Meconium peritonitis

Meconium peritonitis refers to a sterile chemical peritonitis due to intra-uterine bowel perforation and spillage of fetal meconium into the fetal peritoneal cavity. It is a common cause of peritoneal calcification. 

Epidemiology

The estimated prevalence is at ~ 1 in 35,000.

Pathology

The aetiology is thought to be the result of a sterile chemical reaction resulting from bowel perforation in utero. The bowel perforates as a result of bowel obstruction, such as an atresia or meconium ileus. A secondary inflammatory response results in the production of fluid (ascites), fibrosis, calcification and sometimes cyst formation. Usually the perforation seals off and the bowel is intact at birth. Intra-peritoneal meconium usually calcifies, sometimes within 24 hours. 

Classification

At least four types are recognised:

• fibro-adhesive 
• cystic 
• generalised
• healed

Associations

• cystic fibrosis : doesn't usually tend to calcify in these cases due to lack of enzymes
• intestinal atresiae, e.g.
  • anal atresia
  • ileal atresia
  • jejunoileal atresia
• polyhydramnios

Radiographic features

Plain film - abdominal radiograph

• may show intra-abdominal (peritoneal) calcification (can be curvilinear, linear or flocculant)
• may show a mass containing calcification in the context of a meconium pseudocyst
• if the processus vaginalis is patent at the time of perforation, calcification may also be seen in the scrotum.

Ultrasound

• may show highly echogenic linear or clumped foci which represent calcifcation ^3-4
• can also give a snowstorm appearance ⁴ 
• may show fetal ascites (most common antenatal sonographic finding ⁶) and / or polyhydramnios ¹¹
• the abdominal circumference may be increased
• may also show associated anomalies such as dilated fetal bowel and / or meconium pseudocysts

Complications 

• ascites : tends to be more echogenic than simple ascites ¹⁰
• bowel obstruction from formation of fibro-adhesive bands
• meconium pseudocyst formation (a walled-off mass of meconium surrounded by a calcific rim)

Prognosis

When the calcifications are isolated, there generally is a favorable neonatal outcome and intervention is not necessary ¹¹. These cases are thought to represent perforation of bowel that spontaneously heals in utero. Therefore, in the absence of other findings, isolated calcifications can be followed sonographically during pregnancy.