NEC is often characterized by impaired blood flow and ischemia in the intestine. Derangements in intestinal blood flow may arise in the early neonatal period due to prematurity and enteral feeding, two critical risk factors for NEC development. ^{, , ,}

Vascular Development

Improper development of the intestinal vasculature in a preterm neonate plays a significant role in NEC pathogenesis. In vivo vascular phenotyping using the intestinal vascular unit (IVU) provides detailed visualization of vascular changes in experimental NEC. Notably, intestinal microvascular structures are underdeveloped in P1 and P5 mouse pups, possibly explaining inadequate blood flow responses. These findings implicate the underdevelopment of intestinal microvascular structures and a compromised postprandial intestinal blood flow response in the preterm intestine, leading to mucosal hypoxia and ultimately NEC development. Emerging evidence suggests that NEC is associated with defects in the vascular endothelial growth factor system. ^, Understanding changes in the microvasculature can potentially lead to more accurate diagnoses of NEC.

Vascular Endothelial Growth Factor (VEGF)

NEC is associated with dysregulation in the intestinal expression of VEGF, the principal growth factor that modulates angiogenesis. This process is mediated by the interaction of VEGF with VEGFR2, stimulating microvascular permeability and promoting endothelial cell proliferation, migration, and survival. ^, Studies have shown an association between perinatal complications, particularly NEC, and genetic polymorphisms of VEGF. ^, VEGF single-nucleotide polymorphisms linked to reduced plasma levels of VEGF increase the risk of NEC. Both human and mouse breastmilk, which are protective against NEC, contain high concentrations of VEGF. ^, Preclinical studies have shown that NEC stressors such as hypoxia, commensal bacteria, and cold stress downregulate VEGF expression in the intestine. Additionally, the incidence of NEC was higher following the inhibition of VEGFR2 in neonatal mice. Increased bioavailability of VEGF with a human VEGF cDNA plasmid construct has been shown to protect against experimental NEC.

Hypoxia-Inducible Factor α (HIF-1α)

The expression of VEGF is regulated by HIF-1α, a master regulator of hypoxia and ischemia-induced cellular responses, which plays a critical role in regulating oxygen homeostasis in fetal and postnatal life. ^, Hypoxia activates HIF-1α and the transcription of downstream genes involved in mammalian fetal vascular development. The in-utero environment is relatively hypoxic and stimulates angiogenesis signaling pathways necessary for vascular development during fetal life. However, preterm infants are exposed to an oxygen-rich extrauterine environment, leading to the downregulation of HIF-1α and its downstream targets. ^, During the critical period of intestinal vascular development, inadequate HIF-1α-mediated VEGF production and downstream signaling contribute to impaired vascular development and increased susceptibility of the intestine to NEC. This vulnerability is further exacerbated by enteral feeding and the inability of the intestine to meet the increased oxygen demand required for feeding. These findings highlight how premature exposure to oxygen further impairs intestinal vascularization through the loss of HIF-1α induced VEGF expression (Fig. 31.1 ).

Role of impaired microvascular development in preterm neonates during NEC development. Within the hypoxic in-utero environment, hypoxia-inducible factor-1 (HIF-1α), a primary regulator of hypoxia and ischemia-triggered cellular responses, activates angiogenesis pathways via vascular endothelial growth factor (VEGF) and VEGF-Receptor 2 (VEGFR2). This normal microvascular development allows the intestine to meet the increased oxygen development after feeding. Preterm infants are prematurely exposed to the oxygen-rich extrauterine environment that exceeds the physiological oxygen levels for this developmental stage. This leads to inadequate stimulation of VEGF/VEGFR2 signaling via HIF-1α. Impaired microvascular development contributes to an inadequate response to hypoxic or ischemic insults, which is exacerbated with exposure to risk factors such as feeding, as the intestine fails to meet its increased oxygen demand after feeding, thus predisposing the neonate to NEC.

The immune system plays a crucial role in the development of NEC. Decades of research have identified inflammatory mediators of both innate and adaptive immune systems that contribute to NEC development.

The intestinal epithelium is a continuously self-renewing tissue. The small intestinal epithelium is structured into two spatially distinct units: a) crypts that contain highly proliferative stem cells and b) long finger-like projections called villi that host terminally differentiated cells that are postmitotic.

The intestinal stem cells (ISCs) reside at the bottom of the crypts and constantly self-renew, giving rise to progenitors that differentiate into absorptive and secretory cells. ISC function and its mediated intestinal reconstitution are well studied in the context of adult intestinal diseases. However, they remain poorly understood in neonatal intestinal diseases such as NEC. It is unclear whether impaired intestinal regeneration is a pathogenic event in NEC development or occurs as a consequence of NEC. Nonetheless, impaired reconstitution of the mucosa in NEC aggravates bowel necrosis, which may lead to mucosal atrophy and bowel shortening. Clinical studies have shown that, in acute NEC, there is an increase in proliferative cells in areas with both severe and moderate epithelial damage. This proliferative zone is not limited to the small intestinal crypts but extends further along the villi. However, this proliferative response is insufficient to rapidly reverse the severe loss of mucosal lining during NEC.

