Hemolytic Uremic Syndrome

Hemolytic Uremic Syndrome: Overview - Tiny Clots, Big Trouble

• Definition: A critical illness defined by the triad of microangiopathic hemolytic anemia (MAHA), thrombocytopenia (platelet count < 150,000/μL), and acute kidney injury (AKI).

• 📌 Classic Triad: HUS = RAT (Renal failure, Anemia, Thrombocytopenia).

• Key Types & Etiology:

  Feature	Typical (D+) HUS	Atypical (aHUS / D-) HUS
  Prevalence	~90% of cases; common in children < 5 years.	~10% of cases; can occur at any age.
  Main Cause	Shiga toxin-producing E. coli (STEC), esp. O157:H7; Shigella dysenteriae.	Primarily genetic mutations leading to chronic, uncontrolled complement activation. Other triggers: infections (non-enteric), drugs.
  Prodrome	Usually follows acute gastroenteritis with bloody diarrhea (D+).	Often no preceding diarrhea (D-); may have insidious onset or be triggered by infections.

⭐ HUS is the most common cause of acute kidney injury (AKI) in young children.

Hemolytic Uremic Syndrome: Pathophysiology - Endothelial Attack Mode

HUS pathogenesis is driven by Shiga toxin (Stx) from bacteria like EHEC. Stx targets vascular endothelial cells, primarily in renal glomeruli and the brain, initiating a damaging cascade.

• Toxin Action: Stx absorption from the gut allows it to bind endothelial Gb3 receptors.
• Cellular Impact: This triggers endothelial damage, apoptosis, and inflammation.
• Vascular Occlusion: Leads to microthrombi in small vessels (glomeruli).
• Systemic Effects: Results in consumption of platelets (thrombocytopenia), mechanical RBC damage (schistocytes), and ultimately renal ischemia and AKI.

⭐ Shiga toxin (Stx) binds with high affinity to the glycosphingolipid globotriaosylceramide (Gb3) receptor, predominantly expressed on renal glomerular endothelial cells.

Hemolytic Uremic Syndrome: Clinical & Lab Dx - Spotting the Syndrome

Clinical Presentation:

• Prodrome:
  • D+ HUS (Typical): Diarrhea (often bloody) post-gastroenteritis (E. coli O157:H7).
  • D- HUS (Atypical/Pneumococcal): Recent URI (S. pneumoniae).
• Key Symptoms: Pallor, weakness, oliguria/anuria, edema, hypertension.
• Severe Signs: Petechiae/purpura, irritability, seizures (CNS involvement).

Key Laboratory Findings:

Hemolytic Uremic Syndrome: Management & Complications - Damage Control & Outlook

⭐ Antibiotic treatment of EHEC infections is generally avoided as it may increase Shiga toxin release and worsen HUS risk; supportive care is the cornerstone for typical (D+) HUS.

Management: Mainstay is supportive care for D+ HUS.

• Fluid/electrolyte balance (⚠️ caution fluid overload).
• RBC transfusion if Hb <6-7 g/dL.
• Platelet transfusion: ONLY for active, severe bleeding (NOT prophylactic).
• Nutritional support.
• Hypertension control.
• Dialysis for severe AKI.
• aHUS: Eculizumab.
• AVOID: Antibiotics/antimotility agents in D+ HUS.

Complications:

• CNS: Seizures, stroke.
• GI: Colitis, perforation.
• Pancreatitis.
• Renal: CKD, hypertension, proteinuria.

Prognosis: Better in D+ HUS with prompt management.

High-Yield Points - ⚡ Biggest Takeaways

• HUS Triad: Microangiopathic hemolytic anemia (MAHA), thrombocytopenia, and acute kidney injury (AKI).
• Typical HUS (D+ HUS): Caused by Shiga toxin-producing E. coli (STEC), often O157:H7, after bloody diarrhea.
• Pathophysiology: Toxin-mediated endothelial damage causes microthrombi in renal microvasculature.
• Key lab finding: Schistocytes on peripheral smear, indicating MAHA.
• Management: Primarily supportive. Avoid antibiotics and antimotility agents in D+ HUS.
• Atypical HUS (aHUS): Due to complement dysregulation; may need eculizumab.