis. To generate an equation for the defect amount for every OCT sector, we first used linear regression analysis. Correlation analysis was applied for the number of absolute defects we found by using our formulas and the number of the defects of the same patients’ corresponding RNFL sectors’ matched VF location.
Correlation and linear regression analysis were performed using SPSS version 16 software (IBM, Armonk, NY, USA). If the t value calculated by the software was significant, we generated the regression equation by using the parameters calculated by the software. The linear regression equation is shown as
y = a + bx
If was not significant we used exponential function method to generate non-linear equation described by Marquart [11]
The non- linear exponential equation is shown as
y = aebx
where y represents the number of absolute defects and x is the value of the RNFL at the sector, e is equal 2,71, a and b are constants.