Making Sense of IBS

CHAPTER 2 What Is IBS?

When a patient is first diagnosed with a medical condition, one of the health practitioner’s most important goals is to provide as much information as possible about the new diagnosis. This educational process includes carefully conveying the diagnosis to the patient, explaining terms, reviewing the natural history of the disease, and discussing treatment options. It is also an appropriate time to address the patient’s individual concerns and fears.

Many people who have irritable bowel syndrome are inadequately informed about their disorder, are given incomplete information, or are given conflicting or confusing advice. When patients have insufficient or confusing information, they naturally can become frustrated, discouraged, and anxious. Having too little or wrong information can make symptoms seem unmanageable and even overwhelming at times. The fear stemming from believing that you have a seemingly murky medical problem is often much worse than dealing with a well-understood disease. The following case illustrates some of the pitfalls encountered in the new diagnosis for a person who has IBS. The case begins by presenting some of the typical symptoms of IBS.

Dr. Heckelman is a young doctor with a busy internal medicine practice in a large city. He is regarded by his colleagues as an intelligent, hardworking clinician who can accurately diagnose some of the most difficult diseases. He routinely sees 40 to 50 patients a day, and sometimes even more on a busy day. He recently saw Kimberly, a first-year college student at a local university, who came in at the urging of her mother. Kimberly told Dr. Heckelman that during the last several months she’d had lower abdominal discomfort, with a lot of bloating and gassiness, several times a week. In addition, Kimberly had developed diarrhea. She would often have urgent bowel movements that occurred after her abdominal pain began. Kimberly said that this was a change in her health, because as a high school student, she never had abdominal pain and never had a bowel movement more than once a day.

Dr. Heckelman listened to her story for a few minutes and then told her, “You have IBS. It is really nothing to worry about. I want you to get on a regular schedule at school, avoid all milk products, and take one Imodium (loperamide) tablet every morning. Call me in four to six weeks if you’re not feeling better.” Kimberly, a little confused by the rapid pace of the appointment, asked “Well, what is IBS? I’ve never heard of it.” Dr. Heckelman replied, “You have a nervous gut. I’m sure a lot of your classmates have it too. We used to call it spastic colitis. That’s all it is, a spasm. It’s really nothing to worry about. Don’t forget to call in a few weeks if you’re still having problems.” And with that, he rushed from the room on to the next patient, leaving behind a confused and disappointed Kimberly.

What Are Typical Symptoms of IBS?

The two symptoms of IBS that characterize and truly define this disorder are lower abdominal pain or discomfort and disordered bowel habits (constipation, diarrhea, or alternating symptoms of both). Abdominal pain is the hallmark of IBS—if a patient does not have abdominal pain, then he or she cannot be formally diagnosed as having IBS (see the formal definition later in this chapter). The pain or discomfort is usually located in the lower half of the person’s abdomen, not the upper abdomen. Upper abdominal pain is more likely to indicate some abnormality in the structure or function of the stomach, liver, pancreas, or gallbladder.

People who have IBS differ on which of the predominant symptoms—abdominal pain or disordered bowel habits—are more bothersome to them. They may emphasize one or the other to their health care provider, depending on the intensity of the symptom, their reaction to it, and how much the symptom disrupts their life. For example, some people are not really bothered by abdominal pain but are perturbed by urgent diarrhea, which interferes with their job or social activities. Other people are more concerned with abdominal pain and are able to work around the problems of constipation or diarrhea.

The pattern of symptoms with IBS varies considerably from person to person but remains fairly consistent in a given individual, although there may be some variations in the intensity or the frequency of symptoms. Typically, symptoms are intermittent, with symptom-free periods lasting days, weeks, or (rarely) months. However, a small number of people will have daily symptoms without remission.

