Neonatal necrotizing enterocolitis: Pathology and pathogenesis

INTRODUCTION — Necrotizing enterocolitis (NEC) is one of the most common gastrointestinal emergencies in the newborn infant. It is a disorder characterized by ischemic necrosis of the intestinal mucosa, which is associated with severe inflammation, invasion of enteric gas forming organisms, and dissection of gas into the bowel wall and portal venous system [1]. Although early recognition and aggressive treatment of this disorder has improved clinical outcomes, NEC accounts for substantial long-term morbidity in survivors of neonatal intensive care, particularly in preterm very low birth weight (VLBW) infants (BW below 1500 g).

Although first described in 1965, the etiology and pathogenesis of NEC remains uncertain [1]. The pathology and pathogenesis of NEC are reviewed here. The clinical features, management, and prevention of this disorder are discussed separately. (See "Neonatal necrotizing enterocolitis: Clinical features and diagnosis" and "Neonatal necrotizing enterocolitis: Management and prognosis" and "Neonatal necrotizing enterocolitis: Prevention".)

PATHOLOGY — The pathology of NEC is primarily due to changes from severe intestinal inflammation and infarction [2]. The specific findings range from mucosal injury to full-thickness bowel necrosis and perforation (picture 1). The terminal ileum and colon are involved in the majority of cases, although the entire gastrointestinal (GI) tract is affected in severe cases.

On gross examination, the bowel appears distended and hemorrhagic (picture 1). Subserosal collections of gas, seen as pneumatosis intestinalis on abdominal radiographs, occasionally are present along the mesenteric border (figure 1). Gangrenous necrosis occurs on the antimesenteric border, and perforation may be present. As the gut heals, thickening of the bowel walls, fibrinous adhesions, and areas of stenosis appear.

The major histologic findings in NEC are mucosal edema, hemorrhage, and transmural bland necrosis (figure 1). Other findings include acute inflammation, secondary bacterial infiltration, and collections of gas. Vascular thrombi are rare.

PATHOGENESIS: PRETERM INFANT

Overview: Multifactorial process — The pathogenesis of NEC remains unknown. For the 90 percent of cases that occur in preterm infants, available evidence supports a multiple-factorial mechanism that requires the concurrent presence of an immature intestinal tract and immune system (increased susceptibility), triggers that lead to dysbiosis (disruption of the normal intestinal bacterial flora [microbiome], resulting in increased growth of potentially pathogenic bacteria), and an exaggerated inflammatory host response with release of cytokines and chemokines (figure 2):

●Increased susceptibility due to intestinal and immunologic immaturity

●Trigger event or factors contribute to changes in the microbiome of the intestinal tract (microbial dysbiosis and/or primary infection) and disruption of the intestinal mucosal barrier and microvasculature. These include:

•Non-human milk feedings and rarely components in human milk may trigger a sensitivity or allergic response (eg, food protein-induced enterocolitis syndrome, milk protein allergy). (See "Food protein-induced enterocolitis syndrome (FPIES)" and "Milk allergy: Clinical features and diagnosis".)

•Medications that cause intestinal mucosal injury or enhance microbial overgrowth (eg, antibiotics)

•Circulatory instability

•Anemia

●Hyper-inflammatory response mediated by toll-like receptor-4 (TLR-4) innate immunity to trigger events within the intestinal tract

Susceptibility: Immunologic and intestinal immaturity — Preterm infants are susceptible to the development of NEC because immunologic and intestinal immaturity results in a susceptible host for potential mucosal injury. The risk of NEC increases with decreasing gestational age (GA) and is greatest in the extremely preterm infant (GA <28 weeks) [3]. (See "Neonatal necrotizing enterocolitis: Clinical features and diagnosis", section on 'Epidemiology'.)

Mucosal defense in the gut is mediated by several interrelated components, some of which provide a physical barrier and others a biochemical and immunologic barrier [4-7]. The impairment of the mucosal barrier allows bacteria to gain access to deeper tissues resulting in inflammation. (See 'Host immune and inflammatory response' below.)

