SIBO Part I: Clinical Presentation

Welcome back to Run the List, a medical education podcast in internal medicine. As a quick disclaimer, this podcast is made for educational and informational purposes only and should not be understood as medical advice under any circumstances. Before we get to the show, a quick word on the sponsors for today's episode. - Open evidence is the premier AI-powered medical information platform for physicians and medical students. It's like chat GPT for anyone who practices clinical medicine. Whether you have a clinical question, a question that comes up during your literature review, if you have a question that comes up when you're trying to synthesize a topic you're gonna teach, you can just go to openevidence.com and or your question and it'll synthesize the answer for you while also linking to those actual articles. It's an outstanding resource. (upbeat music) - I am Ali Shivi, a current third year medical student Harvard and a recent member of the RTL team. Today we will be talking about small intestinal bacterial overgrowth, also known as SIBO, an increasingly discussed and diagnosed condition in the primary care office and gastroenterology clinic. I have with me Dr. Kumar, a gastroenterologist at Brigham & Women's Hospital, and of course one of our fearless leaders of RTL. - Thank you so much, Ali. It's so great to be back recording with you and especially on such a hot topic as SIBO. I also wanna thank you for all your outstanding work and bringing back our RTL handouts along with your co-medical student, Hannah Shapiro, who is also a third year student at Harvard Medical School. And if I may have the honor as it's been a little bit, so I'd love to say our token line here, let's go ahead and run the list. - And as always, let's start with the case. Miss R is a 45 year old female with a history of obesity, status posts ruined wide gastric bypass, who presents to primary care clinic with two months of worsening bloating, flatulence, and diarrhea. She reports abdominal distension is present upon waking up and worsens after meals. She's concerned that she might have underlying SIBO. Dr. Kumar, can you first explain what SIBO is? - Absolutely, so SIBO or small intestinal bacterial overgrowth is a condition in which bacteria proliferate in the small intestine, which should otherwise be a largely sterile environment where you do not have very much bacteria at all. So we can contrast that with the colon, which is full of bacteria. And think about how, if these bacteria or other species take resonance more proximately in the small intestine, how those bacteria can produce a variety of symptoms for patients dealing with this condition. - That totally makes sense. So how does the overgrowth of bacteria cause the symptoms our patient is experiencing? - That's an excellent question, Ali, and the way I think about this when I'm teaching or even when I'm in clinic and talking to my own patients is that there's three main pathways for bacterial overgrowth of a small intestine to cause symptoms. So the first one is that the small intestinal bacteria that are in this area of the digestive track where they shouldn't be end up breaking down the foods and the nutrients from a recently ingested meal, right? Because the first thing that happens after the food empties from the stomach is that it gets into the small intestine. And normally, those nutrients would be broken down and completely absorbed in the small intestine. But when there's bacteria there, the bacteria actually break down these nutrients. And in the process of breaking down these products, they release gas. And that gas leads to the classic symptoms of abdominal bloating, cramping, distension, as well as flatulence. So the first mechanism is from the bacteria breaking down the food products and releasing gas. Now, the second pathway that I think about is how these small intestinal bacteria, which again are not in the location that they should be, they can actually take up residence right along the mucosal surfaces of a small intestine. And as they line up on the mucosal surfaces, they can interfere with absorption. In these cases, what typically happens is a watery diarrhea ensues because the unabsorbed substances that would have otherwise been absorbed, but instead of remaining in the lumen because they're not getting absorbed being blocked by this bacteria, they're lining the mucosa. Those unabsorbed substances draw water into the intestinal tract a la osmotic diarrhea. So in even more severe cases, you don't only have an osmotic diarrhea, but you can also get stiateria because of small intestinal bacteria actually disrupt absorption of fats. So some patients would see, though, actually complain more of stiateria where they see oil droplets in the stool or they have the classic very difficult to flush stool from the toilet bowl of stiateria. So think about what is happening at the mucosa level in terms of absorption and how that can lead to osmotic as well as stiateria. And the third pathway, and this is typically only in extreme cases, but these same small intestinal bacteria that are competing for nutrients and potentially interfering with absorption at the level of the mucosa, they can actually lead to vitamin and mineral deficiencies. So what can happen is that these bacteria can absorb B12 for themselves and take that away, essentially steal it from the host and cause B12 deficiency. This can also happen with iron deficiency as well, where the bacteria are consuming the iron that was ingested, and instead of the host being able to absorb that iron, the bacteria take it from themselves. And then inversely, it's an interesting phenomenon as if some of these bacteria actually produce folate. And so they can raise the levels of folate above assay when measured from the blood. So a little pearl to share is that if you have a patient who has clinical signs or symptoms of SIBO, let's say they have the abdominal gas, distension and diarrhea, you may also see this hallmark sign on labs where they have B12 deficiency because the SIBO is stealing the B12 from the host and then high folate levels often greater than assay because they are producing folate. So I always, if the patients happen to have had recent anemia labs with a B12 in folate, I look to look at those because though it fits that pattern, low B12 high folate, I'm thinking, okay, my index is suspicion for this being SIBO is even higher. - Awesome, that's a great pearl. So now that we understand the pathophysiology explaining why the constipation of symptoms of SIBO occurs, who actually gets SIBO and are there any risk factors for developing this disease? - Yes, for sure. So the overarching theme as with any proliferation of bacteria in any part of the body is altered motility. So the most common cause of altered GI motility, as we all know is IBS or Iridal bowel syndrome. And that's why there's such a significant overlap between IBS and SIBO. Actually, recent data and literature shows that up to a third of patients who have IBS also had SIBO based on data obtained from those patients. So there's a very high overlap between IBS and SIBO and that's largely driven by the altered GI motility that is characteristic of IBS. There are also