This study found that trend analysis of visual field (VF) global indices may not entirely accurate for measuring rate of progression. Click image to enlarge.

In advanced glaucoma, where there is considerable risk of developing visual disability, estimating rate of progression is vital. However, researchers in Osaka, Japan, believe that trend analysis of visual field (VF) global indices may underestimate the rate of progression in advanced glaucoma due to the influence of test points without detectable sensitivity or blind locations. To test this hypothesis, they compared the rates of change of VF global indices with and without exclusion of undetectable points at various disease stages. They also developed targeted mean total deviation, an average of total deviation values that excludes undetectable locations. The results demonstrated that MD rate of progression is lower in severe glaucoma compared with earlier stages, and this apparent reduction becomes smaller by removing consistently undetected VF locations.

The study assessed 648 eyes of 366 glaucoma patients with eight or more reliable 30-2 standard automated perimetry tests over more than two years. Targeted mean total deviation was calculated as the average of the total deviation values after excluding locations that were consistently undetected (i.e., had a threshold sensitivity value of <0dB). Eyes were classified as early (≥-6dB), moderate (-6dB to -12dB), advanced (-12dB to -20dB) and severe (<-20dB) based on baseline MD. The rates of change of MD and targeted mean total deviation in each stage were statistically compared.

Overall, targeted mean total deviation rate of change strongly correlated with MD rate of change. The MD slope (-0.34dB/year) in severe glaucoma was significantly slower than the targeted mean total deviation slope (-0.42dB/year) and was slower than MD slopes in the other stages. The researchers noted that this difference between MD and targeted mean total deviation slopes was most prominent in eyes with MD values less than -25dB.

“The number of undetectable locations increased as MD decreased (worsened), and more than half of locations were already undetected in baseline tests in severe glaucoma,” they wrote in their paper. “These results support our hypothesis that the lower rate of MD slopes in severe glaucoma is caused by the presence of consistently undetectable, and thus non-progressive, locations.”

On the other hand, MD slope was significantly faster than targeted mean total deviation slope in early and moderate glaucoma. The researchers suggested the small but significant difference between targeted mean total deviation rate of change and MD rate of change in earlier stages may have possibly resulted from the fact that MD is more heavily weighted in central locations while targeted mean total deviation is not.

“It seems clear that conventional MD slope that shows positive slopes in 67% of cases with MD values less than -25dB is not appropriate for estimating the rates of progression in these eyes, and targeted mean total deviation slopes may be a promising alternative,” the researchers concluded.