Diverticulitis doesn’t have a dramatic visible appearance from the outside. Your abdomen may look swollen or distended during a flare, and the lower left side will likely feel tender and firm to the touch. But the real visual story of diverticulitis plays out inside the body, on imaging scans, through a scope, and in changes to your stool. Here’s what it looks like at every level.
What You Can See From the Outside
During an acute episode, most people notice their belly looks bloated or puffy, particularly on the lower left side. In more severe cases, the inflamed section of colon can swell enough that you or a doctor can actually feel it through the abdominal wall as a firm, tender mass. The skin itself doesn’t typically change color or develop a rash. What stands out is the rigidity: if your abdomen becomes board-like and extremely sensitive to any pressure, that signals a more serious complication like a perforation, which needs emergency care.
Diverticulosis vs. Diverticulitis
The distinction matters because a lot of people have the pouches without ever having inflammation. Diverticulosis means small, bulging pouches (called diverticula) have formed along the wall of the colon, most commonly in the sigmoid colon on the lower left. These pouches are usually painless and discovered incidentally during a colonoscopy or CT scan. They look like small, round pockets protruding outward from the colon wall.
Diverticulitis is what happens when one or more of those pouches becomes inflamed or infected. The opening of a pouch gets blocked, bacteria multiply, and the surrounding tissue swells. On imaging, the difference is obvious: diverticulosis shows clean, quiet pouches, while diverticulitis shows angry, thickened bowel wall surrounded by hazy, inflamed fat.
What It Looks Like on a CT Scan
A CT scan is the primary tool doctors use to diagnose diverticulitis, and it produces the clearest picture of what’s happening. The hallmark finding is “fat stranding,” which appears as a hazy, streaky brightness in the fat tissue surrounding the inflamed section of colon. Normally that fat looks uniformly dark on a scan. When it’s streaky and bright, it means inflammation has spread from the pouch into the surrounding tissue.
Alongside fat stranding, the colon wall itself appears thickened. In a healthy colon, the wall is thin and uniform. In diverticulitis, the affected segment swells noticeably. The degree varies: some cases show minimal thickening with faint fat stranding, while others display extensive inflammation affecting a long stretch of bowel. Two other characteristic signs radiologists look for are the “comma sign,” a pocket of fluid that collects at the base of the sigmoid colon’s supporting tissue, and the “centipede sign,” where the blood vessels feeding the area become visibly engorged and prominent.
If the inflammation has progressed to an abscess, the CT scan will show a distinct fluid collection with a bright, enhancing rim around it. These abscesses average about 2.3 cm in diameter but can range from under 1 cm to nearly 7 cm. Smaller ones (under 3 cm) often resolve with antibiotics alone, while larger ones, particularly those over 4 cm, may need to be drained with a needle guided by CT imaging. Some abscesses also contain pockets of trapped air, which appear as bright white spots within the fluid collection.
What It Looks Like Through a Colonoscope
Colonoscopy isn’t typically used to diagnose an active flare because inflating the colon with air during acute inflammation carries risk. But when a scope does capture inflamed diverticula, the appearance is striking. The tissue around the affected pouch looks red and swollen, a combination doctors describe as erythema and edema. Small pinpoint hemorrhages (petechiae) may dot the surface. In more advanced cases, a thick, yellowish discharge of mucus and pus can be seen oozing from a perforated pouch.
After an acute episode resolves, a follow-up colonoscopy is often recommended to rule out colon cancer, which can mimic diverticulitis on a CT scan. At that point, the scope typically shows the diverticula themselves as small, dark openings in the colon wall, sometimes with residual scarring or narrowing from prior inflammation.
Changes in Your Stool
Your bowel movements can change visibly during a diverticulitis flare. Many people notice their stools become narrower or more ribbon-like, because the inflamed, swollen colon creates a tighter passage. Mucus may appear on or mixed into the stool. Some people develop constipation, while others swing toward loose stools or diarrhea.
The most alarming visual change is blood. Diverticular bleeding, which can happen with or without active inflammation, tends to produce sudden, significant amounts of blood in the stool. The blood may appear as bright red clots or darker red, depending on how quickly it moves through the colon. This type of bleeding is distinct from the small streaks you might see with hemorrhoids. It’s heavy enough that it’s hard to miss, and it warrants prompt medical attention.
A low-fiber diet contributes to both the formation of diverticula and the symptoms of flares. Without enough fiber, the colon has to squeeze harder to move small, hard stool through, which increases pressure inside the colon and can worsen existing pouches.
What Surgeons See During an Operation
Most diverticulitis episodes resolve without surgery, but severe or complicated cases sometimes require it. When surgeons open the abdomen or use a camera (laparoscopy), the inflamed sigmoid colon is often wrapped in a layer of fatty tissue called the omentum, which the body drapes over the area like a biological bandage to contain the infection. The affected colon segment looks thickened, reddened, and stiff compared to healthy bowel.
In the most serious cases, a perforated pouch leaks intestinal contents into the abdominal cavity. If the leak is contained, surgeons may find a sealed-off pocket of pus (a localized abscess) nestled in the pelvis. If it’s not contained, the abdominal cavity can fill with either purulent (pus-filled) or feculent (stool-contaminated) fluid. Feculent contamination, classified as Hinchey stage IV, is the most dangerous scenario and requires emergency surgery. A fibrous, yellowish coating of inflammatory material may line the surfaces of nearby organs and the abdominal wall.
Under a microscope, tissue removed during surgery shows thickening of the inner colon lining, depletion of the mucus-producing cells, and sometimes small abscesses within the gland structures of the colon wall. These microscopic changes help pathologists confirm the diagnosis and distinguish diverticulitis from other inflammatory conditions like Crohn’s disease.