Hemolytic Uremic Syndrome

Table of Contents

1. Overview
2. Typical HUS (STEC-HUS)
3. Shiga Toxin Mechanism
4. Atypical HUS (aHUS)
5. Clinical Features & Course
6. Diagnosis
7. Treatment
8. HUS vs. TTP
9. Prognosis & Long-Term Outcomes
10. Key Research Papers
11. Featured Videos

Overview

Hemolytic uremic syndrome (HUS) is a thrombotic microangiopathy (TMA) defined by the triad of microangiopathic hemolytic anemia (MAHA), thrombocytopenia, and acute kidney injury (AKI). Unlike its close relative thrombotic thrombocytopenic purpura (TTP), HUS predominantly injures the kidneys — glomerular capillary endothelial cells bear the brunt — rather than the neurological microvasculature.

HUS is the most common cause of acute kidney failure in children in developed countries, and the majority of pediatric cases follow infection with Shiga toxin–producing Escherichia coli (STEC). The recognition that a common foodborne pathogen could trigger catastrophic kidney failure in children transformed food safety regulation and drove landmark basic-science discoveries in complement biology.

Two principal subtypes are distinguished by cause and outcome: typical HUS (STEC-HUS, approximately 90% of cases) is usually self-limited with excellent recovery; atypical HUS (aHUS, approximately 10%) is driven by complement dysregulation, follows a relapsing or progressive course, and before the complement-blocking era ended in dialysis or death for most patients.

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Typical HUS (STEC-HUS)

Typical HUS — also called STEC-HUS, diarrhea-associated HUS, or D+HUS — is caused by Shiga toxin–producing Escherichia coli (STEC). The dominant serotype responsible in the United States and Europe is O157:H7, which produces both Shiga toxin 1 (Stx1) and the more pathogenic Shiga toxin 2 (Stx2). Additional STEC serotypes — O111:H8, O26:H11, O145, and O104:H4 (responsible for the 2011 German outbreak) — are increasingly recognized.

STEC resides in the intestinal tracts of ruminant livestock, particularly cattle, without causing disease in the animal. Human infection follows ingestion of contaminated beef (especially undercooked ground beef), raw leafy vegetables (spinach, romaine lettuce), unpasteurized apple cider or dairy products, contaminated drinking water, or direct animal contact. Person-to-person fecal-oral transmission occurs readily in daycares and households — a feature that amplifies outbreaks because the infectious dose is exceptionally low, sometimes as few as 50–100 organisms.

The attack rate for HUS following symptomatic STEC O157:H7 infection is approximately 10–15% overall, but rises to 20–30% in children under 5 years of age — a difference explained in part by the higher density of the Shiga toxin receptor Gb3 (globotriaosylceramide) on pediatric renal endothelial cells relative to adults.

Epidemiology: STEC-HUS affects approximately 2,000–3,000 children per year in the United States, with a peak incidence in summer and early autumn corresponding to increased cattle contact and the season of peak STEC prevalence in the environment.

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Shiga Toxin Mechanism

Shiga toxins belong to the family of AB5 toxins: a single enzymatic A-subunit noncovalently associated with a pentameric B-subunit ring. The B-subunit binds with high affinity to the glycolipid receptor Gb3 (globotriaosylceramide, CD77) on the surface of human cells. Gb3 expression is highest on renal glomerular endothelial cells, renal tubular cells, and in the brain — explaining the tissue tropism of HUS.

The molecular sequence from intestinal colonization to kidney failure proceeds through several steps:

1. STEC colonizes the large intestine, attaches to the intestinal epithelium, and produces Stx1/Stx2. The toxins cross the intestinal barrier, aided by local mucosal inflammation and cytokine release, and enter the systemic circulation.
2. Circulating Shiga toxin binds Gb3 on renal glomerular capillary endothelial cells. The AB5 complex is endocytosed via receptor-mediated endocytosis and traffics retrograde through the Golgi to the endoplasmic reticulum.
3. The A-subunit is proteolytically cleaved into an enzymatically active A1 fragment that depurinates a specific adenine residue on 28S ribosomal RNA — the same mechanism used by ricin. This irreversibly inactivates the ribosome, halting protein synthesis and triggering apoptotic cell death.
4. Dying endothelial cells in glomerular capillaries retract, exposing the underlying basement membrane to flowing blood. Platelets adhere to this denuded surface, activate, and aggregate — forming platelet microthrombi that occlude glomerular capillary loops.
5. Occluded glomeruli cannot filter blood. Red blood cells forced through fibrin-platelet thrombi are physically fragmented into schistocytes (helmet cells, triangular forms) — the morphological hallmark of MAHA. Consumed platelets drive thrombocytopenia. Progressive glomerular occlusion leads to oliguria, then anuria, and acute kidney injury.

