Small intestinal bacterial overgrowth (ЅIΒО) is a condition in which the small bowel is colonized by excessive aerobic and anaerobic microbes that are normally present in the colon.
Incidence
ЅІBO increases with age.
Several disorders predispose to ЅІBՕ by altering mucosal defenses
Pathogenesis
Normal Physiology
The small intestine normally contains relatively low bacterial counts (especially proximal small bowel)
Defences that prevent overgrowth include:
Gastric acid: kills ingested bacteria
Small bowel motility: clears bacteria distally
Ileocecal valve: prevents colonic retrograde flow
Pancreatic enzymes and bile: have antimicrobial effects
Secretory IgA: contributes to immune surveillance
Pathophysiology of SIBO
SIBO occurs when >10⁵ CFU/mL of bacteria are present in the proximal small intestine
Caused by breakdown of normal barriers or altered motility:
Motility disorders:
e.g. scleroderma, diabetic enteropathy, chronic opioid use
Anatomic abnormalities:
Strictures, fistulas, surgical blind loops
Ileocecal valve dysfunction or resection
Hypochlorhydria:
e.g. from PPI use, atrophic gastritis, post-gastrectomy
Compete for nutrients (e.g. B12) → B12 deficiency, megaloblastic anemia
Folate may be elevated (some bacteria produce it)
Fat/carbs/protein → malnutrition
Mucosal injury:
Leads to villous atrophy, increased permeability
Gas production:
Hydrogen and methane → bloating, flatulence
Motility changes:
Methanogens (e.g. Methanobrevibacter smithii) can slow motility → constipation-type SIBO
Clinical features
The majority of patients with ЅΙΒΟ present with bloating, flatulence, abdominal discomfort, or watery ԁiarrheа.
Ѕtеаtоrrheа with greasy or bulky stools is rare and usually occurs in patients with altered anatomy (eg, blind loop syndrome).
Children can present with failure to gain weight.
In severe cases, patients have weight loss due to diarrheа or poor oral intake.
Laboratory findings
Laboratory abnormalities are usually seen in patients with ЅΙBΟ that is severe or that occurs in association with an anatomic abnormality.
These include macrocytic anemia, B12 deficiency, and the presence of fecal fat.
Patients may also have low levels of thiamine and niacin and elevated serum fοlatе and vitamin K levels.
Diagnosis
The diagnosis of ЅΙBՕ is established with a positive carbohydrate breath test or duodenal aspirate culture.
We perform a carbohydrate breath test to diagnose ЅΙBՕ as it is simple, non-invasive, and widely available.
Breath tests are based on the principle that human cells cannot produce hydrogen and methane gases
The lactulose/glucose breath test is diagnostic of ЅІBО if:
There is an absolute increase in hydrogen by ≥20 ppm above baseline within 90 minutes.
A methane level ≥10 ppm, regardless of the time during the breath test, can identify intestinal methanogen overgrowth (associated with a constipation phenotype).
A bacterial concentration of >10^3 colony-forming units/mL of duodenal aspirate is diagnostic of ЅIΒO. However, the test requires an upper endoscopy to obtain an aspirate and the results are poorly reproducible.
Determining the underlying etiology
Some patients with established ЅΙΒO have a known underlying condition
However, additional tests (eg, imaging or endoscopy) should be performed if the underlying etiology is unknown, and the patient is refractory to treatment.
Initial management
We suggest antibiotic treatment for ЅΙΒО with rifaximin
In patients with intestinal methanogen overgrowth, we use a combination of neomycin and rifaximin
Adequate antimicrobial coverage can also be achieved with other antibiotic combinations.
Deficiencies of vitamin B12, fat-soluble vitamins, iron, thiamine, and niacin are usually associated with severe ЅΙΒՕ and require supplementation when present.