As noted previously, abdominal pain must be present for a health care provider to diagnose IBS accurately. The abdominal pain should coincide with having a bowel movement (defecation). Abdominal pain related to urination, menstruation, or exertion is not characteristic of IBS and suggests a different diagnosis. The character of the pain varies among people who have IBS, although for individuals it usually remains fairly stable over time. Some people who have IBS describe the pain as “crampy” in nature, whereas others describe it as sharp or burning. The location of the pain may vary from person to person but, again, remains fairly consistent over time in the same person. The abdominal pain of IBS is most likely to occur in the left lower side of the abdomen; it can occur on both sides but is less likely to occur only on the right side of the abdomen. Some people who have IBS have pain in the area above their pubic bone; others describe a deep-seated pain in their pelvis that moves toward the rectum and eventually remains there or pain that moves into their lower back, similar to labor pains. For some people who have IBS, the pain is difficult to locate, because it does not occur in a specific area.

The abdominal pain of IBS generally occurs in association with a bowel movement (either before, during, or after). Some people who have IBS have pain that occurs at other times as well. Unpredictable and unexpected episodes of abdominal pain can be frustrating for people who have IBS. The pain usually represents either a spasm of the smooth muscle that lines the GI tract or a GI tract that is overly sensitive to stretch, or distention. It is not uncommon for people with severe IBS to describe severe, debilitating, daily abdominal pain that develops from the time they wake up in the morning and then disappears at night.

In the United States, the generally accepted pattern of normal bowel activity ranges from three bowel movements a day to three bowel movements a week. People who have IBS have altered patterns of bowel activity. As with lower abdominal pain, the altered patterns of defecation may be variable from person to person but are fairly consistent for each individual. People who have IBS usually have one of three predominant patterns of altered defecation: predominantly constipation, predominantly diarrhea, or alternating constipation and diarrhea. Many people who have IBS and diarrhea find that the first stool in the morning is of normal consistency but subsequent bowel movements become increasingly loose and are associated with significant urgency, abdominal cramping, bloating, and gassiness. The extreme urgency and abdominal cramping may be temporarily relieved by the passage of stool; however, these symptoms often quickly return and precipitate yet another bowel movement. When the episode of diarrhea finally ends, the stool is usually all liquid or mostly mucus, and many people are left feeling exhausted. In contrast, people who have IBS and constipation often report the passage of rocky, hard, pellet-like stools called scybala. They may have symptoms of straining and incomplete evacuation (the feeling that you have not completely emptied your lower colon after having had a bowel movement). Mucus may cover the stools or be passed alone.

People who have IBS also often describe fecal urgency (the sudden urge to go to the bathroom, now!), increased stool frequency (more bowel movements than usual during a given time period), and severe lower abdominal cramps and spasms during the postprandial period (after a meal). These symptoms are just an exaggeration of a normal reflex. Almost everyone has the urge to have a bowel movement after at least one meal per day. This urge is a normal gastrocolic reflex (gastro refers to the stomach, and colic refers to the colon). The gastrocolic reflex develops when food in the stomach stimulates sensory receptors, which then send signals to the colon, telling it to contract. A normal reflex typically occurs 30 to 45 minutes after eating a medium-to-large meal. In people who have IBS, however, especially those who have IBS and diarrhea, this urge can be very exaggerated, and patients can develop an extreme sense of urgency that feels uncontrollable. This heightened gastrocolic reflex may occur within only a few minutes of beginning to eat and may force a person to hurry to the bathroom during the meal. Fecal incontinence occurs in up to 20 percent of people who have IBS and most likely results from extreme fecal urgency in association with repetitive spasms of the lower colon, rectum, and anal canal.

Bloating (a sense of fullness and gassiness in the abdomen) and abdominal distention (a visibly bulging abdomen filled with gas) are common symptoms in people who have IBS. Bloating and distention may reflect the presence of increased amounts of abdominal gas, delayed transit of gas through the GI tract, or, more commonly, increased sensitivity to normal amounts of intestinal gas.

What Is the Definition of IBS?

During the course of the past century, IBS has been given various labels, including nervous colitis, spastic colitis, mucus colitis, unstable colon, and irritable colon. These labels should all be discarded, as they are confusing, imprecise, and inaccurate. In addition, the labels may be distressing to patients, because they can be confused with other disorders, such as ulcerative colitis. In Kimberly’s case, Dr. Heckelman told her that she had IBS without explaining what that term was and inappropriately referred to it as spastic colitis. Providing her with another term didn’t help Kimberly at all, because she still did not know anything about what was causing her symptoms.