Importantly, Shiga toxin also activates complement via the alternative pathway and triggers neutrophil-mediated injury — complicating the picture and explaining why complement inhibition has shown benefit even in some STEC-HUS cases.

Why antibiotics are contraindicated in STEC infection: Bactericidal antibiotics (especially fluoroquinolones and trimethoprim-sulfamethoxazole) cause rapid bacterial lysis, releasing a surge of preformed Shiga toxin into the intestinal lumen. Epidemiological studies consistently show higher rates of HUS in children who receive antibiotics for their STEC diarrheal prodrome. Supportive care without antibiotics is the standard of management.

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Atypical HUS (aHUS)

Atypical HUS is a chronic relapsing disease of complement dysregulation rather than an acute infection-triggered event. The complement system, which normally restricts its destructive activation to pathogen surfaces and foreign cells, loses precision on the endothelial cell surface — leading to continuous, low-grade endothelial injury and TMA.

The alternative complement pathway is in constant low-level activation (the "tick-over" mechanism). Under normal conditions, complement factor H (CFH) circulates in plasma and is the principal regulator that recognizes polyanion-rich host cell surfaces (such as endothelium) and dampens C3 activation on those surfaces. Genetic mutations that disable CFH — or other regulators — allow the alternative pathway to amplify unchecked on the endothelium:

• CFH mutations (25–30% of aHUS) — most common genetic cause; loss-of-function mutations impair endothelial surface regulation; certain CFH mutations affect only the C-terminal recognition domain (the SCR19-20 region that binds polyanion surfaces) without reducing serum CFH levels, making genetic testing essential since routine CFH levels may be normal.
• Membrane cofactor protein (MCP/CD46) mutations (15–20%) — MCP is a transmembrane complement regulator expressed on the surface of all nucleated cells including endothelium; loss-of-function mutations reduce local complement restriction; MCP mutations generally have better prognosis than CFH mutations (less likely to progress to ESRD).
• CFI (complement factor I) mutations (5–10%) — CFI cleaves and inactivates C3b/C4b; loss impairs degradation of surface-bound opsonins.
• C3 gain-of-function mutations (5–10%) — makes C3 resistant to regulation by CFH and CFI.
• CFB gain-of-function mutations (1–4%) — stabilizes the alternative pathway convertase C3bBb, amplifying activation.
• THBD (thrombomodulin) mutations (3–5%) — thrombomodulin normally accelerates CFI-mediated C3b inactivation; mutations impair this; also affects the protein C anticoagulant pathway.
• Anti-CFH antibodies (6–10%, particularly in children) — autoantibodies against CFH have a similar functional consequence to CFH mutations; associated with CFHR1/CFHR3 gene deletion on chromosome 1q31-32; treated with plasma exchange + immunosuppression.

Approximately 30–40% of aHUS cases have no identified mutation despite comprehensive genetic panel testing — reflecting incomplete genetic discovery, variants of uncertain significance, or uncharacterized regulators. Genetic testing nevertheless has major implications for treatment, prognosis, and donor kidney selection for transplant.

Triggers for clinical episodes in genetically predisposed individuals include infections (especially upper respiratory tract infections in children — the "two-hit" model: predisposing mutation + trigger), pregnancy (particularly postpartum, when complement activation is highest), surgery, organ transplantation, and certain drugs (tacrolimus, bevacizumab, quinine, oral contraceptive pills).

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Clinical Features & Course

Typical HUS (STEC-HUS): The illness evolves in two phases separated by roughly 5–10 days:

1. Prodromal diarrheal phase (days 1–5): crampy abdominal pain and watery diarrhea that becomes grossly bloody in 80–90% of cases — the "bloody diarrhea" that prompts medical evaluation. Vomiting is common. Fever is often absent or low-grade early, distinguishing STEC from other bacterial causes of bloody diarrhea. The child may appear well between cramps.
2. HUS phase (days 5–15): as diarrhea improves, the TMA triad emerges — pale (hemolytic anemia), petechiae or purpura (thrombocytopenia), and reduced urine output (AKI). Hypertension is nearly universal once AKI develops. Edema from fluid overload and hypoalbuminemia accumulates.

