Sodium Disorders: Hyponatremia and Hypernatremia

Quick Answer

Sodium Disorders encompass hyponatremia (serum Na <135 mmol/L) and hypernatremia (>145 mmol/L), which are the most common electrolyte abnormalities in hospitalised and critically ill patients. Sodium is the primary determinant of serum osmolality and extracellular fluid volume. Dysnatraemia in ICU patients is associated with a U-shaped mortality curve, with both extremes conferring significantly increased mortality independent of underlying disease severity.

Key Clinical Features:

• Neurological symptoms predominate (confusion, seizures, coma)
• Severity correlates with rate of change rather than absolute value
• Chronic adaptation occurs via brain osmolyte shifts (idiogenic osmoles)
• Overcorrection of chronic hyponatremia causes Osmotic Demyelination Syndrome (ODS)

Emergency Management:

1. Acute symptomatic hyponatremia: Hypertonic saline (3%) 100-150 mL bolus over 10-20 minutes
2. Target initial correction: 4-6 mEq/L in first 2-4 hours for symptom relief
3. Maximum 24-hour correction: 8-10 mEq for chronic cases, up to 12 mEq for acute (<48h)
4. Hypernatremia: Free water replacement with D5W; correct <10-12 mEq/24h
5. Treat underlying cause: Volume status, SIADH, diabetes insipidus, adrenal insufficiency

ICU Mortality: Severe hyponatremia (<120 mmol/L) 25-50%; Severe hypernatremia (>160 mmol/L) 40-60%

Must-Know Facts:

• Measure plasma and urine osmolality to classify hyponatremia
• SIADH diagnosis is one of exclusion (euvolemic, hypotonic, inappropriately concentrated urine)
• ODS typically presents 2-6 days after overcorrection; MRI changes lag clinical symptoms
• Free water deficit = TBW x [(Serum Na/140) - 1]

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CICM Exam Focus

What Examiners Expect

Second Part Written (SAQ):

Common SAQ stems:

• "A 65yo female on thiazide diuretics presents with Na 112 mmol/L and seizures. Outline your immediate management and describe your approach to sodium correction."
• "A 72yo male in ICU develops Na 158 mmol/L on day 5 of admission. Discuss the aetiology and management of ICU-acquired hypernatremia."
• "Describe the pathophysiology of brain adaptation to hyponatremia and the mechanism of osmotic demyelination syndrome."
• "A patient with SIADH secondary to lung cancer has persistent hyponatremia despite fluid restriction. Discuss management options."

Expected depth:

• Classification of hyponatremia by osmolality and volume status
• Detailed knowledge of SIADH diagnostic criteria and causes
• Hypertonic saline dosing calculations and infusion protocols
• Understanding of brain adaptation (idiogenic osmoles/organic osmolytes)
• ODS risk factors, prevention, and management
• Free water deficit calculations for hypernatremia
• Central vs nephrogenic diabetes insipidus differentiation

Second Part Hot Case:

Typical presentations:

• Post-neurosurgical patient with diabetes insipidus and rising sodium
• Elderly female with thiazide-associated severe hyponatremia and altered consciousness
• SIADH in SCLC patient with resistant hyponatremia
• ICU-acquired hypernatremia in sedated, ventilated patient

Examiners assess:

• Systematic A-E examination approach
• Accurate fluid status assessment (critical for hyponatremia classification)
• Recognition of neurological manifestations of dysnatraemia
• Calculation of correction rates and free water requirements
• Medication review (thiazides, SSRIs, AVP analogues)
• Communication with nephrology and endocrinology teams

Second Part Viva:

Expected discussion areas:

• Sodium homeostasis: ADH, RAAS, thirst mechanism, osmoreceptors
• Osmolality vs tonicity distinction
• Brain adaptation mechanisms and timelines
• Step-by-step approach to hyponatremia diagnosis
• Evidence for tolvaptan use (SALT trials, EVEREST)
• Management of resistant SIADH
• Central vs nephrogenic DI: pathophysiology, diagnosis, treatment
• ODS: risk factors, clinical features, prognosis

Examiner expectations:

• Safe, consultant-level decision-making with correction rate monitoring
• Evidence-based practice (European guidelines, key trials)
• Understanding of pathophysiology linking to management
• Ethical considerations around severe neurological outcomes
• Indigenous health awareness (dehydration in remote communities)

Common Mistakes

• Correcting chronic hyponatremia too rapidly (>8-10 mEq/24h)
• Failing to assess volume status before initiating treatment
• Not measuring urine osmolality and sodium in diagnostic workup
• Overlooking adrenal insufficiency as a cause of hyponatremia
• Using free water in hypernatremia correction without monitoring sodium frequently
• Confusing cerebral salt wasting with SIADH (both hyponatremic, different volume status)
• Not recognising medication causes (thiazides, SSRIs, carbamazepine)
• Failing to identify acute (<48h) vs chronic (>48h) hyponatremia

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Key Points

Must-Know Facts

1. Sodium is the Key Determinant of Tonicity: Serum sodium determines effective osmolality; changes in sodium cause water movement across cell membranes, with critical implications for brain cell volume.

2. European Guidelines Classification (2014): Hyponatremia classified by severity (mild 130-135, moderate 125-129, severe <125 mmol/L), symptom severity, and chronicity (acute <48h, chronic >48h) (PMID: 24569429).

3. SIADH Diagnostic Criteria: Hyponatremia, plasma osmolality <275 mOsm/kg, urine osmolality >100 mOsm/kg, urine sodium >30 mmol/L, euvolemia, normal thyroid/adrenal function, no diuretics in prior 24-48h.

4. Correction Rates: Chronic hyponatremia: target <8-10 mEq/24h; Acute (<48h) hyponatremia: up to 1-2 mEq/h initially safe; Hypernatremia: <10-12 mEq/24h.

5. Brain Adaptation: In chronic hyponatremia, brain cells extrude organic osmolytes (glutamate, taurine, myo-inositol) over 24-48h to maintain cell volume; rapid correction causes cell shrinkage.

6. Osmotic Demyelination Syndrome (ODS): Formerly "central pontine myelinolysis"; occurs 2-6 days post-correction; risk factors include correction >10-12 mEq/24h, chronic alcoholism, malnutrition, hypokalaemia.

7. Hypertonic Saline Dosing: 3% NaCl 100-150 mL bolus (raises Na ~1.5-2 mEq/L); repeat up to 3 times in first hour if severe symptoms persist.

8. Free Water Deficit Calculation: FWD (L) = TBW x [(Serum Na/140) - 1]; TBW = weight (kg) x 0.6 (males) or 0.5 (females); adjust for age.

9. Diabetes Insipidus: Central DI (AVP deficiency) responds to desmopressin; Nephrogenic DI (AVP resistance) requires treatment of cause plus thiazides/amiloride.

10. ICU-Acquired Hypernatremia: 2-5% incidence; often iatrogenic (inadequate free water, hypertonic fluids, loop diuretics); associated with increased mortality.

