Haemolytic Uraemic Syndrome (HUS)

1. Overview

Haemolytic Uraemic Syndrome (HUS) is a thrombotic microangiopathy (TMA) characterized by the classic triad of:

1. Microangiopathic Haemolytic Anaemia (MAHA)
2. Thrombocytopenia
3. Acute Kidney Injury (AKI)

HUS represents the most common cause of acute kidney injury requiring dialysis in children, with peak incidence in children under 5 years of age. [1,2] The condition results from endothelial injury leading to microthrombus formation predominantly affecting the renal microvasculature, though systemic manifestations can occur.

There are two major classifications of HUS based on aetiology:

Typical HUS (STEC-HUS or D+HUS): Accounts for approximately 90% of paediatric cases and is caused by Shiga toxin-producing Escherichia coli (STEC), most commonly serotype O157:H7. This form typically follows a prodrome of bloody diarrhoea and has a generally favourable prognosis with supportive care. [3,4]

Atypical HUS (aHUS or D-HUS): Accounts for 5-10% of cases and results from dysregulation of the alternative complement pathway due to genetic mutations or acquired autoantibodies. This form does NOT have a diarrhoeal prodrome, has a poorer prognosis historically, but outcomes have been transformed by complement blockade therapy with eculizumab. [5,6]

The distinction between typical and atypical HUS is critical because:

• Typical STEC-HUS is managed supportively and has good renal recovery rates (> 85%)
• Atypical HUS requires specific complement-targeted therapy and has high risk of progression to end-stage renal disease (ESRD) without treatment
• Atypical HUS has significant recurrence risk and genetic implications for family members

Clinical Pearls

Antibiotics are Contraindicated in STEC-HUS: The use of antibiotics in STEC gastroenteritis is strongly contraindicated as they increase the risk of HUS development 2-17 fold. Bacterial lysis releases stored Shiga toxin from intracellular compartments, precipitating or worsening HUS. This has been demonstrated in multiple observational studies and meta-analyses. [7,8]

Platelets are Rarely Indicated: Platelet transfusion should be avoided unless there is life-threatening active bleeding. Transfused platelets are immediately consumed into microthrombi, potentially worsening microvascular occlusion and thrombotic complications. [9]

Schistocytes are Diagnostic: Fragmented red blood cells (schistocytes, "helmet cells") on peripheral blood film are the hallmark of microangiopathic haemolysis. These result from mechanical shearing of erythrocytes as they traverse fibrin strands in damaged capillaries. [10]

Early Recognition of Atypical HUS is Critical: Any HUS presentation without prodromal diarrhoea, age > 5 years, recurrent episodes, or family history should trigger investigation for atypical HUS with urgent complement studies and consideration of eculizumab therapy. [11]

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2. Epidemiology

Incidence and Prevalence

Typical STEC-HUS:

• Annual incidence: 2.1 per 100,000 children less than 5 years in developed countries [12]
• Geographic variation: Higher in countries with intensive cattle farming
• Accounts for 90% of childhood HUS cases [3]
• Incidence has decreased in some regions due to improved food safety measures

Atypical HUS:

• Annual incidence: 0.23 per 100,000 children [13]
• Accounts for 5-10% of paediatric HUS cases
• More evenly distributed across age groups compared to STEC-HUS
• No clear geographic clustering

Demographics

Age Distribution:

• STEC-HUS: 75% of cases occur in children less than 5 years, with peak at 2-3 years [14]
• Atypical HUS: Bimodal distribution with peaks in infancy (less than 1 year) and young adulthood [15]

Risk Factors for STEC-HUS:

• Young age (less than 5 years) - increased expression of Gb3 receptor
• Consumption of undercooked ground beef (hamburgers)
• Unpasteurized milk and dairy products
• Contact with animals at farms/petting zoos
• Swimming in contaminated water
• Attendance at daycare facilities (person-to-person transmission)
• Use of antimotility agents during STEC gastroenteritis (increases HUS risk)
• Antibiotic exposure during STEC infection [7,8]

Risk Factors for Atypical HUS:

• Genetic predisposition (complement regulatory gene mutations in 50-70%)
• Family history of HUS or unexplained kidney disease
• Pregnancy and postpartum period
• Infections (particularly Streptococcus pneumoniae, influenza)
• Medications (calcineurin inhibitors, chemotherapy agents)
• Malignancy
• Autoimmune diseases

Seasonality

STEC-HUS demonstrates marked summer and early autumn peak (June-September in Northern Hemisphere) corresponding to:

• Outdoor cooking and barbecues
• Farm visits and agricultural shows
• Warmer temperatures promoting bacterial multiplication in food

Atypical HUS shows no seasonal variation, consistent with its non-infectious aetiology.

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3. Aetiology and Pathophysiology

Typical STEC-HUS: Pathophysiological Cascade

The pathogenesis of STEC-HUS involves a multi-step process from bacterial colonization to systemic thrombotic microangiopathy:

Step 1: Bacterial Colonization and Toxin Production

Causative Organisms:

• Escherichia coli O157:H7: Most common serotype (> 70% of cases) [16]
• Non-O157 STEC: O26, O111, O103, O145, O121 (increasing recognition)
• Shigella dysenteriae serotype 1: Rare in developed countries, important in developing world

Transmission Routes:

• Foodborne: Contaminated ground beef, unpasteurized milk/juice, raw vegetables
• Waterborne: Contaminated drinking or recreational water
• Person-to-person: Faecal-oral transmission (infectious dose less than 100 organisms)
• Animal contact: Direct contact with ruminants (cattle, sheep, goats)

Shiga Toxin Structure and Function: Shiga toxin (Stx) exists in two main forms: Stx1 and Stx2 (with subtypes)

• Stx2 is more strongly associated with HUS development [17]
• Each toxin consists of:
  • "A subunit: Enzymatic activity (N-glycosidase) - cleaves 28S ribosomal RNA"
  • "B subunit: Pentamer that binds to globotriaosylceramide (Gb3) receptors"

Step 2: Toxin Absorption and Systemic Dissemination

After bacterial colonization of the colon:

1. Colonic damage: Shiga toxin causes haemorrhagic colitis with bloody diarrhoea
2. Toxin translocation: Shiga toxin crosses the compromised intestinal epithelium
3. Lymphatic/vascular entry: Toxin enters circulation bound to neutrophils and monocytes
4. Systemic distribution: Toxin is delivered to organs with high Gb3 receptor expression

Step 3: Endothelial Targeting and Injury

Gb3 Receptor Distribution:

• Highest expression: Glomerular endothelium, renal tubular epithelium
• Moderate expression: Brain microvasculature, colonic mucosa
• Lower expression: Most other vascular beds

Mechanism of Endothelial Injury:

1. Shiga toxin B subunit binds Gb3 receptors on endothelial cell surface
2. Receptor-mediated endocytosis internalizes the toxin
3. A subunit cleaves 28S ribosomal RNA → protein synthesis inhibition
4. Endothelial cell apoptosis and detachment from basement membrane
5. Subendothelial matrix exposure triggers coagulation cascade

Pro-thrombotic Effects:

• ↑ von Willebrand Factor release
• ↑ Tissue factor expression
• ↑ Platelet adhesion molecule expression (P-selectin)
• ↓ Thrombomodulin expression (reduced protein C activation)
• ↓ Prostacyclin production

Exam Detail: Molecular Pathophysiology Detail:

Shiga Toxin Intracellular Trafficking:

• After Gb3 binding, Shiga toxin undergoes retrograde transport through:
  • Early endosomes → Golgi apparatus → Endoplasmic reticulum
  • ER translocation allows A subunit access to 60S ribosomal subunit
  • Specific cleavage of adenine-4324 in 28S rRNA blocks elongation factor binding
  • Irreversible inhibition of protein synthesis → apoptosis

Why Kidneys are Preferentially Affected:

1. High Gb3 expression: Particularly in glomerular endothelium and podocytes
2. Glomerular filtration: Concentrates circulating toxin
3. Local complement activation: Amplifies injury (see complement interaction below)
4. Cytokine milieu: Renal endothelium particularly sensitive to TNF-α and IL-1

Complement Interaction in STEC-HUS: Even in "typical" STEC-HUS, there is evidence of complement activation:

• Shiga toxin upregulates complement regulators but also activates alternative pathway
• Genetic complement variants (CFH, CFI polymorphisms) associated with worse outcomes in STEC-HUS
• Suggests STEC-HUS and aHUS represent a pathophysiological spectrum rather than discrete entities

Step 4: Microthrombus Formation and Organ Injury

Glomerular Pathology:

• Endothelial swelling and detachment
• Subendothelial widening and deposition of fibrin/platelets
• Mesangiolysis (dissolution of mesangial matrix)
• Glomerular capillary thrombosis → reduced GFR → AKI

Haematological Consequences:

1. Thrombocytopenia: Platelet consumption in microthrombi
2. Microangiopathic haemolysis: RBCs fragmented by fibrin strands → schistocytes
3. Anaemia: Haemolysis + bone marrow suppression + uraemia-related reduced EPO response

Renal Functional Impairment:

• Acute tubular necrosis: Ischaemia from glomerular thrombosis
• Direct tubular toxicity: Shiga toxin damages proximal tubular cells
• Intrarenal vasoconstriction: Endothelin release, reduced NO production
• Volume depletion: Diarrhoeal losses compound prerenal injury

Atypical HUS: Complement Dysregulation

Atypical HUS results from uncontrolled activation of the alternative complement pathway on endothelial surfaces, leading to chronic or recurrent endothelial injury.

