Distension Colonic

A second form of motility in the large intestine is mass movement. Three or four times per day, typically after a meal, a strong propulsive contraction occurs that moves a substantial bolus of chyme forward toward the distal portion of the colon. Mass movements may result in the sudden distension of the rectum that elicits the defecation reflex. 

It is likely that the hormone gastrin, which is secreted by the stomach antral mucosa in response to distension, also plays some role in this effect because gastrin has an excitatory effect on the colon and an inhibitory effect on the ileocecal valve, thus allowing rapid emptying of ileal contents into the cecum. This in turn elicits increased colonic activity. 

Ordinarily, defecation results from the defecation reflexes, which can be described as follows. When the feces enter the rectum, distension of the rectal wall initiates afferent signals that spread through the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus. As the peristaltic wave approaches the anus, the internal anal sphincter is inhibited by the usual phenomenon of receptive relaxation, and if the external anal sphincter is relaxed, defecation will occur. This overall effect is the intrinsic defecation reflex of the colon itself. 

A 41-year-old man developed acute abdominal pain with profuse diarrhea and fever (39°C) while receiving intramuscular fluphenazine decanoate 125 mg once every 3 weeks. During the previous 3 months he had also taken oral alimemazine 50 mg/day, levomeproma-zine 50 mg/day, and amitriptyline 100 mg/day. Colonoscopy showed necrotic ulcers in the mucosa of the sigmoid and descending colon. After three weeks of parenteral nutrition, there was a marked reduction in the colonic lesions and he recovered. Levomepromazine 50 mg/day and fluphenazine decanoate 100 mg/day were reintroduced. Two days later he complained again of abdominal pain, and tomodensi-tometry confirmed distension. 

A 51-year-old woman presented for the second time in 4 months with abdominal pain (21). On this as on the previous occasion, emergency laparotomy was needed and she was found to have patchy infarction of the terminal ileum and the ascending colon. It emerged that she had been taking pseudoephedrine for 2 years, because she found that it reheved her headaches. In fact she had often noticed abdominal pain and distension, with occasional bloody diarrhea, after taking... 

The symptoms of lactose intolerance are caused by the osmotic effect of the unabsorbed lactose, which increases water and sodium levels in the lumen. Unabsorbed lactose, upon reaching the colon, can be fermented by colonic flora, which produces gas, causing abdominal distension and discomfort. A lactose tolerance test has been developed based on the measurement of blood glucose level and the hydrogen level in the breath. However, its usefulness has been questioned as the test is based on a 50 g dose of lactose. 

Toxic megacolon—A segmental or total colonic distension of >6 cm with acute colitis and signs of systemic toxicity. 

Large Intestine Propulsive peristaltic waves in the colon are diminished or abolished after administration of morphine, and tone is increased to the point of spasm. The resulting delay in the passage of bowel contents causes considerable desiccation of the feces, which, in turn, retards their advance through the colon. The amplitude of the nonpropulsive type of rhythmic contractions of the colon usually is enhanced. The tone of the anal sphincter is augmented, and reflex relaxation in response to rectal distension is reduced. These actions, combined with inattention to the normal sensory stimuli for defecation reflex owing to the central actions of the drug, contribute to morphine-induced constipation. 

Fig. l.la,b. Pyloric atresia. Anteroposterior (a) and lateral (b) plain abdominal radiograph in a newborn infant that shows distension of the stomach (st) and absence of air in the small howel and colon, resulting in the characteristic single bubble image... 

Low intestinal obstruction is defined as one occurring in the distal ileum or colon. The symptoms are vomiting, abdominal distension, and failure to pass meconium. 

Plain film shows numerous dilated loops of bowel occupying the entire abdominal cavity, including the pelvic portion, and multiple air-fluid levels in upright film (Fig. 1.19). With this degree of distension the mucosal pattern of the small bowel is effaced and it is impossible to differentiate the small bowel from the colon (Winters et al. 1992). Examination of the colon is then warranted to disclose the presence or absence of a colonic lesion. In ileal atresia, the colon is normally placed but has an abnormally small caliber, the so-called functional microcolon typical of distal small bowel obstruction (Dalla Vecchia et al. 1998) (Fig. 1.19d,e). The presence of pneumoperitoneum indicates that perforation has occurred and a colon examination is contraindicated. Intraperitoneal calcifications, indicative of meconium peritonitis, are not uncommon in ileal atresia. 

Fig. 1.19a-e. Ileal atresia, a-c Supine, lateral, and upright abdominal radiographs show multiple dilated air-filled bowel loops occupying the entire abdominal cavity, with air-fluid levels in the upright radiograph. Note in (b) the absence of air in the rectum. With this degree of distension it is impossible to differentiate the small bowel from the colon. d,e Contrast enema outlines the minute size of the colon corresponding to an unused colon... 

Fig. 1.20a,b. Meconium ileus, a Supine plain radiograph shows marked bowel distension with suggestive evidence of mottled air and feces in ascending colon and terminal ileum, b Upright film shows absence of well-defined air-fiuid levels... 

Fig. 1.29a-d. Colon atresia, a Supine radiograph shows gaseous distension of howel. b,c Contrast enema, anteroposterior and lateral views show abnormally small colon (microcolon) with complete obstruction to retrograde flow of contrast material proximal to the middle transverse portion of colon, d Photograph at operation shows a distended colon up to the site of the atresia (arrow) and a microcolon distal to it... 

Fig. 1.30a-e. Hirschsprung disease. Supine (a), upright (b) and lateral (c) plain radiographs show distension of the bowel corresponding to a low obstruction, with air-fluid levels in the upright film and absence of gas in the rectum in the lateral film. d,e Contrast enema shows a distended colon and an obvious zone of transition (arrows) in the rectosigmoid junction. The caliber of the rectum is very small... 

Fig. 1.33a-d. Meconium plug syndrome. Anteroposterior (a) and lateral (b) plain radiographs show distension of bowel loop consistent with low obstruction. c Isosmolar water-soluble contrast enema, lateral view outlines the characteristic long filling defect (arrows) within the colon, d Clinical image of a meconium plug surgically obtained in a patient in whom several attempts to conservatively solve the obstruction failed... 

In the newborn period the patient may present with a delay or failure to pass meconium, distension of the abdomen, bilious vomiting or signs of neonatal intestinal obstruction. An AXR will show evidence of low obstruction but will be non-specific however, it may allow assessment of the lower spine and in Hirschsprung s disease a lack of air in the distal colon or rectum may he a sign of congenital agan-glionosis (Fig. 6.9a). 

Identification of the colon with sonography is relatively difficult because the colon is filled with gas and feces, rather than fluid. As gas and feces are present in various amounts in the normal colon, the diagnosis of obstruction can be made only when the colon is found to be dilated continuously to the level of the lesion (see Fig. 4.3), where abnormal distension ends abruptly, with the colon distal to it free of gas (Wilson 1994). Since the colon is fixed... 

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