Necrotizing Enterocolitis

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Chapter 33

Necrotizing Enterocolitis

Kathleen M. Dominguez and R. Lawrence Moss

Necrotizing enterocolitis (NEC) is not a new disease. Reports of a disease fitting the clinical characteristics of NEC date to the 1820s in France.¹ The earliest reports in the USA occurred in the early 1960s, when Santulli and colleagues published the first significant surgical experience with NEC.^2,3 They described a disease of low birth weight infants with a high mortality rate, which requires early, aggressive surgical management. Many investigators have devoted careers to better define this challenging disease and improve strategies for treatment and prevention. Despite these efforts, NEC remains a difficult and elusive disease. It remains unclear which premature infants are most at risk and what are the optimal prevention and treatment strategies. Long-term sequelae are easily overlooked in the acute setting, but contribute substantially to the subsequent morbidity and mortality.

Epidemiology

Several large population-based studies have found the incidence of NEC to be approximately 1 per 1000 live births. In select populations, such as infants under 1500 g, the incidence rises to between 2.3–11% (Table 33-1). Both the incidence and case fatality rate of NEC are inversely associated with birth weight.^4–9 Several studies have identified an increased incidence of NEC in black infants, particularly males, a difference that holds true even when adjusting for birth weight. Hispanic infants also show an increased incidence, though to a lesser degree.^4,10

TABLE 33-1

Population-Based Epidemiologic Studies on Necrotizing Enterocolitis

VLBW, very low birth weight.

Despite improvements in other areas of neonatal care, rates of NEC have remained stable for very low birth weight infants (VLBW) infants.¹¹ Mortality remains high, with rates ranging from 15–30%.^4,5,9,10,12 Higher fatality rates are associated with lower birth weight and younger gestational age.^12,13 In a study summarizing trends for mortality and NEC in the USA between 1979 and 1992, the death rate was 12.4 deaths per 100,000 live births.¹³ The highest mortality rate was seen in VLBW infants who were black and male.^4,10,13

Although most cases of NEC are managed medically, 20–40% will require operative intervention.5,7,12,14 Mortality increases up to 50% when surgery is necessary, and has not changed significantly over the past 30 years. The highest risk for mortality in this subgroup is also in the lowest birth weight and youngest gestational age infants.¹⁵

Long-term outcomes in patients requiring operation are worse, with increased complications such as neurodevelopmental delay, growth delay and chronic gastrointestinal problems.¹⁶

Though over 90% of cases are seen in preterm infants, there are occasional reports of NEC developing in full-term infants. Although the clinical and pathologic findings are similar, the initiating factors are likely different. Term infants who develop NEC are more likely to have predisposing risk factors such as congenital heart disease, sepsis, respiratory disease, or reported hypoxic events.17–20 The common feature of these predisposing conditions is reduced mesenteric perfusion.¹⁸ The incidence of NEC in term or near term infants is approximately 0.5 per 1000 live births.²⁰ The mortality rate for term infants with NEC appears to be similar to that of preterm infants with NEC.¹⁸

The morbidity associated with NEC translates into considerable economic burden. In one study, infants with NEC were hospitalized 60 days longer than unaffected preterm infants if operation was needed, and 20 days longer if medical treatment was successful.²¹ In this study, the mean hospital costs were $186,200 greater for surgically treated patients than controls and $73,700 greater for medically treated patients. In addition to the high costs associated with the initial hospitalization, these children often have significant long-term health care needs. The mean cost of care for a child with short bowel syndrome over a five-year period exceeds $1.6M.²²

Pathophysiology

Despite decades of research into the pathogenesis of NEC, a complete understanding of its pathophysiology remains elusive. Classic histologic findings include inflammation, bacterial overgrowth, and coagulation necrosis, and are present in over 90% of surgical specimens.²³ Radiographic findings provide insight into the pathologic process that is unfolding. Pneumatosis intestinalis, or air within the intestinal wall, is thought to be due to gas produced by overgrowth of enteric bacteria.²⁴ Progression to portal venous or lymphatic gas suggests extension of this process along vessels draining the affected intestine. Pneumoperitoneum indicates necrosis with complete disruption of the intestinal wall.

As our understanding of the pathophysiology of NEC evolves, a working model of the multifactorial nature of this disease has emerged. The unifying concept is that NEC represents an exaggerated inflammatory response to an insult. The nature of this insult is not well defined, and may vary among affected infants. It may be a global ischemic insult from congenital heart disease, an infectious insult from abnormal bacterial colonization, an insult related to formula feeding or lack of enteral feeding, or simply the response of translocation of normal bacterial flora in a genetically predisposed host.

Whatever the insult, it leads to a disruption of the intestinal epithelial barrier followed by translocation of bacteria. There is an exaggerated or inappropriate immune response, likely owing to the immature nature of the intestine and immune system. Stress pathways become activated, and pathways that normally suppress the immune system are inhibited. The end result is activation of the host immune system and release of cytokines, leading to a global, detrimental inflammatory response.

The Intestinal Barrier

The pathologic features of NEC suggest that failure of the intestinal barrier is either a cause or result of disease progression. The normal intestinal barrier is composed of both mechanical and non-mechanical factors. The mechanical factors include intestinal peristalsis, the mucous coat, and tight junctions between epithelial cells. Non-mechanical factors include immunologic defenses and cellular homeostasis and regeneration.

Intestinal Motility and Digestion

Intestinal motility develops during the third trimester of pregnancy but may not be fully mature until the eighth month of gestation.25–28 In premature infants, immature motility leads to increased epithelial exposure to potentially noxious substances, and poor clearance of bacteria with subsequent overgrowth. Additionally, the immature intestine has decreased digestion and absorption, which may lead to direct epithelial injury through a lowered pH.^29–31 Newborns have reduced gastric acidity and pancreatic enzyme activity, which may further contribute to impaired digestion of macromolecules and bacterial proliferation.³²

An increased ileal bile acid level may play a role in the pathogenesis of NEC. Bile acids are known to be cytotoxic, resulting in the development of mucosal injury.³³ In premature infants, levels of ileal bile acid-binding protein are lower, leading to increased levels of bile acids in the intestinal lumen and in enterocytes.³⁴ Another risk factor which may contribute is formula feeding, which elicits more toxic bile acids than breast feeding.³⁵

The Mucous Coat

The mucous coat overlying the intestinal epithelium plays a key role in the barrier function. This layer is composed of water, mucin, lipids, and peptides.³⁶ Mucin, a glycoprotein, is secreted by goblet cells in the epithelial layer and concentrates enzymes near the intestinal surface.^37,38 Mucin aids in lubrication, mechanical protection, protection against the acidity of gastric and duodenal secretions,³⁹ and fixation of pathogens.⁴⁰ The effectiveness of mucin is related to maturity.³⁹ Mature mucins have higher viscosity, better pH buffering, and resistance to bacterial breakdown.^39–41 Mucin production and composition changes with gestational age, bacterial challenges, and colonization by commensal organisms.^42–44 Deficits in the production or composition of mucin may contribute to the ability of bacteria to invade the intestinal epithelium and thus contribute to the pathogenesis of NEC.^{32,36,37,45–47}