Normal Alternative Pathway Regulation

The alternative pathway continuously undergoes "tick-over" activation:

1. Spontaneous hydrolysis of C3 → C3(H₂O)
2. Binding of Factor B and cleavage by Factor D → C3 convertase (C3bBb)
3. C3 convertase cleaves more C3 → amplification loop
4. C5 convertase formation → C5b-9 (membrane attack complex)

Regulatory Mechanisms (Prevention of Self-Injury):

• Factor H (CFH): Accelerates decay of C3 convertase, cofactor for Factor I
• Factor I (CFI): Serine protease that cleaves C3b and C4b
• Membrane Cofactor Protein (MCP/CD46): Cell surface regulator of C3b
• Thrombomodulin (THBD): Cofactor for factor I, also activates protein C
• Factor B (CFB): Component of C3 convertase (loss-of-function mutations protective)
• C3: Central protein (some mutations cause hyperactivity)

Genetic Basis of Atypical HUS

Approximately 50-70% of aHUS patients have identifiable genetic abnormalities in complement regulatory genes: [18,19]

Anti-Factor H Antibodies:

• Found in 5-10% of aHUS patients, particularly children [20]
• Associated with homozygous CFHR1 deletion (absence of complement factor H-related proteins)
• Usually responsive to immunosuppression ± plasma exchange
• May respond to eculizumab but antibody depletion often needed

Incomplete Penetrance:

• Only 50% of individuals with pathogenic mutations develop HUS
• Requires additional "trigger" or "second hit":
  • Infections (particularly upper respiratory, gastroenteritis)
  • Pregnancy
  • Medications
  • Trauma/surgery
  • Vaccination (rare, controversial)

Exam Detail: Molecular Mechanisms of Complement Mutations:

CFH Mutations:

• Most mutations cluster in C-terminal domain (SCR19-20) - the surface recognition region
• Loss of binding to glycosaminoglycans and C3b on endothelial surfaces
• Systemic C3 levels typically normal (CFH still functions in fluid phase)
• Impaired protection of endothelium → localized complement activation

MCP Mutations:

• Only affects cell-surface regulation (not fluid phase)
• Reduced protection of patient's own cells
• Phenotype often milder, better renal outcomes
• Does NOT cause high transplant recurrence (transplanted kidney has normal MCP)

C3 Mutations:

• Typically gain-of-function mutations in C3b
• Resist inactivation by Factor H and Factor I
• Persistent C3 convertase activity
• Low serum C3 levels often seen (consumption)

DGKE Mutations:

• Diacylglycerol kinase ε - NOT a complement protein
• Regulates protein kinase C pathway in endothelial cells and platelets
• Loss causes endothelial dysfunction and platelet hyperactivation
• Complement-independent mechanism (eculizumab response variable)
• Early-onset disease (less than 1 year), very high progression to ESRD

Triggers for Atypical HUS Episodes

In genetically predisposed individuals, HUS episodes are often precipitated by:

Infections (Most Common Trigger - 50-60% of cases):

• Upper respiratory tract infections
• Gastroenteritis (viral or bacterial, non-STEC)
• Streptococcus pneumoniae (particularly invasive disease)
• Influenza
• HIV

Pregnancy-Associated aHUS:

• Occurs in 10-20% of aHUS presentations in women of childbearing age [21]
• Usually presents postpartum (within days to weeks)
• High maternal and fetal morbidity/mortality if untreated
• High recurrence risk in subsequent pregnancies (50-80%)
• Difficult to distinguish from preeclampsia/HELLP syndrome

Medications:

• Calcineurin inhibitors (tacrolimus, cyclosporine)
• mTOR inhibitors (sirolimus)
• Anti-VEGF therapies (bevacizumab)
• Chemotherapy (gemcitabine, mitomycin C)
• Quinine
• Oral contraceptives (controversial)

Other Triggers:

• Malignancy
• Autoimmune diseases (SLE, antiphospholipid syndrome)
• Bone marrow/solid organ transplantation
• Major surgery or trauma

Streptococcus pneumoniae-Associated HUS (Sp-HUS)

A distinct entity accounting for 5% of childhood HUS with unique pathophysiology: [22]

Mechanism:

• Pneumococcal neuraminidase cleaves sialic acid from cell surfaces
• Exposes hidden Thomsen-Friedenreich (T) antigen on RBCs, platelets, glomerular cells
• Pre-existing IgM antibodies bind T-antigen → immune-mediated haemolysis and thrombocytopenia
• Positive Direct Coombs Test (DAT) - distinguishing feature from other HUS forms

Clinical Features:

• Usually associated with invasive pneumococcal disease (pneumonia, meningitis, bacteraemia)
• More severe haemolysis than STEC-HUS
• Higher risk of respiratory and neurological complications
• Poor prognosis (mortality 5-10%, ESRD 20-30%)
• Plasma therapy contraindicated (contains anti-T antibodies that worsen haemolysis)

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4. Clinical Presentation

Typical STEC-HUS: Biphasic Presentation

Phase 1: Prodromal Gastroenteritis (Days 1-7)

Gastrointestinal Symptoms:

• Watery diarrhoea: Initial symptom in > 90% [23]
• Progression to bloody diarrhoea: Occurs in 70-95% within 1-2 days
• Abdominal pain: Severe, cramping (often colicky)
• Vomiting: Present in 70-80%
• Fever: Variable, often low-grade or absent (unlike invasive bacterial gastroenteritis)

Duration: Typically 3-7 days before onset of HUS

High-Risk Features During Prodrome:

• Very young age (less than 2 years)
• Marked leukocytosis (WBC > 15-20 × 10⁹/L) - predictor of severe HUS [24]
• Severe, prolonged bloody diarrhoea
• Antibiotic use
• Use of antimotility agents

Phase 2: Haemolytic Uraemic Syndrome (Days 5-13)

Timing: Median onset 6-10 days after start of diarrhoea [25]

Clinical Triad:

1. Microangiopathic Haemolytic Anaemia (MAHA):

• Pallor: Often striking, sudden onset
• Jaundice: Mild (unconjugated hyperbilirubinaemia from haemolysis)
• Tachycardia: Compensatory response to anaemia
• Dark urine: Haemoglobinuria
• Lethargy: Anaemia + uraemia

2. Thrombocytopenia:

• Petechiae/purpura: Usually mild, limited distribution
• Bruising: Easy bruising, minimal trauma
• Bleeding: Uncommon unless platelets less than 20 × 10⁹/L
  • "Sites: GI tract (melaena, haematemesis), epistaxis, haematuria"
  • Major haemorrhage is rare (less than 5%)

3. Acute Kidney Injury:

• Oliguria/anuria: Most common presentation (60-70%)
  • Parents report "dry nappies" or no urination for 12-24 hours
• Haematuria: Microscopic or macroscopic (50-80%)
• Proteinuria: Variable degree
• Oedema: Periorbital, peripheral (from fluid retention)
• Hypertension: Present in 50-70% [26]
  • "Mechanism: Volume overload + activated renin-angiotensin system"
  • Can be severe (> 99th centile for age)

Severity of AKI:

• Mild AKI: Elevated creatinine, maintained urine output (20%)
• Oliguric AKI: Urine output less than 0.5 mL/kg/hr (50-60%)
• Anuric AKI: No urine output > 24 hours (20-30%)
• Dialysis requirement: 50-60% of STEC-HUS patients [27]

Systemic and Extrarenal Manifestations

Neurological Complications (20-40% of Severe Cases) [28]

Spectrum of Severity:

• Mild: Irritability, lethargy, headache
• Moderate: Confusion, altered consciousness, seizures
• Severe: Coma, stroke, cerebral oedema

Mechanisms:

1. Hypertensive encephalopathy: Rapid BP elevation
2. Shiga toxin direct neurotoxicity: Crosses blood-brain barrier, damages neurons
3. CNS thrombotic microangiopathy: Microinfarcts
4. Metabolic disturbances: Uraemia, hyponatraemia, hypoglycaemia

Seizures:

• Occur in 10-30% of hospitalized HUS patients
• Types: Generalized tonic-clonic most common, focal seizures, status epilepticus
• Poor prognostic indicator (associated with higher mortality and long-term neurological sequelae)

Stroke:

• Ischaemic > haemorrhagic
• Cerebral venous sinus thrombosis reported
• Requires urgent neuroimaging (CT/MRI)

Gastrointestinal Complications

Ongoing Colitis:

• Persistent bloody diarrhoea
• Abdominal pain and distension
• Rarely: toxic megacolon

Severe Complications (2-5%):

• Colonic necrosis/perforation: Surgical emergency
• Rectal prolapse: From severe straining
• Intussusception: Inflammatory lead point
• Pancreatitis: Shiga toxin damages pancreatic islet cells
  • May cause transient or permanent diabetes mellitus [29]

