Measurement and Analysis of Central Corneal Thickness at Different Postnatal Stages in Chinese Premature Infants

Purpose The objective of this study was to evaluate central corneal thickness (CCT) in Chinese premature infants at different postnatal stages to study the peak point and analyze influential factors on CCT development. Methods This was a cross-sectional study of premature infants. Initial CCT measurement was taken at 34 weeks of gestational age (GA) and at intervals until 88 weeks of postmenstrual age (PMA) was reached. The comparison and correlation analysis were carried out to access the association of CCT with gender, birth weight (BW), GA, and retinopathy of prematurity (ROP) for each PMA. The premature infants were divided into the thick CCT group and the thin CCT group according to the average CCT at 40 w. And the difference in CCT between the two groups at subsequent 52 w and 64 w was compared. Results A total of 1726 premature infants (3463 measurements) with an average of 2.21 ± 1.57 measurements were included in this study. The CCT decreased from 34 w GA to 52 w PMA (R = 92.36, P < 0.0001) and then reached a plateau (R = 2.541, P=0.3567). Male (P < 0.05), low BW (P < 0.05), and low GA (P < 0.05) were associated with thicker CCT at the early stage of PMA. The premature infants who had thick CCT at 40 w would have thick CCT at 52 w and 64 w accordingly. Conclusions The CCT values of premature infants decreased over time and plateaued at 52 w PMA. Gender, BW, and GA were considered as the influential factors of CCT at the early stage of PMA. Moreover, CCT at 40 w could forecast its development trend at 52 w or 64 w after birth.


Introduction
Central corneal thickness (CCT) has become an important aspect of diagnosing and treating various eye disorders [1].
ere are many techniques for measuring CCT, such as ultrasonic pachymetry, Visante optical coherence tomography, and Fourier-domain optical coherence tomography [2]. e accurate measurement of corneal thickness is vital to evaluate the endothelial function and to manage pediatric glaucoma [3]. Lope et al. demonstrated in their study that the mean CCT was higher in the pediatric glaucoma group [4]. Rebecca et al. also showed that CCT was a phenotypic risk factor for primary open angle glaucoma (POAG) in a genomic study [5].
In general, premature infants were born with immature organs and tissues in their physiological conditions. CCT in premature infants and full-term newborns is significantly thicker compared to adult corneal thickness [6]. Among infants, CCT in premature infants was also found to be thicker than in full terms [7][8][9]. Few studies discussed the CCT development in premature infants during the neonatal period. Choo et al. examined the premature infants at 2 different points, reported a significant decrease in CCT between 1 st at "premature" (PMA between 32 and 36 weeks) and 2 nd at "term" (PMA between 37 and 41 weeks) [6]. Another study also suggested a decrease in CCT from 794 to 559 µm between 24 and 32 w of GA in 56 premature infants [10].
In addition, researches also showed that CCT in premature infants was negatively correlated with GA and birth weight (BW) at early phases, which indicated that premature infants with higher GA and BW would have lower CCT values [11,12]. is agreed to the decrease in CCT with the growth of infants. Overall, however, these studies had a shorter follow-up period and a relatively small sample size. ey also had not discussed whether GA, BW, and presence of retinopathy of prematurity (ROP) were potential influential factors in change in CCT.
Our study aimed to analyze the dynamic change in CCT in a large sample of premature infants within a period of 1 year and to explore whether gender, GA, BW, and presence of ROP could be influential factors in CCT changes.

Patients and Methods
is study was a cross-sectional clinical trial conducted on premature infants at the Affiliated Eye Hospital of Wenzhou Medical University (Hangzhou, China), from February 2014 to April 2018. It followed the tenets of the Declaration of Helsinki. Local ethical approval was obtained from the ethics committee of the Affiliated Eye Hospital of Wenzhou Medical University, Zhejiang, China. Informed consent was obtained from all premature infants' parents. A total of 1726 premature infants were screened for ROP and recruited in this study. e age subjects were screened at 34 w, 36 w, 38 w, 40 w, 42 w, 44 w, 46 w, and 48 w GA and 52 w, 64 w, 76 w, and 88 w postmenstrual age (PMA). Infants with neurological anomalies, systemic syndromes, and other ocular disease such as congenital keratopathy and congenital microphthalmos syndromes were excluded. Gender, BW, GA, PMA, and presence of ROP were recorded at each visit. e follow-up time was 1 year in total.
Prior to ROP screening, the eyelids of infants were opened with a pediatric speculum, followed by instillation of proparacaine hydrochloride 0.5% (Alcaine-Alcon, USA). en, the CCT measurements were performed using handheld corneal pachymeter (PachPen, Accutome, USA). Prior to testing, the probe tip was disinfected using an alcohol swab and was then placed at the central corneal surface gently after complete evaporation of alcohol. ree measurements were taken on each eye using supine position. e average reading was used for analysis. Prophylactic ofloxacin eye ointment was prescribed two times a day in both eyes for three days after the procedure. All operations were completed by one experienced ophthalmologist (Dr. Mao). None of the infants had any sign of adverse effects before, during, or after the procedure.
Statistical analysis was performed using SPSS version 22 (IBM Corp., Armonk, NY, USA). ere was no statistically significant difference between the two eyes. us, the data of the right eye were used for analysis. Mean CCT was expressed in mean ± standard error (SE). Other parameters were recorded as mean ± deviation (SD). Independentsample t-test was used to study association between CCT and gender, BW, GA, and ROP subgroups. One-way ANOVA was used to analyze the CCT at 40 w, 52 w, and 64 w in the thin CCT group and thick CCT group, respectively. A value of P < 0.05 was set for statistical significance.

