Crohn’s Disease Overview

Crohn’s disease is a long‑term inflammatory condition that can affect any part of the digestive tract, most often the last part of the small intestine and the colon. Inflammation is patchy and can extend through the full thickness of the bowel wall. This overview explains likely causes, where and how Crohn’s shows up, and common complications to watch for over time. (niddk.nih.gov)

Key takeaways

• Causes are multifactorial, involving genes, the immune system, the gut microbiome, and environment. Diet and stress do not cause Crohn’s. (niddk.nih.gov)

• Inflammation is patchy and transmural, which helps explain strictures, fistulas, and abscesses. (ncbi.nlm.nih.gov)

• Crohn’s has distinct locations and behavior patterns; behavior can change from inflammatory to stricturing or penetrating over time. (radiopaedia.org)

• Smoking increases the chance of Crohn’s developing and worsens the disease course, including surgery risk. (niddk.nih.gov)

• Blood clots, kidney stones, gallstones, and malnutrition are important extra‑intestinal risks in Crohn’s. (pmc.ncbi.nlm.nih.gov)

What causes Crohn’s disease

Crohn’s arises from an interaction of factors rather than a single cause. Researchers point to an abnormal immune reaction that targets normal gut microbes, in people with susceptible genes, living in environments that influence the microbiome. Diet and stress may aggravate symptoms, but they do not cause Crohn’s. Smoking roughly doubles the chance of developing the disease. (niddk.nih.gov)

Specific genetic variants, including changes in NOD2, ATG16L1, and IL23R, modestly shift risk. No single gene predicts who will get Crohn’s or how severe it will be. Genes interact with exposures such as antibiotics and early‑life factors that shape the microbiome. (pubmed.ncbi.nlm.nih.gov)

Where Crohn’s happens and how it behaves

Crohn’s can involve the mouth to the anus, but it most often affects the terminal ileum and right colon. Inflammation is patchy, with “skip lesions,” and extends through the full bowel wall, not just the lining. These features help distinguish Crohn’s from ulcerative colitis. (ncbi.nlm.nih.gov)

Clinicians describe Crohn’s by location and behavior using the Montreal classification. Locations are ileal (L1), colonic (L2), ileocolonic (L3), and an upper‑gut modifier (L4). Behavior patterns are inflammatory (B1), stricturing (B2), and penetrating (B3), with a perianal modifier (P). These patterns guide testing and treatment choices. (radiopaedia.org)

Behavior can evolve. Many people start with inflammatory disease, and a substantial share develop stricturing or penetrating complications over time. In a population‑based cohort, about one in three developed stricturing or penetrating disease within 5 years, and roughly half did over two decades. Ileal involvement increased the risk of progression. (pmc.ncbi.nlm.nih.gov)

Common complications and what they mean

Transmural, patchy inflammation drives most Crohn’s complications.

• Strictures: Scar‑related narrowing can cause cramping, bloating, and vomiting. Long‑standing strictures may require endoscopic dilation or surgery. (ncbi.nlm.nih.gov)

• Penetrating disease: Deep ulcers can form fistulas (abnormal connections) or abscesses. Fistulas may connect bowel to skin, bladder, or other bowel. Management often combines antibiotics, drainage or setons, and biologic therapy. (gastro.org)

• Perianal disease: Up to one in five adults develops perianal fistulas or abscesses. Best care pairs surgical control of infection with effective medical therapy, often anti‑TNF agents. (gastro.org)

• Malnutrition and micronutrient deficiencies: Active inflammation, reduced intake, and prior surgery increase risks for iron and vitamin D deficiency, among others. Monitoring and dietitian support help prevent anemia and bone loss. (pubmed.ncbi.nlm.nih.gov)

• Kidney stones and gallstones: Ileal inflammation or resection alters bile acid and fat absorption, raising risks for calcium oxalate stones and gallstones. Hydration and tailored diet can help. (ncbi.nlm.nih.gov)

• Blood clots (venous thromboembolism): Risk is about threefold higher than the general population, especially during flares and hospital stays. Inpatients should receive clot‑prevention measures unless contraindicated. (pmc.ncbi.nlm.nih.gov)

• Cancer risks: Long‑standing colonic Crohn’s increases colorectal cancer risk, so people with significant colonic involvement need surveillance colonoscopy on a schedule set by risk factors. Small bowel adenocarcinoma is rare but more common in Crohn’s with small‑bowel involvement. Routine screening for small‑bowel cancer is not recommended. (pubmed.ncbi.nlm.nih.gov)

Complications at a glance

Factors that raise or lower complication risk

• Smoking clearly worsens Crohn’s, increasing flares and the need for surgery. Avoid secondhand smoke as well. Help with quitting is part of good IBD care. (pubmed.ncbi.nlm.nih.gov)

• Nonsteroidal anti‑inflammatory drugs (NSAIDs) can aggravate bowel inflammation and symptoms in some people. Discuss safer pain options. (mayoclinic.org)

• Early, effective control of inflammation lowers the chance of strictures, fistulas, hospitalizations, and steroid dependence. Modern guidelines emphasize appropriate use of biologics for moderate to severe luminal and perianal Crohn’s. (gastro.org)

How this hub fits

• Locations and behavior: See “Where Crohn’s Happens: Locations Behavior (Montreal Classification).” (radiopaedia.org)

• Symptoms: See “Common Symptoms of Crohn’s.”

• Perianal disease: See “Perianal Crohn’s (Fistulas, Abscesses, Skin Tags).” (gastro.org)

• Monitoring and cancer prevention: See “Monitoring Crohn’s Over Time” and “Cancer Prevention in IBD.” (pubmed.ncbi.nlm.nih.gov)

FAQs

Is Crohn’s curable with surgery

No. Surgery treats complications or removes damaged segments, but inflammation can return. After surgery, medicines are often used to reduce recurrence. (cris.maastrichtuniversity.nl)

Does everyone with Crohn’s get strictures or fistulas

No. Risks vary by location and disease activity. Over time, a significant minority develop stricturing or penetrating disease, especially with ileal involvement. (pmc.ncbi.nlm.nih.gov)