Not All Free Air in the NICU is Necrotizing Enterocolitis 

Paul Holtrop MD1, Paras Khandhar MD1, and Nathan Novotny MD1, 2

1Beaumont Children’s, Royal Oak, MI, USA

2Monroe Carell, Jr. Children’s Hospital at Vanderbilt, Nashville, TN, USA

When a tiny preterm baby in our neonatal intensive care unit (NICU) has an intestinal perforation, it is always necrotizing enterocolitis (NEC), right? Not so fast, my friend. Many of these preterm infants who perforate have free air from spontaneous intestinal perforation (SIP) and not NEC. As it turns out, not all free air in the NICU is NEC.  How can you tell the difference and does it matter?  

It can be challenging to tell the difference, but SIP tends to occur earlier in life, often within the first week, whereas NEC usually occurs slightly later. One analysis of a large data set showed the median age of onset of SIP was seven days after birth versus 15 days for NEC (1). SIP often occurs in babies who have never been fed or have only had trophic feeds, while many babies who develop NEC have had significant amounts of enteral nutrition or are on full feeds. SIP often presents with a relatively sudden onset of abdominal distention, not uncommonly with associated hypotension. NEC patients are more likely to have feeding intolerance or bloody stools noted before progressing to intestinal perforation.  

The physical exam can be different, too.  While infants with SIP and NEC will usually both have abdominal distention, those with SIP tend to have some bluish discoloration on the skin, whereas in NEC, the skin on the distended abdomen can be erythematous, with crepitus and induration in severe cases (2,3). Obviously, there is overlap in the physical exam between the two. But think of it this way: with SIP, the perforation comes first and necrosis, with inflammation, comes later.  With NEC, the inflammation and necrosis come first, followed by perforation.

On x-ray, SIP babies usually have just free air, without many other abdominal findings. NEC babies can have dilated intestinal loops, pneumatosis intestinalis, and sometimes a fixed dilated loop of bowel. If the baby has those, especially pneumatosis intestinalis, it is NEC and not SIP. 

The most definitive way to differentiate is by looking at the bowel in the operating room. SIP usually occurs in the terminal ileum and has normal bowel proximal and distal to the perforation – although it can occur in the jejunum or colon – while NEC will show abnormality of a more extensive section of intestine, with ischemic and necrotic areas.  

While the most definitive way is to look at the intestine, we often do not get the chance to see the bowel in these types of infants. For babies who weigh less than 1,000 grams, the treatment of choice if they have free air in the abdomen, regardless of whether it’s NEC or SIP, has usually been to place a peritoneal drain at the bedside. The classic teaching has been that a third never need another intervention, a third fail and need an operation (either acutely or for a stricture, later) and a third succumb. Some pediatric surgeons have seen that drain as just a temporizing measure and believe most will need a laparotomy later. Regardless, that recommendation might be changing. Forthcoming data suggest that even for babies less than 1000 grams, if the preoperative diagnosis is NEC, the better treatment is a laparotomy. If the diagnosis is SIP, placing a peritoneal drain is the way to go and will frequently be the only intervention needed. Stay tuned for more information about that. This would be a huge shift in our thinking of treatment which makes differentiating between the two, prior to intervening, even more important.

Even if there is some diagnostic uncertainty between SIP and NEC, all of our small patients need to be closely monitored. If the infant has SIP, the drain may be all the surgical treatment they need for now. However, in NEC, a drain might not help, and the baby can deteriorate after the drain as the disease progresses. A laparotomy might then be necessary, if the baby can tolerate it and has a reasonable chance of survival.  

So, does it matter if it’s NEC or SIP? If the above unpublished data is confirmed, it matters because it influences your choice of treatment and helps to counsel parents regarding outcomes.  Precision in diagnosis allows for precision in terminology which will give us an opportunity for precision in treatment. Finally, it makes a difference to your hospital statistics, which will please your hospital quality assurance chief. The disease process is different, which makes the terminology different, which (likely) makes the treatment different. Not all free air in the NICU is NEC. 


  1.  Attridge JT, Clark R, Walker MW, Gordon PV. New insights into spontaneous intestinal perforation using a national data set.  (1) SIP is associated with early indomethacin exposure.  J Perinatol 2006: 26:93. 
  2.  Kim, Jae H.  Neonatal necrotizing enterocolitis: clinical features and diagnosis.  In: Kim, Melanie S, and Abrams, Steven A, eds. UpToDate. Accessed July 10, 2020.
  3. Kim ES and Brand ML.  Spontaneous Intestinal Perforation of the Newborn.  In: Kim, Melanie S, and Garcia-Prats, Joseph A, eds. UpToDate. Accessed July 10, 2020. 

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