The ISCs within the crypt have been classified into two subtypes. The first subtype is the active intestinal stem cells that frequently divide and thereby contribute to the majority of tissue regeneration. These cells are wedged between Paneth cells and are identified by the marker leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5). ^, Preterm babies exposed to multiple risk factors such as hypoxia, formula feeding, and bacteria dysbiosis have an increased chance of developing NEC. These risk factors, in combination, lead to reduced Lgr5 expression in the intestinal epithelium, which causes impaired epithelium regeneration. Moreover, studies have discovered that during NEC, increased TLR4 signaling in the Lgr5-positive ISCs of neonatal mice results in reduced ISC proliferation and increased apoptosis. The second subtype of ISCs is the quiescent or reserve intestinal stem cells, which are slow-dividing ISCs present in the +4 to +6 position from the base of the crypt. These quiescent ISCs, though resistant to stress, can be activated when the function of active ISCs is compromised. However, their role in NEC remains unknown.

The ISC function is regulated by both extrinsic and intrinsic factors. A gradient of morphogenic factors such as Wingless (Wnt) and Bone morphogenic protein (BMP) family members determine ISC function to either self-renew or to differentiate along the crypt–villus axis. In the crypts, Paneth cells and the surrounding mesenchyme/myofibroblasts constitute the niche environment. They generate several secretory and/or membrane-bound ligands from the Wnt, BMP, and Notch pathways that tightly regulate ISCs proliferation, maintenance, and differentiation. ^, For instance, TLR4 signaling leads to reduced enterocyte proliferation through inhibition of β-catenin signaling via GSK3β activation. Decreased Wnt/β-catenin signaling impairs ISC activity and intestinal regeneration abilities. To counteract this, administration of Wnt has been shown to maintain intestinal epithelial homeostasis and prevent NEC intestinal injury. Lithocholic acid, a key component of secondary bile acids, exacerbates NEC by inhibiting intestinal cell proliferation through downregulating the Wnt/β-catenin pathway. These findings suggest that Wnt/β-catenin signaling plays a crucial role in intestinal regeneration. Exogenous administration of Heparin-binding EGF-like growth factor (HB-EGF) potentiates recovery from intestinal injury in vitro and in vivo . Therefore, endogenous HB-EGF may be involved in epithelial cell repair, proliferation, and regeneration during recovery from NEC injury.

Absorptive progenitors differentiate into enterocytes that constitute most of the epithelium and mediate nutrient absorption. Secretory progenitors differentiate into goblet cells, tuft cells, enteroendocrine cells, and Paneth cells. The secretory cells are involved in the production of mucin, hormones, and antibacterial agents. All the differentiated cells except for the Paneth cells move upward along the crypt–villus axis and eventually die and slough off into the lumen. Paneth cells move downward toward the crypt and are interspersed between the crypt-based ISCs. The gut microbiota-mediated mesenchymal cells are important for Paneth cell differentiation niches during early postnatal development. Immature Paneth cells have reduced Wnt secretion and are unable to stabilize the ISC niches and maintain regeneration during NEC.

The enteric nervous system (ENS) is a vast neural network that controls the peristaltic movement of the gastrointestinal tract, an important function that pushes food along the entire digestive system. It also interacts greatly with the epithelium and can influence epithelial proliferation, differentiation, and the stem cell niche. With such an important role to play for the proper functioning of the intestine, the ENS has been shown to be involved in NEC as well. Similar to intestinal regeneration, it is not clear whether ENS defects have a causative role in NEC pathogenesis or manifest as a consequence of NEC. Further studies are needed to elucidate the contribution of ENS in NEC development.

Alterations of the ENS in NEC has been reported in various clinical studies looking at the enteric ganglion component of tissues taken from neonates with NEC. Decreased neurons and supporting glia cells were observed in human NEC tissue. Interestingly, the most significant loss of neurons was mainly found in the antimesenteric side of the intestinal wall, where blood supply is the poorest, suggesting that they are affected by ischemic damage. When looking at the intestinal cross-section, the most severe damage on ganglions was observed within the submucosal plexus, which is the closest plexus layer to the epithelium. This observation supports the concept that epithelial damage in NEC takes place first, exposing the mucosa, followed by further damage to the nervous system underneath.

The gut microbiome refers to the community of microorganisms residing in the small and large intestines, engaging in a symbiotic relationship with the human host. ^, Upon birth, the infant’s gut is initially populated by anaerobic species such as Bacteroides, Bifidobacterium , and Clostridium spp. Over time, the gut undergoes maturation, with an increase in bacterial richness and diversity, culminating in an adult-like microbiome typically between 2 and 5 years of age, which is characterized by a predominance of Bacteroidetes and Firmicutes. Nonetheless, in preterm infants, there is a heightened prevalence of Gammaproteobacteria in the intestinal tract, indicating a potential for disease development. ^{, ,} Additionally, a reduced abundance of anaerobes, particularly lactobacilli and Bifidobacterium spp., is associated with an increased risk of developing NEC.

In summary, NEC initiates when an immature intestinal vasculature fails to increase intestinal blood supply in response to enteral feedings, resulting in intestinal ischemia and subsequent epithelial barrier dysfunction, bacterial translocation, and intestinal inflammation. Inflammation then further leads to necrosis, diminished intestinal regeneration, and impaired ENS functioning.

The early suspicion of NEC is based on a cluster of clinical presentations and initial signs and symptoms including feeding intolerance, abdominal distension, gastric retention, emesis, and bloody stools. ^, Following these signs, bloodwork reveals thrombocytopenia and metabolic acidosis, and abdominal radiography and/or ultrasound reveal dilated loops of bowel, intestinal perforation, portal venous gas, and/or pneumatosis intestinalis indicative of bacterial invasion.

