Few ultrasound studies focused on neonatal intestine, possibly because it can be interference by intestinal gas and stool mass. In fact, many intestinal diseases can be diagnosed by US even with interference [10–13]. US has become an important diagnostic method in many children’s diseases, such as intussusception , malrotation of intestine [15, 16] and intestinal polyp . For neonates, US has proven to be of great value in necrotizing enterocolitis . For neonates with SBA, the proximal intestine is often dilated filling with amniotic fluid or milk. Whereas, in order to avoid aspiration and relieve pain, clinicians usually carry out gastrointestinal decompression for neonates when intestinal obstruction was suspected, which not only makes more intestinal gas, but also makes the dilation state relieved and disturbing doctors to find the dilated bowel. The distal intestine of SBA was not affected by swallowing and gastrointestinal decompression. As no amniotic fluid and milk passed, the distal intestine (including the colon and a part of small bowel), remained in a state of diminution for a long time. It only has a little meconium contents and without any gas. These characteristics are easily detected on ultrasound and can be used to distinguish with other types of intestinal obstruction. In brief, the present study examined the distal small bowel and colon characteristics to determine the accuracy of the diagnosis of SBA.
The microcolon has been recognized and proposed by surgeons for nearly 100 years . It has now become an important basis for barium enema in the diagnosis of SBA, but it was barely mentioned on the ultrasound. Although Hao  reviewed the ultrasonographic findings of 19 neonates with SBA and confirmed that US can detect microcolon without gas, the study did not provide the criteria of microcolon, the data on false negative and false positive rates. In order to ensure the consistency, we selected a transverse view of the kidney (usually left kidney) showing the short axis of the colon for measurement. This area was chosen because the anatomic locations of the ascending colon and descending colon are more constant and hardly disturbed by intestinal gas at the specified section. Finding colon requires some experience and patience. Gently pressure if necessary to avoid crying influence. Avoid treating dilated small bowel as colon. In this study, there were 5 neonates with unclear or unexplored colon. In addition to high display rate (93.1%, 67/72), the colonic diameter of the study group was significantly smaller than that of the other two groups, indicating that the colonic diameter has a more intuitive value in the diagnosis of SBA. Combined with the cut-off value of ROC curve, we believe that the ultrasonic colon with diameter less than 6.5mm has the maximum predictive value for SBA, which can be called ultrasonic microcolon. SBA can be ruled out when the colon diameter is greater than 8.5mm on ultrasound.
We assumed that distal micro small bowel might be a secondary change of the SBA due to prolonged complete obstruction. But it is difficult to assess and repeatable measurement, as they perform small, peristalsis slower, no fixed location and often obscured by dilated intestines. For atresia of the ileocecal junction or near the ileocecal junction, the micro small bowel will also be absent or difficult to search . We made a periumbilical scan to find the section with the most small intestine and the least intestinal gas for analysis. The diameter less than 6 mm and without wriggle defined as the micro small bowel. Ideally, all neonates with SBA not only had micro small bowel, but also were gas negative. However, micro small bowel was also present in study group without SBA and control group. It possibly because the neonates has not accumulated food in the intestinal tract soon after birth. Especially some neonates with heavy intestinal stenosis in the study group without SBA, who have a high proportion of micro small bowel.
Gas negative in distal intestine is an important diagnostic basis for SBA. Except for very few individuals has a biliary fistula . In the study group without SBA, neonates without colonic gas were mostly patients with severe intestinal obstruction. In the control group, there was only 1 case of 1-day-old newborn with colon of gas negative. Possibly because the neonate did not eat much after birth, and the gas in the intestinal lumen had not moved to the colon. Since the amount of gas can be changed over time, to find a micro small bowel without gas need more experience which can not be an independent diagnosis for SBA.
Of course, the current findings do not justify the use of colon ultrasound as a substitute for barium enema in the diagnosis of SBA. But our study could provide clinicians and neonates with more options. For neonates with suspected SBA, US can be done firstly, and if SBA can be diagnosed, maybe barium enema should be canceled. The difference in diagnostic accuracy between US with barium enema is what we will study next.
Our study had several limitations. First, although our study spanned two years, we did not have enough cases, including the control group of normal newborns, because they rarely received routine abdominal ultrasound. More clinical data need to be accumulated for comparison in the differentiation of colonic diameter between SBA and other diseases. Second, the amount of micro small bowel is difficult to accurately determine, and the amount of gas in it can be changed over time. The results may vary depending on the doctor's experience. Third, Given the benefits of timely gastrointestinal decompression, we could not require eliminate gastrointestinal decompression in suspicious SBA neonates before US. Therefore, the dilation of proximal bowel was not analyzed in this study. However, barium enema or upper gastrointestinal contrast cannot be performed before ultrasound examination is still required.