Diverticulitis
Diverticulitis is inflammation or infection of diverticula—small pouches that form in the wall of the colon and push outward through weak points in the muscle layer. Having these pouches is called diverticulosis and is extremely common with age; only a minority of people with diverticulosis ever develop diverticulitis. In Western populations, diverticulitis most often affects the sigmoid colon in the lower left abdomen, producing pain, fever, and changes in bowel habits. Most episodes are uncomplicated and improve with outpatient care, while a smaller proportion involve complications such as abscess, perforation, fistula, or obstruction that require more intensive treatment. CT of the abdomen and pelvis is the primary imaging test used to confirm the diagnosis and to determine whether an episode is uncomplicated or complicated.
What is it?
Diverticula are small pouches, usually a few millimeters to a centimeter across, that form when the inner lining of the colon pushes outward through weak points in the muscular wall—typically where small blood vessels penetrate the muscle layer. The presence of these pouches is called diverticulosis. It is one of the most common structural findings in the colon, becomes progressively more common with age, and by itself causes no symptoms in most people.
Diverticulitis occurs when one or more of these pouches becomes inflamed. The traditional explanation was that a diverticulum becomes obstructed by stool, leading to bacterial overgrowth and infection. Current understanding is more nuanced and treats diverticulitis as an inflammatory process influenced by changes in the colonic microbiome, low-grade chronic inflammation of the bowel wall, altered colonic motility, and genetic susceptibility—with infection often a secondary rather than initiating event. This shift in understanding is part of why antibiotics are no longer considered mandatory for every mild episode.
The distribution of diverticula varies by population. In North America and Europe, the sigmoid colon (the S-shaped segment in the lower left abdomen just above the rectum) is involved in the large majority of cases, which is why left lower quadrant pain is the classic presentation. In East Asian populations, right-sided diverticula are considerably more common, and diverticulitis in those patients can produce right lower abdominal pain that closely mimics appendicitis.
Risk factors associated with developing diverticulitis include increasing age, a diet low in fiber and high in red meat, obesity, physical inactivity, smoking, and regular use of NSAIDs (nonsteroidal anti-inflammatory drugs), aspirin, and corticosteroids. Family history and genetic factors also contribute meaningfully—twin and family studies suggest a substantial hereditary component. Immunosuppression (from transplant medications, chemotherapy, chronic corticosteroids, or chronic kidney disease) does not necessarily increase the frequency of episodes but is strongly associated with more severe presentations and a higher likelihood of needing surgery.
A long-standing piece of dietary advice—that people with diverticulosis should avoid nuts, seeds, corn, and popcorn to prevent a seed from lodging in a pouch—has not been supported by evidence. Large prospective studies have found no increased risk of diverticulitis or diverticular bleeding with these foods, and current guidance no longer recommends avoiding them. This is a frequent source of unnecessary dietary restriction.
Diverticulitis is generally divided into two broad categories, and the distinction drives nearly every management decision.
Uncomplicated diverticulitis, which accounts for the large majority of episodes, involves inflammation of the colon wall and surrounding fat without abscess, free perforation, fistula, or obstruction. Most patients with uncomplicated disease can be treated as outpatients.
Complicated diverticulitis involves one or more of the following: an abscess (a walled-off collection of infected fluid), free perforation with spillage into the abdominal cavity and resulting peritonitis, a fistula (an abnormal connection between the colon and another structure, most commonly the bladder), a stricture with bowel obstruction, or extensive phlegmon. The modified Hinchey classification is commonly used to grade severity, from a small confined pericolic abscess through to generalized purulent or fecal peritonitis.
Symptoms typically develop over hours to a few days. The most common presentation is persistent abdominal pain in the lower left quadrant that is constant rather than intermittent, often accompanied by low-grade fever, tenderness on examination, and a change in bowel habits in either direction. Nausea is common; vomiting is less so and can suggest obstruction. When the inflamed segment lies against the bladder, urinary frequency or discomfort can occur. Some patients report a several-day history of milder symptoms before the pain becomes prominent.
