Patent ductus arterious and increased conjugated bilirubin in the second week after birth are independent risk factors for necrotizing enterocolitis in preterm infants: an observational study

Neonatal necrotizing enterocolitis (NEC) is a common critical illness of the gastrointestinal system in neonatal intensive care units, which manifests as ulceration, ischemia, and even necrosis of the intestinal wall tissue. Among neonates younger than 34 weeks in neonatal intensive care units in China, the incidence of NEC is about 3% [1]. NEC is prone to serious complications such as intestinal stricture, short bowel syndrome, and delayed growth and development.

In 1998, Mashall proposed the concept of the “gut-liver axis” [2]. The liver and gallbladder are connected with the intestine through the portal system, and intestinal microecology can affect the immune function of the liver [3]. On the one hand, when the intestine is damaged, the intestinal flora can transfer and enter the portal system; on the other hand, the flora and endotoxin entering the portal system will stimulate the liver macrophage system to release a series of inflammatory factors. This process aggravates the flora disturbance and causes intestinal mucosal damage. The intestinal and hepatobiliary systems form a complex network of interactions [4]. In neonatal bilirubin metabolism, there is special enterohepatic circulation. In this study, the clinical data of preterm infants were collected, and the differences in early serum- conjugated bilirubin between NEC preterm infants and non-NEC preterm infants were compared to understand the effect of serum-conjugated bilirubin on NEC.

NEC is more common in preterm and low birth weight infants than full-term babies [7], in addition, preterm infants usually develop this disease at 2–4 weeks after birth. Symptoms of NEC are often atypical, which makes the early diagnosis of NEC difficult. Therefore clinicians may miss the opportunity for diagnosis and treatment, which leads to an increase in surgical rates. In this study, we included a variety of prenatal and postnatal factors and found that feeding time, PDA, and bilirubin were associated with the occurrence of NEC. Through multivariate analysis, we found that PDA and increased conjugated bilirubin in the 2nd week after birth were associated with NEC. Conjugated bilirubin showed a continuous upward trend in the NEC group before the occurrence of NEC, while the non-NEC group did not show this trend, which is a new finding compared to previous studies. We think that the increased serum-conjugated bilirubin before the occurrence of NEC may have an impact on the occurrence of NEC. The following points are possible mechanisms. (1) Impaired gut physical barrier: Sustained elevation of conjugated bilirubin may indicate the occurrence of cholestasis, where bile acid secretion may also be impaired. A study has shown that normal physiological doses of bile salts excreting into the intestine can promote the proliferation of intestinal mucosal epithelial cells [8]. An animal study [9]showed that in a murine model of cholestasis, the small intestinal mucosa was edematous, the distribution of epithelial tight junction proteins was abnormal, and the permeability of intestinal mucosa was increased. (2) Transfer of intestinal flora: Conjugated bilirubin secreted from the liver into the bile flows to the intestine for excretion. It undergoes microbial catabolism in the intestine and converts to urobilinogen, then forms stercobilinogen for excretion. Elevated serum-conjugated bilirubin may indicate changes in the gut microbiome. Animal studies have shown that in mice with cholestasis, coupled with elevated bilirubin, the diversity of intestinal flora is lower than that in normal mice [10]. (3) Hormonal changes: Decreased bile secretion affects the normal secretion of gastrointestinal hormones [11], resulting in digestive tract dysfunction. These factors accelerate intestinal damage, which may lead to neonatal necrotizing enterocolitis. To sum up, the increase of conjugated bilirubin may cause certain intestinal damage.

In addition, before the diagnosis of NEC, some pathological damage may have occurred in the intestine. The intestinal mucosa is damaged, and gastrointestinal hormones such as cholecystokinin are reduced. This process may affect the normal excretion of bile, resulting in an increase of conjugated bilirubin. The diversity of gut microbiota decreased [12]. When the intestinal flora decreases, the intestinal environment can be alkaline, and the activity of β-glucuronidase increases in this environment [13]. The enterohepatic circulation of bilirubin is active, and more bilirubin is returned to the blood through the portal vein. As a result, serum bilirubin rises before diagnosis of NEC.

We also found that a delay in the time of first milk initiation may affect the occurrence of NEC, but it was not an independent risk factor for NEC. This is similar to the study by Kimak, who found that preterm infants who started breastfeeding for more than 5 days had an increased incidence of NEC compared with preterm infants who started breastfeeding 1 day after birth [14]. Previous studies have shown that delayed feeding was not associated with an increased risk of NEC [15, 16]. A recent Cochrane meta-analysis found that in very preterm and low-birth-weight infants, delayed feeding may slightly reduce feeding intolerance, but at the same time may increase the risk of invasive infection [17]. For premature and low-birth-weight infants, whether enteral feeding should be started as early as possible remains to be discussed.

At the same time, this study showed that PDA was an independent risk factor for NEC. Ndour et al. thought those preterm infants with hemodynamically significant patent ductus arteriosus(hsPDA) and treated with oral ibuprofen within 1 week were prone to gastrointestinal complications (NEC and intestinal perforation) [18]. This is consistent with our conclusion. A meta-analysis found that hsPDA was an independent risk factor for NEC with or without medication [19]. Salvatori et al. found that hypoperfusion due to PDA can influence nutrition because of malabsorption, feeding intolerance or intestinal ischemia [20]. Intestinal microcirculation disorders are one of the factors that cause intestinal necrosis in NEC. PDA affects blood flow, so hsPDA may cause NEC [21].

Since this study is a retrospective study, there are many clinical confounding factors, and the results are limited. In the future, it is necessary to expand the sample size and carry out mechanism research to predict the occurrence and development of the disease.

To sum up, before the diagnosis of NEC, the body may have experienced changes in serum-conjugated bilirubin, showing a continuously increasing trend. The changes in early conjugated bilirubin may be an important indicator for the occurrence of NEC in premature infants. It may provide new ideas for the pathogenesis of NEC.