Diverticula are outpouchings of the colonic mucosa through the muscular wall, and where and why they form explains both diverticulitis and diverticular bleeding. They are common and mostly silent, so their true incidence and prevalence are not known: the figures come from imaging and colonoscopy series rather than from the whole population.
Where and how diverticula form
The outer longitudinal smooth muscle of the colon forms three distinct muscular bands called the taeniae coli: the mesenteric taenia, which lies close to the mesentery, and the anti-mesenteric taeniae, which are the omental and free taeniae.
Diverticula arise in four distinct areas, at the four main branches of the vasa recta: on the two sides of the mesenteric taenia, and on the mesenteric side of the free and omental taeniae. This is why the vasa recta — not the strength of the muscle elsewhere — determines where the wall gives way.
The number of diverticula can vary from a few to more than a hundred, and they also vary in size. Most are about 2–8 mm in diameter, though they can be much larger in rare cases. Their location depends on geography: in Western societies they are more common in the left colon, while in Eastern societies they are more common in the right colon.
Epidemiology and heritable risk
Diverticular disease of the colon was first described in the sigmoid colon in 1849, and diverticulitis was recognised as its most common clinical manifestation in 1899. Diverticulosis is the most commonly reported abnormality at colonoscopy, appearing in more than 40% of reports, and more than two thirds of people have it by the ninth decade of life.
Two factors drive its frequency. Age is the stronger: about 10% of adults younger than 40 years have diverticulosis, compared with more than 66% of adults older than 80 years. Ageing raises the prevalence of the outpouchings, but the occurrence of diverticulitis does not follow the same trend, because diverticulitis does not become more common with age in the way diverticulosis does. Geography is the second: diverticulosis is more common in Western countries, which may reflect their fibre-poor diet compared with other countries.
Diverticular disease was long thought to be purely acquired, but genetic and heritable factors also contribute. Diverticulosis occurs with the same prevalence in men and women, although women may have more episodes of diverticulitis while episodes of bleeding appear similar. It is about 3 times more prevalent in the siblings of a person with diverticulosis than in the general population, and it is significantly more common in monozygotic twins than in the general population. Bleeding is reported most often in African-American people, and diverticulitis most often in Caucasian people.
Why the wall gives way
The exact pathogenesis of diverticular formation is not known, and several factors interact. The most accepted environmental explanation is a low-fibre diet. In populations eating little fibre, intestinal passage time lengthens and stool volume falls, and both of these raise intraluminal pressure in the colon, which favours herniation of the mucosa — diverticulosis. Whether a high-fibre diet prevents diverticular disease has been questioned, and the evidence does not fully settle the point; what the data do suggest is that a fibre-rich diet reduces attacks of acute diverticulitis. Smoking raises the risk of diverticular disease and of hospitalisation for it, and obesity, arterial hypertension and immunosuppression are additional risk factors. Ageing and motility disorders of the colon contribute too.
The colonic wall and its motility
In the diverticular colon both the longitudinal and circular muscles are thickened, and shortening of the longitudinal muscle leads to accordion-like pleating of the folds, called myochosis, which can be seen on colonoscopy. This thickening is not due to hyperplasia or hypertrophy of the muscle fibres but to deposition of elastin fibres in these muscular parts of the colon: in diverticulosis there is a 200% increase of elastin fibre deposition in the taenia muscles. Expression of type III collagen fibres and of inhibitors of metalloproteinase enzymes is also increased. The importance of matrix proteins is supported by the higher prevalence of diverticular disease in patients with Ehlers-Danlos syndrome, Marfan syndrome and other connective tissue disorders.
Studies of colonic motility show that intraluminal pressure is increased at rest and during action potentials, cholinergic stimulation is increased, inhibitory innervation is decreased, colonic contraction becomes segmented rather than continuous, and the tonicity of muscle contraction is increased. The rise in intraluminal pressure and colonic tonicity together is believed to cause diverticulosis.
The gut microbiota is altered as well: patients with diverticulosis have an increase of Bacteroides and Prevotella, while other bacterial groups are decreased.
Diverticulitis
Diverticulitis is inflammation and, sometimes, infection of a diverticulum. The suggested pathophysiological pathway is very similar to that of appendicitis. The neck of the diverticular sac becomes obstructed by faeces; the faeces damage the epithelium of the diverticulum and start the first inflammation; venous drainage is diminished, weakening the immune response; and bacteria accumulate inside the obstructed sac. Once diverticulitis is established, the colon wall can be damaged and micro-perforation can develop, which may evolve into frank perforation.
Diverticular bleeding
Diverticular bleeding is the most common cause of significant lower gastrointestinal bleeding (LGIB) in adults. Bleeding occurs in 10–15% of patients with significant diverticulosis. Although in Western populations most diverticulosis is in the left colon, most diverticular bleeding comes from diverticulosis on the right side of the colon.
The reason is structural: in the dome of the diverticulum, the vasa recta is separated from the lumen only by the mucosal layer, which makes it highly susceptible to damage and bleeding.