Symptoms that suggest a complication include pain that is worsening, spreading, or becoming generalized; high fever with chills; abdominal distension with inability to pass gas or stool; and passing air or stool in the urine or recurrent urinary tract infections, which suggest a colovesical fistula. Generalized abdominal rigidity and severe tenderness suggest peritonitis and constitute an emergency.
An important point about bleeding: diverticular bleeding and diverticulitis are separate manifestations of the same underlying anatomy and typically do not occur together. Diverticular bleeding usually presents as painless passage of a large volume of red or maroon blood, whereas diverticulitis presents with pain and inflammation without significant bleeding. Significant rectal bleeding during a painful episode should prompt consideration of alternative diagnoses.
Diagnosis rests primarily on imaging, because the clinical presentation overlaps with a number of other conditions.
CT of the abdomen and pelvis with intravenous contrast is the reference standard. Characteristic findings include the presence of diverticula, thickening of the colon wall, inflammatory stranding of the fat surrounding the colon, and, in complicated disease, a fluid collection (abscess), gas outside the bowel lumen (perforation), free fluid, an abnormal tract to an adjacent organ (fistula), or bowel dilation proximal to a narrowed segment (obstruction). CT also grades severity, guides whether drainage or surgery is needed, and identifies alternative diagnoses when the clinical picture is atypical. Oral or rectal contrast is used in some protocols to improve delineation of the bowel.
Ultrasound can identify colonic wall thickening and inflamed diverticula in experienced hands and is often used as a first test in younger patients and where radiation avoidance is a priority, though it is more operator-dependent and less reliable for detecting deep abscesses or small perforations.
MRI provides comparable information to CT without ionizing radiation and is the preferred cross-sectional test during pregnancy when ultrasound is inconclusive. It is used less often than CT in routine practice because of availability and scan time.
Blood tests typically include a complete blood count (which may show an elevated white blood cell count), C-reactive protein (which correlates reasonably well with severity and is useful for monitoring), and kidney and liver function. Urinalysis helps evaluate for urinary tract infection and may show abnormalities when the inflamed colon lies adjacent to the bladder or ureter. Pregnancy testing is appropriate in patients of childbearing potential.
Colonoscopy is generally avoided during an acute episode because insufflation of the inflamed segment carries a risk of perforation. It is instead performed after the episode has resolved—typically 6 to 8 weeks later—in patients who have not had a recent adequate colonoscopy, primarily to exclude an underlying colon cancer, which can produce imaging findings that closely resemble diverticulitis. Contemporary evidence suggests the yield is meaningfully higher after complicated episodes than after simple uncomplicated ones, and guidelines increasingly individualize this recommendation rather than applying it universally.
The differential diagnosis includes colon cancer (which can mimic diverticulitis both clinically and on CT), inflammatory bowel disease, infectious colitis, ischemic colitis, irritable bowel syndrome, appendicitis (particularly with right-sided diverticulitis or a long redundant sigmoid), epiploic appendagitis, and, in women, gynecologic conditions including ovarian cysts, torsion, tubo-ovarian abscess, and ectopic pregnancy. Urologic conditions such as ureteral stones can also present similarly.
Important to Know
Management of diverticulitis has changed substantially over the past decade, and several long-standing practices have been revised in light of newer evidence. Care is typically coordinated by primary care clinicians and gastroenterologists, with colorectal surgeons and interventional radiologists involved when complications are present.
The most significant change concerns antibiotics. Multiple randomized trials and subsequent guidelines from major gastroenterology and surgical societies now support selective rather than routine antibiotic use in immunocompetent patients with mild, uncomplicated diverticulitis confirmed on imaging. These patients can often be managed with observation, adequate hydration, oral intake as tolerated, and analgesia, with close follow-up and a clear plan for escalation if symptoms worsen. Antibiotics remain appropriate for patients who are immunosuppressed, frail, or pregnant; who have significant comorbidities, sepsis, or high fever; who have complicated disease; or who fail to improve with observation alone. Where antibiotics are used in outpatients, regimens cover gram-negative and anaerobic organisms—commonly amoxicillin-clavulanate, or a fluoroquinolone combined with metronidazole in patients who cannot take penicillins.