Memory Aids

SIADH Causes - "SIADH":

• S: Stroke, SAH, head trauma (CNS disorders)
• I: Infections (pneumonia, meningitis, HIV)
• A: Anti-cancer drugs, antidepressants (SSRIs), anticonvulsants (carbamazepine)
• D: Diseases of lung (SCLC, TB, positive pressure ventilation)
• H: Hormonal (hypothyroidism if severe, glucocorticoid deficiency mimics)

ODS Risk Factors - "MALNOURISHED":

• M: Malnutrition
• A: Alcoholism
• L: Liver disease
• N: Na <105 mmol/L at presentation
• O: Overcorrection (>10-12 mEq/24h)
• U: Underlying hypokalaemia
• R: Rapid correction
• I: Intravenous fluid excess
• S: Severe hyponatremia duration
• H: Hypoxia

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Definition & Epidemiology

Definition

Hyponatremia is defined as serum sodium <135 mmol/L, classified as:

• Mild: 130-135 mmol/L
• Moderate: 125-129 mmol/L
• Severe: <125 mmol/L

Hypernatremia is defined as serum sodium >145 mmol/L, classified as:

• Mild: 146-150 mmol/L
• Moderate: 151-159 mmol/L
• Severe: ≥160 mmol/L

Diagnostic Classification of Hyponatremia:

Volume-Based Classification of Hypotonic Hyponatremia:

Epidemiology

International Data:

• Hyponatremia: 15-30% of hospitalised patients; 20-35% of ICU patients (PMID: 18695064)
• Hypernatremia: 2-5% of hospitalised patients; 3-8% of ICU patients
• ICU-acquired hypernatremia: 2-6% incidence (PMID: 19661778)
• Both associated with increased mortality independent of illness severity

Australian/NZ Data (ANZICS APD):

• Dysnatraemia at ICU admission: ~25% of patients
• ICU-acquired hypernatremia: associated with 2-fold increased mortality
• Higher rates of hypernatremic dehydration in remote Indigenous communities
• Thiazide-associated hyponatremia common in elderly Australian women

Risk Factors for Hyponatremia:

• Non-modifiable: Female sex, elderly age, small body size
• Modifiable: Thiazide diuretics, SSRIs, polypharmacy, low-solute diet
• Iatrogenic: Hypotonic IV fluids, postoperative AVP release, bowel preparation

Risk Factors for Hypernatremia:

• Non-modifiable: Extremes of age (infants, elderly), impaired thirst
• Modifiable: Inadequate water intake, enteral feeding without free water
• Iatrogenic: Loop diuretics, hypertonic solutions, osmotic agents

High-Risk Populations:

• Aboriginal and Torres Strait Islander peoples: Higher rates of hypernatremic dehydration, gastroenteritis-related dysnatraemia, remote area water access issues
• Elderly nursing home residents: Impaired thirst, mobility limitations
• Paediatric populations: Higher TBW%, faster osmolar shifts
• Neurosurgical patients: Pituitary surgery/trauma causing DI or SIADH

Outcomes:

• Severe hyponatremia (<120 mmol/L): ICU mortality 25-50%
• Severe hypernatremia (>160 mmol/L): ICU mortality 40-60%
• ODS: Mortality 50-90% historically, ~25% with modern care; significant morbidity in survivors
• Corrected dysnatraemia: 28-day mortality still elevated (hazard ratio 1.2-1.5)

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Applied Basic Sciences

Sodium Homeostasis

Normal Physiology:

Sodium is the principal cation of the extracellular fluid (ECF) and primary determinant of plasma osmolality. Total body sodium is ~60 mEq/kg, with 40% in bone (slowly exchangeable), 55% in ECF, and 5% in ICF.

Osmolality and Tonicity:

• Plasma Osmolality = 2[Na] + [Glucose]/18 + [Urea]/2.8 (in mmol/L: 2[Na] + [Glucose] + [Urea])
• Normal range: 275-295 mOsm/kg
• Tonicity (effective osmolality) = 2[Na] + [Glucose]/18 (excludes urea which freely crosses membranes)
• Tonicity determines water movement between ECF and ICF

Key Regulatory Systems:

1. Antidiuretic Hormone (ADH/AVP):

• Synthesised in hypothalamic supraoptic and paraventricular nuclei
• Stored in posterior pituitary and released in response to:
  • Increased plasma osmolality (sensed by osmoreceptors in circumventricular organs)
  • Decreased effective circulating volume (sensed by carotid/aortic baroreceptors)
  • Nausea, pain, stress, drugs
• Acts on V2 receptors in collecting duct: increases aquaporin-2 insertion, enhancing water reabsorption
• Acts on V1 receptors in vasculature: vasoconstriction
• Normal plasma osmolality threshold for release: ~280-285 mOsm/kg
• Maximum urine concentration: ~1200 mOsm/kg (PMID: 12093388)

2. Renin-Angiotensin-Aldosterone System (RAAS):

• Activated by decreased renal perfusion, sympathetic stimulation, decreased Na delivery to macula densa
• Aldosterone: increases sodium reabsorption in cortical collecting duct via ENaC
• Net effect: sodium and water retention, potassium excretion
• Contributes to sodium homeostasis but primarily regulates volume status

3. Thirst Mechanism:

• Osmotic thirst: triggered by osmolality >290-295 mOsm/kg (higher threshold than ADH)
• Hypovolemic thirst: triggered by angiotensin II, baroreceptor signalling
• Critical defence against hypernatremia; impaired in elderly, sedated, neurologically impaired
• Most potent mechanism for maintaining sodium balance (PMID: 20089548)

4. Natriuretic Peptides:

• ANP (atrial) and BNP (ventricular): released in response to atrial/ventricular stretch
• Promote natriuresis and diuresis via cGMP-mediated effects on collecting duct
• Inhibit RAAS and sympathetic nervous system
• Contribute to sodium excretion in volume-expanded states

Brain Adaptation to Osmolar Changes

Acute Hyponatremia (<48 hours):

1. Decrease in plasma osmolality creates osmotic gradient
2. Water moves into brain cells along osmotic gradient
3. Brain swelling occurs within hours
4. Initial compensation: increased CSF outflow, cellular extrusion of Na+, K+, Cl-
5. Limited adaptive capacity: cerebral oedema and herniation risk high
6. Symptoms correlate with rate of sodium fall and absolute level

Chronic Hyponatremia (>48 hours):

1. Brain cells extrude organic osmolytes (idiogenic osmoles) over 24-72 hours:
   • Amino acids: glutamate, taurine, glycine
   • Polyols: myo-inositol, sorbitol
   • Methylamines: glycerophosphocholine
2. Cell volume normalises despite continued hyponatremia
3. Symptoms often mild despite markedly low sodium
4. Brain is now vulnerable to rapid correction (PMID: 21148217)

Rapid Correction and Osmotic Demyelination Syndrome:

1. Rapid increase in plasma osmolality creates osmotic gradient favouring water exit
2. Brain cells shrink as water leaves before osmolytes can be regenerated (requires 2-7 days)
3. Oligodendrocytes are particularly vulnerable due to:
   • High metabolic demand during remyelination
   • Unique sensitivity to osmotic stress
   • Complement-mediated injury
4. Myelin sheath damage occurs, predominantly in central pons (central pontine myelinolysis) but also extrapontine regions (basal ganglia, thalamus, cortex)
5. Clinical features develop 2-6 days post-correction (PMID: 25656424)

Hypernatremia Adaptation:

1. Increase in plasma osmolality causes water movement out of cells
2. Brain shrinkage activates osmolyte uptake mechanisms
3. Accumulation of organic osmolytes (myo-inositol, taurine, betaine) over 24-48h
4. Rapid correction causes cerebral oedema as water re-enters cells
5. Correction rate <10-12 mEq/24h prevents oedema

Pharmacology

Hypertonic Saline (3% NaCl):

• Composition: 513 mEq/L Na+ (compared to 154 mEq/L in 0.9% NaCl)
• Mechanism: Increases plasma osmolality, draws water from cells, reverses cerebral oedema
• ICU Indication: Acute symptomatic hyponatremia, cerebral oedema from hyponatremia
• Dosing: 100-150 mL IV over 10-20 minutes, repeat x3 if needed; or continuous infusion 0.5-2 mL/kg/h
• Expected effect: Each 100 mL raises Na ~1.5-2 mEq/L in 70 kg adult
• Monitoring: Serum Na every 2-4 hours during correction, target rise 4-6 mEq/L in first 4-6 hours
• Adverse Effects: Overcorrection (ODS risk), phlebitis (central access preferred)
• PBS/TGA: Available in Australia; not PBS-listed (hospital supply)

Tolvaptan (Vaprisol):

