Meckel's diverticulum: a vitelline duct remnant
Meckel's diverticulum is a closed, blind-ending pouch of ileum left over when the vitelline duct, the channel between the midgut and the yolk sac, fails to close completely.
Remember
In a young child, painless rectal bleeding points to Meckel's diverticulum, because its ectopic gastric mucosa (stomach-type lining in the wrong place) makes acid that damages the ileum. Inflammation of the same pouch can mimic appendicitis.
In the embryo, the vitelline duct (also called the omphalomesenteric duct) is a temporary channel between the midgut and the yolk sac. It normally closes and disappears before birth. If it fails to close completely, a remnant is left, and the commonest is a Meckel's diverticulum, a closed, blind-ending pouch of ileum.
The pouch arises on the antimesenteric border: the edge of the bowel away from where the mesentery attaches (the fold that carries its blood vessels). Many are silent. Some contain ectopic gastric mucosa, meaning stomach-type lining in the wrong place. That lining makes acid, which damages the neighbouring ileal lining. This can cause ulceration and painless rectal bleeding.
A fibrous remnant of the duct may run from the tip of the pouch to the abdominal wall, so loops of bowel can twist round it or become trapped beneath it, which causes intermittent obstruction. Bleeding is the commoner presentation in younger children, while pain and obstruction are commoner in older ones. Inflammation of the pouch gives pain in the right lower abdomen that can be mistaken for appendicitis. Nothing discharges from the umbilicus (belly button), because the duct has closed at the umbilical end; only the ileal end remains open.
A Meckel's scan (a nuclear medicine scan) can show the gastric mucosa.
Key facts
- Origin
- incomplete closure of the vitelline duct, which normally closes before birth.
- What it is
- a blind-ending pouch of ileum, and the most common congenital anomaly of the gastrointestinal tract.
- Memory aid
- the rule of 2s says that it occurs in 2% of the population, that about 2% of those are symptomatic (cause symptoms), that it is located about 2 feet (60 cm) from the ileocaecal valve (where the ileum joins the caecum), and that the pouch is up to 2 inches (5 cm) long.
- Presentation
- usually under 2 years, with boys affected twice as often as girls.
- Lining
- ileal mucosa, which may contain ectopic tissue of two types, gastric and pancreatic; the gastric type makes acid, but pancreatic tissue does not secrete acid.
- Investigation
- a Meckel's scan (a nuclear medicine scan) can show the gastric mucosa.
Exam traps
- A fibrous vitelline band, a solid cord with no mucosa, can trap bowel and cause obstruction or twisting (volvulus), but it does not cause the pain of an inflamed pouch that can be mistaken for appendicitis.
- A patent vitelline duct (one left open all the way to the skin) gives discharge at the umbilicus, but bleeding from the rectum is not its typical feature.
- Ectopic pancreatic tissue can sit in a Meckel's diverticulum, but it does not secrete acid, so it does not account for painless rectal bleeding.
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