We read with great interest and attention the recent article from Velazquez-Martin and associates regarding the evaluation of ciliary body thickness in eyes of patients with unilateral ocular/oculodermal melanocytosis. They concluded that ciliary body thickness is increased in eyes affected with ocular/oculodermal melanocytosis in comparison with the fellow unaffected eyes. Although the measurements seem to be accurate and the results interesting, we believe that the statistical procedure used by the authors is not appropriate for their study design. Furthermore, some confounding factors were not taken into account that could have a direct effect on the significance of the results.
The authors used a Student t test for their statistical analysis, which requires the assumption of independence between the 2 groups. However, because the controls were the unaffected eyes from the same person, this is a violation of independence. Thus, statistical methods that account for this dependency are required, such as the paired t test. Additionally, the Student t test and the paired t test require that the values in the 2 groups (affected and unaffected eyes) follow a normal distribution. Did the authors verify the normal distribution of the data in the 2 groups before conducting the Student t test? If the values are not normally distributed, which is likely the case in this study because of the limited sample size (12 patients), then the Wilcoxon signed-rank test should have been used. Hence, we believe that the authors should verify their results after conducting a Wilcoxon signed-rank test.
Furthermore, because the ciliary body thickness strongly depends on the refractive error and the axial length of the eye, the authors should provide the values of these 2 parameters for each eye and should compare the mean axial length and the mean refractive error between the affected eyes and the unaffected eyes. Indeed, it has been shown that the ciliary body thickness is significantly higher in eyes with unilateral high axial myopia than in their relatively normal fellow eyes. Thus, these 2 variables could be important confounding factors for the interpretation of the results.
Finally, it would have been interesting to compare statistically the reflectivity between the 2 groups. Indeed, the authors have evaluated the reflectivity with a 5-point scale that could be converted to a numerical scale with the following correspondence: low = 1, medium/low = 2, medium = 3, medium/high = 4, and high = 5. With such numerical scale, we can calculate the mean of reflectivity for each eye, and we can compare the 2 groups. We performed this comparison using the available data in the Supplemental Table. Interestingly, we found a significant difference between the affected eyes (3.98 ± 0.48) and the unaffected eyes (2.92 ± 0.12; P = .0024, Wilcoxon signed-rank test).
We congratulate the authors on their informative article, and we hope these remarks will help them to improve the accuracy of their analysis and to confirm their interesting results.