Making an early and adequate diagnosis of NEC remains one of the primary challenges in neonatology due to rapid onset and variability in clinical presentation of NEC, lack of a reliable and specific biomarker or pathognomonic sign, and shortcomings in the current diagnostic criteria used for NEC. The most definitive diagnosis of NEC is based on histology either through surgery or postmortem. Bell’s staging criteria, first published in 1978 and later modified to increase the number of stages from 3 to 6, ^, has been one of the most widely used classification systems for NEC, which uses systemic, intestinal, and radiological signs. Bell’s modified criteria consist of suspected NEC (Stage IA and IB), mild NEC (Stage IIA), moderate NEC (Stage IIB), and severe NEC (Stage IIIA and IIIB). There are several limitations with Bell’s and modified Bell’s staging criteria. Even though Bell’s staging criteria initially provided a way to stage the severity once an infant had been diagnosed with NEC, it has been adapted as the diagnostic criteria for NEC. The features in this criteria are sometimes subjective, have varying sensitivity and are mostly nonspecific, which can lead to over- or underestimation of NEC. Systemic signs such as temperature instability, lethargy, apnea, bradycardia, poor feeding, or even shock can also occur in separate diseases or other conditions compromising the health of a neonate such as sepsis or spontaneous intestinal perforation (SIP). Most clinicians continue to use Bell’s criteria for diagnosis, despite its limitations, primarily due to the absence of a suitable alternative that has effectively replaced these outdated criteria in clinical practice.

Several attempts have been made to develop standardized criteria for NEC diagnosis.

• 1.The Vermont Oxford Network (VON) definition , developed after Bell’s criteria, defines NEC as a diagnosis at surgery or on postmortem examination or based on clinical and radiographic criteria (features from Bell’s criteria). Infants must have at least one clinical sign (bilious gastric aspirate/emesis, abdominal distension, or occult/gross blood in stool) and one radiographic finding (pneumatosis intestinalis, hepatobiliary gas, or pneumoperitoneum). Infants found at surgery or postmortem examination to have a focal intestinal perforation (SIP) are excluded. Recent reports have noted a declining incidence of NEC in the United States (7.1% in 2005 to 5.2% in 2014) using this definition.

• 2.The Centers for Disease Control and Prevention (CDC) , a US health agency, has a similar case definition for NEC as VON, with some modifications. It requires at least one clinical and one imaging finding, similar to VON. If at least one imaging test finding is equivocal, then clinical correlation with physician documentation of antimicrobial treatment for NEC is required for diagnosis. The CDC definition categorizes NEC as surgical if there is surgical evidence of extensive bowel necrosis (>2 cm of bowel affected), or surgical evidence of pneumatosis intestinalis with or without intestinal perforation.

• 3.The UK Neonatal Collaborative NEC (UKNC-NEC) Study Group developed a point-based GA-specific case definition using a population-based cohort of infants. The NEC score ranges from 1 to 9 and includes 1 point for the presence of abdominal discoloration, tenderness, increased or bilious aspirations and abdominal distention, or ≥1 radiographic signs of pneumoperitoneum, fixed loop, or portal venous gas. Two points were assigned for blood in the stool and 3 points for pneumatosis. Based on the GA group (<30 weeks, 30 to <37 weeks, or ≥37 weeks), infants have score cut points to meet the case definition, ranging from ≥2 points at <30 weeks to ≥4 points required at ≥ 37 weeks. These thresholds correlate with a predicted NEC probability of over 40%. This method showed lower classification errors compared to the VON definition.

• 4.The 2 of 3 rule is a scoring system that diagnoses preterm NEC if an infant with abdominal distention, ileus, and/or bloody stools meets at least two of these criteria: pneumatosis and/or portal air on abdominal radiography (AXR) or ultrasound (AUS) at presentation, persistent platelet count <150,000 for 3 days postdiagnosis, and a postmenstrual age at disease onset consistent with NEC rather than SIP (SIP and surgical NEC are diagnosed earlier, but there is overlap). Infants with known SIP, complex congenital anomalies, those fed less than 80 mL/kg/day, or those ≥36 weeks’ gestation are excluded from this diagnosis.

• 5.The Stanford NEC score is a weighted diagnostic model that incorporates various clinical, laboratory, and radiographic parameters to classify the severity of NEC in infants. It was developed using a 6-center cohort of 520 infants with suspicion of NEC. The score considers the following factors: baseline characteristics (postnatal age, gender, ethnicity), clinical/historic factors (feeding intolerance, ventilation requirement on the day of NEC suspicion), clinical exam findings (abdominal wall discoloration, abdominal tenderness, increased or bilious gastric residuals), laboratory findings (platelet count, blood pH value), and AXR findings (pneumatosis intestinalis, portal venous gas). Each of these parameters is assigned a specific weight or score. By inputting the relevant data, the model generates a cumulative Stanford NEC score ranging from 1 to 9, which can then be used to classify the severity of NEC and determine the risk of disease progression.

• 6.The International Neonatal Consortium (INC) formed a workgroup to develop a new NEC definition. The proposed criteria focus on the timing of onset and clinical and radiographic evidence. Infants must show one of two clinical signs (abdominal distention or hematochezia), onset between the 10th postnatal day and 36 weeks’ postmenstrual age, and at least one of the following: intestinal necrosis at laparotomy, pneumatosis intestinalis or portal venous gas (by AXR or AUS), or signs of vasculitis, coagulopathy, or inflammation without bacterial/fungal/viral infection. This definition distinguishes “preterm NEC,” excluding infants with early intestinal perforation in the first 10 days without specific evidence or those with NEC after 36 weeks’ gestation, isolated feeding intolerance, congenital cyanotic heart disease, or gastroschisis. The report suggests classifying NEC cases that do not meet “preterm NEC” criteria as “atypical NEC” or “term NEC” for clinical research.

