SIADH (Syndrome of Inappropriate ADH) - Adult

1. Overview

The Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) is the most common cause of euvolaemic hyponatraemia, accounting for approximately 30-40% of all cases of hyponatraemia in hospitalised patients. [1] It represents a paradigm of non-osmotic ADH release, where excessive vasopressin (antidiuretic hormone) is secreted despite low serum osmolality, leading to impaired free water excretion and dilutional hyponatraemia. The condition was first described by Schwartz and Bartter in 1957 in patients with bronchogenic carcinoma. [2]

SIADH is fundamentally a diagnosis of exclusion, requiring careful assessment of volume status and systematic exclusion of other causes of hyponatraemia including hypothyroidism, adrenal insufficiency, diuretic use, and renal or cardiac failure. The clinical presentation ranges from asymptomatic biochemical abnormalities detected incidentally to life-threatening neurological emergencies with seizures and coma when sodium falls rapidly or to critically low levels.

The importance of SIADH extends beyond electrolyte disturbance—it often serves as a sentinel marker for serious underlying pathology, particularly malignancy (especially small cell lung cancer), CNS disorders, and pulmonary disease. Recognition and appropriate management are critical, as both the hyponatraemia itself and its over-rapid correction carry significant morbidity and mortality risks. The mortality rate in patients with severe acute symptomatic hyponatraemia approaches 5-10% when untreated, while overly aggressive correction can lead to irreversible osmotic demyelination syndrome. [3]

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2. Epidemiology

Prevalence and Incidence

Hyponatraemia (serum sodium less than 135 mmol/L) is the most common electrolyte abnormality in clinical practice, occurring in 15-30% of hospitalised patients, with SIADH representing the leading cause in euvolaemic cases. [1,4] The true incidence of SIADH in the community is difficult to establish as many cases are asymptomatic and undetected.

Demographics and Risk Factors

Age: SIADH can occur at any age but is more common in older adults. The median age at diagnosis is 65-70 years, reflecting increased prevalence of causative conditions (malignancy, polypharmacy). [5]

Sex: No significant sex predilection for SIADH overall, though specific causes show gender differences (small cell lung cancer more common in males; SSRIs more commonly prescribed in females).

Key Risk Factors:

• Advanced age (> 65 years) - 3-fold increased risk
• Active malignancy - 5-10 fold increased risk
• CNS pathology (stroke, meningitis, SAH)
• Pulmonary disease (pneumonia, TB)
• Polypharmacy (≥5 medications)
• Postoperative state (TURP, spinal surgery)
• HIV infection (5-10% develop SIADH)

Temporal Trends

The incidence of SIADH has increased over the past two decades, largely attributable to: [6]

• Increased SSRI prescribing (prescription rates doubled 2000-2020)
• Aging population with higher baseline hyponatraemia rates
• Improved recognition through routine electrolyte screening
• Increased survival with malignancy (particularly lung cancer)

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3. Aetiology and Pathophysiology

Causes of SIADH

The causes of SIADH are traditionally classified into four major categories: malignancy, CNS disorders, pulmonary disorders, and drugs. A systematic approach to identifying the underlying cause is essential as management depends critically on addressing the primary pathology.

Malignancy (15-30% of cases)

Ectopic ADH Secretion:

• Small Cell Lung Cancer (SCLC): The classic association. 10-15% of SCLC patients develop SIADH at presentation; up to 40% develop it during disease course. [7] SCLC cells synthesize and release vasopressin independently of normal osmoregulatory control.
• Non-small cell lung cancer: Less common (less than 5%) but reported
• Head and neck cancers: Particularly oropharyngeal carcinoma
• Gastrointestinal: Pancreatic carcinoma, duodenal carcinoma
• Genitourinary: Prostate, bladder carcinoma
• Haematological: Lymphoma (Hodgkin's and non-Hodgkin's), leukaemia, thymoma

Mechanism: Neoplastic cells express vasopressin genes and produce biologically active ADH that escapes normal hypothalamic-pituitary control. Tumour-derived ADH is biochemically identical to hypothalamic ADH and binds V2 receptors normally.

