Paediatric AKI (pRIFLE/KDIGO), Haemolytic Uraemic Syndrome (HUS), and Hyperkalaemia/Dysnatraemia Management

Paediatric Acute Kidney Injury, Haemolytic Uraemic Syndrome, and Electrolyte Emergencies

Acute kidney injury (AKI) and electrolyte disturbances represent frequent, high-stakes medical emergencies encountered in pediatric wards and intensive care units. Whether arising as a secondary consequence of severe dehydration, systemic sepsis, or primary microvascular insults such as Haemolytic Uraemic Syndrome (HUS), rapid recognition and pathophysiologically targeted interventions are paramount to avert permanent renal damage and cardiac arrest.

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Paediatric Acute Kidney Injury: Staging and Classification

Acute kidney injury is characterized by a rapid, potentially reversible decline in glomerular filtration rate (GFR) over hours to days, resulting in the retention of nitrogenous waste products (urea, creatinine) and the dysregulation of extracellular fluid volume and electrolytes.

Pediatric Diagnostic Criteria: KDIGO vs pRIFLE

Interpreting renal function in children requires recognizing developmental physiology. In infants and children, serum creatinine reflects muscle mass; therefore, baseline serum creatinine in a healthy infant is low ($18\text{ to }35\ \mu\text{mol/L}$ or $0.2\text{--}0.4\text{ mg/dL}$). A creatinine of $70\ \mu\text{mol/L}$ ($0.8\text{ mg/dL}$) may be markedly abnormal in a toddler; interpret it against baseline, age, height and the clinical course rather than assigning severity from that value alone.

Under KDIGO (Kidney Disease: Improving Global Outcomes) pediatric guidelines, AKI is defined and staged by either serum creatinine rise or oliguria:

The pRIFLE criteria categorize dysfunction into Risk (eGFR decrease by 25%), Injury (50% decrease), Failure (75% decrease or eGFR $<35\text{ mL/min}/1.73\text{ m}^2$), Loss (persistent failure $>4$ weeks), and End-stage (ESRD $>3$ months).

Etiological Classification

1. Prerenal AKI (Functional Azotemia, ~50–60%): Hemodynamic renal hypoperfusion with structurally intact renal parenchyma. Causes include hypovolemic dehydration (severe gastroenteritis, burns), hemorrhage, septic vasodilatory shock, cardiac pump failure, and severe third-spacing (nephrotic syndrome, liver failure).
2. Intrinsic Renal AKI (~30–40%): Structural parenchymal damage:
   • Acute Tubular Necrosis (ATN): Prolonged, uncorrected prerenal ischemia, or exogenous nephrotoxins (aminoglycosides, vancomycin, amphotericin B, cisplatin, intravenous radiocontrast, NSAIDs).
   • Vascular / Microangiopathic: Haemolytic Uraemic Syndrome (HUS), renal vein thrombosis.
   • Glomerular: Acute post-streptococcal glomerulonephritis, rapidly progressive glomerulonephritis (RPGN), lupus nephritis.
   • Acute Interstitial Nephritis (AIN): Drug hypersensitivity (penicillins, cephalosporins, NSAIDs, proton pump inhibitors).
3. Postrenal AKI (Obstructive, ~5–10%): Mechanical obstruction to urinary outflow. Congenital: Posterior Urethral Valves (PUV) in infant males, bilateral pelviureteric junction obstruction (PUJO), bilateral ureteroceles, neurogenic bladder. Acquired: Urolithiasis, tumor lysis syndrome uric acid/xanthine nephropathy, obstructing pelvic malignancy (rhabdomyosarcoma, neuroblastoma).

Differentiating Prerenal Azotemia from Acute Tubular Necrosis

The Fractional Excretion of Sodium (FeNa) measures the percentage of sodium filtered by the glomeruli that is excreted into the urine, directly evaluating tubular reabsorptive capacity:

$\text{FeNa} (\%) = \frac{U_{\text{Na}} \times P_{\text{Cr}}}{P_{\text{Na}} \times U_{\text{Cr}}} \times 100$

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Haemolytic Uraemic Syndrome (HUS)

Haemolytic Uraemic Syndrome is the most common cause of intrinsic acute kidney injury in infants and young children, predominantly affecting children between 6 months and 5 years of age.