Increased susceptibility, which predisposes the preterm infant to NEC, is due the following (figure 3) [4,7,8]:

●Immature mucosal physical barrier with increased permeability and bacterial penetration into the intestinal wall (translocation) compared with term infants.

•The intestinal mucin barrier is not fully developed in the very preterm infant. Mucin, which is secreted by the goblet cells, hampers bacterial epithelial binding and enhances bacterial removal [4,7].

•There is increased permeability most likely due to immaturity of the composition and function of the tight junctions [9]. Tight junctions between epithelial cells maintain the semipermeable property of the intestinal tract.

●Immature immunity and biochemical protection with diminished concentrations of secretory immunoglobulin A (IgA; the major intestinal immunoprotective antibody), mucosal enzymes (eg, pepsin and proteases), and other protective agents (eg, defensins, lactoferrin), and increased gastric pH, which promotes bacterial overgrowth [4,7,9,10].

●Immature gut motility and function. Preterm infants have dysfunctional gastric emptying and decreased bowel motility. This results in delayed transit time, which increases bacterial proliferation and overgrowth [11].

Potential triggers and risk factors — It is postulated that trigger events and environmental factors initiate intestinal injury in a vulnerable host (preterm infant), which prompts a hyper-inflammatory response that cannot be well-regulated due to an immature immune system.

There is good evidence that microbial dysbiosis and non-human milk feeding are risk factors for NEC [11-13]. In addition, more limited evidence suggest that hyperosmolar agents and histamine type 2 (H2) receptor blockers that increase intestinal pH are also environmental risk factors.

The evidence is also unclear whether there is an association between NEC and primary infection, circulatory instability, or anemia and red blood cell (RBC) transfusion.  

Microbial dysbiosis — Increasingly, there is evidence that disruption of commensal bacterial colonization that potentially include pathogenic bacteria (microbial dysbiosis) plays a key role in the development of NEC [8,14-19]. Interestingly, NEC does not occur in utero despite evidence that supports the presence of microbes in meconium, which may indicate that certain pathogenic bacteria are required for the development of NEC. NEC rarely occurs without the presence of feeding, which stimulates the growth of and changes in the intestinal microbiome. The type of feeding (human milk versus bovine-based formula) can have positive or negative effects on the microbiome and subsequent development of NEC. (See 'Milk feeding' below and "Neonatal necrotizing enterocolitis: Prevention", section on 'Human milk feeding'.)

Commensal bacteria play a symbiotic role with the intestine through toll-like receptors by regulating the expression of genes involved in intestinal maturation, function (eg, barrier, digestion, angiogenesis, production of IgA), and protection against more pathologic organisms [8,10,14,20,21]. Longitudinal stool colonization studies have shown that microbial dysbiosis in the absence of primary infection results in mucosal injury that affects intestinal maturation and increases intestinal permeability. The increased intestinal permeability contributes to greater bacterial translocation from the intestinal lumen into the intestinal tissue, leading to an inflammatory response with activation of cytokines, apoptosis, and necrosis [15,17,18]. Additionally, specific bacterial products (postbiotics) such as formate have been associated with NEC through metagenomic and targeted metabolomic analyses [22]. (See 'Host immune and inflammatory response' below.)

Growth factors, such as epidermal growth factor (EGF), are important in intestinal development and preservation of gut barrier function. Animal models and studies in humans suggest a protective role for EGF in the pathogenesis of NEC, including a modulating role in the toll-like receptor 4 pathway of inflammation [23].

It remains uncertain whether early exposure to antibiotic therapy contributes to microbial dysbiosis and risk of NEC. Some studies found that early and prolonged (ie, ≥5 days) antibiotic administration were associated with an increased risk of NEC [24-29]; while other studies reported a lower risk of NEC among neonates who received early antibiotic therapy [30].

The use of probiotic therapy has been proposed to alter the microbial dysbiosis and promote commensal intestinal bacteria, which may inhibit inflammatory pathways and decrease the risk of developing NEC. This is discussed in detail separately. (See "Neonatal necrotizing enterocolitis: Prevention", section on 'Probiotics'.)