CNS Disorders (15-30% of cases)

• Stroke (ischaemic and haemorrhagic): 10-15% develop SIADH
• Subarachnoid haemorrhage: 30-35% incidence, typically developing days 3-10 post-bleed [8]
• Subdural haematoma
• Traumatic brain injury: Moderate-severe TBI
• Infections: Meningitis (bacterial, viral, TB, fungal), encephalitis, brain abscess
• Inflammatory: Guillain-Barré syndrome, acute intermittent porphyria, multiple sclerosis
• Other: Hydrocephalus, psychosis

Mechanism: Direct hypothalamic/posterior pituitary involvement or disruption of normal osmoregulatory pathways. Inflammation, oedema, and haemorrhage can all trigger inappropriate ADH release.

Pulmonary Disorders (10-20% of cases)

• Pneumonia: Particularly atypical organisms
  • "Legionella pneumophila: Classical association, 20-30% of cases develop hyponatraemia [9]"
  • Mycoplasma, viral pneumonia
• Tuberculosis: Both pulmonary and CNS TB
• Aspergillosis
• Pneumothorax
• Mechanical ventilation: Positive pressure ventilation
• Acute respiratory failure
• Lung abscess, empyema

Mechanism: Not fully understood. Proposed mechanisms include direct stimulation of hypothalamic centres via afferent vagal pathways, inflammatory cytokines (IL-6, IL-1β) stimulating ADH release, and intrathoracic pressure changes affecting baroreceptor input.

Drugs (25-35% of cases in hospital settings)

SIADH is one of the most common drug-induced electrolyte disorders. Multiple mechanisms exist: increased ADH release, potentiation of ADH action at the kidney, or reduced water excretion.

Antidepressants and Psychotropics (Most Common):

• SSRIs: Citalopram, escitalopram, fluoxetine, sertraline, paroxetine
  • Risk 0.5-32% depending on age; highest risk > 65 years [10]
  • "Mechanism: Enhanced ADH release via serotonergic pathways"
• SNRIs: Venlafaxine, duloxetine
• Tricyclics: Amitriptyline (less common than SSRIs)
• Antipsychotics: Haloperidol, risperidone, quetiapine
• Mood stabilizers: Carbamazepine (5-10% incidence), oxcarbazepine

Anticonvulsants:

• Carbamazepine: Classic association, dose-dependent
• Sodium valproate, lamotrigine (rare)

Chemotherapy:

• Cyclophosphamide (particularly high-dose)
• Vincristine, vinblastine
• Cisplatin
• Ifosfamide
• Melphalan

Analgesics:

• Opioids: Morphine, fentanyl, tramadol
• NSAIDs (rare): Ibuprofen, indomethacin

Other Drugs:

• MDMA (Ecstasy): Acute severe SIADH in young adults
• Desmopressin (DDAVP): Iatrogenic ADH excess
• Proton pump inhibitors: Rare but reported (omeprazole, pantoprazole)
• Bromocriptine, amiodarone, chlorpropamide (historical)

Other Causes (5-15% of cases)

• Idiopathic: 10-20% of cases, diagnosis of exclusion, more common in elderly
• Post-operative: Particularly TURP, spinal surgery, abdominal surgery
• HIV infection: 5-10% of patients with advanced disease
• Hereditary: Rare gain-of-function mutations in V2 receptor (nephrogenic SIADH)
• Endurance exercise: Marathon running, ultra-endurance events (combination of ADH release and excessive hypotonic fluid intake)

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Pathophysiology

SIADH represents a failure of the normal osmoregulatory system. Understanding the molecular mechanisms is essential for postgraduate examinations and rational therapeutics.

Normal ADH Physiology (Brief Review)

Synthesis and Release:

• ADH (vasopressin) is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus
• Packaged with neurophysin II and transported to the posterior pituitary
• Released into circulation in response to:
  • "Osmotic stimuli: ↑ plasma osmolality (> 295 mOsm/kg) detected by hypothalamic osmoreceptors (most sensitive)"
  • "Non-osmotic stimuli: ↓ blood volume/pressure (> 10% change) detected by atrial and arterial baroreceptors"
  • Nausea (powerful stimulus), pain, stress, hypoxia

Renal Action:

• ADH binds to V2 receptors on the basolateral membrane of principal cells in the collecting duct
• V2 receptor is a G-protein coupled receptor (Gs) → activates adenylyl cyclase → ↑ cAMP → activates PKA
• PKA phosphorylates aquaporin-2 (AQP2) water channels stored in intracellular vesicles
• Phosphorylated AQP2 translocates to the apical (luminal) membrane
• Water moves down osmotic gradient from tubular fluid → cell → interstitium (via constitutive AQP3/4 on basolateral membrane)
• Result: Concentrated urine, water retention

Normal Suppression:

• When plasma osmolality falls less than 280 mOsm/kg, ADH secretion is suppressed
• Collecting duct becomes water-impermeable
• Dilute urine is excreted (osmolality less than 100 mOsm/kg possible)

The SIADH Cascade

Step 1: Inappropriate ADH Secretion

• ADH is released despite low plasma osmolality
• Mechanism varies by cause:
  • "Ectopic production: Tumour cells synthesize ADH autonomously"
  • "Central dysregulation: CNS pathology disrupts hypothalamic osmoreceptors or pituitary function"
  • "Drug-induced: Enhanced hypothalamic release or augmented renal response"
• ADH levels may be frankly elevated or "inappropriately normal" (should be undetectable at low osmolality)

Step 2: Renal Water Retention

• Circulating ADH binds V2 receptors
• AQP2 channels insert into collecting duct apical membrane
• Free water is reabsorbed from collecting duct → bloodstream
• Urine remains inappropriately concentrated (> 100 mOsm/kg, typically 300-600 mOsm/kg)

Step 3: Dilutional Hyponatraemia

• Retained water expands total body water
• Serum sodium concentration falls (dilutional effect)
• Plasma osmolality decreases (less than 275 mOsm/kg)
• Critically: Total body sodium is normal or near-normal; it is the water excess that creates hyponatraemia

Step 4: Volume Expansion and ANP Response

• Increased total body water → mild expansion of extracellular volume (ECF ↑ 5-10%)
• This expansion is usually clinically imperceptible (no oedema)
• Volume expansion stretches atrial myocytes → release of Atrial Natriuretic Peptide (ANP)
• ANP acts on kidneys to increase sodium excretion (natriuresis)

Step 5: Natriuresis ("Salt Wasting")

• ANP-mediated sodium excretion → high urine sodium (typically > 30 mmol/L, often > 40 mmol/L)
• This natriuresis prevents overt volume overload but worsens the hyponatraemia
• Patients reach a new steady state: euvolaemic but hyponatraemic
• This explains the paradox: Low serum sodium but high urine sodium

Step 6: "Escape" from Water Retention

• After several days, water retention plateaus despite ongoing ADH
• Mechanisms incompletely understood but involve:
  • Downregulation of AQP2 channels
  • Increased pressure natriuresis
  • GFR changes
• Patients do not develop progressive water accumulation or oedema (differentiates from heart failure/cirrhosis)

Cerebral Consequences of Hyponatraemia

Acute Hyponatraemia (less than 48 hours):

• Rapid fall in plasma osmolality → osmotic gradient across blood-brain barrier
• Water shifts from extracellular → intracellular compartment (cells swell)
• Cerebral oedema develops (brain swells within fixed cranial vault)
• ↑ Intracranial pressure → brainstem herniation risk
• Symptoms: Headache, nausea, confusion, seizures, coma, respiratory arrest
• Critical threshold: Na less than 120 mmol/L or rapid fall > 10 mmol/L in 24 hours

Chronic Hyponatraemia (> 48 hours):

• Brain adapts to low osmolality via cerebral volume regulation
• Osmolyte extrusion: Brain cells export osmotically active solutes
  • "Acute phase (hours): Export of Na+, K+, Cl- (electrolytes)"
  • "Chronic phase (days): Export of organic osmolytes (taurine, glutamate, glutamine, myoinositol, creatine)"
• Cell volume returns toward normal despite persistent low plasma osmolality
• Patient may be asymptomatic despite Na 115-125 mmol/L
• Consequence: Brain is now adapted to hyponatraemia; rapid correction is dangerous (see osmotic demyelination below)

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Molecular Detail: V2 Receptor Signaling Cascade

Exam Detail: V2 Receptor Structure:

• 7-transmembrane G-protein coupled receptor (GPCR)
• Encoded by AVPR2 gene on chromosome Xq28
• 371 amino acids; primarily expressed on collecting duct principal cells
• Coupled to Gs alpha subunit

Signaling Cascade (High-Yield for Exams):

1. ADH binding → conformational change in V2 receptor
2. Gs activation → GDP-GTP exchange on Gs alpha subunit
3. Adenylyl cyclase activation → ATP → cAMP
4. PKA activation → phosphorylation of multiple targets:
   • AQP2 at Ser256 residue (key phosphorylation site)
   • Phosphorylation triggers vesicle trafficking to apical membrane
5. AQP2 insertion into luminal membrane → water permeability ↑100-fold
6. Water reabsorption via AQP2 (apical) → through cell → AQP3/AQP4 (basolateral) → interstitium
7. Long-term effects: Sustained cAMP → increased AQP2 gene transcription (hours to days)