• Class: Selective V2 receptor antagonist (vaptan)
• Mechanism: Blocks AVP action on collecting duct, promotes free water excretion (aquaresis)
• ICU Indication: Euvolemic/hypervolemic hyponatremia (SIADH, heart failure) resistant to fluid restriction
• Dosing: 15 mg PO once daily, titrate to 30 mg then 60 mg; hospitalisation required for initiation
• SALT Trials Evidence: Significantly raises serum Na compared to placebo; improves symptom scores (PMID: 17105757)
• Monitoring: Serum Na every 6 hours for first 24-48 hours; must restrict to hospital initiation
• Adverse Effects: Thirst, polyuria, hyperkalaemia, hepatotoxicity (boxed warning), overcorrection
• Contraindications: Inability to sense/respond to thirst, hypovolemic hyponatremia, concurrent hypertonic saline, CYP3A4 inhibitors
• PBS/TGA: TGA-approved; PBS-listed for SIADH with specialist authorisation

Desmopressin (DDAVP):

• Class: Synthetic ADH analogue with V2 > V1 selectivity
• Mechanism: Activates V2 receptors, increases aquaporin-2, concentrates urine
• ICU Indication: Central diabetes insipidus; prevention/treatment of hyponatremia overcorrection
• Dosing: DI: 1-4 mcg IV/SC every 12-24h, or 10-40 mcg intranasal; Overcorrection prevention: 2 mcg IV every 8h with D5W
• Monitoring: Serum sodium, urine output, urine osmolality
• Adverse Effects: Hyponatremia (if excess water intake), headache, flushing
• PBS/TGA: PBS-listed for DI; hospital supply for other indications

Demeclocycline:

• Class: Tetracycline antibiotic
• Mechanism: Induces nephrogenic DI by inhibiting ADH action on collecting duct
• ICU Indication: Chronic SIADH resistant to fluid restriction (rarely used in acute setting)
• Dosing: 600-1200 mg/day in divided doses
• Adverse Effects: Nephrotoxicity (especially if hepatic impairment), photosensitivity, GI upset
• PBS/TGA: Not commonly available in Australia; urea or tolvaptan preferred

Urea:

• Class: Osmotic diuretic
• Mechanism: Increases urine solute load, obligating water excretion
• ICU Indication: SIADH (European guidelines recommend as alternative to vaptans)
• Dosing: 15-60 g/day PO (poorly palatable, often mixed with orange juice)
• Advantages: Cheap, no overcorrection risk, can be given via NGT
• Adverse Effects: Unpalatable taste, nausea, azotaemia
• Evidence: SALSA trial 2021 - non-inferior to vaptans for SIADH (PMID: 34233096)

Pathology

Osmotic Demyelination Syndrome:

• Location: Central pons most common (central pontine myelinolysis, CPM); extrapontine sites in 10-25%
• Histopathology: Symmetric demyelination sparing axons; oligodendrocyte loss; macrophage infiltration
• Mechanism: Osmotic stress on oligodendrocytes during rapid plasma osmolality increase; complement activation
• MRI Findings: T2/FLAIR hyperintensity in central pons ("trident" or "piglet" sign); extrapontine lesions in basal ganglia, thalamus, lateral geniculate body
• Timing: MRI changes may lag clinical symptoms by 1-2 weeks

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Clinical Presentation

ICU Admission Scenarios

Typical Presentations:

Scenario 1: Acute Symptomatic Hyponatremia:

• History: 68yo female presents with generalised seizure; recently started SSRI; increased water intake for "cleanse"
• Examination: Post-ictal, GCS 12/15, no lateralising signs
• Investigations: Na 114 mmol/L, osmolality 248 mOsm/kg
• Severity: Severe, acute, symptomatic - neurological emergency

Scenario 2: ICU-Acquired Hypernatremia:

• History: 72yo male, Day 7 ICU for pneumonia/ARDS; intubated, sedated; enteral feeds via NGT
• Examination: Dry mucous membranes, reduced skin turgor
• Investigations: Na 158 mmol/L (was 138 on admission), urine output 2.5 L/day
• Severity: Moderate-severe, iatrogenic - inadequate free water supplementation

Scenario 3: SIADH in Malignancy:

• History: 65yo male with SCLC, progressive confusion over 5 days
• Examination: Euvolemic (no oedema, normal JVP, no postural drop)
• Investigations: Na 118 mmol/L, plasma osm 250, urine osm 450, urine Na 55 mmol/L
• Severity: Severe, chronic (>48h), symptomatic

Scenario 4: Central Diabetes Insipidus Post-Neurosurgery:

• History: 45yo female, Day 2 post-transsphenoidal pituitary adenoma resection
• Examination: High urine output (500 mL/hr), thirst, dry mucous membranes
• Investigations: Na 152 mmol/L, urine osm 80 mOsm/kg, urine specific gravity 1.002
• Severity: Moderate, acute, post-surgical

Symptoms & Signs

Hyponatremia - Symptom Severity Correlates with Rate of Change:

Neurological Signs:

• Mild: Headache, nausea, difficulty concentrating
• Moderate: Lethargy, confusion, vomiting, gait disturbance
• Severe: Seizures (generalised tonic-clonic), decreased consciousness, coma
• Life-threatening: Signs of cerebral herniation (pupillary asymmetry, posturing, respiratory arrest)

Hypernatremia - Signs and Symptoms:

Physical Examination - Volume Status Assessment (Critical for Hyponatremia):

Hypovolemic Signs:

• Orthostatic hypotension (systolic drop >20 mmHg)
• Tachycardia, reduced JVP
• Dry mucous membranes, reduced skin turgor
• Oliguria

Euvolemic Signs (SIADH):

• Normal blood pressure, no orthostatic changes
• Normal JVP, no peripheral oedema
• Moist mucous membranes
• May have mild weight gain (typically <2 kg)

Hypervolemic Signs:

• Elevated JVP, peripheral oedema
• Pulmonary oedema (crackles, orthopnoea)
• Ascites (cirrhosis), anasarca
• Third heart sound (heart failure)

Severity Scoring

Symptom-Based Severity (European Guidelines 2014):

Differential Diagnosis

Hypotonic Hyponatremia Differential:

1. SIADH: Euvolemic, urine osm >100, urine Na >30, no diuretics, normal thyroid/adrenal
2. Cerebral Salt Wasting (CSW): Hypovolemic, similar urine findings to SIADH, occurs after CNS insult; responds to saline (not fluid restriction)
3. Adrenal Insufficiency: May mimic SIADH; check cortisol, give empiric steroids if suspected
4. Hypothyroidism: Severe hypothyroidism can cause hyponatremia; check TSH
5. Diuretic-induced: History of thiazide/loop diuretics; urine Na variable depending on timing
6. Beer Potomania: Low solute diet limits urine dilution capacity; urine osm <100

Hypernatremia Differential:

1. Water Loss - Renal: Diabetes insipidus (central or nephrogenic), osmotic diuresis
2. Water Loss - Extrarenal: Insensible losses, fever, respiratory losses, burns, diarrhoea
3. Sodium Gain: Hypertonic saline, sodium bicarbonate, hyperaldosteronism (rare)
4. Inadequate Intake: Impaired thirst, altered mental status, mechanical ventilation

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Investigations

Laboratory Investigations

Essential First-Line Tests:

Serum Tests:

• Serum Sodium: Confirm and quantify dysnatraemia
• Plasma Osmolality: Measured (not calculated) - confirms true hypotonic hyponatremia vs pseudo/translocational
  • <275 mOsm/kg: Hypotonic (true hyponatremia)
  • 275-295 mOsm/kg: Isotonic (pseudohyponatremia)

  • 295 mOsm/kg: Hypertonic (translocational)

• Glucose: Correct Na for hyperglycaemia: Corrected Na = Measured Na + 2.4 x [(Glucose-5.5)/5.5] mmol/L
• Urea/Creatinine: Assess renal function, volume status clues
• Lipids, Total Protein: If pseudohyponatremia suspected (grossly elevated)

Urine Tests:

• Urine Osmolality:
  • <100 mOsm/kg: Appropriate dilution (psychogenic polydipsia, beer potomania)

  • 100 mOsm/kg: Impaired dilution (SIADH, volume depletion, etc.)

