| Megacolon | |
|---|---|
 | |
| Toxic megacolon associated withulcerative colitis. | |
| Specialty | Gastroenterology  |
Megacolon is an abnormal dilation of thecolon (also called the largeintestine).[1][2] This leads tohypertrophy of the colon.[2] The dilation is often accompanied by aparalysis of theperistaltic movements of the bowel. In more extreme cases, thefeces consolidate into hard masses inside the colon, calledfecalomas (literally,fecal tumor), which can requiresurgery to be removed.
A human colon is considered abnormally enlarged if it has adiameter greater than 12 cm (4.7 in)[3] in thececum (it is usually less than 9 cm [3.5 in][4]), greater than 6.5 cm (2.6 in)[3] in therectosigmoid region and greater than 8 cm (3.1 in)[3] for theascending colon. The transverse colon is usually less than 6 cm (2.4 in) in diameter.[4]
A megacolon can be eitheracute orchronic. It can also be classified according to cause.[5]
External signs and symptoms areconstipation of very long duration,[2] abdominalbloating, abdominal tenderness andtympany,abdominal pain,palpation of hard fecal masses and, intoxic megacolon,fever, low bloodpotassium,tachycardia and may lead toshock.Stercoral ulcers are sometimes observed in chronic megacolon, which may lead to perforation of the intestinal wall in approximately 3% of the cases, leading tosepsis and risk of death.[6][7]
Also calledHirschsprung's disease, it is acongenital disorder of the colon in whichnerve cells of themyenteric plexus in its walls, also known as ganglion cells, are absent. It is a rare disorder (1:5000), with prevalence among males being four times that of females. Hirschsprung's disease develops in thefetus during the early stages ofpregnancy. A genetic predisposition to Hirschsprung's disease has been linked tochromosome 13 where amissense mutation at an ultraconserved region impairs functionality of the W276C receptor. Seven other genes seem to be implicated, however. If untreated, the patient can developenterocolitis.[citation needed]
Toxic megacolon is mainly seen inulcerative colitis andpseudomembranous colitis, two chronicinflammations of the colon (and occasionally, in the other type ofinflammatory bowel disease,Crohn's disease). Its mechanism is incompletely understood. It is probably due to excessive production ofnitric oxide, at least in ulcerative colitis. The prevalence is about the same for both sexes.[citation needed]
In patients with HIV/AIDS,cytomegalovirus (CMV) 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.[10]
Megacolon can be associated withChagas disease.[11] Chagas disease is caused byTrypanosoma cruzi, a flagellateprotozoan transmitted by theassassin bug. Chagas disease 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 alsomegaesophagus) develops in Chagas disease. TheAustrian-Brazilianphysician andpathologistFritz Köberle was the first to propose theneurogenic hypothesis based on the documented destruction of themyenteric plexus in the walls of the intestinal tracts of Chagas patients. In this, the destruction of theautonomic nervous system innervation of the colon leads to a loss of the normalsmooth 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:[citation needed]
WhyT. cruzi causes the destruction remains to be determined. There is evidence for the presence of specificneurotoxins as well as a disorderlyimmune system reaction.[citation needed]
Diagnosis is achieved mainly by plain and contrastedradiographical andultrasound 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 ofradiocontrast 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. Acolonoscopy 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.[12]
Possible treatments include:[13]
There are several surgical approaches to treat megacolon, such as acolectomy[2][14] (removal of the entire colon) with ileorectalanastomosis (ligation of the remaining ileum and rectum segments), or a totalproctocolectomy (removal of colon, sigmoid and rectum) followed byileostomy or followed by ileoanal anastomosis.
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