Variation of straylight between contra lateral eyes - results from 1017 healthy subjects

Abstract

Editor, A large multicentre study on the vision of European drivers (GLARE study) was carried out in five participating clinics in Amsterdam, Salzburg, Tübingen, Barcelona and Antwerp, where 2422 volunteers underwent a series of visual function tests, including tests for visual acuity (VA), visual field (VF), contrast sensitivity, straylight and useful field of view (UFOV). Special attention was paid to intraocular straylight increase because of ageing of the crystalline lens (Van den Berg et al. 2007; Michael et al. 2009; Van Rijn et al. 2009; Nischler et al. 2010). The intraocular straylight parameter (log [s]) was obtained using the compensation comparison technique, as employed in the C-Quant (Oculus GmbH, Wetzlar, Germany) straylight meter. For each eye, two consecutive measurements were taken, starting with the right eye. This technique has proved to give reliable and objective measurements of intraocular straylight (Coppens et al. 2006; Van den Berg et al. 2009). We analysed the straylight values of 1017 healthy phakic individuals from the GLARE study, who were without any ocular pathology such as glaucoma, diabetic retinopathy, macular degeneration, dry eye, amblyopia and cataract (LOCS III >1.5), to further confirm the repeatability of the test and to assess the difference in straylight between contra lateral eyes. We found no statistically significant or clinically relevant difference between straylight repeated measures. The differences between repeated log(s) values had a mean and standard deviation for the right eyes of 0.0042 ± 0.0992 and 0.0034 ± 0.1043 for the left eyes, corresponding to repeated measures standard deviations of 0.0702 and 0.0738, respectively. This yields an average standard deviation for repeated measures of 0.0720 (root mean square of 0.0702 and 0.0738), which remains stable with different levels of straylight (see Fig. 1A). The repeatability obtained is in agreement with the results reported by Coppens et al. (2006), Cerviño et al. (2008), and others. The mean differences are virtually zero and statistically not significant, indicating that no learning effect exists. (A) Scatter plot of repeated measures. Straylight variation between first and second measurement remains stable at all straylight values. –––, x = y. (B) Difference in straylight values between right and left eyes seems to have a tendency to increase with higher values of straylight. –––, mean difference; - - -, 95% confidence interval. (C) Difference in straylight values between right and left eyes as function of age shows a slight tendency to increase with age, until the age of 75, where it seems to drop back. –––, mean difference; - - -, 95% confidence interval. For the straylight differences between right and left eyes, we found a mean and standard deviation of −0.0047 ± 0.1158 log(s), respectively (Fig. 1B). The standard deviation is higher as would be expected from the repeated measures differences reported earlier (0.0720), as the differences between both eyes were based upon averages of two values for each individual eye. According to anova analysis, the statistical significance of this effect is p < 0.0001. Furthermore, the standard deviation of the intra-eye difference progressively increased with age (Fig. 1C), probably due to the asymmetrical evolution of lens ageing, going from 0.0985 around 35 years, 0.1114 from 45 to 60 years, and 0.1277 from 61 years upward. Those differences are statistically significant (p = 0.023). For the group above 75 years of age, the standard deviation of the difference seems to drop back a little (to 0.1154); nonetheless, this difference is not statistically significant (F test, p = 0.16). The mean difference is virtually zero and statistically not significant (paired t-test, p = 0.172), confirming that no learning effect exists. Although the differences between contra lateral eyes are not very large, it is interesting to speculate about the cause. It can be considered that contra lateral eyes from a same person have slightly different straylight levels, just as any other part of the body is slightly asymmetrical to their contra lateral counterpart. It requires a focused study, especially interesting in the <45-year group, what may cause these differences. The standard deviation of the straylight difference between eyes gives us a good base to estimate the acceptable difference in straylight after a procedure in one eye using the value of the contra lateral healthy eye as control.