Milk feeding — More than 90 percent of infants who develop NEC have received enteral feeding [31]. Although both human milk and bovine-based infant formulas can serve as substrates for bacterial proliferation in the gut, the risk of NEC is greater with infant formula. Newborns, especially preterm infants, have not developed the ability to completely digest and absorb nutrients. As a result, bacterial fermentation of milk products of incompletely digested carbohydrates and lipids (eg, reducing substances, organic acids, short chain fatty acids, carbon dioxide, and hydrogen gas) in the intestine of preterm infants may cause mucosal injury [32]. The immature dysmotility in preterm infants can exacerbate this process by delaying transit time. Supportive data was provided by a study using an animal model that demonstrated a mixture of casein, organic acids, and low pH led to mucosal injury with infiltration of cellular elements and vasoactive compounds [33].

Indirect evidence supporting feeding as a risk factor for NEC is provided by the observation that there is an inverse relationship between timing of NEC and GA. For example, the median age at onset in infants with a GA of less than 26 weeks was 23 days compared with a median age of 11 days for more mature infants with a GA of greater than 31 weeks [34]. There are two main possible explanations for this finding. The first proposes that because most preterm infants take longer to begin enteral feeding, it takes them longer to reach a critical amount of feeding mass needed for the development of NEC. Another possibility is that there are changes with development that make an infant more prone to intestinal inflammation or immune dysregulation as they mature.

Although NEC is associated with enteral feeds, it does not appear that feeding-related factors are important in the development of NEC. (See "Neonatal necrotizing enterocolitis: Prevention", section on 'Standardized feeding protocol'.)

Human milk — Human milk, compared with formula, is more protective against NEC in preterm infants because it contains components (not found in formula) that reduce inflammation while not introducing foreign antigens in the GI tract (figure 4). These protective factors within human milk include oligosaccharides, lactoferrin, lysozyme, platelet activating factor acetylhydrolase, secretory IgA, cytokines (interleukin [IL]-10, IL-11), epidermal growth factors (EGFs) [35], nucleotides, glutamine, and antioxidants such as vitamin E, carotene, and glutathione. In addition, human milk microbes (prebiotic) in combination with human milk oligosaccharides (prebiotic) appear to synergically influence the development of the neonatal intestinal microbiome [36]. (See "Neonatal necrotizing enterocolitis: Prevention", section on 'Human milk feeding'.)

Human milk feeding is also more likely associated with growth of nonpathogenic bacteria (commensal bacteria) compared to formula [37]. The mucus coat of the intestine is less affected by human milk, and growth factors within human milk (ie, EGFs) repair disruptions in this layer. Human milk improves intestinal motility, which avoids milk stasis and decreases intestinal permeability. Human milk also stimulates the mucosal defense system so that local immune activation is thwarted.

Medications

Hyperosmolar agents — The administration of hyperosmolar medications and/or formulas can cause mucosal injury and may result in NEC [38-40].

●Oral medications such as multivitamins or phenobarbital contain hypertonic additives that might irritate the intestinal mucosa.

●Instillation of hyperosmolar contrast agents into the bowel for diagnostic radiographic studies can cause mucosal injury because of fluid shifts, bowel distention, and ischemia. Isotonic contrast agents should be used to avoid this complication.

●Hyperosmolar formulas, which are concentrated above recommended strengths or containing multiple additives, should be avoided in the first few weeks after birth and preferably when feeds have reached full volume.

Agents that reduce gastric acidity — Histamine type 2 (H2) receptor antagonists, such as cimetidine, ranitidine, and famotidine, are associated with higher rates of NEC. This was illustrated in a report from the National Institute of Child Health and Human Development (NICHD) Neonatal Research Network [41]. In this study of 11,072 preterm infants (birth weight [BW] 401 to 1500 g) from 1998 to 2001, the incidence of NEC was 7.1 percent. Infants with NEC compared with matched controls were more likely to have received H2 antagonists (odds ratio [OR] 1.71, 95% CI 1.34-2.19). A possible explanation for this finding is that gastric acidity, which may reduce the risk of NEC by inhibiting bacterial growth [42], was lowered by the use of H2 antagonists. A subsequent study reconfirmed the negative effects of acid blockade with increased infection rates in infants on this class of drug [43].