Tight Junctions

Tight junctions create fusion points between epithelial cells, forming an intact yet semipermeable barrier. Mature tight junctions are composed of the transmembrane proteins occludin, claudin, and junctional adhesion protein; these normally present a barrier to diffusion of large molecules.32,48 Tight junctions are not static, but may be altered by disease processes.⁴⁹ Immaturity in the composition of tight junctions likely plays a role in the increased permeability of the epithelium of the newborn intestine.⁵⁰

Cytokines are produced in response to bacteria, and may interfere with tight junctions, promoting the translocation of bacteria.⁵¹ Inflammatory mediators such as tumor necrosis factor, interferon (IFN)-γ, and interleukin (IL)-1β further cause epithelial dysfunction by upregulating inducible nitric oxide synthase (iNOS) leading to the overproduction of nitric oxide (NO), and the generation the reactive nitrogen intermediate peroxynitrite (ONOO⁻). This process has been associated with increased epithelial cell apoptosis and death.⁵² NO has been shown to play a role in mediating the decrease in the localization and expression of tight junction proteins.^49,53 Disruption of tight junctions may lead to increased intestinal permeability, allowing bacterial translocation and activation of the immune system.

Immunologic Defenses of the Gastrointestinal Tract

The gastrointestinal tract contains the largest amount of lymphoid tissue in the body and coordinates the immunologic mechanisms of the adaptive and innate immune systems.⁵⁴ Gut-associated lymphoid tissue consists of several cell types which work in concert to perform antigen presentation and processing^37,55

In neonates, antigen processing and presentation is less efficient, reducing the ability of the immune system to respond to pathogenic organisms. Peyer patches are fewer, smaller, and lack germinal centers.³⁶ Paneth cell activation by bacteria or components of bacterial cell walls leads to secretion of a variety of antibacterial substances, including α-defensins and lysozyme.^56,57 Production of these peptides is decreased in premature infants, and may predispose to bacterial overgrowth, allowing NEC to develop. Following recovery from NEC, Paneth cell hyperplasia occurs, suggesting these cells play an important role in NEC.⁵⁶

IgA is normally synthesized by plasma cells of the lamina propria and secreted into the mucin layer where it binds bacteria and viruses, inhibiting attachment to the epithelium. The newborn lamina propria is largely devoid of the IgA-secreting plasma cells, resulting in deficient secretion until several weeks of age.58,59 Neonates can obtain IgA through passive transfer from breast milk,⁴⁹ but infants who do not receive breast milk lack this important immunoglobulin and its protective effects.

Regenerating the Intestinal Barrier

The pathologic findings of NEC arise not only from alterations in the integrity of the intestinal barrier but also from an impaired ability to regenerate.⁶⁰ Premature infants have a reduced capacity for intestinal repair, likely contributing to the pathogenesis of NEC.

Lipopolysaccharide

Lipopolysaccharide (LPS) is the endotoxin portion of the Gram-negative bacterial cell wall, and is one of the most abundant proinflammatory stimuli. LPS is seen in high levels in NEC.²⁴ LPS impairs intestinal barrier function by inhibiting repair and promoting the release of signaling molecules and proinflammatory mediators such as NO, IFN-γ, cyclooxygenase-2 (COX-2) and RhoA from enterocytes which promote intestinal injury.^49,60–62 LPS causes increased expression and function of integrins on the cell surface, resulting in increased cell adhesion to the basement membrane,⁶³ and compounds the effects of platelet-activating factor (PAF).^64,65

Nitric Oxide

NO is a key mediator of numerous physiologic and pathologic systems, but has been shown to have a paradoxical role in NEC. Low levels of NO are important for maintaining vasodilation; conversely, sustained overproduction of NO can have profound cytopathic effects. The cytopathic effects of NO are believed to be due to toxic nitrogen intermediates, such as ONOO⁻.³²

NO is a highly reactive free radical formed by the conversion of arginine to citrulline by NO synthase (NOS) which exists in three forms: the constitutive form (nNOS), the inducible isoform (iNOS), and the constitutive endothelial isoform (eNOS).⁶⁶ The presence of the constitutive forms of NOS in the gastrointestinal tract suggests that NO has a normal physiologic role in gut function. The eNOS isoform maintains intestinal homeostasis by enhancing mucosal blood flow and maintaining microvascular tone.⁶⁷

When produced by iNOS under inflammatory conditions, the NO level increases up to a million fold,⁶⁷ which can lead to cellular damage and failure of the intestinal barrier. Excess NO overwhelms local scavenging mechanisms and reacts with superoxide anion (O₂⁻) to produce the highly toxic ONOO⁻.^67–69 These effects may be compounded in the presence of high levels of LPS, which leads to increased iNOS expression and function within the intestine.^69–70 Studies have linked NO with the pathogenesis of NEC. The expression of iNOS has been shown to be upregulated in critically ill patients and in patients with NEC.⁵² Conversely, expression is down regulated by the anti-inflammatory cytokine interleukin-10.⁷¹ Excess NO may also inhibit intestinal restitution by blocking enterocyte migration and proliferation.^32,49,72

Platelet-Activating Factor

PAF is potent phospholipid inflammatory mediator that is produced by most cells and tissues.73,74 The cytotoxic effects of PAF are due to initiation of the inflammatory cascade. PAF-induced bowel injury is associated with the production of oxygen-derived free radicals as well as leukocyte migration, activation, and capillary leakage resulting in apoptosis in affected enterocytes.⁷⁵

Various studies have shown the importance of PAF in the pathogenesis of NEC. Higher concentrations of PAF have been found in NEC patients compared with controls.75–77 PAF-acetylhydrolase (AH) activity has been shown to be deficient in sick infants with NEC, and the administration of PAF-AH or a PAF receptor antagonist in animal models of NEC reduces the degree of intestinal injury.74,76,78 PAF-AH is present in maternal breast milk, which may contribute to the protective effect against NEC it provides.⁷⁴

Maintaining Intestinal Barrier Homeostasis

Epidermal Growth Factor

Epidermal growth factor (EGF) is a peptide secreted into the intestinal lumen.⁷⁹ It plays an important role in the development, maturation, and maintenance of gut homeostasis, being active in processes from intestinal repair and adaptation to cell movement and prevention of bacterial translocation.^80–86 EGF has been shown to support maintenance of the intestinal barrier, as well as being active in the down regulation of inflammatory cytokines.^79,86

