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Zahra Derafshi, Hadi Tajalli, Sanitta Thongpang, Justin Williams, John R Hetling; Corneal ERG Topography in Healthy Rat Eyes and Eyes with Focal Retinal Lesions. Invest. Ophthalmol. Vis. Sci. 2014;55(13):6180.
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Spatial differences in ERG potentials recorded from different locations on the cornea (ERG topography) reflect spatial differences in retinal activity, and may therefore have diagnostic value. Multi-electrode electroretinograms (meERG), consisting of 25 simultaneously recorded ERG waveforms, have been recorded from rats using a Contact Lens Electrode Array (CLEAr Lens) to evaluate sensitivity of ERG topography to local retinal lesions.
meERG responses were recorded from seven Long Evans rats following full-field flash stimuli; animals were prepared as for conventional ERG (dark adapted, general anesthesia, pupil dilation, corneal anesthetic). Four out of the seven rats then received local damage (adjacent to optic disk but restricted to one hemisphere) using laser photocoagulation, and a second set of meERG responses was recorded from all rats, resulting in nine healthy eye data sets and 4 lesion eye data sets.
To evaluate spatial symmetry in ERG potentials, the a-wave amplitudes measured on each of the 12 peripheral electrodes were normalized to the average amplitudes on the central five electrodes, which resulted in 12 ratios for each eye. These ratios were averaged (by electrode position) for all nine healthy-eye meERG responses to form a normative data set (containing 12 average ratios). In laser damaged eyes, ratios were reduced compared to healthy eyes in areas of the cornea closest to the area of laser damage at the retina. A cluster analysis was performed, using the 12 ratios as coordinates in a 12-dimensional space, and calculating the Euclidean distance between each response and the normative mean response. Average (± one SD) distances for healthy eyes (using a leave-one-out technique) was 6 ± 2 and for laser damaged eyes was 12 ± 5. Using these distances as the sole metric to distinguish healthy from laser-damaged eyes resulted in the area under an ROC curve of ~90%.
Spatial differences in a-wave amplitudes across the cornea are altered for rat eyes having a local lesion in the retina. Lesions hear the posterior pole of the retina resulted in measureable changes in corneal ERG topography (evaluated by the meERG-derived ratios), yielding good sensitivity and specificity. Corneal ERG topography is a novel source of information, independent of absolute ERG amplitudes, and obtained using relatively simple full-field stimuli.
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