I read with great interest the article by Farid and associates regarding the effect of multifocal intraocular lens (IOL) on nonspecific reduction of mean deviation (MD) upon Humphrey standard achromatic perimetry (SAP) 10-2 testing with Swedish Interactive Threshold Algorithm (SITA) standard thresholds.
Both multifocal and monofocal IOL groups involve optics with aspheric design to correct spherical aberrations. However, it was noted that within either group, some patients received blue-blocking IOL (multifocal [SN6AD1]: 5/22 = 22.7%; monofocal [SN60WF]: 2/15 = 13.3%) and some non-blue-blocking IOL (multifocal [ZMB00, ZMA00]: 17/22 = 77.3%; monofocal [ZCB00, ZA9003]: 13/15 = 86.7%), at a different proportion. It was a controversial issue whether blue-blocking IOLs affect contrast sensitivity (CS) at mesopic and photopic conditions, which might confound SAP performance. A subtle CS change might already alter central 10-2 visual field performance. Pierre and associates noted that the luminance contrast values of patients with yellow-tinted IOLs were significantly lower than those of patients with clear IOLs, in a series of 25 patients. In a second study, Jang and associates found no statistically significant difference on SAP indices, including MD, pattern standard deviation, and glaucoma hemifield test score, when one eye was implanted with blue-blocking IOL and the other non-blue-blocking. These observations have to be interpreted with caution, as Jang studied the central 30 degrees and their subjects had quite a wide range of visual field indices, signifying heterogeneous retinal sensitivity or performance within the subjects.
There is a lack of a reference database showing the inter-test variability of 10-2 fields. Maddess demonstrated that such effect would be more pronounced in the presence of increased scotoma depth, reduced stimulus size to III (employed by Farid and associates), and increased fixation jitter. In another study by Barkana and associates on 26 glaucoma patients, clinically significant variations were noted in individual instances by Bland-Altman analysis. At 95% limits of agreement, difference in MD was 3.4 dB and difference in number of depressed points at least at P < 2% was 11.2, between the first- and second-visit 10-2 SITA standard tests within a 2-month interval. Although Farid and associates studied visual field parameters on normal subjects, it would be much safer to address the issue of inter-test variability, rather than drawing conclusions of between-group MD difference at 1.87 dB based on 1 single field per subject.
Another conclusion made by the authors was the lack of MD improvement in the multifocal IOL group with time despite plausible neuroadaptation mechanisms. The authors should perform serial visual fields on the same subjects and observe a change in parameters, instead of comparing patients having perimetry early post operation to those after 6 months. This can avoid the likely high individual variability to recalibrate after surgery. Learning effect can be minimized if perimetry is introduced to subjects before cataract operation.