Results
A total of 1726 premature newborns (3463 measurements) were included in this study. Subjects were evaluated longitudinally 1 to 7 times (in total 3463 times). 943 were male, and 783 were female. e mean BW was 1556.40 ± 409.63 g (range: 670-3790 g). e mean GA was 30.99 ± 2.22 w (range: 23-39 w) ( Table 1). e mean CCTof premature infants at 34 w was 602.67 ± 57.26 μm, which gradually declined with the increase in PMA. At 52 w of PMA, the mean CCT reached 529.17 ± 38.70 μm (R � 92.36, P < 0.0001) and slowly plateaued at 530 μm (R � 2.541, P � 0.3567) ( Table 2 and Figure 1). e mean CCT of males was consistently higher than that of females during the early period (≤42 w) (all P < 0.05). After 42 w, there was no significant difference between males and females at any PMA (Table 3 and Figure 2(a)). e mean CCT of low BW (<1500 g) was consistently thicker than that of the high BW (>1500 g) during the early period (≤40 w) (all P < 0.05). After 40 w, there was no significant difference between low and high BW at any PMA (Table 4 and Figure 2(b)). e mean CCT of low GA (<30 w) was consistently thicker than that of the high GA (>30 w) during the early period (≤40 w) (all P < 0.05). After 40 w, there was no significant difference between low and high GA at any PMA (Table 5 and Figure 2(c)).
ere was no significant difference between groups with and without ROP at any PMA (Table 6 and Figure 2(d)).
At 40 w, the average CCT was 566.66 ± 53.00 µm in 65 eyes from 65 premature infants. We divided these infants into two groups, with 36 having thin CCT < 566.66 µm and 29 eyes having thick CCT ≥ 566.66 µm. Keep a record of CCT in each infant at 40 w, 52 w, and 64 w. e results showed that two groups both kept a decline all the time (P � 0.005 in the thin CCT group and P < 0.001 in the thick CCT group). Furthermore, CCT change was significantly different between the two groups at 40 w (t � −11.50, P < 0.001), 52 w (t � −4.98, P < 0.001), and 64 w (t � −5.86, P < 0.001) ( Table 7 and Figure 3).

Discussion
Both central and peripheral corneal thickness had been studied in adults in several researches [13,14]; however, very little was known about the CCT in premature infants. We studied the trend of CCT change in premature during the 1year period using a large sample size. is study was so far the largest and longest study on the CCT in premature infants. We found that, at early stage of PMA (before 52 w), the CCT of premature infants decreased gradually from 602.67 ± 57.26 µm to 529.17 ± 38.70 µm and then stabilized from 52 w to one year at approximately 530 μm.
Kirwan demonstrated that CCT in premature infants decreased from 691 µm at 31 w to 605 µm at term [15]. Studies by Jethani et al. had showed the CCT of preterm infants(<260 days) decreased from 620.7 ± 88.8 µm to 534.1 ± 57.6 µm at the end of one year [16]. Our finding at the early stage of PMA was consistent with these studies. With the longer observation period in our study, we were able to 2 Journal of Ophthalmology analyze changes in CCT up to 1 year and map out the trend of change more precisely with a larger sample base. In our study, we discovered that the CCT did not continuously decrease until term. e speed of change decreased at later PMA, and the CCT stabilized after. In Rushood study, the CCT of males was significantly higher than females [17]. In our study, the curve in Figure 2 showed that the CCT of males was slightly thicker than females in the early period. Although CCT at each PMA was higher in males, a statistically significant difference was only found prior to 42 weeks.
Uva et al. found a significant correlation between CCT and BW [12]. Gunay observed a negative correlation between GA, BW, and CCT in premature infants and demonstrated that GA and BW could significantly predict the CCT values [11]. However, Karahan et al. showed that CCT did not correlate with GA or BW in their crosssectional study [8]. Figure 3 in our study showed that the CCT of low BW (≤1500 g) was thicker than high BW (>1500 g). e difference in CCT was not apparent after 40 weeks. e same was true about GA. e CCT of low GA (≤30 weeks) was thicker than high GA (>30 weeks). e        decrease in CCT reached an endpoint at 40 weeks and then kept relatively stable. In addition, presence of ROP did not contribute to change in CCT. In our study, gender, BW, and GA were found to be important influencing factors in change in CCT in premature infants. e reason of a different result might be due to a larger sample data and a longer follow-up period.
Kıvanç et al. conducted a study on comparing corneal thicknesses of prematurely born and full-term early school-aged children according to GA, and it was concluded that CCT in the small GA group was significantly lower than the appropriate GA group. In addition, CCT in the term children was higher than those of the prematurely born children [18]. In our study, premature infants were grouped as the thin CCT group (CCT < 566.66 µm) and the thick CCT group (CCT ≥ 566.66 µm) at 40 w. e two groups hold the similar decreasing rates, and the infants with thick CCT at 40 w also had a thicker CCT at 52 w and 64 w. is indicated that the CCT at 40 w had a correlation with that at one year, which could predict the CCT variation trend during the first year after birth.
is study has several limitations. e result of CCT value at the age of 2 to 3 years, or even older, was not included. e comparison of CCT in premature and full-term newborns or adults was not included in our study, which could be the future direction.

Conclusions
In our study, we found that the mean CCT had a weekly reduction at the early stage of life and then stabilized after 3 months of PMA, and the thick CCTat 40 w would maintain a thicker CCT at 52 w and 64 w, which could predict the variation trend of CCT during the first year after birth. e CCT measurements for each GA and PMA may contribute to a future reference parameter in diagnosing and managing pediatric glaucoma in premature infants.

Data Availability
e data used to support the findings of this study are available from the corresponding author on request.

Disclosure
Chenyi Liu and Hanfei Wu are the co-first authors. is study was performed in the Affiliated Eye Hospital ofWenzhou Medical University (Hangzhou, China).

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.