The name irritable bowel syndrome has led many patients—and physicians—to believe that this disorder is just a vague conglomeration of complaints. Some people suspect that IBS is an easy term for doctors to use if a patient’s symptoms are vague, confusing, or not typical of any other specific disease or if the cause of the symptoms cannot be detected by laboratory tests or diagnostic studies. However, the name remains an appropriate description. First, this disorder truly is a constellation of symptoms (which is why it is called a syndrome) rather than a single isolated symptom. Second, IBS can affect multiple areas of the gastrointestinal (GI) tract and is not just limited to the colon. Third, the intestinal tract of a person who has IBS does at times seem “irritable.” For these reasons, irritable bowel syndrome is an appropriate and descriptive title.

IBS is classified as a functional GI disorder (also called a functional bowel disorder). “Functional” means that even though people who have IBS have symptoms that seem to represent a problem in the GI tract, no testing (laboratory tests, x-ray studies, and endoscopic procedures like colonoscopies) can identify such a problem (such as an ulcer, a blockage, or even a malignancy). “Functional” is an accurate word for IBS because although the GI tract may look normal in patients who have IBS and all currently available relevant tests are normal (see Chapters 7 & 8), it clearly does not always function normally. When there are no abnormal findings on physical examination, and test results are normal, symptoms of abdominal bloating and distention with either constipation or diarrhea can be frustrating and sometimes confusing for patients and physicians. However, like many other medical conditions, a concise definition for IBS does exist, and physicians may use this definition to diagnose IBS in their patients.

The definition of IBS has evolved considerably over the past three decades. In the late 1970s a list of symptoms called the Manning criteria was used to diagnose IBS. The Manning criteria were used to guide clinical research and patient care; generally, the more symptoms (criteria) a patient had, the more likely it was that the patient had IBS. The Manning criteria included:

• Abdominal pain easing after a bowel movement

• Looser bowel movements after the onset of pain

• More frequent bowel movements at the onset of pain

• Distention of the abdomen

• Passage of mucus when having a bowel movement

• Feelings of not having completely emptied after having a bowel movement

In 1989, a group of experts met in Rome and published a revised set of criteria for the diagnosis of IBS. These modifications, identified as the Rome criteria (later called Rome I), were meant to simplify the Manning criteria and to clarify the relationship between the presence of abdominal pain and disordered bowel habits. The Rome I criteria have been revised twice since the original meeting, and clinicians and researchers now use the Rome III criteria to accurately diagnose IBS in men and women. According to the Rome III criteria, people diagnosed with IBS must have had their symptoms begin at least six months before diagnosis and, during the last three months before diagnosis, they must have had at least three days per month in which they experienced abdominal pain or discomfort associated with two or more of the following:

• Symptom relief with defecation

• Change in stool frequency

• Change in stool form (appearance)

In addition, for physicians to accurately diagnose IBS and to categorize it appropriately (IBS with constipation, diarrhea, or mixed symptoms of alternating constipation and diarrhea), their patients should have one or more of the following symptoms at least 25 percent of the time:

• Abnormal stool frequency (less than three times a week)

• Abnormal stool form (lumpy/hard)

• Abnormal stool passage (straining, incomplete evacuation)

• Bloating or feeling of abdominal distention

• Passage of mucus

• Frequent, loose stools

In everyday clinical practice, many physicians use criteria that are less strict and less cumbersome than described above. For example, if a patient has chronic symptoms of lower abdominal pain or discomfort associated with disordered bowel movements, and these symptoms are relieved with defecation, then he or she has IBS. This simpler definition helps minimize the difficulty that patients have trying to remember their bowel habits and GI symptoms during the previous six months. A new definition of the Rome criteria (Rome IV) will likely be published in 2016; this change will be mostly important for physicians and scientists performing research studies.

What Is Causing My Symptoms of IBS?

