Mechanisms of development (pathogenesis)
To understand the pathogenesis, it is necessary to recall the physiological norm: about 8–10 liters of fluid pass through the digestive tract daily. This volume consists of ingested water, as well as secretions from the salivary glands, stomach, pancreas, bile, and intestinal juice. The vast majority of this fluid is reabsorbed in the small intestine. Diarrhea occurs when this balance is disrupted.
Depending on the cause of fluid accumulation in the intestinal lumen, the following types of the syndrome are distinguished:
- Osmotic: osmotically active substances accumulate in the intestine and retain water. A classic example is the use of osmotic laxatives. The main differential feature of this type is that diarrhea completely stops during fasting (withdrawal of enteral nutrition).
- Secretory: there is direct stimulation of water and electrolyte secretion into the intestinal lumen. Causes may include bacterial toxins (cholera toxin), hormonally active substances (gastrin, vasoactive intestinal polypeptide — VIP), bile and fatty acids, as well as certain medications.
- Inflammatory (Exudative): the mucous membrane is damaged, and inflammatory exudate is released into the intestinal lumen. This is characteristic of acute intestinal infections and chronic inflammatory bowel diseases (IBD).
- Hypermotile: occurs due to increased intestinal motility, when contents move through the tract too quickly (typical of irritable bowel syndrome).
Clinical Classification by Course
Based on the duration of the disease, the syndrome is divided into two large groups that require completely different medical approaches.
Acute diarrhea starts suddenly, stool frequency exceeds 3 times a day, and stool consistency becomes liquid. Most often it is self-limiting. It includes the following etiological groups:
- Infectious (viruses, bacteria, parasites) and toxico-infectious (food poisoning).
- Toxic (poisoning with arsenic, mercury, hyperosmolar drugs).
- Neurogenic ("exam diarrhea" triggered by severe psychoemotional stress).
- Metabolic (for example, uremic diarrhea in severe renal failure).
Management tactics: bacteriological stool culture is mandatory to identify the specific pathogen.
Chronic diarrhea lasts for more than one month. This condition requires a prolonged diagnostic workup, as there can be many causes:
- IBD: ulcerative colitis (UC) and Crohn's disease.
- Iatrogenic (drug-induced) diarrhea: occurs with laxative abuse, iron supplements, or magnesium-containing antacids. A separate category is antibiotic-associated diarrhea, in which the destruction of normal microflora leads to the activation of Clostridium difficile and the development of severe pseudomembranous colitis.
- Oncological processes: malignant neoplasms of the gastrointestinal tract.
- Malabsorption syndrome: impaired absorption can be hereditary (celiac disease) or acquired (pancreatic pathology).
- Systemic diseases and infections: hyperthyroidism, HIV infection, giardiasis.
- Functional disorders: irritable bowel syndrome (IBS) — established as a diagnosis of exclusion.
Anamnesis Taking and Topographical Diagnostics
The diagnostic workup begins with a thorough interview. The physician needs to find out the frequency of bowel movements (comparing it to the "pre-illness" norm), the duration of symptoms, and the presence of nocturnal defecation (an important differential sign). Stool color, consistency, and the presence of pathological impurities (mucus, blood) are also evaluated.
Data on the relationship between symptoms and food intake, recent antibiotic use, and possible contact with infectious patients are necessarily collected. Additionally, the presence of associated manifestations is clarified: weight loss, abdominal pain, nausea, and vomiting. At the interview stage, it is extremely important to differentiate true diarrhea from fecal incontinence, as patients often confuse these conditions.
Topographical diagnostics (determining the level of lesion) is possible based on characteristic stool features:
- Small intestine involvement: stool is infrequent but very voluminous (polyfecalia). The appearance of steatorrhea (fatty stool) is characteristic, which directly indicates malabsorption syndrome or pancreatic pathology.
- Distal large intestine involvement: stool is frequent, but passed in small portions. It is accompanied by imperative (urgent) urges to defecate and tenesmus — false, extremely painful urges.
Briefly: answers to topical questions
What pathological impurities in the stool are characteristic of inflammatory (exudative) diarrhea?