EGF is believed to play an important role in the pathogenesis of NEC. Decreased levels of EGF have been demonstrated in the saliva and serum of premature infants with NEC.⁸⁷ Furthermore, studies have shown that salivary levels of EGF in the first two weeks of preterm life may have a predictive value for the occurrence of NEC.⁸⁸ A potentially therapeutic role for EGF was reported in an infant suffering from intestinal necrosis resembling NEC who received a continuous infusion of EGF resulting in complete recovery of the damaged intestine.⁸⁹ These investigators subsequently treated a small group of neonates with stage II and III NEC in a randomized, double-blind, prospective trial with recombinant EGF and found that repair of the intestinal epithelium was seen at four, seven, and 14 days.⁹⁰

Heparin-binding EGF (HB-EGF) is a member of this family of growth factors, and is found in amniotic fluid and breast milk.⁹¹ In animal models of NEC, administration of HB-EGF has been shown to reduce the incidence of bowel injury by 50%, more than double survival,^92–95 and preserve the integrity of the intestinal barrier.⁹⁶ Animals with overexpression of HB-EGF have decreased susceptibility to NEC,⁹⁷ while animals with deletion of the HB-EGF gene have increased susceptibility.^86,97 These effects seem to be at least in part due to the cytoprotective effects of HB-EGF, which serves to protect intestinal stem cells from injury.⁸⁶

EGF and HB-EGF have a role as a potential preventive strategy for NEC. These compounds may have a protective effect by altering the balance of pro- and anti-inflammatory cytokines in the pathogenesis of NEC, and may also play a role in decreasing bacterial translocation from the intestine. Active research is ongoing.

Neonatal Vasculature and the Pathogenesis of NEC

Newborn intestinal circulation is characterized by a low resting vascular resistance.98,99 This results in increased blood flow and oxygen delivery. Control of vascular resistance involves intrinsic and extrinsic control mechanisms.¹⁰⁰ Extrinsic mechanisms are mediated by the autonomic nervous system. The intrinsic regulation is mediated by two vascular effector mechanisms produced and released within the intestine—one vasoconstrictive and one vasodilatory.^101,102 Endothelin (ET)-1 is the primary vasoconstrictor stimulus in the newborn intestine and is produced by the endothelium.^103,104 Although constitutively produced, it can also be stimulated by decreased flow, hypoxia, and various inflammatory cytokines.^105–107 The production of ET-1 is age specific, being greater in younger subjects.¹⁰³

NO is the primary vasodilator stimulus.98,99 eNOS is also continuously produced, but like ET-1 the rate of production can be increased in response to a variety of stimuli.¹⁰¹ In the neonate, the balance of these two products favors vasodilation, generating the characteristic low vascular resistance. In disease states, endothelial dysfunction leads to ET-1 mediated vasoconstriction, causing compromised blood flow, intestinal ischemia, and injury. The vasoconstrictor ET-1 has been linked to intestinal tissue injury in several studies.^103,108 Increased expression of ET-1 has been found in intestine removed from infants with NEC, and the amount of ET-1 increased proportionally to the degree of intestinal injury.¹⁰²

In summary, the intestinal circulation of the newborn is unique, with a dynamic balance between constrictor (ET-1) and dilator (NO) stimuli maintaining basal vascular resistance. Disruption of the intestinal endothelial function can alter the delicate balance, favoring vasoconstriction over the normal state of vasodilation, leading to significant intestinal ischemia and tissue injury.

Bacterial Colonization

NEC is most commonly diagnosed during the second week of life, after intestinal colonization has been established. Bacteria likely play a role in the pathogenesis of NEC, though studies have not identified a single infectious agent. NEC likely arises from an unfavorable balance between commensal and pathogenic bacteria.

Abnormal colonization may alter the balance of pathogenic and beneficial bacteria, favoring an increase in pathogenic bacteria and resulting in a loss of the beneficial role of commensal bacteria. Furthermore, the immature immune system of premature infants may not be able to respond appropriately to normal colonization of bacteria, much less abnormal flora.¹⁰⁹

Clinical Diagnosis

The diagnosis of NEC is based on clinical and radiographic findings. The clinical course can vary from a slow, indolent process to a rapid fatal progression. Early signs are nonspecific, including apnea, bradycardia, lethargy, and temperature instability. Feeding intolerance, demonstrated by high gastric residuals, is the most common gastrointestinal symptom of NEC. The most common presenting sign is abdominal distention. Gross or occult blood in the stool may be found.

Gastrointestinal signs progress from abdominal distention to tenderness suggestive of peritoneal irritation (Fig. 33-1). Palpable loops of intestine may become evident. Localized disease may progress to generalized peritonitis or may worsen in a focal area, including discoloration of the skin and the development of an abdominal mass. When present, the findings of a fixed abdominal mass and erythema of the abdominal wall are strongly predictive of dead bowel; however, these findings occur in only 10% of patients with NEC.¹¹⁰ A sudden need for increased ventilatory support may also serve as a harbinger of NEC.¹¹¹ This is due to increased metabolic requirements combined with increased intra-abdominal pressure.

FIGURE 33-1 This infant has NEC. Note the abdominal distension and abdominal wall erythema.

Confirmation of the diagnosis of NEC combines signs and symptoms with radiologic findings. These findings have been combined into the clinical staging system proposed by Bell that aids in describing the severity of disease (Table 33-2).^112,113

TABLE 33-2

Modified Bell’s Staging for Necrotizing Enterocolitis

Laboratory Studies

Laboratory studies reveal nonspecific indicators of an inflammatory or infectious process such as leukocytosis with bandemia. Thrombocytopenia and metabolic acidosis are also common. A rapid fall in platelet count is a poor prognostic factor.¹¹⁴

Several studies have tried to identify an accurate biochemical marker to identify neonates at risk for NEC, avoiding prolonged periods without enteral nutrition as well as the use of unnecessary tests and antibiotics.¹¹⁵ Serum acute phase proteins and cytokines have been investigated for an association between high levels and the severity of NEC. Increased levels of IL-6, IL-10, and C-reactive protein (CRP) have been documented in premature infants with NEC, with the highest levels of IL-10 in those patients who did not survive.¹¹⁶ CRP has also been associated with NEC when the levels rose quickly after the diagnosis was suspected. A failure of the levels to return to normal has been found to be associated with complications, including abscesses, strictures, and sepsis.¹¹⁷ In a prospective study, CRP levels were elevated in infants with stage II and III NEC and may be useful in discriminating between stage II NEC and other gastrointestinal disorders.¹¹⁸

Multiple other potential markers have been studied—gastrointestinal tonometry, urinary D-lactate levels, exhaled breath hydrogen, endotoxin elevations in stool, plasma intestinal fatty acid-binding proteins—but none of these has yielded the sensitivity or specificity required for a diagnostic tool.61,119–121 Currently, no biochemical markers have been adequately predictive of the patient’s clinical course or outcome to be clinically useful.

