Crohn’s disease has a distinctive appearance on colonoscopy: patchy areas of inflamed, ulcerated tissue separated by stretches of completely normal-looking bowel. This “skip pattern” is one of the most recognizable features and helps distinguish Crohn’s from other inflammatory bowel conditions. What your gastroenterologist sees through the scope, and where they see it, plays a major role in confirming a diagnosis, grading severity, and tracking how well treatment is working.
The Skip Pattern
The hallmark of Crohn’s on colonoscopy is discontinuous inflammation. Instead of a continuous sweep of redness and swelling, the scope reveals inflamed segments with healthy tissue in between. These are called skip lesions. In one study of over 200 Crohn’s patients who underwent imaging, about 25% had clearly identifiable skip lesions in the small bowel alone. This patchy distribution can appear anywhere from the mouth to the anus, though the colonoscope typically evaluates the colon and the very end of the small intestine.
This stands in sharp contrast to ulcerative colitis, where inflammation is uniform and continuous, almost always starting at the rectum and extending upward without gaps. In Crohn’s, the rectum is often spared entirely, and when the colon is involved, it tends to favor the right side. Recognizing this pattern is one of the first steps in telling the two conditions apart.
What the Ulcers Look Like
The earliest visible sign of Crohn’s is usually aphthous ulcers: small, shallow sores surrounded by a halo of redness on otherwise normal-looking tissue. They resemble canker sores and can be as small as a few millimeters. In mild disease, a gastroenterologist might see fewer than five of these scattered across the terminal ileum (the last section of the small intestine before it connects to the colon).
As disease progresses, these small ulcers deepen and connect. Linear ulcers form along the length of the bowel, running in long tracks. When linear ulcers intersect with each other, the surrounding swollen tissue bulges up between them, creating what’s described as a cobblestone appearance. The mucosa looks like an uneven cobblestone street, with raised islands of inflamed tissue separated by deep crisscrossing grooves. This pattern is considered highly characteristic of Crohn’s and is rarely seen in other conditions.
Unlike ulcerative colitis, where inflammation stays in the superficial lining, Crohn’s ulcers can penetrate deep into the bowel wall. The scope can’t see through the full wall thickness, but the depth and sharpness of the ulcers give clues. Deep, knife-like fissures suggest the inflammation extends transmurally, meaning through all layers of the intestine.
Why the Terminal Ileum Gets Special Attention
The terminal ileum is the single most common location for Crohn’s disease. This segment sits at the junction between the small and large intestine and contains dense immune tissue that makes it particularly vulnerable. During a colonoscopy for suspected Crohn’s, the gastroenterologist will almost always advance the scope past the colon and into this area.
In the terminal ileum, Crohn’s typically shows aphthous or linear ulcerations and the classic cobblestone pattern. Even when the colon looks normal, findings in the terminal ileum alone can point toward a diagnosis. Biopsies are routinely taken here, and sometimes from normal-appearing areas too, because microscopic inflammation can exist before it becomes visible.
What Biopsies Reveal
During the colonoscopy, your gastroenterologist will take small tissue samples from both inflamed and healthy-looking areas. Under the microscope, pathologists look for a specific structure called a granuloma: a tight cluster of immune cells that forms when the body walls off something it can’t eliminate. These non-caseating granulomas (meaning they don’t have the dead-center tissue seen in tuberculosis) are considered a hallmark of Crohn’s, but they’re found less often than many people assume.
At the time of initial diagnosis, only about 25% of Crohn’s patients have granulomas in their biopsies. Over time, with repeated biopsies, that number rises to around 37%. So the absence of granulomas doesn’t rule out Crohn’s. Pathologists also look for signs of chronic inflammation, focal damage patterns, and involvement that extends deeper than the surface lining, all of which support the diagnosis even without granulomas.
How Doctors Score What They See
Gastroenterologists don’t just describe Crohn’s in general terms. They assign a formal severity score based on what the scope reveals. The most widely used tool is the Simple Endoscopic Score for Crohn’s Disease (SES-CD), which evaluates five bowel segments: the ileum, right colon, transverse colon, left colon, and rectum.
In each segment, the doctor rates four things on a scale of 0 to 3: the size of ulcers (from none up to larger than 2 cm), the percentage of surface covered by ulcers, the overall extent of affected surface area, and whether any narrowing is present. The scores are added up across all segments, producing a total between 0 and 56. A score under 3 with no visible ulcers is generally considered endoscopic remission, meaning the inflammation has resolved at the mucosal level.
For patients who have had surgery, a separate scoring system called the Rutgeerts score is used. It specifically evaluates the area around the surgical connection point and the new end of the small intestine. The scale runs from i0 (no lesions at all) to i4 (widespread inflammation with large ulcers, nodules, or narrowing). Scores of i0 or i1 suggest minimal recurrence, while i3 and i4 indicate the disease has come back aggressively. Current guidelines recommend this endoscopic check 6 to 12 months after surgery.
Strictures and Fistula Openings
Crohn’s disease causes cumulative damage over time, and the colonoscopy can reveal complications that go beyond surface inflammation. Strictures are areas where the bowel has narrowed, either from active swelling or from scar tissue that has built up over repeated cycles of inflammation and healing. On the scope, a stricture looks like a tightened passage that may be difficult or impossible to advance the camera through. Doctors classify these as single or multiple and note whether the scope can still pass through them.
Strictures shorter than 5 cm that run straight along the bowel and aren’t surrounded by additional complications are the most straightforward to manage. Longer, angled, or ulcerated strictures are more concerning and more likely to need surgical intervention. If a fistula opening is present within 5 cm of a stricture, that also changes the treatment approach significantly. Fistulas appear as small openings in the bowel wall where abnormal tunnels have formed, connecting the intestine to other organs or to the skin surface.
What Remission Looks Like
The goal of Crohn’s treatment is mucosal healing, and the colonoscopy is the primary way to confirm it. When treatment is working well, the previously ulcerated, cobblestoned tissue should look dramatically different. Remission on the scope means an SES-CD score below 3 with no visible ulcers. The bowel wall should appear thin and smooth, without the swelling, redness, or deep grooves seen during active disease.
Achieving this level of healing is associated with better long-term outcomes, including fewer hospitalizations, fewer surgeries, and longer periods of remission. This is why repeat colonoscopies are a standard part of managing Crohn’s, even when you feel well. What you experience as symptom relief doesn’t always match what the scope shows, and ongoing subclinical inflammation can silently cause damage that leads to strictures or other complications down the road.