Diverticulosis looks like small, round pouches or pockets bulging outward from the colon wall. Each pouch, called a diverticulum, typically measures 3 to 10 mm in diameter, roughly the size of a pea. Most people first see them described on a colonoscopy or CT scan report, and the natural question is: what am I actually looking at?
What Diverticula Look Like During a Colonoscopy
During a colonoscopy, diverticula appear as small, dark openings in the colon wall. Imagine looking at the inside of a pink tube and seeing scattered holes or dimples, each one the mouth of a small pouch that extends outward through the muscle layer. The openings are typically round or oval, and the surrounding tissue looks the same healthy pink as the rest of the colon lining. In an uncomplicated case, there’s no redness, swelling, or discharge. The pouches themselves sit on the outside of the colon, so the camera only captures the openings, not the full sac.
Some people have just a handful of these openings scattered along a short stretch of colon. Others have dozens spread across a longer segment. The number and density vary widely, but they almost always cluster in the sigmoid colon, the S-shaped section in the lower left part of your abdomen. This is the narrowest segment of the large intestine, where pressure from stool is highest and the wall is most prone to bulging.
What They Look Like on a CT Scan
On a CT scan, diverticula show up as small outpouchings along the outer edge of the colon wall. They look like tiny bubbles or bumps protruding from the colon’s surface, filled with air, stool, or both. Radiologists can spot them easily because they break the normally smooth outer contour of the bowel.
A healthy, relaxed colon wall measures up to about 3 mm thick. When diverticula are present without any inflammation, the wall thickness stays in that normal range. If the wall appears thicker than 3 mm in a relaxed segment, it can signal that inflammation (diverticulitis) has developed. One specific sign radiologists look for on CT is the “arrowhead sign,” where a small pocket of fluid gets trapped in the swollen neck of a diverticulum, creating a shape that points inward like an arrowhead. That sign suggests early inflammation even before symptoms become severe.
The Anatomy Behind the Appearance
Despite looking like simple bumps or holes, diverticula have a specific structure. The colon wall has several layers: an inner lining (mucosa), a middle cushioning layer (submucosa), a thick muscle layer, and a thin outer covering. In diverticulosis, the inner two layers push through weak spots in the muscle layer, forming a small sac that sits just outside the colon. The thin outer covering stretches over the pouch but the muscle layer does not surround it. This is why they’re technically called “false” diverticula, or pseudodiverticula. They don’t contain all the layers of a true outpouching.
The weak spots where diverticula form aren’t random. They occur where small blood vessels penetrate the muscle layer to supply blood to the inner lining. These natural gaps in the muscle create points of vulnerability, especially when pressure inside the colon pushes outward repeatedly over years.
How Size and Number Vary
Most diverticula fall in the 3 to 10 mm range. At the small end, they’re barely noticeable during a colonoscopy, just a subtle dimple in the wall. At 10 mm, they’re more obvious, with a clearly visible opening and a palpable sac on the outside. In extremely rare cases, a “giant diverticulum” can exceed 4 cm (about 1.5 inches), though this is unusual enough to warrant special attention.
The number of pouches tends to increase with age. Someone in their 40s might have a few small ones in the sigmoid colon. By their 70s or 80s, the same person could have extensive diverticulosis involving much of the left colon. The condition is progressive, but having more diverticula doesn’t necessarily mean more symptoms or higher risk of complications.
What Complications Look Like
Uncomplicated diverticulosis is visually unremarkable. The concern is what it looks like when something goes wrong.
When a diverticulum becomes inflamed (diverticulitis), the tissue around the opening turns red and swollen. A colonoscopy might show redness and puffiness at the mouth of the pouch, or even pus draining from the opening. In some cases, a mass of inflamed granulation tissue forms at the diverticular opening, creating a small, raised bump that can initially be mistaken for a polyp.
When a diverticulum bleeds, the visual signs during colonoscopy range from active spurting of blood from the pouch opening to subtler clues: a visible blood vessel at the base, a blood clot stuck to the opening that won’t wash away, or a dark flat spot marking where recent bleeding occurred. Diverticular bleeding is the most common cause of significant lower intestinal bleeding in older adults, and identifying the exact pouch responsible is one of the main goals of an urgent colonoscopy.
Why You Might Never See It Yourself
Most people with diverticulosis never know they have it until a colonoscopy or imaging study done for another reason reveals the pouches incidentally. About 80% of people with diverticulosis never develop symptoms. The condition is so common after age 60 that it’s often treated as a normal finding on a report rather than a diagnosis requiring action. If your colonoscopy report mentions diverticulosis, what you’re picturing is accurate: small, quiet pockets in the colon wall, sitting there uneventfully in most cases.