Megacystis microcolon intestinal hypoperistalsis syndrome (MMIHS) is a rare congenital condition characterized by abdominal distension caused by a largely dilated non-obstructed urinary bladder (megacystis); very small colon (microcolon); and decreased or absent intestinal movements (intestinal peristalsis). Usual clinical presentation is similar to other neonatal intestinal obstructions: bile stained vomiting and failure to pass meconium (the first bowel movement the baby has). Other intestinal anomalies may be present like intestinal malrotation. Many problems with the urinary tract result from the bladder dysfunction. It is part of a group of conditions caused by changes (mutations) in the ACTG2 gene and is inherited in an autosomal dominant manner. However medical scientists believe that many cases of MMIHS are caused by de novo mutations in the ACTG2 gene (meaning the mutation in the gene happened by mistake during the making of the sperm or egg). There is currently no cure for MMIHS and treatment is supportive. In the majority of patients total parenteral nutrition is required.

Megacystis microcolon intestinal hypoperistalsis syndrome (MMIHS) is primarily characterized by abdominal distention caused by an enlarged bladder (megacystis) and intestinal pseudo-obstruction.The enlarged bladder may be detected prenatally. Children with MMIHS may have various abnormalities of the digestive tract, including microcolon (very small colon), malrotation of the gut, decreased or absent intestinal movements, and short bowel. Additional abnormalities of the urinary tract that have been described include renal dysplasia, hydronephrosis, and enlargement of the ureter. Abnormalities that have been reported in some cases include undescended testes or bilateral streak gonads (underdeveloped gonads), heart anomalies, umbilical hernia, or omphalocele.

ACTG2 gene. ACTG2-related disorders are inherited in an autosomal dominant manner.

In some cases, an affected person inherits the mutation from an affected parent. In other cases, the mutation occurs for the first time in a person with no family history of the condition. This is called a de novo mutation. While the exact proportion of inherited versus de novo mutations is unknown, current data suggest that de novo mutations are common.

When a person with a mutation that causes an autosomal dominant condition has children, each child has a 50% (1 in 2) chance to inherit that mutation. However, the severity of signs and symptoms can vary within a family. If the mutation in a family member with MMIHS has been identified, prenatal testing for pregnancies at increased risk is possible.

While there are reports of longer survival, the prognosis and life expectancy remains poor, and it is still fatal in many cases. The main causes of death include sepsis, malnutrition, or multiple organ failure.