Changes in the refractive status and ocular biometrics in preterm and term infants

Dear Editor, Preterm infants have been noted to have myopia, hyperopia, or astigmatism at the time of birth, depending upon the gestational age (GA) and stage of retinopathy of prematurity (ROP), if present.[1] With the increase in the survival rate of premature infants, the role of ophthalmologists becomes important in screening and treating these conditions. The present study aims to obtain longitudinal data on the refractive status of preterm infants and compare it with that of term infants in the Indian population. All normal babies admitted to the neonatal intensive care unit or ward of a tertiary healthcare center from January 2019 to September 2020 were studied. Parameters such as GA, birth weight (BW), corneal diameter (CD), axial length (AL), and anterior chamber depth (ACD) were documented at birth and at 3rd, 6th, and 12 months of age. The retinoscopic values were also noted. A total of 90 infants (45 preterm and 45 term infants) were enrolled in the study. There were 55 male and 35 female infants in the study, with a male-to-female ratio of 1.2:1 among preterm infants and 2:1 among the term infants. The two groups had no significant difference in baseline characteristics such as GA (231.82 ± 17.61 and 271.56 ± 7.77 days, respectively; P = 0.001) and BW (1545.27 ± 432.17 and 2800 ± 424.74 g, respectively; P = 0.001). The CD, AL, and ACD were significantly larger in term babies [Fig. 1]. At baseline, 81 infants (90%, 40 preterm) had ametropia. Myopia was detected in 25 (55.5%) preterm infants compared to six term infants (13.3%). At the 12-month follow-up, 70 infants (77.7%, 37 preterm) had ametropia and 20 infants (22.3%) had achieved emmetropisation. The spherical equivalents of infants at 12 months of follow-up in the right eye showed a significant difference in scores for preterm (M = −0.622, SD = 1.82) and term (M = 1.05, SD = 1.037, P = <0.001) [Table 1]. The association between the mean spherical equivalent outcome and gestational age showed a positive correlation (spearman rho r = 0.449, P = <0.001) and BW (r = 0.486, P = <0.001) respectively.Figure 1: Shows the trends in the changes of mean spherical equivalent (a), mean axial length (b), mean corneal diameter (c), and mean anterior chamber depth (d) in term and preterm babiesTable 1: Trend in the mean spherical equivalent changeThe emmetropisation process in preterm infants differs from that in term infants. There have been reports of an initial myopic shift and delayed emmetropisation in preterm eyes.[2,3] In this study, there was an increase in the myopic shift till 3 months, followed by a decrease till 12 months in preterm babies. Thus, 84.4% of preterm babies remained ametropic at 1 year. All the term babies moved toward emmetropisation. A similar observation has been reported by Cook et al.[4] where myopia increased till 2 years of age in preterm babies. Astigmatism showed a rapid decline in the term infants compared to the preterm group. Preterms are reported to have against-the-rule astigmatism, supported by the present study.[2] The AL and CD increased linearly in both preterm and term babies. The ACD among preterm infants decreased at 3 months and then increased, while term infants showed a continuous increase. No previous studies have observed such a change in ACD in preterm infants. The limitations of this study are the absence of corneal curvature and lens thickness analysis, which would have provided more insights into the emmetropisation process. However, most previous studies were cross-sectional, whereas this study was longitudinal, analyzing the changes in the ocular biometrics with time in the same cohort. To conclude, preterm babies tend to have more myopia and astigmatism compared to term babies with significant failure to achieve emmetropisation. The uncorrected refractive error will lead to subnormal visual outcomes even with normal anatomy. The follow-up of preterm babies with or without ROP is essential for early refractive correction and amblyopia treatment. Financial support and sponsorship: Nil. Conflicts of interest: There are no conflicts of interest.