The current definitions of NEC have several limitations, which is why a universally acceptable definition is still lacking. The criteria used across the various definitions are highly variable and subjective. There is no consensus on the specific clinical, laboratory, and radiographic parameters that should be included in the definition. Pneumatosis intestinalis, a key radiographic finding in most NEC definitions, can be challenging to ascertain accurately. There is often uncertainty in interpreting the presence or absence of pneumatosis, ^, and the use of AUS has limited sensitivity for this finding. Furthermore, limited studies have compared the diagnostic validity of different NEC definitions against a gold-standard diagnosis. There is no generally agreed-upon method to determine the gold standard for NEC, making it difficult to assess the performance of existing definitions. Some definitions may be more suitable for clinical diagnosis, while others may be better suited for population-based surveillance or research studies. There is ongoing research to overcome these limitations, refine existing NEC definitions, and develop a consensus definition for NEC.

Traditionally, AXR has been the standard and most widely employed radiological assessment for diagnosis and guiding the management of NEC. The characteristic findings on AXR include abnormal gas pattern, pneumatosis intestinalis (PI), portal venous gas (PVG), and pneumoperitoneum (Figs. 31.2 and 21.3 ). An abnormal gas pattern with dilated, asymmetric bowel loops and paucity of gas in other areas is a common early finding in NEC. ^, This may progress to a fixed, dilated loop that persists over serial x-rays, indicating bowel necrosis. PI or gas within the bowel wall is seen in 50%–60% of NEC cases and appears as a linear or bubbly radiolucency outlining the bowel wall, giving a “soap bubble” appearance. ^, This pathognomonic finding represents gas produced by bacteria within the necrotic bowel wall. PVG seen as branching radiolucencies within the liver is another specific but late finding of advanced NEC. ^, The presence of free intraperitoneal air, seen as an oblong radiolucency over the liver and abdominal contents on supine views or as an air-fluid level on left lateral decubitus views, indicates bowel perforation and is an ominous sign requiring urgent surgical intervention. ^, AXR has long been the mainstay of diagnosis of definite NEC (PI) and provides the only universally accepted indication for operation (pneumoperitoneum).

Plain radiographs in premature neonate with clinical signs of NEC. Left panel: Supine and right panel: Lateral shoot-through radiographs show typical findings in NEC. There is diffuse distension of the intestines with gas, diffuse intramural gas, and a large amount of portal venous gas. There is, however, no evidence of free gas.

Plain radiographs in three different premature neonates with pneumoperitoneum due to perforated NEC. Left panel: Supine radiograph shows diffuse distension of intestine with gas and a large amount of free gas (football sign). Right panels: Top lateral shoot through radiograph shows small amounts of free gas seen as triangles of gas between the more rounded loops of intestine with gas. Bottom lateral shoot through radiograph shows a very small amount of free gas as a linear radiolucency anterior to the liver ( arrow ).

While AXR findings are highly suggestive of NEC, their absence does not exclude the diagnosis, and they are not seen consistently in patients with early, treatable conditions. AUS has become increasingly recognized as a more sensitive and comprehensive imaging tool for evaluating gut injury and in particular, NEC. ^, AUS provides dynamic imaging of the bowel as well as real-time assessment of gut motility and perfusion, bowel wall thickening and thinning, abdominal fluid, as well as earlier and more sensitive detection of PVG and pneumatosis intestinalis. ^, AUS can detect early signs of NEC (bowel wall thickening, increased perfusion, and variable peristalsis), as well as more advanced signs (bowel wall thinning, decreased/absent perfusion, absent peristalsis, fluid collections/complex, intraabdominal fluid), which suggest impending bowel perforation (Fig. 31.4 ). These finding can inform the clinician’s decision for earlier intervention before significant clinical deterioration. In addition, the introduction of color Doppler ultrasound to assess intestinal viability and perfusion in NEC, first described in 2005, is a new advancement that could improve the early detection of ischemic or necrotic bowel loops. ^{, ,} Hence, AUS is increasingly used as an adjunct to AXR in the diagnosis and management of NEC. A 2018 systematic review and meta-analysis reported that AUS features had sensitivities below 70% and specificities largely above 80% for diagnosing definite NEC (Bell stage ≥ II). Several AUS features including focal fluid collections, complex ascites, absent peristalsis, and pneumoperitoneum were significantly associated with failed medical treatment and need for surgery. ^, Furthermore, definitive NEC diagnostic criteria based solely on AUS are not yet well established. Current evidence supports a valuable role for AUS as an adjunct to AXR for NEC diagnosis and management.

Sonographic image of abdomen in a premature neonate shows large amount of intraluminal gas (curvilinear hyperechoic white lines ) and small bubbles of intramural gas (hyperechoic focus external to intraluminal gas).

Due to the multitude of bioactive factors present in human breast milk, breastfeeding and the impact of breastmilk on incidence of NEC has been thoroughly investigated as a preventative mechanism. The use of human milk in preterm infant feeds has been identified as the most effective strategy for protection against development of NEC. Various studies have demonstrated a significant reduction in incidence of NEC among preterm infants when using their mother’s own breastmilk in comparison to infants receiving formula feeds. ^, Furthermore, similar benefits have been demonstrated utilizing donor human breast milk in preterm infants for whom maternal breastmilk is unavailable. ^{, ,} A systematic review and meta-analysis revealed that formula feeding significantly increased the risk of NEC in preterm and low birth weight infants and concluded that the beneficial effect of donor breast milk was observed even when given as a supplement to maternal breast milk. The proposed mechanisms by which human breast milk exerts these protective effects is by lowering gastric pH, enhancing intestinal motility, and transferring various antiinflammatory factors to the preterm infant, such as sodium nitrate and oligosaccharide, which reduce mesenteric endothelial inflammation. Additionally, and perhaps most importantly, human breast milk introduces prebiotic and probiotic components that promote growth of nonpathogenic probiotic microorganisms that alter gut microbiota and subsequently decrease microbial dysbiosis. ^, Recently, studies have sought to evaluate the role of breast milk-derived extracellular vesicles (EVs) in preventing NEC. Experimental studies on rodent models of NEC have shown that milk-derived EVs promote intestinal epithelial cell growth, stimulate intestinal stem cell and goblet cell activity, and prevent the development of experimental NEC. ^, These promising results from experimental models present exciting opportunities for future clinical application in humans.

