What is acute diverticulitis and what causes it?

What is Acute Diverticulitis and What Causes It?

Acute diverticulitis is inflammation and/or infection of colonic diverticula (abnormal outpouchings of the intestinal wall), occurring when diverticula that were previously asymptomatic become inflamed. 1, 2, 3

Definition and Pathophysiology

Acute diverticulitis develops when pre-existing diverticula—which are outpouchings originating from the intestinal lumen—become inflamed and potentially infected. 3 The condition represents a progression from diverticulosis (the mere presence of diverticula without inflammation) to an active inflammatory state. 1, 2

The infectious process in acute diverticulitis involves the colonic microbiota, requiring coverage for Gram-positive and Gram-negative bacteria, as well as anaerobes. 1 Most cases represent community-acquired infections. 1

Epidemiology and Incidence

• The annual incidence in the United States is approximately 180 cases per 100,000 persons, resulting in approximately 200,000 hospital admissions annually. 1, 2, 3
• Healthcare costs exceed $6.3 billion annually in the United States. 3
• Only 1-4% of patients with diverticulosis will develop acute diverticulitis in their lifetime, though an estimated 5-10% develop the condition according to other sources. 1, 2, 3
• The incidence in younger adults (ages 40-49) increased by 132% from 1980 through 2007. 2
• Left-sided colonic diverticulitis predominates in Western countries, while right-sided diverticulitis is more common in Asian populations. 1

Risk Factors and Causes

Multiple modifiable and non-modifiable factors predispose patients to developing acute diverticulitis: 3

Non-Modifiable Risk Factors:

• Age older than 65 years 3
• Genetic factors, including variants in the tumor necrosis factor superfamily member 15 (TNFSF15) gene 3
• Connective tissue diseases such as polycystic kidney disease, Marfan syndrome, or Ehlers-Danlos syndrome 3

Modifiable Risk Factors:

• Body mass index ≥30 kg/m² (obesity) 3, 4
• Smoking 3, 4
• Use of opioids, corticosteroids, and nonsteroidal anti-inflammatory medications 3, 5
• Hypertension 3
• Type 2 diabetes 3
• Lack of physical activity 5, 4
• Diet (low fiber intake) 5, 4

Clinical Classification

Acute diverticulitis is classified into two main categories based on the extent of disease: 1

Uncomplicated Diverticulitis (approximately 85-88% of cases):

• The infection only involves the colon and does not extend to the peritoneum 1
• CT findings show diverticula, thickening of the colon wall, and increased density of the pericolic fat 1, 2, 3

Complicated Diverticulitis (approximately 12-15% of cases):

• The infectious process proceeds beyond the colon 1
• Includes abscess formation, phlegmon, fistula, obstruction, bleeding, or perforation 1, 3, 4
• Subdivided by the WSES classification into stages based on abscess size, presence of distant gas, and extent of peritoneal involvement 1

Clinical Presentation

The most common presenting symptom is acute or subacute abdominal pain, typically located in the left lower quadrant. 2 Other manifestations include:

• Fever 2, 4
• Changes in bowel habits 2
• Nausea and vomiting 3, 4
• Abdominal tenderness and distention 4
• Anorexia, constipation, diarrhea, and dysuria 4
• Elevated white blood cell count and/or C-reactive protein (CRP) 1, 2

Clinical diagnosis alone lacks accuracy, with positive and negative predictive values of only 0.65 and 0.98, respectively. 1 A clinical decision rule suggests high probability of diverticulitis when all three criteria are present: (1) direct tenderness only in the left lower quadrant, (2) CRP >50 mg/L, and (3) absence of vomiting. 1

Prognosis

• Mortality following medical therapy is approximately 1%, rising to 4% in-hospital mortality for those requiring surgery 5
• Postoperative mortality is 0.5% for elective colon resection and 10.6% for emergent colon resection 3
• Recurrence rates following medical management vary from 13% to 36% 5
• The risk of complicated diverticulitis is highest with the first presentation 2
• Approximately 5% of patients may experience smoldering diverticulitis, characterized by persistent abdominal pain and continued inflammation 2