IBS — Irritable Bowel Syndrome
Diagnosis and Definition
Irritable bowel syndrome (IBS) is a common chronic intestinal disorder characterized by abdominal discomfort and altered bowel habits. These symptoms occur in the absence of “structural or biochemical abnormalities.” It is estimated that up to 20% of the population of the United States has symptoms suggestive of IBS. Multiple comorbidities, the high cost of medical utilization, and diminished productivity and quality of life all may be found in association with irritable bowel syndrome. Despite extensive research, there is no specific test that can diagnose this condition. In clinical practice, a diagnosis of IBS is accomplished after performing a careful medical history, including a system assessment using established diagnostic criteria, a complete physical examination, and limited laboratory testing. A flexible sigmoidoscopy or colonoscopy is often suggested; the choice of these evaluations depends on the age and risk factors of the individual patient. The clinician must carefully assess the patient for any signs and symptoms of organic disease. So-called red flags or findings suggestive of an alternative diagnosis include the presence of fever, anemia, age greater than 50 years at symptom onset, severe diarrhea associated with dehydration, rectal bleeding, symptoms that awaken the patient, and a family history of colon cancer or inflammatory bowel disease. Laboratory findings including anemia, elevated C-reactive protein, thyroid-stimulating hormone (TSH) abnormalities, electrolyte disturbances, or hypoalbuminemia should prompt other evaluations, as they are not features of irritable bowel syndrome. Once organic disorders have been eliminated from the differential diagnosis, established diagnostic criteria assist with the diagnosis of IBS. The most recent of these parameters, the Rome II criteria (Table: Rome II criteria for irritable bowel syndrome (IBS) diagnosis), resulted from the consensus of an expert working team in 1998. When used in clinical practice, diagnostic criteria for irritable bowel syndrome have been determined to be accurate and reproducible in these patients. The use of diagnostic criteria for IBS accomplishes several important goals. Accurate diagnosis assists in limiting the excessive use of testing and even abdominal surgery, to which these patients are often subjected. A secure diagnosis of IBS is beneficial to the physician-patient relationship by providing patients with reassurance that potentially life-threatening organic disorders are not the cause of their symptoms.
Table: Rome II criteria for irritable bowel syndrome (IBS) diagnosis*
|Abdominal discomfort or pain for 12 weeks or more (consecutive or nonconsecutive) with at least two of these features:|
|Relieved with defecation|
|Onset associated with a change of stool frequency|
|Onset associated with a change in stool form|
* In the absence of structural or metabolic abnormalities to explain the symptoms.
These criteria may also help the clinician, working with the patient, to establish appropriate parameters to evaluate the effects of various treatments. Finally, the consistent use of diagnostic criteria such as Rome II is required for evidence-based clinical trials designed to investigate the epidemiology, pathophysiology, and treatments of irritable bowel syndrome.
Clinicians and researchers have also found it useful to subclassify patients with IBS. The major subclasses of IBS are the constipation-predominant, diarrhea-predominant, and alternating diarrhea and constipation forms of the condition. Although some authors have questioned the usefulness and durability of this subclassification system since many patients will ultimately have alternating symptoms, management strategies for the various subgroups are quite different. The Rome II group has also established diagnostic criteria for functional constipation, including the passage of fewer than three stools per week, frequent straining, incomplete evacuation, passage of hard or lumpy stools, symptoms resembling anorectal blockage, and the use of manual maneuvers to assist with evacuation. When these defining factors for functional constipation are combined with Rome II criteria for irritable bowel syndrome, specific criteria for constipation predominant IBS are also established (Table: Rome II criteria for functional constipation).
Data related to the epidemiology, pathophysiology, and comorbidities of irritable bowel syndrome are best discussed in general terms, as is the approach in the following discussion. This chapter focuses specifically on constipation-predominant IBS. A description of pharmacotherapy for IBS will be restricted to the specific subgroup of patients with constipation-predominant irritable bowel syndrome.