Extrarenal complications in STEC-HUS:

• Neurological (15–20%): seizures, altered consciousness, stroke — cerebral microvascular involvement (Gb3 expressed on brain endothelium); most common serious complication after AKI.
• Gastrointestinal: colitis, bowel necrosis, intussusception, rectal prolapse; severe colitis can mimic surgical abdomen — imaging before intervention is essential.
• Cardiac: myocarditis from Stx-mediated injury; cardiomyopathy in severe cases.
• Pancreatitis: Stx receptor expressed on pancreatic acinar cells; transient hyperglycemia in 10–15% (from beta-cell injury).

Atypical HUS: presentation is more insidious and variable. AKI is often the dominant presenting feature, without a clear infectious prodrome. Episodes may be triggered by an identifiable event (infection, pregnancy) or occur spontaneously. Before eculizumab, the natural history was grim: approximately 50% reached end-stage renal disease (ESRD) within one year of first episode, with 25% mortality in the acute phase. Recurrence after kidney transplant was frequent (50–90% depending on genetic mutation), destroying transplanted kidneys.

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Diagnosis

The diagnostic evaluation serves two goals: confirm TMA and identify its cause (because the cause determines whether antibiotics are dangerous, whether eculizumab is indicated, and whether kidney transplant is safe).

Initial laboratory evaluation:

• Complete blood count with differential: thrombocytopenia (usually <100,000/µL), anemia (Hgb often 6–9 g/dL in STEC-HUS).
• Peripheral blood smear: schistocytes (fragmented red cells — helmet cells, triangular forms) confirm MAHA. Absence of schistocytes should prompt reconsideration of diagnosis.
• Hemolysis markers: elevated LDH (>600 IU/L typical), elevated indirect bilirubin, low or absent haptoglobin, elevated reticulocyte count, negative direct antiglobulin test (Coombs) — distinguishing MAHA from immune-mediated hemolysis.
• Renal function: serum creatinine, BUN, electrolytes (hyperkalemia, metabolic acidosis in AKI), urinalysis (hematuria, proteinuria).
• ADAMTS13 activity: critical to distinguish from TTP. ADAMTS13 <10% = TTP; normal ADAMTS13 (>10%, usually >50%) in HUS. Send before plasma exchange if initiated empirically.

STEC-specific testing:

• Stool culture: STEC O157:H7 requires sorbitol-MacConkey agar (SMAC) — sorbitol non-fermenting colonies are tested for O157 antigen. Non-O157 STEC requires PCR or specific culture techniques.
• Stool PCR (Shiga toxin genes stx1/stx2): more sensitive than culture, detects non-O157 STEC, and remains positive for days after antibiotics (though antibiotics are contraindicated). Most preferred by current guidelines.
• Serum anti-LPS antibody titers (O157 and non-O157 panels): useful retrospectively after stool has cleared; may identify STEC cause in children who present with established HUS after the diarrheal prodrome has resolved.

aHUS evaluation: when STEC testing is negative and TTP (ADAMTS13) has been excluded, pursue:

• Complement panel: C3, C4, factor H, factor I, factor B levels (abnormal in 30–50% depending on mutation type).
• Anti-CFH antibodies: particularly important in children aged 6–12 months to 5 years (peak age for anti-CFH–associated aHUS).
• Genetic complement panel: next-generation sequencing of CFH, MCP/CD46, CFI, C3, CFB, THBD, and CFHR1-5 (complement factor H–related proteins) — results typically take weeks but guide transplant planning.
• STEC/pneumococcal serology and cold agglutinins: rule out pneumococcal HUS (rare but important — Streptococcus pneumoniae neuraminidase unmasks Thomsen-Friedenreich antigen on red cells/platelets, causing TMA in infants and toddlers; transfuse only washed RBCs to avoid plasma-borne anti-T antibodies).

