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Editors Selection IGR 8-4

Electrophysiology: Blue-on-yellow checkerboards mfVEP

Comment by Vittorio Porciatti on:

23857 Identifying preperimetric functional loss in glaucoma: a blue-on-yellow multifocal visual evoked potentials study, Arvind H; Graham S; Leaney J et al., Ophthalmology, 2009; 116: 1134-1141

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In early glaucoma, demonstration of functional loss that corresponds to observed structural changes may lend more confidence in the diagnosis. Arvind et al. (650) recruited 30 patients with at least one glaucomatous optic disc but bilaterally normal white-onwhite standard automated perimetry (SAP). All patients had ≥ 6/9 corrected visual acuity and no posterior subcapsular opacities. Optic discs were graded by two examiners based on stereo-photographs, and RNFL thickness (Stratus OCT, fast RNFL program) was also measured. In order to find evidence of functional loss, the authors recorded multifocal VEP (mfVEP) in all patients, in response to blue-onyellow checkerboards (BonYmfVEP) presented sequentially to either eye. The rationale of using blue-on-yellow stimuli was analogue to that used for Short Wavelength Automated Perimetry (SWAP); that is, neurons selectively activated by the blue stimuli are relatively sparse in the retina and are thought to have little functional reserve. BonYmfVEP of each eye were used to calculate inter-eye asymmetries in amplitude, which were compared with those of a database of 60 normal controls. Fourteen of 30 patients demonstrated significant abnormality of BonYmfVEP amplitude asymmetry. The sign of amplitude asymmetry corresponded to the eye with the worse disc, and the mfVEP defect was consistent with topographical location of disc abnormalities in 13 of 14 patients. Mean asymmetry of RNFL thickness was significantly higher in patients with mfVEP defects compared to those without mfVEP defects. Inter-eye asymmetries in mfVEP latency were inconsistent with optic disc topography.

Achromatic-contrast mfVEP have been already shown by several investigators to have a role in glaucoma detection, also taking into account inter-eye asymmetry. According to Arvind et al., amplitude asymmetry of BonYmfVEP seems to be a promising tool to identify functional loss in preperimetric glaucoma. This conclusion needs to be taken with some caution, however. One should consider that the mfVEP is very variable among subjects. In order to reduce variability, the authors normalized the mfVEP amplitude to the underlying EEG amplitude. To further reduce variability, inter-eye amplitude difference was normalized to the sum of the amplitude of the two eyes, thereby isolating inter-eye amplitude asymmetry. While inter-eye asymmetry may signify glaucoma even before normal structural-functional measures are outside the normal range of values, asymmetry is present only in a fraction of cases. In this study, the patient population was selected as having rim changes truly suggestive of glaucoma, and with optic disc asymmetry with identifiable location of the worst rim. BonYmfVEP inter-eye asymmetries correctly identified less than half of the patients. In less ideal conditions (e.g., subjects with less asymmetric discs and early cataracts), the BonYmfVEP may identify functional defects in a smaller percentage of patients. In this study, SWAP was not performed. It would have been important to know the percentage of abnormalities in a corresponding perimetric test.

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