Crohn's disease is a chronic transmural inflammatory bowel disease that can affect any segment of the digestive tract from mouth to anus, with the terminal ileum and right colon involved in roughly 70% of cases. About 3 million US adults live with inflammatory bowel disease (IBD), of whom an estimated half have Crohn's; incidence is rising fastest in newly industrializing regions of Asia, South America, and the Middle East (Ng 2017).
Crohn's disease (ICD-10: K50) is a chronic, immune-mediated inflammatory disorder characterized by transmural granulomatous inflammation that can affect any part of the gastrointestinal tract from the mouth to the anus, with discontinuous involvement known as skip lesions. The terminal ileum and proximal colon are the most common sites (ileocolonic disease, ~50% of patients), followed by isolated ileal disease (~30%) and isolated colonic disease (~20%). Inflammation extends through the full thickness of the bowel wall, which produces the disease's signature complications — fibrostenotic strictures, sinus tracts, fistulas (entero-enteric, entero-cutaneous, perianal), and abscesses — distinguishing Crohn's from ulcerative colitis, where inflammation is limited to the colonic mucosa and is continuous. Disease behavior is classified by the Montreal classification across three axes: age at diagnosis (A1 below 16, A2 17-40, A3 above 40), location (L1 ileal, L2 colonic, L3 ileocolonic, L4 upper GI), and behavior (B1 inflammatory non-stricturing, B2 stricturing, B3 penetrating, with a 'p' modifier for perianal disease).
The key symptoms of Crohn's Disease are: Chronic diarrhea persisting more than 4 weeks, typically 3-10 loose stools per day, often without overt blood when disease is ileal — present in 70-90% of patients at diagnosis., Crampy abdominal pain, classically in the right lower quadrant in ileal disease, often worse after meals and partially relieved by passing stool or gas., Unintentional weight loss of 5-15% of body weight over the months preceding diagnosis, driven by malabsorption, reduced intake from food-related pain, and systemic inflammation., Fatigue that is disproportionate to activity, related to anemia, chronic inflammation, iron deficiency, and disrupted sleep — reported by 70-80% of patients., Low-grade fever during active flares (typically 37.5-38.5 degrees Celsius), often without an identifiable infection., Perianal disease in 20-30% of patients overall and up to 80% of those with colonic Crohn's — skin tags, fissures, fistula openings draining pus, and recurrent perianal abscesses., Bloody diarrhea in roughly 40% of those with colonic involvement; less common in pure ileal disease..
Diagnosis of Crohn's disease combines clinical history, biomarkers, endoscopy with biopsy, and cross-sectional imaging — no single test is sufficient. A young adult with chronic diarrhea, abdominal pain, weight loss, and elevated fecal calprotectin should be referred for ileocolonoscopy, which is the cornerstone investigation. Calprotectin above 250 micrograms per gram has roughly 90% sensitivity for active intestinal inflammation and helps separate Crohn's from irritable bowel syndrome, where calprotectin is typically normal. C-reactive protein is elevated in roughly 70% of active Crohn's but is non-specific; anti-Saccharomyces cerevisiae antibodies (ASCA) are positive in 60-70% of Crohn's and rarely in ulcerative colitis, while perinuclear anti-neutrophil cytoplasmic antibodies (p-ANCA) show the opposite pattern. At ileocolonoscopy, the classic findings are deep aphthous ulcers, longitudinal serpiginous ulcers, cobblestoning, skip lesions, and terminal ileal involvement with a normal-appearing rectum. Biopsies from involved and uninvolved segments are obtained; non-caseating granulomas are pathognomonic when seen but appear in only 15-30% of cases. Magnetic resonance enterography (MRE) is preferred over CT enterography (CTE) for assessing small-bowel extent and complications because it avoids radiation; both have over 90% sensitivity for clinically significant ileal disease. MRI of the pelvis is mandatory whenever perianal disease is suspected, to map fistula tracts and abscesses. Capsule endoscopy is reserved for cases with normal ileocolonoscopy but a high clinical suspicion of small-bowel Crohn's — capsule retention rates of 1-2% require ruling out strictures first. Differentiating Crohn's from intestinal tuberculosis, Behcet's disease, lymphoma, and chronic infections remains critical, particularly in regions where TB is endemic.
Crohn's is a lifelong disease but, with modern treat-to-target therapy, most patients live full lives with periods of sustained remission. About 30-50% of patients achieve and maintain steroid-free clinical remission long-term on effective biologic therapy. The historical lifetime surgery rate of about 50% has fallen to under 30% in cohorts treated with early biologics. Higher-risk features at diagnosis — young age, deep ulcers, ileal disease with stricturing or penetrating behavior, perianal involvement, smoking, and need for steroids — predict worse outcomes; lower-risk patients with isolated colonic inflammatory disease often do well on conventional therapy. All-cause mortality is only modestly elevated (standardized mortality ratio approximately 1.4) and is mostly driven by complications of penetrating disease and post-operative events, not the underlying inflammation. The decisive prognostic factor is access to specialist IBD care and adherence to monitoring and therapy.
A gastroenterologist with IBD experience should be involved at diagnosis to confirm the disease, stage it accurately with the Montreal classification, identify high-risk features that warrant early biologic therapy, and set up the treat-to-target monitoring schedule. Specialist referral is essential when biologic therapy is being considered, when perianal disease is present, when there is loss of response to current treatment, when surgery is being planned, and during pregnancy or pre-conception counseling.
Find specialists →Symptom improvement after starting steroids or anti-TNF induction typically begins within 1-2 weeks, with clinical remission achievable in 8-12 weeks for most responders. Endoscopic healing follows clinical response by months — repeat ileocolonoscopy at 6-12 months after initiating biologic therapy guides continuation or escalation. After ileocecal resection, most patients are eating normally within 1-2 weeks and back to baseline activity by 6 weeks; endoscopic recurrence at the anastomosis can begin within 6-12 months without prophylactic therapy.
Regular moderate aerobic and resistance exercise (150 minutes per week) is safe in stable Crohn's, supports bone density during steroid courses, and improves fatigue and mood. During severe flares, scale back to gentle walking and stretching until energy and symptoms allow more. Patients with active perianal disease should avoid prolonged sitting on hard surfaces and high-friction activities until fistulas are controlled.
Look for board certification in gastroenterology, ideally with advanced IBD training or a dedicated IBD practice, and access to a multidisciplinary IBD service that includes a colorectal surgeon, an IBD nurse specialist, a dietitian, and on-site MRI and endoscopy. Ask whether the clinic uses therapeutic drug monitoring for biologics, how often they monitor calprotectin, and how they manage perianal disease. Continuity matters — Crohn's is a multi-decade relationship.
Medically reviewed by AIHealz Medical Editorial Board · May 12, 2026
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