The Classic Diagnostic Triad

1. Microangiopathic Hemolytic Anemia (MAHA):
   • Non-immune, mechanical intravascular red cell shearing across microvascular platelet thrombi.
   • Coombs test (Direct Antiglobulin Test, DAT) is strictly NEGATIVE.
   • Peripheral blood film demonstrates marked red cell fragmentation: schistocytes (helmet cells, triangular cells, burr cells).
   • Marked biochemical hemolysis: Extremely elevated lactate dehydrogenase (LDH), unconjugated hyperbilirubinemia, and absent haptoglobin.
2. Thrombocytopenia:
   • Platelet count typically $<50\text{--}100 \times 10^9\text{/L}$ due to consumptive trapping in endothelial microthrombi.
   • Coagulation Profile: Prothrombin Time (PT), activated Partial Thromboplastin Time (aPTT), and fibrinogen are NORMAL (or near-normal), distinguishing HUS from Disseminated Intravascular Coagulation (DIC).
3. Acute Kidney Injury (AKI):
   • Ranges from microscopic hematuria and subnephrotic proteinuria to oliguric/anuric renal failure with marked azotemia and hypertension.

Typical / Enteropathic HUS (STEC-HUS / D+ HUS)

Accounts for $>90\%$ of pediatric HUS cases.

• Etiology: Infection with Shiga toxin-producing Escherichia coli (STEC), predominantly serotype O157:H7 (as well as O104:H4, O26, O111), and occasionally Shigella dysenteriae type 1.
• Transmission: Ingestion of contaminated, undercooked beef (minced beef), unpasteurized cow's milk or fruit juices, contaminated recreational water, or direct contact with farm livestock in petting zoos.
• Pathophysiological Cascade:
  1. STEC colonizes the large bowel, producing Shiga toxins (Stx1 and Stx2).
  2. The toxin incites acute hemorrhagic colitis (severe abdominal cramping and copious bloody diarrhea).
  3. Shiga toxin enters the mucosal microvasculature, travels bound to neutrophils in the systemic bloodstream, and targets vascular beds expressing the glycolipid receptor Globotriaosylceramide (Gb3).
  4. In the human kidney, glomerular capillary endothelial cells express exceptionally high densities of Gb3 receptors.
  5. Toxin internalizes, cleaves 28S ribosomal RNA, arrests protein synthesis, and triggers endothelial cell apoptosis and detachment.
  6. Subendothelial collagen exposure triggers massive localized platelet activation, localized thrombin generation, and deposition of microvascular platelet-fibrin thrombi.

                 STEC O157:H7 Ingestion
                           |
          Hemorrhagic Colitis (Bloody Diarrhea)
                           |
            Shiga Toxin (Stx2) into Blood
                           |
       Binds to Gb3 Receptors on Renal Glomerular
                    Endothelium
                           |
         Endothelial Cell Swelling and Apoptosis
                           |
    +----------------------+----------------------+
    |                                             |
Platelet Adhesion & Microthrombi           Narrowed Glomerular Capillaries
    |                                             |
    +----------------------+----------------------+
                           |
         +-----------------+-----------------+
         |                 |                 |
   [Consumption]     [Mechanical RBC]   [Glomerular Ischemia]
  Thrombocytopenia      Shearing           Acute Kidney
  (Normal PT/aPTT)    Schistocytes            Injury
                      (Coombs Neg)       (Oliguria/Azotemia)

Interventions to Avoid in Routine STEC Management

Avoid the following routine interventions in suspected or proven STEC enteritis/HUS:

1. Do Not Use Antibiotics to Treat STEC Enteritis:
   • Antibiotics may increase the risk of HUS and are not recommended for STEC infection itself. Treat another suspected serious bacterial infection when clinically indicated, with specialist advice.
2. ANTIMOTILITY AGENTS ARE STRICTLY CONTRAINDICATED:
   • Agents that slow bowel motility (e.g., Loperamide, Diphenoxylate-atropine, opioids) paralyze intestinal peristalsis. This prolongs mucosal contact time with the bacteria and toxins, dramatically increases systemic toxin absorption, and worsens both neurological complications and renal necrosis.
3. ROUTINE PLATELET TRANSFUSIONS ARE STRICTLY CONTRAINDICATED:
   • Platelet transfusions must NEVER be administered purely to correct a low laboratory platelet count, regardless of how low the number is (even $<10 \times 10^9\text{/L}$).
   • Routine platelet transfusion is avoided because of concern that it may worsen microvascular thrombosis; a low count alone is not an indication. Platelets are reserved exclusively for active, life-threatening internal hemorrhage (e.g., intracranial hemorrhage) or prior to invasive emergency surgery.

