Megacolon

Overview
Megacolon is an abnormal dilatation of the colon (a part of the large intestines) that is not caused by mechanical obstruction. The dilatation is often accompanied by a paralysis of the peristaltic movements of the bowel, resulting in chronic constipation. In more extreme cases, the feces consolidate into hard masses inside the colon, called fecalomas (literally, fecal tumor), which 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

A megacolon can be either acute or chronic. It can also be classified according to etiology.

Etiology

 * Congenital or aganglionic megacolon
 * Acquired megacolon, of which there are several possible etiologies:
 * Idiopathic megacolon
 * Toxic megacolon
 * Megacolon secondary to infection
 * Other neurologic, systemic and metabolic diseases

Aganglionic megacolon
Also called Hirschsprung's disease, it is a congenital disorder of the colon in which nerve cells of the myoenteric or Auerbach's plexus in its walls, also known as ganglion cells, are absent. It is a rare disorder (1:5 000), with prevalence among males being four times that of females. Hirschsprung’s disease develops in the fetus during the early stages of pregnancy. The exact genetic cause remains unsolved, although in familial cases (in which families have multiple affected patients), it seems to exhibit autosomal dominant transmission, with a gene called RET, in chromosome 10, being dominant. Other 7 seven genes seem to be implicated, however. If untreated, the patient can develop enterocolitis.

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.

Megacolon in Chagas disease
In Central and South America, the most common incidence of chronic megacolon is that observed in ca. 20% of patients affected with Chagas disease, caused by Trypanosoma cruzi, a flagellate protozoan transmitted by the feces of an hematophagous insect, the assassin bugs, or by contamination through blood transfusion or pregnancy. There are several theories on how megacolon (and also megaesophagus) develops in Chagas disease. The Austrian-Brazilian physician and pathologist Fritz Köberle was the first to propose a coherent hypothesis based on the documented destruction of the Auerbach's plexus in the walls of the intestinal tracts of Chagas patients, the so-called neurogenic hypothesis. 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) they were strongly reduced all over the digestive tract; 2) that 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 (muscular annular contraction) in those parts where a strong force is necessary to impel the luminal bolus of feces; and 4) Idiopathic megacolon and Chagas megacolon appear to have the same etiology, namely the degeneration of the Auerbach's myoenteric plexus.

Why T. cruzi causes the destruction, however, remains to be elucidated: there are evidences for the presence of specific neurotoxins as well as a disordely immune system reaction.

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 shock. Stercorary ulcers are sometimes observed in chronic megacolon, which may lead to perforation of the intestinal wall in ca. 3% of the cases, leading to sepsis and risk of death.

Diagnosis
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 etiologies. In this test, the patient swallows a water soluble bolus of radio-opaque contrast and films are obtained 1, 3 and 5 hours later. Patients with colonic inertia show the marker spread the large intestines, while patients with outlet obstruction exhibit show 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:
 * In stable cases, use of laxatives and bulking agents, as well as modifications in diet and stool habits are effective.
 * Corticosteroids and other anti-inflammatory medication is used in toxic megacolon.
 * Desimpaction of feces and decompression using anorectal and nasogastric tubes.
 * When megacolon worsens and the conservative measures fail to restore transit, surgery may be necessary.

There are several surgical approaches to treat megacolon, such as a total abdominal colectomy (removal of the entire colon) with ileorectal anastomosis (ligation of the remaining ileus and rectum segments), or a total proctocolectomy (removal of colon, sigmoid and rectum) followed by ileostomy or followed by ileoanal anastomosis.