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John Hamilton, Antony Theogene, Rejean Munger, Stuart Coupland; Refractive Impact on the Espion Multifocal ERG in Clinical Practice. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5850. doi: https://doi.org/.
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The multifocal electroretinogram (mERG) enables simultaneous recording of central, localized electroretinal function. A significant difference in the response density of the mERG has been suggested for every 2-diopter change in refraction, which may be attributable to changes in the retinal image size induced by the corrective lens used. There is currently no clear concensus in regards to the importance of correction during mERG testing, and no study has ever investigated the refractive impact on the Diagnosys Espion mERG. The purpose of this project is to investigate the influence of refraction, or defocus, on Espion mERG response.
The refractive impact on mERG results was studied monocularly in 30 healthy volunteers. Multifocal ERGs were recorded according to ISCEV standards using a 61 hexagonal elements stimulus projected on the central 30 degrees surrounding the fovea. Recordings were obtained using DTL electrodes with a corrective lens in front of a cycloplegic eye. Reference ear clip and ground forehead electrodes were used. Magnification caused by the trial lens was counteracted through software by changing the stimulus size. Participants were subject to three testing conditions: (1) uncorrected with plano lens, (2) corrected without magnification adjustment, (3) corrected with magnification adjustment. Near correction involved distance Rx plus +3.00 add. The difference between conditions 1&3 were attributable to optical defocus, whereas the difference between conditions 2&3 were a result of magnification. Tests were done over three sessions on different days. Each subject's ring average results were analyzed for P1 response density amplitude and timing differences between each refractive condition.
There was a significant effect of defocus on mERG P1 amplitude but not timing (p<0.01). Magnification induced by the corrective lens used also significantly affected mERG P1 amplitude but not peak latency (p<0.05). The effects of image blur and magnification were greatest in ring 1 and decreased peripherally.
Both image blur and magnification significantly affect Espion mERG P1 amplitude but not peak latency. The Espion mERG stimulus must be focused and scaled accordingly to accurately assess central retinal function. This study demonstrates the importance of refraction towards standardizing testing conditions and optimizing mERG response in clinical practice.
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