Meconium peritonitis (MP) is a sterile chemical peritonitis induced by meconium extruding into the peritoneal cavity through a perforation in the intestine (most commonly the ileum), proximal to the site of obstruction and causing peritoneal irritation.

The etiology is thought to be the result of a sterile chemical peritonitis resulting from a small bowel perforation in utero, possibly due to a meconium ileus, intussusception, an internal bowel hernia, atresia, Meckel’s diverticulum, imperforate anus, peritoneal bands, intrauterine fetal congenital infections caused by CMV or rubella, vascular accident or some other unknown complication. 

Associations: cystic fibrosis (up to 40% association with MP), intestinal atresia, polyhydramnios.

Ultrasound features

A prenatal diagnosis of MP can be established by ultrasound findings of

• fetal ascites: hypoechoic
• intra-abdominal calcified lesions: if there are calcified lesions present cytic fibrosis is unlikely
• dilated bowel loops: may be due to atresia
• dilated stomach: may be due to ileus
• pseudocyst formation: omentum walls off the meconium collection, resulting in the formation of a cyctic mass
• dense mass: the intense chemical reaction causes the formation of a dense mass with calcium deposits that eventually seal off the perforation (the fibroadhesive type)
• hydrocele/vulvar swelling: due to passing down of meconium through the patent processus vaginalis
• polyhydramnios: lack of swallowing of amniotic fluid

Outcome

In the majority of cases, the perforation has closed by the time of birth. Mortality is said to be high in the cases in which the perforation persists until the time of birth.

The neonatal outcomes of MP are much better if it is detected prenatally than if it is diagnosed after birth. In addition, the ultrasound features of MP vary with gestational age. Fetal ascites, polyhydramnios and dilated bowel loops can resolve spontaneously throughout the prenatal course.

Early onset of MP has been associated with lower survival rates and poorer neonatal outcomes.