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|Classification and external resources|
Colonic pseudo-obstruction is characterized by massive dilatation of the cecum (diameter > 10 cm) and right colon on abdominal X-ray. It is a type of megacolon, sometimes referred to as "acute megacolon," to distinguish it from toxic megacolon.
Ogilvie's syndrome may occur after surgery, especially following coronary artery bypass surgery and total joint replacement. It is also seen with neurologic disorders, serious infections, cardiorespiratory insufficiency, and metabolic disturbances. Drugs that disturb colonic motility (e.g., anticholinergics or opioid analgesics) contribute to the development of this condition.
The exact mechanism behind the acute colonic pseudo-obstruction is not known. The probable explanation is imbalance in the regulation of colonic motor activity by the autonomic nervous system.
Acute megacolon develops because of abnormal intestinal motility. Normal colonic motility requires integration of myogenic, neural, and hormonal influences. The enteric nervous system is independent but is connected to the central nervous system by sympathetic and parasympathetic nerves. The targets of the enteric neurons are muscle cells, secretory cells, endocrine cells, microvasculature, and inflammatory cells. The neurons in the enteric plexuses are stimulated by a food bolus, which both distends the gut and stimulates the mucosal surface, leading to the release of factors that stimulate interneurons. The stimulated interneurons transmit excitatory signals proximally, which cause contraction and inhibitory signals distally, and these in turn cause relaxation. These signals are transmitted by the neurotransmitters acetylcholine and serotonin, among others.
Acute megacolon can also lead to ischemic necrosis in massively dilated intestinal segments. This is explained by Pascal's principle and Laplaces's law. Pascal's principle states that the pressure within a cylindrical structure is the same, regardless of whether the structure is dilated, and Laplace's law states that the wall tension of a cylinder is equal to the pressure within the cylinder multiplied by the radius. Therefore, a dilated intestinal segment has a greater wall tension than a nondilated segment; if the dilatation and tension are sufficiently great, blood flow may be obstructed and ischemia of the bowel will occur. 
Signs and symptoms
It usually resolves with conservative therapy stopping oral ingestions, i.e. nil per os and a nasogastric tube, but may require colonoscopic decompression which is successful in 70% of the cases. A study published in the New England Journal of Medicine showed that neostigmine is a potent pharmacological way of decompressing the colon. According to the American Society for Gastrointestinal Endoscopy (ASGE), it should be considered prior to colonoscopic decompression. The use of neostigmine is not without risk since it can induce bradyarrhythmia and bronchospasms. Therefore atropine should be within immediate reach when this therapy is used.
It is a serious medical disorder and the mortality rate can be as high as 30%. The high mortality rate is likely a measure that this syndrome is seen in critically ill patients, rather than this syndrome being in itself lethal, although it can also present in otherwise healthy individuals (especially if the disorder was induced by pharmocologic agents). Drug induced megacolon (i.e. from Clozapine) has been associated with mortality as high as 27.5%. <ref="NEJM_2"/>
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