As most cases of NEC occur in preterm infants, the bacterial flora, which is present in the intestines of premature infants, likely plays a role in the development of NEC. Early microbial dysbiosis has been associated with the occurrence of NEC, and the introduction of probiotics to the premature gut has been identified as an avenue of treatment. Probiotics are enteral supplements that contain beneficial live microorganisms. They promote the growth and colonization of commensal intestinal bacteria, decrease overgrowth of pathogenic microorganisms, and are believed to decrease bacterial translocation and inflammation. Studies comparing use of probiotics to placebo have revealed a significant reduction in incidence of NEC. Moreover, the use of probiotics has been associated with reduced overall mortality, length of hospitalization, and time to reaching full enteral feeds. ^{, , ,} Specifically, evidence supports the use of multistrain probiotics containing lactobacillus and Bifidobacterium in preterm, very preterm, and very low birth weight infants. Although these results are promising, controversy remains regarding the systematic use of probiotics due to a lack of standardization of bacterial strains, regulatory challenges, and a hypothetical risk of sepsis with the introduction of live bacteria in infants with immature immune systems. Notably, in 2023 the US Food and Drug Administration (FDA) issued a notice urging providers to exercise caution when utilizing probiotics in very preterm or VLBW infants due to the risk of bacteremia and fungemia ( https://www.fda.gov/news-events/press-announcements/fda-raises-concerns-about-probiotic-products-sold-use-hospitalized-preterm-infants ). Furthermore, the FDA stated that no probiotic product has received approval for use in infants, thereby contributing to the controversy surrounding probiotic use for the prevention of NEC.

It is known that prematurity and the associated immaturity of the neonatal intestine is the primary risk factor for NEC. As a result, treatments to prevent preterm birth and promote maturation of the intestine are of particular interest. Although the use of corticosteroids in preterm infants is controversial, evidence suggests the utility of prenatally administered corticosteroids to reduce morbidity and mortality of preterm birth, as well as to decrease incidence of NEC in newborns. A meta-analysis of 15 randomized controlled trials noted an effective reduction in incidence of NEC following prenatal application of glucocorticoids with no significant harm to mother or infant. Furthermore, in addition to a decreased incidence of NEC, antenatal corticosteroid exposure has been associated with a lower risk of intraventricular hemorrhage, respiratory distress syndrome, and retinopathy of prematurity in preterm infants. However, there is no standardized protocol for corticosteroid administration to specifically prevent NEC, and recommendations for antenatal corticosteroids in pregnant individuals who are at high risk for preterm delivery are dictated by the treating obstetrician.

The introduction of early enteral feeds to premature infants has many positive effects such as increased weight gain, decreased parenteral nutrition requirements, and early stimulation of the immature gut. However, hypothetically early and rapid escalation in volume of feeds could overwhelm the immature gastrointestinal tract and subsequently promote pathologic bacterial growth, resulting in translocation and the development of NEC. Although slow feeding has been proposed as a preventative strategy, current data refutes this and suggests that slow advancement of enteral feeds has a negligible role in reducing risk of NEC in preterm infants. The administration of minimal enteral feeds (15–20 mL/kg/d) within 1–3 days of life has not been associated with an increased incidence of NEC when compared to fasting. Rather, it has been suggested that this may be beneficial in priming the intestinal tract. Due to limited evidence, the role of a feeding protocol in reducing NEC is debated.

The prevention of bowel ischemia and hypoxia is important in preventing NEC and improving clinical outcomes in affected infants to preserve the maximum amount of viable bowel. It has been demonstrated that the intestines of infants with NEC reveal microcirculatory injury. In order to improve blood flow to the immature intestine, various strategies have been proposed such as use of prostaglandin E2 to improve intestinal blood flow, increasing NO production through supplementation with arginine acids, and use of remote ischemic conditioning (RIC). Although most of these studies remain in the preclinical phase, RIC is a noninvasive method for intestinal protection that is currently being tested in phase II clinical trials. RIC involves limiting blood flow to the periphery for brief, reversible periods, thereby stimulating endogenous protective signaling mediators that are transmitted from the organ/tissue under ischemic conditions to the target organ (i.e., intestine). This subsequently improves blood flow and reduces intestinal damage as shown in experimental conditions of NEC. Furthermore, in preterm infants, conditions like patent ductus arteriosus (PDA) can cause significant hemodynamic alterations, which further impair gastrointestinal tissue perfusion. Evidence has suggested that opting for alternate medications for treatment of PDA may improve oxygenation. Avoiding indomethacin and instead utilizing ibuprofen, for example, has demonstrated an attenuated effect on mesenteric blood flow, and oral formulations of nonsteroidal antiinflammatory drugs (NSAIDs), rather than IV, have been associated with a reduced incidence of NEC.

