Anna Brouwer

Effects of DTL position on amplitude and implicit time Introduction Since the introduction of the Dawson Trick Litzkov (DTL) electrode 1 , its use in recording electroretinograms (ERGs) has spread. One of the main advantages of the DTL is that it is much more comfortable to wear compared to conventional electrodes, such as contact lens electrodes. 2–4 Originally, the ERG was mainly used for diagnosing retinal diseases such as retinal dystrophies, where the ERG is frequently severely abnormal. However, now that the ERG is increasingly used for monitoring disease, more subtle ERG changes become important. Therefore, one must be aware of factors that may affect the ERG results, other than disease or treatment. Particularly factors that influence the inter- session variability are important. Factors that may influence the absolute ERG results, but will have little effect on inter-session variability since they do not differ between sessions, include gender, 5 refraction, 6 and ocular pigmentation. 7 Other factors that can affect the intersession- variability may be minimized by always incorporating International Society for Clinical Electrophysiology of Vision (ISCEV) standards. These include duration of dark- and light adaptation, flash strengths and pupil size. 8 Besides these, there are factors that also affect the intersession variability but are more difficult to address. These include: DTL position, 9–12 media opacities, 13 and age. 14–16 When the DTL electrode was introduced, it was said that its position was ‘only a little disturbed by blinking’. 1 However, since then several reports state that the DTL position can shift, which may significantly affect the recorded amplitudes. When the DTL is positioned on the cornea, the highest amplitudes can be recorded, which decrease as the DTL is positioned more toward the fornix. 17 Despite these differences in amplitudes, the latest ISCEV standard does not advice which DTL position should be used. The most common DTL positions are: the lower lid position (LLP) and the fornix position (FP). 8 In our clinic, reference values are based on ERG measurements recorded at the LLP. This position was advised by the manufacturer for two reasons. First, it would ensure high amplitudes, without much risk of scratching the cornea. Second, blinking would have little effect on the DTL position. In some cases however, we have seen the DTL shift toward the fornix during an ERG recording. Some studies advise using the FP, because it would shift less easily and therefore lead to a more stable recording. The FP would yield lower amplitudes but the ERG would be less variable. 9–11 However, these studies were conducted in relatively small groups of healthy volunteers. These healthy volunteers are probably often coworkers who know how to cooperate during an ERG since they are familiar with ERG 141 7