I read and reviewed the article entitled “Swept-source optical coherence tomography correlations between retina and choroid before and after vitrectomy for epiretinal membranes,” by Michalewska and associates, with interest. The authors examined the patients that had pars plana vitrectomy with epiretinal membrane (ERM) removal and internal limiting membrane peeling owing to idiopathic ERM twice in the preoperative period (9-12 months and 1 week before surgery) and 3 times in the postoperative period (1 week, 6 months, and 12 months after surgery). A significant increase was determined in the visual acuity of the patients 12 months after surgery. The authors did not determine any significant difference between pre- and postoperative central retinal thicknesses; however, they found a significant decrease particularly in choroidal thickness (CT) in postoperative month 6, and this decrease stayed stable in postoperative month 12. I appreciate the work of the authors; however, I would like to ask for further details and contribute to the article.

Choroidal thickness shows significant diurnal variation. The choroid could increase its thickness by 50% in an hour and quadruple its thickness in a few days. Kee and associates showed that the choroid can thin very rapidly, by about 100 μm in 3-4 hours in chicks. Usui and associates studied subfoveal choroidal thickness (SFCT) in healthy subjects, and measured SFCT every 3 hours over a 24-hour period. They found that the mean SFCT (271.9 ± 103.5 μm) was the thinnest at 6 PM, and it was the thickest (290.8 ± 110.8 μm) at 3 AM. In that study, Usui and associates showed that diurnal variation of CT might be up to 65 (range: 8-65) μm, and SFCT was negatively correlated with systolic blood pressure (SBP) in all eyes.

In another study, Tan and associates evaluated CT in humans at 2-hour intervals between 9:00 AM and 5:00 PM, and determined significant differences in CT among all measurement points. Tan and associates found mean diurnal amplitude of CT as 33.7 ± 21.5 μm (range: 3-67 μm). The change in CT was also correlated with changes in SBP.

Moreover, Chakraborty and associates measured axial length (AL), intraocular pressure (IOP), and CT of the participants at 3-hour intervals, 10 different times over 2 consecutive days beginning at 9 AM, the final measurement being at 9 PM. In that study, it was determined that CT, IOP, AL, and vitreous chamber depth showed significant diurnal variations.

It has been known that AL and IOP have significant effects on CT. Therefore, AL and IOP measurements should be taken into consideration when CT is measured in studies.

On the other hand, a number of local and systemic physiological/pathologic conditions affect CT. Since the average age of the patients was 68 years in the study of Michalewska and associates, I suspect presence of comorbid systemic diseases in the participants. Therefore, I ask the authors whether they take presence of any systemic diseases or use of drugs that could affect CT measurements into consideration. I also wonder about the sleep and exercise statuses of the patients, as well as consumption of alcohol or caffeinated/noncaffeinated beverages, before optical coherence tomography (OCT). I am also curious about whether the body mass index of the patients, the results of their systemic blood pressure measurements, and the lighting conditions of the test room are taken into consideration, since all those parameters significantly affect CT.

In the light of all those data, one should expect significant effects of the physiological fluctuation of CT on test and statistical results. I recommend measuring OCT in both pre- and postoperative periods at the same time point of the day. Finally, I suggest that the authors optimize all local, systemic, and environmental parameters that could affect the results of such a comprehensive study.