Background
Megacolon, like megarectum, is a descriptive term. Megacolon denotes dilatation of the colon that is not caused by a mechanical obstruction. While the definition of megacolon has varied in the literature, most use a cecum measurement of greater than 12 cm in diameter to define megacolon. Adding to this definition, because the diameter of the large intestine is different in different areas, measurements of greater than 6.5 cm for the rectosigmoid region and greater than 8 cm for the ascending colon may also be significant.
Megacolon may be divided into three categories by acuity of onset, as follows: (1) acute megacolon (pseudoobstruction, Ogilvie syndrome); (2) chronic megacolon, which includes congenital, acquired, and idiopathic causes; and (3) toxic megacolon.
The Gastroenterology section of the Medscape Drugs & Diseases journal contains three articles devoted to megacolon, and they are separated based on the three aforementioned categories (see also Differentials). This article is devoted to the acute development of megacolon.
Acute colonic pseudoobstruction (ACPO) is a rare condition first reported in 1948 by Sir William Ogilvie. It is characterized by acute colonic dilatation in the absence of an intrinsic mechanical obstruction or an extrinsic inflammatory process (toxic megacolon). [1, 2]
Pathophysiology
The pathophysiology underlying acute colonic pseudoobstruction (ACPO) (acute megacolon, Ogilvie syndrome) remains unresolved. Limited human studies have investigated the pathophysiology underlying ACPO; few, if any, animal models have been found. However, multiple risk factors have been described that appear to be associated with ACPO, with the presumption that dysregulation of colonic motor activity as a possible underlying mechanism. [3] It is well known that the colonic distensibility changes in the presence of luminal contents. For example, fatty acids infused into the cecum appear to reduce the volume of the proximal large bowel. Long-term opiate narcotic use (including diphenoxylate and loperamide) may lead to colonic dilatation and may limit the ability of the colon to contract when dilated.
Many pathophysiologic mechanisms have been proposed in an attempt to explain the altered motility seen in acute megacolon; these include the following [3] :
-
Imalance of autonomic innervation (the most suggested mechanism): 1) Excess sympathetic motor input (failure to contract), 2) decrease in parasympathetic motor input to the distal colon (failure to relax and atonic segments), and 3) excess parasympathetic motor input (failure to relax)
-
Disruption of colo-colonic (spinal and ganglionic) inhibitory reflex arcs: Abnormal motility in one colonic segment may lead to dilatation of other segments [4]
-
Intrinsic colonic dysfunction (limited evidence): 1) impairment of the interstitial cells of Cajal (ICC) and enteric nervous system (ENS) function, and 2) dysregulation of nitric oxide neurotransmitter
-
Excess stimulation of peripheral opioid receptors by endogenous and/or exogenous opioids (initially, excess activation, followed by prolonged inhibition)
Etiology
Causes of acute megacolon (acute colonic pseudoobstruction [ACPO], Ogilvie syndrome) include, but are not limited to, the following:
-
Metabolic abnormality(ies), including renal failure, hypothyroidism, and hyperthyroidism
-
Electrolyte disturbances, which can alter autonomic function and predict poor clinical response to neostigmine
-
Chronic illness/diseases which can dysregulate neurotransmitters and disrupt the interstitial cells of Cajal (ICC) and enteric nervous system (ENS)
-
Medications, of which multiple drugs has been described that alter colonic motor activity, including anticholinergics, antidiarrheals, opioids, calcium channel blockers, antipsychotics, digitalis, clonidine, dexmedetomidine, and others
-
Pregnancy and obstetric causes, which are most commonly associated with cesarian section surgery (possibly due to alteration of autonomic innervation); hormonal changes during pregnancy may have an effect on the colonic tone
-
Inflammatory bowel disease, including ulcerative colitis and Crohn disease.
Note that in any setting, a mechanical cause (eg, a tumor) and a toxic cause (eg, acute ulcerative colitis) must be first ruled out because the treatments are very different for these conditions.
Epidemiology
Acute colonic pseudoobstruction (ACPO) (acute megacolon, Ogilvie syndrome) is rare disease. A national US hospital admissions database reported an estimated incidence rate of approximately 100 cases out of 100,000 inpatient admissions per year. [8]
Race-, sex-, and age-related demographics
Race has not been documented to play a role in acute megacolon.
Most patients are males over the age of 60 years [9] ; however, the increased incidence seen in older populations is more likely a reflection of their medical and surgical comorbidities rather than age alone.
Prognosis
Morbidity/mortality
Prognosis of patients with acute colonic pseudoobstruction (ACPO) (acute megacolon, Ogilvie syndrome) is determined in part by the underlying cause of the megacolon, the presence of any comorbidities, and the development of complications.
The mortality rate associated with ACPO declined from 9.4% in 1998 to 6.4% in 2011. [8]
In the absence of complications, mortality is approximately 15% as compared to up to 45% in patients with bowel ischemia or spontaneous perforation of nontoxic megacolon. However, most patients respond to treatment prior to the onset of this complication. [10, 11]
Complications
The most common complications of ACPO are bowel ischemia and perforation, which occur in up to 20% of cases. In a study by Vanek and Al-Salti, the perforation rates were 0% for cecal diameters of less than 12 cm, 7% for cecal diameters of 12-14 cm, and 23% for cecal diameters greater than 14 cm. [9]
Mortality has also been shown to increase with a delay of decompression therapy.
If surgical therapy is required, mortality increases based on older age, the presence of comorbidities, and decreased functional status.