Cardiovascular Complications

• Hypertension: See above (50-70%)
• Myocardial involvement: Rare
  • Troponin elevation
  • ECG changes
  • Heart failure (from volume overload ± direct myocardial involvement)
• Pericarditis/pericardial effusion: Rare

Electrolyte and Metabolic Derangements

• Hyperkalemia: Life-threatening, especially in anuric patients
• Hyponatraemia: Dilutional (fluid overload) or SIADH
• Metabolic acidosis: From renal failure + diarrhoeal bicarbonate loss
• Hypocalcaemia: Common in AKI
• Hyperphosphataemia: Reduced renal excretion
• Hyperglycaemia or Hypoglycaemia: Pancreatic involvement

Atypical HUS: Distinct Presentation

Key Distinguishing Features

Clinical Presentation Patterns of aHUS

Acute Severe Presentation:

• Rapid onset of triad (hours to days)
• Often following infection or other trigger
• Can present with severe hypertension, seizures, reduced consciousness
• May be mistaken for acute glomerulonephritis initially

Insidious Onset:

• Gradual onset over weeks
• Non-specific symptoms: fatigue, reduced appetite, pallor
• Incidental finding of anaemia, thrombocytopenia, elevated creatinine
• May present with chronic kidney disease already established

Relapsing-Remitting:

• Recurrent episodes of HUS separated by periods of remission
• Triggers often identifiable (infections, pregnancy, medications)
• Progressive renal decline with each episode if untreated

Pregnancy-Associated aHUS:

• Presents in 3rd trimester or postpartum (often week 1-4 postpartum)
• Difficult to distinguish from HELLP syndrome or severe preeclampsia
• Key differentiator: persistence/worsening after delivery suggests aHUS
• Requires urgent complement studies and eculizumab consideration

Examination Findings: General Approach

General Inspection:

• Appearance: Lethargic, pale, unwell, possibly jaundiced
• Hydration status: Dehydrated (prodromal losses) OR overloaded (anuric phase)
• Respiratory: Tachypnoea (acidosis, pulmonary oedema)

Cardiovascular:

• Heart rate: Tachycardia (anaemia, hypovolemia)
• Blood pressure: CRITICAL to measure accurately with appropriate cuff size
  • Compare to centile charts for age/sex/height
  • Hypertension present in 50-70%
• JVP: Elevated if fluid overloaded (difficult to assess in young children)
• Heart sounds: S3 gallop if fluid overloaded

Respiratory:

• Work of breathing: Kussmaul breathing (metabolic acidosis)
• Chest auscultation: Crackles if pulmonary oedema

Abdominal:

• Inspection: Distension (colitis, ascites)
• Palpation: Generalized tenderness, particularly left lower quadrant
• Hepatosplenomegaly: Rare (if present, consider alternative diagnoses like TTP)

Skin:

• Pallor: Marked (anaemia)
• Jaundice: Mild (haemolysis)
• Petechiae/purpura: Usually limited, non-blanching
• Oedema: Periorbital (often first sign), pedal, sacral, scrotal

Neurological:

• Consciousness level: GCS assessment
• Seizures: Observe for ongoing/recurrent seizure activity
• Focal neurology: Examine for signs of stroke
• Fundoscopy: Papilloedema (hypertensive encephalopathy, cerebral oedema)

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5. Differential Diagnosis

The differential diagnosis of a child presenting with the triad of anaemia, thrombocytopenia, and AKI includes:

1. Thrombotic Thrombocytopenic Purpura (TTP)

Key Differentiators:

Classical TTP "Pentad" (present in only 40%):

1. Microangiopathic haemolytic anaemia
2. Thrombocytopenia
3. Neurological symptoms
4. Renal impairment
5. Fever

ADAMTS13 Testing:

• ADAMTS13 activity less than 10% diagnostic of TTP
• HUS patients have normal ADAMTS13 (> 50%)
• Result may take days - do not delay plasma exchange if TTP suspected

2. Disseminated Intravascular Coagulation (DIC)

Differentiators:

• Coagulation abnormal in DIC: Prolonged PT/APTT, low fibrinogen, elevated D-dimer
• HUS: Normal PT/APTT, normal fibrinogen (unless severe)
• Clinical context: DIC occurs with sepsis, malignancy, trauma, obstetric emergencies
• Schistocytes: Less prominent in DIC

3. Immune Thrombocytopenia (ITP)

Differentiators:

• ITP: Isolated thrombocytopenia, NO haemolysis, NO renal impairment
• Normal haemoglobin (unless bleeding)
• Normal renal function
• No schistocytes on blood film
• May follow viral illness (like HUS prodrome, can cause diagnostic confusion)

4. Acute Post-Streptococcal Glomerulonephritis (APSGN)

Similarities: Hypertension, oedema, renal impairment, haematuria after infection

Differentiators:

5. Haemolytic Disease (Other Causes)

G6PD Deficiency:

• Haemolysis after oxidative stress (fava beans, medications, infection)
• No thrombocytopenia, no renal impairment
• Bite cells, blister cells on film (not schistocytes)

Autoimmune Haemolytic Anaemia (AIHA):

• Positive Direct Coombs Test (DAT) - key differentiator (HUS is DAT negative EXCEPT Sp-HUS)
• No thrombocytopenia (unless Evans syndrome)
• No renal impairment
• Spherocytes on film

6. Malignant Hypertension/Hypertensive Emergency

Can cause secondary TMA:

• Severe BP elevation can damage endothelium → TMA
• Distinction from primary HUS can be difficult
• Requires careful history: pre-existing hypertension? Renal disease? Medications?

7. Drug-Induced TMA

Medications implicated:

• Calcineurin inhibitors, chemotherapy (gemcitabine, mitomycin), quinine, anti-VEGF agents
• Requires detailed medication history

8. Systemic Conditions with TMA

• Systemic Lupus Erythematosus (SLE)
• Antiphospholipid syndrome
• Scleroderma renal crisis
• Malignancy-associated TMA

Diagnostic Approach to TMA

Child with Anaemia + Thrombocytopenia + AKI
                    ↓
        Check Blood Film for Schistocytes
                    ↓
        Schistocytes Present → TMA Confirmed
                    ↓
            Assess for HUS vs TTP:
    ┌───────────────┴───────────────┐
    ↓                               ↓
History of Diarrhoea?          No Diarrhoea?
    ↓                               ↓
STEC-HUS likely              Atypical HUS vs TTP
- Stool culture/PCR          - Send ADAMTS13
- Supportive care            - Send complement studies
                            - Consider urgent plasma exchange
                            - Consider eculizumab

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6. Investigations

Early, comprehensive investigation is essential for diagnosis, assessment of severity, and guiding management.

Initial Assessment: Diagnostic Triad Confirmation

1. Full Blood Count (FBC)

Haemoglobin:

• Anaemia: Typically moderate to severe (Hb 60-90 g/L at presentation) [30]
• Rate of decline may be rapid (5-10 g/L per day in active haemolysis)
• Severity correlates with disease severity and need for transfusion

Platelet Count:

• Thrombocytopenia: Usually 20-100 × 10⁹/L
• Severe thrombocytopenia (less than 20) less common but increases bleeding risk
• Platelet count often first to recover (within 7-14 days)

White Blood Cell Count:

• Leukocytosis: Often marked (15-30 × 10⁹/L)
• Predictive value: WBC > 20 × 10⁹/L during prodrome predicts severe HUS requiring dialysis [24]
• Neutrophilia predominant
• Left shift may be present (stress response, not necessarily infection)

Blood Film Examination - CRITICAL:

Must specifically request "blood film for schistocytes"

Schistocytes (Fragmented RBCs):

• Diagnostic hallmark of MAHA
• Helmet cells, triangle cells, fragmented cells
• Typically > 1% of RBCs considered significant (normal less than 0.5%)
• Quantification: Mild (1-2%), Moderate (2-5%), Severe (> 5%)

Other Features:

• Polychromasia: Reflects reticulocytosis (bone marrow response)
• Nucleated RBCs: Stress erythropoiesis
• Anisocytosis and poikilocytosis: Variation in RBC size and shape
• Burr cells (echinocytes): Common in uraemia

Exam Detail: Why Schistocytes Form: Microthrombi create fibrin strands across vessel lumens → RBCs undergo mechanical shearing as they attempt to deform through narrowed vessels → fragmentation creates schistocytes. The spleen removes some fragments; others remain in circulation. The bone marrow attempts to compensate by releasing immature RBCs (reticulocytes) → polychromasia.

