Choroidal thickness alterations in obstructive sleep apnea–hypopnea syndrome (OSAS)

Sir,

We would like to congratulate Xin et al¹ on their prospective, observational case–control study of retinal and choroidal thickness (CT) evaluation by spectral domain optical coherence tomography in patients with obstructive sleep apnea–hypopnea syndrome (OSAS). The study presents, for the first time, CT alterations in OSAS patients; however, we think that some important issues need more thorough discussion.

The choroid of the eye is one of the most highly vascularized tissues of the body, and it is the major blood supply to the retina.² It is known that the choroid is prone to suffer from microvascular atherosclerotic changes and changes inherent to other microvascular systems.³ However, the authors did not exclude the subjects having systemic diseases except diabetes, and about 50% of the participants were indicated to have hypertension. Also, hyperlipidemic subjects and smokers were not excluded, although both of them were shown to affect CT.^{4, 5} As body mass index of OSAS group in their study is significantly higher than that of controls and systemic diseases are often comorbid with OSAS, expecting hypercholesterolemia more in OSAS group is reasonable. It is obvious that including patients having hypertension and other systemic diseases may affect the conclusion of the CT studies. Although the number of the patients and controls having the hypertension are matched, severity and duration of the systemic diseases could not be standardized. Therefore, it should be kept in mind that, CT alterations seen in Xin et al’s study might be due to the underlying systemic diseases and concomitant treatment in patients.

Not only systemic diseases, but also ocular conditions including axial length and refractive error are known to influence CT.⁶ The authors stated in their study that age and diopter were corrected using covariance analysis before comparing CT between the groups. However, spherical equivalent refraction is not stable throughout life. Myopic shifts can occur especially in elderly patients because of nuclear cataract progression. In this study, OSAS patients tended to be older than controls, although not significantly. So, considering axial length measurements instead of refractive status would be more accurate particularly in old population. Thus, we also wonder that whether the authors paid attention to the concomitant nuclear sclerosis in their patients that might influence the refractive status and consequently the results of their study.