Keywords

1 Introduction

Anal incontinence (AI) is defined as the inability to delay evacuation until socially convenient (urge incontinence) or, more in general, as the involuntary passage of gut contents through the anus (passive incontinence or soiling/fecal seepage, the latter defining leakage of stool or mucus that occurs with normal continence and evacuation) [1]. Its prevalence is likely underestimated, due to embarrassment. Although it is known to increase with age, young individuals can also be affected, with no differences between males and females. Numerous idiopathic or secondary neuromuscular disorders of the pelvic floor and/or anal sphincter and/or rectum can be involved in causing its onset and are extensively described in this volume. Although diarrhea is more likely associated with incontinence, also severe constipation with fecal impaction and related pseudo-diarrhea can precipitate borderline anatomical and functional anorectal abnormalities.

The irritable bowel syndrome (IBS) is the most frequent clinical condition characterized by bowel abnormalities including diarrhea (IBS-D), constipation (IBS-C) and mixed bowel habits (IBS-M) [2]. AI has been reported to affect 15–20% of patients with IBS in a multicenter study [3]. Similarly, fecal incontinence occurring at least once per week was reported in 20% of IBS patients in a single-center study from the UK [4]. Compared with those who did not report AI, IBS patients affected by this severe complication had more frequently IBS-D, had undergone more gastroenterological consultations, and they were older, more frequently males, with higher body mass index and, not surprisingly, had more anxiety and depression [4]. Still, these figures could be markedly underestimated, since a prevalence as high as 60% was reported in another study carried out at a secondary care level in the UK and one-fourth of included patients admitted never having disclosed their incontinence before [5]. Appropriate management of IBS is required to control incontinence in affected individuals. We will briefly summarize the current view on IBS with a specific focus on the role of changes and modulation of gut microbiota.

2 Clinical Features

IBS is a functional gastrointestinal disorder (FGID), now termed disorder of gut-brain interaction (DGBI) [6], characterized by symptoms including abdominal pain associated with a change in stool form or frequency [2]. IBS diagnosis is based on the patient’s self-reported symptoms and no validated biomarkers for diagnosis are available.

IBS is defined by symptom-based diagnostic criteria known as the “Rome criteria”, which include recurrent abdominal pain, on average at least 1 day per week in the last 3 months, associated with 2 or more of the following: related to defecation, associated with a change in frequency of stool, associated with a change in form (appearance) of stool [2]. According to the Bristol Stool Form Scale, patients are categorized in IBS-D, IBS-C, and IBS-M [2].

Supportive symptoms include defecation straining, feeling of incomplete bowel movement, urgency, passing mucus, and bloating. IBS patients also often complain of mood problems, other gastrointestinal symptoms such as heartburn and extra-intestinal symptoms such as fibromyalgia, headache, back pain, and genitourinary symptoms such as, in women, the so-called pelvic pain referred to as a bladder pain, worsening of symptoms during menstruation, dyspareunia or other gynecologic symptoms. AI is reported by up to 20% of IBS patients [3, 4]. These symptoms increase the severity of IBS and may be associated with psychological factors.

3 Epidemiology and Risk Factors

IBS prevalence is highly variable among different geographical regions with averages around 10% and it is higher in women than men and in subjects aged less than 50 years old [7]. The peak incidence of IBS was observed in the third and fourth decades of life [7], although in certain countries the condition is more prevalent in younger men aged 16–30 years. Even if it is not a life-threatening condition, IBS impacts significantly quality of life and places a considerable burden both on the individual sufferers and society as a whole. Due to the relevant reduction of quality of life, patients would sacrifice on average up to 15 years of their remaining life expectancy for an immediate cure. It has been estimated that the annual direct and indirect costs related to IBS are estimated to be up to €8 billion in Europe [8].

An ascertained risk factor for IBS is a previous enteric infection [9, 10], either bacterial, viral or protozoal, which is now termed postinfection IBS (PI-IBS) [11]. A recent metanalysis [12] showed a fourfold risk increase in developing IBS in individuals exposed to an enteric infection, additionally reporting that female sex, severe infection course, antibiotic intake and previous psychological comorbidities were associated to the development of this condition. Interestingly, a long-term follow-up study over a 16-year period reported an IBS prevalence of 36.8% in a cohort with culture-proven Salmonella enteritidis infection [9].

