Irritable Bowel Syndrome

Etiology

The etiology of IBS is broad and not clearly understood. However, as below in the pathophysiology section, motility, visceral sensation, brain-gut interaction, and psychosocial distress can all play a role in the development of IBS.

Epidemiology

Nearly 12 percent of patients seek medical care in primary care practices for IBS related complaints.[1][2] Studies have demonstrated that the prevalence of IBS ranges between ten and fifteen percent; however, the majority of these patients do not seek medical care.[1] IBS is most prevalent in South America at approximately 21 percent and least prevalent in Southeast Asia at 7 percent.[3][4] In the United States, Canada, and Isreal, IBS symptoms are 1.5 to 2 times more prevalent among women than men.[5] Moreover, women are more likely to report abdominal pain and constipation, whereas men are more likely to report diarrhea.[5] The prevalence of IBS also decreases with age.[3] IBS can also be broken down into more specific diagnoses, which include IBS with diarrhea (IBS-D), IBS with constipation (IBS-C), and IBS with mixed bowel patterns (IBS-M). The prevalence of these three diagnoses differs in the United States versus Europe. In the United States, there is an equal distribution of these diagnoses, whereas, in Europe, IBS-C or IBS-M can be more prevalent.[6]

Pathophysiology

The pathophysiology of IBS is broad and includes abnormalities involving motility, visceral sensation, brain-gut interaction, and psychosocial distress.[3] One of these can usually be demonstrated in the majority of IBS patients; however, not all symptoms can be attributed to them.[3] Recent studies have also shown altered gut immune activation and intestinal and colonic microbiome are associated with IBS[3][7][8]. Environmental contributors to IBS include early life stressors, food intolerance, antibiotics, and enteric infections.[3] Patients often complain that IBS symptoms are related to food intake. However, a true food allergen has a limited contribution to IBS.[3][9]

Histopathology

Histopathology examination of the intestinal mucosa in those with IBS can show chronic inflammatory cells, mast cells, enteroendocrine cells, and enteric nerves.[10]. IBS-D is typically associated with a greater increase in mucosal T-lymphocytes than IBS-C.[10][11] Moreover, there can be an increased number of nerve fibers that stain positive for neuron-specific enolase, substance P, and 5-HT.[10][12] There also appears to be a significantly increased density of nerve fibers around mast cells.[10][12]

History and Physical

IBS typically consists of abdominal pain or discomfort, altered bowel habits along with constipation, diarrhea, or both. Other complaints in patients with IBS include bloating, distention, symptoms brought on by food intake, and a change in pain location and stool pattern with time.[3]. Concerning features would include onset after the age of 50 years old, severe or progressive symptoms, unexplained weight loss, nocturnal diarrhea, rectal bleeding, iron deficiency anemia, or a family history of organic gastrointestinal diseases such as colon cancer, celiac disease, or inflammatory bowel disease.[3] Additional history that would be important would include travel and social history.

The Rome IV criteria are used to diagnose IBD, which requires at least 3 days a month in the last 3 months associated with 2 or more of the following: improvement in abdominal pain or discomfort with defecation, onset associated with a change in frequency of stool, and/or an onset accompanied by a change in form or appearance of stool[3].

Evaluation

If no alarm findings exist (weight loss, hematochezia, iron deficiency), routine diagnostic testing is not recommended.[1] If symptoms are not typical of IBS or alarm symptoms are present, then a complete blood cell count, comprehensive metabolic panel, inflammatory markers such as erythrocyte sedimentation rate or C-reactive protein, and thyroid stimulating hormone level should be checked.[1] If diarrhea is predominant, fecal leukocytes and stool tests for Clostridium difficile, Giardia, and Cryptosporidium, when appropriate, should be ordered.[1] Testing for celiac disease may be needed as well, and a tissue transglutaminase or TTG-IgA can be obtained.[1] A colonoscopy may be beneficial when there is a family history of inflammatory bowel disease, colon cancer, or alarm symptoms.[1] If the patient has diarrhea, random biopsies should be done on colonoscopy.[1]

Treatment / Management

One of the most important goals in the management of IBS patients is to develop a trusting patient-physician relationship by actively listening, showing empathy, and setting realistic expectations for treatment.[3][13] IBS is a symptom-based disorder, and thus treatment goals are aimed at resolving symptoms such as pain, bloating, cramping, and diarrhea or constipation.[3] For constipation, fiber supplements and laxatives can be helpful whereas, in those with diarrhea, medications such as loperamide or probiotics can be helpful.[3] Moreover, increased physical activity can increase colonic transit time and improve symptoms.[3][14] Patients also often associate food intake with IBS symptoms. Foods such as wheat products, onions, fruits, vegetables, sorbitol, and some dairy can include short-chain, poorly absorbed, highly fermentable carbohydrates, which are known as FODMAPs. FODMAPs have been associated with increased gastrointestinal symptoms in IBS patients.[3] Patients with constant and chronic abdominal symptoms oftentimes can have a response to low dose tricyclic antidepressants (TCAs) or serotonin reuptake inhibitors (SSRIs).[1] Alosetron can be used to treat IBS-D in females but can cause ischemic colitis.

Rifaximin is a nonabsorbable broad-spectrum antibiotic sometimes used to treat patients with irritable bowel syndrome. Patients with irritable bowel syndrome who take rifaximin have less abdominal pain and diarrhea. The effectiveness of rifaximin in patients with IBS is used as evidence to support the idea that bacterial overgrowth plays a role in the etiology of IBS.

Differential Diagnosis

The differential diagnosis of IBS is broad and ultimately depends on whether the patient has predominant diarrhea or constipation.  If a patient has IBS with diarrhea, the differentials includes lactose intolerance, caffeine intake, alcohol intake, gastrointestinal infections (Giardia, Amoeba, HIV), inflammatory bowel disease, medication-induced diarrhea (antibiotic use, proton pump inhibitor, nonsteroidal anti-inflammatory drugs, ACE inhibitor, chemotherapy), celiac disease, malignancies, colorectal cancer, hyperthyroidism, VIPoma, and ischemic colitis.[15] If constipation is the predominant symptoms, then the differentials can include inadequate fiber intake, immobility, Parkinson's disease, multiple sclerosis, spinal injury, diabetes, hypothyroidism, hypercalcemia, medication-induced (opiates, calcium-channel blockers, antidepressants, clonidine), malignancies, bowel obstruction, endometriosis, and diverticular disease. If a patient's history indicates one of these diseases, then appropriate lab testing should be pursued.[15]

Prognosis

IBS has a good prognosis, and the diagnosis is unlikely to change on follow-up.[16] The use of ambulatory health services by IBS patients can be reduced if a positive physician-patient interaction is developed.[16]