Exam Pearl: If asked to describe the blood film in HUS, structure answer as:

1. RBC morphology: Schistocytes (fragments, helmet cells), polychromasia (reticulocytosis), anisocytosis
2. Platelet count: Reduced (estimate from film - normally 7-20 per high-power field)
3. WBC: Elevated (state if present)

2. Reticulocyte Count

• Elevated in active haemolysis (5-15%, normal less than 2%)
• Reflects bone marrow response to anaemia
• If LOW despite anaemia: Consider concurrent bone marrow suppression (uraemia, viral infection)

3. Markers of Haemolysis

Lactate Dehydrogenase (LDH):

• Markedly elevated (often 2-10 times upper limit of normal)
• Released from lysed RBCs and damaged tissues
• Useful for monitoring disease activity
• Decline indicates resolution

Haptoglobin:

• Low or undetectable (less than 0.3 g/L)
• Haptoglobin binds free haemoglobin → complex cleared by liver
• Intravascular haemolysis depletes haptoglobin

Unconjugated Bilirubin:

• Mildly elevated (typically 30-100 µmol/L)
• From haem breakdown
• Conjugated bilirubin remains normal (no hepatic dysfunction)

Direct Coombs Test (DAT):

• NEGATIVE in STEC-HUS and aHUS (non-immune haemolysis)
• POSITIVE in Sp-HUS (immune-mediated T-antigen exposure)
• Essential test to differentiate Sp-HUS from other forms

4. Renal Function

Urea and Creatinine:

• Elevated in proportion to AKI severity
• Creatinine may rise rapidly (50-100 µmol/L per day in severe cases)
• Urea often disproportionately elevated (urea:creatinine ratio > 80-100) - suggests prerenal component from dehydration

Electrolytes:

• Sodium: Hyponatraemia common (dilutional from SIADH or fluid overload)
• Potassium: HYPERKALEMIA - life-threatening, requires urgent treatment
  • "Risk factors: Anuria, acidosis, tissue breakdown, haemolysis (K+ release from lysed RBCs)"
  • Levels > 6.5 mmol/L require urgent intervention
• Bicarbonate: Low (metabolic acidosis from renal failure + diarrhoeal losses)
• Calcium: Hypocalcaemia (reduced 1,25-vitamin D, hyperphosphataemia)
• Phosphate: Hyperphosphataemia (reduced renal excretion)

eGFR:

• Calculated eGFR often severely reduced (less than 30 mL/min/1.73m²)
• Creatinine-based eGFR may underestimate true GFR in acute setting
• Useful for longitudinal monitoring

5. Urinalysis

Dipstick:

• Blood: Positive (haematuria in 50-80%) - microscopic or macroscopic
• Protein: Variable (1+ to 3+), rarely nephrotic-range
• Leukocytes: May be present (not indicative of UTI in absence of nitrites/clinical features)

Microscopy:

• RBCs: Dysmorphic RBCs (glomerular origin)
• RBC casts: Glomerular injury
• Granular casts: Tubular injury
• WBCs: Inflammatory component

Urine Protein:Creatinine Ratio (uPCR):

• Usually elevated but rarely > 200 mg/mmol (nephrotic range)

Microbiological Investigations

For STEC-HUS Diagnosis

Stool Culture:

• MUST specifically request STEC/E. coli O157 testing - not routine culture
• Sorbitol-MacConkey agar for O157:H7 (non-sorbitol fermenting)
• Sensitivity declines rapidly: 80% positive if tested within 6 days of diarrhoea onset, less than 25% after 1 week [31]
• Negative stool does not exclude STEC-HUS

Stool PCR for Shiga Toxin Genes:

• More sensitive than culture (detects toxin genes even if bacteria no longer viable)
• Can detect non-O157 STEC
• Faster turnaround than culture (24-48 hours)
• Increasingly used as first-line test

Serology:

• Anti-LPS (lipopolysaccharide) antibodies to E. coli O157
• Useful if stool negative but HUS suspected (seroconversion over 2-4 weeks)
• Confirms recent STEC infection retrospectively

Blood Culture:

• NOT routinely positive in STEC-HUS (toxin-mediated, not bacteraemic illness)
• Perform if fever/sepsis suspected (secondary infection, alternative diagnosis)

Investigations to Differentiate aHUS from STEC-HUS

If aHUS suspected (no diarrhoeal prodrome, atypical age, recurrent episodes, family history):

Complement Studies (URGENT)

Serum Complement Levels:

• C3: May be low in C3 mutations, CFI deficiency; often normal in CFH/MCP mutations
• C4: Typically normal (alternative pathway mediated)
• Factor H level: Low in CFH deficiency
• Factor I level: Low in CFI deficiency

Interpretation Challenges:

• Complement levels may be normal even in genetic aHUS (particularly CFH/MCP mutations)
• Acute phase reaction can elevate complement proteins
• Normal complement does NOT exclude aHUS

Genetic Testing

Indications for Genetic Testing:

• Any suspected aHUS (no diarrhoeal prodrome)
• Age less than 6 months or > 5 years
• Recurrent HUS
• Family history of HUS or unexplained renal disease
• Planning renal transplantation (risk stratification)

Genes to Test: Full complement panel: CFH, CFI, CFB, C3, MCP, THBD, DGKE, CFHR1-5

Turnaround Time:

• Standard: 4-8 weeks
• Some centers offer rapid turnaround (1-2 weeks) for acute cases

Yield:

• Pathogenic mutation found in 50-70%
• Variants of uncertain significance (VUS) common - interpretation challenging

Anti-Factor H Antibody Testing

Indications:

• All suspected aHUS, particularly children
• May be more common in certain populations

Interpretation:

• Titre > 1000 AU/mL considered significant
• Associated with CFHR1 deletion (test simultaneously)

ADAMTS13 Activity

Indications:

• All patients with TMA to differentiate HUS from TTP
• Particularly if neurological features predominate
• Adult patients

Results:

• less than 10%: Diagnostic of TTP → plasma exchange urgent
• 10-50%: Borderline, repeat/clinical correlation
• > 50%: Excludes TTP, consistent with HUS

Timing:

• Send BEFORE plasma therapy if possible (plasma infusion normalizes ADAMTS13)
• If already started plasma: Can measure ADAMTS13 inhibitor antibodies

Additional Investigations Based on Clinical Features

Imaging

Renal Ultrasound:

• Indications: All HUS patients (baseline assessment)
• Findings in HUS:
  • Kidneys often normal size or enlarged (oedematous)
  • Increased echogenicity (cortical and medullary)
  • Normal or reduced corticomedullary differentiation
  • Exclude obstruction, structural abnormalities
• Prognostic value: Prolonged increased echogenicity suggests cortical necrosis → poor recovery

Abdominal X-ray/CT:

• Indications: Severe abdominal pain, peritonism, distension
• Assess for: Toxic megacolon, perforation, pneumatosis, bowel necrosis

Neuroimaging

Indications:

• Seizures
• Reduced consciousness (GCS less than 13)
• Focal neurological signs
• Severe uncontrolled hypertension

CT Brain:

• First-line in acute setting (rapid, assess for haemorrhage, mass effect)
• Findings: Oedema, infarction, haemorrhage, posterior reversible encephalopathy syndrome (PRES)

MRI Brain:

• More sensitive for ischaemic injury, microinfarcts
• Findings: Cortical/subcortical infarcts (particularly basal ganglia), PRES changes

Electroencephalogram (EEG)

• Indications: Seizures, altered consciousness
• Assess for ongoing seizure activity, encephalopathy

Cardiac Assessment

ECG:

• Essential in hyperkalemia (K+ > 6 mmol/L)
• Look for: Tall tented T waves, widened QRS, prolonged PR, loss of P waves, sine wave (pre-arrest rhythm)

Echocardiogram:

• Indications: Suspected heart failure, pericardial effusion, myocarditis
• Assess: LV function, pericardial fluid, valvular function

Monitoring Investigations

Once diagnosis established, regular monitoring required:

Daily (During Acute Phase):

• FBC (Hb, platelets, WCC)
• U&Es (particularly K+)
• Venous blood gas (acidosis)
• Urine output (strict fluid balance)

Every 2-3 Days:

• LDH (marker of haemolysis activity)
• Reticulocyte count (bone marrow response)

Weekly:

• Liver function tests
• Bone profile (Ca, PO4)
• Blood pressure monitoring (4-6 hourly minimum)

Exam Detail: ### Investigations Summary Table for Viva Preparation

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7. Management

Management of HUS is primarily supportive for STEC-HUS, with specific complement-targeted therapy for atypical HUS. The goals are to support the patient through the acute phase while kidneys recover, manage complications, and prevent long-term sequelae.

Initial Management: Acute Presentation

1. Admission and Isolation

All patients with suspected or confirmed HUS require hospital admission.