4 Diagnosis

The most recent guidelines recommend a positive diagnosis of IBS simply using symptom-based diagnostic criteria included in the “Rome criteria”, in the absence of warning signs. In order to assess the presence of rectal disorders including both AI and outlet obstruction, clinicians should also ask about bladder and urinary symptoms and the need to strain or digitate the anal canal or vagina or support the perineum to facilitate the evacuation of stool [13].

In the presence of warning signs, a thorough evaluation is required that includes colonoscopy and other symptom-guided diagnostic tests [14]. A proctological examination should be performed to identify signs of pelvic floor and defecatory disorders, which should be further investigated by anorectal manometry, balloon expulsion test and defecography [14].

Among the laboratory tests suggested, a full blood count, C-reactive protein and serological screening for celiac disease should be performed. Differential diagnoses that should be considered include microscopic colitis, Crohn’s disease, bile acid diarrhea and small intestinal bacterial overgrowth for patients with IBS-D, and chronic constipation (without pain) for those with IBS-C.

5 Pathophysiology

In the past, IBS was considered to be the consequence of dysregulation of the brain-gut axis, associated with psychosocial factors, including stress, altered motility, and visceral hypersensitivity [6]. Current evidence views IBS as a disorder characterized by a complex interplay involving several peripheral and central factors, including: genetic predisposition, alterations in gastrointestinal sensory-motor function, intestinal dysbiosis, increased intestinal permeability, mucosal low-grade inflammation or immune activation, neuroendocrine abnormalities, food sensitivity/allergy and psychosocial factors [14, 15]. The main features of the current understanding of IBS pathophysiology are summarized in Fig. 19.1 [14, 15].

Fig. 19.1
A pathophysiological illustration of irritable bowel syndrome. Bile acids, dysbiosis, infections, and allergies increase the tight junction permeability between C D 3 plus T cells. Increase in eosinophils and mast cells. Below, there is the enteric nervous system. A cerebrospinal structure on the left is labeled increased sensory perception.

Main pathophysiological mechanisms of intestinal dysfunction and symptom perception in patients with irritable bowel syndrome

Full size image

There are at least three clinical scenarios that link IBS with the gut microbiota, all characterized by disruption of the balance between the host and the intestinal microbial ecosystem: (1) more than 10% of patients with IBS develop their symptoms after infectious gastroenteritis (PI-IBS) [11, 15]; (2) disruption of gut microbiota with systemic antibiotics increases the risk of IBS; (3) treatments aimed at modifying gut microbiota composition can improve symptoms in IBS.

Growing evidence indicates that dysbiosis, occurring when the diversity, composition, and/or functions of the intestinal microbial ecosystem are disrupted, could contribute to the alteration of the usual intestinal functions, with implications in the development, progression, and symptom flare-up of several diseases [15]. Among these, AI and IBS are both characterized by dysbiosis associated with impairment of the intestinal physiology [16].

6 Gut Microbiota Characterization

The human gut harbors a community of about 1014 microorganisms including bacteria, archaea, microeukaryotes (i.e., fungi and protists) and viruses, resulting in a mutually beneficial relationship with the host. The gut microbiota participates in digestive functions, shapes the host immune system, modulates host metabolisms, influences local and systemic processes (i.e., vitamin intake and nutrient metabolism), and protects against pathogens.

A reduced microbial diversity characterizes patients with IBS-D, suggesting that the microbial species involved in maintaining homeostasis may be missing [17]. An association between lower diversity and looser stool has been reported, suggesting a potential relevance in the pathophysiology of AI [18].

Most studies agree in showing increased Firmicutes and decreased Bacteroidetes in IBS. Bacteroides are increased in patients with IBS-D and associated with mucosal low-grade inflammation. Conversely, protective bacteria belong to Clostridiales, especially Faecalibacterium prausnitzii, an anaerobic butyrate-producing bacteria which has already been extensively studied for its favorable role in controlling inflammation and maintaining gut-barrier homeostasis. Reduced diversity, reduced exhaled methane, relative reduction of Methanobacteriales and Prevotella enterotype and abundance of Bacteroides enterotype have been reported to be linked to symptom severity.

