Causes of Hemolytic Uremic Syndrome (HUS)
HUS is fundamentally caused by three distinct mechanisms: Shiga toxin-producing bacterial infections (typical HUS), complement system dysregulation (atypical HUS), or secondary triggers in the setting of underlying conditions.
Primary Etiologic Categories
1. Typical HUS (STEC-HUS) - Most Common Form
- Shiga toxin-producing Escherichia coli (STEC) is the predominant cause, particularly serotypes O157 and O26 1, 2
- Accounts for approximately 78-85% of pediatric HUS cases 3
- Follows gastrointestinal infection with bloody diarrhea as the typical presentation
- The Shiga toxin directly damages endothelial cells, initiating the thrombotic microangiopathy cascade 2
2. Atypical HUS (aHUS) - Complement-Mediated
Genetic causes (60% of aHUS cases have identifiable mutations) 1:
- Complement factor H (CFH) mutations
- C3 mutations
- CD46 (MCP) mutations
- Complement factor I (CFI) mutations
- Thrombomodulin (THBD) mutations
- Complement factor B (CFB) mutations
- CFHR1-5 gene variants
- Non-complement genes: DGKE (diacylglycerol kinase epsilon), WT1 1, 2
Acquired autoimmune causes:
- Anti-CFH antibodies causing complement dysregulation 1
- More common in children than adults with aHUS
Critical point: 40-50% of aHUS patients have no identifiable genetic mutation or autoantibody, yet still have complement-mediated disease 1
3. Secondary HUS - Triggered by Specific Conditions
Infectious triggers 1:
- Streptococcus pneumoniae (neuraminidase-mediated) 3
- Viral infections: H1N1 influenza, influenza A, HIV, SARS-CoV-2, adenovirus 1, 4
- These may unmask underlying complement abnormalities or directly cause endothelial injury
Medication-induced 1:
- Certain cytotoxic drugs
- Calcineurin inhibitors
- Other nephrotoxic agents
Pregnancy-related 1:
- HELLP syndrome overlap
- Pregnancy as a trigger for underlying complement disorders
Malignancy-associated 1:
- Direct tumor effects or chemotherapy-related
Transplantation-related 1:
- Renal transplantation can trigger aHUS recurrence or de novo disease
- Hematopoietic stem cell transplantation 3
Autoimmune diseases 5:
- Systemic lupus erythematosus
- Antiphospholipid syndrome
- Other systemic autoimmune conditions
Malignant hypertension 1:
- Can trigger or coexist with aHUS
Metabolic disorders 2:
- Cobalamin C deficiency
- Disorders of cobalamin metabolism
Pathophysiologic Framework
The common final pathway involves 2:
- Endothelial cell injury (from toxin, complement, or other trigger)
- Intravascular hemolysis
- Platelet activation and aggregation
- Formation of microthrombi in small vessels
- Tissue ischemia and organ damage (predominantly kidney)
This creates a vicious cycle where complement activation perpetuates endothelial damage, which further activates platelets and complement 2.
Critical Clinical Distinctions
Multifactorial nature of aHUS: The disease requires both a genetic/acquired predisposition AND an environmental trigger for manifestation due to incomplete penetrance of complement mutations 1. The most frequent triggers are infections and pregnancy, but the triggering event may be confusing diagnostically 1.
Important caveat: If HUS persists after removal of a suspected secondary cause (e.g., infection, medication), reconsider the diagnosis—the "trigger" may actually be unmasking underlying complement dysregulation rather than being the primary cause 1.
Age is not a predictor: aHUS can occur at any age, though typical STEC-HUS predominantly affects children aged 3-5 years 1, 3.