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M. Gauvin, J.-M. Lina, J. Racine, P. Lachapelle; Quantifying Residual ERGs With the Discrete Wavelet Transform : Beyond Amplitude and Time Measurements. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1489.
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With time, degenerative retinopathies, such as RP, will gradually modify the amplitude and morphology of the ERG signal to the point where it will become indistinguishable from noise, thus making it almost impossible to extract diagnostically and prognostically significant information from the time-amplitude domain analysis despite clinical evidence of visually guided behaviours. The purpose of this study was to examine if the use of the discrete wavelet transform (DWT) approach could help identify remnant features of the normal ERG in residual responses.
DWT analysis was performed on photopic ERGs obtained from normal subjects [n=75; photopic hill (PH): 15 flash stimuli (from -0.78 to 2.84 log cd.sec.m-2); background: 30 cd.m-2] and patients (n=65; flash intensity: 0.69 log cd.sec.m-2) presenting with a variety of retinal disorders and severely depressed ERGs (<20% of normal amplitude). DWT was performed on each ERG waves of normal PH and key descriptors (Holder exponent and wavelet coefficient maxima) were characterized and used to analyse the pathological ERGs.
Analysis of the normal PH reveals that the Holder exponent follows a parabola that starts at 0.763 log µV2.Hz-2, peaks at 1.061 log µV2.Hz-2 and ends at 0.707 log µV2.Hz-2, while some of the local wavelet maxima (corresponding to the b-wave and OPs) seems to follow a PH like distribution (that starts at 3.1 log µV, peaks at 4.87 log µV and reaches a quasi-plateau at nearly 4 log µV) or a logistic growth function (a-wave) that starts at 2.65 log µV and reaches its maximum value at 4.73 log µV. In half of the pathological ERGs, the wavelet maxima matched that found in normal ERGs evoked at low stimulus intensities (a-wave >= 2.65 log µV.ms, b-wave >= 3.1 log µV.ms), while in the nearly extinguished ERGs category (<10% of normal) the wavelet coefficient was between 0.8 and 2.5 log µV.
The ERG descriptors found with DWT represent a useful tool to identify subtle features of residual ERGs. This method should allow a more accurate quantification of ERG responses, especially those in the low-voltage range. Use of this analytical approach should also facilitate follow-up estimates of disease progression in patients initially presenting with a nearly extinguished ERG or identify diagnostically significant frequency domain changes in pathological ERGs; a feature not currently used in clinical electroretinography.
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