Kulkarni and associates present an important study by correlating different scoring systems of visual field defects in glaucoma patients with their vision-related functional impairment. Not only did they implement the most commonly used questionnaire instrument in ophthalmology, the 25-item National Eye Institute Visual Function Questionnaire (NEI VFQ-25), functional impairment also was assessed by performance-based measurements (the Assessment of Disability Related to Vision Test [ADREV]). This is a very important and relatively new approach to obtain an idea of the real-life functional impairment of the glaucoma patient. The authors report that the simple visual field scoring with the mean defect (MD) performs very well with the functional impairment in terms of NEI VFQ-25 and ADREV scores. With the data presented in Tables 2 and 3, MD is correlated moderately with the NEI VFQ-25 composite score ( r = 0.42 and r = 0.39 for better and worse eye, respectively) and the ADREV score ( r = 0.47 and r = 0.40 for better and worse eye, respectively). Looking at these data, it is hard to understand the authors statement, that “correlations with the worse eye were far weaker” than for the better eye. With regard to the MD data above, this equals a difference in the correlation coefficient of 0.03 for the NEI VFQ-25 and 0.07 for the ADREV.

In a recent publication by Gothwal and associates using the glaucoma-specific questionnaire Glaucoma Activity Limitation 10, the correlation between MD and Glaucoma Activity Limitation 10score was −0.40 for the better eye and −0.35 for the worse eye, and therefore in a comparable range with the results of the present study. However, there was a far better correlation of the Glaucoma Activity Limitation 10score with the visual acuity of the worse eye ( r = 0.49) than with the visual acuity of the better eye ( r = 0.35). In our own study evaluating the Glaucoma Activity Limitation 9 (GAL-9) questionnaire, we also found a better correlation between the GAL-9 score and the visual acuity of the worse eye ( r = 0.42) than with the visual acuity of the better eye ( r = 0.32). However, regarding the MD, in our study the GAL-9 score was correlated even better with the MD of the worse eye ( r = −0.45) than with the MD of the better eye ( r = −0.30; the differences in prefixes are the result of the Rasch-based scaling method, which led to higher GAL-9 values for more impaired glaucoma patients). Furthermore, the authors should have discussed that it is very likely, although not yet well examined, how functional impairment of glaucoma patients is influenced by the patterns of visual field defects.

Having these data in mind, one should be very cautious about stating that the worse eye had “far weaker” influence on the glaucoma patient’s functional impairment. It is much more likely that the worse eye has a very similar influence on quality of life or functional impairment compared with the better eye. The quality-of-life research in ophthalmology regarding better and worse eye has just begun, and one of the things we have understood until now is that quality of life is driven not only by the better eye, with the influence of the worse eye being underestimated.