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K.R. Alexander, A.S. Rajagopalan, G.A. Fishman; Relationship Between the High–Frequency Flicker ERG and the Derived Parameters of Cone Phototransduction in Retinitis Pigmentosa . Invest. Ophthalmol. Vis. Sci. 2005;46(13):514.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: ERG responses to a flicker frequency near 30 Hz are often used clinically to evaluate the integrity of the cone system in retinal diseases, but these responses can be influenced by the properties of postreceptoral neurons. The present study investigated the relationship between high–frequency ERG responses and the derived parameters of the activation phase of cone phototransduction in a group of patients with retinitis pigmentosa (RP) to determine whether the high–frequency ERG can provide an index of the integrity of the cone photoreceptors. Methods: Light–adapted, full–field ERGs were obtained from 15 patients with various forms of RP and from 12 visually normal, age–equivalent control subjects. Responses were recorded to four xenon flash luminances (2.8 to 4 log scot td–s) presented against an achromatic rod–desensitizing adapting field. ERG responses were also measured to middle–wavelength, sinusoidally flickering light of 100% contrast, at frequencies ranging from 8 to 100 Hz, presented against a short–wavelength, rod desensitizing adapting field. The leading edges of the a–waves of the brief flash responses were fit with a delayed Gaussian model of cone phototransduction, and the parameters Rmp3 (maximum response amplitude) and S (sensitivity) were derived. The amplitudes and phases of the ERG responses at the stimulus fundamental frequencies were obtained from spectral analysis. Results: The majority of patients with RP showed a reduction in Rmp3, and approximately half showed a reduction in S as well. At temporal frequencies of 45 Hz and higher, the log fundamental amplitudes of the patients with RP were reduced uniformly below those of the control subjects. For both the patients with RP and the control subjects, the mean log amplitude of the flicker ERG across temporal frequencies of 45 Hz and higher was correlated significantly with the sum of log Rmp3 and log S (r2 = 0.85, 0.86, for RP patients and control subjects, respectively; p < 0.001). This relationship was not observed at temporal frequencies below 45 Hz. Conclusions: The properties of cone phototransduction are the primary determinant of ERG amplitude at flicker frequencies of 45 Hz and higher, even though the cone photoreceptors themselves appear to make little direct contribution to the ERG under these conditions. The high–frequency flicker ERG may thus provide a more direct assessment of the integrity of the cone photoreceptors in retinal diseases than the lower flicker frequencies conventionally used.
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