Purpose:
We evaluated the global visual field indices mean deviation (MD) and visual field index (VFI) in glaucoma patients to observe the behavior of VFI around MD values of -20 dB.
Methods:
We collated the visual field indices in eyes from patients whose visual field (VF) time series had two consecutive VFs where the MD values had changed from less than to greater than -20.9 decibels or vice versa. We only considered cases where the patient’s age (from the 1st to the 2nd test) stayed within the age-matched normal decile to systematically eliminate any change seen in the MD due to a change from the age-normal values alone. The visual field index (VFI) and mean deviation (MD) values were calculated with StatPacTM.
Results:
The VF indices from consecutive visual field readings (n=33 pairs) had an average time period difference of 0.69±0.6 years. The average absolute difference between the pairs for MD and VFI were 3.4±2.9 dB and 26.6%±12%, respectively. The Figure is a scatterplot showing the relationship between the two differences, with boxplots showing the range of values. The red box shows those points whose MD changed by less than 2.0 MD (mean dB=1.3 +0.4 dB) with the change in VFI (mean =15.7 + 4.5%).
Conclusions:
A method to estimate the rate of visual progression in glaucoma, VFI, has been reported. Here we detect a drastic change in VFI when shifting from a MD greater than -20 dB to a MD less than -20 dB. Specifically, if a 100% drop in VFI roughly corresponds to a mean deviation defect of -30dB, then the slope is in the magnitude of 3.3% drop per -1dB. However, here we see a 3x-10x magnification change in slope sensitivity near the -20dB cutoff. A linear regression of the VFI has been suggested to represent the global visual field rate of progression; however it should be noted that a large change or discontinuity in slope may occur for calculating the VFI around MDs of -20dB boundary.