Aquaporin Biology:

• AQP2: Vasopressin-regulated, apical membrane (rate-limiting step)
• AQP3 and AQP4: Constitutively expressed, basolateral membrane
• AQP2 mutations cause nephrogenic diabetes insipidus
• Chronic ADH excess → AQP2 upregulation → why SIADH persists even with adaptation

Pharmacological Targeting:

• Tolvaptan, Conivaptan: Competitive V2 receptor antagonists ("vaptans")
• Block ADH binding → no cAMP generation → no AQP2 insertion → aquaresis (water diuresis without natriuresis)

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Variants of SIADH

Not all SIADH is identical. Four subtypes based on ADH secretion patterns:

Type A (40%): Erratic ADH secretion, no relationship to osmolality (e.g., ectopic production by tumours)

Type B (30%): ADH secretion responds to osmotic stimuli but "set point" is lowered (reset osmostat)

Type C (15%): ADH secretion normal but renal sensitivity to ADH is increased (nephrogenic SIADH, rare V2 receptor mutations)

Type D (15%): No detectable ADH but antidiuretic effect present (hypothesised alternative antidiuretic factors or V2 receptor activating antibodies)

Reset Osmostat (Type B)

A clinically important variant, particularly in pregnancy, malnutrition, and tuberculosis.

• Osmoregulation functions normally but at a lower set point (e.g., 260 mOsm/kg instead of 285)
• Patients maintain stable sodium (e.g., 125-130 mmol/L) indefinitely
• ADH is appropriately suppressed if osmolality falls below the new set point
• Water load can be excreted (unlike classic SIADH)
• Clinical clue: Chronic stable hyponatraemia, patient asymptomatic

Management Implication: Attempts to raise sodium to normal are futile and may be harmful; the new set point is defended. Accept chronic mild hyponatraemia if asymptomatic.

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4. Clinical Presentation

The clinical presentation of SIADH depends primarily on: (1) the absolute serum sodium level, (2) the rate of decline, and (3) the patient's age and comorbidities. Acute severe hyponatraemia is a medical emergency; chronic mild hyponatraemia may be entirely asymptomatic.

Symptom Severity Spectrum

Rate of Decline is Critical:

• Acute drop (> 10 mmol/L in 24 hrs): Symptoms severe even at Na 125 mmol/L
• Chronic slow decline: May tolerate Na 115 mmol/L with minimal symptoms (cerebral adaptation)

Symptoms by System

Neurological (Predominant)

Hyponatraemia is fundamentally a neurological disorder due to cerebral oedema and neuronal dysfunction.

Mild (130-135 mmol/L):

• Often asymptomatic
• Subtle cognitive slowing, impaired concentration
• Increased falls risk in elderly (gait unsteadiness, impaired attention)

Moderate (120-129 mmol/L):

• Headache (dull, diffuse)
• Nausea and vomiting (often prominent, sign of rising ICP)
• Confusion, disorientation
• Lethargy, somnolence
• Gait ataxia, unsteadiness
• Muscle cramps

Severe (less than 120 mmol/L):

• Seizures (generalised tonic-clonic) – often first presentation
• Reduced GCS / Coma
• Respiratory depression (brainstem involvement)
• Cheyne-Stokes breathing
• Decorticate/decerebrate posturing
• Respiratory arrest (terminal)

Red Flag Features (Immediate Action Required):

• Seizure activity
• GCS less than 14 or dropping
• Severe persistent vomiting (suggests imminent herniation)
• Abnormal breathing pattern

Gastrointestinal

• Nausea (early and common)
• Vomiting (ominous if severe/persistent)
• Anorexia

Musculoskeletal

• Muscle cramps
• Generalised weakness

Chronic Sequelae

Even asymptomatic chronic mild hyponatraemia (125-135 mmol/L) has consequences: [11]

• Increased falls risk (1.67-fold increased odds)
• Osteoporosis and fractures (bone demineralisation)
• Cognitive impairment (reversible with correction)
• Gait disturbance

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Clinical Examination Findings

Volume Status Assessment (Critical)

SIADH is by definition a euvolaemic hyponatraemia. Accurate volume assessment is essential.

