Sir,

In an article regarding changes in subfoveal choroidal thickness (SFCT) in patients with central serous chorioretinopathy,1 Drs Kang and Kim described decreases in SFCT of 39.9 and 66.9 μm respectively, following observation and treatment with reduced-fluence photodynamic therapy.

We would like to highlight some aspects of the study design that may have bearings on the interpretation of these results. Although the authors mentioned diurnal variation in the discussion,1 it does not appear from the description of the methods that this was accounted for in the study design. Earlier studies2, 3 have demonstrated significant diurnal variation of SFCT measured using spectral-domain optical coherence tomography. In these papers, the amplitude (difference between the maximum and minimum choroidal thickness) exceeded 30 μm,2, 3 which is similar in magnitude to the change reported in the observation group in the current paper. Furthermore, when subjects with thicker choroids (defined as ≥400 μm) were sub-analyzed in one paper,2 the mean amplitude was even larger (43.1 μm) with a maximum of 59 μm. In addition to within-subject diurnal variation between the initial and follow-up reviews, it is also important in this study to consider the effects of potentially different measurement times between the two groups of patients (ie, within-group variation), which might have had an effect on the mean choroidal thickness of each group.

Another point of interest is whether the authors utilized the eye-tracking feature of the Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany) in performing the successive OCT scans between visits. As the choroid is known to exhibit spatial variation in thickness throughout the macula,4, 5 a minor change in the OCT scan position may result in differences in choroidal thickness measurements, which are sufficient to influence the comparison of SFCT. These concerns could have been mitigated by the use of the eye-tracking function that is also available on the Spectralis OCT, which we believe is an important methodological consideration in longitudinal measurements of choroidal thickness.

In conclusion, we congratulate the authors on an interesting paper, and urge investigators to consider the impact of diurnal variation of choroidal thickness on the results of such studies.