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.
Eighteen patients with RP and nine visually normal subjects had a series of three unilateral ERGs, with an intervisit 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 intervisit percent difference was compared between stimulus intensities. Intervisit 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 intervisit 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.
Threshold for significant increase or decrease in intervisit 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 intervisit 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 intervisit 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.
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 intervisit variability of ERG amplitudes. These findings suggest that a percentage of patients with RP with appreciably lower baseline ERG amplitudes may manifest greater intervisit ERG amplitude variability than patients with RP with higher baseline amplitudes or controls. Stimulus intensity had no clinically significant effect on intervisit amplitude variability.