Euvolaemia (SIADH):

• No peripheral oedema
• Normal JVP (not elevated)
• Moist mucous membranes
• Normal skin turgor
• No postural drop in BP
• Absence of tachycardia
• Patients may have gained 2-3 kg (mild ECF expansion) but this is clinically imperceptible

If Hypovolaemic Signs Present (suggests alternative diagnosis):

• Dry mucous membranes → Dehydration
• Postural hypotension → Volume depletion
• Tachycardia → Hypovolaemia
• Low JVP → Consider renal salt wasting, GI losses, diuretics

If Hypervolaemic Signs Present (suggests alternative diagnosis):

• Peripheral oedema → Heart failure, cirrhosis, nephrotic syndrome
• Elevated JVP → Heart failure
• Ascites, pleural effusions → Cirrhosis, heart failure

Diagnostic Pitfall: Distinguishing SIADH from cerebral salt wasting (CSW) in neurosurgical patients is challenging. Both present post-SAH with hyponatraemia and high urine sodium. Key difference: CSW is truly hypovolaemic (volume depletion), SIADH is euvolaemic. Fluid balance charts and serial weights help differentiate.

Neurological Examination

• GCS: Quantify conscious level
• Pupillary responses: Check for brainstem signs
• Focal neurology: Stroke, CNS mass lesion
• Meningism: Meningitis/SAH
• Reflexes: Brisk reflexes in acute, depressed in severe

Respiratory Examination

• Signs of consolidation: Pneumonia (crackles, bronchial breathing, dullness)
• Signs of malignancy: Fixed monophonic wheeze, clubbing, cachexia, lymphadenopathy

Other

• Stigmata of hypothyroidism: Dry skin, bradycardia, slow-relaxing reflexes (exclude myxoedema)
• Pigmentation: Adrenal insufficiency (exclude Addison's disease)

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5. Differential Diagnosis

Hyponatraemia is a common endpoint of multiple pathologies. SIADH must be differentiated from other causes systematically.

Classification of Hyponatraemia by Volume Status

Key Differentials (Must Not Miss)

1. Hypothyroidism (Severe Myxoedema)

Why it mimics SIADH:

• Euvolaemic hyponatraemia
• Inappropriately concentrated urine
• High urine sodium
• Mechanism: Reduced cardiac output → non-osmotic ADH release; reduced GFR → impaired free water excretion

Distinguishing Features:

• Bradycardia, dry skin, delayed relaxation of reflexes
• Elevated TSH, low free T4
• Always check TSH before diagnosing SIADH

Management: Thyroxine replacement corrects hyponatraemia

2. Adrenal Insufficiency (Addison's Disease)

Why it mimics SIADH:

• Euvolaemic or mildly hypovolaemic hyponatraemia
• High urine sodium (aldosterone deficiency → renal salt wasting)
• Mechanism: Cortisol deficiency → non-osmotic ADH release; aldosterone deficiency → renal Na loss

Distinguishing Features:

• Hyperpigmentation (buccal, palmar creases, scars)
• Hyperkalaemia (SIADH should have normal K+)
• Hypoglycaemia
• Postural hypotension
• Low cortisol (9am less than 100 nmol/L), failed synacthen test
• Always check 9am cortisol before diagnosing SIADH

Management: Hydrocortisone replacement corrects hyponatraemia

3. Cerebral Salt Wasting (CSW)

Clinical Context: Neurosurgical patients, especially post-subarachnoid haemorrhage (days 3-10).

Why it mimics SIADH:

• Hyponatraemia
• High urine sodium (> 30 mmol/L)
• Same triggers (SAH, TBI, neurosurgery)

Distinguishing Features:

• True hypovolaemia: Postural BP drop, tachycardia, negative fluid balance
• Elevated haematocrit (haemoconcentration)
• Mechanism: Elevated BNP/ANP → renal salt wasting → volume depletion

Management: 0.9% saline (fluid replacement), NOT fluid restriction

• Critical distinction: Fluid restriction (correct for SIADH) worsens CSW

Clinical Approach in Neurosurgery: When uncertain, safer to assume CSW (give fluids) than SIADH (restrict fluids and risk cerebral hypoperfusion)

4. Psychogenic Polydipsia (Primary Polydipsia)

Clinical Context: Psychiatric patients, institutionalised individuals, schizophrenia.

Mechanism: Excessive water intake (> 10-15 L/day) overwhelms renal excretion capacity

Distinguishing Features:

• Dilute urine (osmolality less than 100 mOsm/kg) – key difference
• Low urine sodium (less than 20 mmol/L)
• History of compulsive water drinking
• Normal ADH levels

Management: Water restriction, treat underlying psychiatric disorder

5. Diuretic-Induced Hyponatraemia

Clinical Context: Thiazide diuretics (bendroflumethiazide, indapamide) > loop diuretics.