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Treatment

Typical HUS (STEC-HUS) — supportive care:

• Fluid management: meticulous volume balance is the central intervention. Hypovolemia worsens renal ischemia; volume overload causes hypertension, pulmonary edema, and hyponatremia. Early aggressive IV hydration (normal saline, 20 mL/kg bolus if hypovolemic at presentation) reduces severity of AKI in early studies and current guidelines, though definitive RCT data are lacking.
• Dialysis: approximately 50–60% of hospitalized children require acute dialysis (peritoneal dialysis preferred in small children; hemodialysis or continuous renal replacement therapy [CRRT] in older children/adults). Indications: anuria >24h, severe hyperkalemia, acidosis, pulmonary edema, uremic encephalopathy.
• Antihypertensives: ACE inhibitors and ARBs (which reduce intraglomerular pressure) are generally avoided acutely due to risk of further reducing GFR; calcium channel blockers are often used.
• Transfusion: packed red blood cells for symptomatic anemia (Hgb <6 g/dL or hemodynamic compromise); platelet transfusions are controversial and generally avoided unless active bleeding or invasive procedures — transfused platelets may fuel microvascular thrombus propagation.
• No antibiotics, no antimotility agents, no antiplatelets: antibiotics increase Stx release (see Shiga toxin mechanism); antimotility agents delay Stx clearance; antiplatelets are unstudied and potentially harmful.

Atypical HUS — eculizumab and ravulizumab:

Eculizumab (Soliris) is a monoclonal antibody that binds complement C5 and blocks its cleavage into C5a (the anaphylatoxin that amplifies inflammation) and C5b (the initiating component of the membrane attack complex). By blocking terminal complement activation, eculizumab halts the endothelial injury cycle in aHUS.

• FDA approved for aHUS in 2011 — a transformative approval that shifted prognosis from near-certain ESRD to near-complete renal recovery in most patients if treatment is initiated early.
• Administered intravenously every 1–2 weeks (induction) then every 2 weeks (maintenance).
• Clinical response: thrombocytopenia and hemolysis resolve within days to weeks; renal recovery follows over weeks to months.
• Mandatory meningococcal vaccination before starting: blockade of terminal complement eliminates a key defense against encapsulated bacteria. All patients must receive meningococcal vaccines (MenACWY + MenB) at least 2 weeks prior, and if treatment is urgent, antimicrobial prophylaxis (penicillin V or ciprofloxacin) must be given until 2 weeks post-vaccination.

Ravulizumab (Ultomiris) — a longer-acting anti-C5 antibody (half-life ~50 days vs. ~12 days for eculizumab) approved for aHUS in 2019; allows 8-week dosing intervals after induction, reducing the burden of frequent infusions while maintaining equivalent efficacy.

Plasma exchange (PEX): used empirically before the cause is confirmed (to treat possible TTP while awaiting ADAMTS13) or as a bridge in aHUS when eculizumab is not immediately available. PEX removes anti-CFH autoantibodies and replenishes CFH protein in CFH-mutation HUS. Its role in aHUS has been largely supplanted by complement blockade.

Kidney transplantation: major complication in aHUS before eculizumab — recurrence in the transplanted kidney was common (50–90% in CFH-mutation aHUS, leading to allograft loss in most). With prophylactic perioperative eculizumab continued post-transplant, outcomes have dramatically improved and are now comparable to other kidney diseases.

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HUS vs. TTP

Both HUS and TTP are thrombotic microangiopathies presenting with MAHA and thrombocytopenia, and they were historically confused under the same umbrella. Distinguishing them is critical because treatment differs fundamentally:

• Predominant organ injured: HUS → KIDNEYS (AKI is the defining complication; neurological involvement is secondary); TTP → BRAIN/NEUROLOGICAL SYSTEM (confusion, seizures, stroke; renal involvement is mild or absent in classic TTP).
• ADAMTS13 activity: HUS → NORMAL (>10%, usually >50%); TTP → SEVERELY REDUCED (<10% activity) due to autoantibody inhibition (acquired TTP) or biallelic ADAMTS13 mutation (congenital TTP).
• Pathological mechanism: HUS → Shiga toxin (typical) or complement dysregulation (atypical) injuring endothelial cells → platelet microthrombi from exposed subendothelium; TTP → ADAMTS13 deficiency → failure to cleave ultralarge von Willebrand factor (ULVWF) multimers → ULVWF captures and activates platelets forming microthrombi.
• Age distribution: STEC-HUS → predominantly children under 5 years; TTP → predominantly adults 20–50 years, female predominance.
• Preceding diarrhea: STEC-HUS → bloody diarrheal prodrome in 90%; TTP → no diarrheal prodrome.
• Treatment: HUS → supportive (STEC) or eculizumab (aHUS); TTP → plasma exchange + corticosteroids + rituximab ± caplacizumab (ADAMTS13 replacement and inhibitor removal). Plasma exchange alone is inadequate for aHUS and unnecessary for STEC-HUS.