Supportive Management of STEC-HUS

• Phase-Appropriate Fluid Management: Early isotonic IV hydration during STEC diarrhea may reduce renal injury. Once HUS is established, assess perfusion, urine output and fluid balance; correct hypovolemia, but restrict and tailor fluids in oligo/anuria or overload with nephrology input.
• Red Blood Cell Transfusion: Administer packed red blood cells (washed, leukodepleted) cautiously when hemoglobin plummets below $60\text{ to }70\text{ g/L}$ or in the presence of cardiopulmonary compromise.
• Renal Replacement Therapy (RRT): Approximately 50% of children with STEC-HUS require dialysis during the acute phase. Peritoneal dialysis (PD) is preferred in young children; continuous venovenous hemofiltration (CVVH) or hemodialysis is utilized in older children or hypercatabolic states.
• Long-Term Prognosis: Acute mortality is $<2\%$. While $>85\%$ recover normal renal function initially, up to 30% develop long-term chronic sequelae (hypertension, proteinuria, or chronic kidney disease) years later, requiring yearly nephrology surveillance.

Atypical HUS (aHUS / Complement-Mediated HUS)

Accounts for 5% to 10% of cases; carries an ominous prognosis if untreated, with up to 50% progressing to ESRD.

• Etiology: Uncontrolled, continuous activation of the alternative complement pathway caused by loss-of-function genetic mutations in complement regulatory proteins (Factor H [CFH], Factor I [CFI], Membrane Cofactor Protein [MCP/CD46]), gain-of-function mutations in C3 or Factor B, or acquired neutralizing anti-Factor H autoantibodies.
• Clinical Presentation: Occurs at any age, typically without a preceding diarrheal prodrome; marked by insidious, recurrent episodes of thrombotic microangiopathy.
• Definitive Targeted Therapy: Eculizumab (or Ravulizumab), a humanized monoclonal antibody directed against complement protein C5, halting the formation of the cytolytic membrane attack complex (MAC, C5b-9). Eculizumab has revolutionized aHUS, inducing complete remission in $>80\%$. Mandatory meningococcal vaccination and continuous antibiotic prophylaxis are required before initiating therapy.

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Hyperkalaemia: Emergency Resuscitation Protocol

Hyperkalaemia is the most lethal electrolyte emergency in pediatric nephrology, disrupting myocardial resting membrane potentials and precipitating fatal ventricular arrhythmias.

• Reference Intervals: Children: $3.5\text{--}5.0\text{ mmol/L}$; Neonates: $3.5\text{--}6.0\text{ mmol/L}$.
• Emergency Threshold: Severe hyperkalaemia ($K^+ >6.5\text{ mmol/L}$) requires urgent treatment and ECG monitoring. Give IV calcium for hyperkalaemic ECG conduction abnormalities; initiate potassium shifting and removal promptly.

Sequential Electrocardiographic (ECG) Changes

   Normal ECG            Peaked T-Waves            PR Prolongation / Loss of P        Sine Wave / VF

       R                       R                               R
      / \                     / \                             / \                     /\    /\
     /   \     T             /   \     /\                    /   \                   /  \  /  \
    /     \   / \           /     \   /  \                  /     \                 /    \/    \
   P       \_/   \         P       \_/    \                /       \_---\_         /      \    \
--+---------------+-------+----------------+--------------+---------------+-------+--------+----+---

1. Tall, symmetrical, peaked ("tented") T waves with a narrow base (earliest finding; prominent in precordial leads $V_2\text{--}V_4$).
2. Prolongation of the PR interval, accompanied by progressive flattening and eventual disappearance of the P wave.
3. Widening of the QRS complex, ST segment depression, and intraventricular conduction delays.
4. Sine Wave Pattern: The widened QRS complex fuses directly with the peaked T wave, producing a smooth biphasic undulating waveform—the harbinger of imminent cardiac arrest.
5. Ventricular fibrillation, pulseless electrical activity (PEA), or asystole.

Emergency Pharmacotherapy: RCUK-Based Non-Arrest Protocol and Adjuncts

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Dysnatraemia: Pathophysiology and Correction Guidelines

Sodium disorders reflect imbalances in free body water relative to total body solute. Because the brain cannot rapidly adjust to sudden osmolar shifts, dysnatraemias carry immense neurological vulnerability.

