|Classification and external resources|
Megacolon is an abnormal dilation of the colon (a part of the large intestines). The dilatation 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 (literally, fecal tumor), 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 (it is normally less than 9 cm), 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.
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. 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.
- Congenital or aganglionic megacolon
- Acquired megacolon, of which there are several possible etiologies:
Aganglionic megacolon 
Also called Hirschsprung's disease, it is a congenital disorder of the colon in which nerve cells of the myenteric 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. Seven other genes seem to be implicated, however. If untreated, the patient can develop enterocolitis.
Risperidone, 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.
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 ca. 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 (for example garapa). 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.
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 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.
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.
- Antibiotics are used for bacterial infections such as oral Vancomycin for Clostridium Difficile
- Disimpaction 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 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.
See also 
- "megacolon" at Dorland's Medical Dictionary
- Megacolon, Chronic at eMedicine
- Horton KM, Corl FM, Fishman EK (2000). "CT evaluation of the colon: inflammatory disease". Radiographics 20 (2): 399–418. PMID 10715339.
- Porter NH (1961). "Megacolon: a physiological study". Proc. R. Soc. Med. 54: 1043–7. PMID 14488085.
- Lim DK, Mahendran R (2002). "Risperidone and megacolon". Singapore Med J 43 (10): 530–2. PMID 12587709.
- Koeberle F (1963). "Enteromegaly and cardiomegaly in Chagas disease". Gut 4 (4): 399–405. doi:10.1136/gut.4.4.399. PMC 1413478. PMID 14084752.
- Stabile G, Kamm MA, Hawley PR, Lennard-Jones JE (1991). "Colectomy for idiopathic megarectum and megacolon". Gut 32 (12): 1538–40. doi:10.1136/gut.32.12.1538. PMC 1379258. PMID 1773963.
- Hirschsprung's disease
- Megacolon resources
- MedlinePlus Encyclopedia Toxic megacolon
- Megacolon in cats