The correlation of RNFL with VF abnormalities has already been reported in the literature.
21 22 In our study, we compared structural and functional parameters of glaucoma patients with and without VF abnormalities. The discriminating power of OCT has been described between normal subjects and patients who have glaucoma.
23 24 We compared our data to those from normal subjects, but focused mostly on our patients with PPG and POAG (i.e., we tried to estimate to what degree RNFL structural loss is responsible for VF changes). First we analyzed the RNFL AVG versus different VF parameters. We found no correlation in the PPG group, but a significant correlation in the POAG eyes. Based on this difference, we refined our grouping criteria with another parameter, dividing all eyes according to the best discriminating parameter, PSD, with a cutoff value of 1.9 dB. Subsequent correlation and regression test results suggested that eyes having a milder VF defect (i.e., PSD below 1.9 dB) were very similar to the eyes of the preperimetric group. Nouri-Mahdavi et al.
25 described that in early glaucoma, with their group average MD of −2.9 dB, no correlation was found between MD/PSD and RNFL AVG thickness using the OCT 2000 (Carl Zeiss Meditec, Inc.). It has been noted that in the earlier stages of glaucoma a so-called functional reserve seems to exist, but this may be only a result of the logarithmic decibel scaling of the VF or due to the effect of the logarithmic scale, which minimizes sensitivity changes at high dB levels but maximizes changes at low dB levels.
26 Our study found that this might indeed be the case, due to the overlap of both RNFL thickness and VF sensitivity distribution in normal, preperimetric and early stages of glaucoma. It is notable that almost all the eyes in the preperimetric group had an RNFL thickness of 68.0 μm or more, and separation of our patients based on PSD showed an almost identical minimum RNFL AVG (70.73 μm). Using OCT, the average RNFL thickness in normal eyes has been reported to vary from 90 to 128 μm.
9 15 16 22 24 27 28 We found an average RNFL thickness of 96.48 ± 8.24 μm in our normal group. Thus, a mean value of RFNL thickness of approximately 70 μm—even considering its wide range of interindividual variability—may represent a profound threshold value in glaucomatous structural changes. To our knowledge, there are no published data estimating RNFL loss in the progression of glaucoma in the presence of VF defects, as measured by StratusOCT. Our study suggests that all normal, preperimetric, or early glaucomatous eyes could be found with AVG above 68.0 μm, whereas RNFL/VF changes become significant in eyes with AVG below this value. This observation is also consistent with postmortem histologic measurements in patients with glaucoma, indicating that at least 25% to 40% of retinal ganglion cells were lost before abnormalities were statistically detected by automated visual field testing.
29 30