Background
Necrotizing enterocolitis (NEC) is a serious digestive tract disease which occurs more frequently in very preterm infants (VPIs) during the neonatal period. According to the data of several systematic reviews [
1,
2], the incidence of NEC is 7% and the mortality is 20%~30% in premature infants with a birth weight of 500 ~ 1500 g, and there has been no evident improvement in these statistics in recent years. A study enrolling 10,823 VPIs from 57 tertiary neonatal intensive care units (NICUs) of the Chinese Neonatal Network in 2019 showed that the survival rate of VPIs was 95.4%, and the incidence of NEC was 4.9%; notably, the incidence of NEC increased with decreasing gestational age [
3]. In 90% cases, NEC occurred in VPIs with a gestational age of < 32 weeks [
4]. The risk factors for NEC vary with the time of onset. Short et al. [
5]. Proposed an onset time of ≤ 7 days after birth to represent early-onset NEC (EO-NEC) and an onset time of > 7 days after birth as late-onset NEC (LO-NEC). Considering the small amount of enteral nutrition in the early postnatal period, the causes of EO-NEC are mostly related to perinatal factors, such as asphyxia and intrauterine infection. Conversely, LO-NEC is closely related to high-risk factors in the NICU, such as improper feeding, intestinal flora imbalance, long fasting time, and late-onset infection [
6]. Most NEC cases occur 13 ~ 23 days after birth, which represents LO-NEC [
2]. There is a paucity of data on the related factors of those cases occurring between 8 ~ 14 days after birth, which also belong to LO-NEC. Therefore, analyzing the high-risk factors for LO-NEC with different onset times to prevent the occurrence of NEC has immense guiding significance in clinical practice.
In terms of nutrition, we explored the high-risk factors for LO-NEC and its impact on the short-term clinical outcomes among VPIs through a retrospective analysis of the clinical data from 28 Chinese NICUs in our study. We hope the findings will help further optimize the treatment strategy for premature infants in the NICU, particularly with improving the nutrition management, reducing the incidence of LO-NEC, and providing a theoretical basis for improving the short-term prognosis of VPIs with LO-NEC.
Discussion
LO-NEC is one of the main causes of death in premature infants one week after birth [
17]. The early clinical manifestations and X-ray imaging results of NEC are often nonspecific, and the progress is rapid. By the time its diagnosis is established, the condition is often serious and generally presents with intestinal necrosis and perforation, which require surgical treatment. Many of the survivors present with short bowel syndrome, which can widely be complicated with intestinal stenosis, PNAC, EUGR, and other short-term adverse prognoses and even with neurocognitive dysfunction in the long term. Therefore, the prevention of this disease is the most critical, and to this end, it is necessary to recognize the high-risk factors for NEC early and intervene promptly, which can greatly improve the clinical outcome of premature infants.
At present, LO-NEC is considered to result from multiple factors which are mainly related to an altered intestinal barrier immune response to feeding and the developing microbiome in premature infants; LO-NEC development can also be attributed to hypoxia, hemodynamic instability, and infection [
1,
18]. In our study, compared to the non-LO-NEC group, the LO-NEC group had a higher incidence of anemia, although there was no difference after adjustment. Anemia can impair splanchnic perfusion, resulting in tissue hypoxia. Anemia can also impair the normal maturation of vascular autoregulation in the premature intestine, predisposing to ischemic injury, and possibly, NEC [
19]. Therefore, the early prevention and treatment of anemia is encouraged in the NICU. Potential means of preventing anemia include delayed cord clamping and methods for limiting phlebotomy losses.
