Microbiology · Year 2 · from Microbiology
Case 1: Hemolytic Uremic Syndrome
Presentation
A 4-year-old girl is brought to the emergency department with a 5-day history of bloody diarrhea. The illness began with watery diarrhea and abdominal cramps after a family barbecue where she ate a hamburger. Over the past 2 days, the diarrhea became grossly bloody. Today her mother noticed decreased urine output and the child appears pale. Vital signs show temperature 37.8°C, heart rate 120 bpm, and blood pressure 100/70 mmHg. Laboratory studies reveal hemoglobin 7.2 g/dL with schistocytes on peripheral smear, platelet count 45,000/μL, creatinine 3.2 mg/dL, and LDH 1,850 U/L.
Clinical Image
Peripheral blood smear demonstrating fragmented red blood cells (schistocytes) characteristic of microangiopathic hemolytic anemia in hemolytic uremic syndrome.
Image Source: Lecture image - hematology laboratory findings
Questions
- What is the most likely diagnosis, and what organism is most commonly responsible?
- What is the mechanism by which this organism causes the clinical syndrome?
- Why are antibiotics contraindicated in this condition?
- What are the key supportive management strategies for this patient?
Answers
- Diagnosis and organism: The most likely diagnosis is hemolytic uremic syndrome (HUS) caused by Shiga toxin-producing Escherichia coli (STEC), most commonly serotype O157:H7. The clinical triad of HUS includes microangiopathic hemolytic anemia (schistocytes, elevated LDH), thrombocytopenia, and acute kidney injury. The history of bloody diarrhea following consumption of undercooked ground beef is classic for EHEC/STEC infection.
- Pathogenic mechanism: EHEC produces Shiga toxin (Stx), which has an A-B structure. The B subunit binds to globotriaosylceramide (Gb3) receptors on glomerular endothelial cells, and the A subunit cleaves ribosomal RNA to halt protein synthesis and cause cell death. Endothelial damage triggers microvascular thrombosis, platelet consumption, and red blood cell fragmentation as they pass through damaged vessels. The resulting microvascular occlusion in the kidneys causes acute kidney injury.
- Antibiotic contraindication: Antibiotics are contraindicated because bacterial killing may increase the release of Shiga toxin from lysed bacteria, potentially worsening the risk of HUS. Studies have shown that antibiotic treatment of EHEC infection is associated with increased progression to HUS. Treatment should be supportive, with close monitoring for HUS development.
- Supportive management: Management includes aggressive intravenous fluid resuscitation to maintain renal perfusion, careful electrolyte management (especially potassium in the setting of acute kidney injury), blood transfusions for symptomatic anemia (avoiding platelets unless active bleeding, as they may worsen microvascular thrombosis), and renal replacement therapy (dialysis) if indicated. Close monitoring of fluid balance, blood pressure, and neurologic status is essential.