Our understanding of the underlying body processes that cause IBS (that is, the abnormal physiology, or the pathophysiology, of IBS) has changed considerably during the last half-century. In the 1940s and 1950s, the medical community thought that IBS was a nervous disorder of the GI tract, which is why many people who have IBS were said to have “nervous colitis.” Dr. Thomas Almy, who was regarded as a leading figure in gastroenterology at the time, was one of the first physicians to propose a connection between the brain and the GI tract. This concept, now called the brain-gut axis, led to a tremendous leap in our understanding of the pathophysiology of IBS (see Figure 2.1).

In his early experiments, Dr. Almy performed rigid sigmoidoscopy (an examination of the lower portion of the colon; see Chapter 8) on healthy volunteers and recorded a variety of information, including their respiratory rate (number of breaths per minute), heart rate, and blood pressure. He also recorded how many times their colons contracted during a certain period of time. After observing a volunteer’s colon for some time, Dr. Almy gave the person some stressful news. Almost immediately, there were significant changes in the person’s heart rate, blood pressure, and respiratory rate. These changes were not surprising, because stress has long been known to affect these bodily functions. What was not expected, given the information available at the time, was that the colon would also rapidly respond to stress, by changing its pattern of contraction. Shortly afterward, Dr. Almy told the volunteers that the stressful information was incorrect, at which point the heart rate, respiratory rate, and blood pressure all returned to baseline, as did the pattern of colonic contractions. This early experiment was one of the first to demonstrate the strong connection between the brain and the gut. Most people are probably not surprised by the existence of the brain-gut connection. Who has not felt a “sinking feeling” in the gut on hearing bad news or experienced “butterflies” in their stomach in anticipation of a stressful event?

More recently, researchers have found that people who have IBS process sensations from the GI tract differently than patients who do not have IBS. As Dr. Almy’s and these experiments have shown, IBS is a complex disorder in which multiple physiologic processes are involved. The three main processes involved in the generation of IBS symptoms include abnormalities in gut motility (the movement of food and liquids through the GI tract, also called peristalsis), alterations in sensory function of the GI tract (how nerves sense things within the gut), and changes in the way the brain processes sensory information from the GI tract (part of the brain-gut axis). The realization that the gut and the brain are intimately connected now plays a central role in the theory of IBS. This interplay between the central nervous system (CNS) and the GI tract, the brain-gut axis, is described in detail in Figure 2.1.

Figure 2.1. The Brain-Gut Axis
The brain and the gut are intimately connected via a pathway of nerves that lead from the gut to the brain (A) and from the brain to the gut (B). This bidirectional information highway is called the brain-gut axis. One of the largest nerves that connects the brain and the gut, the vagus nerve, is 90 percent sensory in nature. This proves what people who have IBS can attest—that the gut truly is a sensory organ. In a healthy gut, contractions in the GI tract are regular and not typically felt or sensed, and the areas of the brain involved in monitoring GI tract motility and sensation generally function at a low level of activation.

Gut Motility Abnormalities

For many years, the medical community thought that IBS was simply a case of abnormal motility of the GI tract. The term motility refers to how things move. Gastrointestinal motility is a complicated process. When GI motility is normal, foods and liquids are propelled easily through the GI tract, from the point of food ingestion at the mouth to the expulsion of waste at the rectum. Normal GI tract motility depends on normal functioning of the muscles and nerves within the GI tract.

The muscle in the GI tract is called smooth muscle (in contrast to the striated muscle seen in muscles that attach to the skeleton, and cardiac muscle seen in the heart). The smooth muscle of the GI tract forms a tube approximately 25 to 30 feet long that stretches from the mouth to the rectum. This tube is designed to propel contents through the GI tract (see Chapter 6).

More so than muscle function, normal motility relies on an intact and functioning nervous system. The human nervous system is generally described as having several distinct parts: the central nervous system, which includes the brain and the spinal cord, and the peripheral nervous system, which includes the somatic nervous system and the autonomic nervous system (see Figure 2.2). The somatic nervous system includes all of the nerves that supply skeletal muscles—these are the muscles that you can voluntarily control. The autonomic nervous system (ANS) functions autonomously, or without conscious thought; the nerves of this system regulate heart rate, blood pressure, sweating, and GI function. Nerves of the ANS originate within the spinal cord and ganglia (collections of nerve cell bodies); these nerves are found extensively within the abdominal cavity. The ANS can be broken down further into the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system (ENS; see Figure 2.3).