Table: Rome II criteria for functional constipation*
|At least 12 weeks (consecutive or nonconsecutive) in the preceding 12 months of two or more of the following:|
|Straining for more than 25% of defecations|
|Lumpy or hard stools for more than 25% of defecations|
|Sensation of incomplete evacuations in more than 25% of defecations|
|Sensation of anorectal blockage or blockage in more than 25% of defecations|
|Manual maneuvers to facilitate more than 25% of defections (including digital evacuation, support of the pelvic floor, etc).|
|Fewer than three defections per week|
* In the absolute definition of functional constipation, no loose stools are present and the patient has insufficient criteria for IBS.
Epidemiology and Health Care Costs
Although approximately one in six residents of the United States has symptoms suggestive of irritable bowel syndrome, the majority of these symptomatic individuals do not seek medical attention. Furthermore, in the United States, IBS occurs twice as often in women than in men. However, four times as many American females with symptoms of IBS consult a physician for medical care. The converse appears to be true in countries such as India and Sri Lanka, where less than one third of patients with irritable bowel syndrome are female.
Of additional interest, one in eight visits to family practitioners involves management of IBS, and IBS patients comprise up to 50% of consultations to specialty gastroenterology practices. The overall prevalence of IBS appears to be similar in Caucasian and African-Americans, but U.S. residents of Hispanic background are less likely to consult physicians for symptoms of irritable bowel syndrome than are their Caucasian counterparts. Familial aggregation has also been described in IBS. Patients with IBS, particularly those with severe symptoms of the condition, are frequent seekers of medical care, creating a large burden on the medical system. For example, patients with IBS symptoms have an increased number of other health complaints, compared to patients without irritable bowel syndrome. These complaints result in increased overall health care utilization and more physician visits for irritable bowel syndrome patients. When translated into actual terms, an estimated 3.5 million physician visits per year in the United States occur for symptoms of IBS, and more than 2 million drug prescriptions are written annually for this condition.
Irritable bowel syndrome patients have more than 10 times as many overall physician visits for gastrointestinal complaints compared to the general population and more than twice the overall number of visits to physicians for non-gastrointestinal complaints. It has been estimated that the annual health care costs (including both direct and indirect costs) attributed to irritable bowel syndrome in the United States amount to more than $20 billion per year. Patients with IBS in health maintenance organizations are responsible for $1000 more in health care costs annually when compared to other patients.
Patients with irritable bowel syndrome are frequently off from work; one study reported 13.4 days compared to the average 4.9 days per year. Additionally, IBS is a cause of discontinuing work in a significant number of patients, particularly those with severe symptoms. Quality-of-life studies have been performed comparing patients with IBS to patients with organic diseases such as diabetes and psychiatric disorders, including major depression. These studies have demonstrated that scores for physical functioning in patients with irritable bowel syndrome closely resemble diminished scores seen in patients with chronic medical disorders. However, lower quality-of-life scores for social and mental function are consistently seen in IBS, resembling patients with decreased quality-of-life scores reported by individuals suffering from depression.
Since its description more than 150 years ago, clinicians and scientists have been puzzled over the cause(s) of irritable bowel syndrome. Current basic and clinical research studies have suggested that this syndrome is a disorder associated with physiologic alterations of several organ systems including the gut, spinal cord, and brain; altered signaling among these systems fuels and perpetuates the condition. Just as the gastrointestinal tract sends signals to the brain in response to a variety of local stimuli, sensory input to the brain affects the function of the gut. Integrated signaling occurs between the gut and brain; these connected activities are influenced by a variety of factors. Increased intestinal motility and mucosal hyperemia induced by stressful or painful experiences is an example of alteration of gut function induced by the brain and spinal cord. Examples of altered central nervous system functioning as a response to gastrointestinal stimuli seen in IBS include changes in the processing of painful and noxious gut stimuli in the brain, as described below.
Psychological disturbances and past experiences of physical or sexual abuse are risk factors for irritable bowel syndrome. Abnormal integration of the intestinal sensory, motor, and autonomic systems, coupled with the appropriate psychological milieu, ultimately may produce functional-type gut disorders. Neural connections from the brain to the gut allow extrinsic information (including visual, olfactory, and auditory stimulation) and cognitive stimuli such as emotional responses, to subsequently interact with the intestinal nervous system. Neurotransmitters are involved in all of the aforementioned neuronal activities.
