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Developmental
Abnormalities of the bowel |
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The most common congenital malformations
affecting the alimentary tract are:
• Oesophageal atresia with tracheo-oesophageal fistula. The most common pattern
is that in which the
upper end of the oesophagus is intact but ends in a blind pouch. There is a
central area of complete
atresia and the lower oesophagus is normal at the gastro-oesophageal junction,
but tapers proximally and
communicates with the trachea.
• Omphalocele is the result of failure of the intestine to return to the
abdominal cavity
during normal rotation of the mid-gut.
Development of the whole gut outside the abdominal cavity is termed 'eventration'.
• Gastroschisis is due to a defect in the anterior abdominal wall, resulting in
protrusion of the viscera into
the amniotic cavity.
• Failure of rotation of the gut as it returns to the abdomen causes several
patterns of malformation. In
non-rotation, the small bowel lies on the right of the abdomen, and the colon on
the left. Other forms of
malrotation lead to obstruction of bowel by twisting of the gut on itself (volvulus).
• Intestinal atresia is most common in the ileum, but also occurs higher in the
bowel in the duodenum. It
causes intestinal obstruction.
• Meckel's diverticulum is caused by an abnormality of the yolk stalk. Other
degrees of abnormality
result in tethering of the ileum to the umbilicus by a fibrous band, development
of a cyst in the
fibrous band,or persistence of a sinus track between the ileum and the
umbilicus.
• Intestinal duplications are caused by developmental abnormalities of
canalization of the gut.
There are two patterns: closed cystic duplications (most common) and
communicating tubular duplications (unusual).
• Imperforate anus is more common in males than in females, and is often
associated with other abnormalities.
The anal canal may end blindly or, more often, opens into the urethra or vagina.
A more common form is anorectal
agenesis, in which the rectum ends blindly above the anal canal, with a fistula
into the vagina or urethra.
These defects are caused by defective separation of the cloaca by the urorectal
septum.
Intussusception occurs when one segment of bowel invaginates into another
Intussusception occurs when one portion of bowel Invaginates into the adjoining
segment. The most
common site is at the ileocaecal valve, with the ileum invaginating into the
caecum. This pattern
is most common in children. In the rare adult cases a structural abnormality of
the small intestinal
wall is almost invariably responsible for precipitating the invagination, and
benign tumours such as leiomyoma
or lipoma are important predisposing factors.
The structural lesion is carried by peristalsis into the adjoining bowel, and
the bowel segment
is invaginated. This leads to venous congestion of the invaginated portion,
causing bleeding
from the mucosa, as well as intestinal obstruction.
If the bowel remains invaginated infarction of the area takes place.
Meconium ileus and rectal prolapse are manifestations of cystic fibrosis
One of the common manifestations of cystic fibrosis is meconium ileus,
occurring in 10-20% of cases. It is caused by abnormally viscid mucin forming a
mass
in the bowel lumen, which cannot be expelled
Intestinal obstruction may also occur after the neonatal period, mainly due to
obstruction
by thick mucus at the ileocaecal valve.
Another common manifestation of cystic fibrosis is rectal prolapse, believednto be the result of bulky stools caused by malabsorption. Other gastrointestinal
manifestations of cystic fibrosis are the result of pancreatic disease which
leads
to failure of the production of digestive enzymes, causing malabsorption.
Hirschsprung's disease is caused by absence of normal myenteric plexus in the
bowel wall
In Hirschsprung's disease there is failure of the normal motility of the bowel
due
to absence of the normal myenteric plexus.
The defect always involves the rectum and may be present in continuity for
a variable distance along the bowel.
Some cases involve just a short segment of rectum, others the rectosigmoid
segment
and, in rare cases, there may be total colonic or total intestinal aganglionosis.
Macroscopically the abnormally innervated bowel segment is narrowed, whereas the
proximal bowel is dilated and shows hypertrophy of the muscle layers.
The disease usually presents in early childhood with failure to pass meconium
or,
later, with persistent constipation.
Diagnosis is made by biopsy of the rectal mucosa.
Rare cases of abnormal bowel motility in childhood are caused by primary
abnormalities of visceral smooth muscle (visceral myopathies) or visceral axons
(visceral neuropathies). |
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