Retinopathy of prematurity (ROP) is characterized by abnormal proliferation of retinal blood vessels in the eyes of premature infants. The incidence and severity of ROP increase with decreasing gestational age and birth weight, posing a significant threat to infant vision. Severe cases may lead to retinal detachment and subsequent blindness (1–4). ROP is linked to elevated levels of vascular endothelial growth factor (VEGF) caused by local hypoxia, promoting aberrant formation of blood vessels in newborns. After birth, premature infants experience a deficiency in the supply of IGF-1 from the placenta and lack autonomous production, thereby impacting the development of retinal blood vessels. Key factors contributing to ROP include premature birth, low birth weight, oxygen therapy, anemia, blood transfusions, sepsis, metabolic acidosis, respiratory distress syndrome (RDS), apnea, and maternal pregnancy complications (5, 6). As our understanding of ultra-premature infants advances, early identification of high-risk factors for ROP becomes crucial for prevention and treatment, ultimately reducing the incidence of blindness and improving the quality of life for premature infants (7).

Vitamin D, a biologically active fat-soluble vitamin synthesized endogenously, not only regulates calcium and phosphorus metabolism but also exhibits antioxidant and anti-inflammatory effects. Additionally, it plays a role in anti-angiogenesis, cell growth, and differentiation. Eye tissues contain vitamin D receptors and 1-hydroxylase, indicating a local regulatory role (8). Given that placental transfer of vitamin D mainly occurs during the third trimester, vitamin D deficiency is a concern, particularly in premature infants with a gestational age <32 weeks (9, 10). However, there is limited research on the association between vitamin D and ROP. Some studies suggested that lower 25(OH)D levels in premature infants with a gestational age <32 weeks after birth might be related to ROP (11–13).

This study aims to collect 25(OH)D levels in premature infants with a gestational age <32 weeks at 1 month after birth and retrospectively analyze clinical data to explore the relationship between serum 25(OH)D levels and ROP. The findings provide valuable insights for the clinical prevention and treatment of ROP in premature infants.

SPSS 29.0 statistical software was utilized for comprehensive data analysis, while GraphPad Prism 10 software facilitated graphical representation. For measurement data conforming to a normal distribution, mean ± standard deviation (x ± s) was employed. Group comparisons were conducted using independent samples t-test and single-factor ANOVA. In cases where measurement data deviated from normal distribution, the results are presented as median (interquartile range) M (Q1, Q3), and the Mann–Whitney U rank sum test was applied for between-group comparisons.

Count data was expressed as case (percentage) n (%), and the χ²-test or Fisher's exact probability method was employed for group comparisons. Single-factor analysis helped identify statistically significant influencing factors as independent variables, with ROP occurrence serving as the dependent variable in the logistic regression model for multi-factor analysis. A significance threshold of p < 0.05 was considered indicative of statistically significant differences.

ROP is one of the leading causes of blindness in preterm infants. The development of fetal retinal blood vessels initiates from the optic disc at approximately 16 weeks of gestation and progresses towards the periphery. By about 32 weeks of gestation, these vessels reach the lateral peripheral part. Notably, the temporal retinal blood vessels do not achieve full maturity until the fetus reaches full term. The immaturity of retinal blood vessels is more pronounced in infants with younger gestational ages, correlating with an increased likelihood of ROP development. In this study, the incidence of ROP in premature infants with a gestational age <32 weeks was found to be 25.35%, a figure that aligns closely with the results reported in both domestic and foreign studies (16, 19–21).

The etiology and pathogenesis of Retinopathy of Prematurity (ROP) involve the intricate interplay of multiple factors. Notably, preterm birth and low body weight stand out as foundational contributors to the development of ROP (19, 21). Cytokines that play a crucial role in promoting retinal development are relatively scarce in early pregnancy and do not witness a significant increase until the later stages of pregnancy. Among these cytokines, insulin-like growth factor-1 (IGF-1) emerges as a key regulator of the vascular endothelial system, playing a pivotal role in the occurrence and progression of ROP (22, 23). After birth, premature infants experience a deficiency in the supply of IGF-1 from the placenta and lack autonomous production, thereby impacting the development of retinal blood vessels. The findings of this study corroborate these insights, revealing that children in the ROP group had lower gestational ages and birth weights compared to those in the non-ROP group. It is evident that low birth weight serves as a high-risk factor for the onset of ROP.

