Irritable bowel syndrome (IBS) is considered a functional disorder because it involves the failure of the large intestine to function as it should. It is not a disease, because no identifiable physical abnormality or intestinal damage can be found in patients with IBS. In addition, no single cause of IBS has been identified. Rather, IBS appears to involve unusual sensitivity and muscle activity in the large intestine. This affects the movement of stool and gas through the intestines, causing a variety of symptoms to occur. Stress, emotions and diet can trigger symptoms in patients with IBS.

The term "syndrome" is used to describe a pattern of symptoms that tend to occur together. IBS symptoms include abdominal pain, cramping, bloating, gas, diarrhea and constipation. Abdominal pain that occurs with diarrhea, constipation or both is the most common symptom of IBS. The pain occurs in episodes (not continually) and is often relieved with a bowel movement. Patients with IBS may also experience the sensation of incomplete defecation after a bowel movement. IBS is a chronic condition, in which symptoms can recur throughout a person’s life.

During normal digestion, wave-like muscle contractions (peristalsis) within the intestines occur a few times each day, helping to move contents through the digestive tract before being expelled as waste through the anus. Muscles, nerves and hormones work together to create peristalsis, which maintains normal digestion.

For patients with IBS, nerves and muscles in the large intestine are hypersensitive and can suddenly and involuntarily contract (spasm), speeding digestive content through the intestines and causing diarrhea. Intestinal muscles may also temporarily stop contracting, preventing content from moving forward and causing constipation.

Functional problems that appear to be involved in IBS include:

• Increased sensitivity of the large intestine. In patients with IBS, the large intestine appears to react strongly to stimuli (e.g., anxiety, stress, certain foods) that do not bother most people. For patients with IBS, intestinal nerves may perceive the normal sensations of gas or movement of stool through the digestive tract as excessive and painful.
  
  
• Rapid or delayed transit of stool. In patients with IBS, stool may travel too quickly or in too large an amount for fluid to be properly absorbed by the intestines. Too much liquid in stool causes diarrhea. Or, slowed movement of stool can lead to too much fluid being absorbed, causing constipation.
  
  
• Abnormal muscle contractions. Intestinal muscles may spasm or temporarily stop contracting. Patients with IBS may experience strong contractions that can cause cramping and push stool through the digestive tract too quickly, leading to diarrhea. Spasms may trap gas or stool in one area, leaving it unable to move forward, which can cause cramping, constipation and abdominal pain. This may not occur in all patients with IBS. It is unclear whether this is a cause or symptom of IBS.

The immune system may also be involved. In some rare cases, IBS may develop after severe gastrointestinal infections such as salmonella or campylobacter pylori.

IBS is a very common disorder. According to the National Institutes of Health (NIH), one in five Americans has the disorder, making it one of the most common disorders diagnosed by physicians. IBS affects more than 58 million people in the United States, according to the American College of Gastroenterology (ACG). It occurs more frequently in women than in men, and typically begins around age 20. Many people with IBS do not seek medical attention.

IBS does not permanently harm the large intestine, cause cancer or intestinal bleeding, or lead to other serious diseases. It is not related to inflammatory bowel diseases such as Crohn's disease and ulcerative colitis (both of which cause inflammation and ulcers within the digestive tract). Very few patients with IBS are later diagnosed with some other gastrointestinal condition. In addition, patients with IBS tend to live as long as those in the general population.

However, IBS can alter a person’s lifestyle and impair quality of life. Patients with IBS may experience mild to severe forms of the condition. When mild, IBS symptoms can be annoying and inconvenient. When severe, the symptoms of IBS can be debilitating, preventing people from working, traveling or enjoy social events.