Anna Brouwer

The ICC showed a good reliability between the two DTL locations for both the amplitude and the implicit time of the b-wave (ICC 0.842 - 0.973), with the ICC of the implicit time showing especially good reliability (Table 3). The ICC of the a-wave showed less good reliability. The ICC of the weakest flash strengths were as low as 0.584. Bland Altman plots show similar results (see Supplemental figure 2). Regarding the amount of variance, amplitude ranges of both positions were quite large, but they were significantly larger in the LLP compared to the FP. However, the CoV did not differ between the two DTL positions, implying that the relative variability was not statistically significantly different between the two positions (Table 4). Discussion The aim of this study was to investigate whether one of two commonly used DTL positions is superior for monitoring purposes in patients. We found no differences in implicit times. We did find differences in amplitudes: in the LLP the amplitudes were between 1.12 and 1.19 times higher compared to the FP and the ranges were larger as well. These differences are lower but still comparable to other reports, where mean amplitudes in the LLP were between 20-31 % higher compared to the FP. 10,12,19 However, these previous reports did not investigate whether there was a statistically significant difference in relative variability. Some investigate if the variability, or amount of agreement (ICC), differs between the two positions. 10 While others report that the variance decreases by 20% in the FP. If the relative variability, or relative standard deviation decreases as well, is not investigated. 12 This study shows that the relative variability was not statistically significantly different between the two DTL positions. It is difficult to compare our results to the literature, because most studies do not investigate DTL position but investigate the DTL electrode to another type of electrode. Also, some reports investigate DTL position in different test such as the multifocal ERG 10 or pattern ERG. 19 Kurtenbach et al did investigate the effects of DTL position on the full-field ERG. However they investigated a smaller group (13 subjects) and did not measure the ERG on the same day. The differences they found were slightly larger than in our study, with a mean difference in amplitude of 20% (SD 9.71%) for the b-wave and 27% (SD 17,7%) for the a-wave of the LA 3.0 cds/m 2 . The smaller sample size and the relatively large SD could explain the differences with our study. Also, they did not investigate differences in the relative variability. We found the differences in amplitude between the LLP and FP to be proportional, because the higher amplitude in the LLP is accompanied by a larger range, and the lower amplitude 148 Chapter 7