Outpatient management is appropriate for most patients with uncomplicated disease who can tolerate oral intake, have adequate pain control, have reliable follow-up, and do not have significant comorbidity or immunosuppression. A clear liquid or low-residue diet is often recommended initially with advancement as symptoms improve, though evidence for strict dietary restriction during an episode is limited.
Hospitalization with intravenous fluids, intravenous antibiotics, and bowel rest is appropriate for patients with complicated disease, significant systemic illness, inability to tolerate oral intake, uncontrolled pain, immunosuppression, or failure of outpatient management.
Abscesses are managed according to size and clinical response. Small pericolic abscesses often resolve with antibiotics alone. Larger abscesses—commonly using a threshold in the range of 3 to 4 cm, though this is individualized—are typically treated with image-guided percutaneous drainage by an interventional radiologist, which frequently allows the acute episode to settle without emergency surgery and permits any subsequent operation to be performed electively under better conditions.
Emergency surgery is required for free perforation with generalized peritonitis, for uncontrolled sepsis, and for patients who deteriorate despite maximal medical therapy or drainage. Options include resection with primary anastomosis (with or without a protective stoma) and the Hartmann procedure (resection with an end colostomy and later reversal). Contemporary evidence supports primary anastomosis in appropriately selected patients, with a higher rate of eventual stoma-free outcome compared with the Hartmann approach. Laparoscopic lavage has been studied for purulent peritonitis but remains selectively applied.
Elective surgery after recovery is no longer recommended on the basis of episode count alone. The historical rule of resecting after two attacks has been replaced by an individualized decision that weighs the frequency and severity of episodes, the presence of complications such as fistula or stricture, ongoing symptoms between episodes, immunosuppression, the effect on quality of life, and the patient’s own preferences and surgical risk. Fistulas and symptomatic strictures generally do warrant elective resection.
Recurrence is common but not inevitable. Roughly one in five patients experiences a recurrence within about a decade after a first episode. Most recurrences are uncomplicated, and the historical belief that each subsequent episode is progressively more dangerous has not been borne out—complications are in fact most likely at the first presentation.
Longer-term prevention focuses on modifiable factors: a diet high in fiber from fruits, vegetables, and whole grains; maintaining a healthy weight; regular physical activity; smoking cessation; and limiting regular NSAID use where clinically possible. As noted above, nuts, seeds, corn, and popcorn do not need to be avoided. Evidence for mesalamine, rifaximin, and probiotics in preventing recurrence has been inconsistent, and these are not routinely recommended.
Two related conditions are worth distinguishing. Smoldering diverticulitis refers to persistent low-grade symptoms that continue after an acute episode rather than resolving, and may warrant reassessment or surgical consideration. Segmental colitis associated with diverticulosis (SCAD) is inflammation of the mucosa in a segment containing diverticula, which can resemble inflammatory bowel disease and is managed differently from acute diverticulitis.
Diverticulitis during pregnancy is uncommon but does occur. Ultrasound is generally the first imaging test, with MRI preferred when cross-sectional imaging is required, and management is coordinated between obstetrics, surgery, and gastroenterology.
Care is typically coordinated by primary care clinicians and gastroenterologists, with colorectal surgery and interventional radiology involved as needed. Imaging, laboratory, and clinical findings are interpreted alongside the patient’s symptoms, examination, and broader clinical context rather than in isolation.
Patient education plays an important role. Understanding the difference between diverticulosis and diverticulitis, the rationale for imaging, why antibiotics may reasonably be withheld in mild cases, the reason for a follow-up colonoscopy when one is recommended, which dietary restrictions are and are not supported by evidence, and the warning signs that require urgent evaluation all contribute to appropriate care and reduce unnecessary anxiety.
Red flag symptoms include severe abdominal pain that is worsening or spreading, generalized abdominal tenderness or rigidity, high fever with chills, signs of sepsis (severe illness, low blood pressure, rapid heart rate, confusion), persistent vomiting, abdominal distension with inability to pass gas or stool, significant rectal bleeding, passing air or stool in the urine, inability to keep down fluids, and rapid clinical deterioration. These warrant immediate emergency evaluation, as they may indicate perforation, peritonitis, abscess, obstruction, fistula, or sepsis.