When aberrant actions are present in these systems, patients develop symptomatic gut dysfunction as shown in Figure: The brain-gut connection. The most consistently demonstrated abnormality in patients with IBS is the presence of visceral hyperalgesia. Patients with IBS exhibit a markedly decreased threshold for the sensation of painful stimuli in the gut when compared to controls.
**Ehrenpreis ED, Burns EA, Hoffman C. The AFP guide to diagnosis and treatment of the irritable bowel syndrome
The most commonly utilized method for demonstrating intestinal hypersensitivity involves the analysis of symptoms produced by the inflation of balloons within the rectosigmoid colon. Multiple research trials have demonstrated that patients with irritable bowel syndrome experience discomfort after inflation of smaller volumes of fluid or air into the rectosigmoid colon than do controls. In fact, the threshold for the initial sensation of discomfort seen at low volumes as well as the development of rectosigmoid pain with larger balloon volumes are both much lower in individuals with IBS compared to normal controls. These types of studies appear to establish the presence of a specific defect of the sensory portion of the enteric nervous system in patients with irritable bowel syndrome. Further studies have demonstrated that this rectosigmoid hypersensitivity may also reflect abnormalities in specific portions of the brain that are involved in the regulation of painful stimuli. For example, studies comparing normal individuals with IBS patients using rectosigmoid balloon inflation performed with simultaneous positron emission tomography (PET) scanning have shown that the anterior cingulate gyrus, a portion of the brain normally involved in the downregulation of noxious stimuli, has decreased activity in IBS patients. Of interest, repeated distention of the rectosigmoid colon results in stimulation of pre-frontal cortex in irritable bowel syndrome patients but not in normal volunteers. The prefrontal cortex is responsible for anticipation of unpleasant stimuli. These data suggest that IBS patients have altered brain function, characterized by enhanced anticipation of unpleasant gastrointestinal stimuli coupled with an inability to downregulate unpleasant sensations produced by these stimuli. These findings validate the concept that complex alterations in the brain-gut axis play an important role in symptoms experienced by patients with IBS. ”
Several studies have demonstrated that irritable bowel syndrome may occur as a late consequence of gastrointestinal infection, including traveler’s diarrhea and acute bacterial gastroenteritis. One study has shown that the likelihood of developing IBS after an intestinal bacterial infection was approximately 10 times higher than that of the general population. Histologic changes, such as the heightened presence of chronic inflammatory cells in the lamina propria, have been demonstrated after a colonic bacterial infection. Alteration of local immune function of the gastrointestinal tract following bacterial infections have also been demonstrated.
Gut dysfunction, including rectosigmoid hypersensitivity and increased intestinal motility, occur for at least 6 months in all individuals after a bacterial gastroenteritis. However, only a small percentage (for example, 5% of individuals having a salmonella gastroenteritis) will develop the chronic symptoms of irritable bowel syndrome by actually experiencing the sensation of altered gut function. The appropriate psychological milieu, including anxiety or stress, or a history of childhood trauma, appears to be characteristic of developing IBS.
Psychosocial factors are clearly important in the development of irritable bowel syndrome. One study has demonstrated that approximately 50% of individuals with IBS meet the criteria for psychiatric diagnoses, compared to less than 20% of patients without IBS. Anxiety, depression, and somatization disorders are the most common psychiatric diagnoses in individuals with irritable bowel syndrome. Patients who have symptoms of IBS and psychosocial disturbances are more likely to complain of abdominal pain and to seek medical therapy
Unfortunately, increased seemingly unnecessary abdominal surgeries are also seen in these patients. Insightful studies by Drossman et al have demonstrated that a significant number of women with functional gut disorders report a history of physical or sexual abuse.