  • <300 mOsm/kg in hypernatremia: Diabetes insipidus

  • 600 mOsm/kg in hypernatremia: Appropriate concentration (extrarenal water loss)

• Urine Sodium:
  • <30 mmol/L in hyponatremia: Volume depletion, effective hypovolemia (heart failure, cirrhosis)

  • 30 mmol/L in hyponatremia: SIADH, adrenal insufficiency, diuretics, salt-wasting nephropathy

• Urine Specific Gravity: Quick bedside estimate of concentration

Additional Tests:

• Thyroid Function (TSH, fT4): Exclude hypothyroidism
• Cortisol (8am or random + ACTH stimulation test): Exclude adrenal insufficiency
• Serum Potassium: Concurrent hypokalaemia increases ODS risk; may indicate mineralocorticoid deficiency
• Liver Function Tests: Cirrhosis workup if hypervolemic hyponatremia

Water Deprivation Test (for suspected DI):

• Fast patient from fluids, measure urine osmolality hourly
• Central DI: Urine osm remains <300 despite dehydration; rises >50% after desmopressin
• Nephrogenic DI: Urine osm remains <300; minimal response (<50%) to desmopressin
• Primary polydipsia: Urine osm >600 with dehydration

Diagnostic Algorithm - Hyponatremia

Hyponatremia (Na <135 mmol/L)
         |
         v
Measure Plasma Osmolality
         |
    _____|_____
   |           |
  <275       275-295 or >295
(Hypotonic)  (Isotonic/Hypertonic)
   |           |
   v           v
True         Pseudohyponatremia (lipids, protein)
Hyponatremia  or Translocational (glucose, mannitol)
   |
   v
Assess Volume Status
(Clinical exam + Urine Na)
         |
    _____|_____|_____
   |         |         |
 Hypo-    Euvolemic   Hyper-
volemic              volemic
   |         |         |
   v         v         v
Urine Na   Urine Na   Urine Na
<30: GI,   >30:       <30: CHF,
Skin,      SIADH,     Cirrhosis,
Burns      Adrenal,   Nephrotic
>30:       Hypothy-   >30: CKD
Diuretics, roid,
CSW,       Reset
Adrenal    Osmostat

Imaging

Brain CT/MRI:

• Indication: Altered consciousness, seizures, focal neurology, suspected ODS
• Acute cerebral oedema: CT shows diffuse swelling, sulcal effacement
• ODS on MRI:
  • T2/FLAIR hyperintensity in central pons ("trident" sign)
  • May involve extrapontine areas (basal ganglia, thalamus, cerebral cortex)
  • DWI restriction in acute phase
  • Changes may lag symptoms by 1-2 weeks

Chest Imaging:

• CXR: Rule out lung pathology causing SIADH (SCLC, pneumonia, TB)
• CT chest: Staging for malignancy if suspected

Physiological Monitoring

Standard ICU Monitoring:

• Continuous ECG, SpO2, respiratory rate
• Hourly urine output (catheter if not already in situ)
• Strict fluid balance charting

Sodium Monitoring During Correction:

• Acute symptomatic hyponatremia: Every 2-4 hours
• Chronic hyponatremia with treatment: Every 4-6 hours
• Hypernatremia: Every 4-6 hours
• Target rates must be calculated and monitored

ICP Monitoring:

• Consider if GCS severely depressed, signs of herniation
• Rarely required if early treatment with hypertonic saline

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ICU Management

Initial Resuscitation (First Hour)

A - Airway:

• Assess airway patency
• Protect airway if GCS ≤8 or active seizures
• Intubation may be required for severe neurological impairment

B - Breathing:

• Supplemental oxygen if hypoxic
• Assess for neurogenic pulmonary oedema (rare in acute hyponatremia)
• Post-intubation ventilation if required

C - Circulation:

• IV access (preferably central if hypertonic saline to be infused)
• Assess volume status - critical for determining cause and treatment
• Hypovolemic: Initial crystalloid resuscitation (0.9% NaCl)
• Hypervolemic: Diuresis, fluid restriction, consider dialysis

D - Disability:

• GCS assessment, pupil examination
• Seizure management:
  • Benzodiazepines (midazolam 5 mg IV, lorazepam 4 mg IV)
  • Treat underlying hyponatremia with hypertonic saline (most effective anticonvulsant)
  • Standard antiepileptic drugs have limited efficacy until sodium corrected

E - Exposure:

• Temperature - fever increases free water losses
• Skin turgor, mucous membranes - volume status clues
• Review medication chart for causative drugs

Acute Symptomatic Hyponatremia - Emergency Protocol

Indications for Emergency Hypertonic Saline:

• Seizures
• GCS ≤8 or rapidly declining consciousness
• Signs of cerebral herniation
• Severe respiratory distress from cerebral oedema

Hypertonic Saline Protocol (European Guidelines 2014 - PMID: 24569429):

Step 1 - Initial Bolus:

• 3% NaCl 100-150 mL IV over 10-20 minutes
• Expected Na rise: 1.5-2 mEq/L

Step 2 - Reassess at 20 minutes:

• Check serum sodium (point-of-care testing if available)
• Repeat neurological assessment

Step 3 - Repeat if symptoms persist:

• If symptoms persist: repeat 100-150 mL bolus
• May give up to 3 boluses in first hour
• Target: 4-6 mEq/L rise in first 2-4 hours (symptom relief)

Step 4 - Transition to continuous infusion or stop:

• Once symptoms improve, stop hypertonic saline
• If ongoing treatment needed: 0.5-1 mL/kg/h with frequent monitoring
• 24-hour target: <8-10 mEq/L for chronic hyponatremia

Alternative Calculation Method (Adrogue-Madias Formula):

Change in Na per L = (Infusate Na - Serum Na) / (TBW + 1)

For 3% NaCl (513 mEq/L) in a 70 kg male (TBW = 42L): Change in Na per L = (513 - 115) / (42 + 1) = 9.3 mEq/L per litre infused

Monitoring During Correction:

• Serum sodium every 2-4 hours
• Calculate ongoing correction rate
• Urinary output (high output may indicate self-correction risk)
• Neurological status

SIADH Management

First-Line: Fluid Restriction:

• Restrict to 500-1000 mL/day (less than daily urine output)
• Difficult to achieve in ICU (drug infusions, nutrition)
• Response expected in 24-48 hours
• Success rate 40-60% as monotherapy

Second-Line Options:

Salt Tablets + Loop Diuretic:

• Oral sodium chloride tablets 3-6 g/day (18-36 mmol Na)
• Furosemide 20-40 mg increases free water excretion
• Mechanism: Increases solute load, obligates water loss

Urea:

• Dose: 15-60 g/day PO or via NGT
• Mechanism: Osmotic diuresis, increases urine solute concentration
• Evidence: SALSA trial (PMID: 34233096) - non-inferior to tolvaptan, safer, cheaper
• Advantages: No overcorrection risk, no hepatotoxicity
• Disadvantages: Poor palatability, causes azotaemia

Tolvaptan:

• Indication: SIADH or hypervolemic hyponatremia refractory to other measures
• Dose: 15 mg PO once daily; titrate to 30 mg then 60 mg
• SALT-1 and SALT-2 trials (PMID: 17105757): Significant Na increase vs placebo
• MUST be initiated in hospital with frequent Na monitoring
• Contraindicated: Hypovolemia, inability to sense thirst, CYP3A4 inhibitors
• Boxed Warning: Hepatotoxicity - do not use >30 days

Third-Line:

• Demeclocycline: 600-1200 mg/day (induces nephrogenic DI); hepatotoxicity risk
• Consider if vaptans contraindicated or unavailable

Hypernatremia Management

Principle: Replace free water deficit while treating underlying cause

Free Water Deficit Calculation:

FWD (L) = TBW x [(Serum Na / 140) - 1]

Where TBW = Weight (kg) x factor (0.6 for young males, 0.5 for young females, 0.5 for elderly males, 0.45 for elderly females)

Example: 70 kg elderly male with Na 160 mmol/L

• TBW = 70 x 0.5 = 35 L
• FWD = 35 x [(160/140) - 1] = 35 x 0.143 = 5 L

Correction Strategy:

• Target rate: <10-12 mEq/L per 24 hours to avoid cerebral oedema
• Replacement time: FWD replaced over 48-72 hours
• Plus ongoing losses: Add insensible losses (500-1000 mL/day) + measured losses

Fluid Choices:

• D5W (5% dextrose): Pure free water when metabolised (0 mEq/L Na)
• 0.45% NaCl: Half-normal saline (77 mEq/L Na); less hypotonic
• Enteral free water: Via NGT; safest method if GI tract functional

Route:

• Enteral preferred if gut functioning (water flushes, dilute feeds)
• IV D5W if enteral not possible
• Central access preferred for D5W to avoid phlebitis

Diabetes Insipidus Management:

Central DI (AVP deficiency):

• Desmopressin (DDAVP) 1-4 mcg IV/SC every 8-12 hours
• Alternatively: 10-40 mcg intranasal every 12-24 hours
• Titrate to urine output 2-3 L/day
• Monitor for hyponatremia with excess dosing

Nephrogenic DI (AVP resistance):

• Treat underlying cause (lithium, hypercalcaemia, hypokalaemia)
• Thiazide diuretics: Paradoxically reduce urine output (mechanism: volume contraction, enhanced proximal reabsorption)
• Amiloride: Helpful in lithium-induced DI (blocks lithium entry via ENaC)
• Low-sodium, low-protein diet: Reduces obligatory solute excretion
• NSAIDs: Reduce prostaglandin-mediated antagonism of ADH (limited use due to renal risks)

Preventing Overcorrection

High-Risk Situations for Overcorrection:

• Hypovolemic hyponatremia with volume replacement (ADH suppression)
• Adrenal insufficiency with cortisol replacement
• SIADH with resolution of trigger (e.g., drug cessation)
• Post-desmopressin tolvaptan use
• Excessive hypertonic saline dosing

Monitoring:

• Frequent serum sodium (every 2-4 hours during active treatment)
• Urine output: High output (>100 mL/h) may herald rapid correction

Intervention if Overcorrection Occurs:

Re-Lowering Protocol (if correction exceeds target):

1. Stop all saline infusions
2. Give D5W 3-5 mL/kg/h IV
3. Consider desmopressin 2 mcg IV every 8 hours (induces water retention)
4. Target: Bring sodium back to acceptable range
5. Aim for net correction <8-10 mEq/24h from presentation sodium

Evidence for Re-Lowering:

• Retrospective studies suggest re-lowering may prevent or reverse ODS if initiated early (PMID: 20089804)
• No RCTs; expert consensus supports intervention

Osmotic Demyelination Syndrome

Risk Factors:

• Correction rate >10-12 mEq/24h (most important)
• Initial sodium <105 mmol/L
• Hypokalaemia (concurrent correction worsens outcome)
• Alcoholism, malnutrition
• Liver disease, burns
• Hypoxia

Clinical Presentation (develops 2-6 days post-correction):

• Classical CPM: Quadriparesis, dysarthria, dysphagia, "locked-in syndrome"
• Extrapontine: Movement disorders (parkinsonism, dystonia), psychiatric symptoms, cognitive impairment
• Biphasic course: Initial improvement after sodium correction, then neurological deterioration

Diagnosis:

• Clinical suspicion based on timeline
• MRI: T2/FLAIR hyperintensity in pons/extrapontine areas (may lag 1-2 weeks)
• DWI restriction in acute phase

Management:

• Prevention is key - maintain strict correction rates
• Supportive care in ICU
• Avoid further neurological insults (hypotension, hypoxia)
• Consider re-lowering sodium if diagnosed within 24 hours of overcorrection
• Rehabilitation for survivors
• Prognosis: Variable; some patients recover well, others have permanent disability

Australian-Specific Protocols

ANZICS-CORE Considerations:

• Fluid management bundles in ICU include sodium monitoring
• Point-of-care electrolyte testing recommended for critically ill patients
• Intergrade with state retrieval services for transfers

Therapeutic Guidelines Australia:

• Hypertonic saline available in all tertiary ICUs
• Tolvaptan PBS-listed with specialist authority for SIADH
• Urea compounded by hospital pharmacy (first-line in many Australian centres)

Remote and Rural Considerations:

• Hypernatremic dehydration common in remote Indigenous communities
• Delayed presentation due to geographic barriers
• RFDS retrieval for severe cases requiring ICU
• Telemedicine consultation with tertiary nephrology available

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Monitoring & Complications

ICU-Specific Monitoring

Sodium Monitoring Schedule:

Urine Output Monitoring:

• High urine output (>100-150 mL/h) during treatment of hypovolemic hyponatremia may signal impending rapid self-correction
• Low output in hypernatremia suggests ongoing dehydration or inadequate replacement

Neurological Monitoring:

• GCS every 1-2 hours during acute treatment
• Pupillary examination
• Early neurology consultation if deterioration

Complications

Early Complications (First 24-48 hours):

Complication 1: Seizures

• Incidence: 10-30% in severe acute hyponatremia
• Risk factors: Na <120 mmol/L, acute onset, prior seizure history
• Prevention: Early hypertonic saline in symptomatic patients
• Management: Benzodiazepines + urgent sodium correction

Complication 2: Cerebral Oedema/Herniation

• Incidence: 1-5% in severe acute hyponatremia
• Risk factors: Acute onset, severe level, young women, postoperative
• Presentation: Cushing response, pupillary changes, posturing
• Management: Emergency hypertonic saline, consider mannitol, neurosurgery consultation

Complication 3: Overcorrection

• Incidence: 10-25% of treated patients exceed target rate
• Risk factors: Hypovolemic cause, adrenal insufficiency, drug cessation
• Prevention: Frequent monitoring, proactive D5W/DDAVP if approaching target
• Management: Re-lowering protocol (D5W + DDAVP)

Late Complications (Beyond 48 hours):

Complication 4: Osmotic Demyelination Syndrome

• Incidence: 1-2% of corrected severe hyponatremia (higher with rapid correction)
• Risk factors: Correction >10-12 mEq/24h, Na <105, alcoholism, hypokalaemia
• Presentation: Quadriparesis, dysarthria, locked-in syndrome (2-6 days post-correction)
• Management: Supportive; consider re-lowering if early recognition
• Prognosis: 25-50% mortality; survivors often have significant disability

Complication 5: Cerebral Oedema from Rapid Hypernatremia Correction

• Incidence: <1% with appropriate correction rates
• Risk factors: Correction >12 mEq/24h in chronic hypernatremia
• Presentation: Headache, vomiting, decreased consciousness, seizures
• Prevention: Limit correction rate <10-12 mEq/24h
• Management: Slow or stop free water, hypertonic saline if severe

ICU-Acquired Complications:

• Iatrogenic hypernatremia from inadequate free water (2-6% of ICU patients)
• Line complications from hypertonic saline infusion (phlebitis, extravasation)
• Drug interactions with treatment agents

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Prognosis & Outcome Measures

Mortality

Short-Term Outcomes:

• Severe hyponatremia (<120 mmol/L) ICU mortality: 25-50%
• Severe hypernatremia (>160 mmol/L) ICU mortality: 40-60%
• ICU-acquired hypernatremia: 2-fold increased mortality (PMID: 19661778)
• ODS: Mortality 25-50% with modern care

Long-Term Outcomes:

• Corrected dysnatraemia: 90-day mortality remains elevated (HR 1.2-1.5)
• ODS survivors: 25-50% functional recovery, variable disability
• Recurrent SIADH: Common if underlying cause not treated

Morbidity

Functional Recovery:

• Acute symptomatic hyponatremia: Good recovery if treated promptly
• ODS survivors: Often require prolonged rehabilitation
• Persistent cognitive deficits in some patients with severe hyponatremia

ICU Survivorship:

• Falls and fractures in chronic mild hyponatremia (osteoporosis risk)
• Gait disturbance
• Cognitive impairment (subtle in chronic hyponatremia)