Radiographic Findings

Plain Films

The cornerstone of the radiographic diagnosis of NEC relies on plain radiographs. The most specific radiographic finding is pneumatosis intestinalis, as seen in Figure 33-2. Other radiographic findings include air–fluid levels, gas-filled loops of bowel, persistently dilated loops of bowel, thickened bowel walls, portal venous gas, and pneumoperitoneum. Although most commonly seen in NEC, pneumatosis intestinalis has also been reported in cases of Hirschsprung enterocolitis, severe diarrhea, and carbohydrate intolerance. Portal venous gas (Fig. 33-3) is a less common radiographic finding but is generally considered a poor prognostic sign. This finding is associated with twice the incidence of diffuse or ‘pan’ necrosis and a significantly lower survival rate.¹²² Nevertheless, many patients with portal venous gas recover fully with medical management.

FIGURE 33-2 Pneumatosis intestinalis is the classic radiographic finding in NEC. This finding may be cystic or linear (arrow), and may be seen in a focal intestinal segment or diffusely throughout the bowel.

FIGURE 33-3 Portal venous gas (arrow) is demonstrated on this abdominal radiograph. This finding is considered a poor prognostic sign. This baby also has widespread pneumatosis intestinalis.

Other Imaging Modalities

Studies have examined ultrasonography (US) as an adjunctive measure for the diagnosis and management of infants with NEC. Abdominal ultrasound evaluation emerged as a potential modality in the treatment of NEC after a report in 2005 that assessed bowel viability using color Doppler imaging in neonates with NEC.¹²³ This publication established critical data for bowel wall thickness, echogenicity, peristalsis, and perfusion in both normal neonates and those with NEC. Additional studies corroborated the usefulness of ultrasound as a means of diagnosing NEC.^124,125 ultrasound offers some potential advantages over plain films in that it can depict bowel wall thickness and echogenicity, free and focal fluid collections, peristalsis, and the presence or absence of bowel wall perfusion by using Doppler imaging.^126,127

The presence of pneumatosis on plain abdominal radiographs helps clinch the diagnosis, but mild findings, such as the lack of intramural gas, makes the diagnosis more difficult. ultrasound may be a useful adjunct in this population because it may allow detection of small amounts of intramural gas not visible on plain films or changes in bowel wall thickness, peristalsis, or perfusion that could confirm or exclude the diagnosis of NEC.¹²⁸ The time frame for when to perform ultrasound initially or when to use it during follow-up has not been established.

In addition to assisting with the diagnosis in difficult cases, ultrasound has been suggested as an adjunct modality in two other groups of patients: those in whom the evolution of changes in the radiographs does not match the clinical course and those whose condition is deteriorating without evidence of pneumatosis on plain films.¹²⁸ Finally, ultrasound may be useful in helping to decide the appropriate time to re-initiate and advance feeding.¹²³ However, at this time, ultrasound does not yet have a well-defined or established role in the management of NEC.

In the acute setting, contrast examinations of the gastrointestinal tract, computed tomography, and magnetic resonance imaging have not been found to be useful modalities in clinical practice.129–133

Differential Diagnosis

The most clinically relevant differential diagnosis in a premature infant with abdominal distention is distinguishing between NEC and sepsis with ileus. In the absence of clinical signs of peritonitis or radiographic signs of NEC, the two conditions may be indistinguishable and only differentiated after observing the clinical course. The differential diagnosis also includes other conditions that may cause abdominal distention, such as Hirschsprung disease, ileal atresia, volvulus, meconium ileus, and intussusception.

A subset of premature infants presents with bowel perforation while not exhibiting other symptoms of NEC nor pneumatosis on radiographs. Some investigators have defined this as spontaneous, isolated, or focal intestinal perforation (FIP). FIP tends to occur in low birth weight infants, usually the first seven to ten days of life, and is sometimes associated with indomethacin treatment.134–140 Whether these infants have a limited form of NEC or a distinct entity is controversial. Some reports contend that FIP is a different disease than NEC, but definitive evidence is lacking.^139–142 As expected, neonates with an isolated bowel perforation have better outcomes in the absence of extensive disease.^{138,143–145}

Medical Management

Medical management of NEC begins with bowel rest, gastric decompression, intravenous fluid resuscitation, and broad-spectrum antibiotic therapy, including anaerobic coverage. Blood, urine, and sputum cultures should be obtained before the initiation of antibiotic therapy. A critical component of medical management is ongoing close observation with serial abdominal examinations and radiographs. As long as the clinical situation is stable or improving, expectant management can continue. Clinical deterioration or worsening radiographic features may indicate the need to consider surgical intervention.

Experimental Medical Treatments: HB-EGF

Endogenous HB-EGF is increased in response to hypoxia, stress, and during wound healing.146–151 HB-EGF mRNA is induced after intestinal ischemia/reperfusion injury in vivo¹⁵² and is involved in epithelial cell repair, proliferation, and regeneration in the early stages after injury.¹⁵³ Based on these findings, it has been theorized that exogenous HB-EGF may also play a role protecting the intestinal mucosa from injury.

Multiple studies have demonstrated that exogenous administration of HB-EGF can protect cells and organs from injury both in vitro and in vivo. HB-EGF can protect enterocytes from proinflammatory cytokine-induced apoptosis.¹⁵⁴ Intestinal epithelial cells pretreated with HB-EGF before hypoxia showed less necrosis with maintenance of the cytoskeletal structure and improved recovery ability.¹⁵⁵ HB-EGF also downregulates the production of NO^156,157 and blocks NF-κB activation in intestinal epithelial cells after cytokine stimulation.¹⁵⁷ In a neonatal rat model of NEC, the administration of HB-EGF reduced the severity and incidence of NEC with preservation of gut barrier integrity.⁹² Studies have also shown that treatment with HB-EGF decreases the overproduction of IL-18 and increases the production of anti-inflammatory IL-10.¹⁵⁸ HB-EGF is the only compound with imminent plans for investigation in humans. A host of other therapeutic agents have shown promise but not yet reached the stage of clinical testing.

Surgical Management

Although many infants can be managed medically, 20–40% will require operative intervention. In some cases, indication for operation develops during the medical management, while in others it is found at presentation. The only absolute indication for drainage or exploration is evidence of intestinal perforation either on an abdominal radiograph (Fig. 33-4) or via paracentesis that is positive for stool or bile.¹⁵⁹ Relative indications for operation include deterioration in the infant’s clinical condition despite maximal medical management. Such signs can include oliguria, hypotension, worsening metabolic acidosis, worsening thrombocytopenia, leukopenia or leukocytosis, and ventilatory failure. Relative radiographic indicators for operation include portal venous gas or persistently abnormal ‘fixed’ loops of bowel on serial radiographs.

