Diarrhea

Podcast Episode Introduction

Welcome back to GI Insights, the podcast dedicated to bringing clarity to complex clinical topics in gastroenterology. Today, we're tackling one of the most universal challenges in all of medicine: the patient with diarrhea. It’s a chief complaint that can signify anything from a benign, self-limited illness to a life-threatening condition. The differential is vast, and it’s easy to get lost in a sea of unnecessary tests or, worse, miss a critical diagnosis.

The goal of this episode is to move beyond rote memorization and equip you with a clear, systematic, and actionable mental framework for triaging, diagnosing, and managing these patients. We'll build a clinical reasoning model that you can apply at the bedside, whether you're in the emergency department, a primary care clinic, or on the GI consult service. Over the next few minutes, we’ll cover the core frameworks for initial triage, the five pathophysiologic buckets that organize your differential, a step-by-step diagnostic pathway, a special deep dive into malabsorption, and finally, a pragmatic treatment ladder for when the initial workup is unrevealing. Let’s get started.

1. The First Sort: Triage, Time, and Temperature

The initial moments of a patient encounter are critical for triage. Before diving into complex pathophysiology, the clinician's first job is to determine acuity and chronicity. These two factors—how sick is the patient right now, and for how long has this been going on?—dictate the entire pace and direction of the workup, telling you whether to admit, scope urgently, or proceed with a measured outpatient evaluation.

Critical Red Flags

The first and most important question is "sick or not sick?" The presence of any of the following alarm features should trigger a more urgent evaluation, and often, admission.

• Hemodynamic Instability: Hypotension, tachycardia, or marked volume depletion.
• Systemic Toxicity: High fever or severe abdominal pain, especially with peritoneal signs.
• Significant Bleeding: Grossly bloody stool.
• Host Factors: Immunocompromised status (e.g., transplant recipients, patients on chemotherapy) or extremes of age.
• Key Historical Clues: Unintentional weight loss or nocturnal diarrhea that awakens the patient from sleep.

Define the Clinical Time Course

The next crucial sort is based on duration. This simple classification helps narrow the differential diagnosis significantly.

• Acute Diarrhea: Symptoms lasting less than 14 days. This is most often infectious, toxin-mediated, or drug-induced.
• Persistent Diarrhea: Symptoms lasting between 14 and 30 days. This category often includes unresolved infections, post-infectious syndromes, or the initial presentation of inflammatory bowel disease (IBD).
• Chronic Diarrhea: Symptoms lasting more than 4 weeks. For these patients, thinking in pathophysiologic categories becomes essential.

Once you’ve established that the patient is stable and their condition is chronic, the next step is to classify the type of diarrhea to systematically narrow the differential.

2. The Core Mental Model: The Five Pathophysiologic Buckets

The "five buckets" framework is the central organizing principle for any chronic diarrhea workup. Rather than chasing down dozens of individual diagnoses, the goal is to first identify the primary pathophysiologic category the patient falls into. This is the single most important step, as it directly informs the subsequent diagnostic strategy, from which stool tests to order to whether an endoscopy is required.

2.1. Osmotic Diarrhea

This type of diarrhea occurs when non-absorbed, osmotically active solutes remain in the intestinal lumen, pulling water in with them. The classic analogy is a sponge in the gut. Because it is driven by ingested substances, it characteristically improves or resolves completely with fasting.

• Hallmarks and Examples
  • Clinical Clues: Smaller stool volume, symptoms cease with fasting, high stool osmotic gap (>100 mOsm/kg).
  • Representative Examples: Lactose intolerance, magnesium-containing antacids or supplements, poorly absorbed sugar alcohols (sorbitol, mannitol, xylitol), and SIBO with carb malabsorption.

2.2. Secretory Diarrhea

Secretory diarrhea is caused by a net increase in intestinal secretion of electrolytes and water, or an inhibition of normal absorption. Unlike osmotic diarrhea, it is driven by internal processes and is not dependent on what the patient eats. This results in a high-volume, watery diarrhea that persists even when the patient is fasting.

• Hallmarks and Examples
  • Clinical Clues: Large stool volume, persists with fasting, often occurs at night, low stool osmotic gap (<50 mOsm/kg).
  • Representative Examples: Bile acid malabsorption (e.g., post-cholecystectomy), microscopic colitis, hormone-secreting tumors (e.g., VIPoma), stimulant laxative abuse, and post-surgical short bowel or high output ileostomy.

2.3. Inflammatory / Exudative Diarrhea

This category results from disruption of the mucosal lining of the intestine, leading to an exudation of mucus, protein, and blood into the lumen. These patients are often systemically unwell and present with symptoms beyond just loose stool, such as fever and abdominal pain.

• Hallmarks and Examples
  • Clinical Clues: Blood and/or mucus in the stool, fever, abdominal pain, elevated fecal calprotectin or lactoferrin.
  • Representative Examples: Inflammatory bowel disease (Crohn's disease, ulcerative colitis), invasive infections (e.g., Shigella, Campylobacter, C. difficile), ischemic colitis, and radiation colitis.

2.4. Fatty Diarrhea (Malabsorption / Maldigestion)

Also known as steatorrhea, this type of diarrhea is caused by the failure of normal fat digestion (maldigestion) or absorption (malabsorption). Because fat is a major source of calories, patients with significant steatorrhea almost always present with weight loss and deficiencies of fat-soluble vitamins.

• Hallmarks and Examples
  • Clinical Clues: Bulky, pale, greasy, foul-smelling stools that are difficult to flush; associated with weight loss and nutritional deficiencies.
  • Representative Examples: Pancreatic exocrine insufficiency (maldigestion), celiac disease (malabsorption), extensive small bowel Crohn's disease, and lymphatic obstruction.

2.5. Disordered Motility / Functional Diarrhea

In this category, the primary problem is altered intestinal transit time, often without any identifiable mucosal inflammation or structural abnormality. This is frequently a diagnosis of exclusion after ruling out the other four buckets, and there is significant overlap with Irritable Bowel Syndrome with Diarrhea (IBS-D).

• Hallmarks and Examples
  • Clinical Clues: Normal labs, imaging, and endoscopy; symptoms often overlap with classic IBS criteria.
  • Representative Examples: IBS-D, hyperthyroidism, diabetic autonomic neuropathy, and post-vagotomy states.