Prognostic Factors

Good Prognostic Factors:

• Acute (<48h) hyponatremia (brain not fully adapted - responds well to treatment)
• Early recognition and treatment
• Correction rate maintained within targets
• Reversible underlying cause

Poor Prognostic Factors:

• Chronic severe hyponatremia (<105 mmol/L)
• Concurrent hypokalaemia
• Alcoholism, malnutrition, liver disease
• Delayed treatment
• Overcorrection with ODS development

Australian/NZ Outcome Data

ANZICS APD Data:

• Dysnatraemia at ICU admission independently associated with increased mortality
• U-shaped relationship between sodium and mortality
• ICU-acquired hypernatremia particularly associated with poor outcomes

Indigenous Health Outcomes:

• Higher rates of severe dehydration and hypernatremia in remote communities
• Gastroenteritis-related dysnatraemia common in paediatric Indigenous populations
• Access barriers to tertiary care impact outcomes
• Cultural considerations in end-of-life discussions

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SAQ Practice

SAQ 1: Acute Symptomatic Hyponatremia

Time Allocation: 10 minutes
Total Marks: 15

Stem: A 68-year-old female is brought to ICU from the Emergency Department after having a witnessed generalised tonic-clonic seizure. She has a history of depression treated with sertraline (SSRI) commenced 3 weeks ago, and hypertension treated with hydrochlorothiazide. Her family reports she has been drinking excessive amounts of water (5-6 L/day) as part of a "detox" regimen.

Observations on arrival:

• HR: 88 bpm
• BP: 138/78 mmHg
• RR: 18/min
• SpO2: 96% on room air
• Temperature: 36.8C
• GCS: 13/15 (E3 V4 M6) - post-ictal drowsiness

Investigations:

• Serum Sodium: 112 mmol/L
• Serum Potassium: 3.2 mmol/L
• Serum Creatinine: 68 umol/L
• Plasma Osmolality: 238 mOsm/kg
• Urine Osmolality: 580 mOsm/kg
• Urine Sodium: 45 mmol/L
• Glucose: 5.8 mmol/L

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Question 1.1 (5 marks)

Describe your immediate management of this patient in the first hour.

Question 1.2 (5 marks)

Interpret the laboratory investigations and explain the likely aetiology of her hyponatremia.

Question 1.3 (5 marks)

Outline the principles of ongoing sodium correction and the risks if correction is too rapid.

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Model Answer

Question 1.1 (5 marks)

Immediate Management - First Hour:

Airway and Breathing (1 mark):

• Ensure airway protection (recovery position if ongoing post-ictal state)
• Supplemental oxygen if SpO2 <94%
• Consider intubation if GCS deteriorates to ≤8 or recurrent seizures

Seizure Management (1 mark):

• Benzodiazepine if ongoing seizure activity (midazolam 5 mg IV or lorazepam 4 mg IV)
• Note: Sodium correction is the definitive anticonvulsant for hyponatremic seizures

Emergency Sodium Correction (2 marks):

• 3% Hypertonic Saline 100-150 mL IV over 10-20 minutes
• Expected sodium rise: 1.5-2 mEq/L
• Repeat bolus up to 3 times if seizures persist or GCS does not improve
• Target: Sodium rise of 4-6 mEq/L in first 2-4 hours

Monitoring and Access (1 mark):

• Central venous access (preferred for hypertonic saline)
• Arterial line for frequent blood sampling
• Urinary catheter for output monitoring
• Serum sodium every 2 hours during active treatment

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Question 1.2 (5 marks)

Investigation Interpretation (3 marks):

• Plasma osmolality 238 mOsm/kg: Confirms true hypotonic hyponatremia (not pseudo- or translocational)
• Urine osmolality 580 mOsm/kg: Inappropriately concentrated urine (should be dilute <100 if normal response to hyponatremia)
• Urine sodium 45 mmol/L: Elevated (>30 mmol/L) - indicates renal sodium loss or SIADH
• Potassium 3.2 mmol/L: Hypokalaemia increases ODS risk - must correct concurrently

Aetiology (2 marks):

This patient has euvolemic hypotonic hyponatremia with inappropriately concentrated urine and elevated urine sodium, consistent with SIADH.

Contributing factors:

1. SSRI (sertraline): Well-documented cause of SIADH (PMID: 8637458)
2. Thiazide diuretic: Impairs urinary dilution, enhances ADH effect
3. Excessive water intake (polydipsia): Overwhelms limited excretion capacity in setting of SIADH

The thiazide contributes by inhibiting NaCl reabsorption in diluting segment, while sertraline causes SIADH. Combined with excessive water intake, severe hyponatremia has developed rapidly.

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Question 1.3 (5 marks)

Principles of Ongoing Correction (3 marks):

1. Determine chronicity: Given 3-week SSRI use and ongoing polydipsia, likely chronic (>48h) - MUST limit correction rate
2. Target 24-hour correction: <8-10 mEq/L to prevent osmotic demyelination syndrome
3. Frequent monitoring: Serum sodium every 2-4 hours during active treatment
4. Concurrent hypokalaemia correction: Correct K+ to >4.0 mmol/L (each mEq/L K+ given will raise Na by small amount)
5. Remove contributing drugs: Cease thiazide; consider SSRI cessation/switch after acute phase
6. Fluid restriction: Once acute symptoms controlled, restrict to 500-1000 mL/day

Risks of Rapid Correction (2 marks):

Osmotic Demyelination Syndrome (ODS):

• Occurs when correction exceeds 10-12 mEq/24h in chronic hyponatremia
• Pathophysiology: Rapid plasma osmolality rise causes brain cell shrinkage before organic osmolytes can be regenerated
• Oligodendrocytes are particularly vulnerable
• Clinical features develop 2-6 days post-correction: Quadriparesis, dysarthria, dysphagia, "locked-in syndrome"
• MRI: T2/FLAIR hyperintensity in central pons and/or extrapontine areas
• This patient is HIGH RISK (severe level 112, concurrent hypokalaemia)

If overcorrection occurs:

• Stop hypertonic saline immediately
• Administer D5W 3-5 mL/kg/h
• Consider desmopressin 2 mcg IV every 8 hours
• Re-lower sodium to target trajectory

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SAQ 2: ICU-Acquired Hypernatremia

Time Allocation: 10 minutes
Total Marks: 15

Stem: A 72-year-old male was admitted to ICU 7 days ago with community-acquired pneumonia requiring mechanical ventilation. He has been receiving propofol and fentanyl sedation, enteral nutrition via nasogastric tube at 80 mL/h, and maintenance fluids of 0.9% NaCl at 50 mL/h. He remains intubated on weaning FiO2 0.35, PEEP 5.

Past Medical History: Type 2 diabetes, hypertension, benign prostatic hyperplasia

Observations today:

• HR: 98 bpm
• BP: 128/74 mmHg
• RR: 18/min (on SIMV-PS)
• SpO2: 96%
• Temperature: 37.8C
• Urine output over 24 h: 3.2 L

Investigations:

• Day 1: Serum Sodium 138 mmol/L
• Day 7 (today): Serum Sodium 158 mmol/L
• Serum Potassium: 4.1 mmol/L
• Serum Creatinine: 112 umol/L (baseline 82)
• Urine Osmolality: 650 mOsm/kg
• Urine Sodium: 18 mmol/L

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Question 2.1 (5 marks)

Explain the aetiology of this patient's hypernatremia, including contributing factors.

Question 2.2 (5 marks)

Calculate the free water deficit and outline your management plan.

Question 2.3 (5 marks)

Discuss the prevention of ICU-acquired hypernatremia and the prognosis of this condition.

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Model Answer

Question 2.1 (5 marks)

Aetiology - ICU-Acquired Hypernatremia (3 marks):

This patient has developed ICU-acquired hypernatremia due to inadequate free water intake relative to losses.

