The results of this study indicated that individuals who had recovered from COVID-19 demonstrated a reduction in corneal ECD and the count of HEX cells (pleomorphism), along with an increase in cell CV (polymegathism). These findings were in line with those of Oren et al., Erdem et al., and Kaushik et al.'s studies.
Oren et al. conducted a study on 68 individuals, comprising 34 healthy subjects and 34 individuals who had previously contracted and recuperated from COVID-19. They observed a reduction in the corneal endothelial cells and an increase in cell CV in the infected group compared to the healthy group, based on cell morphology. Furthermore, a decline in HEX cells and an elevation in central corneal thickness were noted, potentially attributable to impaired sodium-potassium pump function [15].
Erdem et al. carried out a study involving 80 patients who had recovered from COVID-19 and 72 healthy individuals. They documented a reduction in endothelial cells within the recovered group, as well as a rise in the CV of endothelial cells. Additionally, there was a decrease in HEX cells and an increase in central corneal thickness [16]. Furthermore, Kaushik et al. performed a study on 129 patients recovered from COVID-19 disease and 123 healthy people as a control group. They reported a reduction in the corneal ECD and the number of HEX cells (pleomorphism) and an increase in the CV of cells (polymegatism) and the central corneal thickness [17].
In a separate investigation, Elshalkami et al. examined 64 right eyes of individuals who had recovered from COVID-19, along with 53 right eyes of healthy individuals as a control group. They found no significant difference in the evaluated parameters between the two groups, which could be attributed to improper grouping of the affected patients, inaccuracies in the timing of the examinations, or errors in the devices utilized for the research [18].
Although contaminated droplets and bodily fluids can readily infect the conjunctival epithelium, it remains uncertain whether COVID-19-related conjunctivitis stems from a primary eye infection or if it arises from the retrograde travel of viral particles from the upper respiratory tract to the nasolacrimal duct [19]. Meduri et al. reported that the deterioration of eye symptoms at the beginning of the COVID-19 disease is due to changes in the tear layer [7].
Freni et al. found that 72% of COVID-19 patients suffered from dry eyes and their Schirmer's test was changed [20]. In the current study, the central corneal thickness increased in affected individuals compared to healthy individuals, and the number of HEX cells decreased in affected individuals, which was also consistent with the findings of other researchers [15–17].
Ma et al. showed that SARS-CoV-2 infection had a stronger association with the cornea than with the conjunctiva [21]. Despite the lower risk of COVID-19 transmission through the cornea, it should still be taken into account [22]. The primary role of endothelial cells is to maintain corneal transparency, which can be compromised by aging, intraocular surgery, and certain systemic illnesses [23–25].
Assessing central corneal thickness is a method for measuring endothelial cell function. Studies have suggested that a reduction in ECD and impaired sodium-potassium pump function can lead to an increase in central corneal thickness, possibly caused by elevated intraocular pressure and inflammation [26, 27].