Diverticuli (plural of diverticulum) are outpouchings of the colon wall. Similar to hernias in the abdominal wall diverticuli develop through weakened points of a muscle layer. In the colon the muscle layer opens gaps for blood vessels to cross and feed the mucosa and return to the body the products of absorption.  
It is thought that a combination of weakening of the colonic wall and increased pressure inside the colon lead to these outpouchings.  The increased pressure is thought to develop from lack of fiber in the diet resulting in less bulk in the colon forcing it to work harder to move the stool column along.  The weakening is probably due to loss of collagen fibers in the colonic wall.
Each diverticulum becomes a structure very much like the appendix: they can become blocked with stool, infection ensues (diverticulitis) and then can rupture. Ruptures can remain contained in the form of an abscess or can break loose into the peritoneal cavity and produce peritonitis.
In very rare instances the abscess smolders for some time and drills a hole into the bladder or the vagina. These patients develop fistulas, colon to bladder (colovesical) result in the passage of gas and stool mixed with urination, and colon to vagina (colovaginal) result in gas and stool passing through the vagina.
Diverticuli are commonly found in either xRays or colonoscopies done for other reasons. It is estimated that 50 percent of Americans by age 60 and nearly all by age 80 have diverticuli in the colon. Eighty percent of these patients never experience any symptoms. Fifteen percent present with recurrent pain and 5% develop the complications already mentioned: abscess, fistula, peritonitis, and bleeding.  
It has been noted that there are two groups of patients who develop diverticulitis: the patient over the age of sixty who may have sporadic attacks that quickly respond to antibiotic therapy, and at the other extreme, the middle aged patient, late thirties or early forties, who develops severe diverticulitis and quickly escalates to complications.  It is easy to see how the elderly can develop both the increased pressure through many years of a diet low in fiber and the weakening of the colon by loss of collagen.
Conversely, the more aggressive course in younger patients suggests a different mechanism in the development of diverticuli.  Ongoing studies are suggesting some possible genetic differences in collagen metabolism among people that can result in weakening of the colonic wall as well as weakening of the wall of blood vessels leading to aneurysms.
The treatment of diverticular disease depends on the type of presentation.  In cases of diverticuli producing either no symptoms or just mild pain an increase in the amount of fiber in the diet (fruits, vegetables, grains, and legumes) and other measures to avoid constipation (increased fluid intake, psyllium seeds, stool softeners).  Patients who present with more intense pain, especially in the left lower side of the abdomen, fever and malaise have progressed to diverticulitis. 
Depending on the findings on physical exam, blood work and a CT scan may be in order.  Cases of mild diverticulitis may be treated with antibiotics by mouth, e.g., metronidazole and ciprofloxacion. More severe cases require admission to the hospital for intravenous antibiotics and observation. 
Most patients respond well to antibiotic therapy.  A group of patients, however, will have improvement of their symptoms but only temporarily.  Depending on the severity and frequency with which diverticulitis recurs surgery becomes the treatment of choice.
The other group of patients is that of complicated diverticulitis: abscess, obstruction, fistula, and perforation (bleeding is less common).  In this group surgery becomes the only option.  Patients with an abscess may benefit from a drain catheter placed in radiology under imaging guidance (ultrasound or CT). 
This allows for clearing of infection from the abdomen before surgery, which in turn makes reconnection of the bowel safe.  In cases of peritonitis, or abscesses than cannot be drained, and in those that cannot receive a proper bowel preparation due to obstruction the best course of action is the removal of the disease colon by surgery and creation of a temporary colostomy.  This colostomy is reversed 8 to 12 weeks later once the infection and inflammation has cleared up from the abdomen.
Except for the rare case of diverticulitis is localized to the right side of the colon all patients with diverticulitis will require removal of the sigmoid colon (sigmoidectomy or sigmoid resection).  Eighty percent of all diverticuli in the colon reside in the sigmoid colon.  Even though 20% of diverticuli are left behind the recurrence of diverticulitis is low because removal of the sigmoid colon facilitates the flow of stool and possibly reduces the increased pressures leading to diverticulitis.
Fortunately, most patients who require surgery for diverticulitis nowadays can have a laparoscopic sigmoid resection and reconnection in the same operation, this is known as “primary anastomosis” as opposed to the staged surgery starting with a “Hartmann’s procedure” and then reversal of the colostomy.

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