Necrotizing Enterocolitis and Short Bowel Syndrome

Defining Necrotizing Enterocolitis: A Conundrum

It is evident that what we have termed NEC is not a discrete entity. This lack of a clear phenotypic and pathophysiologic description thwarts progress. The underlying reasons are multifactorial and include:

• 1.

  Greater emphasis and support for the smallest, most preterm infants who have the highest propensity to develop intestinal injury.

• 2.

  Intestinal injury seen in the smallest, most immature infants occurs at a different time after birth than in more mature infants.

• 3.

  Many different forms of intestinal injury in the neonate are consolidated under the all-encompassing term NEC; however, preventative and therapeutic procedures are not specifically aimed at these subentities of intestinal injury.

• 4.

  Animal models used in studies of pathophysiology and preclinical preventative and treatment strategies do not accurately reflect the disease in preterm infants.

• 5.

  Multifactorial pathophysiology of these individual-specific injuries placed under the umbrella of NEC has not been clearly elucidated.

If progress is to be made in the prevention and treatment of the various forms of intestinal injuries termed NEC, a better classification system is in order. Numerous retrospective observational studies have suffered from classifying different pathophysiologic entities as NEC. Because of this, databases include infants with spontaneous intestinal perforations (SIPs), ischemic bowel disease secondary to cardiac anomalies, congenital intestinal abnormalities, and food protein intolerance enterocolitis syndrome (FPIES). These all have different pathophysiologic features. Furthermore, the staging system for NEC, developed about five decades ago, has less current utility due to the greater degree of immature infants seen in today’s intensive care setting.

NEC Stages

Variants of a staging system originally developed in the 1970s by Dr. Martin Bell have been relied upon to guide the management of NEC. These have been incorporated into databases such as the Vermont Oxford Network (VON), but it is becoming clear that this system has numerous pitfalls and may cause considerable confusion and not be as useful as intended. Numerous previous chapters have listed these stages, but we will not because they lead to confusion and, in the authors’ opinion, should no longer be used. For example, in stage 1 NEC, intestinal necrosis is implied but not validated. Many of the symptoms and signs in this stage are seen in extremely low birthweight (ELBW) infants without any clear bowel pathology. Stage 2 is largely dependent on radiographic diagnosis, which provides no clear evidence for necrosis of the bowel and can be misread. Stage 3 largely depends on stronger evidence of bowel injury, which is optimally diagnosed by direct examination of the bowel by intraoperative inspection and/or histologically. However, when reliance for the diagnosis of stage 3 NEC is placed on radiographic diagnosis of pneumoperitoneum and is treated with surgical drain placement without direct visualization, the differentiation between true bowel necrosis and SIP, a distinct entity, is not possible. This matters in terms of most appropriate treatment.

Newly emerging artificial intelligence technologies such as machine learning may help us better differentiate these entities and provide a more accurate definition of NEC.

Animal Models

In the early years of NEC research, hypothermia, severe hypoxia, and infection were considered to be stressors that led to NEC. These stressors were associated with difficult birth. However, studies have shown that low Apgar scores, when corrected for prematurity, are not associated with NEC. Furthermore, in the most preterm infants, in whom NEC occurs most frequently, the disease tends not to develop until several weeks after birth, hence undermining hypoxia and ischemia as significant pathophysiologic triggers for classic NEC. Despite these caveats, the most common animal model used to study NEC involves some combination of inducing hypoxia, exposing the animal to a cold environment, infusing pathogenic bacteria or proinflammatory bacterial components such a lipopolysaccharide (LPS), and feeding the animals with a formula that is very different than the milk provided by the mother. It is highly unlikely that this model is relevant to NEC seen in the human preterm baby.

Piglets have been used as models for NEC largely because the porcine gastrointestinal (GI) tract shares similarities to that of humans, but there are also important differences. In contrast to the human neonate, one critical difference is that piglets require mother's colostrum or infusion of immunoglobulin G (IgG) to prevent death from a sepsis-like syndrome.

