Performance of the Visual Field Index in Glaucoma Patients With Moderately Advanced Visual Field Loss

PurposeTo explore the relationship between the visual field index (VFI) and the visual field mean deviation (MD) in glaucoma patients with moderately advanced perimetric damage and to identify the magnitude of the boundary effect of VFI that occurred when the VFI estimation strategy changed from pattern deviation probability value to total deviation probability value as the MD crossed −20 dB in longitudinal visual field (VF) series.DesignA retrospective cohort study of longitudinal data analysis.MethodsThe MD and VFI values obtained from VF tests conducted on 148 eyes of 148 glaucoma patients having an MD around −20 dB were studied. A total of 1286 VFs with MD values within the range of −16 dB to −24 dB were included. The eyes were divided into 2 groups, with the first having serial MDs all better than or all worse than −20 dB and the second with serial MDs crossing the −20 dB value. Change in MD (ΔMD) was defined as the absolute difference between the MD values of 2 consecutive VFs. Based on the 2 VFI values of the same VFs, the absolute value of change in VFI (ΔVFI) was calculated.ResultsThe means (± standard deviation) for the ΔVFI were 4.17% (± 3.3%) in the group of eyes with MDs on either side of −20 dB, and were 15.8% (± 8.4%) in the group with MDs crossing −20 dB (P < .0001). For ΔVFI/ΔMD, these values were 6.8%/dB (± 10.5%) when the range of MD falls on either side of −20 dB, and 7.9%/dB (± 6.2%) when the range of MD crosses the −20 dB values (P = .042).ConclusionsThe values of the VFI become highly variable in serial VFs of eyes with MDs crossing −20 dB, in comparison to those VFIs associated with MDs on either side of −20 dB. The likelihood for this effect is the change from use of pattern deviation probability value to total deviation probability value in the points included in the calculation of VFI at −20 dB of MD. The development of indices to measure VF rates that are free from this boundary effect in moderately advanced glaucoma is desirable.