In clinical practice, when a patient presents with TMA and the cause is unclear, ADAMTS13 testing is sent immediately and plasma exchange is often started empirically to treat possible TTP while awaiting results. Once ADAMTS13 returns normal and STEC testing is positive, antibiotics are withheld and supportive care follows.

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Prognosis & Long-Term Outcomes

STEC-HUS: overall prognosis is favorable with modern supportive care. Acute mortality is under 5% in contemporary series. Approximately 65–85% of children recover normal renal function by 1 year; however, long-term sequelae are more common than once believed:

• Proteinuria persisting at 5 years: approximately 30–40%.
• Hypertension at 5 years: 10–25%.
• Reduced GFR (<80 mL/min/1.73m²) at 5–10 years: 20–30%.
• ESRD: approximately 5% overall, higher in those requiring prolonged dialysis in the acute phase.
• The 2011 German O104:H4 STEC outbreak (3,842 cases, 845 HUS cases, predominantly adults) demonstrated that STEC-HUS is not exclusively pediatric and that severe neurological complications occur in adult-onset cases.

Predictors of worse renal outcome include: prolonged oligoanuria (>10 days), need for dialysis, initial white blood cell count >20,000/µL (marker of severe toxin-mediated inflammation), and neurological involvement.

aHUS: pre-eculizumab era — grim (50% ESRD at 1 year; 25% death in the acute episode; near-universal graft loss in transplant). With early eculizumab, the majority of patients recover renal function; long-term continuous treatment is typically required (particularly for CFH mutations) to prevent relapse. Ongoing research focuses on whether treatment can safely be stopped — the STOPECU study showed that complement biomarker-guided treatment discontinuation is feasible in a proportion of patients.

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Key Research Papers

1. Tarr PI et al. "Shiga-toxin-producing Escherichia coli and haemolytic uraemic syndrome." Lancet. 2005;365(9464):1073–86. PMID 15781103
2. Noris M, Remuzzi G. "Atypical hemolytic-uremic syndrome." N Engl J Med. 2009;361(17):1676–87. PMID 19846853
3. Legendre CM et al. "Terminal complement inhibitor eculizumab in atypical hemolytic-uremic syndrome." N Engl J Med. 2013;368(23):2169–81. PMID 23738544
4. Karpman D et al. "Pathophysiology of typical hemolytic uremic syndrome." Semin Thromb Hemost. 2010;36(6):575–85. PMID 20865642
5. Frank C et al. "Epidemic profile of Shiga-toxin-producing Escherichia coli O104:H4 outbreak in Germany." N Engl J Med. 2011;365(19):1771–80. PMID 22029753
6. Loirat C, Frémeaux-Bacchi V. "Atypical hemolytic uremic syndrome." Orphanet J Rare Dis. 2011;6:60. PMID 21902819
7. Wong CS et al. "The risk of hemolytic-uremic syndrome after antibiotic treatment of Escherichia coli O157:H7 infections." N Engl J Med. 2000;342(26):1930–6. PMID 10874060
8. Fremeaux-Bacchi V et al. "Genetics and outcome of atypical hemolytic uremic syndrome: a nationwide French series comparing children and adults." Clin J Am Soc Nephrol. 2013;8(4):554–62. PMID 23258793
9. Kato H et al. "Analysis of complement-system mutations in patients with atypical hemolytic uremic syndrome and STEC-HUS." Clin J Am Soc Nephrol. 2012;7(7):1084–91. PMID 22595829
10. Nester CM et al. "Pre-emptive eculizumab and plasmapheresis for renal transplant in atypical hemolytic uremic syndrome." Clin J Am Soc Nephrol. 2011;6(6):1488–94. PMID 21527646
11. Mache CJ et al. "Complement inhibitor eculizumab in atypical hemolytic uremic syndrome." Clin J Am Soc Nephrol. 2009;4(8):1312–6. PMID 19608713
12. Garg AX et al. "Long-term renal prognosis of diarrhea-associated hemolytic uremic syndrome: a systematic review, meta-analysis, and meta-regression." JAMA. 2003;290(10):1360–70. PMID 12966129

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Connections

• Hematology
• Thrombotic Thrombocytopenic Purpura (TTP)
• Disseminated Intravascular Coagulation
• Thrombocytopenia
• Anemia
• Acute Kidney Injury
• Paroxysmal Nocturnal Hemoglobinuria
• Antiphospholipid Syndrome

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Featured Videos