Acute diverticulitis is a sudden inflammation of small pouches (diverticula) that form in the wall of the colon, most often in the lower left side. It happens when one or more of these pouches develops a tiny tear, allowing bacteria to escape through the colon wall and trigger an inflammatory response. About 65% of people in Western countries have these pouches by age 85, though fewer than 10% of people under 40 do, and only a fraction of those with pouches ever develop an inflamed episode.
How Diverticula Form and Become Inflamed
Diverticula are small, balloon-like bulges that push outward through weak spots in the colon wall. Having them is called diverticulosis, and it’s extremely common with age. Most people never know they have them.
The trouble starts when one of these pouches gets blocked, often by a small piece of stool. That blockage causes irritation, reduced blood flow to the tissue, and eventually a micro-perforation, a tiny breach in the colon wall. Bacteria that normally live harmlessly inside the colon can then cross that barrier, setting off inflammation and sometimes a full-blown infection. In milder cases, nearby tissue or organs seal off the tiny hole on their own. In more serious cases, the perforation can lead to abscesses, widespread infection in the abdominal cavity, or other complications.
The underlying reasons some people develop diverticulitis and others don’t involve a mix of factors. Changes in the gut microbiome, specifically a drop in beneficial bacteria that produce short-chain fatty acids and an increase in more aggressive bacteria, can weaken the colon’s mucosal barrier and make inflammation more likely.
Common Symptoms
Left lower abdominal pain is the hallmark symptom, showing up in about 70% of cases. The pain is typically constant rather than crampy, and it can range from moderate to severe. In a small number of people, particularly those of Asian descent, the pain may instead localize to the right lower abdomen.
Beyond the pain, you may experience fever, nausea, vomiting, bloating, gas, or a noticeable change in bowel habits, often constipation. On physical examination, your abdomen may be tender or distended, and sometimes a firm, tender mass can be felt. More severe cases involving perforation or widespread infection can produce signs of sepsis: high fever, rapid heart rate, and significant abdominal rigidity.
What Raises Your Risk
Age is the single biggest factor. The pouches themselves become far more common as you get older, and so do flare-ups. Obesity and smoking are also strongly linked to diverticulitis. Red meat consumption in a Western-style diet has been associated with a higher incidence of episodes, while regular physical activity appears to reduce the risk of complications.
Genetics play a surprisingly large role. Research suggests that 40 to 50% of the susceptibility to diverticular disease is inherited. Certain connective tissue disorders, including Ehlers-Danlos syndrome and Marfan syndrome, carry an elevated risk because they affect the structural integrity of the colon wall.
Some medications can also contribute. NSAIDs and aspirin are associated with diverticular bleeding. Steroids and opiates may promote the formation of additional diverticula. If you regularly take any of these, that’s worth discussing with your doctor in the context of your digestive health.
How It’s Diagnosed
Doctors suspect acute diverticulitis based on your symptoms: constant abdominal pain (usually left-sided), fever, and tenderness on examination. But the gold standard for confirming the diagnosis is a contrast CT scan. The scan can show the inflamed diverticula, wall thickening, and any complications like abscesses or perforation. It’s also essential for ruling out other conditions that mimic diverticulitis, such as appendicitis, ovarian problems, or colon cancer.
If a CT scan with contrast dye isn’t an option for you, alternatives include a non-contrast CT, an MRI, or an ultrasound. Blood tests showing elevated inflammatory markers support the diagnosis but can’t confirm it on their own.
Uncomplicated vs. Complicated Disease
Doctors classify acute diverticulitis into two broad categories, and the distinction matters because it determines your treatment.
Uncomplicated diverticulitis means inflammation without perforation, abscess, or other structural damage. This is the majority of cases. Complicated diverticulitis involves one or more of those problems and is graded using a staging system originally developed by surgeon Earl Hinchey. In simplified terms:
- Stage I: Inflammation confined to the area around the diverticulum, possibly with a small, localized abscess.
- Stage II: A larger abscess extending into the pelvis or abdomen.
- Stage III: A ruptured abscess causing pus to spread throughout the abdominal cavity (purulent peritonitis).
- Stage IV: A free perforation spilling stool into the abdominal cavity (fecal peritonitis), the most dangerous scenario.
Potential Complications
Abscesses are the most common complication, detected in up to 30% of acute diverticulitis cases. These are walled-off collections of infected fluid that can form near the colon or deeper in the pelvis. Smaller abscesses often resolve with antibiotics alone, while larger ones typically need to be drained.
Fistulas, abnormal connections between the colon and a neighboring organ, develop in about 14% of cases. The most common type connects the colon to the bladder, which can cause recurrent urinary tract infections or air in the urine. Fistulas can also form between the colon and the small intestine, uterus, or skin.
Bowel obstruction occurs in 10 to 20% of cases, usually partial rather than complete. Repeated episodes of inflammation cause scarring and thickening of the colon wall, which can gradually narrow the passageway. Severe obstruction is rare during an acute episode but can develop over time in people with chronic or recurrent disease.
Treatment for Uncomplicated Cases
If you have uncomplicated diverticulitis and are otherwise healthy, you may not need antibiotics at all. Current guidelines from both NICE and European gastroenterology societies recommend against antibiotics for uncomplicated cases in people who are not immunocompromised and don’t appear systemically unwell. Evidence shows no significant benefit in recovery time or complication rates from routine antibiotic use in these patients.
Antibiotics are reserved for people who are systemically unwell (fever, elevated heart rate, signs of spreading infection), have weakened immune systems, or have complicated disease with abscesses or perforation.
For mild cases managed at home, the initial approach focuses on giving your colon a break. Your doctor may recommend a clear liquid diet for a few days, including broths, plain gelatin, and clear juices. Most people start feeling noticeably better within two to three days. From there, you gradually reintroduce low-fiber foods as tolerated, then slowly work back up to a high-fiber diet over several weeks.
When Surgery Becomes Necessary
Most episodes of acute diverticulitis resolve without surgery. Emergency surgery is needed in specific, serious situations: free perforation with signs of peritonitis, failure of conservative treatment with worsening infection or sepsis, or uncontrollable diverticular bleeding that can’t be managed with other interventions.
Elective surgery, planned in advance and performed after the acute episode resolves, may be recommended if you develop a fistula to the bladder or other organs, a significant narrowing of the colon causing symptoms, or recurrent episodes that substantially reduce your quality of life. The procedure typically involves removing the affected segment of the sigmoid colon.
Surgery is not recommended for a first episode of uncomplicated diverticulitis, for small abscesses that respond to conservative treatment, or for diverticulosis that has never caused symptoms.