Mechanism: Impaired diluting capacity in distal tubule; volume depletion → ADH release

Distinguishing Features:

• Recent diuretic initiation or dose increase
• May be hypovolaemic (postural symptoms)
• Hypokalaemia often coexists
• High urine sodium initially (diuretic effect)

Management: Stop diuretic, cautious 0.9% saline if hypovolaemic

6. Heart Failure, Cirrhosis (Hypervolaemic Hyponatraemia)

Mechanism: Reduced effective arterial blood volume → baroreceptor-mediated ADH release

Distinguishing Features:

• Oedema, ascites (clinically hypervolaemic)
• Elevated JVP (heart failure)
• Low urine sodium (less than 20 mmol/L) – key difference from SIADH
• Known cardiac or liver disease

Management: Treat underlying heart failure/cirrhosis; fluid restriction; vaptans (tolvaptan) in refractory cases

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Diagnostic Approach Summary

HYPONATRAEMIA (Na less than 135 mmol/L)
         ↓
   Assess Volume Status
         ↓
    EUVOLAEMIC?
         ↓
      YES
         ↓
Check: TSH, 9am Cortisol, Paired Osms, Urine Na
         ↓
  ┌──────┴──────────┐
TSH↑            Cortisol↓
  ↓                  ↓
Hypothyroidism   Addison's
  │
  └→ Both Normal + UOsm > 100 + UNa > 30 → SIADH

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6. Diagnostic Criteria

The diagnosis of SIADH requires fulfilment of specific criteria and systematic exclusion of alternative causes.

Bartter-Schwartz Criteria (Classic)

The original diagnostic criteria proposed in 1967, remain the cornerstone:

1. Hyponatraemia: Serum sodium less than 135 mmol/L (usually less than 130 mmol/L)
2. Low plasma osmolality: less than 275 mOsm/kg (indicates true hypotonic hyponatraemia)
3. Inappropriately concentrated urine: Urine osmolality > 100 mOsm/kg (often > 300 mOsm/kg)
4. Elevated urine sodium: > 30 mmol/L (typically > 40 mmol/L) on normal salt/water intake
5. Clinical euvolaemia: No signs of hypo- or hypervolaemia
6. Normal renal function: Excludes CKD as cause
7. Normal adrenal function: 9am cortisol normal or Synacthen test normal
8. Normal thyroid function: TSH and free T4 normal

Modern Diagnostic Approach (ESE Guidelines 2014) [12]

Essential Criteria (all must be present):

• Serum osmolality less than 275 mOsm/kg
• Urine osmolality > 100 mOsm/kg (inappropriately concentrated for low plasma osmolality)
• Clinical euvolaemia
• Urine sodium > 30 mmol/L with normal dietary salt intake
• Exclusion of: hypothyroidism, hypocortisolism, diuretic use

Supportive Criteria (strengthen diagnosis):

• Serum uric acid less than 0.24 mmol/L (fractional excretion of urate ↑ due to volume expansion)
• Serum urea less than 3.6 mmol/L (dilutional, increased clearance)
• Failure to correct Na after 0.9% saline infusion
• Correction of Na after fluid restriction

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7. Investigations

Initial Investigations

Serum Tests

Urea and Electrolytes:

• Sodium: less than 135 mmol/L (severity guides urgency)
• Potassium: Usually normal (if ↓ think diuretics; if ↑ think Addison's)
• Urea: Often low (less than 3.6 mmol/L) – dilutional effect + increased clearance
• Creatinine: Normal (exclude CKD)

Serum Osmolality:

• Calculated: 2×Na + Glucose + Urea (all in mmol/L)
• Measured (laboratory): More accurate
• SIADH: less than 275 mOsm/kg (hypotonic hyponatraemia)
• Osmolal gap > 10 suggests pseudohyponatraemia or presence of unmeasured osmoles (ethanol, mannitol, ethylene glycol)

Glucose: Hyperglycaemia causes factitious hyponatraemia (Na falls ~2 mmol/L per 5 mmol/L rise in glucose > 5 mmol/L). Correct or use measured osmolality.

