How Atypical HUS Presents
Atypical HUS presents with the classic triad of microangiopathic hemolytic anemia (non-immune), thrombocytopenia, and acute kidney injury, though critically, all three features may not be present simultaneously at onset in nearly 50% of cases 1.
Core Clinical Features
The hallmark presentation includes:
- Microangiopathic hemolytic anemia: Non-immune hemolysis confirmed by negative direct Coombs test, elevated LDH, reduced haptoglobin, elevated indirect bilirubin, with or without schistocytes on peripheral smear 1
- Thrombocytopenia: Platelets <150,000/mm³ or a 25% reduction from baseline 1
- Renal involvement: Hematuria and/or proteinuria and/or elevated creatinine 1
Critical Diagnostic Pitfall
The absence of schistocytes should NOT exclude TMA diagnosis due to low test sensitivity 1. Similarly, schistocytes >1% strongly favor TMA, but their absence doesn't rule it out 1.
Age and Timing Considerations
aHUS can occur at any age—age is not a predictor 1. However:
- In infants <1 year old: Consider complement-unrelated gene mutations (DGKE, WT1) or metabolic causes like methylmalonic acidemia with homocystinuria (MMACHC) 1
- Children have higher mortality than adults, with prognosis heavily dependent on genetic background 1
Distinguishing Features from STEC-HUS
In pediatric patients with diarrhea:
- Short duration of diarrhea OR simultaneous onset of diarrhea and HUS suggests aHUS rather than STEC-HUS 1
- STEC-HUS typically appears 4-5 days after diarrhea onset 1
Neurological Manifestations
Neurological involvement occurs in 10-20% of patients 1 and may include:
- Seizures
- Encephalopathy
- Motor symptoms and generalized weakness
- Vision changes
Brain MRI findings: Bilateral symmetric hyperintensities in basal ganglia on FLAIR/T2 sequences are suggestive of TMA; other affected areas include cerebral peduncles, caudate nuclei, putamen, thalami, hippocampi, insulae, and brainstem 1.
Special Populations
Post-Transplant Presentation
13% of transplant patients lack significant platelet reduction and 38% lack significant anemia or thrombocytopenia 1. The absence of marked thrombocytopenia or anemia should NOT exclude TMA diagnosis in this setting 1.
Pregnancy-Associated aHUS
Women may develop aHUS triggered by pregnancy, presenting with the same triad of features 1.
Essential Initial Workup
When anemia plus thrombocytopenia present in the emergency setting, immediately obtain 1:
- Haptoglobin (reduced)
- Indirect bilirubin (elevated)
- LDH (elevated)
- Direct Coombs test (negative)
- Creatinine
- Urinalysis for hematuria/proteinuria
Urgent ADAMTS13 activity testing is compulsory to distinguish from TTP (ADAMTS13 <10 IU/dL in TTP) 1. Additionally, test stool for verocytotoxin-producing E. coli (VTEC) 1.
Genetic Background
While 60% of cases have identifiable complement gene variants, 40-50% have no identifiable mutation or immunologic background—aHUS can still be diagnosed without identifiable mutations 1. Gene variants are more common than autoantibodies in adults 1.