Inflammatory bowel disease (IBD) is a chronic condition of inflammation or irritation that may occur in many different places throughout the digestive tract.

Some general terms are used to describe inflammation of the intestines. Colitis is the inflammation or irritation of the large intestine. Enteritis describes the same symptoms when they occur in the small intestine. Some types of inflammation are named for their cause. For example, radiation colitis or radiation enteritis is caused by radiation treatment received for various cancers of the abdomen. Infectious colitis or enteritis is caused by bacteria or viruses. The inflammation caused by ischemic colitis occurs when not enough oxygen-rich blood flows to the bowel.

The two primary types of IBD are Crohn's disease (CD) and ulcerative colitis (UC).

CD causes inflammation and ulceration anywhere in the digestive tract, from the mouth to the anus. Most commonly, it occurs in either the small intestine, large intestine or both. All layers of the intestinal lining can be affected. Inflammation may be confined to one location, or it may occur in various places, with normal areas between areas that are inflamed.

Various forms of CD are distinguished from one another by where in the digestive tract they occur. These include:

• Ileocolitis. The most common form of CD, with symptoms occurring in both the small intestine’s ileum and the large intestine.
  
  
• Ileitis. When inflammation occurs in the small intestine’s ileum.
  
  
• Gastroduodenal CD. Affects the stomach and small intestine’s duodenum.
  
  
• Jejunoileitis. Inflammation of the small intestine’s jejunum and ileum.
  
  
• Crohn's colitis. Red, irritated tissue and inflammation that occurs in the colon only.

UC causes inflammation and ulceration only in the inner lining of the large intestine. In most cases, inflammation starts in the rectum and extends up the colon. UC may include various conditions that can occur throughout the large intestine. These conditions include:

• Ulcerative proctitis. Inflammation occurs in the rectum and anus. It is a common and typically mild form of UC, but may be a precursor to more serious forms of UC.
  
  
• Left-sided colitis (sometimes referred to as limited or distal colitis). Occurs along the left side, from the rectum into the colon.
  
  
• Pancolitis. Affects the entire colon. Serious complications, such as intestinal perforation and massive bleeding, can result, requiring surgery.
  
  
• Fulminant colitis. A rare but life-threatening condition that affects the entire colon, causing severe pain, diarrhea and abdominal distention. It may also cause toxic megacolon (severely distended colon) or colon rupture.

Additional conditions involving inflammation of the bowels include:

• Microscopic colitis. Chronic inflammation in the colon. Named for the only method to view the inflammation – by examination of biopsied tissue under a microscope (affected areas appear normal during endoscopy). Symptoms start gradually and become long-term. They include chronic, watery diarrhea without blood (unlike both CD and UC), abdominal pain, cramping and dehydration. Microscopic colitis does not increase a patient’s risk of colon cancer.

Microscopic colitis includes:

• • Collagenous colitis. Where the lining of the colon develops a thick layer of non-elastic tissue made of a protein called collagen (connective tissue). Collagenous colitis develops primarily in women in their 50s and older.
    
    
  • Lymphocytic colitis. Where the lining of the colon accumulates lymphocytes (white blood cells that fight infection and disease). These cells may also appear in a patient with collagenous colitis.
    
    
• Diversion colitis. Inflammation that affects the lower part of the large intestine (the part diverted from the digestive process) after surgery performed to remove the colon or rectum in order to alter the passage of stool through the bowels. These surgeries are sometimes performed to treat cancer, UC and diverticulitis. Symptoms usually appear within a year of surgery and include abdominal pain and blood or mucus in the stool. The symptoms are usually mild and do not require treatment. Reattaching the separated portions of the intestine can sometimes resolve the symptoms of diversion colitis.
  
  
• Pseudomembranous colitis (also called necrotizing colitis). Occurs when the natural balance of bacteria in the intestines is disrupted, leading to inflammation and destruction of intestinal tissue. Antibiotics typically trigger this bacterial imbalance, although chemotherapy may also cause pseudomembranous colitis.
  
  
• Necrotizing enterocolitis. Can occur in either the small or large intestine, although it most commonly affects the small intestine’s ileum. It is primarily a disease of infants, particularly premature or sick newborns, in which intestinal tissue dies and falls away from the intestinal wall. Although the cause is unknown, necrotizing enterocolitis appears to occur due to the bowel’s inability to produce enough protective mucus in the intestines. Bacteria may also play a role. Surgery is often required. According to the National Institutes of Health, approximately 25 percent of infants with necrotizing enterocolitis die of the disease.

IBD differs from irritable bowel syndrome (IBS) in both its diagnosis and symptoms. IBD is intestinal inflammation that can be seen via a barium x-ray or colonoscopy. IBS is a functional disorder – no inflammation is seen and IBS cannot be identified by diagnostic tests. Both involve symptoms of abdominal cramping and pain, as well as diarrhea. However, IBD can include rectal bleeding, fever and an elevated white blood cell count, whereas IBS does not.