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C.R. Seaman, K.G. Locke, D.C. Hood, D.G. Birch; Rapid Clinical Assessment of A-Wave Photoreceptor Function . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1884.
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Purpose: Parameters relating to the phototransduction process can be obtained from the ERG a-wave in response to intense flashes. The parameter, Rmax, is a measure of the maximum receptor response from the retina and S is a measure of receptor gain or sensitivity. Present techniques require access to digital waveforms and computational modeling. Our goal was to determine a quick, easy, and accessible method for analyzing a-waves. Methods: Full-field ERG testing was conducted on 40 patients with retinitis pigmentosa with detectable rod and cone a-waves and on 40 normal subjects. Four white flashes, 4.4, 4.0, 3.7 and 3.2 log scot td-s were presented first to the dark-adapted eye and then in the presence of a background light (40 cd/m2). The "gold standards" for the rod isolated S and Rmax values were parameters derived from fits to the phototransduction model in Matlab holding td constant at 3.2 ms. For the fast graphical method, one of the rod isolated waves from these high intensity flashes (4.0 log scot td-s) was used. A diagonal line was drawn through the leading edge of the rod isolated a-wave and a second line was drawn horizontally through the maximum deflection. The maximum response (Amax) was read directly from the y-axis (amplitude) and the intersection between the diagonal line, x-axis (time), and the horizontal line gave tAmax. Results: The average difference between log Amax and log Rmax was 0.02 log µV and 95% of all differences lay within 0.06 log unit of the mean. Amax highly correlated with Rmax both in patients (r2 = 0.99) and in normal subjects (r2 = 0.95). The sensitivity parameter tAmax derived from the graphical approach correlated well with S both in patients (r2 = 0.71) and in normal subjects (r2 = 0.76). Conclusions: A single intense flash produces valid parameters describing photoreceptor sensitivity and maximum rod receptor response. This method for analyzing ERG a-waves will benefit clinicians by decreasing the amount of time required both to test patients and to interpret their a-wave data.
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