Meconium ileus

Pathogenesis and development of obstruction

Normally, meconium passes easily through the fetal intestine. However, in this condition, it becomes excessively sticky and viscid. In the distal ileum, this meconium desiccates, becomes compacted, and forms an impassable plug. This leads to the development of high-grade obstruction.

Proximal to the obstruction, the small bowel loops are markedly dilated. They are filled with green-black masses resembling the consistency of thick toothpaste. Because the meconium cannot pass further and does not reach the large intestine, the colon remains unused. Consequently, a microcolon (microcolon) develops—a functionally underdeveloped large bowel of extremely narrow caliber. Clinically, the disease manifests with symptoms of low intestinal obstruction.

Diagnostic imaging: from radiography to enema

To establish an accurate diagnosis, a comprehensive imaging approach is used to assess the level of obstruction and the condition of the distal segments:

• Plain radiography (non-contrast): Dilatation of small bowel loops of varying diameters is visualized. A specific sign is the "soap bubble" appearance (soap bubble appearance), classically localized in the right iliac fossa. It occurs due to the mixture of accumulated intestinal gas with thick meconium.
• Diagnostic contrast enema: Allows visualization of a nearly empty microcolon. The main objective is to advance the contrast agent retrogradely into the terminal ileum. Multiple filling defects are found there, which represent meconium pellets or plugs. With successful reflux of contrast beyond the obstruction zone, dilated ileal loops become visible.

Management and water-soluble contrast

Contrast enema serves both diagnostic and therapeutic purposes. In uncomplicated cases, it allows non-surgical resolution of the obstruction.

A water-soluble contrast agent is introduced into the intestinal lumen. Due to high intraluminal osmotic pressure, fluid is drawn into the intestine, increasing the volume of liquid contents, liquefying the obstructing meconium, and facilitating its passage. Diluted Gastrografin (Gastrografin) mixed with water in a 1:2 or 1:4 ratio is commonly used.

Important note: In small or low-birth-weight newborns, the use of Gastrografin can provoke a significant and dangerous fluid shift in the body. In such cases, it is safer to use other iodine-containing contrast agents. If the infant is in a stable condition, the contrast enema procedure can be repeated as needed until the obstruction is completely resolved.

Complicated course

Approximately 50% of patients present with complicated meconium ileus. The course of the disease may be exacerbated by intrauterine volvulus, intestinal atresias, ischemic wall necrosis, and perforation. Immediate surgical intervention is required.

If the bowel wall perforates prenatally, sterile meconium enters the abdominal cavity, causing an intense inflammatory reaction — chemical meconium peritonitis. Subsequently, this process culminates in fibrosis and calcification. Another complication is a meconium pseudocyst, which forms when intestinal loops rendered ischemic by volvulus adhere to each other and necrose, while a dense fibrous wall forms around them.

Radiological markers of complications:

• Presence of intra-abdominal calcifications (may also be identified in the scrotum).
• Calcification of the bowel wall itself.
• Pronounced air-fluid levels (air-fluid levels).
• Visualization of soft tissue density masses (pseudocysts).

Differential diagnosis with atresias

A similar clinical picture can be caused by atresias resulting from in utero vascular accidents:

1. Colonic atresia: Extremely rare (believed to be triggered by compression of the bowel wall by the closing umbilical ring). Symptoms appear after a few feedings (vomiting, abdominal distention, absence of meconium). Radiography reveals a massively dilated bowel loop just proximal to the atresia. Contrast enema demonstrates a distal microcolon and complete cessation of retrograde contrast flow at the site of atresia.
2. Distal ileal atresia: A consequence of a late-gestation vascular event (part of thejejunoileal atresia spectrum). Plain radiography shows a low obstruction. During the enema, contrast cannot reflux into the small bowel, and a microcolon is identified in the large bowel. Nuance: if the vascular insult occurred quite late, a small amount of meconium may be found in the colon.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Pathogenesis of meconium pseudocyst formation during intrauterine volvulus.
• The spectrum of jejunoileal atresia and the impact of the timing of the vascular event on the volume of meconium in the colon.
• Colonic atresia: anatomical features of the umbilical ring as a risk factor.
• Differential radiological diagnosis of low and high intestinal obstruction in newborns.

Other Topics in "Pediatric Diagnostic Radiology"

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