Figure 2.2. The Nervous System—An Overview
The human nervous system can be classified into two major subdivisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, consisting of the brain and spinal cord, is safely encased within the skull and the spinal column. The brain is involved in conscious thought, emotions, memory, movement, and sensation. The spinal cord is a bundle of sensory and motor nerves that carry information back and forth from the brain to the rest of the body. The PNS also can be divided into two parts, the somatic nervous system and the autonomic nervous system. The somatic nervous system receives sensations from the skin, joints, and muscles and transmits this information to the brain. It also carries signals from the brain to the skeletal muscle system and joints to initiate and coordinate voluntary movement. The autonomic nervous system is described in the caption for Figure 2.3.

The ENS is a network of nerve cells and connections that line the GI tract (see Figure 2.4). The ENS is often called the “second brain” because there are more nerve cells in the ENS than there are in the spinal cord. The ENS is what makes your gut work effortlessly and without any conscious thought—food, liquids, and nutrients are propelled down your GI tract without you ever having to think about it. You only become aware of the ENS not working properly when you develop symptoms such as bloating, constipation, diarrhea, or abdominal pain.

Technological advances in the 1970s enabled researchers to directly measure motility patterns of the stomach and small intestine. When people who have IBS participate in motility studies of the stomach and small intestine, they are sometimes found to have unusual patterns of activity called discrete clustered contractions. These discrete clustered contractions are isolated bursts of rhythmic contractions and are typically found in the small intestine. The contractions can be associated with episodes of abdominal pain in some people who have IBS. Other people who have IBS have prolonged muscle contractions within the colon or small intestine, or they have severe contractions within the colon, especially after a meal (see Figure 2.5). These contractions may also be associated with episodes of abdominal pain. Although people who have IBS may experience different patterns of abnormal GI motility, no single pattern is routinely found in all people with this disorder. In general, the symptoms of IBS and the alterations in GI motility that are associated with them reflect an exaggeration of normal patterns of GI motility. This means that everybody—people who do and do not have IBS—has similar patterns of motility in the GI tract, although some of these patterns are exaggerated and amplified in people who have IBS.

Figure 2.3. The Autonomic Nervous System
The autonomic nervous system (ANS) was formally recognized and described over 100 years ago. The ANS is responsible for automatic responses in the body (ones that happen without our thinking about them) such as breathing and our heart beating. The ANS can be broken down into three sections: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. The sympathetic nervous system originates in the spinal cord, and the parasympathetic nervous system originates in the brainstem and the lower spinal cord. In the GI tract, the sympathetic nervous system is generally responsible for slowing down motility, while the parasympathetic nervous system is generally responsible for speeding it up.

Figure 2.4. The Layers of the GI Tract
There are two layers of smooth muscle in the GI tract, the inner layer, which is circular muscle, and the outer layer, containing longitudinal muscle. These layers are involved in the muscular contractions (peristalsis) required to move materials through the GI tract. Two other layers contain the nerves of the enteric nervous system (ENS). The submucosal plexus is a highly complex pathway of interconnected nerve cells and their processes. It lies between the circular muscle layer and the innermost layer, the mucosa. The submucosal plexus processes sensations within the GI tract. The myenteric plexus, between the two muscle layers, is primarily involved in coordinating peristalsis. The mucosa contains cells that produce and secrete mucus and other cells that absorb fluid and nutrients.

Figure 2.5. Colonic Motility before and after a Meal
A, Healthy volunteers. On the left side of the panel, the motility of the colon is stable, without any contractions. After the person eats a meal, smooth muscle contractions occur as part of the normal gastrocolic (stomach-colon) reflex. The contractions are of modest strength and do not cause pain or discomfort. B, Patients who have IBS. Occasional scattered contractions are noted on the left side of the panel, even before a meal. These contractions are felt as painful in some people. After eating a meal, many people who have IBS have very strong, high-amplitude contractions that can be uncomfortable or painful. Some experience urgent diarrhea and cramps immediately after eating a meal, and in these persons, the contractions in the colon may be excessively strong and prolonged.

