The result of our study demonstrated that in diabetic patients without and with DR, corneal endothelial parameters were not statistically significant difference. There was a significant relationship between CV and the duration of the disease with age variable control.

The cornea with altered morphology and functionality is known to be more susceptible to pathologies like recurrent corneal erosions, and impaired corneal sensitivity following trauma or surgical insult leading to recurrent ulceration with impaired healing [1, 2, 17]. So recognition of any potential endothelial dysfunction before the surgery potentially can be associated with more positive surgical outcomes [18]. Corneal endothelial cell parameters can be helpful indexes before referring patients for cataract or refractive surgery [9, 18].

The possible explanation for corneal endothelial changes in DM patients is multifactorial including impairment of apical junctions on the endothelial cells, impairment of physical barriers of corneal cell and altered permeability of corneal cell due to reduced Na + /K + ATPase activity pump in the endothelial cells [19, 20] Diabetic cornea especially with high glucose can lead cellular swelling due to increased sorbitol inside the cells due to increased activity of aldose reductase [19, 21].

Having exact data about the number and morphology of endothelial cells before cataract surgery reduces the risk of endothelial injury, especially in patients with DM [11]. In contrast to our study, limited studies have addressed the association of the severity of DR with the altered corneal endothelial parameters [12, 13].

A recent study by Ashok Jha et al. demonstrated DM patients had significantly an altered morphology including increased polymegathism, decreased cell density, and hexagonality when compared with healthy controls [12]. The effect of severity of DR and corneal parameters can be indirect via the effect of factors like duration, the severity of DM, age, etc. [12].

Since there was no endothelial cells proliferation with aging, on one hand, the number of corneal endothelial will be decreased and on the other hand cells size will be increased to compensate for the lack of lost cells.

In each ocular surgery, the CV number should be in the normal range to ensure that it does not occur decompensation after the ocular surgery [8, 9]. In the current study, it was shown that there were no significant differences in CV number in NPDR and PDR groups.

Choo et al. in 2010, did not show any correlation to the duration of DM, hemoglobin A1c level, and severity of DR [12]. In contrast to the study of Choo et al., in our study, there is a significant relationship between CV and the duration of the disease with age variable control that is a predictable and acceptable finding regarding increasing the adverse effect of DM in all organs with increasing the duration of diabetes [22]. This differentiation may be due to different ethnicity between Iranian and Japanese populations and differences in duration of disease in the enrolled population.

The findings of a study of Nurdan Gamze Taşlı et.al [23] about corneal specular microscopy in patients with type-2 DM demonstrated an increase in the stage of DR, alterations in corneal findings also increased. In our study, marginally association was obtained in the 60–65 years age groups for CV.

The possible explanation for the absence of statistically significant differences between other parameters of endothelial changes and severity of DM can be attributed to a relatively small sample size of our study and may be associated with ethnic differences [23].

Although existence of DR is important factor for alternation of corneal endothelial parameters, factor associated with clinical course of diseases are important factors for this alternation. The finding of Yoo Jin Kim and Tae Gi Kim suggest that DM affects corneal endothelial cell in older age and those with long-standing DM and higher HbA1c [24].

The importance of our study lies on evaluation of possible association of diabetic retinopathy with corneal endothelial parameters in diabetic patients. In most previous study with case–control design, normal population considered as a control group but in our study in both group the patient had DM and existence of retinopathy was as independent variable.

The study of El-Agamy et al. included patients without DR, eyes with NPDR, and PDR. The results of their study demonstrated ECD was significantly lower in the diabetic cornea than in control group and CV was higher in diabetic cornea. The diabetic cornea group had lower percentage of hexagonal cells than the control group, but the difference was not statistically significant [25].

Although our study has a suitable data analysis regarding different age groups and different DR grades for evaluation of DR on corneal endothelial parameters effect, there is some limitation including relatively small sample size, absence of normal population group as normal control, absence of level of glycosylated hemoglobin (HbA1c) and absence of data about corneal thickness.