OSAS is a syndrome characterized by recurrent apnea or hypopnea episodes during sleep, and frequently decrease in blood oxygen saturation (SpO2). Optic disc damage can be occured by loss of ganglion cells. The hypoxia secondary to OSAS-induced imbalance between mediators; nitric oxide (NO) and endothelin can cause this ganglion cell loss. 16
High IOP is the most important risk factor for the glaucoma. The higher IOP causes a change in the ONH structures, especially makes RNFL thinning. A study by Karakucuk et al.17 found that four patients have glaucome and the prevalence of glaucoma in OSAS patients was 12.9% (n=4/31). They also found a positive correlation between IOP and AHI. In our study we found that IOP values in OSAS group was significantly higher than in controls. IOP is important risk factor for glaucoma and glaucomatous ONH damage. At the same time there is no correlation between AHI groups.
Another importent parameter is increase vertical cup/disk (C/D) ratio in glaucomatous optic nerve. A few study were compared vertical C/D ratio between OSAS and control. Sergi et al18 did not find any difference in the cup/disk ratio between the OSAS patients and the control group. They found a significant correlation between AHI and the cup/disk ratio. 18 In contrast with their study, our study shows that the vertical cup/disk ratio significantly higher in OSAS group than in controls without OSAS.
In glaucoma, RNFL thickness decreases progressively. This thinning can be present in eyes of patients with glaucoma before detectable changes occur in the visual field19,20. If a decrease in the RNFL can be detected, it gives an alert to the risk for developing glaucoma. Firstly Kargi et al. 21 were reported RNFL thickness decreases in patients with OSAS. Conversely, a study performed in Turkish population by Adam et al. 22reported that there is no difference in RNFL thickness between healthy and OSAS subjects. Another study by Ferrandez et al. 23reported that RNFL thickness did not differ significantly between OSAS and control groups. Similarly in our study, we detected no correlation between OSAS patients and control group in RNFL parameters.
In addition to the measurement of peripapillary RNFL thickness by OCT, measurement of the macular ganglion cell complex thickness has been used in several ocular and neurological diseases, particularly glaucoma. Ferrandez et al24 showed that neither ganglion cell layer (GCL) nor ganglion cell inner plexiform layer (GC-IPL) thickness were reduced in OSAS patients compared with healthy controls. Conversely, Huseyinoglu et al. 25 found GCL thickness decreases in patients with severe OSAS compared to the control group. In addition, there was a significant negative correlation between AHI and GCL thickness. 24 Kara26 reported that the GC-IPL thickness in patients with severe OSAS was significantly lower than that of controls. In our study we found no significant correlation in GCL thickness between OSAS and control group.
The retina is metabolically very active tissue in the human body, and it has a double blood supply from the central retinal artery and the choroid. Certain characteristics of the choroid have been studied histologically. Spaide et al. 27 described a "enhanced deep imaging" (EDI) technique to monitor the full thickness of the choroid and this optimize the parameters of OCT. Karalezli et al28 showed that the median choroidal thickness was statistically thinner in the eye of patients with severe OSAS than that of the controls. On the other hand Tonini et al.29 reported that choroidal vascular response to hypoxia does not affected in the patients with OSAS. In a study Yuvacı et al. 5 although differences in choroidal thickness were observed, no significant differences were found between the control group and the other OSAS groups. Similarly in our study although central choroidal thickness were thinner in OSAS group but datas were not statistically significant.
The different results in several studies may be because of the calibrations of measuring instruments, the duration of OSAS, the demographic characteristics of the participants, the vascular dysregulation present in patients with OSAS. In addition to these general factors, there are some limitations of our study and the major one is the limited sample of patients. A larger cohort study can gives us more meaningful analysis on the relation between the severity of OSAS and posterior ocular findings. Another limitation was we were the exact timing of the development of OSAS was not known, which has a certain influence on the degree of hypoxia. The results of our study may be compared after longterm follow up and after CPAP treatment.
In summary we found that peripapillary RNFL, GCC, central choroidal thickness did not change with OSAS patients or the severity of OSAS. Further more we found that IOP, C/D ratio and vertical C/D ratio of patients were all significantly higher than control group. These association between OSAS and glaucome supports the several previous studies. However multicenter longterm cohort studies are still needed to assess the definite changes of RNFL thickness, GCC and CCT in OSAS patients.