Alterations in Sensory Function

As discussed previously, lower abdominal pain is a critical part of the definition of IBS. When evaluating people who have symptoms of IBS, physicians first try to understand why they are experiencing chronic abdominal pain. In the past, these people were often subjected to multiple tests, including blood work, x-rays, computed tomography (CT) scans, barium enemas, and colonoscopies. When all of these tests came back normal for people who have IBS, physicians often told them that the pain was “all in their head.”

Physicians now know that people who have IBS have an increased sensitivity to pain within the GI tract. The concept of altered sensitivity to pain within the GI tract is called visceral hypersensitivity. The term “viscera” refers to internal organs within the body (stomach, colon, etc.). In essence, visceral hypersensitivity means that people who have IBS generally have a lower threshold for pain in the GI tract than do people who do not have IBS.

Researchers have completed many different studies that demonstrate the concept of visceral hypersensitivity. In several of these studies, they used a special technique involving balloon distention of the GI tract to measure the heightened sensitivity. During this procedure, a small balloon is placed in the GI tract and gradually inflated. People who have IBS sense the balloon being inflated (distended) much earlier in the process (that is, at very low levels of balloon inflation) than do people who do not have IBS (see Figure 2.6). In addition, people who have IBS also describe the distention as more painful.

These experiments show that people who have IBS sense painful stimuli within the gut at much lower levels, and with more pain, than people who do not have IBS. Some health care providers use the analogy that people who have IBS can hear a radio (their GI tract) at even the lowest volume, whereas people who do not have IBS need to turn the volume up to hear all of the signals from their GI tract. In essence, people who have IBS have particularly acute sensitivity: they sense things in their gut that other people do not.

Visceral hypersensitivity may lead people who have IBS to interpret normal sensations in their GI tract as abnormal and painful. The misinterpretation of normal sensations as painful is a condition called allodynia. Health care providers do not know why some people who have IBS have allodynia and others do not. One theory is that a previous illness, infection, or surgery could have somehow injured the sensory nerves from the GI tract and made them more sensitive.

Figure 2.6. Visceral Hypersensitivity of the GI Tract
People who have IBS are frequently overly sensitive to stimulation from within the GI tract. Hypersensitivity in the GI tract can be easily assessed by inflating a small balloon in the GI tract and measuring a patient’s response. The graph above records how healthy volunteers and patients who have IBS compared in their responses to balloon distention of the rectum.

A small percentage of healthy volunteers first began to sense the balloon inflation as painful after approximately 80 to 100 ml of air had been instilled into the balloon. It was not until approximately 160 to 180 ml of air had been instilled into the balloon that a larger percentage of normal volunteers began to feel the inflated balloon as a painful sensation.

In contrast, many patients who had IBS could sense the balloon inflation at very low levels. About a third of patients who had IBS reported that at even modest levels of balloon inflation (100 ml) the pressure is very uncomfortable. Similar results were obtained when the balloon was inflated in the esophagus, stomach, small intestine, or colon. These findings are all consistent with the notion that people who have IBS are hypersensitive in the GI tract.

Influences of the Brain on the Gut

People who have IBS may also process sensory information abnormally in places outside of the GI tract. In particular, they may process sensory information differently in the brain than do people who do not have IBS. As part of a study published several years ago, people who had IBS underwent a special x-ray study of the brain called a positron emission tomography (PET) scan. PET scans measure the metabolism of individual organs and cells and thus can be used to measure the activity of a specific organ. During the study, researchers performed PET scans before and during balloon distention of a person’s rectum. These images of the brain were then compared to those of people without IBS who underwent similar testing. The results showed that people who had IBS had increased activity in the prefrontal cortex, which is an area of the brain associated with anxiety and increased vigilance. In addition, people who had IBS had less activity in the anterior cingulate cortex, an area important for opioid (narcotic) binding.