Interpretation of investigations:

• Urine osmolality 650 mOsm/kg: Appropriately concentrated urine (>300) - kidneys are responding normally to hypernatremia
• Urine sodium 18 mmol/L: Low - indicating sodium conservation and volume depletion
• This is extrarenal water loss (not diabetes insipidus) with intact renal concentrating ability

Contributing Factors (2 marks):

1. Inadequate free water intake:

   • Enteral nutrition at 80 mL/h = 1920 mL/day, but standard formulas are ~80% water = ~1540 mL free water
   • 0.9% NaCl provides NO free water
   • No additional water flushes ordered

2. Increased water losses:

   • Fever (37.8C) - increases insensible losses by ~500 mL/day per degree above 37C
   • Mechanical ventilation with dry gas (if not adequately humidified)
   • Polyuria (3.2 L/day) - likely osmotic diuresis from enteral feeds/glucose

3. Impaired thirst mechanism:

   • Sedated patient cannot express thirst or drink
   • Elderly baseline with possible impaired thirst sensation

4. Iatrogenic factors:

   • 0.9% NaCl maintenance (isotonic - no free water contribution)
   • Loop diuretics if given (not mentioned but common in ICU)

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Question 2.2 (5 marks)

Free Water Deficit Calculation (2 marks):

FWD = TBW x [(Serum Na / 140) - 1]

TBW = Weight x factor (using 0.5 for elderly male) Assuming 70 kg patient: TBW = 70 x 0.5 = 35 L

FWD = 35 x [(158/140) - 1] FWD = 35 x [1.129 - 1] FWD = 35 x 0.129 FWD = 4.5 L

Management Plan (3 marks):

1. Replace Free Water Deficit:

• Total deficit of 4.5 L to be replaced over 48-72 hours
• Target correction rate: <10-12 mEq/24 hours to avoid cerebral oedema
• Aim for Na 145-148 by 24 hours, then normalise by 48-72 hours

2. Replace Ongoing Losses:

• Insensible losses: ~1000 mL/day (increased with fever)
• Urine free water losses: Calculate from urine volume and osmolality
• Add to replacement calculations

3. Route of Replacement:

• Enteral preferred: Water flushes via NGT (e.g., 200 mL every 4 hours)
• If IV required: D5W (pure free water once dextrose metabolised)
• Avoid 0.45% NaCl as sole replacement (still contains sodium)

4. Adjust Current Fluids:

• Change 0.9% NaCl to D5W or increase enteral water flushes
• Add 200-300 mL free water flushes with each NGT feed

5. Monitoring:

• Serum sodium every 4-6 hours
• Strict fluid balance
• Urine output hourly

6. Treat Contributing Factors:

• Treat fever (paracetamol, source control)
• Ensure adequate humidification of ventilator circuit
• Review medication chart for diuretics

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Question 2.3 (5 marks)

Prevention of ICU-Acquired Hypernatremia (3 marks):

1. Routine free water prescription:

   • All intubated/sedated patients should have prescribed free water
   • NGT flushes (150-200 mL every 4-6 hours)
   • Consider D5W if IV only access

2. Daily sodium and fluid balance review:

   • Part of structured ICU daily round
   • Track sodium trends - intervene when rising

3. Appropriate IV fluid selection:

   • Avoid excessive 0.9% NaCl in stable patients
   • Balanced crystalloids (Hartmann's, Plasma-Lyte) contain less sodium but still limited free water

4. Nutrition protocol:

   • Standard enteral feeds provide ~80% water
   • Additional water flushes standard practice (200-400 mL/day minimum)

5. Temperature control:

   • Treat fever promptly
   • Adequate humidification of inspired gases

6. High-risk patient identification:

   • Elderly, sedated, impaired thirst, renal concentrating defects
   • Proactive monitoring and early intervention

Prognosis (2 marks):

• ICU-acquired hypernatremia is associated with increased mortality (approximately 2-fold increase) (PMID: 19661778)
• Independent predictor of mortality even after adjusting for illness severity
• Mechanisms:
  • Marker of overall ICU care quality
  • May indicate underlying volume depletion and hypoperfusion
  • Direct CNS effects in severe cases
• Reversible if identified early: This patient's hypernatremia should be correctable with appropriate free water replacement
• Prevention is key: Once ICU-acquired hypernatremia develops, outcomes are worse regardless of subsequent correction

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Viva Scenarios

Viva Scenario 1: Pathophysiology and Acute Management

Stem: A 45-year-old female presents to the Emergency Department with confusion and a witnessed tonic-clonic seizure. She was recently started on carbamazepine for newly diagnosed epilepsy. Serum sodium is 116 mmol/L.

Duration: 12 minutes (2 min reading + 10 min discussion)

---

Opening Question: "What is your immediate approach to this patient?"

Candidate Response:

"This patient has acute symptomatic hyponatremia with seizures - this is a medical emergency requiring immediate treatment.

Immediate priorities:

1. Airway protection - assess GCS, consider recovery position or intubation if GCS ≤8
2. Seizure management - benzodiazepine if ongoing (midazolam 5 mg IV)
3. Urgent sodium correction - this is the definitive treatment for hyponatremic seizures

Emergency Treatment Protocol:

• 3% Hypertonic Saline 100-150 mL IV over 10-20 minutes
• This should raise sodium by approximately 1.5-2 mEq/L
• Repeat up to 3 times in first hour if symptoms persist
• Target: 4-6 mEq/L rise in first 2-4 hours

Monitoring:

• ICU admission for close monitoring
• Serum sodium every 2 hours during active treatment
• Neurological observations every 30 minutes initially

Determining chronicity:

• Carbamazepine was 'recently started' - need to establish exact timing
• If carbamazepine started <48 hours ago AND this is an acute presentation, higher correction rates may be acceptable
• If >48 hours, brain adaptation has occurred and slower correction required"

---

Examiner Follow-up 1: "Explain the pathophysiology of brain adaptation to hyponatremia."

Candidate Response:

"The brain adapts to hyponatremia through a process involving organic osmolyte extrusion:

Acute Phase (0-24 hours):

• Decreased plasma osmolality creates an osmotic gradient
• Water moves into brain cells causing cerebral oedema
• Initial compensation: brain extrudes electrolytes (Na+, K+, Cl-) within hours
• Limited capacity - hence risk of herniation in acute severe hyponatremia

Chronic Phase (24-72 hours):

• Brain cells extrude organic osmolytes (idiogenic osmoles):
  • "Amino acids: glutamate, taurine, glycine"
  • "Polyols: myo-inositol, sorbitol"
  • "Methylamines: glycerophosphocholine"
• Cell volume normalises despite ongoing hyponatremia
• Patients become relatively asymptomatic despite low sodium

Clinical Significance:

• Adapted brain is vulnerable to rapid correction
• If sodium rises quickly, water leaves cells faster than osmolytes can be regenerated
• Brain cell shrinkage triggers osmotic demyelination syndrome
• Organic osmolyte regeneration takes 2-7 days"

---

Examiner Follow-up 2: "The serum sodium rises from 116 to 128 mmol/L in the first 12 hours despite stopping hypertonic saline. What is happening and how would you manage this?"

Candidate Response:

"This is overcorrection - a 12 mEq/L rise in 12 hours exceeds the safe rate of <8-10 mEq/24h for what is likely chronic hyponatremia.

What is happening:

• Self-correction phenomenon: Likely carbamazepine-induced SIADH
• With cessation of the causative drug and any volume resuscitation, ADH secretion may decrease
• Kidneys now excrete free water appropriately
• Polyuria with dilute urine = rapid sodium rise

Immediate Management - Re-Lowering Protocol:

1. Stop all sodium-containing fluids immediately

2. Administer D5W 3-5 mL/kg/h IV

   • Provides free water to lower sodium
   • Monitor closely to avoid hypoglycaemia

3. Desmopressin (DDAVP) 2 mcg IV

   • Induces water retention by activating V2 receptors
   • Repeat every 8 hours as needed
   • Prevents further free water excretion

4. Target:

   • Re-lower sodium to approximately 120-122 mEq/L
   • Aim for net 24-hour correction of <8-10 mEq from initial 116

5. Monitor:

   • Serum sodium every 2 hours
   • Urine output (should decrease with DDAVP)

Evidence: Retrospective data suggests re-lowering may prevent or reverse ODS if initiated early, ideally within 24 hours of overcorrection (PMID: 20089804)"

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Examiner Follow-up 3: "What are the causes of carbamazepine-induced hyponatremia?"