Hyponatraemia (Serum $\text{Na}^+ <135\text{ mmol/L}$)

Classified by extracellular volume status:

• Hypovolaemic Hyponatraemia: Loss of sodium exceeds loss of water (severe gastroenteritis with vomiting/diarrhea, burns, third-spacing, salt-losing congenital adrenal hyperplasia). Urine $\text{Na} <20\text{ mmol/L}$ indicates extra-renal loss; urine $\text{Na} >20\text{ mmol/L}$ indicates renal salt wasting.
• Euvolaemic Hyponatraemia: Normal extracellular fluid volume with excess free water. Prototype: Syndrome of Inappropriate Antidiuretic Hormone (SIADH), triggered by meningitis, encephalitis, severe pneumonia, pain, or nausea. Characterized by inappropriately concentrated urine (urine osmolality $>100\text{ mOsm/kg}$, urine $\text{Na} >30\text{--}40\text{ mmol/L}$) in the presence of plasma hypoosmolality ($<275\text{ mOsm/kg}$). Treatment is strict fluid restriction.
• Hypervolaemic Hyponatraemia: Water retention exceeds sodium retention (congestive heart failure, nephrotic syndrome, acute renal failure). Treatment: Fluid restriction and loop diuretics.

Emergency Management of Acute Symptomatic Hyponatraemic Encephalopathy

When serum sodium plummets rapidly ($<48\text{ hours}$), water moves along an osmotic gradient into astrocytes, generating acute cerebral edema, raised intracranial pressure, projectile vomiting, intractable seizures, tentorial herniation, and coma.

• EMERGENCY INTERVENTION:
  • Infuse Intravenous 3% Hypertonic Saline at $3\text{ to }5\text{ mL/kg}$ infused over 10 to 15 minutes.
  • May repeat once or twice if seizures persist, aiming to acutely elevate serum sodium by $3\text{ to }5\text{ mmol/L}$. This initial partial correction reduces cerebral edema and may control symptoms. Reassess after each bolus and measure sodium at least hourly during acute correction.
• THE CEILING OF CORRECTION (PREVENTING OSMOTIC DEMYELINATION):
  • In chronic hyponatraemia ($>48\text{ hours}$ duration), brain cells adapt by shedding intracellular organic osmolytes (myo-inositol, glutamate, taurine).
  • If chronic hyponatraemia is corrected too rapidly, water is drawn rapidly out of brain tissue, shrinking oligodendrocytes and triggering Osmotic Demyelination Syndrome (ODS) / Central Pontine Myelinolysis (CPM).
  • Clinical features of ODS: Dysarthria, dysphagia, horizontal gaze paralysis, spastic quadriparesis, pseudobulbar palsy, and "locked-in syndrome", typically appearing 2 to 6 days after the rapid correction.
  • MANDATORY CORRECTION RATE LIMIT: The rate of serum sodium correction in chronic hyponatraemia must NEVER exceed 8 to 10 mmol/L in 24 hours (and $\le 18\text{ mmol/L}$ in 48 hours).

Hypernatraemia (Serum $\text{Na}^+ >145\text{ mmol/L}$)

Hypernatraemic dehydration represents a catastrophic hyperosmolar state, typically caused by severe viral gastroenteritis with high insensible losses, central/nephrogenic diabetes insipidus, or improper infant formula preparation (adding excess powder or table salt).

• Clinical Presentation: Doughy, velvety skin turgor, extreme unquenchable thirst, high-pitched shrill cry, hyperreflexia, hyperthermia, and profound lethargy.
• Pathophysiology and Idiogenic Osmoles:
  • Hypertonicity draws water out of neurons, causing acute brain shrinkage and mechanical traction on delicate bridging dural veins, potentially triggering subdural hematomas or sagittal sinus thrombosis.
  • Over 24 to 48 hours, brain astrocytes synthesize protective intracellular "idiogenic osmoles" (amino acids, polyols) to pull water back and normalize intracellular brain volume.
• THE DANGER OF RAPID CORRECTION: FATAL CEREBRAL EDEMA:
  • If hypotonic fluids are administered rapidly, extracellular osmolality plummets faster than idiogenic osmoles can be degraded.
  • Water surges into the brain cells along the osmotic gradient, precipitating malignant cerebral edema, intractable convulsions, brainstem herniation, and death.
• SAFE REHYDRATION PROTOCOL:
  • Assess and treat shock first. Replace the deficit slowly, usually over at least 48 hours; $\text{Na}^+\ge 170\text{ mmol/L}$ requires specialist care and individualized correction over several days.
  • Start with 0.9% sodium chloride with 5% glucose for deficit replacement. Reassess sodium and fluid balance frequently; use hypotonic fluid only after expert review of the response.
  • MANDATORY REDUCTION CEILING: The rate of serum sodium decline must NOT exceed 0.5 mmol/L per hour (or maximum $10\text{ to }12\text{ mmol/L}$ in 24 hours).
  • Monitor serum sodium concentrations every 2 to 4 hours, adjusting infusion rates dynamically to enforce this safe ceiling.