Merck Manual

Please confirm that you are a health care professional

honeypot link

Overview of Inflammatory Bowel Disease


Aaron E. Walfish

, MD, Mount Sinai Medical Center;

Rafael Antonio Ching Companioni

, MD

Last full review/revision Jun 2020| Content last modified Jun 2020
Click here for Patient Education
Topic Resources

Inflammation results from a cell-mediated immune response in the gastrointestinal mucosa. The precise etiology of inflammatory bowel disease unknown, but evidence suggests that the normal intestinal flora trigger an abnormal immune reaction in patients with a multifactorial genetic predisposition (perhaps involving abnormal epithelial barriers and mucosal immune defenses). No specific environmental, dietary, or infectious causes have been identified. The immune reaction involves the release of inflammatory mediators, including cytokines, interleukins, and tumor necrosis factor.

Although Crohn disease and ulcerative colitis are similar, they can be distinguished in most cases (see Table: Differentiating Crohn Disease and Ulcerative Colitis Differentiating Crohn Disease and Ulcerative Colitis Inflammatory bowel disease (IBD), which includes Crohn disease and ulcerative colitis, is a relapsing and remitting condition characterized by chronic inflammation at various sites in the gastrointestinal... read more ). About 10% of colitis cases are not initially distinguishable and are termed unclassified; if a surgical pathologic specimen cannot be classified, it is termed indeterminate colitis. The term colitis applies only to inflammatory disease of the colon (eg, ulcerative, granulomatous, ischemic, radiation-induced, infectious). Spastic (mucous) colitis is a misnomer sometimes applied to a functional disorder, irritable bowel syndrome Irritable Bowel Syndrome (IBS) Irritable bowel syndrome is characterized by recurrent abdominal discomfort or pain with at least two of the following characteristics: relation to defecation, association with a change in frequency... read more .



Inflammatory bowel disease (IBD) affects people of all ages but usually begins before age 30, with peak incidence from 14 to 24. IBD may have a second smaller peak between ages 50 and 70; however, this later peak may include some cases of ischemic colitis.

IBD is most common among people of Northern European and Anglo-Saxon origin and is 2 to 4 times more common among Ashkenazi Jews than non-Jewish whites from the same geographic location. The incidence is lower in central and southern Europe and lower still in South America, Asia, and Africa. However, the incidence is increasing among blacks and Latin Americans living in North America. Both sexes are equally affected. First-degree relatives of patients with IBD have a 4- to 20-fold increased risk; their absolute risk may be as high as 7%. Familial tendency is much higher in Crohn disease than in ulcerative colitis. Several gene mutations conferring a higher risk of Crohn disease (and some possibly related to ulcerative colitis) have been identified.

Cigarette smoking seems to contribute to development or exacerbation of Crohn disease but decreases risk of ulcerative colitis. Appendectomy done to treat appendicitis also appears to lower the risk of ulcerative colitis. Nonsteroidal anti-inflammatory drugs (NSAIDs) may exacerbate IBD. Oral contraceptives may increase the risk of Crohn disease. Some data suggest that perinatal illness and the use of antibiotics in childhood may be associated with an increased risk of IBD.

For unclear reasons, people who have a higher socioeconomic status may have an increased risk of Crohn disease.

Pearls & Pitfalls

  • Cigarette smoking decreases the risk of ulcerative colitis.

Extraintestinal Manifestations

Crohn disease and ulcerative colitis both affect organs other than the intestines. Most extraintestinal manifestations are more common in ulcerative colitis and Crohn colitis than in Crohn disease limited to the small bowel. Extraintestinal manifestations of inflammatory bowel disease are categorized in 3 ways:

2. Disorders that are clearly associated with IBD but appear independently of IBD activity: These disorders include ankylosing spondylitis, Ankylosing Spondylitis Ankylosing spondylitis is the prototypical spondyloarthropathy and a systemic disorder characterized by inflammation of the axial skeleton, large peripheral joints, and digits; nocturnal back... read more Ankylosing Spondylitis sacroiliitis, uveitis Overview of Uveitis Uveitis is defined as inflammation of the uveal tract—the iris, ciliary body, and choroid. However, the retina and fluid within the anterior chamber and vitreous are often involved as well.... read more Overview of Uveitis , pyoderma gangrenosum Pyoderma Gangrenosum Pyoderma gangrenosum is a chronic, neutrophilic, progressive skin necrosis of unknown etiology often associated with systemic illness and sometimes skin injury. Diagnosis is clinical. Treatment... read more Pyoderma Gangrenosum , and primary sclerosing cholangitis Primary Sclerosing Cholangitis (PSC) Primary sclerosing cholangitis (PSC) is patchy inflammation, fibrosis, and strictures of the bile ducts that has no known cause. However, 80% of patients also have inflammatory bowel disease... read more . Ankylosing spondylitis occurs more commonly in IBD patients with human leukocyte antigen B27 (HLA-B27). Most patients with spinal or sacroiliac involvement have evidence of uveitis and vice versa. Primary sclerosing cholangitis, which is a risk factor for cancer of the biliary tract, is strongly associated with ulcerative colitis and Crohn colitis. Cholangitis may appear before or concurrently with the bowel disease or even 20 years after colectomy. Liver disease (eg, fatty liver, autoimmune hepatitis, pericholangitis, cirrhosis) occurs in 3 to 5% of patients, although minor abnormalities in liver tests are more common. Some of these conditions (eg, primary sclerosing cholangitis) may precede IBD by many years and, when diagnosed, should prompt an evaluation for IBD.

3. Disorders that are consequences of disrupted bowel physiology: These disorders occur mainly in severe Crohn disease of the small bowel. Malabsorption Overview of Malabsorption Malabsorption is inadequate assimilation of dietary substances due to defects in digestion, absorption, or transport. Malabsorption can affect macronutrients (eg, proteins, carbohydrates, fats)... read more may result from extensive ileal resection and cause deficiencies of fat-soluble vitamins, vitamin B12, or minerals, resulting in anemia, hypocalcemia, hypomagnesemia, clotting disorders, and bone demineralization. In children, malabsorption retards growth and development. Other disorders include kidney stones resulting from excessive dietary oxalate absorption, hydroureter and hydronephrosis resulting from ureteral compression by the intestinal inflammatory process, gallstones resulting from impaired ileal reabsorption of bile salts, and amyloidosis secondary to long-standing inflammatory and suppurative disease.

Thromboembolic disease may occur as a result of multiple factors in all 3 categories.

Treatment of Inflammatory Bowel Disease

Supportive care

Most patients and their families are interested in diet and stress management. Although there are anecdotal reports of clinical improvement on certain diets, including one with rigid carbohydrate restrictions, controlled trials have shown no consistent benefit. Stress management may be helpful.

Health Maintenance in Inflammatory Bowel Disease


Screening tests

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Click here for Patient Education
NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
Professionals also read
Test your knowledge
Esophageal Foreign Bodies
Which of the following is the most common site of foreign body impaction subsequent to ingestion? 
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID