We read with great interest the recent article by Turk and associates. The article provides valuable information on the normative values of the peripapillary retinal nerve fiber layer (RNFL) and macular thickness in healthy children obtained by spectral-domain optical coherence tomography (SD-OCT). The authors found that there was no significant correlation between the average RNFL thickness and axial length. However, recent studies have demonstrated that RNFL thickness measured with SD-OCT decreases as axial length increases. The differences between Turk and associates’ study and other studies may be attributable to different age and ethnicity, as well as to the following reasons.
Firstly, as acknowledged by the authors, the axial length range (from 21.23 to 26.06 mm) may not be large enough to allow a statistically significant correlation between axial length and RNFL thickness. The sample previously investigated by Savini and associates, for example, had a larger range (from 20.64 to 28.85 mm). We encourage the authors to repeat the study and include shorter and longer eyes (with a similar distribution among short, medium, and longer eyes).
Secondly, age might have been a confounding variable: it is hard to think that the optic nerve of a 6-year-old baby has the same anatomic configuration as that of a 16-year-old boy. Since RNFL thickness is influenced by age, correlation between RNFL thickness and axial length should be adjusted for age. The solution is multiple linear regression analysis, with RNFL thickness as the dependent variable and age and axial length as influencing variables, like Bendschneider and associates recently did.
Last but not least, the camera magnification in the fundus imaging system of the Spectralis may be different than that of Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, California, USA), which was used in previous studies correlating axial length with RNFL thickness, although this correlation has already been reported also with the Spectralis.
It would be interesting to see the authors’ viewpoint regarding these differences.
Furthermore, the authors should use their data and the appropriate statistical methods to establish a normative database with respect to RNFL, macular thickness, and macular volume for healthy pediatric Turks. Reporting the mean value, standard deviation, and range is not sufficient for the clinician in order to assess whether the measurement of the single eye should be considered “normal.” When establishing the normative thresholds, the upper limit of normal values is defined as one-half the interquartile range (IQR) above the third quartile (Q3), or Q3 + 1.5 IQR based on the Tukey method. These newly constructed normative databases are more useful for detection and verification of optic nerve diseases and glaucoma than the authors’ result.