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CS Barnes, KR Alexander, S Grover, GA Fishman; Cone ON-pathway defect contributes to ERG implicit time delays in carriers of X-linked retinitis pigmentosa (XLRP) . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1198.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Carriers of XLRP frequently show prolonged implicit times in the clinical electroretinogram (ERG) response to a 32-Hz flickering stimulus. This study tested the hypothesis that a response attenuation within the cone depolarizing (ON) bipolar cell (DBC) pathway is a major contributing factor to the prolonged implicit times of XLRP carriers. This hypothesis was based on several considerations, including a recent vector summation model of the ERG response fundamental (Kondo & Sieving, 2001), which demonstrated that a relative attenuation of the DBC response component induces a phase lag (corresponding to a prolonged implicit time) in the fundamental of the 32-Hz flicker ERG response. Methods: Full-field light-adapted ERGs were recorded from 8 carriers of XLRP (ages 35-64) and 12 controls (ages 34-64). Stimuli included 4- and 8-Hz sawtooth flicker to elicit ON and OFF responses, and sinewave flicker at temporal frequencies ranging from 8 to 96 Hz. Amplitudes and phases of the ERG responses to the sinewave stimuli were derived from spectral analysis and were analyzed within the framework of the vector summation model. Results: The carriers of XLRP showed a range of ERG amplitudes at 32 Hz, from normal to markedly subnormal. There was a statistically significant correlation (r = 0.91, p < 0.01) between their log fundamental response amplitudes at 32 Hz and their response phases at this frequency. Further, the carriers' response phases at 32 Hz were correlated significantly (r = 0.90, p < 0.01) with the ratios of their response amplitudes at 32 vs. 12 Hz, such that a reduced amplitude ratio was associated with a greater phase delay. The reduced amplitude ratio was interpreted as evidence for an attenuated response within the DBC pathway, based on the vector summation model. Additional evidence that an ON-pathway defect contributed to the ERG response delays was provided by the observation that the b-wave response to rapid-on sawtooth flicker was more affected than the d-wave response to rapid-off flicker in those patients who had phase lags at 32 Hz. Conclusion: The sinewave and sawtooth flicker ERG results indicated that carriers of XLRP can have a response attenuation within the DBC pathway. This response attenuation appears to be a primary determinant of their prolonged flicker ERG implicit times.
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