Clinical experience suggests that mild-to-moderate gut dysfunction in IBS occurs primarily from disturbances in both gut and brain biology, while psychosocial abnormalities predominate in patients with the most severe forms of irritable bowel syndrome.
Serotonin and the Gastrointestinal Tract
Serotonin (5-hydroxytryptamine, 5-HT) functions as both a neurotransmitter as well as a paracrine chemical in the bowel. Serotonin is found throughout the gastrointestinal tract and resides within enterochromafin cells. It is also found within nerve fibers in the enteric nervous system. Enterochromafin cells within the gut are responsible for the production of more than 95% of the body stores of serotonin. The remainder of serotonin is located within the brain and spinal cord. There are three subtypes of serotonin receptors found within the gastrointestinal tract: 5-HTl, 5-HT3, and 5-HT4. Increased serotonin concentration in the serum and mucosa has been demonstrated in women with diarrhea-predominant irritable bowel syndrome. The direct affect of the application of serotonin to the intestine increases intestinal motility and secretion; therefore, this neurotransmitter has attracted a great deal of attention as a putative mediator of the symptoms of IBS. Additionally, laboratory studies and clinical trials have shown that alteration of serotonin receptors with various pharmacologic agents have a direct effect on visceral sensation. It is with this background that modulators of intestinal serotonin receptors have been employed for the development of new treatments for IBS. Additional research is being directed toward the development of medications that will affect peripheral nerve terminal receptors of visceral afferent neurons, such as opioid receptors, ion channels, neuroreceptors within the spinal cord such as opioid, glutamate, calcitonin gene-related peptide, and natural killer cell 1 (NK-1), as well as receptors within the brainstem and prefrontal cortex, including dopamine, acetyl-choline, and epinephrine.
As previously mentioned, a large number of patients with symptoms of irritable bowel syndrome do not report these symptoms to their physicians. Moreover, some symptomatic patients who visit a physician do not require specific therapy but rather derive a great deal of benefit from explanation of their diagnoses and factors that worsen IBS, including stress and possibly dietary factors. A stepwise approach, beginning with the establishment of a trusting physician-patient relationship is required for effective management of IBS. A summary of this approach is shown in Table: Stepwise approach for the treatment of patients with IBS.
The health care provider should reassure patients with irritable bowel syndrome that their condition will not degenerate into other life-threatening disorders, that it most likely represents a physiologic abnormality involving the intestinal motor and sensory system, and that IBS can correlate with psychosocial disorders that require treatment when present. At this point, determining the severity of the condition may help with planning future therapeutic regimens. For example, the majority of patients with severe excruciating abdominal pain and symptoms that adversely affect the overall quality of life should be investigated for a history of physical or sexual abuse or the presence of the aforementioned psychiatric disorders. Early involvement in psychological or psychiatric care may be highly beneficial in these patients. If a physiologic disturbance of the gastrointestinal tract appears to predominate, utilization of pharmacotherapy is justified and may be highly beneficial.
Table: Stepwise approach for the treatment of patients with IBS
|Establish physician-patient relationship|
|Healthy habits, exercise|
After the diagnosis of IBS has been made, symptomatic improvement may be achieved using dietary modification. One study has shown that 48% of patients with irritable bowel syndrome benefit from the elimination diet, with the gradual reintroduction of foods that do not cause additional symptoms. Elimination of lactose is recommended, as up to 60% of the population in the United States has lactose intolerance and these symptoms can mimic IBS. Both caffeinated and decaffeinated coffee have been demonstrated to stimulate motor activity of the rectosigmoid colon in normal volunteers. Sorbitol, a sugar alcohol that is a common ingredient of sugar-free candies, medicines, and antacids, may produce bloating, diarrhea, and gas, mimicking irritable bowel syndrome. Establishment of exercise and stress management routines is often also recommended. A method for determining whether life stresses, dietary components, and other psychosocial factors exacerbate IBS symptoms involves the use of a symptom diary for 2 to 6 weeks. This may be used to determine if dietary factors and stressful events are associated with worsening of IBS symptoms. These journal entries may then be used for recommending dietary exclusions, stress management, and psychological counseling.