Serum Uric Acid:

• Typically low (less than 0.24 mmol/L) in SIADH
• Mechanism: Volume expansion → increased GFR and fractional excretion of urate
• Not diagnostic but supportive

Lipids: Severe hypertriglyceridaemia or hypercholesterolaemia causes pseudohyponatraemia (artefactual low Na on indirect ion-selective electrodes; true serum Na is normal). Now rare with direct ISE methods.

Urine Tests (Paired with Serum)

Critical to collect simultaneously with serum sample.

Urine Osmolality:

• SIADH: > 100 mOsm/kg (usually 300-600 mOsm/kg)
• Key concept: In true hyponatraemia, osmoreceptors should suppress ADH → dilute urine (less than 100 mOsm/kg). If urine is concentrated despite low plasma osmolality → inappropriate ADH.

Urine Sodium:

• SIADH: > 30 mmol/L (often > 40 mmol/L)
• Reflects ANP-mediated natriuresis due to volume expansion
• Important: Patient must be on normal salt diet; if salt-restricted, UNa may be less than 30 even in SIADH

Spot Urine Sodium:Potassium Ratio (Furst Formula):

• If (UNa + UK) > Serum Na → predicts positive electrolyte-free water clearance
• Suggests hyponatraemia will worsen with standard IV fluids (0.9% saline)
• Useful in deciding whether saline or fluid restriction

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Exclusion of Mimics (Mandatory)

Thyroid Function Tests

• TSH and Free T4
• Exclude severe hypothyroidism

Adrenal Function Tests

• 9am Serum Cortisol:

  • 450 nmol/L: Adrenal insufficiency excluded

  • 100-450 nmol/L: Indeterminate, proceed to Synacthen test
  • less than 100 nmol/L: Adrenal insufficiency likely
• Short Synacthen Test (if cortisol indeterminate):
  • 250 mcg tetracosactide IM/IV
  • Cortisol at 0, 30, 60 min
  • "Normal: 30 or 60 min cortisol > 500 nmol/L"

Drug History

• Comprehensive medication review for SIADH-inducing drugs

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Identifying the Underlying Cause

Once SIADH diagnosed, systematic search for aetiology:

Malignancy Screen

Chest X-Ray (First-Line):

• Lung mass (SCLC, NSCLC)
• Consolidation (pneumonia)
• Hilar lymphadenopathy

CT Chest with Contrast (if CXR abnormal or high suspicion):

• Characterise lung lesion
• Mediastinal staging

Other Imaging:

• CT/MRI head if CNS signs
• CT abdomen/pelvis if GI malignancy suspected
• PET-CT if occult malignancy search

Infection Screen

If Pneumonia Suspected:

• Sputum culture
• Blood cultures
• Pneumococcal/Legionella urinary antigens
• Atypical serology (Mycoplasma, Legionella)
• HIV test (if risk factors)

If CNS Infection Suspected:

• Lumbar puncture (after imaging excludes mass/↑ICP): Cell count, protein, glucose, Gram stain, culture, TB PCR, viral PCR

CNS Imaging

CT/MRI Brain (if neurological signs, trauma, suspected stroke/bleed):

• Acute stroke
• Subarachnoid haemorrhage
• Subdural haematoma
• Mass lesion
• Meningitis/encephalitis (enhancement, oedema)

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Advanced Investigations (Rarely Needed)

ADH Level (Copeptin):

• Not routinely measured (most labs don't offer)
• Copeptin (ADH precursor) more stable
• In SIADH: Detectable ADH despite low osmolality (should be undetectable)
• Main use: Differentiating SIADH (ADH elevated/normal) from primary polydipsia (ADH suppressed)

Water Deprivation Test: Never needed for SIADH diagnosis (used for diabetes insipidus)

Fluid Restriction Test:

• Limit fluids to 500-800 ml/day
• If Na rises → confirms ADH-mediated water retention (SIADH)
• If Na unchanged → alternative cause

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8. Management

Management of SIADH involves three parallel strategies: (1) treating the underlying cause, (2) correcting the hyponatraemia safely, and (3) preventing overcorrection. The approach differs dramatically based on acuity and severity.

Classification by Urgency

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Emergency Management (Severe Symptomatic Hyponatraemia)

Indications:

• Seizures
• Coma or GCS ≤12
• Respiratory compromise
• Any severe neurological symptoms attributable to hyponatraemia

Goal: Rapid initial rise of 4-6 mmol/L to terminate acute symptoms (NOT to normalise sodium).