STEC-HUS Patients:

• Barrier nursing/contact isolation until:
  • Two negative stool samples (> 48 hours apart)
  • Alternative diagnosis confirmed
• Rationale: STEC is highly infectious (infectious dose less than 100 organisms)
• Protect other vulnerable hospitalized children

Setting:

• Pediatric Intensive Care Unit (PICU): Severe cases (seizures, anuria, severe hypertension, respiratory compromise)
• High-Dependency Unit/Specialist Nephrology Ward: Most cases
• Tertiary pediatric nephrology center: All cases ideally, or early transfer

2. Immediate Assessment and Stabilization

Airway, Breathing, Circulation:

• Assess for respiratory distress (pulmonary oedema, acidosis)
• Secure IV access (may need central line for dialysis, monitoring, medication)
• Continuous cardiorespiratory monitoring

Urgent Interventions:

Life-Threatening Hyperkalemia (K+ > 6.5 mmol/L or ECG changes):

Immediate management:

1. Calcium gluconate 10%: 0.5 mL/kg IV over 5-10 min (max 20 mL)
   • Cardiac membrane stabilization (does NOT lower K+)
   • Repeat if persistent ECG changes
2. Insulin-glucose: 0.1 units/kg soluble insulin + 0.5 g/kg glucose (5 mL/kg 10% dextrose) IV over 30 min
   • Shifts K+ intracellularly
   • Onset 15-30 min, duration 4-6 hours
   • Monitor blood glucose closely (risk of hypoglycaemia)
3. Salbutamol: 5 mg nebulized or 4 mcg/kg IV (max 250 mcg)
   • Shifts K+ intracellularly via β2-agonist effect
   • Onset 30 min
4. Sodium bicarbonate: 1-2 mmol/kg IV over 30-60 min
   • If concurrent metabolic acidosis (pH less than 7.2)
   • Shifts K+ intracellularly
5. Calcium resonium: 1 g/kg PO/PR
   • Ion-exchange resin (slow onset, hours)
   • Limited efficacy in acute setting
6. Dialysis: Definitive treatment if above measures fail or K+ > 7.5 mmol/L

Severe Hypertension (> 99th centile + symptoms OR > 99th centile + 30 mmHg):

Treat urgently but avoid precipitous BP reduction (risk of cerebral hypoperfusion):

• Target: Reduce BP by maximum 25% over first 8 hours
• First-line agents:
  • "Amlodipine: 0.1-0.2 mg/kg PO once daily (max 10 mg)"
  • "Labetalol: 1-3 mg/kg/day PO in 2 divided doses, or 0.2-1 mg/kg/hr IV infusion"
  • "Nifedipine: 0.25-0.5 mg/kg PO (short-acting, use with caution)"
• Severe/resistant hypertension:
  • IV labetalol infusion
  • "Sodium nitroprusside: 0.5-8 mcg/kg/min IV (requires ICU, risk of cyanide toxicity)"

Seizures:

• First-line: Benzodiazepines (lorazepam 0.1 mg/kg IV or buccal midazolam 0.5 mg/kg)
• Second-line: Levetiracetam 20-40 mg/kg IV or phenytoin 20 mg/kg IV
• Correct underlying causes: Hypertension, hyponatraemia, hypoglycaemia
• Neuroimaging once stabilized

Pulmonary Oedema:

• Sit upright, high-flow oxygen
• Urgent dialysis (fluid removal)
• Furosemide has limited efficacy in anuric patients

3. Fluid Management

Fluid management is one of the most challenging aspects of HUS care, requiring careful individualization:

Principles:

• Assess volume status carefully (clinical examination, fluid balance, weight)
• Phases of illness require different approaches:
  • "Early/prodromal phase: Dehydration from diarrhoea → fluid resuscitation"
  • "Oliguric/anuric phase: Risk of fluid overload → restriction"

Dehydrated Patient (Early Phase):

• IV fluid resuscitation: 0.9% saline or Hartmann's 10-20 mL/kg boluses
• Aim to restore euvolaemia
• Monitor urine output response
• Evidence suggests early aggressive hydration during diarrhoeal phase may reduce HUS severity [32]

Euvolaemic/Overloaded Patient (Oliguric/Anuric Phase):

• Fluid restriction: Insensible losses (400 mL/m²/day) + urine output + other measurable losses
• Strict fluid balance charting (hourly in/out)
• Daily weights (same time, same scale, ideally same conditions)
• Consider concentrated medication preparations to reduce fluid volume

Monitoring:

• Clinical: Oedema, JVP, lung auscultation
• Weight: > 5% increase suggests fluid overload
• Serum sodium: Hyponatraemia suggests fluid overload

Exam Detail: Calculation of Fluid Requirements:

Insensible Losses (Simplified):

• 400 mL/m²/day (approximately 15-20 mL/kg/day for most children)

Body Surface Area (BSA) Calculation:

• Mosteller formula: BSA (m²) = √[(height (cm) × weight (kg)) / 3600]

Example:

• Child: 3 years, 15 kg, 95 cm
• BSA = √[(95 × 15) / 3600] = 0.63 m²
• Insensible losses = 400 × 0.63 = 252 mL/day
• If anuric: Total maintenance = 252 mL/day (~10 mL/hour)
• If urine output 100 mL/day: Total = 352 mL/day (~15 mL/hour)

Viva Pearl: Explain that fluid management aims to maintain euvolaemia: "In the anuric phase, I would restrict fluids to insensible losses plus measured outputs. I'd perform daily weights, strict fluid balance, and clinical assessment for signs of overload or dehydration. If fluid overload develops, I'd proceed to dialysis."

4. Medications to AVOID

CONTRAINDICATED:

1. Antibiotics (in STEC-HUS):

• Multiple studies confirm increased HUS risk with antibiotic use during STEC gastroenteritis [7,8]
• Mechanism: Bacterial lysis → toxin release
• Particularly quinolones, TMP-SMX, beta-lactams
• Meta-analysis: OR 2.24 (95% CI 1.14-4.39) for HUS development with antibiotics [33]
• Exception: If bacteraemic/septic shock, antibiotics necessary (choose carefully, avoid those that promote lysis)

2. Antimotility Agents:

• Loperamide, diphenoxylate
• Slow intestinal transit → prolonged toxin exposure
• Increased HUS risk

3. NSAIDs:

• Reduce renal perfusion (prostaglandin inhibition)
• Exacerbate AKI
• Increase bleeding risk with thrombocytopenia

4. ACE Inhibitors/Angiotensin Receptor Blockers (ARBs):

• Cause efferent arteriolar vasodilation → reduced GFR
• Risk of hyperkalemia
• May be needed for hypertension control later (once past acute phase)

RARELY INDICATED:

Platelet Transfusion:

• Only if life-threatening bleeding (e.g., intracranial haemorrhage, major GI bleed)
• NOT for prophylaxis based on platelet count alone
• Risks: Transfused platelets consumed into microthrombi, worsening TMA

Red Cell Transfusion:

• Only if symptomatic anaemia (SOB, tachycardia, heart failure) or Hb less than 60 g/L
• Transfusion thresholds lower than usual paediatric practice
• Use packed RBCs (reduce volume load)
• Transfuse slowly (risk of fluid overload in anuric patients)
• Volume: 5-10 mL/kg over 4 hours (may need dialysis before/after if anuric)

5. Nutritional Support

Challenges:

• Reduced appetite (uraemia, illness)
• Fluid restriction (limits enteral/parenteral nutrition volumes)
• Electrolyte constraints (restrict K+, PO4)

Approach:

• Enteral feeding preferred if tolerated (NG tube if needed)
• High calorie, low protein diet initially (reduce uraemia, limit K+)
• Restrict potassium (less than 1-2 mmol/kg/day) and phosphate
• Sodium restriction if hypertensive/fluid overloaded
• Consider parenteral nutrition if prolonged poor intake/intestinal complications
• Involve pediatric dietitian early

6. Renal Replacement Therapy (RRT)

Approximately 50-60% of STEC-HUS patients require dialysis. [27]

Indications for Dialysis:

Absolute:

• Anuria > 24 hours
• Severe hyperkalemia (> 7 mmol/L or refractory to medical management)
• Severe fluid overload with pulmonary oedema
• Severe metabolic acidosis (pH less than 7.1) unresponsive to bicarbonate
• Uraemic encephalopathy (confusion, seizures from uraemia)
• Pericarditis (uraemic)

Relative:

• Oliguria with rising creatinine
• Moderate hyperkalemia (6-7 mmol/L) with increasing trend
• BUN > 30-35 mmol/L (uraemic symptoms likely)

Modality Choice:

Peritoneal Dialysis (PD):

• Most common in children (particularly less than 20 kg)
• Advantages: Gentler, continuous fluid/solute removal, no vascular access needed
• Disadvantages: Risk of peritonitis, may be difficult if recent abdominal surgery/colitis complications
• Technique: Acute PD catheter insertion → manual or automated cycles

Haemodialysis (HD):

• Preferred in larger children/adolescents or if PD contraindicated
• Advantages: Rapid correction of electrolytes/uraemia, rapid fluid removal
• Disadvantages: Requires vascular access (tunneled catheter), haemodynamic instability risk, anticoagulation
• Usually intermittent HD (3-4 times per week initially)

Continuous Renal Replacement Therapy (CRRT):

• Reserved for PICU patients with haemodynamic instability
• Continuous venovenous haemofiltration (CVVH)

Duration of Dialysis:

• Median duration: 2-3 weeks in STEC-HUS [34]
• Range: Few days to several months (rarely permanent)
• Continue until adequate urine output returns and biochemistry stabilized

7. Monitoring During Acute Phase

Clinical Monitoring:

• Blood pressure: Every 4-6 hours (or continuous if severe hypertension)
• Urine output: Hourly (strict input/output chart)
• Weight: Daily (same time each day)
• Neurological observations: GCS, seizure monitoring
• Fluid balance: Cumulative balance chart

Laboratory Monitoring:

Daily:

• FBC (Hb, platelets, WCC)
• U&Es (Na, K, urea, creatinine)
• Venous blood gas (pH, bicarbonate, lactate)

Every 2-3 Days:

• LDH (haemolysis marker)
• Reticulocyte count
• Liver function tests

Weekly:

• Bone profile (calcium, phosphate)
• Magnesium
• Blood film review (schistocyte monitoring)

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8. Specific Therapy for Atypical HUS: Eculizumab

The development of eculizumab, a humanized monoclonal antibody against complement component C5, has transformed the prognosis of atypical HUS from a disease with 50% mortality/ESRD to one with excellent renal recovery rates. [35,36]

Mechanism of Action

Eculizumab binds to complement protein C5, preventing its cleavage into C5a and C5b, thereby:

• Blocking C5a production: Prevents anaphylatoxin-mediated inflammation
• Blocking C5b-9 (MAC) formation: Prevents membrane attack complex-mediated cell lysis

Result: Inhibits terminal complement activation without affecting early complement components (C3) needed for opsonization and immune defense.