This study confirmed that low GA and grade III ~ IV NRDS were high-risk factors for LO-NEC occurring between 8 ~ 14d. A lower GA with a higher incidence of grade III ~ IV NRDS is associated with intestinal immaturity and vulnerability to various pathological factors, consequently leading to NEC [
20]. In VPIs with grade III ~ IV NRDS, immature lungs have damaged gas exchange function and intestinal tissue oxygenation disorder, which lead to NEC. In our study, all grade III-IV NRDS patients have received IMV treatment. Compared to the LO-NEC group occurring after 14 d, higher proportion of grade III ~ IV NRDS and longer IMV duration were found in the LO-NEC group occurring between 8 ~ 14d. It has been hypothesized that indwelling endotracheal tube may compromise the infants’ mucosal barriers increasing the risk of infection [
21]. Moreover, variations in intrathoracic pressure during IMV had a major impact and affect systemic venous return, right ventricular preload, left ventricular preload, right ventricular afterload, left ventricular afterload and myocardial contracility, which may affect blood supply to the intestine and consequently increase the risk of NEC [
22].
A long fasting time may lead to intestinal mucosal atrophy, intestinal dysfunction, abnormal intestinal flora colonization, and excessive intestinal inflammatory response in premature infants, which are high-risk factors for NEC [
23]. Notably, enteral feeding can prevent intestinal atrophy, enhance mucosal adaptability, and stimulate intestinal movement and growth. This study shows that early feeding is a protective factor against LO-NEC occurring between 8 ~ 14d. A prospective randomized controlled trial showed that VPIs given trophic feeding within 24 h of birth had faster body weight growth and that this approach helped prevent NEC [
16]. In agreement with previous studies, the present study also confirmed that breastfeeding was a protective factor for LO-NEC. The protective effect of breastfeeding on NEC reportedly has a dose–effect relationship. Lapidaire et al. confirmed that every 10% increase in breast milk intake was associated with a ~ 12% reduction in the incidence of NEC [
24]. Miller et al. [
25]. also reported a more evident protective effect when the intake of breast milk was ≥ 50% of the total intake. However, the overall breastfeeding ratio in this study was not high because most NICUs in China did not receive breast milk during the COVID-19 pandemic; this may also explain the incidence of LO-NEC being slightly higher in this study (8.4%) than in other studies (7.0%) [
2].
The results of this study showed that a higher cumulative dose of the MCT/LCT emulsion in the first week was a risk factor for LO-NEC occurring between 8 ~ 14d; however, the cumulative dose of SMOF did not statistically significantly differ between the two groups. Some investigators have pointed out that the use of lipid emulsion impairs monocyte, lymphocyte, and neutrophil functions, and these changes seemed to be related to the quantity and rate of lipid administration; conversely, the use of n-3 fatty acids (represented by SMOF) produces less immunosuppressive eicosanoids [
26]. A major finding of a previous study was that compared to the standard dose of intravenous lipid emulsion (starting from 0.5 g/kg/day
−1 and gradually increased by 1 g/kg/day to a maximum of 3.5 g/kg/day), a restricted dose of intravenous lipid emulsion (1 g/kg/day) significantly increased the rate of bacterial clearance in preterm infants with blood stream infections [
27]. A significant increase in the levels of n-6 long-chain polyunsaturated fatty acids (LCPUFAs), proinflammatory lipid metabolites, such as arachidonic acid-containing glycerophospholipids, and eicosanoids, such as prostaglandin A2 (PGA2), was observed in the intestinal tissue of NEC pigs [
28]. The MCT/LCT emulsion contains a high proportion of ω-6 LCPUFAs (ω-6:ω-3 = 7:1). ω-6 Arachidonic acid (AA), a metabolite of LCPUFA, is a precursor of inflammatory mediators. It can catalyze the formation of PG and TBX
via cyclooxygenase, reduce and weaken the phagocytosis of neutrophils, promote oxidative stress injury, increase the production of inflammatory mediators, and damage the vascular endothelial system and immunosuppression [
29], thus resulting in an increased risk of NEC. In addition, ω-6 LCPUFAs metabolize to produce a strong vasoconstrictor, TXA2, whereas the vasodilator PGI2 is significantly reduced; these changes constrict the blood vessels and form thrombi in severe cases [
25,
29], thus affecting intestinal blood flow and inducing NEC. This study also confirmed that early feeding was a protective factor for LO-NEC occurring between 8 ~ 14d, whereas a long fasting time was a risk factor for LO-NEC. A long fasting time and late feeding of VPIs will inevitably increase the cumulative dose of the MCT/LCT emulsion ingested intravenously in the first week, which may further increase the risk of NEC. Therefore, enteral nutrition should be started as soon as possible to minimize the dosage of the MCT/LCT emulsion in the first week. Notably, giving SMOF lipid emulsion with less ω-6 LCPUFAs may be beneficial to reduce the incidence of LO-NEC. Similarly, regarding nutrition aspects, lower early feeding rate and breastfeeding rate, more MCT/LCT emulsion intake, slower growth velocity were found in the LO-NEC group occurring between 8 ~ 14d when compared to the LO-NEC group occurring after 14 d. These results may suggest that nutrition plays a role in affecting the timing of NEC occurrence, which is worth further exploration.