The findings of this PET study confirmed that people who have IBS process and interpret gut sensations differently than do people who do not have IBS. One theory as to why this occurs is that people who have IBS cannot block out painful sensations as well as people who do not have IBS. Another theory is that people who have IBS have difficulty separating out, or discriminating, the normal sensations of gut motility that we all have from the abnormal sensations that may arise from an overly strong contraction or spasm in the GI tract. Thus, the nervous systems of people who have IBS misinterpret the normal sensations as painful or unpleasant. This theory of misinterpretation becomes relevant later in the book when we discuss treatments for IBS (see Chapter 13).

Finally, it is now well recognized that other stimulation, such as stress, anxiety, or depression, may regulate sensory processing in the brain and thus influence a person’s perception of pain. These findings have significant implications, especially in regard to treatment of IBS. IBS treatment that focuses only on the GI tract may not be nearly as successful as a multisystem approach that treats both the GI tract and the central nervous system (see Chapters 13, 14, 16, and 17 for more information about the treatment of IBS).

What Is the Brain-Gut Axis?

As mentioned previously, research studies and physicians’ observations during the last several decades have discovered a significant and critical connection between the brain (the central nervous system) and the gut (the GI tract) in people who have IBS (see Figure 2.7). Although some physicians suspected a connection between the brain and the gut as many as one hundred years ago, Dr. Almy’s study was one of the first to scientifically demonstrate this connection in humans.

The brain-gut axis can best be described as an information highway that connects these two vital structures. This information highway is not unidirectional, meaning that it does not run only from the gut to the brain, or only from the brain to the gut. Rather, messages are transmitted bidirectionally. Information about GI function, motility, and visceral sensation is constantly sent from the gut to the brain. In the other direction, information about emotions, mood, conscious and subconscious thoughts, and sensations elsewhere in the body is constantly sent from the brain to the gut.

Sensory information within the GI tract is first recorded by sensory nerves that line the gut wall. These specialized cells that collect information about sensations in the GI tract are called sensory afferent neurons. They send the collected information through the spinal cord and up into the brain. Within the brain, there are many specialized structures responsible for collecting sensory information from the gut. These structures may relay the information to other areas of the brain, where the information can be grouped with, and interpreted alongside, information from other parts of the body. It is here, within the brain, that external influences such as stress can affect the interpretation of signals from the gut or the content of messages sent to the gut. Signals from the brain are sent to the gut by a series of nerves, including the vagus nerve and the sympathetic and parasympathetic nerves. Through these pathways, information is constantly being sent back and forth between the brain and the gut.

Figure 2.7. The Brain-Gut Axis in Patients Who Have IBS
In people who have IBS, the brain-gut axis may be more active than in people who do not have the disease. Increased activity in the gut is common in patients who have IBS; contractions may be more frequent and stronger. As messages are sent from the gut to the brain via any of the millions of sensory nerves in the GI tract (via pathway A), increased sensations of discomfort or pain register in the brain. This increased brain activity may in turn lead to an increase in the number, type, or intensity of signals to the gut (pathway B). These signals may then further stimulate gut motility, worsening pain or causing diarrhea.

When this intricate and delicate system functions normally, gut motility occurs effortlessly, digestion occurs painlessly, and neither constipation nor diarrhea dominates a person’s GI function. When this interconnected pathway is disrupted or malfunctions, GI dysfunction is bound to occur and cause the typical symptoms of IBS: abdominal pain, bloating, and constipation or diarrhea.

Summary

• IBS remains widely misunderstood. Misconceptions and misperceptions about IBS are common.

• IBS is defined by the presence of abdominal pain or discomfort in association with disordered defecation (that is, either constipation or diarrhea or both). The sensation of abdominal pain or discomfort is a key part of the definition of IBS.

• Other typical symptoms of IBS include bloating, gassiness, abdominal distention, feelings of extreme urgency to use the bathroom, excessive straining while having a bowel movement, feelings of incomplete evacuation after having had a bowel movement, and the passage of mucus during evacuation.

• IBS symptoms develop from abnormalities in both gut motility and visceral sensitivity. In some cases the GI tract contracts too quickly and too forcefully; in other cases, it may not contract enough. Nearly all people who have IBS are hypersensitive in their GI tract, which means that they sense things too well in their gut.

• The brain-gut axis plays a critical role in IBS.