The purpose of this study was to evaluate potential changes in admission- and incidence rates of patients with acute diverticulitis during the 25 years of study. We found significant changes with a 2.9-fold increase in the admission rate and a 2.6-fold increase in the incidence rate.

The admission rates increased with age and by time (Table 2). However, the increase in females slowed during the later years, as can be seen in Fig 5. There was a male predominance among patients younger than 55 years, shifting to female predominance in the older age groups. The life expectancy of the Norwegian population is increasing, and the number of elderly patients in need for treatment of age-dependent diseases will continue to increase. This confirms that patients with acute colonic diverticulitis place increased burden on healthcare.

Throughout the study period, CT scans became more commonly used for patients hospitalized with acute abdominal conditions. In this series on patients with acute diverticulitis, CT was used for diagnosis in 7 % at the beginning and in 75 % towards the end of the 25 years. Thus, it seems likely that some patients with acute colonic diverticulitis could have been given an unspecified diagnosis of acute abdomen during the earlier years of the study.

However, during the first 10 years of this study, fewer patients were discharged from our hospital with an unspecified diagnosis of acute abdomen compared to the later 15 years. A Swedish study of 3,349 patients with acute abdomen showed that acute diverticulitis differed from nonspecific abdominal pain in both clinical presentation and laboratory investigations [23].

Very few have investigated the admission rates of all types of acute colonic diverticulitis. Two large investigations from USA [18, 24] studied hospital admissions for acute diverticulitis between 1998 and 2005. The rate of admission increased from 61.8/100,000 to 75.5/100,000 during the study period. These admission rates were even higher than found in the present study, which was 51.1/100,000 during 2008–2012. In patients managed in the USA from 2002 to 2007, the admission rate increased by 9.5 % [25]. Obesity, a more sedentary life and different diets may predispose one for acute diverticulitis [26], and these factors may in part explain the differences in incidence rates between different populations. The overall incidence of perforated diverticulitis found in the present study, 2.5/100,000 person-years, compared well with a study from Finland that found an increase from 2.4/100,000 in 1986 to 3.8/100,000 in 2000 [17] and two studies from UK that found incidence rates of 2.7/100,000 person-years between 1990 and 2005 [16] and 3.5/100,000 between 1995 and 2000 [27]. In the present study, most of the perforations, 89 %, occurred during the first admission for acute colonic diverticulitis. This was in accordance with other recent studies [19, 28, 29]. Elective resection of the sigmoid colon to prevent perforation after the first episode would be unnecessary for most patients, since the risk of free perforation is highest at the index episode [11, 30]. We are not able to identify the patients who present with perforation without prior symptoms of diverticulitis.

A recurrence rate of 25–30 % is usually reported [30–32], which compares well with that of the present study, although rates as low as 6.1 % have been reported [33]. Different ways to indicate recurrence, whether as an absolute rate or as an estimated rate using Kaplan-Meier analysis, may explain some of the reported differences. Moreover, in countries with higher incidence rates, one might also expect higher recurrence rates of this disease.

The relation between incidence rates of an acute disease in the population and admission rates to hospital for this disease is complex. Factors other than disease incidence may influence hospital admissions [34]. The threshold to seek medical examination, advice and treatment vary from person to person and may change with age, periods of time and geographical area. The decisive factor is likely the severity of the acute disease as perceived by the patient. Trivial infections require no treatment or can be managed by general practitioners, while more serious cases are admitted to hospital. The present study was limited to hospitalized patients.

One can argue that in recent years, a lower threshold for admission of less severe cases of acute diverticulitis might have led to the increased admission rates. This may in part be true. However, the study found that the median values of CRP in patients with uncomplicated diverticulitis increased significantly during the 25 years, both for values measured at admission as well as for the maximum measured values during the hospital stay. CRP varies with the severity of the diverticulitis and may aid to predict perforation [35]. The Poisson regression analyses showed a significant increase of admission rates during the 25 years, which could not be attributed only to an increased population or more elderly inhabitants.

A strong indication of a real increase in the incidence of acute diverticulitis in the population was the significant increase of the most severe forms of the disease, acute diverticulitis with perforation and purulent or faecal peritonitis. In our area, such a serious condition would result in referral to hospital, and it was also so during the early periods of the study. The incidence rate of perforated diverticulitis with peritonitis increased by a factor of 3.2 from 1988–92 to 2008–12; this compared well with the general increase of acute diverticulitis by a factor of 2.9. We expect that the change in incidence of perforated acute diverticulitis corresponds to a general increase in incidence of acute diverticulitis. On the other hand, the increase of diverticular perforations might partly be due to an increased use of NSAIDS, which is known to be associated with perforated diverticular disease [12].

Weaknesses and limitations of the study

In a prospective study, a set of diagnostic tests and criteria can be implemented to ensure the diagnosis. In this retrospective study, we were not able to demand CT scan or other tests to confirm the diagnosis for every patient. Some cases might be missing if their discharge diagnosis codes had been intra-abdominal abscess or peritonitis, without also adding the diagnosis code for colon diverticulitis. We were not aware of a change in admission policy for acute diverticulitis during the study period. If more patients had been treated at home early in the study period, this might have affected the outcome of the study. Patients from this area were not included in the study if they had been treated for acute diverticulitis at another hospital while, for example, travelling. Likewise, patients who did not live here and were admitted for an acute abdominal condition at our hospital were included in the study.

Strengths of the study

Our understanding of this disease [36] has been improved by the recent, extensive reports that have based their data on administrative data. The present study validated the administrative data by examining the case records of every patient, thus excluding patients with diverticular disease who did not have acute colonic diverticulitis.

Levanger Hospital serves a localized area with a relatively stable population, which makes this area suitable for epidemiological studies [37]. Patients with acute abdominal conditions, like acute diverticulitis, are admitted to one hospital that has had continuity in admission policy for acute diverticulitis during the 25 years of study. Because the patients in this series were recruited from a long period of time, this allowed assessment of trends, which might be difficult to discover within a shorter observation period. Very few studies have addressed this disease over such an extended period of time.