Recent advances in metabolomics and metagenomics analyses of microbiota have led to the idea that symptoms could be caused by modification of the function more than the structure of gut microbiota. A meta-analysis identified low levels of fecal propionate and butyrate in IBS-C and a higher proportion of butyrate in IBS-D, compared to controls [19]. Other studies suggested a role of SCFAs-producing bacteria such as Ruminococcaceae, unknown Clostridiales and Erysipelotrichaceae in dysbiosis of IBS-D patients. A decreased availability of butyrate represents a potential attractive therapeutic target in IBS, as suggested by a recent study showing that Lactobacillus paracasei CNCM I-1572 improves symptoms and modulates gut microbiota structure and function through increased levels of acetate and butyrate in patients with IBS [20]. A recent trial aimed at assessing stool metabolome and microbiome in female patients with AI and the potential implication of metabolites and microbial composition to predict response to therapy remains to be fully analyzed [21]. All together these data constitute the rationale for pharmacological and nonpharmacological treatments aimed at modulating the intestinal microbiota in patients with IBS and AI.

7 Gut Microbiota Modulation

Compelling evidence suggests a key therapeutic role of microbial modification, including diet, prebiotics, probiotics, poorly-absorbable antibiotics or fecal microbiota transplantation (FMT), in IBS.

The dietary approach represents an interesting first-line treatment opportunity for IBS with a potential impact on gut microbiome. A low fermentable oligo-, di-, and monosaccharide and polyol (FODMAP) diet reduces fermentation and improves global symptoms as well as abdominal pain in patients with IBS, particularly with IBS-D [22]. Notably, the low FODMAP diet has been reported to benefit both stool consistency and fecal incontinence in patients with AI associated with loose stool [23].

Increasing interest exists on the role of prebiotics, probiotics, and symbiotics in the management of IBS. Unfortunately, the clinical evidence of probiotic efficacy in disease states is still inconclusive. Although international guidelines suggest against the use of probiotics in IBS [24], recent well-performed trials using well-defined end-points show promising results [20]. Based on these data, the British Society of Gastroenterology (BSG) guidelines on the management of IBS suggest that probiotics, as a group, may be an effective treatment for global symptoms and abdominal pain in IBS [25]. Further studies are needed to clarify definitely the potential role of probiotics in the management of patients with IBS and AI.

Rifaximin, a poorly-absorbable nonsystemic antibiotic, was found to be more effective than placebo in reducing IBS global symptoms, bloating, abdominal pain and loose or watery stools of patients with IBS without constipation [25]. Furthermore, repeat treatment with rifaximin was found to be effective and safe, without developing bacterial resistance. The main international guidelines on IBS suggest the use of rifaximin to treat IBS-D [24].

Fecal microbiota transplantation (FMT)—the process of transferring intestinal microbiota from a healthy donor into the gastrointestinal tract of a patient with dysbiosis, which proved to be an effective treatment of recurrent Clostridioides difficile infection—has been evaluated in IBS with mixed results [26]. Recently, a large single-center trial including IBS of all subtypes showed that FMT (30 g and 60 g), acquired from a single superdonor in excellent health and delivered into the distal duodenum, was significantly more effective than placebo (autologous FMT) [27].

8 Conclusions

IBS is a frequent clinical condition characterized by pain and stool abnormalities. IBS-D represents the most frequent cause of chronic or recurrent diarrhea in industrialized countries, thus constituting a major threat for AI patients. IBS-D and AI also seem to share some common pathophysiological mechanisms. Treating AI without taking into consideration bowel functions as a whole would represent a major mistake. Thus, recognizing the treatable underlying causes of IBS is mandatory for a correct therapeutic management of these complex cases. Our understanding of IBS pathophysiology has enormously improved over the last two decades and several effective therapeutic approaches are available.