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M.F. Marmor, G. Hochstein, F.E. Molnar; Intrasession Variability of the Full–Field ERG . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1655.
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Variability of the normal ERG makes it difficult to follow small changes in retinal disease, and to study new therapeutics or toxicity. As a first step towards minimizing this variability, we have studied the stability of ERG responses within single recording sessions.
ISCEV Standard ERGs were recorded from 8 healthy volunteers aged 19 – 32, and scotopic "maximal" and photopic "cone" b–wave amplitudes were measured. Multiple flashes were presented at different interflash intervals and after different periods of dark– and light–adaptation. The stability of the stimulus flash was measured with a photometer.
Under ISCEV standard conditions, the coefficient of variability for repeated stimuli was near 3% for scotopic responses and 5% for photopic responses. Relative to the "standard" 10 sec. interflash interval, the scotopic b–wave was significantly larger using a shorter 5 sec. interflash interval, and was slightly smaller with intervals of 20 sec. Increasing dark adaptation time from the "standard" 20 min. to 30 and 40 min. resulted in significantly larger b–wave amplitudes; there was a similar (but not statistically significant) rise in amplitude when light–adaptation time increased from 10 min. to 15 and 20 min. The flash stimulus showed roughly 2% variance in illuminance with repeated discharges, within the interflash intervals studied. Parallel recording of flash intensity and b–wave amplitude showed only very weak correlation, indicating that these variations are largely independent.
Intrasession variability is much less than the 20 – 30% intersession variability reported in the literature. Nonetheless, it represents a basis of variability that will affect all records, and that may be minimized if the causes can be determined. Review of the original ERG tracings showed that baseline drift and occasional unusual waveforms are contributing factors. Variations in adaptation time and interflash interval, which are not uncommon in a clinical setting, may be contributors to intersession variability. The unexpected finding of larger b–waves after shorter interflash stimulation intervals may reflect differing adaptation characteristics among inner and outer retinal generators that summate to produce the ERG response.
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