A new born girl not passed meconium for 48 hrs, has abdominal distention and vomiting. Initial investigation of choice would be:
More than 90% of full term newborn infants pass meconium within the first 24 hrs. The possibility of intestinal obstruction is considered if an infant does not pass meconium for 24-36 hrs.
Differential diagnosis would include causes of low intestinal obstruction.

- Hirschsprung disease
- Beal or colonic atresia/stenosis
- Meconium ileus
- Meconium plug syndrome
- Neonatal small left colon syndrome

The first investigation is always a plain abdominal radiograph (supine and left lateral decubitus view)
Next investigation is a contrast enema also k/a lower GI series (presence of any perforation can be ruled out from plain X-rays)
Barium can be used but water-soluble contrast agents are used commonly
Contrast enema helps to some extent in differentiating among the causes of low intestinal obstructions.

Hirschsprung's disease-

Barium enema usually shows a transition zone between dilated proximal colon and a smaller-caliber obstructed distal colon caused by the nonrelaxation of the agaglionic bowel. However, a normal study does not exclude Hirschsprung's disease. Diagnosis is confirmed by rectal biopsy.

Meconium ilens-

Meconium ileus occurs when meconium becomes inspissated and obstructs the distal ileum.
The condition is usually a manifestation of cystic fibrosis. Meconium ileus is usually taken to be synonymous with cystic fibrosis until proven otherwise, and meconium ileus is generally the earliest manifestation of the disease.
Contrast enema study typically demonstrates microcolon, which is a reflection of underused bowel.

Meconium plug syndrome

Meconium plug syndrome, also termed functional immaturity of the colon, is a transient disorder of the newborn colon characterized by delayed passage (>24-48 h) of meconium and intestinal dilatation . Currently, meconium plug syndrome is understood as a transient functional disorder of the colon resulting from immaturity of the myenteric plexus nerve cells or their hormonal receptors.


Contrast enema demonstrates the retained meconium as a filling defect or plug that produces a double-contrast effect.

Usually, the colon is normal or may be mildly enlarged and filled with meconium. A change in the colon's diameter at the splenic flexure may be seen and is indistinguishable from that observed in Hirschsprung disease, although in the latter disorder the transition zone usually is in the rectosigmoid.
Meconium plug syndrome is a diagnosis of exclusion. Contrast enema usually eliminates congenital small bowel obstruction and rare colon abnormalities such as atresia or duplication. The primary differential consideration is Hirschsprung disease, which is diagnosed eventually in approximately 10-30% of patients with apparent meconium plug syndrome

Neonatal small left colon syndrome —

Colonic obstruction with a characteristic caliber reduction in the sigmoid and descending colon unrelated to meconium inspissation or aganglionosis. This condition has a frequent association with maternal gestational diabetes mellitus and seldom requires surgical intervention.
A contrast enema demonstrated a narrowed descending and sigmoid colon, with a caliber transition at the splenic flexure.
Manometry
Genotyping for cystic fibrosis
Lower GI contrast study
Serum trypsin immunoblot