One rodent model uses a chemical inhibitor of intestinal Paneth cells and ingestion of pathogenic Klebsiella during the later stages of preweaning. Whether this model expresses fidelity to the disease seen in humans is unlikely because it uses damage to a single cell type to induce disease, whereas it is more likely that true inflammatory NEC involves a multifactorial pathophysiologic cascade.

“Classic” Necrotizing Enterocolitis Pathophysiology

As we discuss what is considered the most “classic” form of NEC, it is critical to remember that some of our knowledge is derived from patient databases that are diluted with “imposters” as well as animal and cell culture studies that may not accurately simulate the disease seen in human preterm infants. The focus on single pathophysiologic pathways may also be misleading because this disease involves multifactorial predispositions along with pathophysiologic cascades culminating in intestinal necrosis that may best be understood using rapidly developing systems network-based illustrations.

One of the forms of intestinal injury that has been termed NEC involves acute antecedent inflammation but is also associated with several other factors. Factors that are associated with this inflammatory injury include characteristics associated with intestinal immaturity: intestinal barrier immaturity, immature immune response, and an immature regulation of intestinal blood flow.

Based on observations of concordance in twins, there is suggestion that there is a genetic component to the pathogenesis of this form of intestinal injury. However, genome-wide association studies (GWAS) and exome-sequencing based studies are limited by the lack of adequately powered replication cohorts to validate the accuracy of these discoveries.

Intestinal Microbiota, Mucosal Immune System, and Vascular Immaturities

The environment affects the taxonomy and function of intestinal microbiota. Although a precise pathophysiologic cascade has not been based on solid mechanistic evidence, an overall likely scenario has been envisioned ( Fig. 64.1 ).

Intestinal Mucosal Immune System

An in-depth review of intestinal mucosal immune system development is beyond the scope of this chapter. The reader is referred to more comprehensive reviews of this topic. Here we will summarize some of the main components as they are related to development of NEC.

Although most attention is paid to the developing intestinal immune system after birth, it is becoming obvious that there is considerable development that occurs prior to parturition in the fetus, and this is a largely unexplored area. The development of the intestinal mucosal immune system is programmed to develop in different phases. These phases can be disrupted by various environmental circumstances, which we will discuss after our brief summary of some of the main components of the intestinal mucosal immune system.

The first layer of defense encountered by a pathogen or foreign antigen is the acidic environment of the stomach. Although primarily designed for digestive processes, these enzymes also are able to destroy many pathogens and immunogenic proteins. On the small intestinal surface, there are additional defense mechanisms. These defense mechanisms include production of mucus by goblet cells, which hinders microbial adherence in concert with polymeric secretory immunoglobulin A (sIgA) that binds luminal antigens. Peristalsis is key because it keeps the antigen-antibody complexes moving aborally to be subsequently eliminated in the stool. Paneth cells from the bottom of the crypts secrete antimicrobial peptides (AMPs). Interepithelial junctions contain various intercellular proteins that maintain junctional integrity and keep the epithelial cells in close proximity to prevent paracellular passage. Cell types such as dendritic cells, undifferentiated and differentiated T cells, and plasma cells interact in a manner that depends on the type of stimulatory milieu present in the intestine for the incitement of proinflammatory and antiinflammatory responses, as well as providing a homeostatic balance between these processes. Dendritic cells are of the macrophage-monocyte lineage that capture luminal antigens, process inflammatory signals, and then migrate to secondary intestinal lymphoid organs to interact with T-cell lymphocytes.