Indications for Eculizumab

Definite Indications:

1. Confirmed atypical HUS (genetic mutation or high clinical suspicion)
2. STEC-HUS with severe neurological involvement (case reports suggest benefit)
3. Recurrent HUS (any form)
4. Pre-emptive therapy in transplant recipients with aHUS history

Consider in:

• Uncertain HUS type with severe disease (may treat empirically while awaiting complement studies)
• Pregnancy-associated HUS not responding to delivery/supportive care

Starting Eculizumab:

• Do NOT wait for genetic confirmation - takes weeks
• Clinical diagnosis + complement studies sent are sufficient
• Earlier initiation → better outcomes

Dosing Regimen

Induction Phase:

Maintenance Phase:

Route: IV infusion over 35 minutes

Duration of Therapy

Controversial - options include:

Lifelong Therapy:

• Recommended for genetic aHUS (ongoing complement dysregulation)
• Prevents recurrence (50-80% risk off therapy)

Finite Therapy:

• Consider in acquired aHUS (anti-FH antibodies after immunosuppression)
• Some centers attempt withdrawal after 6-12 months disease-free
• Requires very close monitoring

Discontinuation Protocol (if attempted):

• Only after sustained remission (normal Hb, platelets, creatinine, no proteinuria)
• Taper dose gradually (increase interval)
• Intensive monitoring (weekly bloods for 1 month, then monthly for 6 months)
• Patient/family education re: symptoms of recurrence
• Immediate access to eculizumab if relapse

Response to Eculizumab

Expected Response:

• Platelet count improvement: Within 7 days (earliest marker)
• Haemolysis cessation: LDH normalization within 2-4 weeks
• Renal recovery: Slower, may take weeks to months
• Complete haematological remission: 80-90% of patients [37]

Meningococcal Prophylaxis - CRITICAL

Eculizumab increases meningococcal infection risk 1000-2000 fold due to impaired MAC-mediated killing of Neisseria meningitidis.

Mandatory Preventive Measures:

1. Vaccination (BEFORE eculizumab if not emergency):

• Meningococcal ACWY conjugate vaccine (Menveo, Menactra)
• Meningococcal B vaccine (Bexsero, Trumenba)
• Pneumococcal vaccine (PCV13 or PPSV23)
• Haemophilus influenzae type b (if not previously vaccinated)

Timing:

• Ideally ≥2 weeks before eculizumab
• In emergency, give vaccine same day as first dose

2. Antibiotic Prophylaxis:

• Duration: Continue throughout eculizumab therapy + at least 2 weeks after last dose
• Options:
  • "Penicillin V: 250-500 mg PO twice daily (children)"
  • "Amoxicillin: 10-20 mg/kg/day (alternative)"
  • "Azithromycin: 10 mg/kg once weekly (if penicillin allergic)"

3. Patient/Family Education:

• Seek immediate medical attention for fever, headache, neck stiffness, rash
• Provide emergency card/letter
• Inform all healthcare providers of eculizumab use

4. Early Antibiotic Treatment:

• Any suspected meningococcal infection → immediate IV ceftriaxone (do not wait for confirmation)

Plasma Therapy: Limited Role

Plasma Exchange (PLEX) or Plasma Infusion:

Historical Role:

• Before eculizumab, plasma therapy was first-line for aHUS
• Theory: Replaces deficient complement regulators (Factor H, I), removes antibodies

Current Role - VERY LIMITED:

1. Bridge to eculizumab (if delay in accessing drug)
2. Anti-Factor H antibody-mediated aHUS (removes antibodies + immunosuppression)
3. Uncertain diagnosis (HUS vs TTP) - start PLEX urgently while ADAMTS13 pending
4. STEC-HUS: NO ROLE (no proven benefit, may harm) [38]

Regimen (if used):

• Daily PLEX (1.5 plasma volumes) until platelet count > 150 × 10⁹/L for 2 days
• Then alternate days for 2 weeks, then taper

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9. Complications

HUS can affect multiple organ systems beyond the kidney. Early recognition and management of complications is critical.

Renal Complications

Acute Phase

1. Anuria/Severe Oliguria:

• Most common acute renal complication (60-70%)
• Requires dialysis (see above)

2. Severe Hypertension:

• Mechanisms: Volume overload, RAAS activation, endothelial dysfunction
• Complications: Hypertensive encephalopathy, seizures, PRES, heart failure
• Management: Antihypertensives (amlodipine, labetalol), dialysis for fluid removal

3. Electrolyte Disturbances:

• Hyperkalemia (life-threatening)
• Hyponatraemia (dilutional, SIADH)
• Hypocalcaemia, hyperphosphataemia
• Metabolic acidosis

4. Acute Tubular Necrosis:

• Ischaemic injury from glomerular thrombosis
• Direct toxin effect (STEC-HUS)
• Usually recovers but may prolong dialysis requirement

Chronic/Long-Term Renal Complications

1. Chronic Kidney Disease (CKD):

• STEC-HUS: 5-30% develop CKD (stage 2-5) [39,40]
  • "Stage 3-5 CKD: 5-10%"
  • "ESRD requiring transplant: 3-5%"
• Atypical HUS (untreated): 50% progress to ESRD
• Atypical HUS (eculizumab-treated): less than 10% ESRD [36]

Predictors of CKD Development:

• Prolonged anuria (> 2 weeks)
• Severe CNS involvement
• Need for dialysis > 4 weeks
• Atypical HUS
• Recurrent episodes
• Persistently elevated creatinine at discharge

2. Hypertension:

• Persistent hypertension in 10-20% at 1 year [41]
• Requires long-term antihypertensive therapy
• Annual BP monitoring essential

3. Proteinuria:

• Persistent low-grade proteinuria in 20-30%
• Marker of ongoing glomerular damage
• May progress to CKD even if creatinine initially normal

4. Hyperfiltration Injury:

• Remaining nephrons undergo hyperfiltration to compensate for lost units
• May lead to progressive CKD over years/decades
• Mechanism of late CKD development

5. End-Stage Renal Disease (ESRD):

• Overall ESRD risk: 5-10% in STEC-HUS, up to 50% in untreated aHUS
• Time to ESRD: Immediate (acute cortical necrosis) to decades later (progressive CKD)
• Transplantation considerations:
  • "STEC-HUS: Excellent transplant outcomes, no recurrence"
  • "Atypical HUS: High recurrence risk (50-90% depending on genotype) without eculizumab"
  • Pre-emptive eculizumab recommended for aHUS transplant recipients

Neurological Complications (20-50% of Severe Cases) [28]

1. Seizures:

• Incidence: 10-30% of hospitalized HUS patients
• Types: Generalized tonic-clonic, focal, status epilepticus
• Mechanisms:
  • Hypertensive encephalopathy
  • Cerebral thrombotic microangiopathy (microinfarcts)
  • Shiga toxin neurotoxicity
  • Metabolic disturbances (hyponatraemia, uraemia, hypocalcaemia)
• Management:
  • "Acute: Benzodiazepines → levetiracetam/phenytoin"
  • Control hypertension, correct electrolytes
  • Neuroimaging (CT/MRI)
  • Maintenance anti-epileptics may be needed

2. Posterior Reversible Encephalopathy Syndrome (PRES):

• Vasogenic oedema affecting posterior cerebral hemispheres
• Clinical: Headache, visual disturbances, altered consciousness, seizures
• Imaging: MRI shows T2/FLAIR hyperintensity in occipital/parietal lobes
• Management: BP control (usually reversible if treated)

3. Stroke:

• Ischaemic (more common) or haemorrhagic
• Cerebral venous sinus thrombosis
• Focal neurological deficits
• Requires urgent neuroimaging and neurology involvement

4. Encephalopathy:

• Uraemic encephalopathy (if dialysis delayed)
• Shiga toxin-mediated direct neurotoxicity
• Global cerebral oedema
• Ranges from confusion/irritability to coma

5. Long-Term Neurological Sequelae:

• Persistent in 10-25% with CNS involvement [42]
• Types: Cognitive impairment, learning difficulties, motor deficits, epilepsy, behavioral changes
• Requires long-term neurodevelopmental follow-up