Additional therapy for irritable bowel syndrome is warranted when patients and their physicians have determined that the condition has adversely affected the patient’s quality of life. Initial treatment for patients with constipation-predominant IBS usually involves increasing dietary fiber intake and the addition of commercial bulking agents. Fiber decreases total gut transit time and may lessen colonic contractility Fiber supplementation has been demonstrated to be beneficial in several studies of patients with constipation-predominant irritable bowel syndrome. A variety of fiber supplements are available commercially and include synthetic fibers containing calcium polycarbophil (Fibercon ® Wyeth Pharmaceuticals, Madison, NJ; Equalactin Newmark Laboratories, Edison, NJ). Soluble fibers occur naturally in a number of fruits and grains, including apples, oranges, apricots, prunes, and oat bran. Psyllium, the active component in Metamucil ® and Konsyl ® (Proctor and Gamble, Mason, OH) is also a soluble fiber. Methylcellulose, the active ingredient of Citrucel ®, is an insoluble fiber that theoretically may produce less gas and bloating than soluble fibers in some patients. Over-the-counter laxatives may help some patients with constipation-predominant IBS. Osmotic laxatives, including milk of magnesia and mineral oil, and stool softeners such as docusate sodium, may be safely and effectively administered in some patients. Polyethylene glycol in a balanced electrolyte solution (Miralax™, Braintree Laboratories, Braintree, MA) is a prescription medication that is very similar to a colonoscopy preparation solution. This is administered as a 16-g dose in 8 oz of fluid once or twice a day. This agent has proven to be effective in patients with refractory constipation. Furthermore, antispasmodic therapy including anticholinergic agents may be beneficial for pain relief. Due to the fact that these are constipating agents, antispasmodics have a more limited role in constipation-predominant irritable bowel syndrome. Antidepressants have been utilized for the treatment of IBS and other functional bowel disorders, and their effectiveness appears to occur primarily due to central nervous system activity Tricyclic antidepressants, which have anticholinergic properties, have been demonstrated to improve abdominal pain and diarrhea in patients with IBS. These drugs often produce constipation and should be used carefully in patients with constipation-predominant IBS. Selective serotonin reuptake inhibitors (SSRIs) may cause either diarrhea or constipation and have been utilized as clinical treatment for IBS and other functional bowel disorders. Evidence-based studies proving the beneficial effects of most of these treatments of irritable bowel syndrome are lacking at this time.
Recently, the 5-HT4 receptor agonist tegaserod (Zelnorm ®; Novartis Pharmaceuticals, East Hanover, NJ) has been approved by the Food and Drug Administration (FDA) for the treatment of constipation-predominant irritable bowel syndrome. This drug, which binds the 5-HT4 receptors in the gastrointestinal tract with high efficiency, has been shown in laboratory studies to stimulate intestinal peristalsis and secretion and to reduce visceral sensitivity Tegaserod also stimulates the release of other neurotransmitters including calcitonin generelated peptide, which may contribute to its effects on gastrointestinal function. Physiologic studies have demonstrated that tegaserod enhances basal motor activity and corrects intestinal motility in patients with constipation-predominant IBS. Three randomized, placebo-controlled double-blind clinical trials involving 2471 female patients with constipation-predominant IBS who ingested either tegaserod (6mg b.i.d.) or placebo for 12 weeks were performed prior to approval of this drug. These studies demonstrated that tegaserod therapy results in the overall relief of discomfort and other symptomatology in female patients with constipation-predominant irritable bowel syndrome. Additionally, patients on tegaserod experienced more frequent bowel movements, decreased abdominal pain, and improvement in stool consistency. The most common self-limiting adverse reaction experienced in subjects consuming tegaserod was diarrhea. Future therapies for constipation-predominant IBS include the development of additional 5-HT4 agonists such as prucalopride and renzapride. Other gut-directed therapies being investigated for IBS include 5-HTl receptor agonists, kappa opioid receptor agonists, somatostatin analogues, as well as antagonists of neurokinin and tachykinin.