Thirty-seven eyes of 34 patients in whom the MD of the VFs crossed −20 dB fulfilled the inclusion criteria. Mean age (± standard deviation) of the patients was 65 ± 11 years. Twenty-three patients (67.7%) were male. MD, pattern standard deviation (PSD), and VFI of the included VFs are shown in
Table 1. The change in MD in the VF pairs ranged from 0.24 dB to 3.7 dB (median: 1.49 dB; IQR: 1.02 to 2.64) while the decrease in VFI when the MD crossed −20 dB showed a wide variability ranging from 3% to 33% (median: 15%; IQR: 10 to 18,
Fig. 2a). Median duration between the VFs of the pair was 1.5 years (IQR: 0.9 to 2.2 years). Twenty-eight stable glaucoma eyes from 26 patients in whom the MD crossed −10 dB were chosen as control eyes. Mean age (± standard deviation) of the control patients was 58 ± 11 years. Seventeen patients (65.4%) were male. The MD and VFI change in the VF pairs of control eyes ranged from 0.22 to 3.14 dB (median: 1.68 dB; IQR: 1.07 to 2.13) and 1% to 8% (median: 4%; IQR: 2–5), respectively (
Fig. 2b). Median duration between the VFs of the pair was 1 year (IQR: 0.6 to 1.3 years). The variability in VFI was significantly less around a MD of −10 dB compared to that around −20 dB (
P < 0.001, Wilcoxon rank-sum test).
We first evaluated if the change in VFI in the VF pairs where the MD crossed −20 dB was explained by the change in MD. For every dB change in MD, VFI changed by 2.6% (P = 0.01). However, change in MD explained only 16% of the variability of the VFI change (coefficient of determination, R 2 = 0.16).
We then investigated if the VFI change when the MD crossed −20 dB was explained by the difference in the number of normal points between the PDPP of the VF with MD better than −20 dB and the TDPP of the VF with MD worse than −20 dB.
Table 2 shows the number of normal points in the PDPP of the VF with MD better than −20 dB and the TDPP of the VF with MD worse than −20 dB. For every one normal point less in the TDPP of the VF with MD worse than −20 dB compared to the PDPP of the VF with MD better than −20 dB, the VFI decreased by 0.9% (
Fig. 3). Difference in the number of normal points between PDPP and TDPP of the VF pairs explained 47% of the variability in VFI change, and together with the change in MD, it explained 65% of the variability in VFI change. Change in MD was not associated (
P = 0.93) with the difference in the number of normal points between PDPP and TDPP of the VF pairs.
Knowing that the difference in the number of normal points between PDPP and TDPP influenced the VFI change, we went on to investigate how the eccentricity of these normal points influenced the VFI change.
Table 3 shows the multivariate model evaluating this. Evaluating the difference in the number of normal points zone-wise between the PDPP of VF with MD better than −20 dB and the TDPP of VF with MD worse than −20 dB, along with the MD change, explained 81% of the variability of VFI change. Change in MD was not associated with the difference in the number of normal points between the PDPP and TDPP in any zone (
P > 0.2 for all comparisons). The regression formula derived from
Table 3 to account for the change in VFI could be thus written as −6.49 + 2.67* MD change + 1.97* difference in the number of normal points in zone 1 + 1.49* difference in the number of normal points in zone 2 + 0.78* difference in the number of normal points in zone 3 + 0.66* difference in the number of normal points in zone 4.
Figure 4 shows an example of the MD of a stable glaucoma patient crossing −20 dB during the follow-up visits. While the MD changed very little from −19.77 dB to −20.01 dB, the VFI showed a drastic change from 50% to 35%. When the MD subsequently got better than −20 dB, the VFI again got back to 51%.
Figure 5 shows the PDPP of the VF with MD better than −20 dB and the TDPP of the VF with MD worse than −20 dB of the eye seen in
Figure 4. The MD change between these two VFs is 0.24 dB, and the difference in the number of normal points between the PDPP and TDPP in zones 1, 2, 3, and 4 are 0, 4, 10, and 7, respectively. Applying the regression equation in Table 3, the VFI change explained by the change in MD and the change in the VFI estimation strategy (from PDPP to TDPP) comes up to 13% (−6.49 + 2.67*0.24 + 1.97*0 + 1.49*4 + 0.78*10 + 0.66*7) and the corrected VFI to 48% instead of the estimated VFI of 35%.