What is TMA (Thrombotic Microangiopathy)?
TMA is a life-threatening pathological condition characterized by the triad of microangiopathic hemolytic anemia (non-immune hemolysis with red blood cell fragmentation), thrombocytopenia (platelet consumption), and ischemic organ damage caused by microvascular endothelial injury and thrombosis. 1, 2
Core Pathophysiology
TMA results from endothelial damage in small blood vessels leading to:
- Platelet consumption → thrombocytopenia (typically <150,000/mm³ or 25% reduction)
- Mechanical red cell destruction → microangiopathic hemolytic anemia with schistocytes (fragmented RBCs)
- Microvascular thrombosis → end-organ ischemia, particularly affecting kidneys, brain, and other organs 1, 3
The endothelial injury triggers excessive platelet adhesion and aggregation, forming microthrombi throughout the body that cause tissue ischemia and organ dysfunction 4.
Essential Diagnostic Features
When you see anemia plus thrombocytopenia, immediately check:
- Haptoglobin (reduced)
- LDH (elevated)
- Indirect bilirubin (elevated)
- Direct Coombs test (negative - confirms non-immune hemolysis) 1
The diagnostic triad requires:
- Non-immune hemolytic anemia with negative Coombs, elevated LDH, reduced haptoglobin, ± schistocytes
- Thrombocytopenia
- Organ involvement - particularly renal (hematuria, proteinuria, elevated creatinine) 1
Critical Caveat on Schistocytes
Absence of schistocytes does NOT exclude TMA - the test has low sensitivity, so do not wait for schistocyte confirmation before acting 1. While >1% schistocytes strongly supports TMA, their absence should not delay diagnosis or treatment.
Major TMA Categories
TMA encompasses multiple distinct disorders with different pathophysiologies:
Thrombotic Thrombocytopenic Purpura (TTP)
- Caused by severe ADAMTS-13 deficiency (<10 IU/dL or <10% of normal)
- Immune-mediated (iTTP): autoantibodies against ADAMTS-13 4
- Congenital (cTTP): ADAMTS13 gene mutations 4
- ADAMTS-13 normally cleaves ultra-large von Willebrand factor; deficiency causes UL-VWF accumulation, triggering excessive platelet aggregation 4
Hemolytic Uremic Syndrome (HUS)
- Shiga toxin-associated HUS (STEC-HUS): follows E. coli infection, typically 4-5 days after diarrhea onset 1, 3
- Atypical HUS (aHUS): complement-mediated, due to dysregulation of alternative complement pathway 1, 3
Secondary TMA
- Drug-induced (calcineurin inhibitors, mTOR inhibitors) 2
- Transplant-associated (0.8-14% of kidney transplant recipients) 2
- Pregnancy-related
- Malignancy-associated
- Infection-associated 3, 5
Immediate Diagnostic Algorithm
When TMA is suspected:
Urgent ADAMTS-13 activity testing - must be done immediately to distinguish TTP from other TMAs 1
- <10% activity = TTP (requires immediate plasma exchange)
- Normal activity = consider other TMA causes
Stool testing for verocytotoxin-producing E. coli (VTEC) - to identify STEC-HUS 1
In children <1 year: consider DGKE, WT1 mutations, and cobalamin metabolism defects (MMACHC) 1
Neurological symptoms present (10-20% of aHUS cases): obtain EEG and brain MRI looking for bilateral basal ganglia hyperintensities on FLAIR/T2 sequences 1
Why This Matters Clinically
TMA is a medical emergency with high mortality if untreated - historical mortality for iTTP was 15.7% before plasma exchange, reduced to 3.9% with treatment 6. The global incidence is only 1-6 cases per million per year for iTTP 6, making it rare but deadly when missed.
Different TMA types require completely different treatments:
- TTP requires immediate plasma exchange, corticosteroids, and caplacizumab 4, 6
- aHUS may require complement inhibitors 2, 3
- STEC-HUS is primarily supportive 3
The key is rapid recognition of the TMA syndrome (the triad), followed by urgent differentiation of the underlying cause through ADAMTS-13 testing and clinical context - do not delay treatment while waiting for complete diagnostic workup if TTP is suspected 7, 6.