other diseases that affect gut motility that we should also consider in patients who are at risk for SIBO. And that includes conditions such as diabetes, melodis, radiation and erytis, crones and erytis, scleroderma and amyloidosis. So that's a bigger box and some are more common than others, but it's always important to just consider what is the underlying motility of my patients digestive tract and does that put them at risk for SIBO? - That makes sense. If peristalsis is not consistent, this provides an excellent environment for bacteria to stay in one place and replicate. - Right, exactly. So, and then stasis can occur not just because of altered motility, but also due to anatomical variation. And that's true for the patient that you described in this M who has a history of a Ruin Y gastric bypass. And so with the Ruin Y gastric bypass, there is this formation of a blind pouch within the newly altered GI tract. And within that blind pouch, there is stasis and hence an increased risk for bacteria to proliferate in that area. Another example would be stasis due to an illiostricture in crones disease. We already mentioned crones disease as a possible risk factor recebo because of altered motility. But also, if there is a narrowing, stool is not gonna travel through that area as readily. And so that decrease in motility and also just stasis from hanging above the stretcher can lead to increased risk recebo. Another common, actually fairly common cause that I found in my patients is identifying a small bowel of diverticula. We think about diverticulosis primarily in a large intestine, but it can also be present in the small intestine. And you can imagine if there's an outpouching in the small intestine that almost shields is shielded from the natural transit through the lumen, bacteria can proliferate in that little outpouching within the small intestine. So the bacteria like to hang out there as well. So we talked mainly about stasis in this segment, but the other anatomical variants to consider are fischulas. So think about fischulizing crones disease where you're connecting the large intestine to the small intestine and how bacteria can move retrograde from the large to the small intestine via the fischula. Or also, many patients who have had surgery such as a right hemicollectomy, they will lose the illiosecal valve that otherwise prevents backflow or reflux of colonic contents into the small intestine. So loss of that valve will allow bacteria to directly flow upwards from the large intestine to the small intestine and cause SIBO in those patients. - Great review of other comorbidities we should be thinking about when SIBO is on the differential. I've also heard about there being a risk of SIBO when there is long-term PPI use. What is the mechanism behind this relationship? - Right, so as you can imagine, stomach acid is one of the GI defenses against bacterial growth that the lower pH is actually an environment in which bacteria do not grow well in. And so when patients are put on long-term PPI, the reduction in stomach acid impairs this host defense and allows bacteria to grow more approximately in the small intestine. So it's important to think about SIBO in your patients who are on long-term PPI and coming to you with symptoms like bloating or diarrhea. And then also it can occur outside of PPI use in patients who have extensive atrophic gastritis where they've lost or damaged many of their pridele cells from an autoimmune process. The lack of pridele cells producing acid will lead to this again low acid state that can put them at risk for bacterial overgrowth. So to review, we've identified altered GI motility, altered anatomy and low stomach acid production as causes for SIBO. Are there any other categories we should be considering? - Yes, there are two more risk factors for SIBO that we should generally be aware of. The first big bucket is anyone with immunodeficiency. So things we think about here are patients with CV, ID or common variable immune deficiency, IgA deficiency and/or HIV or AIDS. Just the general risk factor of being immunodeficient will of course put these patients at higher risk for any type of bacterial proliferation. And that includes that bacterial proliferation within the small intestine. The second category includes patients who have altered bile or digestive enzyme composition. So the main disorder is that we think about in this category are patients with cirrhosis as well as patients with chronic pancreatitis. Both of these patient categories cirrhosis and chronic pancreatitis have this issue with altered bile digestive enzyme composition which can lead to the proliferation of bacteria in the small intestine. - Fantastic. We covered a lot today in terms of understanding why SIBO happens, why patients present with the symptoms they do and who might be at risk. Before we go, can you summarize the most important takeaways for our listeners? - Yes, absolutely. Let's do three key takeaways here. So the first key takeaway is just understanding that SIBO or small intestinal bacteria overgrowth is due to a pathological increase in the amount of bacteria in the small bowel, which otherwise should be a sterile environment and contrast that with the colon which we discussed earlier is full of bacteria. So the issue is when these bacteria start proliferating in the small intestine where they shouldn't be. The second main takeaway that I'd like to share is that these small-bat intestinal bacteria can produce a variety of symptoms but the main ones are due to increased gas production and decreased mucosal absorption. And the combination of the increased gas production and the decreased mucosal absorption could lead to the classic symptoms of bloating, abdominis, comfort and diarrhea, which is usually a watery diarrhea due to osmotic diarrhea from the osmolytes remaining in the intestine as opposed to being absorbed. But then as we talked about earlier, there are instances where patients will present more of its diarrhea due to fat absorption. And my last key takeaway is that the major risk factor for SIBO is altered motility in the GI tract and that's most commonly due to IBS but also consider abnormal anatomy whether that be strictures or officializing disease or small intestinal diverticula or post-surgical such as Ruin-Y or loss of illicycle valve and then also the gastric hypochlorhydria which basically means a low acid state which can be due to long-term PPI use or excessive atrophic gastritis leading to pridele cell death. So those are the main risk factors that I want you to think about when you're seeing patients who you believe may have SIBO because it gives you an sense of why they're presenting with this condition at this time. - Thank you, Dr. Kumar, for all your SIBO insights and to our audience, we hope you enjoyed the first episode of our three part series on SIBO. We will be back soon to discuss SIBO diagnosis and treatment. - Thank you so much, Ally, you were an awesome guest host and thank you again for helping to resurrect our handouts. We'll definitely have one for this episode and like Ally said, we're looking forward to coming back to discuss the next two episodes on SIBO on diagnosis and then treatment. (upbeat music) (upbeat music)