The medical management of NEC aims to provide supportive care, prevent further intestinal injury, and promote healing. For Bell stage I (suspected NEC), supportive medical management alone is typically provided. In Bell stage II (proven NEC), initial treatment usually involves medical management. If the patient does not respond to medical treatment, surgical intervention is considered. Patients with Bell stage III (advanced NEC) may require medical management and inotropic support. However, neonates who develop intestinal perforation, suspected bowel necrosis, or fail to respond to medical treatment need surgical intervention. Among very low birth weight (VLBW) infants, 27%–52% require surgical intervention.

The medical management of NEC involves bowel rest and nutritional support, broad-spectrum antibiotics, and systemic respiratory and cardiovascular support. Enteral feeding is discontinued, and the infant is kept nil per os (NPO) to allow the inflamed intestine to rest and heal. Parenteral nutrition is initiated to provide essential nutrients and calories intravenously, avoiding further intestinal injury from enteral feeds. ^, The duration of fasting depends on the clinical, laboratory, and imaging findings. Recent evidence suggests that early reintroduction of enteral feeding (within 7–14 days) is associated with fewer complications, shorter antibiotic therapy duration, faster progression to nutritional goals, and shorter hospitalization compared to prolonged fasting. ^, Prolonged fasting can lead to negative consequences such as nutrient deficiencies, intestinal mucosal atrophy, bacterial overgrowth, and increased risks associated with prolonged parenteral nutrition. Surgical NEC typically requires prolonged use of parenteral nutrition (>21 days). Infants with less remaining bowel length after resection may require longer durations of parenteral nutrition. The need for parenteral nutrition support >28 days after surgical intervention is associated with increased 1-year mortality. The duration of parenteral nutrition can be an indirect measure of intestinal failure and the ability to tolerate enteral nutrition. The primary metabolic complication associated with parenteral nutrition is intestinal failure associated liver disease (IFALD).

Broad-spectrum antibiotics are administered to reduce bacterial overgrowth and translocation. The typical antibiotic regimen includes a combination of ampicillin or a third-generation cephalosporin, an aminoglycoside, and metronidazole to cover anaerobic organisms. The duration of antibiotic therapy is guided by clinical improvement and resolution of laboratory abnormalities. Antibiotic duration for both medical and surgical NEC remains variable within and among NICUs. Traditionally, an antibiotic duration of 7–10 days for medical NEC and 14 days for surgical NEC is given. ^, A short (10-day) course of narrow-spectrum antibiotics (ampicillin, gentamicin, and metronidazole) achieved outcomes comparable to broader-spectrum antibiotics and longer treatment durations in one study. Another report found that extended antibiotic courses in both medical and surgical NEC were linked to prolonged time to full feeds and length of hospital stay. There is a need for standardized antibiotic protocols to improve NEC management and promote intestinal recovery.

Infants with NEC often require respiratory support, ranging from supplemental oxygen to mechanical ventilation and nasal continuous positive airway pressure (NCPAP), depending on disease severity. Preterm infants with NEC requiring surgical intervention have a higher need for mechanical ventilation or NCPAP compared to those with medical NEC. Furthermore, VLBW infants who require more respiratory support before gastrointestinal symptoms are more likely to develop NEC. Cardiovascular support with intravenous fluids and inotropic agents may be necessary to maintain adequate perfusion and treat shock or sepsis. In cases of significant abdominal distension or pneumoperitoneum, abdominal decompression with a nasogastric or orogastric tube (or, in the case of pneumoperitoneum, emergent needle decompression) may be necessary to relieve pressure and prevent respiratory compromise.

The percentage of infants with NEC who develop advanced disease requiring surgical intervention varies according to different studies. In 2023 the Canadian Neonatal Network (CNN) reported that the incidence of infants with NEC, <33 weeks of gestational age, who underwent surgery was 50%. In addition, in our experience in Toronto, 27% of neonates with medical NEC progress to more advanced disease requiring surgery (unpublished data).

The goal of surgery for NEC in the acute phase of the disease is to resect the gangrenous and perforated intestine, while preserving as much as possible intestinal length. During the acute phase of the disease, it is important to excise the necrotic intestine to minimize generalized inflammation, which leads to severe neurological derangements ^, and ultimately death. During the recovery phase from NEC, surgery is indicated to remove NEC-related intestinal strictures.

The absolute indications for surgery include :

• (1)

  Pneumoperitoneum indicative of intestinal perforation

• (2)

  Clinical deterioration despite maximal medical treatment

• (3)

  Abdominal mass with persistent intestinal obstruction or sepsis

• (4)

  Development of intestinal stricture

Several authors have proposed different indications for surgery, though some remain debated. Occasionally, a laparotomy is conducted due to increased abdominal tenderness, distension, or discoloration of the abdominal wall. The differential diagnosis in these situations includes midgut volvulus, Hirschsprung’s enterocolitis, and, rarely, intussusception. Additionally, a fixed dilated intestinal loop, defined by its persistent location and configuration for more than 24 hours, has been suggested as a surgical indication. However, about half of the patients with this finding recover without surgery. The presence of gas in the portal vein on an abdominal radiograph has also been suggested as an indication for surgery, although this is not universally accepted and tends to be associated with a relatively poor prognosis.

The authors do not insert a peritoneal drain unless the infant is < 1000g body weight, has pneumoperitoneum, difficult ventilation, instability, and on transportation to a surgical center with pediatric surgical facilities.

The preferred approach is to perform a laparotomy (Fig. 31.5 ) regardless of gestational age or body weight. Various operative options at laparotomy are considered in relation to extent of the disease and patient stability:

• SIP : if this is diagnosed intraoperatively, a resection and anastomosis is performed.

