Meconium is the material found in the intestine in a newborn. It consists of succus entericus that is made up of bile salts, bile acids, and debris that is shed from the intestinal mucosa during intrauterine life. It is normally evacuated within 6 hours after birth or sooner in utero as a result of a vagal response to perinatal stress.
Normally, meconium is invisible on a radiograph. It occasionally has a mottled appearance on abdominal radiographs during the first 2 days of life. By convention, 4 GI conditions have the term meconium in their name: meconium ileus, meconium ileus–equivalent syndrome, meconium peritonitis, and meconium plug syndrome.
In meconium ileus, low or distal intestinal obstruction results from the impaction of thick, tenacious meconium in the distal small bowel. In addition, complications such as ileal atresia or stenosis, ileal perforation, meconium peritonitis, and volvulus with or without pseudocyst formation can occur in association with meconium ileus. [1, 2, 3, 4, 5, 6]
Meconium peritonitis may be incidentally detected on abdominal radiographs. Clinically, patients may present because of bowel obstruction caused by fibroadhesive bands, which are the result of the inflammatory peritoneal reaction. The bowel itself may be intact, with the perforation having healed, but bowel atresias are often found in association. If the processus vaginalis is patent at the time the perforation occurs, calcification or hernias may involve the scrotum. Ascites may also be present.
Meconium ileus–equivalent syndrome
The condition is important, as it can be the presenting feature of cystic fibrosis in childhood and even in early adult life. Moreover, the operative mortality and morbidity rates are high. Recurrent bowel obstruction (which is often correlated with poor compliance with medication for cystic fibrosis) may manifest as recurrent colicky abdominal pain, often in the right upper quadrant. In the older infant or child, chronic constipation can be a problem, and intestinal obstruction can occur secondary to fecal impaction. These patients may also present with intussusceptions.
Meconium plug syndrome
Functional colonic obstruction in the full-term neonate is another name for meconium plug syndrome. Although this abnormality is found mostly in term infants, Mees et al reported that 3 of their 4 patients were premature. Most infants with this form of colonic obstruction present within their first 24-36 hours of life. Findings include abdominal distention, bilious vomiting, and failure to initiate the normal passage of meconium. 
Radiography is the preferred examination for evaluating cases of meconium ileus, meconium peritonitis, meconium ileus–equivalent syndrome, and meconium plug syndrome. 
Radiography is the preferred examination for evaluating cases of meconium ileus, meconium peritonitis, meconium ileus–equivalent syndrome, and meconium plug syndrome.  See the images below.
Contrast enema examination can help in differentiating meconium plug syndrome from meconium ileus or ileal atresia. In these latter conditions, microcolon is seen, and only occasionally does an infant with meconium ileus have a picture similar to that seen in meconium plug syndrome.  See the images below.
Plain abdominal radiographs frequently demonstrate a low small-bowel obstruction with numerous air-filled loops of bowel. Although plain imaging findings may suggest the diagnosis, with or without a family history of cystic fibrosis, contrast enema examination of the colon is required to establish a diagnosis of meconium ileus. [10, 11, 12]
Water-soluble contrast material is typically used and a variety of water-soluble contrast agents have been used. In the past, Gastrografin has been used, but this must be diluted to at least 3:1 (water to Gastrografin, as recommended by the manufacturer). However, the use of Gastrografin is now controversial, as deaths from fulminant colitis and dehydration have been reported. 
The Cystic Fibrosis Foundation Consensus Conference on GI disorders concluded that there is no scientific evidence that hyperosmolar Gastrografin enema is any better than an iso-osmolar or hypo-osmolar enema. However, opinion remains divided on this topic, and Gastrografin is used safely by very experienced pediatric radiologists when diluted appropriately.
Other nonionic contrast agents (eg, Hypaque, Omnipaque) are preferred by many radiologists and have less risk of dehydration or colitis. Because of potential fluid shifts, hypovolemia is a risk and therefore adequate hydration and electrolyte balance are essential before, during, and after the procedure. This monitoring should prevent significant fluid shifts. If perforation of bowel and leak of contrast or Gastrografin into the peritoneum occurs, more rapid fluid shifts may result and close monitoring by a neonatologist is recommended.
Acetylcysteine (a mucolytic) can be mixed with the enema to aid passage of the very sticky meconium, but the efficacy of this is not yet proven.
Because of the tenacious meconium and the abnormality of mucous-gland secretion, air-fluid levels are often absent. However, images in some infants may demonstrate air-fluid levels, especially in those with complications such as volvulus and stenosis or atresia.
Although the absence of air-fluid levels strongly suggests meconium ileus, air-fluid levels do not exclude it. In some cases of meconium ileus, distention of the obstructed small bowel can be enormous, and the findings can be easily misinterpreted as distal colonic distention. In other cases, a so-called soap-bubble effect of gas mixed with meconium, may be seen.
Although the soap-bubble effect is not pathognomonic of the condition, it is commonly seen with meconium ileus. However, it can also be seen with ileal atresia, colonic atresia, Hirschsprung disease, and meconium plug syndrome.
