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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- NEC is most common in premature and very low birth weight infants after feeds are established
- Classic findings: feeding intolerance, abdominal distension, bilious emesis, bloody stools, lethargy, and temperature instability
- Abdominal X-ray: pneumatosis intestinalis is the classic diagnostic finding; portal venous gas and pneumoperitoneum indicate severe disease
- Management: stop feeds, gastric decompression, IV fluids, broad-spectrum antibiotics, serial exams/imaging, and surgery for perforation or necrosis
- Human milk feeding and standardized feeding protocols reduce NEC risk
Overview
Necrotizing enterocolitis is a neonatal gastrointestinal emergency characterized by intestinal inflammation, bacterial invasion, ischemia, and necrosis. It is strongly associated with prematurity, immature intestinal barrier function, dysbiosis, and enteral feeding. Disease ranges from mild feeding intolerance to fulminant perforation and septic shock. Step 2 CK commonly tests the radiographic finding of pneumatosis intestinalis and the immediate management of bowel rest plus broad-spectrum antibiotics.
Epidemiology
NEC primarily affects preterm infants, particularly those with very low birth weight. It usually occurs in the second to third week of life after feeds have started. Risk factors include prematurity, formula feeding, intestinal hypoperfusion, congenital heart disease, sepsis, and dysbiosis. Mortality is substantial in surgical NEC and extremely low birth weight infants.
Clinical Features
Symptoms
Feeding intolerance, increased gastric residuals, or poor feeding
Abdominal distension, tenderness, or discoloration
Bilious emesis or vomiting
Bloody stools or guaiac-positive stool
Lethargy, apnea, temperature instability, or shock
Signs
Distended abdomen with decreased bowel sounds
Abdominal wall erythema, edema, discoloration, or palpable mass
Hypotension, poor perfusion, metabolic acidosis, or respiratory deterioration
Thrombocytopenia or disseminated intravascular coagulation in severe disease
Pneumoperitoneum indicates perforation and surgical emergency
Investigations
First-line
Abdominal radiographsSupine and left lateral decubitus/cross-table views; pneumatosis intestinalis is classic, portal venous gas suggests severe disease, free air indicates perforation
CBC and plateletsLeukopenia/leukocytosis and thrombocytopenia suggest severe inflammation or sepsis
Blood culture and sepsis labsNEC overlaps with neonatal sepsis; culture before antibiotics if possible
Blood gas, lactate, electrolytesMetabolic acidosis, hyponatremia, and rising lactate suggest severe disease or necrosis
Second-line
Abdominal ultrasoundCan detect portal venous gas, bowel wall perfusion, complex ascites, and focal fluid collections when X-ray is equivocal
Serial abdominal exams and imagingTrack progression, perforation, and response to therapy
Coagulation studiesEvaluate DIC in ill infants or those needing surgery
Specialist
Pediatric surgeryConsult early for suspected NEC; urgent surgery for perforation, necrotic bowel, or clinical deterioration despite maximal medical therapy
Neonatology/NICUAll suspected NEC requires NICU-level monitoring and multidisciplinary care
1
Immediate medical management
- Stop enteral feeds immediately
- NPO with orogastric/nasogastric decompression
- IV fluids, electrolyte correction, parenteral nutrition, and hemodynamic support
- Broad-spectrum IV antibiotics covering gram-positive, gram-negative, and anaerobic organisms
2
Monitoring
- Serial abdominal examinations, girth measurements, vital signs, urine output, and perfusion
- Repeat abdominal radiographs every 6-12 hours initially if unstable or progressive
- Trend CBC, platelets, blood gas, lactate, electrolytes, and inflammatory markers
3
Surgical indications
- Pneumoperitoneum/free intraperitoneal air is an absolute indication for surgery
- Clinical deterioration, fixed abdominal mass, portal venous gas with instability, worsening acidosis, or refractory thrombocytopenia suggests necrosis
- Options include peritoneal drain in selected extremely low birth weight infants or laparotomy with resection/ostomy
4
Prevention
- Prefer mother’s own milk or donor human milk when available
- Use standardized feeding advancement protocols in preterm infants
- Avoid unnecessary antibiotics and acid suppression when possible
Complications
- Intestinal perforation: Free air and peritonitis require urgent surgical management
- Septic shock: Bacterial translocation and necrosis can cause rapid cardiovascular collapse
- Short bowel syndrome: Extensive resection can cause long-term malabsorption and parenteral nutrition dependence
- Strictures: Post-NEC intestinal strictures can present weeks later with obstruction
- Neurodevelopmental impairment: Severe NEC and prematurity are associated with adverse long-term outcomes
USMLE Step 2 CK Exam Tips
- 1Premature infant + feeds recently started + abdominal distension + bloody stool = NEC
- 2Pneumatosis intestinalis is the classic X-ray finding
- 3Free air under the diaphragm or pneumoperitoneum = perforation = surgery
- 4First step is stop feeds, NPO, NG/OG decompression, IV fluids, and broad-spectrum antibiotics
- 5NEC is not treated by changing formula and observing at home
- 6Human milk is protective; formula feeding is a risk factor
- 7Portal venous gas is ominous but pneumoperitoneum is the cleanest surgical trigger on exams
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