Isaac W Chay1, Louis W Lim1, Kai Xiong Cheong1, Colin S Tan1,2. 1. National Healthcare Group Eye Institute, Tan Tock Seng Hospital, Singapore. 2. Fundus Image Reading Center, National Healthcare Group Eye Institute, Singapore.
Sir,We read with interest the article by Narendran et al.,[1] reporting that choroidal thickness measurements obtained using a spectral-domain optical coherence tomography (SD-OCT) device (Spectralis OCT; Heidelberg Engineering, Heidelberg, Germany) and a swept-source OCT (SS-OCT) device (Topcon deep range imaging [DRI] OCT-1; Topcon Inc., Tokyo, Japan) were comparable.In their prospective cohort of 31 normal participants, the authors found that the mean subfoveal choroidal thickness measurements of the SD-OCT and SS-OCT devices were 325.3 and 319.5 μm, respectively.[1] Similar results have also been reported in other studies on subfoveal choroidal thickness in healthy volunteers.[234] In addition, it has been reported that choroidal thickness measurements obtained from SD-OCT and SS-OCT devices are also comparable among eyes with retinal diseases. In a study of 64 patients with retinal diseases measured using the same two OCT devices, the mean difference between an SD-OCT and SS-OCT device was 8.4 μm (P = 0.014), which was slightly more than the difference among healthy eyes.[2]In Table 1 of the article by Narendran et al.,[1] the authors cited a study by Tan et al.[5] involving fifty eyes measured using the same SD-OCT and SS-OCT devices and quoted a mean difference of 19 μm between the two devices. We would like to point out an important difference in that study. The choroidal thicknesses were central subfield choroidal thicknesses, instead of subfoveal point choroidal thickness measured in the other papers.[1234] In addition, that study actually reported a mean central subfield choroidal thickness of 312.4 μm for the Spectralis OCT, compared with 263.1 μm using the automated segmentation features of the DRI OCT-1, which resulted in a mean difference of 49.3 μm (P < 0.001). The difference of 19 μm that is reported in Table 1 of the current article actually refers to the choroidal thicknesses that were obtained after manual adjustment of the segmentation boundaries on the DRI OCT-1 by trained reading center graders. This illustrates the impact of segmentation boundaries on choroidal thickness measurements.In summary, we agree that the subfoveal choroidal thickness measurements are comparable between the studied SD-OCT and SS-OCT devices. However, it is also important to note that the presence of uncorrected automatic segmentation errors could increase the variability of choroidal thickness measurements between SD-OCT and SS-OCT devices.
Financial support and sponsorship
Dr. Tan receives research funding from the National Medical Research Council Transition Award (Code: NMRC/TA/0039/2015). Dr. Tan also receives conference support from Bayer (South East Asia) Pte. Ltd., Heidelberg Engineering (Heidelberg, Germany), and Novartis (Singapore).