• Focal NEC : if the infant is stable, resection and primary anastomosis is performed. A stoma is created if the patient is unstable during surgery or if the distal intestine cannot be visualized.

• Multifocal NEC : if the distal intestine is viable a resection and primary anastomosis is performed. In case the infant is unstable, there is bleeding during dissection of the intestine, or the distal intestine is questionable or not visualized, resection of gangrenous intestine is performed plus stoma creation. An alternative is to perform a diverting jejunostomy. Clip and drop is also considered if the infant is very unstable or the disease is affecting many sections of the intestine.

• Pan-Intestinal : in case the disease is extensively involving the small and large intestine, a proximal diverting jejunostomy is performed, or a clip and drop technique is considered to minimize the risk of developing short bowel syndrome.

• Total intestinal gangrene : when the entire GI tract is gangrenous with no spared segment of small intestine consideration is given to withdrawal of treatment after discussing this option with the parents.

Authors preferred operative management.

Adapted from Pierro A, Hall N. Surgical treatments of infants with necrotizing enterocolitis. Sem Neonatol . 2003.

Emerging developments in the treatment of NEC include remote ischemic conditioning (RIC) and stem cell therapy.

RIC (discussed briefly above) is emerging as a promising noninvasive treatment strategy for the early stages of NEC. RIC involves the application of brief, intermittent cycles of ischemia and reperfusion to a distant organ or tissue, typically a limb, to protect against ischemia-reperfusion injury in another organ. In the clinical setting, this is achieved by inflating and deflating a blood pressure cuff around a limb. The procedure activates endogenous protective signaling pathways, creating two distinct temporal phases of protection: an initial wave lasting a few hours and a second wave lasting up to 72 hours. ​​ Preclinical studies have demonstrated that RIC can improve intestinal blood flow and counteract the poor intestinal hemodynamic response to feeding seen in premature infants. Recent preclinical studies have demonstrated that RIC, when administered in early stages of the disease, reduced intestinal injury and inflammation associated with NEC and prolonged survival. The mechanism of action was directed toward improving intestinal microcirculation via hydrogen sulfide and NO-mediated vasodilation. Clinical translation of these findings is underway. A phase I safety study conducted at The Hospital for Sick Children in Toronto demonstrated that RIC could be safely administered to preterm infants with suspected or confirmed NEC without adverse effects. Following this, a multicenter phase II feasibility randomized controlled trial (RCT) is in progress, aiming to evaluate the feasibility of RIC administration in a clinical setting. This trial involves preterm neonates diagnosed with medical NEC who are randomized to receive either RIC or no RIC within 24 hours of diagnosis; both study arms continue to receive the standard of care for NEC. The primary outcome is the feasibility of recruitment, randomization, and delivery of the intervention.

Stem cell therapy represents another novel approach for treating NEC. There is significant interest in using stem cells and stem cell-derived products due to their regenerative potential and ability to modulate inflammation. Two primary sources of stem cells used in NEC research are bone marrow-derived mesenchymal stem cells (BM-MSC) and amniotic fluid stem cells (AFSC).

BM-MSC have shown promise in reducing the incidence and severity of NEC. Studies have demonstrated that BM-MSCs can decrease histopathological damage, improve weight gain, and enhance gut barrier function in experimental models of NEC. Both intraperitoneal and intravenous routes of administration have been investigated, with intravenous administration showing higher integration without significant differences in reducing histopathological damage and improving intestinal barrier function and survival.

AFSC, derived from embryonic and fetal tissues, have mesenchymal features and proliferative capabilities similar to BM-MSCs. In 2001, a mesenchymal subpopulation of AFSC was identified that proliferated more rapidly in vitro compared to adult and fetal cells. In 2003, AFSC with similar multilineage differentiation potential as BM-MSC was described. Later, pluripotent stem cells expressing the Oct4 marker were found in amniotic fluid. By 2007, AFSC capable of differentiating into lineages from all three embryonic germ layers were reported. The therapeutic effects of AFSC have been demonstrated in preclinical models of NEC. In a rat model, AFSC improved survival, clinical status, and gut structure. AFSC were recruited to the injured gut, providing protection via a COX-2 dependent paracrine mechanism, promoting migration of COX-2-positive cells to intestinal crypts to release prostaglandins, which decreased inflammation and enterocyte apoptosis while promoting epithelial proliferation. ^, AFSC also reduced ascites and abdominal distention in NEC rats. Other studies confirmed AFSC benefits, showing reduced NEC incidence and enhanced intestinal barrier function in rats. ^, In a neonatal mouse model of NEC, AFSC activated the Wnt/β-Catenin pathway, crucial for ISC regulation and impaired during NEC, thus restoring Lgr5+-ISCs and epithelial regeneration. ^, AFSC can be safely collected, cultured, and expanded, ^{, ,} making them a promising treatment for NEC by promoting intestinal regeneration.

Current advancements in the field highlight the promise of regenerative medicine for NEC, but significant challenges remain. While stem cell-based interventions have shown feasibility in related neonatal conditions, they have yet to be thoroughly evaluated in neonates with NEC. The NEC-ACCELERATOR (NECrotizing enterocolitis-ACceleration of CELl-basEd & Regenerative medicine Advancements for Translation in neOnates) is a multidisciplinary and evidence-based approach with the aim to streamline the clinical translation of these therapies, addressing barriers such as trial design, regulatory compliance, and parent/caregiver engagement, thereby enhancing the potential for these innovative treatments to reach clinical practice.