Unproven risk factors

Primary infection — Less frequently, NEC has been reported to be associated with primary invasions of the gut by pathogenic enteric bacteria. In these small case series, bacterial organisms usually found in the distal GI tract have been recovered from the blood and peritoneal cavities of patients with NEC. These organisms include Escherichia coli, Klebsiella pneumoniae, Pseudomonas, and Clostridioides difficile [44-46].

In addition to these bacteria, viral and fungal pathogens have been isolated in some sporadic cases of NEC and in epidemic outbreaks [47-49]. Virus particles have been isolated from the stools of infants with NEC and their caregivers, including their mothers, the midwives, and the nursery nursing staff. Injury caused by infectious agents may be aggravated by inflammatory mediators. (See 'Host immune and inflammatory response' below.)

The use of molecular techniques may prove to be a useful tool in evaluating the role of infection in the pathogenesis of NEC. In a small prospective study, stool samples from 12 infants less than 34 weeks gestation who were admitted to the neonatal intensive care unit (NICU) were analyzed using polymerase chain reaction (PCR) and gel electrophoresis [50]. Clostridium perfringens was recognized during the first two weeks of life in the three infants who later developed NEC, but not in the nine infants who did not.

Circulatory instability — It has long been proposed that ischemic insult to the GI tract is a key contributor to NEC. However, most preterm infants with NEC have not had an obvious perinatal hypoxic-ischemic event [51]. Circulatory events have been implicated by observational case series in term infants, including patients with congenital heart disease [52,53], polycythemia [54,55], and RBC [56-58] and exchange transfusions [59]. A diminished blood supply to the gut may contribute to the pathogenesis of NEC in infants exposed to cocaine [60]. In addition, there are small studies that have reported insufficient intestinal perfusion either antenatally or postnatally [52,61].

These observations are inadequate to conclusively show that ischemia is a contributing factor in the development of NEC in preterm infants. Nevertheless, it still is thought that a subtle reduction in blood flow and subsequent reperfusion occurring in response to hypoxia may contribute to bowel injury.

Anemia and red blood cell transfusion — Although data suggest an association between the development of NEC, anemia, and red blood cell (RBC) transfusion, the nature of the association remains unclear. Observational studies and a systematic review noted an association between red blood cell (RBC) transfusions and increased risk of NEC [56-58,62-66]. In contrast, a large multicenter observational cohort study reported that severe anemia and not RBC transfusion was associated with NEC [67].

In this study of 598 very low birth weight (VLBW) infants (BW ≤1500 g), multivariable analysis showed no difference in the rate of NEC (defined as Bell stage 2 or greater) between infants exposed to transfusions within a week of developing NEC versus those who were not exposed. In contrast, the risk of NEC was increased in infants with severe anemia (hemoglobin level ≤8 g/dL) within a week of developing NEC compared with those who did not have severe anemia (adjusted cause-specific hazard ratio [HR] 5.99, 95% CI 2-18). Unlike previous reports, this study adjusted for confounding factors including BW, center, breastfeeding for less than 10 days, illness severity, and duration of antibiotic therapy.

These results suggest that clinically-significant anemia is associated with an increased risk for NEC, and that RBC transfusion is either a surrogate marker for anemia or potentiates bowel injury when severe anemia is present. However, the observational nature of this study prohibits the ability to determine causation between NEC and significant anemia, and also does not account for all potential confounding factors (eg, long-term breastfeeding beyond 10 days). A subsequent review of the literature reported that the pooled data from the included published studies failed to demonstrate an association between NEC and RBC transfusion [68]. Within the same review, a pooled analysis of three trials examining the relationship between NEC and different transfusion thresholds reported less frequent NEC among infants in the liberal versus restrictive transfusion groups. However, the overall quality of the evidence was rated as low to very low because of inconsistency among the studies, the lack of a consistent definition, and heterogeneity amongst the patient groups.

In another study, blood transfusions were associated with increases in intestinal fatty acid binding protein and may suggest a possible mechanism for transfusion related intestinal injury [69].

