Abdominal distention is common in neonates. The challenge is differentiating pathological causes of abdominal distension from benign ones. But first, we need to consider what, exactly, defines a distended abdomen.
What is a distended abdomen?
There is no universal definition. It can be described simply as an increase in abdominal girth. Some clinicians use a more precise description, defining a distended abdomen as when the abdomen is protruding above the xiphisternum in an infant lying flat on its back.
In most clinical settings, a pragmatic description is a measurable difference in abdominal girth compared to a baseline.
Regardless of definition, the most important question to consider whilst considering your differentials is whether abdominal distension is an isolated finding or whether there are associated signs or symptoms.
Are there any associated gastrointestinal symptoms?
The most serious associated symptom is bilious emesis. Bile-stained vomit suggests bowel obstruction and requires urgent surgical review. GI symptoms must be correlated to examination findings. A distended and angry-looking abdomen which is taut with prominent abdominal veins, skin discolouration and absent bowel sounds requires acute intervention. A distended but soft abdomen with active bowel sounds is considerably more reassuring. Symptoms of milky vomit and increased gastric aspirates lean towards feeding intolerance and dysmotility rather than surgical pathology.
How is their general exam?
Neonates tend to deteriorate very quickly, particularly premature neonates. Tachycardia, hypotension, tachypnea, apnea, temperature instability and irritability or lethargy suggest an infective cause. These babies need a septic work-up and broad-spectrum antibiotics.
Are they feeding well?
Neonates, especially premature infants, have immature intestines. Any change in formula or addition of fortification can affect intestinal motility and feeding tolerance. The method of feeding and volume of milk can also affect tolerance. Neonates receiving orogastric or nasogastric tube feeds may receive their milk faster than their bodies can digest it, and their abdomen may become distended.
Have they been passing stool?
The stool is a good proxy for intestinal motility. Delay in the passage of meconium, or a neonate who previously passed stool but hasn’t for a few days, suggests an obstructive aetiology. The quality and the content of stool should also be assessed – is there blood or mucus present; is the stool hard or watery?
Are they receiving any ventilatory support?
Continuous positive airway pressure (CPAP) or other non-invasive ventilation can cause excessive air to be pushed into the stomach and intestines causing gross abdominal distension and occasionally milky vomits.
Case 1: Neonatal Enterocolitis
A 29-weeker, currently on day 28 of life, on CPAP ventilation has increasing amounts of bilious aspirates from the NG tube and a distended abdomen. He has been hypothermic with more frequent apnoea, bradycardia and desaturations. His abdomen is taut with distended veins and discolouration.
Necrotising enterocolitis (NEC) occurs when sections of bowel tissue undergo inflammation leading to bacterial invasion and necrosis. This may lead to bowel perforation and subsequently peritonitis, sepsis and death. 90% of babies are preterm, and most are very low birth weight infants with growth restriction. NEC has a 20-40% mortality rate and of those who survive, 25% develop long-term sequelae.
The exact mechanism is not well understood. Gastrointestinal immaturity in premature infants, together with the poor gastrointestinal blood supply in infants with growth restriction, are probably causative. Other risk factors include enteral feeding, formula feeding, bowel ischaemia, polycythaemia and recent surgery.
Infants with NEC are usually systemically unwell with poor perfusion, temperature instability, apnoea and/or bradycardia, hypotension and lethargy. Other features include bilious aspirates or vomits and sometimes bloody stools. Increasing abdominal distension, with distended veins, discolouration or erythema are classics, often with reduced bowel sounds or a palpable abdominal mass.
Once NEC is suspected, perform baseline investigations promptly. These may demonstrate thrombocytopenia, neutropenia, elevated CRP and metabolic acidosis. An abdominal x-ray including a lateral decubitus view should be done to look for dilated loops of the bowel, pneumatosis intestinalis, portal venous air and pneumoperitoneum.
