This study demonstrated that the corneal endothelial cell density and the central corneal thickness decreased with aging. Although the corneal endothelial cell density and other morphology have been reported in previous studies, direct comparison of the results among studies is limited due to various evaluation methods and study populations. The results from most studies showed a trend toward decreasing cell density (Table 4) and increasing cell variation with aging. High CEC density (> 4200 cells/mm2) had been reported just after birth and in infants, but these values rapidly decreased during childhood and then decreased slowly after the age of 18 [10].This evidence supports the fact that corneal endothelium lacks proliferative ability, resulting in reduction of these cells with age.
The mean ECD in this study was 2732 ± 258 cell/mm2, which was similar to the results from other studies using non-contact specular microscope in normal eyes (Table 4).These included Filipino (2798 ± 307) [11] , Malay (2648 ± 310) [12], Turkish (2732 ± 307) [13], Egyptian (2647 ± 387) [14], Nigerian (2610 ± 371) [15], and Indian (2525 ± 337) [16]. The mean ECD in Thai eyes was lower than those of Japanese (2943 ± 387) [17] and Chinese (2932 ± 363) [18], but higher than the results of Iranian (1961 ± 457) [19].The corneal diameter has been postulated to responsible for the variation of ECD in various populations as the corneal diameter might be inversely proportional to endothelial cell density as the Indian and American had less ECD than the Japanese people [16, 20].However, one study using the confocal microscopy and the Orbscan corneal topography did not find the correlation between ECD and corneal diameter in the elderly eyes [21].The different results among studies could be due to the different specular microscope used in each study as well.
The annual endothelial cell loss rate in this study was 0.23%, which was similar to the results in Chinese (0.3%) [18], Indian (0.3%) [16], Egyptian (0.3%) [14], and Japanese (0.25%) [17], while this was lower than those from the Middle East and Caucasians i.e. Iran (0.6%) [19]. Yunliang et al. reported the annual cell loss of 0.3% in a normal Chinese population [18]. However, they noted a variation in the cell loss rate in different age groups with a higher loss of cells in the younger age groups. They also suggested that an exponential function to determine the rate of cell loss might be more appropriate than using linear regression analysis. Niederer et al. used in vivo confocal microscopy with contact method for studying corneal morphology and found that the annual rate of CEC cell loss was 0.5% [22].
Aging also influenced other corneal parameters. This study found that age had a direct correlation with the variation of cell size and cell area, and had an inverse correlation with hexagonality. The negative impacts of age on the cell variation (Indian, Chinese, Malay, Filipino) [11, 12, 16, 18], cell size (Malay, Chinese, Indian, Iranian, Turkish, Filipino) [11, 13, 16, 18], and cell shape (Indian, Chinese, Turkish) [13, 16, 18] were previously reported from different study populations. One study in Lithuania did not show the correlation of age on the CV and hexagonality [23].
For the influence of gender on CECs, after adjustment for age this study found that males had a significant inverse correlation with CV and direct correlation with hexagonality. This means that CECs in males are supposed to have fewer variables in size and had more hexagonal shape than in females. However, there was no difference of the ECD among genders. Another study reported that men’s corneal endothelial cells are more regular and have more hexagonal cells [24].These findings were different from studies in the Filipino [11] and Japanese population [17], in which women had significantly greater ECD than that in men. Some studies including Malay [12], Turkish [13], Iranian [19], and Egyptian [14] found no significant differences of mean ECD between genders.The disagreement between studies indicates that the influence of sex on corneal endothelial cells still requires more studies to justify the findings.
Corneal thickness, another important indicator of corneal health and changes in corneal endothelial function, becomes more important in determining the IOP and in planning for refractive surgery. In general, the cornea becomes edematous if the CECs decrease or loss of function. This study demonstrated that the CCT decreased with increasing age and this was similar to studies in Lithuania [23], Turkish [13], and Egyptian [14] populations (Table 4). The decrease CCT with aging may be due to the degenerative changes in corneal structures such as the thinning of corneal stroma, nerve, epithelium, or the CEC body which needs further study to investigate the answer.
This present study found that there was no correlation between CCT and the ECD after adjustment for age. Müller et al. investigated the ECD and corneal thickness in different areas of the cornea in elderly eyes using the confocal microscopy and corneal topography (Orbscan II). They found that ECD significantly correlated with CCT and corneal curvature [21].They suggested that in an older population, low ECD values would be expected in thinner and /or steeper cornea. Results from a population-based study in Japan, using the ultrasound pachymeter (USP) for evaluating the CCT in adult volunteers (age more than 40), found that CCT significantly correlated with ECD even though they suggested that the results may not be clinically significant (r = 0.071) [17]. Although the USP is a frequently used pachymeter, the main disadvantage of this device is the variation among examiners due to the inaccuracy of the probe alignment causing errors in measurement [25].Our study showed that there was a significantly difference of CCT between the right and left eye, which might be caused by the measurement errors such as the eye’s misalignment. Previous studies found that CCT varies over the day (circadian CCT) as the cornea is thicker in the morning and gradually become thinner. This may reflect the change in corneal metabolism occurring during the night with the increase lactate and corneal swelling. To reduce this error, CCT and ECD should be measured at the same time of the day [26, 27]. In addition, the CCT measurement may be affected if performs after applanation tonometry, even though previous study found no significant influence [28]. Nevertheless, the difference in CCT between eyes found in this study (10 μm) may not be clinically significant.
This study used the non-contact specular microscopy that could assess both the corneal endothelial morphology and the central corneal thickness. This non-contact device has advantages of reducing the risk of corneal epithelial injury, transmission of infection, artifacts resulting from corneal manipulation and also providing comfort for the volunteers. However, there were some limitations of this study. First; the ECD in different areas of the cornea were not investigated as well as the relationship of corneal ECD and other parameters such as corneal diameter and curvature, axial length, anterior chamber depth, and refractive errors. Second; this study may be confounded by mild degree and asymptomatic dry eye subjects. As previous study found that corneal ECD significantly decreased in dry eye patients and correlated with clinical severity [6]. The possible mechanisms for endothelial cell loss supposed to be due to the reduced corneal nerve and the associated inflammation in dry eye disease [6, 29]. Nonetheless, the authors noted that ECD might not be affected in mild cases of dry eye which are commonly encountered in clinical practice [6].Third; this study did not adjust the effect of IOP on CCT. Last; there may be some other potential confounding factors such as smoking or nutrition. Therefore, further prospective longitudinal studies are required to evaluate their effects on the change of corneal endothelial cells with aging.