Management of anemia and RBC transfusion in preterm neonates at risk for NEC is discussed separately. (See "Neonatal necrotizing enterocolitis: Prevention", section on 'Prevent severe anemia/Decrease exposure to RBC transfusion'.)

The indications and administration of red blood cell transfusions in the newborn are discussed in greater detail separately. (See "Red blood cell (RBC) transfusions in the neonate".)

Host immune and inflammatory response — Increasingly, data show that an exaggerated inflammatory immune response initiated by the innate immune system to environmental triggers is an important pathogenic component in the development of NEC. In very preterm infants (GA <32 weeks), there is a deficiency in antibody-mediated immunity, as immunoglobulins do not cross the placenta until the second trimester with a linear increase in levels as pregnancy progresses [7,70]. As a result, the innate immune system is the primary immunologic protective system in very preterm infants. Toll-like receptors (TLR) found on the apical border of intestinal epithelial cells are a major component of innate immunity, including TLR-4, which binds to lipopolysaccharide (LPS, a component of gram-negative bacteria). In patients with NEC, exaggerated and uncontrolled TLR signaling induced by infectious agents, particularly gram-negative bacteria and mucosal irritants, results in marked inflammation leading to the release of cytokines and chemokines. These inflammatory mediators further aggravate mucosal injury, increase vascular permeability, and attract inflammatory cells, resulting in apoptosis and cellular necrosis of the bowel [10,71]. Data have shown that levels of cytokines are increased in preterm infants with NEC and correlate with the severity of the NEC disease and include tumor necrosis factor (TNF), interleukins (IL-1, IL-6, IL-8, IL-10, IL-12, and IL-18), and platelet activating factor (PAF). These inflammatory mediators increase vascular permeability and attract inflammatory cells [10,72-75].

This cascade of cellular events may be mediated at least in part by PAF, which is synthesized by many cell types, including macrophages, neutrophils, eosinophils, and endothelial cells [76]. In animal models, administration of PAF causes intestinal necrosis as well as systemic hypotension, capillary leakage, pulmonary hypertension, neutropenia, and thrombocytopenia [76]. In humans, plasma concentrations of PAF are higher in infants taking milk feedings, however, acetylhydrolase present in human milk breaks down PAF [76]. Plasma acetylhydrolase activity is lower in infants with NEC compared with controls and may contribute to the increased concentrations of PAF [72].

Chorioamnionitis — Chorioamnionitis is an antenatal proinflammatory process involving the placenta and fetal membrane, which may play a contributing role in the pathogenesis of NEC. However, data are inconclusive whether it is a significant clinical risk factor, as illustrated by the following:

●Meta-analysis of studies examining the relationship between NEC and chorioamnionitis confirmed by histology found no significant association between histologic-confirmed chorioamnionitis and NEC [77]. However, a subgroup analysis demonstrated an association between NEC and histologic-confirmed chorioamnionitis that included fetal involvement.

●Subsequently published retrospective studies have reported conflicting results regarding the contribution of chorioamnionitis in the pathogenesis of NEC [78,79].

Further investigation is needed to determine whether there is a significant clinical role of chorioamnionitis in the pathogenesis of NEC.

Intestinal vascular resistance — Mucosal injury and inflammation may alter the balance of endogenous vasoactive agents, which may contribute to the development of NEC with changes in the intestinal vascular resistance and microcirculation [6,80-82]. Data are limited on the roles of nitric oxide, the primary vasodilator, and endothelin-1, a potent vasoconstrictor, in the pathogenesis of NEC.

●Endothelin-1 (ET-1) – In a pathologic study of resected intestinal specimens from infants with suspected NEC, there were increased levels of ET-1 in areas that demonstrated histological findings consistent with NEC as compared with unaffected tissue from the same specimen [82]. In addition, arterioles showed increased vasoconstriction in affected compared with unaffected tissue that was reversed by an endothelin receptor antagonist.

●Nitric oxide (NO) – The limited data available on the role of NO are conflicting.

