Diverticular disease (DD) is an age-related disorder of the large bowel which may affect half of the population over the age of 65 in the UK. This high prevalence ranks it as one of the most common bowel disorders in western nations. The majority of patients remain asymptomatic but there are associated life-threatening co-morbidities, which, given the large numbers of people with DD, translates into a considerable number of deaths per annum. Despite this public health burden, relatively little seems to be known about either the mechanisms of development or causality. In the 1970s, a model of DD formulated the concept that diverticula occur as a consequence of pressure-induced damage to the colon wall amongst those with a low intake of dietary fiber. In this review, we have examined the evidence regarding the influence of ageing, diet, inflammation and genetics on DD development. We argue that the evidence supporting the barotrauma hypothesis is largely anecdotal. We have also identified several gaps in the knowledge base which need to be filled before we can complete a model for the etiology of diverticular disease.

Diverticula are herniations through the colonic wall and are therefore likely to be due to a weakness within the wall, an alteration in intracolonic pressures or a combination of these factors. This article reviews these aspects in relation to the pathogenesis of colonic diverticula.

Medline and Science Citation Index searches were performed to locate English language articles relating to the pathogenesis of colonic diverticulosis published since 1960. Manual cross-referencing was also performed and some historical articles were included.

Several theories now exist about the development of colonic diverticula. The majority of the evidence suggests that the morphological changes are the response to a lifelong consumption of a low-residue diet. However, there are complex relations between colonic structure, motility and dietary factors, and it is likely that all of these (and possibly genetic influences) play a role in the pathogenesis to a greater or lesser degree.

Diverticular disease is worldwide in distribution, but the incidence is highest in industrialised countries. It is associated with aging and low intake of dietary fibre. There is a broad range of clinical manifestations--from asymptomatic diverticula to life-threatening complications. Elderly patients often present with complicated diverticular disease, and may lack typical symptoms and signs. Treatment includes fibre supplementation, drugs or antibiotics for complications, and surgery for refractory disease. Proper diagnosis and treatment requires knowledge of the full range of presentations and careful selection and timing of medical versus surgical intervention.

Diverticular disease of the sigmoid colon is an increasingly common clinical problem in the ageing population of western industrialised countries but the mechanism by which the disease develops remains unknown. The muscular abnormality is the most striking and consistent feature and this has been studied by light and electron microscopy in 25 surgical specimens of uncomplicated diverticular disease and in 25 controls. This is the first ultrastructural study of human colonic muscle to be published and shows that the muscle cells in diverticular disease are normal; neither hypertrophy nor hyperplasia is present. There is, however, an increase in the elastin content of the taeniae coli by greater than 200% compared with controls: elastin is laid down between the muscle cells and the normal fascicular pattern of the taeniae coli is distorted. There is no alteration in the elastin content of the circular muscle. As elastin is laid down in a contracted form, this elastosis may be responsible for the shortening or 'contracture' of the taeniae which in turn leads to the characteristic concertina-like corrugation of the circular muscle. Such a structural change could explain the altered behaviour of the colon wall in diverticular disease and its failure to change on treatment with bran.

1 Pharmacological experiments were performed on circular and longitudinal muscle strips of sigmoid colon from diverticular disease specimens, and their responses compared with those of similar muscle strips from sigmoid colon resected from carcinoma (control). 2 The longitudinal muscle strips from diverticular disease specimens were significantly less responsive to acetylcholine, histamine or noradrenaline than control longitudinal muscle strips. 3 The responses of the diverticular circular muscle strips showed a small decrease to acetylcholine, a small increase to noradrenaline and no change to histamine when compared to control strips.

Spontaneous motor action of fresly prepared circular and longitudinal muscle strips were examined in two separated trials. Bowel musculature of diverticular patients were compared with those of normal subjects. Finally all explantations were examined histologically.

1. There was no difference in electrical activity and tension development regarding circular muscle strips of patients suffering from D.D. and those of normal subjects. 2. The longitudinal muscles show however significant differences in contrast to normal subjects; to sum up D.D. presents us a spastic and contracted taenia. 3. The histological investigations did not explain the different electromechanical results. There were however remarcable ribbons only in the longitudinal muscle of D.D.--known from myoma uteri. The results of the electrophysiological analysis have importance for our surgical concept in the treatment of peridiverticulitis. They justify the horizontal myotomy in the left, functionally disturbed colon descendens additional to the resection as an preventive procedure.

The association of diverticulosis with a thickened muscle wall in the pelvic colon is well known. There appeared to be a possibility that this muscular thickening might give rise to the rectosigmoidal mucosal puckering often seen through a sigmoidoscope. In 278 colonic motility studies, mucosal puckering was correlated with patient age, diverticulosis, and the symptomatology of the irritable colon syndrome. The prevalence of mucosal puckering increased with age at the same rate as that of diverticulosis, but the different percentage levels were reached some 25 years earlier in life. Mucosal puckering, which persisted in all subsequent examinations, showed no correlation with the clinical characteristics of irritable colon syndrome. Resistance of the bowel wall to distention was significantly greater in association with puckered rectosigmoidal mucosa than when the mucosa appeared smooth. It is concluded that rectosigmoidal mucosal puckering is probably caused by thickening of the underlying muscle; it is a precursor of diverticulois, and patients who have it should be kept on a high-residue diet.

Herniation of the glandular epithelium into the submucosa has been observed in 11 out of 27 cases of chronic ulcerative colitis. Glandular herniation was associated with thickening of the muscularis mucosae, with interruption of the muscularis mucosae by lymphoid follicles, and, in five of the 11 cases, with significant crowding of the glands of the mucosa. This study strongly suggests that sustained contraction of the muscularis mucosae, which has been shown by others to be a major feature of chronic ulcerative colitis, is the prime factor in the formation of downgrowths or herniations of the glandular epithelium into the submucosa. Comparison of the cases in which cancer developed with those where there was glandular herniation led to the conclusion that they are independent associations of chronic ulcerative colitis, and that glandular herniation plays no part in the development of dysplasia or cancer.

The rationale of transverse taeniamyotomy in the treatment of diverticular disease of the colon, with or without inflammatory or other complications, is discussed. The technique of the operation is described, multiple transverse incisions being made at 2-cm intervals through the two thickened antimesenteric taeniae coli throughout the affected length of bowel. The indications and results in 18 patients are tabulated and discussed. Transverse taeniamyotomy is a simple procedure, rapidly performed, with minimal morbidity, and is well tolerated even by elderly, poor-risk patients. The results so far have been promising.