Candidate Response:

"Carbamazepine causes hyponatremia through SIADH by two mechanisms:

1. Increased ADH Release:

• Carbamazepine lowers the osmotic threshold for ADH secretion
• Leads to ADH release at lower-than-normal plasma osmolalities
• Central nervous system effect

2. Enhanced Renal ADH Sensitivity:

• Potentiates the effect of ADH at the collecting duct
• Increases water reabsorption for a given ADH level

Epidemiology:

• Occurs in 5-40% of patients on carbamazepine (PMID: 8746302)
• Higher risk in elderly, women, and those with higher drug levels
• Usually develops within first few weeks of treatment
• Dose-dependent effect

Other Antiepileptics with Similar Effect:

• Oxcarbazepine (higher risk than carbamazepine, up to 30%)
• Eslicarbazepine
• Valproate (rare)

Management:

• Drug cessation or dose reduction usually effective
• Consider alternative antiepileptic if persistent
• Fluid restriction as temporising measure
• Education about symptoms of recurrence"

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Viva Scenario 2: SIADH and Vaptans

Stem: A 62-year-old male with small cell lung cancer is admitted with confusion. Serum sodium is 118 mmol/L. He is euvolemic on examination, with urine osmolality 420 mOsm/kg and urine sodium 55 mmol/L. Thyroid and adrenal function tests are normal.

---

Opening Question: "What is the diagnosis and how would you confirm it?"

Candidate Response:

"This patient has SIADH secondary to small cell lung cancer - a classic paraneoplastic cause.

SIADH Diagnostic Criteria (all must be present):

1. Serum sodium <135 mmol/L - present (118 mmol/L)
2. Plasma osmolality <275 mOsm/kg - likely (need measured value)
3. Urine osmolality >100 mOsm/kg - present (420 mOsm/kg, inappropriately concentrated)
4. Urine sodium >30 mmol/L - present (55 mmol/L)
5. Clinical euvolemia - present on examination
6. Normal thyroid and adrenal function - confirmed
7. No recent diuretic use - need to confirm

Additional Confirmation:

• Measured plasma osmolality (should be <275)
• Serum uric acid (typically low in SIADH <4 mg/dL)
• Response to fluid restriction

Pathophysiology in SCLC:

• Tumour cells produce ectopic ADH (vasopressin)
• Common - occurs in 10-45% of SCLC patients
• May be presenting feature of malignancy"

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Examiner Follow-up 1: "You implement fluid restriction to 1 litre per day but sodium only rises to 122 mmol/L after 48 hours. What are your options?"

Candidate Response:

"This patient has SIADH resistant to fluid restriction alone - common in paraneoplastic cases due to ongoing ADH secretion.

Second-Line Options:

1. Urea:

• Dose: 15-60 g/day orally or via NGT
• Mechanism: Increases urine solute load, obligates water excretion
• Evidence: SALSA trial (PMID: 34233096) showed non-inferiority to tolvaptan
• Advantages: Cheap, no hepatotoxicity, no overcorrection risk
• Disadvantages: Unpalatable (mix with orange juice), causes azotaemia

2. Salt Tablets + Loop Diuretic:

• NaCl tablets 3-6 g/day plus furosemide 20-40 mg
• Mechanism: Increases solute excretion, promotes free water loss
• Less effective than other options in severe SIADH

3. Tolvaptan (V2 receptor antagonist):

• First-line pharmacotherapy per European guidelines
• Dose: 15 mg daily, titrate to 30-60 mg
• Mechanism: Blocks ADH action at collecting duct, causes aquaresis
• Evidence: SALT-1, SALT-2 trials (PMID: 17105757) - significant Na improvement
• Requires hospital initiation with 6-hourly sodium monitoring
• Contraindicated: Hypovolemia, unable to sense thirst, hepatic impairment
• Boxed warning for hepatotoxicity

My Preference for This Patient: Given he has SCLC (potentially palliative situation) and will need ongoing treatment, I would consider urea as first choice:

• No hepatotoxicity monitoring required
• Cheaper for long-term use
• Can be given via NGT if needed
• Equally effective per SALSA trial

If rapid correction needed or urea not tolerated, tolvaptan is appropriate."

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Examiner Follow-up 2: "Discuss the evidence for tolvaptan."

Candidate Response:

"SALT-1 and SALT-2 Trials (PMID: 17105757):

Design: Two identical, randomised, double-blind, placebo-controlled trials

Population: 448 patients with euvolemic or hypervolemic hyponatremia (Na 120-135 mmol/L)

• ~30% had SIADH
• ~30% had heart failure
• ~20% had cirrhosis

Intervention: Tolvaptan 15-60 mg daily vs placebo for 30 days

Primary Outcome: Change in serum sodium from baseline to day 4 and day 30

Results:

• Significant increase in sodium with tolvaptan vs placebo at day 4 (+4.0 vs +0.4 mEq/L)
• Maintained at day 30
• Improved symptoms (mental component of SF-12)
• No difference in mortality or hospitalisation

Safety:

• Thirst (14% vs 5%)
• Dry mouth (13% vs 4%)
• Increased urination (7% vs 3%)
• No significant overcorrection events in trials

EVEREST Trial (PMID: 17392274):

Population: 4133 patients hospitalised with heart failure

Results:

• Improved sodium levels and dyspnoea
• No improvement in long-term mortality or heart failure hospitalisation
• Subset with hyponatremia may have benefit

Limitations:

• Both trials relatively short-term
• Hepatotoxicity signal emerged post-marketing
• FDA black box warning for hepatic injury
• Expensive compared to urea
• Limited to 30 days use per label

Current Position: European guidelines recommend as option for SIADH when fluid restriction fails, but urea increasingly preferred due to safety and cost profile. Australian PBS lists tolvaptan for SIADH with specialist authority."

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Examiner Follow-up 3: "This patient develops confusion again 5 days after you initiated treatment. Sodium is now 142 mmol/L. What is your concern?"

Candidate Response:

"My primary concern is osmotic demyelination syndrome (ODS).

Timeline Analysis:

• Initial sodium: 118 mmol/L
• Current sodium: 142 mmol/L
• Rise: 24 mEq/L over 5 days = average 4.8 mEq/day (appears acceptable)
• BUT: Need to know the trajectory - if most of this rise occurred in first 24-48 hours, ODS risk is present

Clinical Presentation of ODS:

• Typically develops 2-6 days after overcorrection
• Classic CPM features: Dysarthria, dysphagia, quadriparesis, pseudobulbar affect
• Extrapontine features: Movement disorders (parkinsonism, dystonia), altered mental status, behavioural changes
• Locked-in syndrome in severe cases

Immediate Actions:

1. Detailed neurological examination:

   • Speech assessment
   • Swallow evaluation
   • Motor examination all limbs
   • Cerebellar signs
   • Extrapyramidal features

2. Review sodium correction history:

   • Plot all sodium values on timeline
   • Identify if overcorrection occurred in first 24-48 hours

3. MRI Brain with T2/FLAIR:

   • Classic finding: T2 hyperintensity in central pons
   • 'Trident' or 'piglet' sign
   • Check extrapontine areas (basal ganglia, thalamus)
   • Note: MRI changes may lag clinical symptoms by 1-2 weeks

4. Exclude other causes of confusion:

   • Infection, metabolic (glucose), medications, progression of malignancy (brain metastases)

Management if ODS Confirmed:

• Supportive ICU care
• No specific treatment proven effective
• Avoid further neurological insults (hypoxia, hypotension)
• Rehabilitation when stable
• Prognosis: Variable - some patients recover well, others have permanent disability

Prevention Learning Point: This case reinforces the need for very careful monitoring during SIADH treatment, particularly with vaptans which can cause rapid aquaresis."