FIGURE 33-4 Free air (arrows) is seen on this radiograph. This finding is an indication of perforation, and is considered an absolute indication for intervention, whether drainage or exploration.

Ideally, surgical intervention would occur when intestinal gangrene is imminent but before actual perforation or necrosis actually occurs. However, this ideal time for intervention is often difficult to identify. One study has tried to evaluate the sensitivity and specificity of 12 different findings to identify early indicators for operation.¹¹⁰ Three findings had a specificity and positive predictive value (PPV) close to 100% with prevalence greater than 10%. These findings were deemed the ‘best’ indications and included portal venous gas and a positive paracentesis (Table 33-3). Three indicators had specificity and PPV close to 100% but prevalence less than 10% and were considered ‘good’ indicators, including a fixed loop on an abdominal radiograph, erythema of the abdominal wall, and a palpable abdominal mass. One indicator, severe pneumatosis, was deemed fair because it had a specificity and PPV above 90% and 20% prevalence. The five remaining indicators were considered poor because the specificities were less than 90% and the PPVs less than 80%. This probability analysis may be useful in the complex decision-making process when an operation is being considered.

TABLE 33-3

Probability Analysis of Various Indications for Operation in Necrotizing Enterocolitis

NEC can affect any segment of the gastrointestinal tract. Most commonly, both large and small bowel are involved.²³ Isolated small intestinal lesions occur with the next greatest frequency. It is as common to have a single affected area as to have multiple-segment disease.^23,160,161 A small subgroup of NEC patients develop massive necrosis of the entire intestine, known as ‘NEC totalis.’¹⁶⁰

Traditional operative management has consisted either of laparotomy with limited resection of the affected bowel with creation of stomas or of primary peritoneal drainage. Much of the attention of surgical investigators of NEC has focused on the relative benefits of these approaches.

Peritoneal drainage was first reported in 1977 as salvage treatment for perforation in VLBW infants who were believed to be too unstable for laparotomy (Fig. 33-5).¹⁶² Initially intended as a temporizing procedure in the sickest and smallest patients, this treatment has evolved into a widely utilized option as primary treatment of perforated NEC. After many years of conflicting results comparing outcomes of the two approaches, a meta-analysis was attempted to synthesize these disparate data. This study found such significant bias in the assignment of patients to one treatment or another that the two options could not be adequately compared.¹⁶³ A need existed for a prospective randomized controlled trial.

FIGURE 33-5 A micropremature infant with NEC and perforation is shown with her corresponding abdominal radiograph. A percutaneous drain (arrows) has been placed in the right lower quadrant for drainage of the intestinal perforation. Note that the drain is placed at a position below the level of the umbilicus to avoid injury to the lower edge of the right lobe of the liver.

Three prospective studies have compared laparotomy to peritoneal drainage. The NICHD Neonatal Research Network conducted a prospective observational cohort study at 16 centers.¹⁶⁴ This study included 156 infants with either NEC or FIP. Overall 50% (n = 78) of the patients died and 72% (n = 112) either died or had some element of neurologic impairment at 18 to 22 months. The babies in this study were not randomized to their treatment groups. The treating surgeons and neonatologists chose which therapy to use for each infant. However, unlike other nonrandomized studies, extensive prospective data were collected, allowing for risk-adjusted multivariable regression analyses. This strategy enabled the investigators to account for the differences between the treatment groups. The odds ratio for death after adjusting for differences in the two treatment groups was 0.97 for laparotomy compared with peritoneal drainage (95% confidence interval [CI]: 0.43–2.20). The odds ratio for the combined outcome of death or neurodevelopmental impairment at 18 to 22 months was 0.44 for laparotomy compared with drainage (95% CI: 0.16–1.2). Although not statistically significant, there is some suggestion in this study that overall outcomes at 18 to 22 months of age may be improved by laparotomy rather than drainage.

The first randomized trial evaluating laparotomy versus peritoneal drainage was the NECSTEPS trial.¹⁶⁵ In this trial, 117 VLBW infants at 15 North American tertiary care centers were randomized to either treatment group. The primary outcome variable was mortality at 90 days. There was no difference in mortality at 90 days between the two treatment groups (34.5% vs 35.5%). Need for parenteral nutrition at 90 days and the length of hospitalization were also similar between the two groups. This study focused on short-term outcomes; within those limits, results suggest that the method of surgical intervention does not impact the outcome.

The second randomized trial comparing laparotomy and peritoneal drainage in infants with perforated NEC was the NET trial.¹⁶⁶ This trial was a multinational trial conducted at 31 centers in 13 countries. The primary outcome variable was mortality at 1 and 6 months. Sixty-nine patients weighing less than 1000 g were enrolled and randomized. There was a trend toward better survival in the laparotomy group (65% survival) compared with the drainage group (51%), with a relative risk of mortality of 0.5 (95% CI: 0.2–1.5). These findings were not statistically significant. The authors concluded that there was no evidence from the trial to support the benefit of primary peritoneal drainage in extremely low birth weight (LBW) infants with intestinal perforation.

Overall, both of these randomized trials suggest that the method of surgical management does not affect the ultimate outcome of infants with perforated NEC. The impact of choice of operation on the outcome of infants, who underwent operation for an indication other than perforation, is not known. Most commonly, these infants are treated with laparotomy.

An additional randomized trial is currently underway. The NEST trial is designed to compare long-term outcomes in extremely LBW infants (≤1000 g) with necrotizing enterocolitis or isolated intestinal perforation treated by either laparotomy or peritoneal drainage. The primary outcome is death or neurodevelopmental impairment at 18–22 months corrected gestational age. Results are expected in the fall of 2015.¹⁶⁷

When laparotomy is performed, stomas are usually created. Because of concerns about the high morbidity associated with enterostomies (Box 33-1), a few centers have advocated primary anastomosis at the time of initial laparotomy. The data to support such management are nonrandomized and retrospective. In actuality, the majority of stomal complications are easily managed and early closure is well tolerated.¹⁶⁸ One study found that survival was 72% with intestinal diversion but only 48% in those undergoing primary anastomosis.¹⁶⁹

Diffuse intestinal involvement poses the most difficult situation for the surgeon. Those infants who survive may develop short bowel syndrome and have some level of dependency on total parenteral nutrition given the extensive amount of affected bowel. Surgical strategies focus on trying to preserve as much intestine as possible while still resecting enough bowel to stabilize the patient. Second-look laparotomies have been proposed as a way to minimize the amount of bowel resected.¹⁷⁰ The ‘clip and drop-back’ technique is another option with a similar strategy.¹⁷¹ All nonviable intestine is resected initially but no ostomies or anastomoses are created. Blind-ending segments are left in the abdomen, and continuity is restored or ostomies are created on re-exploration in 48 to 72 hours. Proximal diversion alone has also been used to treat ‘pan’ necrosis without reported worsened survival and with recovery of much of the bowel by the time of ostomy closure.¹⁷² None of these approaches has been prospectively evaluated; therefore, no single technique can be strongly advocated.

