Gastroenteritis for the USMLE Step 1 Exam
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Gastroenteritis for USMLE Step 1
Definition
- Gastroenteritis: Inflammation of the gastrointestinal tract due to infectious agents, leading to diarrhea, vomiting, and abdominal discomfort.
- Common causes: Viruses, bacteria, and parasites.
- Non-infectious causes: Toxins, drugs, and food allergies.
Etiology
- Viral Gastroenteritis:
- Norovirus: The leading cause in adults, transmitted via contaminated food, water, or direct contact.
- Frequently occurs in outbreaks (e.g., cruise ships, schools).
- Rotavirus: Major cause of severe diarrhea in infants and young children; vaccination has reduced its prevalence.
- Adenovirus and Astrovirus: Less common, usually affect children and immunocompromised individuals.
- Bacterial Gastroenteritis:
- Escherichia coli (E. coli):
- ETEC: Causes watery diarrhea, mainly in travelers.
- STEC: Produces Shiga toxin, leading to bloody diarrhea and hemolytic uremic syndrome (HUS).
- Shigella: Highly contagious, causes dysentery (bloody diarrhea).
- Salmonella: Transmitted via contaminated poultry, eggs, and dairy; causes gastroenteritis or enteric fever.
- Campylobacter jejuni: Associated with undercooked poultry; can lead to post-infectious complications such as Guillain-Barré syndrome.
- Clostridioides difficile (C. diff): Causes pseudomembranous colitis, typically after antibiotic use.
- Parasitic Gastroenteritis:
- Giardia lamblia: Acquired from contaminated water, causes prolonged diarrhea with malabsorption.
- Entamoeba histolytica: Causes amebic dysentery and can lead to liver abscesses.
Pathophysiology
- Viral Gastroenteritis:
- Causes damage to enterocytes, leading to impaired absorption of water and electrolytes.
- Rotavirus produces NSP4 toxin, leading to increased chloride secretion and watery diarrhea.
- Bacterial Gastroenteritis:
- Toxin-mediated: ETEC and Vibrio cholerae increase cAMP or cGMP, causing chloride secretion and watery diarrhea.
- Invasive bacteria: Shigella and Campylobacter invade the intestinal mucosa, leading to inflammation and bloody diarrhea.
- Parasitic Gastroenteritis:
- Giardia leads to villous atrophy and malabsorption, while Entamoeba histolytica invades the colonic wall, causing ulceration and dysentery.
Clinical Features
- General Symptoms:
- Diarrhea: May be watery (viral or toxin-mediated bacterial causes) or bloody (invasive bacterial causes).
- Vomiting: More common in viral causes (e.g., Norovirus).
- Abdominal pain and cramping: Common in both viral and bacterial gastroenteritis.
- Fever: Typically present in bacterial gastroenteritis.
- Dehydration: Key concern, especially in children and the elderly.
- Specific Findings:
- Rotavirus: Watery diarrhea and vomiting in infants.
- STEC: Bloody diarrhea with risk of HUS (hemolytic anemia, thrombocytopenia, acute kidney injury).
- C. difficile: Severe watery diarrhea post-antibiotic use, pseudomembranous colitis in severe cases.
Diagnosis
- Clinical Evaluation: Based on history of exposure, recent travel, food consumption, and symptom duration.
- Stool Culture: Recommended for bloody diarrhea or prolonged symptoms.
- PCR Testing: Rapidly identifies viral and bacterial pathogens.
- C. difficile Toxin Assay: Performed in cases of diarrhea after recent antibiotic use.
- Stool Ova and Parasites: Useful in prolonged diarrhea, especially after travel.
Treatment
- Rehydration:
- Oral Rehydration Therapy (ORT): Preferred in most cases.
- Intravenous Fluids: Reserved for severe dehydration or shock.
- Pharmacological Treatment:
- Antibiotics: Reserved for bacterial infections (e.g., Shigella, C. difficile).
- Antimotility Agents: Can be used cautiously in non-bloody diarrhea; avoid in suspected C. difficile infection.
- Probiotics: May reduce the duration of diarrhea in viral gastroenteritis.
Complications
- Dehydration: Major concern, especially in infants, the elderly, and immunocompromised patients.
- Hemolytic Uremic Syndrome (HUS): Associated with STEC infection, presents with hemolysis, thrombocytopenia, and acute kidney injury.
- Guillain-Barré Syndrome: Rare neurological complication of Campylobacter infection.
- Toxic Megacolon: Seen in severe C. difficile infections.
Prevention
- Vaccination: Routine rotavirus vaccination for infants.
- Hygiene: Handwashing, safe food handling, and clean water practices.
- Traveler’s Precautions: Avoiding high-risk foods and water in endemic regions; vaccines for cholera and typhoid in some cases.
Key Points
- Common causes of gastroenteritis include viral agents (Norovirus, Rotavirus), bacteria (E. coli, Shigella, Salmonella), and parasites (Giardia, Entamoeba).
- Key symptoms: Diarrhea (watery or bloody), vomiting, abdominal pain, and dehydration.
- Rehydration is the mainstay of treatment; antibiotics are used selectively for bacterial infections.
- Diagnosis is clinical, with stool testing for severe or prolonged cases.
- Complications include dehydration, hemolytic uremic syndrome (STEC), and Guillain-Barré syndrome (Campylobacter).