Избыточный бактериальный рост в тонком кишечнике: причины, симптомы и лечение.

Small intestinal bacterial overgrowth (SIBO), also termed bacterial overgrowth, or small bowel bacterial overgrowth syndrome (SBBOS), is a disorder of excessive bacterial growth in the small intestine. Unlike the colon (or large bowel), which is rich with bacteria, the small bowel usually has fewer than 100,000 organisms per millilitre. Patients with bacterial overgrowth typically develop symptoms which may include nausea, bloating, vomiting, diarrhea, malnutrition, weight loss and malabsorption, which is caused by a number of mechanisms. The diagnosis of bacterial overgrowth is made by a number of techniques, with the gold standard being an aspirate from the jejunum that grows in excess of 105 bacteria per millilitre. Risk factors for the development of bacterial overgrowth include dysmotility; anatomical disturbances in the bowel, including fistulae, diverticula and blind loops created after surgery, and resection of the ileo cecal valve; gastroenteritis induced alterations to the small intestine; and the use of certain medications, including proton pump inhibitors. Small bowel bacterial overgrowth syndrome is treated with an elemental diet or antibiotics, which may be given in a cyclic fashion to prevent tolerance to the antibiotics, sometimes followed by prokinetic drugs to prevent recurrence if dysmotility is a suspected cause.

Symptoms traditionally linked to SIBO include bloating, diarrhea, constipation, and abdominal pain/discomfort. Steatorrhea may be seen in more severe cases. constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. SIBO also causes an increased permeability of the small intestine. Some patients may lose weight. Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where fats are not properly absorbed and spill into the stool, may also occur. Laboratory test results may include elevated folate, and, less commonly, vitamin B12 deficiency or other nutritional deficiencies.

Some studies reported up to 80% of patients with irritable bowel syndrome (IBS) have SIBO (using the hydrogen breath test). IBS D is associated with elevated hydrogen numbers on breath tests while IBS C is associated with elevated methane numbers on breath tests. Subsequent studies demonstrated statistically significant reduction in IBS symptoms following therapy for SIBO. There is general consensus that breath tests are abnormal in IBS; however, the disagreement lies in whether this is representative of SIBO.

Certain people are more predisposed to the development of bacterial overgrowth because of certain risk factors. These factors can be grouped into four categories: (1) disordered motility or movement of the small bowel or anatomical changes that lead to stasis, (2) disorders in the immune system, (3) interference with the production of proteolytic enzymes, gastric acid, or bile, and (4) conditions that cause more bacteria from the colon to enter the small bowel. This may affect results from hydrogen methane breath testing. Absence or impairment of the migrating motor complex (MMC), a cyclical motility pattern in the small intestine, and phase III of the MMC in particular, is associated with the development of SIBO. Problems with motility may either be diffuse, or localized to particular areas. MMC impairment may be a result of post infectious irritable bowel syndrome, drug use, or intestinal pseudo obstruction among other causes. There is an overlap in findings between tropical sprue, post infectious irritable bowel syndrome and small intestinal bacterial overgrowth in the pathophysiology of the three conditions and also SIBO can similarly sometimes be triggered by an acute gastrointestinal infection. As of 2020, there is still controversy about the role of SIBO in the pathogenesis of common functional symptoms such as those considered to be components of irritable bowel syndrome. cause diffuse slowing of the bowel, leading to increased bacterial concentrations. More commonly, the small bowel may have anatomical problems, such as out pouchings known as diverticula that can cause bacteria to accumulate. After surgery involving the stomach and duodenum (most commonly with Billroth II antrectomy), a blind loop may be formed, leading to stasis of flow of intestinal contents. This can cause overgrowth, and is termed blind loop syndrome. Systemic or metabolic disorders may lead to conditions allowing bacterial overgrowth as well. For example, diabetes can cause intestinal neuropathy, pancreatitis leading to pancreatic insufficiency can impair digestive enzyme production, and bile may be affected as part of cirrhosis of the liver. The use of proton pump inhibitors, a class of medication used to reduce stomach acid, is associated with an increased risk of developing SIBO. Finally, abnormal connections between the bacteria rich colon and the small bowel can increase the bacterial load in the small bowel. Patients with Crohn's disease or other diseases of the ileum may require surgery that removes the ileocecal valve connecting the small and large bowel; this leads to an increased reflux of bacteria into the small bowel. After bariatric surgery for obesity, connections between the stomach and the ileum can be formed, which may increase bacterial load in the small bowel.

The diagnosis of bacterial overgrowth can be made by physicians in various ways. Malabsorption can be detected by a test called the D xylose absorption test. Xylose is a sugar that does not require enzymes to be digested. The D xylose test involves having a patient drink a certain quantity of D xylose, and measuring levels in the urine and blood; if there is no evidence of D xylose in the urine and blood, it suggests that the small bowel is not absorbing properly (as opposed to problems with enzymes required for digestion). The gold standard for detection of bacterial overgrowth is the aspiration of more than 105 bacteria per millilitre from the small bowel. The normal small bowel has less than 104 bacteria per millilitre. Some experts however, consider aspiration of more than 103 positive if the flora is predominately colonic type bacteria as these types of bacteria are considered pathological in excessive numbers in the small intestine. The reliability of aspiration in the diagnosis of SIBO has been questioned as SIBO can be patchy and the reproducibility can be as low as 38 percent. Breath tests have their own reliability problems with a high rate of false positive. Some doctors factor in a patients' response to treatment as part of the diagnosis. There is insufficient evidence to support the use of inflammatory markers, such as fecal calprotectin, to detect SIBO. The main obstacle to accurately define SIBO is limited understanding of the normal intestinal microbial population. Future advances in sampling technology and techniques for counting bacterial populations and their metabolites should provide much needed clarity. In addition to the archaeon, a few bacteria can also produce methane, such as members of the Clostridium and Bacteroides genus. Production of methane therefore, may not be bacterial, nor limited to the small intestine, and it has been proposed that the condition should be classified as a separate 'intestinal methanogen overgrowth' (IMO). A variety of antibiotics, including tetracycline, amoxicillin clavulanate, metronidazole, neomycin, cephalexin and trimethoprim sulfamethoxazole have been used; however, the best evidence is for the use of rifaximin, a poorly absorbed antibiotic. A combination of probiotic strains has been found to produce better results than therapy with the antibiotic drug metronidazole. An elemental diet has been shown to be highly effective for eliminating SIBO with a two week diet demonstrating 80% efficacy and a three week diet demonstrating 85% efficacy. An elemental diet works via providing nutrition for the individual while depriving the bacteria of a food source. Additional treatment options include the use of prokinetic drugs such as 5 HT4 receptor agonists or motilin agonists to extend the SIBO free period after treatment with an elemental diet or antibiotics. A diet void of certain foods that feed the bacteria can help alleviate the symptoms.