Outcomes

Recurrence

NEC recurs in approximately 5% of cases.173,174 There is no apparent correlation between the site of disease and the site of recurrence. Usually recurrent disease can be managed nonoperatively.^{159,174–179}

Length of Hospitalization

Hospital stays are longer for infants who suffer from NEC when compared with other infants of the same gestational age. Furthermore, those who require operation tend to have even longer hospitalization. Several studies have shown hospitalizations averaging two to three months for medically treated NEC and four to five months for surgically treated NEC.180–182 There was no difference in length of hospitalization between the two treatment groups in the NECSTEPS trial.¹⁶⁵

Mortality

Estimates of mortality from NEC have remained steady over the past two decades at 15–30% despite the fact that the postsurfactant era has led to a rise in the incidence of disease.4,5,9,10,12,13,180,181,183,184 Operative mortality has improved with a decline from 70% mortality in the 1960s¹⁸⁵ to more recent rates of 20–50%.^{5,10,13,180,183,186,187} The main predictor of mortality in NEC is gestational age. The highest mortality occurs in the youngest, smallest infants, and black male VLBW infants are at greatest risk. Additionally, infants with a greater extent of bowel affected by the disease tend to also have a higher mortality rate.^188,189

Gastrointestinal Outcomes

Short Bowel Syndrome

NEC is the leading disease responsible for short bowel syndrome (SBS) in children, accounting for half of all pediatric cases. Furthermore, SBS develops in a fourth of all patients who suffer from NEC.¹⁹⁰ SBS develops when an infant is left with inadequate functional intestine to absorb the nutrients required for growth. This can result from resection at the time of operation or from poor function of the remaining intestine. Traditional teaching based on early reports suggests that a minimum of 40 cm of small intestine is required for a patient to have a chance of weaning from total parenteral nutrition.¹⁹¹ Despite these observations, experience has shown that the function of the intestine is much more important to this disease than the specific length of intestine.

The portion of intestine resected is also important for subsequent gastrointestinal functioning. Patients with ileal or jejunal disease have a higher mortality rate than those with colonic disease.187,188,192,193 Patients with extensive jejunal resection fare better than those with extensive ileal resection. This is due to the differing abilities of the intestinal regions to undergo adaptation. The ileum has the greatest capacity to adapt and increase its absorptive capacity.

Preservation of the ileocecal valve has been considered important for minimizing the risk of SBS.¹⁹⁴ Some studies have suggested that dependence on parenteral nutrition is lessened when the ileocecal valve was preserved,^195–198 but others have found no difference.^{180,199–202} It appears that the actual length and functional capacity of the remaining ileum is far more important than the presence of the valve itself.

Stoma Complications

Creation of a properly constructed stoma can be life-saving in the management of NEC. Stomas are used for both decompression and diversion. Enterostomies can be fraught with early and late complications. A number of strategies have been proposed for optimal stoma creation, including what type of stoma to create as well as how to exteriorize the stoma. End stomas, double-barrel (Mikulicz) stomas, and loop enterostomy have all been advocated. Small studies comparing complication rates between these various strategies have not found differences in complications, including retraction, prolapse, hernias, or wound infections.203–205 Many surgeons exteriorize the stoma and mucous fistula through the incision, some at one end, others at opposite ends of the incision. Others advocate a separate incision, citing concerns about increased wound infection and difficulties attaching stoma appliances. Another consideration for a separate incision is whether the stoma needs to remain for a prolonged period of time. Most surgeons do not recommend maturing a stoma owing to potential interference with an already tenuous blood supply.²⁰³

Stomal complications can lead to significant morbidity. Studies have shown complication rates exceeding 50%.187,206–209 The most serious complications include prolapse, stricture, and retraction, all of which may require surgical intervention. Proximal jejunostomies can cause significant electrolyte and fluid losses that can lead to problems with fluid balance and weight gain.^194,210 Fluid losses from jejunostomies can also cause peristomal skin complications. Despite these problems, with an aggressive approach to fluid and electrolyte replacement and meticulous skin care, proximal jejunostomies can be a viable option for the management of NEC.^194,211

The timing of enterostomy closure remains controversial. Recommendations vary from as early as one month to as late as four months after operation.212–215 Most suggest waiting one to two months after the initial operation, and until a weight of 2000 g is reached, as long as adequate feeding and growth is being maintained.^194,210,213 Earlier closure may be necessary with very proximal stomas due to fluid and electrolyte losses and the inability to gain weight. Coexisting medical problems must also be considered in determining the optimal time for closure.

Intestinal Strictures

Intestinal strictures are a common occurrence after NEC. The incidence of stricture has been reported from 12–35%, and occurs in patients who have been managed medically or surgically.131,207,216–221 This incidence does not differ between patients treated by primary anastomosis or enterostomies.^{159,186,187,199,206,207,212,215–227} In one series of patients treated for severe NEC by proximal diverting enterostomy, the incidence was 55%.²¹⁷ Most post-NEC strictures occur in the colon, specifically the left colon (Fig. 33-6).^{160,212,219,226,228}

FIGURE 33-6 This contrast enema demonstrates a stricture (arrow) in the ascending colon after nonoperative management for NEC. NEC strictures may occur anywhere in the intestinal tract, but are most common in the left colon. These can develop whether the baby was treated medically or surgically.

Resection of strictures is usually needed, although not all lesions are symptomatic and spontaneous resolution has been reported.131,224,229,230 Other approaches have been proposed, including close radiographic follow-up of asymptomatic patients.¹³¹ Balloon dilatation is an option for focal, nonobstructing lesions, but has not been widely utilized.²³⁰

Patients treated by laparotomy and stoma for NEC should undergo routine imaging of the distal intestine before enterostomy closure to evaluate for a possible stricture. Patients managed medically, by peritoneal drainage, or with primary anastomosis may also develop strictures. Some patients remain asymptomatic and others present acutely in distress due to perforation.²¹⁹ Thus, some surgeons advocate contrast studies in all NEC patients.^{131,218,219,221} The potential for false-negative results and the invasiveness of the procedure prevent this practice from being commonly used.

Little is known about the long-term impact of stricture formation after NEC. Infants who require ostomy closure may have strictures addressed at that time. For those managed without ostomies, stricture resection requires a new operation with subsequent prolongation of recovery and time to full enteral feeding. Resection of additional bowel may also ultimately impact gastrointestinal outcomes.

