Gastroenteritis for the USMLE Step 2 Exam
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Gastroenteritis for USMLE Step 2
Definition
- Gastroenteritis: Acute inflammation of the gastrointestinal tract, typically due to infectious agents, causing diarrhea, vomiting, and abdominal discomfort.
- Common causes include viruses, bacteria, and parasites.
- Can also be caused by toxins, food allergies, or medications.
Etiology
- Viral Gastroenteritis:
- Norovirus: Leading cause in adults, highly contagious and commonly seen in outbreaks (e.g., cruise ships, nursing homes).
- Rotavirus: Primary cause of severe diarrhea in infants and young children; vaccination has significantly reduced its incidence.
- Adenovirus and Astrovirus: Mostly affect children, with a milder disease course.
- Bacterial Gastroenteritis:
- Escherichia coli (E. coli):
- ETEC: Causes watery diarrhea in travelers (traveler’s diarrhea).
- STEC: Causes bloody diarrhea and hemolytic uremic syndrome (HUS) due to Shiga toxin.
- Shigella: Highly contagious, leading to dysentery (bloody diarrhea); transmitted via contaminated water or food.
- Campylobacter jejuni: Common cause of bacterial gastroenteritis, associated with undercooked poultry. Post-infectious complications include Guillain-Barré syndrome.
- Salmonella: Found in contaminated poultry, eggs, and dairy products. Causes gastroenteritis or enteric fever (S. Typhi).
- Clostridioides difficile: Associated with antibiotic use; causes pseudomembranous colitis.
- Parasitic Gastroenteritis:
- Giardia lamblia: Acquired from contaminated water, causing chronic diarrhea and malabsorption.
- Entamoeba histolytica: Causes amebic dysentery and liver abscesses, typically seen in areas with poor sanitation.
Pathophysiology
- Viral Gastroenteritis:
- Primarily affects the small intestine, leading to impaired water and electrolyte absorption.
- Rotavirus produces NSP4 toxin, which induces chloride secretion, leading to watery diarrhea.
- Bacterial Gastroenteritis:
- Toxin-mediated: ETEC and Vibrio cholerae produce toxins that increase intracellular cAMP or cGMP, leading to chloride and water secretion.
- Invasive pathogens: Shigella, Salmonella, and Campylobacter invade the intestinal mucosa, causing inflammation, ulceration, and bloody diarrhea.
- Parasitic Gastroenteritis:
- Giardia causes villous atrophy, impairing nutrient absorption.
- Entamoeba histolytica invades the colonic wall, causing flask-shaped ulcers and dysentery.
Clinical Features
- General Symptoms:
- Diarrhea: Can be watery (viral or toxin-mediated) or bloody (invasive bacterial infections).
- Vomiting: Prominent in viral infections, especially Norovirus.
- Abdominal pain: Common in both viral and bacterial gastroenteritis.
- Fever: Often present in bacterial gastroenteritis.
- Dehydration: Particularly severe in infants, elderly, and immunocompromised individuals.
- Specific Findings:
- Rotavirus: Watery diarrhea in infants, often accompanied by vomiting and fever.
- STEC: Bloody diarrhea without fever, risk of hemolytic uremic syndrome (HUS), characterized by acute kidney injury, hemolytic anemia, and thrombocytopenia.
- C. difficile: Watery diarrhea after antibiotic use; may cause pseudomembranous colitis in severe cases.
Diagnosis
- Clinical Evaluation: History of exposure, travel, food intake, and recent antibiotic use.
- Laboratory Tests:
- Stool Culture: Indicated for severe, persistent, or bloody diarrhea.
- PCR Testing: Rapid identification of viral and bacterial pathogens.
- C. difficile Toxin Assay: Used in cases of diarrhea following antibiotic use.
- Stool Ova and Parasites: Ordered when parasitic infection is suspected, especially in prolonged cases or travel history.
Treatment
- Rehydration:
- Oral Rehydration Therapy (ORT): First-line for mild to moderate dehydration.
- Intravenous Fluids: Required for severe dehydration or when oral intake is not feasible.
- Pharmacological Treatment:
- Antibiotics: Reserved for bacterial gastroenteritis.
- Shigella: Fluoroquinolones or azithromycin.
- C. difficile: Oral vancomycin or fidaxomicin.
- Traveler’s Diarrhea: Azithromycin or fluoroquinolones in moderate to severe cases.
- Antimotility Agents: Can be used cautiously for non-bloody diarrhea; contraindicated in suspected C. difficile or STEC infections.
- Probiotics: Can reduce the duration of symptoms in viral gastroenteritis.
Complications
- Dehydration: The most common and severe complication, especially in infants and elderly.
- Hemolytic Uremic Syndrome (HUS): Associated with STEC infection, presents with renal failure, hemolysis, and thrombocytopenia.
- Guillain-Barré Syndrome: Linked to Campylobacter infection, presents as ascending paralysis.
- Toxic Megacolon: A complication of severe C. difficile infection.
Prevention
- Vaccination: Routine rotavirus vaccination in infants.
- Hygiene Measures: Proper handwashing, food handling, and clean water practices are crucial to preventing gastroenteritis.
- Traveler’s Precautions: Avoid consumption of unsafe water and food in endemic areas.
Key Points
- Gastroenteritis is commonly viral (Norovirus, Rotavirus), bacterial (E. coli, Shigella, Campylobacter), or parasitic (Giardia, Entamoeba).
- Symptoms include diarrhea (watery or bloody), vomiting, abdominal pain, and dehydration.
- Diagnosis is clinical, with stool tests performed for severe or prolonged cases.
- Treatment centers around rehydration, with antibiotics used selectively for bacterial causes.
- Complications include dehydration, hemolytic uremic syndrome (STEC), and Guillain-Barré syndrome (Campylobacter).