Gastrointestinal Complications

1. Prolonged/Severe Colitis:

• Persistent bloody diarrhoea
• Abdominal pain, distension
• Protein-losing enteropathy

2. Toxic Megacolon:

• Colonic dilatation > 6 cm
• Risk of perforation
• Requires surgical consultation

3. Colonic Perforation/Necrosis:

• Rare (less than 2%) but life-threatening
• Presents with peritonism, free air on imaging
• Requires emergency laparotomy, bowel resection

4. Rectal Prolapse:

• From severe straining with bloody diarrhoea
• Usually self-limiting once diarrhoea resolves

5. Pancreatitis:

• Shiga toxin damages pancreatic islet cells
• Acute pancreatitis: Elevated amylase/lipase, abdominal pain
• Diabetes mellitus: Transient hyperglycaemia common; permanent Type 1 DM rare but reported [29]

6. Hepatobiliary:

• Transaminitis (mild, usually resolves)
• Rare: Hepatic thrombosis, cholangiopathy

Haematological Complications

1. Severe Anaemia:

• Symptomatic (heart failure, SOB, collapse)
• May require transfusion (risks in anuric patients)

2. Severe Thrombocytopenia:

• Bleeding complications (rare unless platelets less than 10-20 × 10⁹/L)
• Sites: GI, epistaxis, intracranial haemorrhage (very rare)

3. Prolonged Haemolysis:

• Usually resolves within 2-4 weeks
• Persistent haemolysis suggests ongoing TMA (consider aHUS, re-evaluate diagnosis)

Cardiovascular Complications

1. Hypertension:

• See above (acute and chronic)
• Hypertensive emergencies requiring ICU care

2. Myocardial Involvement:

• Rare, but reported: myocarditis, troponin elevation, heart failure
• Shiga toxin direct myocardial toxicity theorized

3. Pericarditis/Pericardial Effusion:

• Uraemic pericarditis (if dialysis delayed)
• Usually responds to dialysis

4. Heart Failure:

• From volume overload (anuric phase)
• Requires dialysis for fluid removal

Infectious Complications

1. Secondary Bacterial Infection:

• Risk factors: Dialysis catheters, immunosuppression (uraemia), prolonged hospitalization
• Types: Bacteraemia, peritonitis (PD), line-related sepsis, pneumonia, UTI
• Requires prompt antibiotics (choose carefully, avoid nephrotoxins)

2. Viral Reactivation:

• CMV, EBV (particularly if immunosuppressed, transplant)

3. Fungal Infections:

• Rare, but increased risk with prolonged catheters, ICU stay

Death

Mortality:

• STEC-HUS: 3-5% acute mortality [43]
  • Higher in severe cases with CNS involvement (10-15%)
• Atypical HUS (pre-eculizumab era): 10-15% acute mortality
• Atypical HUS (eculizumab era): less than 5%

Causes of Death:

• CNS complications (stroke, cerebral oedema, status epilepticus)
• Multi-organ failure
• Cardiac complications (arrhythmia from hyperkalemia, heart failure)
• Sepsis
• GI complications (perforation, bowel ischaemia)

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10. Prognosis and Outcomes

Prognosis varies significantly based on HUS type, disease severity, and management.

Typical STEC-HUS: Generally Favorable

Acute Phase Outcomes

Mortality: 3-5% [43]

• Lower in centers with pediatric nephrology/PICU expertise

Dialysis Requirement: 50-60%

• Median duration: 2-3 weeks
• Most discontinue dialysis within 4-8 weeks

Complete Recovery: 70-80%

• Normal renal function (eGFR > 90 mL/min/1.73m²)
• No proteinuria
• Normal blood pressure

Long-Term Outcomes (Follow-Up 5-20 Years)

Renal Function:

Proteinuria: 20-30% have persistent low-grade proteinuria (marker of ongoing glomerular damage)

Hypertension: 10-20% require long-term antihypertensive therapy

Hyperfiltration Injury:

• Even "recovered" patients may develop progressive CKD years/decades later
• Mechanism: Reduced nephron mass → hyperfiltration in remaining nephrons → glomerulosclerosis
• All HUS patients require lifelong renal monitoring

Predictors of Poor Outcome in STEC-HUS

Poor Prognostic Factors:

1. Prolonged anuria (> 2 weeks)
2. Severe CNS involvement (seizures, stroke, coma)
3. Need for dialysis (especially if > 3-4 weeks)
4. Very young age (less than 6 months) or older age (> 5 years)
5. Marked leukocytosis (WCC > 20 × 10⁹/L) [24]
6. Low C3 or complement abnormalities (suggests combined STEC + complement susceptibility)
7. Recurrent episodes (rare in pure STEC-HUS, suggests atypical component)
8. Severe extra-renal manifestations (pancreatitis, cardiac involvement)

Atypical HUS: Transformed by Eculizumab

Pre-Eculizumab Era (Historical)

Acute Outcomes:

• Mortality: 10-15%
• Progression to ESRD or death: 50% within 1 year [44]
• Dialysis-dependent: 30-40%

Transplant Outcomes:

• Recurrence in transplant: 50-90% (depending on genotype)
• Graft loss from recurrence: 80-90%

Genotype-Specific Outcomes (Untreated):

Post-Eculizumab Era

Acute Outcomes (With Early Eculizumab):

• Complete haematological remission: 80-90% [37]
• Dialysis independence: 80-85% (if started early)
• Mortality: less than 5%

Long-Term Outcomes:

• Event-free survival (no death, dialysis, TMA): 80-85% at 2 years [36]
• Renal function preservation: Majority maintain or improve eGFR
• Recurrence prevention: Near-complete (while on therapy)

Transplant Outcomes (With Eculizumab):

• Recurrence rate: less than 5% (with pre-emptive eculizumab)
• Graft survival: Excellent (equivalent to non-HUS patients)

Impact of Treatment Timing:

• Eculizumab within 7 days of presentation: 90% dialysis independence
• Eculizumab after 4 weeks: 30% dialysis independence
• Key message: Early recognition and treatment critical

Streptococcus pneumoniae-Associated HUS

Prognosis: Worse than STEC-HUS, better than untreated aHUS

• Mortality: 5-10%
• ESRD: 20-30% [22]
• Severity related to invasive pneumococcal disease severity

Pregnancy Outcomes in HUS Survivors

Women with History of STEC-HUS:

• Generally favorable pregnancy outcomes
• Increased monitoring recommended (BP, proteinuria, renal function)
• Small increased risk of preeclampsia

Women with History of Atypical HUS:

• High risk of pregnancy-associated recurrence (50-80% if not on eculizumab) [21]
• Pre-conception counseling essential
• Continue eculizumab throughout pregnancy if on therapy
• Consider prophylactic eculizumab even if in remission (discuss risks/benefits)
• Close maternal-fetal medicine and nephrology co-management

Quality of Life

Physical Health:

• Most recovered children have normal activity levels
• Those with CKD may have reduced exercise tolerance, growth delay

Neurodevelopmental:

• 10-25% of children with CNS involvement have long-term deficits [42]
• School performance may be affected
• Requires ongoing neurodevelopmental assessment

Psychological:

• PTSD from traumatic illness/hospitalization
• Anxiety regarding recurrence (particularly aHUS)
• Family stress

Socioeconomic:

• Hospital costs significant (ICU, dialysis, eculizumab)
• Eculizumab extremely expensive (~£500,000/year) - funding challenges

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11. Prevention and Screening

Primary Prevention of STEC-HUS

Public Health Measures:

1. Food safety:

   • Cook ground beef thoroughly (internal temperature > 71°C/160°F)
   • Avoid unpasteurized milk and dairy products
   • Avoid unpasteurized juices
   • Wash raw vegetables/fruits thoroughly
   • Prevent cross-contamination (separate cutting boards for meat)

2. Water safety:

   • Adequate chlorination of public water supplies
   • Avoid swallowing water while swimming in lakes/rivers
   • Boil water advisories during outbreaks

3. Animal contact:

   • Hand hygiene after petting zoo visits or farm contact
   • Supervise young children around animals
   • Designated eating areas away from animal contact areas

4. Infection control:

   • Hand hygiene in childcare settings
   • Exclude children with bloody diarrhoea from daycare until stool clearance
   • Education of parents and childcare workers

Individual Prevention:

• Thorough hand washing (soap and water for 20 seconds, especially after toilet, before eating)
• Avoid high-risk foods in young children
• Food preparation hygiene

Secondary Prevention: Reducing HUS Risk in STEC Gastroenteritis

When STEC Gastroenteritis Diagnosed:

AVOID:

• Antibiotics (increase HUS risk 2-17 fold) [7,8,33]
• Antimotility agents (loperamide, etc.)