Intestinal epithelial cells, macrophages, and other intestinal cells carry both cell membrane and intracellular receptors involved in inflammatory responses; the best known of these being the Toll-like receptors (TLRs). TLRs recognize and bind to highly conserved and specific pathogen-associated molecular patterns (PAMPs) or microbe-associated molecular patterns (MAMPs). TLR4, one of the receptors that has been provided considerable attention in the pathogenesis of NEC, is preferentially expressed by enterocytes within crypts. The intracellular pathway commonly associated with TLR4 and some of the other TLRs includes activation of nuclear factor kappa B (NF-κB), which is a critical transcription factor inciting inflammatory responses in the developing intestine.

The fetus and preterm infant usually respond to luminal microbial stimuli with inflammation. This response can be beneficial. However, an abnormal intestinal microbial profile associated with an excessive immature inflammatory response is a critical pathophysiologic component of the “classic” form of NEC (see Fig. 64.1 ). When in proper balance, they contribute to innate immunity by maintaining a healthy epithelial barrier. However, imbalances may occur. Specific microbial components such as LPS stimulate the production of proinflammatory cytokines and initiate a downstream signaling cascade that may upset the balance of proinflammatory and tolerizing mechanisms. These intestinal mucosal interactions are complex and require additional investigation to attain clinical diagnostic and therapeutic relevance. Antibodies in the intestinal tract are also of importance. Studies of secretory IgA appear to play a role in prevention of intestinal injury in both humans and animal models.

Microbial Colonization and Dysbiosis

It is becoming increasingly recognized that the microbial environment of the intestine plays an important role in inflammatory bowel injury in preterm infants. Microbes harbor various agonists that interact with the aforementioned intestinal receptors, which incite an inflammatory response. Of interest, these same inflammatory agonists can also induce tolerance and produce various metabolites that may be either injurious or protective to the intestine depending on dose and presence of other mediators.

Over the past two decades, research stemming from the Human Microbiome Project has allowed for molecular identification of many microbes that are difficult to culture when using conventional techniques. Evaluation of fecal microbiota from infants in whom NEC was diagnosed and compared with control samples suggests that NEC is associated with altered intestinal microbial taxa prior to the development of the disease. From these studies, an imbalance in microbes with differing inflammatory and metabolic potentials is associated with pathogenesis. The microbial ecology of the intestine appears to be an important feature that predisposes to intestinal inflammatory injury such as that seen in NEC. However, to develop preventative approaches, we need a better understanding of how these microbes and their metabolites interact with the developing intestine. Such studies should use newly emerging multiomic approaches integrated with prospective randomized studies exploring different environmental conditions, as will be discussed next.

Antibiotics

Antibiotics are routinely prescribed in very low birthweight (VLBW) neonates after birth due to fear that preterm delivery is secondary to infection. This is despite the low incidence of culture-positive, early-onset sepsis. There is considerable evidence showing that antibiotics in the early perinatal and neonatal period, even when provided for short periods, are associated with intestinal dysbiosis, which has been linked to short-term adverse outcomes that include bronchopulmonary dysplasia, late-onset sepsis, NEC, and retinopathy of prematurity. On the other hand, one large observational study suggests a lower incidence of NEC after early antibiotic use. Whether or not such routine antibiotic use is warranted can be at least partially addressed with a prospective, randomized, multicenter, appropriately powered trial. Such a trial should take into account not only clinical outcomes but also multiomic features, which will provide an improved mechanistic framework for pathogenesis and prevention.

Human Milk

It is very difficult to perform controlled randomized studies between human milk (especially that derived directly from the infant’s own mother) versus formula. Nevertheless, studies in human infants comparing the relation of type of diet to intestinal barrier function have demonstrated increased intestinal permeability in those receiving formula. This suggests either a compromise in intestinal barrier function from the cows’ milk or intestinal protection from the human milk. Infants fed predominantly breast milk also have a reduced incidence of infections and are less likely to develop NEC compared with infants fed formula. The numerous bioactive factors present in fresh breast milk include live microbes, immunoglobulins, cells, enzymes, growth factors, oligosaccharides, and IgA, to name a few, all of which may play a protective role against NEC and other intestinal injuries.