Anna Brouwer

Summary, discussion & future perspectives Effects of DTL position on the ERG In Chapter 7 we investigated the effects of the Dawson-Trick-Litzkow electrode (DTL) position on the ERG, because some studies state that the fornix position (FP) would be better suited for monitoring purposes than the lower lid position (LLP). The LLP is the position that is used in the UMCU. The FP would supposedly be better, because reference values with the FP have a smaller range than the LLP. 8–10 To investigate the difference between the LLP and the FP, we measured the light adapted ERG twice in our uveitis cohort. Similar to previous studies, we observed that the median amplitudes were lower in the FP, compared to the LLP. 10,11 The interquartile ranges (IQR) were also smaller in the FP. However, all these differences between the FP and the LLP were proportional. To illustrate this, we calculated the coefficient of variation (CoV), also known as the relative standard deviation. We observed no significant differences between the CoV of the two DTL positions. This means that although the absolute confidence intervals (CIs) are smaller in the FP, they are proportionally smaller. Therefore, we concluded that when the ERG is used for monitoring purposes, a smaller CI in the FP is not a valid reason to choose the FP over the LLP, because this CI is proportionally smaller. When physicians choose a DTL position for their local protocols, different aspects need to be taken into account. The FP is more comfortable for patients, 10 but the LLP yields larger responses. This means that when it is important to be able to record very small ERG responses, which can be observed in retinal dystrophies, the LLP can be preferred. When patient comfort is considered as more important, the FP may be chosen. But above all, it is important to check that the DTL position stays at the same position during an ERG measurement and that it is the same position that was used for reference values. 163 8