Due to the underdeveloped respiratory system and nervous system of premature infants, they often experience frequent apnea or neonatal respiratory distress syndrome, necessitating high-concentration oxygen therapy for survival. The immature retinal blood vessels in these infants are highly sensitive to oxygen, and exposure to high concentrations can lead to vasoconstriction and, in severe cases, occlusion, resulting in relative hypoxia in the retina. The intricate interplay of various active factors under these conditions promotes the generation of numerous new blood vessels, culminating in the development of Retinopathy of Prematurity (ROP) (24, 25). The findings of this study align with these mechanisms, indicating that children with ROP had higher durations of invasive mechanical ventilation, non-invasive mechanical ventilation, and oxygen therapy compared to those without ROP. Notably, invasive ventilation time emerged as an independent risk factor for the occurrence of ROP. de las Rivas Ramírez et al. retrospectively collected ROP data from preterm infants treated in Regional University Hospital of Málaga, in the multivariate analysis, weight and mechanical ventilation duration, and late-onset sepsis were independently associated with the development of ROP (26).

This study identifies sepsis as a significant risk factor for Retinopathy of Prematurity (ROP). Sepsis poses an increased risk for ROP, with inflammatory factors playing a pivotal role in this association (27, 28). During infections, premature infants release substantial amounts of cytokines, including IL-6 and TNF-α. These cytokines can inflict damage on endothelial cells or disrupt vascular function, subsequently stimulating retinal vascular endothelial cells to produce vascular endothelial growth factor (VEGF), thereby contributing to the onset of ROP (29–31). Infections in premature infants may also induce hemodynamic changes, influencing retinal blood perfusion and resulting in heightened retinal ischemia, ultimately leading to the development of ROP.

Antenatal steroids emerge as protective factors against Retinopathy of Prematurity (ROP), effectively reducing the risk of its occurrence (32, 33). Notably, this study revealed a notably low rate of antenatal steroid administration in the ROP group, standing at only 34.5%. The “European Guidelines for the Management of Neonatal Respiratory Distress Syndrome” advocate for the administration of antenatal steroids to all pregnant women with a gestational age of less than 34 weeks and those at risk for preterm birth. The administration of glucocorticoids aids in promoting fetal lung maturation, decreasing the postpartum oxygen requirement, and consequently mitigating the risk of ROP. Additionally, antenatal steroids exhibit inhibitory effects on the production of oxidative stress and inflammatory factors, indirectly contributing to the prevention of ROP.

Vitamin D undergoes hydroxylation in the human liver to form 25-(OH)D. This metabolite, characterized by a stable structure and a long half-life, serves as the optimal indicator for assessing the nutritional status of vitamin D (34). Studies by Alsalem et al. (8) have identified the presence of vitamin D receptors and hydroxylase in ocular cells, including corneal epithelial and retinal cells. These cells have the capacity to convert vitamin D from an inactive form to its active form. Given that intrauterine placental transport of vitamin D predominantly occurs in the third trimester, vitamin D deficiency is prevalent among premature infants. Despite routine vitamin D supplementation, our study reveals an overall deficiency in 25-(OH)D levels among premature infants in our hospital. The serum 25-(OH)D concentration in the ROP group at one month is significantly lower than that in the non-ROP group, exhibiting a consistent pattern, and that had significant difference between treated group and untreated group. A study conducted in Iran (35) found an association between low vitamin D levels in premature infants on the first day after birth and ROP. The severity of ROP increased with greater vitamin D deficiency. Another study from India (36) demonstrated persistent vitamin D deficiency in premature infants with ROP at 4 weeks after birth compared to those without ROP. The current understanding of the pathogenesis of vitamin D in retinopathy encompasses several aspects: Antioxidant effect: Vitamin D's antioxidant properties mitigate oxidative stress damage to the retina and optic nerve by preserving mitochondrial function and neutralizing free radicals (37). Anti-inflammatory and immunomodulatory effects: Studies have demonstrated that vitamin D can modulate T cell subset proportions in retinopathy, inhibit inflammatory response signaling pathways, and decrease inflammatory cell infiltration, thereby reducing damage to the retina and optic nerve (38, 39). Anti-angiogenic effect: Deregulation of vascular endothelial growth factor (VEGF) levels leads to (ROP). VIT-D binding domains are present in the promoter regions of VEGF, thus, VIT-D supplementation might aid in restoring VEGF levels, especially in pathologic conditions secondary to low VEGF levels (40). Jamali et al. (41) have demonstrated that vitamin D's inhibitory effect on retinal neovascularization is dependent on the expression of vitamin D receptors. Experimental studies in a mouse model of oxygen-induced ischemic retinopathy revealed that mice treated with calcitriol exhibited a significant decrease in VEGF expression, retinal neovascularization, and blood-retinal barrier permeability (42, 43). Investigation of serum vitamin D levels is recommended, Early correction of vitamin D deficiency may lead to reduction of RoP in premature infants (35).

In conclusion, this study highlights that Vitamin D, lower birth weight, long-term invasive mechanical ventilation, and sepsis were associated with incidence of ROP in preterm infants.

Standardizing the administration of antenatal steroids, providing judicious vitamin D supplementation, and closely monitoring serum 25-(OH)D levels can be instrumental in mitigating the incidence and severity of ROP in this vulnerable population.