Anna Brouwer

18 Chapter 1 conjunctiva, or skin of the lower eyelid. Over the years multiple types of active electrodes have been developed. Each has their own advantages and disadvantages and none is determined as the gold standard by ISCEV. Initially, the main goal of electro-physicists was to record as high amplitudes as possible. These are best recorded at the top of cornea. 52 Therefore, for a long time Burrey-Allen contactlens electrodes were the most often used electrodes. While they yield high amplitudes, the disadvantage of these electrodes is that they are quite uncomfortable for patients to wear. Also, there is a chance of damage to the cornea due to scratching. To increase patient comfort, several other electrodes were developed. One of these is the Dawson-Trick-Litzkov (DTL) electrode. In this thesis all ERG recordings were performed with DTL electrodes. When the DTL was first introduced, its developers stated that it was possible to use it without a local anesthetic. 53 However, it is our experience that it is advisable to use some local anesthetics to ensure a better patient comfort during the ERG recording. Besides a better patient comfort, the DTL electrode is usually disposable, which decreases the possibility of an infection. Lastly, with DTL the chance of significant damage to the cornea is small. A disadvantage of the DTL electrode is that it can shift its position during an ERG recording. The position of the DTL can affect the recorded ERG response. In chapter 6 we will look further into the effects of DTL electrode position and the ERG. Factors other than disease that can influence the ERG Besides retinal diseases, many other factors can influence the recorded ERG as well. Some of these factors can be (easily) addressed by adhering to the ISCEV standards. These include the duration of dark- and light adaptation, flash strength and pupil size. 38 Light and dark adaptation appear to have an exponential effect on the ERG responses. In the first minutes most of the effect is reached, thereafter additional adaptation time yields only slightly larger responses. 54 The standard adaptation time for dark adaptation is 20 minutes, for light adaptation 10 minutes. Several patient specific factors can also significantly affect the recorded ERG response, but these are less important when the ERG is used for monitoring purposes, because they are expected to remain stable over the sessions. Examples of these factors are axial length, ocular pigmentation and gender. 55–58 Pupil size affects how much light can reach the retina. If the amount of light that can reach the retina is reduced, this should be taken into account when an ERG is reviewed. Therefore, a recorded ERG with a small pupil size may correspond to a weaker flash strength than the one which was used. Pupil sizes were therefore recorded in the prospective studies of this thesis.