The authors would like to thank Drs Huang and Zhang for their interest in the study on assessment of the morphology and vascular layers of choroid in eyes with Stargardt disease compared to healthy eyes, and appreciate their insightful comments. The concerns raised include the effect of diurnal variation and axial length on choroidal analysis.
As pointed out, studies suggest that choroidal thickness may vary based on the time of the day it is assessed. It appears that subfoveal choroidal thickness in healthy eyes is thinnest in the evening and thickest in early morning. In the authors’ study, all patients with Stargardt disease and all healthy control subjects underwent imaging during the morning and afternoon clinics (between ∼8:30 AM and ∼3:00 PM) at the New England Eye Center, Boston, Massachusetts, USA. Since imaging time did not vary drastically for both the Stargardt disease patients and healthy (control) subjects, the authors believe that diurnal variation was an unlikely variable affecting their results. Even so, if slight variations in choroidal thickness did exist within the ∼7-hour period that imaging was performed, it is expected that they were equally distributed across the diseased eyes and healthy controls, making the comparative choroidal analysis in their study effective.
As rightly suggested, axial length may play an important role in the assessment of choroidal thickness. Wei and associates suggested that besides age, axial length (or myopic refractive error) appears to be the most influential parameter affecting the subfoveal choroidal thickness measurements. This study evaluated eyes with a wide range of refractive errors from −20.00 diopters to +7.00 diopters and found that for every 1 mm increase in axial length, the subfoveal choroidal thickness decreases by 32 μm. In the authors’ study, one of the exclusion criteria for Stargardt disease patients was a myopic refractive error of −6.00 diopters or worse. A similar exclusion criterion was applied for the healthy (control) subjects. Since the authors’ study was retrospective in design, axial length data were not available for all subjects. However, mean myopic refractive error of eyes with Stargardt disease was −1.65 (1.15–2.25) diopters and that of the healthy (control) eyes was −1.25 (1.00–1.60) diopters ( P = .83; Table 1 in the original manuscript). The authors expect that with the degree of myopia in their subjects, there would be minimal, if any, variations in the axial length. Even so, as for the diurnal variation, if slight differences in axial length did exist, it is expected that they were equally distributed across the diseased eyes and healthy controls, making the comparative choroidal analysis in their study valid.
In conclusion, the authors described changes in choroidal morphology and vascular layers in eyes with Stargardt disease compared to healthy (control) eyes. All subjects (both diseased and control) were imaged during a similar time of the day. The control group was age matched and refraction matched to the diseased group. Hence, the authors believe that diurnal variation and axial length, although important considerations, were likely insignificant contributors to the comparative choroidal analysis in their study.