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Crohn's Disease

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Medical Author:

Adam Schoenfeld, MD

Medical Author:

George Y. Wu, MD, PhD

Medical Editor:

Jay W. Marks, MD

Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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What is Crohn's disease?

Crohn's disease (also spelled Crohn disease) is a chronic inflammatory disease of the intestines. It primarily causes ulcerations (breaks in the lining) of the small and large intestines, but can affect the digestive system anywhere from the mouth to the anus. It is named after the physician who described the disease in 1932. It also is called granulomatous enteritis or colitis, regional enteritis, ileitis, or terminal ileitis.

Crohn's disease is related closely to another chronic inflammatory condition that involves only the colon called ulcerative colitis. Together, Crohn's disease and ulcerative colitis are frequently referred to as inflammatory bowel disease (IBD). Ulcerative colitis and Crohn's disease have no medical cure. Once the diseases begin, they tend to fluctuate between periods of inactivity (remission) and activity (relapse).

Inflammatory bowel disease affects approximately 500,000 to two million people in the United States. Men and women are affected equally. Americans of Jewish European descent are 4 to 5 times more likely to develop IBD than the general population. IBD has historically been considered predominately disease of Caucasians, but there has been an increase in reported cases in African Americans suffering from IBD. The prevalence appears to be lower among Hispanic and Asian populations. IBD most commonly begins during adolescence and early adulthood (usually between the ages of 15 and 35). There is a small second peak of newly-diagnosed cases after age 50. The number of new cases (incidence) and number of cases (prevalence) of Crohn's disease in the United States are rising, although the reason for this is not completely understood.

Crohn's disease tends to be more common in relatives of patients with Crohn's disease. If a person has a relative with the disease, his/her risk of developing the disease is estimated to be at least 10 times that of the general population and 30 times greater if the relative with Crohn's disease is a sibling. It also is more common among relatives of patients with ulcerative colitis.

What causes Crohn's disease?

The cause of Crohn's disease is unknown. Some scientists suspect that infection by certain bacteria, such as strains of mycobacterium, may be the cause of Crohn's disease. To date, however, there has been no convincing evidence that the disease is caused by infection per se. Crohn's disease is not contagious. Although diet may affect the symptoms in patients with Crohn's disease, it is unlikely that diet is responsible for the disease.

Activation of the immune system in the intestines appears to be important in IBD. The immune system is composed of immune cells and the proteins that these immune cells produce. Normally, these cells and proteins defend the body against harmful bacteria, viruses, fungi, and other foreign invaders. Activation of the immune system causes inflammation within the tissues where the activation occurs. (Inflammation is an important mechanism of defense used by the immune system.)

Normally, the immune system is activated only when the body is exposed to harmful invaders. In individuals with IBD, however, the immune system is abnormally and chronically activated in the absence of any known invader. The continued abnormal activation of the immune system results in chronic inflammation and ulceration. The susceptibility to abnormal activation of the immune system is genetically inherited. Thus, first degree relatives (brothers, sisters, children, and parents) of people with IBD are more likely to develop these diseases. Recently a gene called NOD2 has been identified as being associated with Crohn's disease. This gene is important in determining how the body responds to some bacterial products. Individuals with mutations in this gene are more susceptible to developing Crohn's disease.

Other genes are still being discovered and studied which are important in understanding the pathogenesis of Crohn's disease including autophagy related 16-like 1 gene (ATG 16L1) and IRGM, which both contribute to macrophage defects and have been identified with the Genome-Wide Association study². In this regard, there have also been studies which show that in the intestines of individuals with Crohn's disease, there are higher levels of a certain type of bacterium, E. coli, which might play a role in the pathogenesis¹. One postulated mechanism by which this could occur is though a genetically determined¹ defect in elimination of the E. coli, by intestinal mucosal macrophages. The exact roles that these various factors play in the development of this disease remain unclear.

How does Crohn's disease affect the intestines?

In the early stages, Crohn's disease causes small, scattered, shallow, crater-like ulcerations (erosions) on the inner surface of the bowel. These erosions are called aphthous ulcers. With time, the erosions become deeper and larger, ultimately becoming true ulcers (which are deeper than erosions), and causing scarring and stiffness of the bowel. As the disease progresses, the bowel becomes increasingly narrowed, and ultimately can become obstructed. Deep ulcers can puncture holes in the wall of the bowel, and bacteria from within the bowel can spread to infect adjacent organs and the surrounding abdominal cavity.

When Crohn's disease narrows the small intestine to the point of obstruction, the flow of the contents through the intestine ceases. Sometimes, the obstruction can be caused suddenly by poorly-digestible fruit or vegetables that plug the already-narrowed segment of the intestine. When the intestine is obstructed, digesting food, fluid and gas from the stomach and the small intestine cannot pass into the colon. The symptoms of small intestinal obstruction then appear, including severe abdominal cramps, nausea, vomiting, and abdominal distention. Obstruction of the small intestine is much more likely since the small intestine is much narrower than the colon.

Deep ulcers can puncture holes in the walls of the small intestine and the colon, and create a tunnel between the intestine and adjacent organs. If the ulcer tunnel reaches an adjacent empty space inside the abdominal cavity, a collection of infected pus (an abdominal abscess) is formed. Individuals with abdominal abscesses can develop tender abdominal masses, high fevers, and abdominal pain.

• When the ulcer tunnels into an adjacent organ, a channel (fistula) is formed.
  
  
• The formation of a fistula between the intestine and the bladder (enteric-vesicular fistula) can cause frequent urinary tract infections and the passage of gas and feces during urination.
  
  
• When a fistula develops between the intestine and the skin (enteric-cutaneous fistula), pus and mucous emerge from a small painful opening on the skin of the abdomen.
  
  
• The development of a fistula between the colon and the vagina (colonic-vaginal fistula) causes gas and feces to emerge through the vagina.
  
  
• The presence of a fistula from the intestines to the anus (anal fistula) leads to a discharge of mucous and pus from the fistula's opening around the anus.

How is Crohn's disease different from ulcerative colitis?

While ulcerative colitis causes inflammation only in the colon (colitis) and/or the rectum (proctitis), Crohn's disease may cause inflammation in the colon, rectum, small intestine (jejunum and ileum), and, occasionally, even the stomach, mouth, and esophagus.

The patterns of inflammation in Crohn's disease are different from ulcerative colitis. Except in the most severe cases, the inflammation of ulcerative colitis tends to involve the superficial layers of the inner lining of the bowel. The inflammation also tends to be diffuse and uniform (all of the lining in the affected segment of the intestine is inflamed.)

Unlike ulcerative colitis, the inflammation of Crohn's disease is concentrated in some areas more than others, and involves layers of the bowel that are deeper than the superficial inner layers. Therefore, the affected segment(s) of bowel in Crohn's disease often is studded with deeper ulcers with normal lining between these ulcers.