NEC occurring in VPIs may increase the risk of short-term adverse outcomes. Shah et al. [
30]. reported that NEC was associated with an increased risk of ROP, BPD, and PVL. Compared with the non-LO-NEC group, our study found that the incidence of EUGR, LOS, PNAC, and MBDP in the LO-NEC group was 1.6, 1.5, 2.4, and 2.6 times higher, respectively. A foreign study has pointed out that more than half of NEC premature infants have EUGR [
31]. A recent domestic study also confirmed that the risk of development of EUGR was 5 times higher in very low-birth-weight infants with confirmed NEC, regardless of whether it was treated surgically [
32]. The reasons were considered to be related to delayed feeding, longer fasting time, slower GV, longer time to regain BW, longer time to reach total enteral nutrition, and longer time to reach a total caloric intake of 110 kcal/kg/d in the LO-NEC group. Less nutrient intake and malabsorption caused by short bowel syndrome or intestinal stenosis after intestinal resection also increase the incidence of EUGR [
33]. Due to NEC, the parenteral nutrition (PN) time is prolonged with decreased intestinal peristalsis, which may lead to intestinal flora translocation through the damaged intestinal wall, causing bacteria and toxins to invade, which leads to an increased risk of LOS. In addition, endotoxin invades the portal venous system, leading to the activation of liver Kupffer cells in the early stage, which results in hepatocyte damage and cholestasis [
34]. A study using a mouse model found that when there was intestinal injury and the PN time was prolonged, which lead to hepatocyte injury and PNAC occurrence, neither the intestinal injury nor the PN prolongation alone was sufficient to trigger the above results, suggesting that the combined effect of intestinal injury and PN prolongation led to the occurrence of PNAC [
35]. NEC and PNAC in premature infants delayed establishment of enteral nutrition, and a PN time of > 4 weeks are reportedly high-risk factors for MBDP [
36].
This study had several limitations. First, this study lacks information regarding some important aspects, such as chorioamnionitis during pregnancy, delayed cord clamping, hypoxic events, impaired placental function, and the use of probiotics. These confounding factors may have influenced the clinical outcomes. Second, the data presented here come from a prospective multicenter study on factors influencing VPIs-EUGR in China, which excluded patients who died at the initial stage and during hospitalization. Therefore, the effect of NEC on death as a clinical outcome was not studied. Finally, nutrition management strategies of each center are slightly different, particularly enteral nutrition, and therefore, the research results may be biased.
Acknowledgements
We would like to thank all participants of the Chinese Multicenter EUGR Collaborative Group, which consisted of: Department of Neonatology, Women and Children’s Hospital, School of Medicine, Xiamen University, Xiamen, Fujian, China (Kun Yao Hong, Yao Zhu, Wei Shen, Li-Xia Tang, Zhi Zheng, and Xin-Zhu Lin); Department of Neonatology, The Third Affiliated Hospital of Guangzhou Medical University, Guangzhou, Guangdong, China (Fan Wu, Qian-Xin Tian, and Qi-Liang Cui); Department of Pediatrics, Shengjing Hospital of China Medical University, Shenyang, Liaoning, China (Jian Mao, Yuan Yuan, and Ling Ren); Department of Neonatology, Guiyang Maternity and Child Health Hospital/Guiyang Children’s Hospital, Guiyang, Guizhou, China (Ling Liu, Bi-Zhen Shi and Yu-Mei Wang); Department of Pediatrics, Peking University Third Hospital, Beijing, China (Yan-Mei Chang, Jing-Hui Zhang, and Xiao-Mei Tong); Department of Neonatology, Children’s Hospital of Fudan University, Shanghai, China (Rong Zhang, Yan Zhu); Department of Neonatology, Maternal and Child Hospital of Guangdong Province, Guangzhou, Guangdong, China (Xiu-Zhen Ye, Jing-Jing Zou); Department of Neonatology, General Hospital of Ningxia Medical University, Yinchuan, Ningxia, China (Yin-Ping Qiu, Yu-Huai Li, and Bao-Yin Zhao); Department of Neonatology, Children’s Hospital of Hebei Province, Shijiazhuang, Hebei, China (Shu-Hua Liu and Li Ma); Department of Neonatology, Children’ Hospital of Nanjing Medical University, Nanjing, Jiangsu, China (Ying Xu and Rui Cheng); Department of Neonatology, The First Hospital of Jilin University, Changchun, Jilin, China (Wen-Li Zhou and Hui Wu); Department of Neonatology, Quanzhou Maternity and Children’s Hospital, Quanzhou, Fujian, China (Zhi-Yong Liu and Dong-Mei Chen); Department of Pediatrics, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei, China (Jin-Zhi Gao, Jing Liu, and Ling Chen); Department of Neonatology, Liaocheng People’s Hospital, Liaocheng, Shandong, China (Cong Li, Chun-Yan Yang, and Ping Xu); Department of Neonatology, the Affiliate Hospital of Inner Mongolia Medical University, Hohhot, Inner Mongolia, China (Ya-Yu Zhang, Si-Le Hu, and Hua Mei); Department of Neonatology, Suzhou Municipal Hospital, Suzhou, Jiangsu, China (Zu-Ming Yang, Zong-Tai Feng, and San-Nan Wang); Department of Neonatology, The Third Affiliated Hospital of Zhengzhou University, Zhengzhou, Henan, China (Er-Yan Meng, Li-Hong Shang, and Fa-Lin Xu); Department of Neonatology, Chengdu Women’ and Children’s Central Hospital, School of Medicine, University of Electronic Science and Technology of China, Chengdu, Sichuan, China (Shaoping Ouand Rong Ju); Department of Neonatology, Hunan Children’s Hospital, Changsha, Hunan, China (Gui-Nan Li and Juan Yi); Department of Neonatology, People’s Hospital of Xinjiang Uygur Autonomous Region, Urumqi, Xinjiang, China (Long Li and Yong-Qiao Liu); Department of Neonatology, Guangzhou Women and Children’s Medical Center, Guangzhou, Guangdong, China (Zhe Zhang and Mei-Gui Wu); Department of Neonatology, Shanghai Children’s Medical Center, Shanghai, China (Fei Bei and Ye Liu); Department of Neonatology, Children’s Hospital of Chongqing Medical University, Chongqing, China (Chun Deng and Hui-Jie Yang); Department of Neonatology, The First People’s Hospital of Yulin, Yulin, Guangxi, China (Ping Suand Shi-Feng Chen); Department of Neonatology, the People’s Hospital of Baoji, Baoji, Shanxi, China (Ling-Ying Luo and Lin-Lin Wang); Department of Pediatrics, Affiliated Hospital of Qingdao University, Qingdao, Shandong, China (Xiao-Hong Liu and Li-Hua Yan); Departments of Neonatology, Shandong Provincial Hospital Affiliated to Shandong First Medical University, Jinan, Shandong, China (Li-Jun Wang and Xiao-Kang Wang); and Departments of Neonatology, Xi’an Children’s Hospital, Xi’an, Shanxi, China (Shu-Qun Yu and Qiao-Mian Zhu). This study was also supported by Project of Clinical key specialty of Fujian Province (Specialty in neonatology).
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