Megacolon


Megacolon is an abnormal dilation of the colon. The dilation is often accompanied by a paralysis of the peristaltic movements of the bowel. In more extreme cases, the feces consolidate into hard masses inside the colon, called fecalomas, which can require surgery to be removed.
A human colon is considered abnormally enlarged if it has a diameter greater than 12 cm in the cecum, greater than 6.5 cm in the rectosigmoid region and greater than 8 cm for the ascending colon. The transverse colon is usually less than 6 cm in diameter.
A megacolon can be either acute or chronic. It can also be classified according to cause.

Signs and symptoms

External signs and symptoms are constipation of very long duration, abdominal bloating, abdominal tenderness and tympany, abdominal pain, palpation of hard fecal masses and, in toxic megacolon, fever, low blood potassium, tachycardia and may lead to shock. Stercoral ulcers are sometimes observed in chronic megacolon, which may lead to perforation of the intestinal wall in approximately 3% of the cases, leading to sepsis and risk of death.

Cause

Also called Hirschsprung's disease, it is a congenital disorder of the colon in which nerve cells of the myenteric plexus in its walls, also known as ganglion cells, are absent. It is a rare disorder, with prevalence among males being four times that of females. Hirschsprung's disease develops in the fetus during the early stages of pregnancy. A genetic predisposition to Hirschsprung's disease has been linked to chromosome 13 where a missense mutation at an ultraconserved region impairs functionality of the W276C receptor. Seven other genes seem to be implicated, however. If untreated, the patient can develop enterocolitis.

Medication

, an anti-psychotic medication, can result in megacolon.

Toxic megacolon

Toxic megacolon is mainly seen in ulcerative colitis and pseudomembranous colitis, two chronic inflammations of the colon. Its mechanism is incompletely understood. It is probably due to an excessive production of nitric oxide, at least in ulcerative colitis. The prevalence is about the same for both sexes.
In patients with HIV/AIDS, cytomegalovirus colitis is the leading cause of toxic megacolon and emergency laparotomy. CMV may also increase the risk of toxic megacolon in non-HIV/AIDS patients with IBD.

Chagas disease

Megacolon can be associated with Chagas disease.
In Central and South America, the most common incidence of chronic megacolon is that observed in about 20% of patients affected with Chagas disease. Chagas is caused by Trypanosoma cruzi, a flagellate protozoan transmitted by the feces of a hematophagous insect, the assassin bug, when it feeds. Chagas can also be acquired congenitally, through blood transfusion or organ transplant, and rarely through contaminated food. There are several theories on how megacolon develops in Chagas disease. The Austrian-Brazilian physician and pathologist Fritz Köberle was the first to propose the neurogenic hypothesis based on the documented destruction of the myenteric plexus in the walls of the intestinal tracts of Chagas patients. In this, the destruction of the autonomic nervous system innervation of the colon leads to a loss of the normal smooth muscle tone of the wall and subsequent gradual dilation. His research proved that, by extensively quantifying the number of neurons of the autonomic nervous system in the Auerbach's plexus, that:
  1. neurons were strongly reduced all over the digestive tract;
  2. megacolon appeared only when there was a reduction of over 80% of the number of neurons
  3. these pathologies appeared as a result of the disruption of the neurally integrated control of peristalsis in those parts where a strong force is necessary to impel the luminal bolus of feces
  4. idiopathic megacolon and Chagas megacolon appear to have the same cause, namely the degeneration of the myenteric plexus.
Why T. cruzi causes the destruction, however, remains to be determined. There is evidence for the presence of specific neurotoxins as well as a disorderly immune system reaction.

Diagnosis

is achieved mainly by plain and contrasted radiographical and ultrasound imaging. Colonic marker transit studies are useful to distinguish colonic inertia from functional outlet obstruction causes. In this test, the patient swallows a water-soluble bolus of radiocontrast agent and films are obtained 1, 3 and 5 days later. Patients with colonic inertia show the marker spread throughout the large intestines, while patients with outlet obstruction exhibit slow accumulations of markers in some places. A colonoscopy can also be used to rule out mechanical obstructive causes. Anorectal manometry may help to differentiate acquired from congenital forms. Rectal biopsy is recommended to make a final diagnosis of Hirschsprung disease.

Treatment

Possible treatments include:
There are several surgical approaches to treat megacolon, such as a colectomy with ileorectal anastomosis, or a total proctocolectomy followed by ileostomy or followed by ileoanal anastomosis.