CONSIDER:

• Early aggressive IV hydration: Evidence suggests fluid expansion during diarrhoeal phase may reduce HUS incidence/severity [32]
  • "Observational data: IV fluids during prodrome associated with reduced oliguria/dialysis need"
  • Mechanism: "Washout" of toxin, maintain renal perfusion, prevent ATN
  • Randomized trial data limited
  • "Current practice: Many centers advocate early hydration in confirmed/suspected STEC"

Monitoring:

• Daily clinical assessment for signs of HUS (pallor, oliguria, bruising)
• Consider daily FBC, U&Es if high-risk features (age less than 2 years, WCC > 20, severe disease)

Tertiary Prevention: Preventing Complications/Recurrence

In Acute HUS:

• Prompt recognition and supportive care (prevent ESRD, death)
• Early eculizumab for aHUS (prevent progression)
• Meticulous BP control (prevent CNS complications)
• Appropriate dialysis (prevent metabolic complications)

In aHUS:

• Eculizumab continuation (prevent recurrence - 50-80% risk off therapy)
• If eculizumab discontinued, very close monitoring
• Family screening (genetic testing for first-degree relatives)
• Pre-conception counseling for women

Screening and Surveillance

Acute Phase Monitoring:

• See above (Investigations and Management sections)

Post-Acute Follow-Up (All HUS Patients):

First Year:

• Weeks 1-4: Weekly FBC, U&Es, BP
• Months 2-6: Monthly clinic visits (BP, urinalysis, U&Es)
• Months 6-12: Every 2-3 months

Long-Term (Lifelong):

• Annual minimum: BP, urinalysis, U&Es, eGFR
• Additional if abnormalities: Urine PCR, renal ultrasound

Red Flags for CKD Progression:

• Persistent/increasing proteinuria
• Hypertension
• Declining eGFR
• Refer to pediatric nephrology

Genetic Screening:

• First-degree relatives of aHUS patients should be offered genetic screening
• Identifies at-risk individuals
• Allows pre-emptive counseling, monitoring, trigger avoidance
• Consider pre-emptive eculizumab in high-risk scenarios (pregnancy, transplant)

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12. Key Guidelines

International Guidelines and Consensus Statements

1. European Pediatric Study Group for HUS (2016) [45]

• Comprehensive guideline covering diagnosis and management of typical and atypical HUS
• Key recommendations:
  • Supportive care is cornerstone for STEC-HUS
  • Avoid antibiotics in STEC gastroenteritis
  • Eculizumab first-line for atypical HUS
  • Genetic testing for all suspected aHUS

2. Kidney Disease: Improving Global Outcomes (KDIGO) - AKI Guideline

• Criteria for AKI diagnosis and staging
• Indications for renal replacement therapy
• Applicable to HUS-related AKI

3. European Consensus on Atypical HUS (2017) [46]

• Diagnostic criteria for aHUS
• Genetic testing recommendations
• Eculizumab dosing and duration
• Transplantation in aHUS

4. UK Renal Association Clinical Practice Guidelines

• Acute kidney injury management
• Renal replacement therapy in children

5. RCPCH Guidance on STEC-HUS

• Antibiotic avoidance in suspected/confirmed STEC
• Public health notification requirements
• Infection control measures

Guideline-Based Management Summary

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13. Common Exam Questions

Typical MCQ/SBA Scenarios

1. Classic Presentation: "A 3-year-old child presents with pallor, reduced urine output, and petechiae 7 days after an episode of bloody diarrhoea. Blood tests show Hb 75 g/L, platelets 45 × 10⁹/L, and creatinine 250 μmol/L. What is the most likely diagnosis?"

• Answer: Haemolytic Uraemic Syndrome (STEC-HUS)

2. Antibiotic Contraindication: "A 2-year-old with bloody diarrhoea and confirmed E. coli O157:H7 is reviewed. What is the most appropriate management?"

• Answer: Supportive care, avoid antibiotics (increase HUS risk)

3. Diagnostic Blood Film: "A child with anaemia, thrombocytopenia, and acute kidney injury has a blood film showing fragmented red cells. What is the name of these cells?"

• Answer: Schistocytes (diagnostic of microangiopathic haemolysis)

4. Atypical vs. Typical HUS: "Which feature would most suggest atypical HUS rather than typical STEC-HUS?"

• Answer: No diarrhoeal prodrome / Family history / Recurrent episodes / Age less than 6 months or > 5 years

5. Life-Threatening Complication: "A child with HUS has ECG showing tall tented T-waves and widened QRS. Blood shows K+ 7.2 mmol/L. What is the most urgent first-line treatment?"

• Answer: IV calcium gluconate (cardiac membrane stabilization)

6. Eculizumab Mechanism: "Eculizumab is a monoclonal antibody that blocks which complement component?"

• Answer: C5 (prevents C5b-9 formation)

7. Essential Prophylaxis: "A child with atypical HUS is started on eculizumab. What is the most important concurrent intervention?"

• Answer: Meningococcal vaccination + antibiotic prophylaxis

8. Differentiating TTP: "Which investigation would best differentiate TTP from HUS in an adult with thrombotic microangiopathy?"

• Answer: ADAMTS13 activity (less than 10% diagnostic of TTP)

Viva Voce Scenarios

Opening Statement for HUS: "Haemolytic Uraemic Syndrome is a thrombotic microangiopathy characterized by the triad of microangiopathic haemolytic anaemia, thrombocytopenia, and acute kidney injury. It is the most common cause of acute renal failure requiring dialysis in children. The majority of cases are typical STEC-HUS caused by Shiga toxin-producing E. coli, while 5-10% are atypical HUS due to complement dysregulation."

Viva Question: "Describe your approach to a child presenting with bloody diarrhoea, pallor, and oliguria."

Model Answer: "I would approach this systematically, as this presentation is highly suggestive of Haemolytic Uraemic Syndrome following STEC gastroenteritis.

Initial Assessment: I would perform a focused history to establish the timeline of symptoms, ask about food exposures (undercooked meat, unpasteurized products), animal contacts, and assess for features of complications such as seizures or severe hypertension. On examination, I would assess hydration status, measure blood pressure accurately, look for pallor, petechiae, oedema, and perform a thorough neurological examination.

Investigations: I would order urgent bloods including FBC (expecting anaemia, thrombocytopenia), blood film specifically requesting assessment for schistocytes (diagnostic of microangiopathic haemolysis), U&Es (elevated creatinine, hyperkalemia), LDH (markedly elevated in haemolysis), and haptoglobin (low). I would send stool for culture and Shiga toxin PCR, urinalysis, and perform a Direct Coombs Test which should be negative in STEC-HUS.

Immediate Management: I would admit the child to a specialist pediatric nephrology unit with barrier nursing. I would avoid antibiotics and antimotility agents. I would carefully assess fluid status and either rehydrate if dehydrated or restrict fluids if oliguric. I would manage electrolytes urgently, particularly if hyperkalemia is present. If blood pressure is elevated, I would start antihypertensive therapy with a calcium channel blocker or labetalol. I would assess for dialysis indications such as anuria, severe hyperkalemia, or fluid overload.

Ongoing Care: I would provide supportive care with close monitoring of blood pressure, fluid balance, and daily bloods. Approximately 50-60% will require dialysis. I would monitor for complications, particularly neurological (seizures, encephalopathy). Most children recover with supportive care within 2-4 weeks, though lifelong renal follow-up is required."

Viva Question: "When would you consider eculizumab therapy?"

Model Answer: "Eculizumab, a monoclonal antibody against C5, is indicated for atypical HUS caused by complement dysregulation. I would consider it in the following scenarios:

1. Confirmed or highly suspected atypical HUS: No diarrhoeal prodrome, atypical age, family history, recurrent episodes, or genetic mutation identified.

2. Severe STEC-HUS with neurological involvement: Although evidence is limited, case reports suggest benefit in severe cases.

3. Recurrent HUS: Any form with recurrent episodes.

I would NOT wait for genetic confirmation before starting eculizumab if aHUS is suspected clinically, as earlier treatment leads to better outcomes. Before administration, I must ensure meningococcal vaccination and start antibiotic prophylaxis due to the 1000-2000 fold increased risk of meningococcal infection. The response is usually rapid, with platelet recovery within days and cessation of haemolysis within 2-4 weeks."

Common Mistakes to Avoid

❌ Giving antibiotics for bloody diarrhoea without considering STEC (increases HUS risk)

❌ Transfusing platelets prophylactically (worsens thrombotic microangiopathy)

❌ Over-aggressive fluid resuscitation in oliguric phase (causes pulmonary oedema)

❌ Missing atypical HUS (failure to send complement studies, genetic testing)

❌ Starting eculizumab without meningococcal prophylaxis (life-threatening meningococcal infection)

❌ Diagnosing ITP or AIHA (failing to check renal function and blood film for schistocytes)

❌ Plasma exchange for STEC-HUS (no proven benefit, delays appropriate care)

❌ Forgetting lifelong follow-up (all HUS patients need ongoing renal surveillance)

Key Facts for Rapid Recall

Triad: MAHA + Thrombocytopenia + AKI

Most common cause in children: E. coli O157:H7

Blood film: Schistocytes (diagnostic)

Coombs Test: Negative (except Sp-HUS)

Contraindicated: Antibiotics, antimotility agents

Dialysis: Required in 50-60%

Atypical HUS: Complement dysregulation, no diarrhoea, eculizumab first-line

Eculizumab target: C5 (blocks C5b-9 formation)

Eculizumab risk: Meningococcal infection (mandatory vaccination + prophylaxis)

Long-term: Lifelong renal follow-up required

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