These children can deteriorate rapidly. Keep the infant warm by nursing in an incubator, supine with their head slightly elevated. They need continuous cardiac and saturation monitoring as well as temperature control. A nasogastric tube should be inserted for decompression, aspirated at regular intervals and left on free drainage. Monitor the type and amount of aspirate. Keep these infants nil by mouth. They should be given IV fluids equal to the maintenance requirements plus replacement volumes of NG aspirates.
Once the infant has been stabilised, they should be referred for definitive management. Before referring for a paediatric surgical opinion, determine the severity of the disease with a scale such as the Modified Bell’s Staging Criteria.
Start triple antibiotics, such as vancomycin, gentamicin and metronidazole, as well gut rest.
Approximately 1 in 4 infants with NEC require surgery. Surgical complications include short gut syndrome, strictures and abscess formation.
Case 2: Bowel Obstruction
A 32-weeker, currently on day 5 of life, is noted to have increasing feeding intolerance with new onset bilious vomiting. He was born in spontaneous labour due to maternal cervical insufficiency, with good Apgars. He was commenced on enteral feeds from Day 1 of life. He has not yet passed meconium.
Bowel obstruction is a common newborn surgical emergency requiring early diagnosis and prompt treatment. Any delay may result in the loss of large amounts of bowel. Signs of obstruction include vomiting, with or without bile-stained material, increased gastric residuals, failure to pass meconium in the first 24 hours, abdominal distension and absent or decreased bowel sounds. Common causes of neonatal bowel obstruction include:
Occurs in 1:5,000-10,000 live births, associated with Down Syndrome in 25% of cases and is more common in males. It is caused by a membranous or interrupted-type lesion at the level of the papilla (Ampulla) of Vater. In 80% of cases, the papilla of Vater opens into the proximal duodenum causing bilious vomits. Polyhydramnios is a common antenatal finding. Abdominal x-ray usually shows a characteristic ‘double-bubble’ appearance.
Midgut malrotation and volvulus
Occurs in 1:6,000 live births, often associated with other conditions such as duodenal atresia or congenital diaphragmatic hernia. It is a result of abnormal gut rotation as it returns to the abdominal cavity during embryogenesis. Usually presents initially with bilious vomiting during the first week of life with abdominal distension being minimal until a much later stage. Bowels can become strangulated at any age, and infants quickly deteriorate once midgut ischaemia occurs. An abdominal x-ray may be normal but an abdominal ultrasound demonstrating malposition of the superior mesenteric vessels is diagnostic. An upper gastrointestinal contrast study is the investigation of choice. Symptomatic infants require immediate surgery.
Usually presents with abdominal distention with bilious vomiting within the first 24 hours after birth. The more proximal the lesion, the earlier the bile-stained vomiting. Abdominal x-ray shows air-fluid levels proximal to the lesion with possible calcification due to meconium peritonitis.
Occurs in 15% of newborns with cystic fibrosis; at least 90% of newborns with meconium ileus have cystic fibrosis. It is caused by thick tenacious meconium in the bowel, which leads to obstruction. 50% of infants may have associated volvulus, jejuno-ileal atresia, bowel perforation and/or meconium peritonitis. Meconium ileus usually presents with early abdominal distension and bilious vomiting. Mucus plugs may be evacuated on rectal examination. The abdominal x-ray usually shows distended loops with thickened bowel walls with an occasional ground-glass sign – meconium mixed with swallowed air. Calcification, free air or air-fluid levels suggests perforation. Uncomplicated cases may be successfully treated with hypertonic enemas but immediate surgery is indicated for complicated cases or when conservative treatment fails.
Meconium plug syndrome
Occurs in 1:500-1,000 live births. It is the most common neonatal cause of functional distal intestinal obstruction. It is caused by inspissated meconium within the distal colon or rectum. Meconium plug syndrome generally presents with marked abdominal distension and failure to pass meconium. Abdominal x-rays shows intestinal dilatation. A contrast enema is diagnostic, showing outlines of the meconium plug. The enema is often curative, with the plugs being evacuated.
