Short-term inter-visit variability of erg amplitudes in normal subjects and patients with retinitis pigmentosa.

PURPOSE: To evaluate the short-term test/retest variability in visually normal subjects and patients with retinitis pigmentosa (RP), and to assess the effect of stimulus intensity and baseline amplitude on electroretinogram (ERG) variability.
METHODS: Eighteen patients with RP and nine visually normal subjects had a series of three unilateral ERGs, with an inter-visit interval of no less than 2 days and no more than 2 weeks. Responses to dark-adapted and both light-adapted single flash and 32 Hz flicker stimuli were recorded from a dilated eye over a range of stimulus intensities. B-wave amplitudes were compared to baseline amplitudes recorded at initial visit, and the resulting inter-visit percent difference was compared between stimulus intensities. Inter-visit variability was determined by one-way repeated measures analysis of variance using a 95% confidence interval to calculate threshold criteria for significant change. Analysis of variance followed by Bonferroni test for pairwise comparison was used to test for differences in inter-visit variability between two RP patient subgroups (higher versus lower baseline amplitudes) and visually normal subjects. The effect of stimulus intensity on amplitude reproducibility was also assessed.
RESULTS: Threshold for significant increase or decrease in inter-visit ERG amplitudes at a 95% confidence level for patients with RP and visually normal subjects was often at or above 25% and not infrequently at or above 40% for certain stimuli and test conditions. While no statistical difference in inter-visit variability was demonstrated between visually normal subjects and patients with RP who were arbitrarily categorized as having relatively higher baseline amplitudes, there was a difference between each of these two groups and a smaller group of patients with RP categorized as having lower baseline amplitudes. Although the authors could not demonstrate that percent inter-visit differences varied with stimulus intensity in either controls or patients with RP, the 32 Hz flicker stimulus generally produced less amplitude variability than either dark- or light-adapted single flash stimuli in patients with RP.
CONCLUSIONS: When using ERG amplitude for monitoring either the natural history of functional loss in retinal disease or as an outcome measure during a therapeutic trial, it becomes vital to define inter-visit variability of ERG amplitudes. These findings suggest that a percentage of patients with RP with appreciably lower baseline ERG amplitudes may manifest greater inter-visit ERG amplitude variability than patients with RP with higher baseline amplitudes or controls. Stimulus intensity had no clinically significant effect on inter-visit amplitude variability.