Growth

Several small observational studies have shown that children treated for NEC fall below the 50th percentile for height and weight even into their grade-school years.231–233 This growth retardation seems particularly to affect those who suffered from stage III NEC.^234,235 The problem is much more severe in children who develop SBS as a result of NEC. Long-term evaluations of growth are required to evaluate the impact that birth weight, NEC severity, operative strategy, and subsequent outcomes may play in ultimate growth outcomes for these children.

Neurodevelopmental Outcomes

In infants surviving NEC, adverse neurodevelopmental outcomes remain an important challenge. In 1980, a groundbreaking study reported that less than half of children surviving NEC were neurodevelopmentally normal at three-year follow-up.²³⁶ Subsequently, multiple observational studies have cited intellectual delays,²³⁷ moderate-to-severe developmental delay with speech and motor impairment,¹⁸³ developmental delay requiring special educational classes,²³¹ and delays in locomotor, hearing and speech, intellectual performance, and social skills.²³⁴

A large multicenter cohort study from the NICHD Neonatal Research Network evaluated neurodevelopmental outcomes in 1100 extremely LBW survivors.²³⁸ This study confirmed that NEC is associated with increased odds of having a delayed score on psychomotor developmental assessment as well as increased odds of cerebral palsy. This study found that almost all of these abnormalities occurred in patients who required an operation for NEC.²³⁹

Two systematic reviews have confirmed the increased risk of neurodevelopmental impairment in VLBW infants who develop NEC (Fig. 33-7).^240,241 The risk for those treated surgically is nearly twice the risk for those treated medically. Most infants with NEC who are treated medically develop like age-matched premature infants without NEC, whereas those requiring an operation have an increased risk of poor neurodevelopmental outcomes.

FIGURE 33-7 Summary data from systematic reviews examining the relationship between NEC and neurodevelopmental outcome.

Prevention

Prevention of NEC has become a major focus of research because management strategies have had little impact on mortality. Prevention has focused on two main aspects of care: feeding strategies and infectious characteristics of the gastrointestinal tract.

Human Milk

Human milk has a variety of antimicrobial, anti-inflammatory, and immunomodulating properties. Additionally, it furnishes the mucosa with IgA antibodies as well as macrophages, lactoferrin, and EGF. Because of these properties, it has been postulated that breast milk is protective against developing NEC. There are major challenges in evaluating the efficacy of human milk in preventing this disease. One difficulty lies in a lack of standardization of what is considered human milk: maternal preterm milk, donor preterm milk, donor term milk, and unfortified or fortified human milk. Another lies in the fact that many infants receive supplementation with formula or donor milk when there is insufficient maternal milk. Thus, determining how much human milk is fed and what constitutes human milk feeding versus formula feeding can be difficult. In addition, recent practices in the neonatal intensive care unit (NICU) often involve fortifying human milk to increase the caloric and nutritional content.

Four small (n = 36–81 patients) randomized or quasi-randomized studies in the 1980s evaluated the incidence of NEC in infants fed either human milk or formula.242–245 None of these studies showed a statistically significant difference in NEC between the two groups, though all had a lower incidence in infants who received human milk.

Another randomized study was done as part of a larger prospective observational trial in the UK.²⁴⁶ The study included 159 infants who were fed exclusively either donor breast milk or preterm formula. The incidence of NEC was slightly lower in the breast milk group, but the odds ratio did not reach significance. These investigators looked at all 926 patients who were part of the prospective observational study. They divided the patients into three groups based on what they had been fed: formula only, formula plus mother’s milk, and human milk only. The formula group had a 10% incidence of NEC versus 4% in the human milk only group. When only considering confirmed cases of NEC, the formula group had 7% versus 1% in the human milk group.

Two systematic reviews and meta-analyses have subsequently been published which found that human milk reduces the risk of NEC with findings of a relative risk of 0.21 (95% CI: 0.06–0.76) in one and 0.25 (95% CI: 0.06–0.98) in the other.247,248 However, these findings must be evaluated with some caution. The studies included in the meta-analyses had significant variation in the incidence of NEC (0–20%), the type of human milk (donor, maternal, term, or preterm), and the timing of feeding. Furthermore, one of the included studies was not truly randomized but used an alternate assignment allocation and none of the studies was blinded to allocation or outcome. Also, the criteria for NEC was not uniform among the studies.

More recently, a randomized multi-institutional study compared a human milk-based diet with a bovine milk-based diet in preterm infants. Mother’s milk was used when available. Otherwise donor breast milk was used and fortified with human milk fortifier when enteral intake reached either 40 mL/kg/day or 100 mL/kg/day. These infants were compared to infants receiving bovine milk-based formula, with fortification started once intake was 100 mL/kg/day. A total of 207 infants were enrolled. Duration of parenteral nutrition, length of hospital stay, sepsis and growth were not significantly different between groups; however, the incidence of NEC was significantly lower with a human milk-based diet (5.8% vs 15.9%, p = 0.02). The odds ratio for developing NEC on an exclusively human milk-based diet was 0.23 (95% CI 0.8–0.66; p = 0.007) or a relative risk reduction of 77%.²⁴⁹ Other significant findings included a lower combined risk of death or NEC, and a lower risk of NEC requiring operation with human milk.

Fortification of human milk is commonly practiced to improve caloric and nutrient content of the human milk received. Fortification has been shown to result in increases in weight, length, and head circumference. A Cochrane review did not show an increase in NEC in the group fed fortified milk.²⁵⁰ Different proprietary preparations of fortifier exist, and little research has been done to compare outcomes or to determine the optimal composition of fortifiers.

Feeding Strategies

Early initiation of enteral feedings in preterm infants helps to promote growth and decreases the need for parenteral nutrition. There have been concerns that early feeding may be associated with an increased risk of NEC. A systematic review found only two small, randomized studies with a total of 82 infants.²⁵¹ In these studies, early feeding had no effect on the incidence of NEC. Given the small sample size, important effects of either strategy may have been missed. In one large prospective study, early feeding of human milk appeared protective against NEC while early feeding of formula was linked to an increased incidence.²⁴⁶ These investigators found that for each day earlier formula feeds were started, the risk of developing NEC increased 20%. Among those infants who received breast milk, there was no association between the day of life that feeding was initiated and the risk of NEC.

After starting enteral feeding, the rate at which to advance feeds is another controversy. Rapid advancement is advocated by some for infants to quickly regain their birth weight and achieve full enteral nutrition.²⁵² In one randomized trial, the study was terminated early due to a higher incidence of NEC in the group that had their feeds rapidly advanced.²⁵³ Results from this study were confounded by a questionable randomization model, an unusually high incidence of NEC, early termination of the study, and exclusion of four patients who died or developed intestinal perforation. A systematic review of rapid versus slow advancement of feeds examined 372 patients in three separate trials and did not find any difference in the relative risk of developing NEC.²⁵² The ideal rate of feeding advancement remains unclear.