A contrast enema study typically demonstrates microcolon, which is a reflection of underused bowel. A microcolon can be due to other etiologies besides cystic fibrosis, such as ileal atresia or any other complete intrauterine obstruction of the distal small bowel. However, when contrast agent refluxes into the small bowel, meconium pellets distending the distal ileum are usually identified, and the diagnosis of meconium ileus is established.
Meconium peritonitis may be incidentally detected on abdominal radiographs. The extruded meconium may or may not become calcified, and when no calcification is present, the radiograph may only suggest fluid in the abdomen. Contrast enema examination can help in differentiating meconium plug syndrome from meconium ileus or ileal atresia.
In some cases, fluid and meconium can pass into the chest, presumably through congenital communications, resulting in meconium thorax. When calcification is seen, the diagnosis can usually be established. This calcification can be amorphous and irregular or curvilinear, with the latter suggesting cystic loculation or coating of the peritoneum. The term cystic or pseudocystic meconium peritonitis has been applied to this finding.
It is uncommon to encounter an older child with residual calcifications.
Only occasionally does an infant with meconium ileus have a picture similar to that seen in meconium plug syndrome.
Meconium ileus–equivalent syndrome
Plain abdominal images show a mottled appearance (which may simulate meconium) in dilated loops of small bowel. Other plain imaging findings include a bubbly, granular appearance predominantly in the right side of the abdomen, with dilated small bowel and fluid levels due to small-bowel obstruction.
Water-soluble contrast enemas are often helpful in excluding the other possible causes of symptoms and can be used to treat this condition. If the diagnosis can be made radiologically, conservative medical treatment should be pursued vigorously, and surgery is undertaken only if it is unavoidable. 
Meconium plug syndrome
Meconium plug and small left-colon syndromes are diagnosed radiologically, and contrast enema examination is required. Plain images are nonspecific and usually show findings of a low small-bowel obstruction. Erect abdominal radiographs, albeit seldom necessary in the newborn, show a paucity of air-fluid levels, and in most cases, no gas is seen in the colon.
Water-soluble contrast enemas can be curative with passage of the meconium.
If air is introduced into the rectum from below (eg, on rectal examination), rectal gas may be visualized. In this setting, meconium in the rectum can erroneously suggest a small presacral mass. However, if air entirely surrounds the meconium mass, it will outline the contour of the mass and suggests the correct diagnosis.
If peristalsis forces gas in from the small bowel into the colon and mixes with the meconium, a granular or bubbly appearance mimicking the findings of pneumatosis cystoides intestinalis and necrotizing enterocolitis (NEC) results. If a bubbly pattern is seen within the first 12 hours of life, meconium plug syndrome should be considered, but if it arises after 12-18 hours, NEC is more likely. NEC typically occurs in preterm infants.
Contrast enema examination shows a characteristic appearance of the colon, with contrast material outlining the solid column of inspissated meconium and the wall of the colon for a double-contrast effect.
After the meconium is passed, the part of the colon from which it was evacuated may appear narrowed, and a transition zone mimicking that seen in Hirschsprung disease may be noted.
In any of these cases, meconium proximal to the apparent transition zone may take the form of a solid, cylindrical mass extending to the cecum or lumps of solid meconium may be scattered throughout the colon.
In patients with intermediate findings, individual assessment is most important.
The small, contracted portion of the descending colon in these infants has led to the term neonatal small left-colon syndrome.
Differentiating Hirschsprung disease from meconium plug or small left-colon syndrome can be a problem, and one must completely exclude Hirschsprung disease before either of these diagnoses is confirmed. 
The initial clinical and radiologic findings may be similar, but infants with Hirschsprung disease invariably return with either constipation or diarrhea. Once treated, those with meconium plug or small left-colon syndrome remain healthy. Plain images in infants with Hirschsprung disease are more likely to show numerous air-fluid levels on the erect view. During an enema, abnormal contractions with bowel wall irregularity may be seen in the narrowed, aganglionic bowel of Hirschsprung disease, but the bowel wall typically has a smooth contour in the meconium plug setting.
Radiologic signs of meconium ileus include enlarged bowel loops at 17-18 weeks' gestational age or a mass with proximal bowel distention, which is suggestive of cystic meconium peritonitis on prenatal sonography. [5, 6]
Meconium ileus–equivalent syndrome
Rare cases of intussusception as a complication of cystic fibrosis may be easily diagnosed with ultrasonography.
Sonography is seldom necessary, as the findings on plain radiographs are usually diagnostic.
Calcified meconium can be seen during in utero sonography. Ultrasonography may be useful after birth, when cystic masses are present and often appear circumscribed and heterogeneous.
Fluid (sonolucent areas) may be seen in the cyst, but usually, increased echogenicity resulting from debris and calcifications is seen. The wall of the cyst may be thin or thick, and one may note loops of fluid-filled bowel bound to the matrix of the associated adhesions.
With free-floating meconium in the abdomen, multiple speckled echoes are seen. These result in the snowstorm configuration.
Calcifications in the scrotum produce echogenic masses.