Pathological impurities in inflammatory diarrhea indicate tissue destruction and exudate release due to damage to the intestinal mucosa. Characteristic impurities include:
- Mucus — released in large bowel involvement (acute shigellosis, ulcerative colitis).
- Blood — may appear as streaks in shigellosis, bright red blood in ulcerative colitis, or uniformly colored mucus resembling "raspberry jelly" in hemorrhages and necrosis.
- Pus — detected in ulcerative colitis, severe acute colitis, and hemorrhagic processes, sometimes forming a "rectal spit" (stool consisting exclusively of mucus, blood, and pus).
What hormonally active tumors can cause severe secretory diarrhea?
It directly follows from the sources:
- In secretory diarrhea, etiological factors can be hormonally active substances: gastrin and vasoactive intestinal polypeptide (VIP).
- Carcinoid tumors are associated with carcinoid syndrome, one of the clinical signs of which is diarrhea.
- For carcinoid tumors of the small intestine, the secretion of serotonin and other vasoactive substances (histamine, dopamine, kallikrein) is indicated.
- For thymic carcinoid, carcinoid syndrome with asthma attacks, pronounced cyanosis, and diarrhea has been described.
Specific names of tumors producing VIP or gastrin are not provided in the sources.
What extraintestinal systemic and endocrine diseases can cause chronic diarrhea?
The sources directly indicate endocrine and systemic causes of chronic diarrhea as:
- Hyperthyroidism.
- HIV infection (AIDS).
Separately, the sources describe diarrhea in diabetes mellitus as diabetic enteropathy as part of gastrointestinal autonomic neuropathy, but diabetes mellitus is not listed in the provided list of causes of chronic diarrhea.
On the basis of what criteria is the diagnosis of diarrhea-predominant irritable bowel syndrome (IBS-D) established?
The diagnosis of irritable bowel syndrome (IBS) is a diagnosis of exclusion and is established based on the Rome IV criteria. The main sign is recurrent abdominal pain occurring at least 1 time per week, characterized by two or more criteria:
- related to defecation;
- associated with a change in stool frequency;
- associated with a change in stool form.
The time criterion requires symptoms to have been present for the last 3 months with a total disease duration of at least 6 months. The diarrhea-predominant variant is classified based on the nature of changes using the Bristol Stool Form Scale.
Which specific bacterial pathogens are most commonly the cause of acute infectious diarrhea?
According to sources, the most frequent bacterial cause of acute intestinal infections / acute infectious diarrhea is pathogenic gram-negative agents, among which are indicated:
- Salmonella: Salmonella typhimurium, S. enteritidis.
- Shigella: Shigella flexneri, S. sonnei.
- Campylobacter: Campylobacter jejuni, C. coli, C. laridis, C. fetus.
- Escherichia: enteropathogenic, enterotoxigenic, and enteroinvasive classes.
- Yersinia: Yersinia enterocolitica serotypes O3, O5, O6, O9.
For the differential diagnosis of osmotic and secretory diarrhea, use the "fasting test": osmotic diarrhea stops when food is withheld, whereas secretory diarrhea persists regardless of food intake.
Class and exam questions
What is the main difference between stool in small bowel versus large bowel lesions?
In small bowel pathology, stool is voluminous (polyfecal) and may contain fat (steatorrhea). When the distal large intestine is affected, stool is frequent, in scant portions, and accompanied by painful tenesmus.
What is antibiotic-associated diarrhea?
This is a complication of antibiotic therapy in which the destruction of normal intestinal microflora leads to the overgrowth of Clostridium difficile, resulting in pseudomembranous colitis.
Why is a stool bacterial culture performed in cases of diarrhea?
This is a mandatory stage in the diagnosis of acute diarrhea, necessary for verifying (accurately identifying) the causative agent of the infectious process.
What else to study on this topic
- Mechanisms and consequences of malabsorption syndrome
- Differential diagnosis of IBD (Crohn's disease and ulcerative colitis)
- Pathogenesis of pseudomembranous colitis (Clostridium difficile infection)
- Diagnostic criteria for irritable bowel syndrome (IBS)
- Assessment of pathological stool admixtures: clinical significance
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