Amino Acid Supplementation

Arginine is the sole substrate for nitric oxide synthase. NO plays an important role in proper functioning of the gastrointestinal tract. Supplementation with arginine has been considered as a potential preventive measure for avoiding NEC. One randomized controlled trial of 152 preterm infants found a significant reduction in NEC in those infants receiving this supplementation.²⁵⁴ This small study was unable to evaluate stage II and III NEC independently; further research on this possible preventive measure needs to be performed to confirm these results.²⁵⁵

A deficiency of serum glutamine has also been correlated with NEC.²⁵⁶ Glutamine is fuel for enterocytes and promotes the growth and integrity of the intestinal epithelium. Glutamine has also been postulated as having protective effects against NEC. Two large multicenter randomized trials have failed to show a benefit for those who received glutamine.^257,258 A Cochrane systematic review of five trials also did not show any benefit to the infants receiving glutamine.²⁵⁹

Oral Antibiotics

Given the role of bacteria in the pathogenesis of NEC, enteral antibiotics have been considered as possible NEC prophylaxis. The use of antibiotics also increases the potential for the development of resistant bacteria. Five randomized trials have examined the effects of prophylactic therapy with enteral antibiotics. No individual study found a significant reduction in NEC; however, when the five were combined together in a meta-analysis, there was a significant reduction in the incidence of NEC in those who received the antibiotics.²⁶⁰ Unfortunately, these studies did not report on the potential harmful effects of this use of enteral antibiotics. Without sufficient evidence regarding the safety of using enteral antibiotics as prophylaxis, an endorsement for this prophylactic measure cannot be made.

Probiotics

Probiotics are live microbial supplements that colonize the gastrointestinal tract. They have been proposed as a means of protecting against NEC. These supplements contain potentially beneficial bacteria or yeasts, most commonly Lactobacillus, Bifidobacterium, and Streptococcus strains.²⁶¹ They can enhance the mucosal barrier by reducing permeability, increasing mucus production, inhibiting bacterial translocation, and strengthening tight junctions.^42,262–267 Colonization with these organisms can reduce the ability of pathogenic bacteria to adhere to the intestinal mucosa.^268,269 Probiotics have also been shown to increase the production of mucosal IgA and short-chain fatty acids that help the immature immune system.^270–272 Additionally, they decrease intestinal inflammation through the reduction of proinflammatory cytokines, the increase of anti-inflammatory cytokines, and the increase of cytokine production by T-cells.^273–276

Studies have examined the ability of probiotics to normalize intestinal flora and to prevent NEC. One randomized controlled trial showed that administration of Bifidobacterium breve within the first 24 hours and continued for 28 days can change the intestinal colonization rates, with increased levels of Lactobacillus and decreased counts of Enterobacter.²⁷⁷ Another study showed that administering Bifidobacterium probiotics to preterm infants lowered the levels of pathogenic species such as Enterobacter and Clostridium in their intestines compared with controls who did not receive the probiotics.²⁷⁸ These studies all suggest that the use of probiotic supplementation can influence intestinal colonization.

Once it was shown that such changes in intestinal flora could be manipulated, the next step lay in determining what clinical effect this might have on these preterm infants. One large prospective cohort study using historical controls evaluated whether newborns given Lactobacillus acidophilus and Bifidobacterium infantis would have reduced rates of NEC.²⁷⁹ They studied 1237 infants over one year with a mean gestational age of 35 weeks and mean birth weight of 2040 g. These infants were treated with each probiotic daily until discharge. The results were compared with historical controls from the preceding year. During the treatment year, the incidence of NEC was 3% compared with 6.6% the year before (p < 0.0002). Furthermore, no side effects were noted.

Subsequently, there have been many studies analyzing the effects of probiotic supplementation in preterm infants. The first meta-analysis in 2007 reviewed seven randomized controlled trials involving a total of 690 infants who received no treatment and 703 who received probiotics (Table 33-4).²⁸⁰ The relative risk for NEC in the group that received probiotics was 0.36 (95% CI: 0.20–0.65). These results must be considered with caution because this meta-analysis combined studies that had many significant differences. This heterogeneity normally would preclude the method of meta-analysis.²⁹¹ Considerable variability among the studies in the demographics of the patients, the age at commencement of treatment, and the type, dose, and duration of probiotic treatment existed. Though this meta-analysis found no increased risk of sepsis in the treatment group, side effects of probiotics were not adequately addressed due to lack of power to detect serious infections. Given the high-risk population in which these probiotics would be used, the issue of safety is crucial.

TABLE 33-4

Clinical Trials Evaluating Probiotics and The Incidence of Necrotizing Enterocolitis ^a

BB, Bifidobacterium breve; BBB, Bifidobacterium bifidus; BBL, Bifidobacterium lactis; BI, Bifidobacterium infantis; BLF, bovine lactoferrin; LBA, Lactobacillus acidophilus; LBC, Lactobacillus casei; LBG, locust bean gum; LGG, Lactobacillus rhamnosus GG; SB, Saccharomyces boulardii; ST, Streptococcus thermophilus.

^a See references for further information.

A 2012 meta-analysis included 20 studies with a total of 3816 patients. For preterm VLBW patients receiving probiotic supplements, the incidence of stage II or higher NEC was 3% compared to 7.4% in the placebo group, with a RR of 0.33 (95% CI, .024–0.46; p < 0.00001).²⁹² Due to differences in the type of probiotics used in the studies, subgroup analysis was performed; the three main probiotic agents (Bifidobacteria, Lactobacillus, and Bifidobacteria and Lactobacillus) all showed a significant risk reduction. Overall mortality was reduced in the group receiving probiotics as well: RR of 0.56 (95% CI 0.43–0.73; p < 0.0001). In regards to safety of probiotics, fourteen of the trials reported data for culture positive sepsis. In VLBW infants receiving probiotics, there was no difference in the risk of culture positive sepsis.

These studies suggest that probiotics are safe and effective in reducing the incidence of NEC and mortality without increasing the risk of sepsis. Despite the many studies available in the literature, there is heterogeneity in type of probiotic, timing of therapy, and dosing. Long-term effects are also not well studied. While probiotics are a very promising preventative strategy, questions still need to be addressed.

Epidermal Growth Factor

As discussed earlier, EGF plays an important role in the pathogenesis of NEC. Premature infants in general, and infants with NEC specifically, have been shown to have decreased salivary concentrations of EGF.⁸⁷ Because EGF is known to support the maintenance of the intestinal barrier and downregulate proinflammatory cytokines,¹²² its use has been postulated to help prevent NEC. A preliminary study of neonates diagnosed with NEC has shown that administration of EGF will promote the repair of the intestinal epithelium. In animal models, supplementation with EGF has decreased the incidence of NEC.²⁹³ HB-EGF has been shown to have similar effects. Trials to test both EGF and HB-EGF as preventive strategies are planned.²⁹⁴