Pathology of the midgut
Due to the large changes in shape during the embryonic period in this region there is a multitude of disorders that can occur, either isolated or combined with other abnormalities. An exhaustive list of all the disorders that could occur is here not possible.
Disorders associated with the omphalomesenteric duct are encountered the most frequently. Some 2-4% of the population exhibits Meckel's diverticulum.
It is most often a sac with a dead end that lies ca. 50 cm cranial to the iliocecal valve and represents the remnant of the omphalomesenteric duct. Usually it is discovered accidentally but a Meckel's divertculum can become infected or exhibit ectopic tissue.
With an omphalocele intestinal loops remain in the umbilical coelom and are not repositioned into the abdominal cavity. Typically the intestinal loops are coated with amnion and peritoneum.
An omphalocele is not to be confused with an umbilical hernia, which is found mainly in premature births.
The reason for an umbilical hernia is a weak abdominal musculature that is not able to hold back the intraperitoneal contents of the abdominal cavity. In contrast to an omphalocele an umbilical hernia is always covered with skin.
Malrotation and coecal elevation
Sometimes, when the intestinal loops return back into the abdominal cavity out of the physiological umbilical hernia, no or an incomplete rotation occurs. This can remain asymptomatic or occasionally lead to a volvulus or other form of strangulation.
Just as in the esophagus and the duodenum an incomplete recanalization of the intestinal lumen can also occur in the other regions of the intestines after the first trimester of the pregnancy.
Aganglionic megacolon (Hirschsprung's disease)
Hirschsprung's disease (often called a congenital megacolon) is a congenital defect of ganglion cells in the large intestine. In a certain portion of the large intestine nerve cells are absent that normally stimulate the muscles to contract and thus further transport the intestinal content. Both Auerbach's and Meissner's plexus are affected. The intestine is thinner in this region and cannot expand very far. Excrement remains at this location causing preceding intestinal sections to make expansions that are often gigantic. It is these expansions, which are actually a consequence of the disorder, that have given this illness the name congenital megacolon. An absent relaxation of the anal sphincter muscle is also a frequent accompaniment. Most often boys are affected. The disease occurs with a frequency of 1:5'000.
In newborns this disorder gives rise to acute infirmities, as are also characteristic in an ileus. The abdomens of the affected children are bloated and hard – they suffer from vomiting and constipation. Stools can be produced that range from voluminous to pencil-lead thin and stinking. In older children a chronic clinical picture of intestinal paralysis with constipation and bloated abdomen often develops. Therapy consists in a surgical removal of the affected section of the large intestine.