•In one report, inducible NO synthase (iNOS) gene and protein expression were detected more often in apical enterocytes in infants undergoing surgical resection for NEC than for intestinal atresia or other noninflammatory conditions [81]. These results would suggest that increased NO may contribute to failure of gut barrier function, bacterial translocation, and direct intestinal damage.

•In contrast, another study reported that although endothelial NO synthase (eNOS) was present in specimens from infants undergoing surgical resection for NEC and control patients with intestinal atresia, eNOS function was impaired, resulting in decreased NO production in arterioles from NEC specimens compared with controls [83]. Arterioles from patients with NEC failed to normally dilate in response to acetylcholine stimulation or increased arteriolar blood flow.

PATHOGENESIS: TERM INFANT — NEC in term infants or infants with birth weight (BW) >2500 g is generally associated with predisposing or underlying condition, such as congenital heart disease, primary gastrointestinal defects perinatal asphyxia, polycythemia, sepsis, hypotension, endocrine disorders, and respiratory disease [84-92]. In addition, there is evidence that infants with neonatal abstinence syndrome may be at risk for NEC [85].

Although the underlying mechanism for NEC in term infants is unknown, it is postulated impaired mesenteric oxygenation due to poor perfusion or cyanosis is a major predisposing factor that contribute to the development of NEC, along with non-human milk feeding [52,85,89,91]. As a result, the pathogenesis of NEC in term infants likely differs from the process that occurs in preterm infants.

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Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Newborn necrotizing enterocolitis (The Basics)")

SUMMARY

●Pathology of necrotizing enterocolitis (NEC) – NEC is the most common gastrointestinal emergency in the newborn infant. It is a disorder manifested by ischemic necrosis of the intestinal mucosa associated with intense transmural inflammation, invasion of enteric gas-forming organisms, and dissection of gas into the muscularis and portal venous system. The pathologic findings of NEC vary depending upon the progression of the disease. In general, the bowel appears distended and hemorrhagic on gross examination (picture 1). The major histologic findings include mucosal edema, hemorrhage, and transmural bland necrosis (figure 1). (See 'Pathology' above.)

●Pathogenesis in preterm infants – NEC occurs primarily in preterm infants. Although the pathogenesis remains unknown, it is most likely due to a multifactorial process that requires the concurrent presence of a susceptible host, environmental triggers that lead to microbial dysbiosis (disruption of commensal bacterial colonization with pathogenetic bacterial overgrowth), and mucosal injury and an exaggerated inflammatory host response. (See 'Overview: Multifactorial process' above.)

•Susceptibility – The risk of NEC increases with decreasing gestational age and is greatest in extremely preterm infants (gestational age [GA] <28 weeks) due to the immunologic and intestinal immaturity. Impairment of the physical, biochemical, and immunologic barriers decreases host resistance to microbial dysbiosis and mucosal injury, and predisposes the preterm infant to NEC. (See 'Susceptibility: Immunologic and intestinal immaturity' above.)

•Risk factors and triggers – Risk factors for NEC include prolonged exposure to antibiotics, non-human milk feeding, and administration of hyperosmolar agents or medications that reduce gastric acidity. These factors result in microbial dysbiosis and an increased likelihood of mucosal injury triggering a hyper-inflammatory response. (See 'Microbial dysbiosis' above.)

•Host inflammatory response – An exaggerated and uncontrolled increase in toll-like receptor (TLR, a component of the innate immune system) signaling induced by infectious agents and mucosal irritants results in marked inflammation due to the release of cytokines and chemokines. These inflammatory mediators further aggravate mucosal injury, increase vascular permeability and attract inflammatory cells, resulting in apoptosis and tissue necrosis (the pathologic findings of NEC). (See 'Host immune and inflammatory response' above.)

●NEC in term infants – In term infants, NEC is generally associated with a predisposing or underlying condition, such as congenital heart disease, perinatal asphyxia, polycythemia, sepsis, hypotension, endocrine disorders, and respiratory disease. Although the underlying mechanism is unknown, it is postulated that decreased oxygen delivery to the intestinal and non-human milk feeding are risk factors for NEC. (See 'Pathogenesis: Term infant' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Richard J Schanler, MD, who contributed to an earlier version of this topic review.