Crohn’s disease has a distinctive visual signature, both inside and outside the body. Internally, it creates a patchwork of inflamed and healthy tissue that doctors can spot during colonoscopy or on imaging scans. Externally, it can show up as skin lesions, mouth sores, and changes around the anus. What you see depends on where you look and how far the disease has progressed.
Inside the Gut: Cobblestoning and Skip Lesions
The most recognizable feature of Crohn’s during a colonoscopy is the “cobblestone” appearance of the intestinal lining. Deep ulcers crisscross the surface while swollen tissue pushes up between them, creating a pattern that genuinely resembles a cobblestone road. This happens as inflammation and swelling beneath the surface worsen and carve shallow grooves, leaving raised nodules across a continuous stretch of bowel.
Unlike ulcerative colitis, which inflames the colon in one unbroken stretch, Crohn’s skips around. Patches of raw, ulcerated tissue sit right next to sections of perfectly normal-looking mucosa. These are called skip lesions, and they’re one of the clearest visual clues that a doctor is looking at Crohn’s rather than another inflammatory bowel disease. The ulcers themselves tend to have sharp, well-defined borders and often run lengthwise along the intestine in parallel lines. Multiple ulcers can appear side by side, and as the disease progresses over years, they typically deepen.
Roughly 45% of people with Crohn’s have disease in both the small bowel and colon, about 30% have it only in the small bowel (especially the last section, the terminal ileum), and around 20% have it only in the colon. So the location of these visual findings varies widely from person to person.
What Imaging Scans Reveal
On CT scans and MRI, the hallmark of Crohn’s is a thickened bowel wall. Normal bowel wall measures about 2 mm thick. In Crohn’s, half of patients with colon involvement have wall thickness exceeding 5 mm, which helps distinguish it from ulcerative colitis, where the wall rarely gets that thick. Imaging also picks up fat stranding, a hazy brightness in the tissue surrounding the inflamed bowel, and increased blood flow to the intestinal wall during active flares.
One classic finding on older barium X-rays, still referenced today, is the “string sign of Kantor.” A section of intestine narrows so dramatically from inflammation, spasm, and scarring that only a thin trickle of contrast dye passes through, looking like a frayed cotton string on the image. This represents a stricture, where the bowel has tightened enough to partially block the passage of food.
Strictures come in two forms that look different on imaging. Inflammatory strictures show bright enhancement on contrast MRI and increased blood flow on ultrasound, signals that the narrowing comes from active swelling that medication can potentially reverse. Fibrotic strictures appear darker on MRI with minimal enhancement, indicating the tissue has scarred and hardened permanently. These often require surgery. Telling the two apart matters enormously for treatment decisions.
What Stool Looks Like
Crohn’s changes what you see in the toilet. Diarrhea is common, sometimes persistent for days or weeks. Visible mucus often appears as white or yellow streaks on the surface of stool. Blood can show up as well, though it’s typically less prominent than in ulcerative colitis. When the disease affects the lower colon or rectum, bright red blood is more likely. When it’s higher up in the small intestine, stool may simply look darker than usual.
Perianal Signs
The area around the anus is one of the most visible external indicators of Crohn’s. Perianal disease encompasses a range of findings: skin tags that are often larger and more prominent than typical ones, caused by chronic lymph fluid swelling (lymphedema). Anal fissures, which are small tears in the skin lining the anus, are common. Deep ulcers can develop inside the anal canal itself.
Fistulas are another telltale sign. These are abnormal tunnels that form between the intestine and the skin surface near the anus, sometimes draining pus or mucus through small openings. Abscesses, which are painful, swollen pockets of infection, can develop alongside them. In some cases, the anal canal narrows into a stricture, making bowel movements difficult and painful. These perianal changes sometimes appear before any gut symptoms do, making them an early visual clue.
Mouth and Oral Changes
Crohn’s can affect the mouth, and in some cases oral symptoms are the first sign of the disease. The most common oral finding is aphthous ulcers: small, shallow sores with white centers and red halos, often clustered on the soft palate, inner cheeks, and the folds where the gums meet the cheeks. These look similar to ordinary canker sores but tend to be more persistent and numerous.
A more distinctive finding is cobblestone papules on the inner cheeks and the tissue behind the molars. These are flesh-colored or pink bumps that cluster together, creating the same cobblestone texture seen inside the intestine. The lips, gums, and the grooves between the gums and cheeks are the most common sites. In children with Crohn’s, oral changes are especially prevalent, appearing in an estimated 50 to 80% of pediatric patients. Gum inflammation occurs in about 25% of cases, while the cobblestone papules show up in roughly 6%.
Skin Manifestations
Two skin conditions are closely linked to Crohn’s and have very different appearances. Erythema nodosum produces tender, red or purple bumps typically on the shins. They look like deep bruises and feel firm and warm to the touch. These tend to flare alongside intestinal symptoms.
Pyoderma gangrenosum is rarer but more dramatic. It starts as a small bump that can easily be mistaken for a spider bite. Within days, it expands into a large, painful open wound with irregular, undermined edges. The legs are the most common location, though it can develop anywhere, including around surgical incision sites. Multiple sores can merge into one larger ulcer. Even after healing, pyoderma gangrenosum often leaves behind permanent scarring and patches of darkened or lightened skin.
Under the Microscope
When a pathologist examines a tissue biopsy from someone with Crohn’s, they’re looking for a specific structure: clusters of immune cells called granulomas. These are tiny, organized clumps of at least five specialized immune cells, sometimes with large multi-nucleated cells mixed in. They appear as small round collections distinct from the surrounding tissue. Up to 60% of Crohn’s patients have these granulomas in their biopsies, and their presence is considered a hallmark of the disease. Notably, finding more and larger granulomas in the deeper layers of the bowel wall is associated with the stricturing form of Crohn’s, where the intestine narrows over time.
The absence of granulomas doesn’t rule out Crohn’s. The remaining 40% of patients never show them on biopsy, which is one reason diagnosis often depends on the full picture: the visual pattern during colonoscopy, the distribution on imaging, the biopsy findings, and the clinical symptoms taken together.