Although EDI-OCT has confirmed morphological choroidal changes in DM eyes, there are few research to investigate the change of choriocapillaris, Sattler’s and Haller’s layers in DR eyes, especially in DR with ME eyes. In this study, EDI-OCT was employed to evaluate the change of MCT in inner and outer layers. Our study indicated that the whole, outer and inner layers of MCT in DR eyes were significantly thinner than that in control. Compared with DR without ME eyes, the whole and outer layers of MCT in DR with ME eyes were significantly decreased. These data indicate that in DR eyes, MCT was thinner than that of healthy subjects, and this reduction was due to decrease of choroidal thickness in both inner and outer layers. Moreover, in DR with ME eyes, the MCT was even thinner than that in without ME eyes and this decrease mainly occured in the outer layer of choroid.
The changes in choroidal thickness of DR eyes have been conducted by several researchers. Whereas, the reports of relationship between DR and MCT are still controversial. In this research, it was found a significant reduction of MCT in DR eyes as compared to that in health subjects. This is in agreement with reports that MCT was significantly thinner in middle NPDR, PDR eyes [6, 10]. Ours and others results corroborate with previous reports that reduction of choroidal blood flow velocity is associated with severe DR [9, 17]. On the contrary, Kim et al. reported the increase of subfoveal choroidal thickness in DR eyes compared to that in the normal subjects. The subfoveal choroid was even thicker in the eyes with DME than that in the eyes without DME and thickest choroid was observed in eyes with subretinal detachment-type of DME [16]. On the other hand, Xu et al. did not find any significant association between MCT and DR changes [15]. The discrepancy may be due to the different enrolled criterions of patients. The MCT of DR is influenced by a variety of factors. Several researches have certified that panretinal photocoagulation, intravitreal anti-VEGF therapy, and intravitreal triamcinolone acetonide injection could affect MCT [18–20]. To eliminate these factors, only DR eyes without any history of ocular treatment were enrolled in this research.
The studies with laser doppler flowmetry and histopathological suggest that the early diabetic choroidal changes most likely occur in the choriocapillaris [17, 21], a vascular layer with a minimal thickness. Histological studies also revealed basement membrane thickening of the choroidal vasculature, and loss of choriocapillaris in donor eyes with DR [7]. Accordingly, our results suggest that the reduction of thickness in both inner and outer layers occurred in DR eyes.
The correlation analysis of the mean changes of MCT and mean change of BCVA showed a significant positive correlation. Our study suggest the reduction of MCT may be one of the major causes of impairing BCVA by disturbing the metabolism of outer layer retina in the macular area. In diabetic patients, the reduction of MCT might induce retinal hypoxia because of the degeneration of choriocapillaris [8]. Hypoxia could then increase the expression of vascular endothelial growth factor in RPE cells, pericytes, and microvascular endothelial cells, that induces dysfunction of the blood-retinal barrier and causes the basic pathological change of DR [22]. Thus, choriocapillaris degeneration, should be one of the important factors causing the impairment of visual function in diabetes patients [8].
In our study, we demonstrated that increasing age was significantly associated with decreasing MCT in whole and outer layers in control group. This is in consistent with a study that described a decrease of macular choroidal thickness, of approximately 3 microns per year of age [23]. Besides, our multivariate regression analyses showed a negative correlation between CMT and the whole and outer layers of MCT in DR eyes. Because the choroidal circulation makes an indispensable part of metabolic exchange in the outer retina, especially in macula avascular region, the impairment of choroidal circulation may be an important factor caused impairment of central macular retina.
Our research indicate that choroidal thickness in whole, inner and outer layers was significantly reduced in DR eyes compared to that in control. Nevertheless, the underlying mechanism of choroidal thinning remains to be fully elucidated. The choroid is vascular structure with innervation by abundant neurons, mainly under control by autonomic nervous system. There were a few studies investigating the relationship between choroidal blood flow and choroical thickness. Zengin et al. found a significant reduction in choroidal thickness after ingestion of nicotine, which was caused by decreasing choroidal blood flow due to the vasoconstrictor action [24]. Similarly, sildenafil citrate and coffee were found to cause the change of MCT due to their vasodilatory or vasoconstrictive effects on choroidal circulation [25] [26]. Thus, the activation of the autonomic nervous system might conduce decreasing in choroidal thickness through vasoconstriction of the choroidal vessels [27]. Therefore, it is necessary to illustrate the mechanisms of autonomic nervous system in decreasing choroidal layers of DR eyes in future.
There are several limitations in our study. Firstly, in this research only the central macular choroidal thickness was measured, while parafoveal choroidal thickness varied in each patients [28], further studies are needed to figure out layer thickness in broad spectrum. Secondly, the number of participants was limited, which may affect the correlation analysis outcome. Thirdly, previous researches have shown that hypertensionand dyslipidemia, may affect the choroidal thickness [29, 30]. While, in this research these factors were not included as variable factors. Fourthly, there was lack of histopathological results to corroborate our results.
In conclusion, our study demonstrated the whole, inner and outer layer of MCT had significantly decreased in DR eyes, especially in DR with ME eyes. This researchsuggestthat the change of MCT is closelycorrelated with DR and the thinning of choroidal layer, at least from one aspect, may reflect progression of diabetic choroidopathy. Our results indicate that the pathological change of MCT caused impairment of BCVA. DEI-OCT as a novel non-invasive instrument, is vital to investigate choroidal change in DR.We should pay attention to the pathological change of DR not only in retina but also in chorod in our clinical practics and research. Further better understanding of the pathological change in diabetes, would be essential for the devepment of novel therapeutice mthod to combate this leading blindness disease.