Occurs in 1:4,000 live births, caused by abnormal innervation of the colon. It can affect the anal sphincter specifically or extend throughout the colon into the small bowel. 80% of cases present in the first 6 weeks of life with a male predominance (4:1). A classic presentation is a failure to pass meconium in the first 24 hours with gradual onset of abdominal distension and vomiting. Definitive diagnosis is via rectal biopsy with acetylcholinesterase staining showing the absence of ganglion cells. The most serious complication is enterocolitis.
Like infants with NEC, these children can deteriorate very rapidly. Keep the infant warm by nursing in an isolette in the supine position with the head slightly elevated. A nasogastric tube should decompress the abdomen
Case 3: Dysmotility of Prematurity
A ventilated 26-weeker, currently on Day 14 of life, is noted to have increasing abdominal distension and milky vomits. There has been no change to ventilatory settings and investigations were otherwise unremarkable. He has been passing pellet-like stool every 2-3 days. On examination, he is active on handling, and well-perfused but has a grossly distended abdomen.
Gastrointestinal dysmotility is common in preterm infants. It usually manifests as increased gastric residue before a feed, abdominal distension (with or without vomiting), or constipation. Preterm infants have less efficient gastric emptying and slower intestinal transit than term infants. Gastric emptying is delayed due to immaturity of duodenal motor function and incoordination between the antrum and duodenum. Total gut transit time varies between 1 to 5 days in preterm infants compared to 4 to 12 hours in adults.
Infants with dysmotility of prematurity usually present with isolated feeding intolerance and otherwise appear well. Inflammatory markers and the abdominal x-ray are normal. As dysmotility is physiological, these infants usually grow out of it as they mature.
Modifying feeding regimes can help, initially with an increased frequency of smaller feeds, hourly or 2-hourly. If there is ongoing intolerance, some experts consider continuous pump feeding.
Prokinetic medications, such as metoclopramide and erythromycin, are sometimes used to enhance the coordination and propulsive movement of the gastrointestinal tract. However, the evidence around these agents is conflicting and potentially serious adverse effects have been reported. Prokinetic agents in preterm infants should be limited to protracted feed intolerance and under cautious monitoring.
Case 4: CPAP Belly
An ex-28-weeker, currently on day 21 of life, on nasal CPAP, is noted to have increasing abdominal distension. She is otherwise tolerating her feeds and passing soft stools regularly. She has large amounts of air aspirates from her NG tube. She is well-perfused, and active on handling, with mildly increased work of breathing. Her abdomen is grossly distended but with normal overlying skin and active bowel sounds.
The non-invasive ventilation movement has led to prolonged periods of non-invasive positive pressure exposure for preterm infants. There are two internal pathways for gas to flow: the trachea and the oesophagus. Gas generally finds its way down the path of least resistance. With lung disease, lung compliance decreases and air easily travels down the oesophagus. Enteral feeding tubes also provide a wedge to the closed oesophagus. Gas slowly fills the GI tract, and thus the term CPAP belly was coined.
The most common feature of a CPAP belly is a relatively well infant on positive pressure ventilation, who is mostly tolerating feeds, and passing stools but with a distended abdomen. A large amount of air might be aspirated from the NG tube. Septic workups are benign and abdominal x-rays show a fairly even distribution of dilated bowels throughout the gastrointestinal tract.
Although this condition is benign, CPAP belly does have clinical implications. The distended bowel can lead to disruption of the gut mucosal barrier, allowing for bacterial translocation into the bloodstream and sepsis. Air in the intestine can be dangerous if there is dysmotility or partial obstruction from a meconium plug. Therefore, it is important to try to keep the gas out of the gastrointestinal tract. Manoeuvres include optimising PEEP and lung compliance to create a path of least resistance into the lung, feeding the gut early to allow an alternate route for gas excretion, vigilance in gastric suctioning and timely radiographs to evaluate marked distension.
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