Using a nationwide registry, this study demonstrated that a single episode of sepsis without NEC was not associated with NDI at 18–24 months of CA in very preterm infants. In contrast, infants with multiple episodes of sepsis or with both sepsis and NEC were at an increased risk of NDI. The multiple-hit theory, which posits that repeated insults can lead to cumulative brain injury, is a key concept in understanding the pathophysiology of brain injury in preterm infants.11 Thus, distinguishing between single and multiple episodes of sepsis and/or NEC may be an important approach when examining the effects of systemic inflammation on neurodevelopmental outcomes in preterm infants.

Despite evidence linking systemic inflammation in preterm infants with adverse neurodevelopmental outcomes, several studies have reported no discernible association between sepsis and NDI in this population. A recent retrospective study involving preterm infants born at <30 weeks of gestation in the Netherlands showed that episodes of sepsis were not associated with NDI at 2 years.8 Another retrospective study involving VLBW infants in Japan reported that sepsis or NEC was not associated with death or NDI after discharge.9 A single-center cohort study involving extremely low-birth-weight infants in the US also reported that sepsis or NEC was not associated with impaired neurosensory development.10 However, in these studies, multiple or recurrent episodes of sepsis or NEC were not considered.

In contrast, several studies have further categorized the study population separately into those who experience both sepsis and NEC or those who experience sepsis but not NEC. A study by the NICHD Neonatal Research Network reported that infants who experienced sepsis and NEC showed the highest risk for NDI.4 Data from the Extremely Low Gestational Age Newborn Study revealed that developmental indices were lowest in infants who experienced bacteremia with medical or surgical NEC.12

Cerebral white matter injury plays a crucial role in NDI in preterm infants with sepsis or NEC.6 White matter injury is attributed to pre-myelinating oligodendrocytes, which are vulnerable to insults, such as hypoxia/ischemia and infection/inflammation during this period.20 One of the key hypotheses in white matter injury is the multiple-hit theory, which suggests that the risk of white matter injury escalates with cumulative exposure to multiple perinatal risk factors.11,21,22 In a study that analyzed 133 newborns born before 34 weeks of gestation, infants exposed to multiple episodes of culture-positive infections had a higher risk of progressive white matter injury on magnetic resonance imaging than those exposed to one or fewer infections.23

The most significant observation in this study was that infants who experienced only a single episode of sepsis did not show an increased risk of NDI. Although the detrimental effects on the white matter may accumulate with repeated systemic inflammation, a single incident may not provoke clinically discernable outcomes. As previous studies have revealed an increased risk of impaired neurodevelopment in patients with sepsis4,5,12 or reported no association between sepsis and neurodevelopment9,10 without considering the number of episodes of sepsis or NEC, the findings of the present study may have important implications. Recently, a single-center retrospective study that explored the association between coagulase-negative staphylococcal sepsis and neurodevelopmental outcomes, excluding infants with additional episodes of sepsis, showed no association between sepsis and white matter injury or adverse neurodevelopmental outcomes.24 Additionally, we analyzed the Sepsis+/NEC− group by categorizing patients based on the type of pathogen to investigate whether the degree of impact on NDI differs according to the infectious agent. Among the patients in this group, 1397 were infected with gram-positive bacteria, 338 with gram-negative bacteria, and 95 with fungi. When compared to the group with no events (Sepsis−/NEC−) using multivariate logistic regression, the odds ratios for NDI and NDI or death were not significantly different across all pathogen types (data not shown).

For patients who experienced NEC without sepsis, the results of the present study did not reveal an increased risk of NDI, a finding that contrasts with the prevailing view in existing research that NEC generally elevates the risk of NDI.12,25,26 First, these studies did not strictly exclude sepsis cases in patients with NEC. As one- or two-thirds of preterm infants with NEC also experience sepsis, it is necessary to distinguish the additional effects of sepsis to understand the effect of NEC on NDI.12,13 Martin et al. categorized infants with NEC with or without bacteremia and found no increased risk of NDI in infants with medical NEC without bacteremia.12 Second, this might be attributed to the group allocation of NEC, as we did not further categorize NEC as medical or surgical. Surgical NEC usually shows a higher degree of systemic inflammation than medical NEC, and the risk of impaired neurodevelopment is higher in surgical NEC.26,27,28 In the sub-cohort analysis, after excluding those who experienced sepsis, preterm infants with medical NEC showed no increased risk of NDI and NDI or death, whereas those with surgical NEC showed an increased risk of NDI and NDI or death (Supplementary Fig. 1).

This study had some limitations. First, although lumbar puncture is commonly performed in neonates with sepsis, the KNN database does not include specific information on whether it was conducted at the time of sepsis onset, limiting the interpretation of our findings. Cases of meningitis were excluded based solely on the reported data, which may not fully account for undiagnosed cases in the absence of lumbar puncture. Second, the follow-up rate of 55.6% at 18–24 months of CA for neurodevelopmental assessment was low, and different modalities were combined to define NDI. Furthermore, data on the severity of each event were not collected from the nationwide registry. The severity of the systemic inflammatory response experienced by patients during sepsis or NEC could vary widely, and the more severe the inflammatory response, the more significant the likelihood of white matter injury could be.8,29 A more detailed and insightful analysis of neurodevelopmental outcomes could have been achieved if data on inflammatory markers, detailed neuroimaging findings—including the degree of white matter injury and brain volume—and the extent of exposure to hypotension or hypoxia had been available. Moreover, genetic predisposition may affect susceptibility to inflammation or the likelihood of brain injury; however, such influences could not be accounted for in this study. However, a strength of this study is the use of a nationwide registry of preterm infants, allowing for the adjustment of various confounding factors, including prenatal inflammation, such as histologic chorioamnionitis, to analyze the association between postnatal systemic inflammation and neurodevelopmental outcomes in very preterm infants.

In conclusion, this study utilized a nationwide registry and observed that a single episode of sepsis was not associated with NDI and NDI or death. However, multiple episodes of sepsis and/or NEC significantly increased the risk of NDI and NDI or death in the study population. Although postnatal systemic inflammation was associated with adverse neurodevelopmental outcomes, a single episode of sepsis may not increase the risk of NDI in preterm infants. Further studies that would consider the degree of systemic inflammation in a single episode of sepsis and neurodevelopment are required.