In the 1990s, new approaches emerged for both the obstetric management of preterm birth and the neonatal care of the prematurely born infant. These included the widespread use of antenatal corticosteroids for women at risk for preterm delivery, surfactant for the prevention and treatment of neonatal respiratory distress syndrome, and postnatal steroids for chronic lung disease. Furthermore, structural changes in the healthcare system have resulted in the deregionalization of perinatal and neonatal care. Various studies explored if these strategies not only impacted the incidence of NEC among the VLBW preterm population but also their mortality in this time period. Despite these interventions, these multicenter cohort studies in the United States, Australia, and the UK showed that NEC incidence had decreased but mortality had remained high, ranging from 10%–30% and increasing to 40%–80% among VLBW infants who develop intestinal ischemia and necrosis within 48 hours of diagnosis.

Over the last 20 years, postnatal steroid use has declined, most infants are ventilated, and continuous positive airway pressure without ventilation increased advancements in the management of NEC. A large cohort study analyzed over 88 million live births from 1999–2020 in North America, revealing some important trends in NEC-related infant mortality rates. Overall NEC mortality across all years was 10.2 per 100,000 live births. NEC mortality declined by 7.7% per year from 2007 to 2012 but showed no further decline after 2012. ^, Stratifying across race, in 2020, NEC mortality was 16.1 per 100,000 for Black infants and 6.4 per 100,000 for White infants, with a Black-to-White ratio of 2.5. Further stratification of mortality rates into surgically or medically treated subgroups, adding important information regarding specific risk factors carrying greater mortality. ^, The overall mortality rate reported for infants with confirmed NEC (Bell stage 2a or higher) was 23.5%. However, the mortality rate was significantly higher (50.9%) for very low birth weight infants (<1000g) who underwent surgical intervention for NEC. These findings align with historical data reporting overall NEC mortality rates between 10%–30%. The high and unchanged mortality despite advancing neonatal intensive care and surgical management points to the lack of precise and specific treatment that targets the underlying pathophysiological features of NEC.

Intestinal failure (IF) is a major complication following NEC, with the incidence varying widely from 2% in medically treated NEC infants to as high as 42% in those requiring surgical intervention. A Swedish cohort study found IF to be 15 times more prevalent in infants with NEC compared to preterm infants without NEC. Alarmingly, up to 9% of NEC survivors continue to require parenteral nutrition for IF at 1 year of age, underscoring the long-term nutritional challenges faced by these infants. Significant risk factors for developing IF after NEC include lower gestational age, lower birth weight, greater extent of bowel resection (>45 cm), resection of the ileocecal valve, reduced colonic remnant length, higher illness severity scores, being small for gestational age, and a history of abdominal surgery beyond NEC. The consequences of IF in NEC survivors are grave, with high mortality rates, parenteral nutrition-associated liver disease, micronutrient deficiencies, recurrent infections, chronic diarrhea, poor growth, and osteopenia. While early enteral feeding, use of breast milk, and elemental formulas may help reduce the risk of IF, data on preventive strategies remains limited. Collectively, IF is a frequent and devastating complication of NEC, especially in very preterm infants requiring extensive surgical intervention, necessitating early identification of high-risk infants and development of targeted interventions to improve outcomes.

NEC has long been recognized for its immediate physical ramifications in neonates, but emerging research emphasizes its profound implications on the neurodevelopment of these infants. NDI is defined as types of impairments that influence how the brain functions and alters neurological development, causing difficulties in social, cognitive, and emotional functioning. A systematic review and metanalysis showed that a significant body of research, spanning 31 studies, has established the pronounced incidence of NDI in survivors of NEC. Alarmingly, the overall incidence of NDI in infants with NEC was found to be 40%, a rate significantly elevated compared to premature age-matched controls without NEC (∼10%). ^,

Among the various NDIs observed, cerebral palsy (CP) stands out prominently. It manifests in a substantial 17% of NEC babies, a figure that starkly contrasts with the 7% incidence in premature age-matched controls. The academic progression of NEC survivors presents additional concerns. Numerous studies reveal that these children encounter significant challenges in reading scores, cognitive indices, language skills, memory capabilities, and adaptive behavior. These issues manifest in real-world challenges: 28% of NEC survivors need special education and 21% require speech therapy.

Among the studies exploring the dichotomy between surgical and medical NEC management, a pronounced trend emerges: the former group manifests a significantly heightened incidence of NDI. Specifically, in a meta-analysis of 10 pertinent studies, infants necessitating surgical intervention for NEC displayed an NDI prevalence of 43%, in stark contrast to the 27% observed in their medically managed counterparts. ^,

MRI scans have furthered our understanding of the long-term brain implications associated with NEC. When preterm infants were examined using near-term structural MRI, those with a prior history of NEC showed brain abnormalities at a rate of 43%. A deeper delve into the MRI data highlights significant disparities based on the type and severity of NEC. Specifically, infants with surgical NEC or those with spontaneous intestinal perforation displayed a significantly heightened white matter injury compared to those with medically managed NEC or age-matched controls. ^, In fact, among the preterm infants with surgical NEC, the magnitude of white matter injury surpassed even those with spontaneous intestinal perforation. The pronounced neurological consequences associated with NEC, as revealed by advanced imaging techniques, advocate for proactive interdisciplinary care and research to optimize outcomes in this vulnerable population.

In summary, while NEC remains a multifaceted condition with numerous systemic implications, its association with cerebral and neurological lesions, particularly in the white matter and ventricle, is undeniable. These findings underscore the necessity for meticulous neurological monitoring in NEC survivors and reiterate the intricate interplay between intestinal and neurological health.