Hypertonic Saline Protocol

First-Line: 3% Saline Bolus

• 100 ml of 3% saline IV over 10 minutes
• Repeat up to 2-3 times if symptoms persist (check Na after each bolus)
• Expected rise: ~2 mmol/L per 100 ml bolus
• Stop when: Symptoms resolve OR Na risen by 5 mmol/L OR Na reaches 120 mmol/L

Alternative: 2.7% Saline Infusion (if 3% unavailable)

• 150-200 ml over 20 minutes
• Monitor closely

Monitoring:

• Continuous clinical observation (GCS, seizure activity)
• Check serum sodium every 1-2 hours initially
• ECG monitoring (arrhythmia risk with rapid changes)
• ICU/HDU setting

Adjunctive Measures:

• Anticonvulsants if seizures (lorazepam/levetiracetam; avoid phenytoin as less effective in hyponatraemic seizures)
• Airway protection if GCS ≤8

Critical Safety Limit:

• Do not exceed 10 mmol/L rise in first 24 hours
• Do not exceed 18 mmol/L rise in first 48 hours
• Risk of osmotic demyelination syndrome increases exponentially above these limits [13]

---

Non-Emergency Management (Mild-Moderate, Asymptomatic)

First-Line: Fluid Restriction

Rationale: Reduce free water intake below renal water excretion capacity → negative water balance → Na rises.

Restriction Targets:

• Severe restriction: 500-800 ml/day (all fluids including IV)
• Moderate restriction: 1000-1200 ml/day
• Typically start with 1000 ml/day and adjust

Efficacy:

• Success rate: 60-70% if Urine Osm less than 500 mOsm/kg
• Predictor of failure: Urine osmolality > 500-600 mOsm/kg (very concentrated urine → unable to excrete sufficient water)
• Expected rise: 1-2 mmol/L per day if effective

Monitoring:

• Daily weights (expect 0.5-1 kg loss)
• Serum sodium daily initially
• Urine output (should maintain output despite restriction)

Challenges:

• Poor compliance: Thirst is distressing, particularly if chronic
• Inpatient logistics: IV medications in large volumes defeat restriction
• Use concentrated drug formulations where possible

Duration:

• Continue until underlying cause resolved OR chronic therapy initiated

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Second-Line Pharmacological Therapies

When fluid restriction fails or is impractical:

Tolvaptan (V2 Receptor Antagonist)

Mechanism: Competitive antagonist at V2 receptor → blocks ADH action → aquaresis (water diuresis without sodium loss).

Indications (NICE Approved):

• SIADH refractory to fluid restriction
• Symptomatic hyponatraemia
• Na less than 125 mmol/L

Dosing:

• Initial: 15 mg once daily (morning, with or without food)
• Titrate up after 24 hours if insufficient response: 15 → 30 → 60 mg daily
• Maximum: 60 mg/day

Efficacy:

• SALT-1 and SALT-2 Trials (NEJM 2006): [14]
  • RCTs, n=448 patients with euvolaemic/hypervolaemic hyponatraemia
  • Tolvaptan vs placebo
  • "Results: Significant increase in serum sodium at day 4 and day 30 (mean rise 4-6 mmol/L greater than placebo)"
  • AUC for serum sodium significantly higher
  • Improved mental component scores on QoL
• Onset: 2-4 hours (monitor sodium at 6-8 hours post-dose)
• Effect duration: 24 hours

Monitoring (Critical):

• Frequent sodium checks: 6, 12, 24 hours after first dose, then daily
• Risk of overcorrection: Tolvaptan can cause rapid rise → osmotic demyelination risk
• If Na rises > 6-8 mmol/L in 24 hours → stop tolvaptan and consider giving hypotonic fluids or desmopressin to re-lower sodium
• Initiate in hospital only with close monitoring

Adverse Effects:

• Thirst, dry mouth (common, due to aquaresis)
• Polyuria (mechanism of action)
• Hepatotoxicity (black box warning in USA; limit use to 30 days, monitor LFTs)
• Hypernatraemia (if overcorrection)

Contraindications:

• Hypovolaemic hyponatraemia
• Anuric renal failure
• Severe hepatic impairment
• Pregnancy

Cost: Expensive (£70-100 per day), limits use

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Demeclocycline (Tetracycline Antibiotic)

Mechanism: Induces nephrogenic diabetes insipidus by antagonising ADH action at collecting duct (mechanism unclear, likely interferes with cAMP generation).

Dosing:

• 600-1200 mg/day in divided doses (300 mg BD or TDS)
• Onset of action: 3-7 days (slow)

Efficacy:

• Effective in 60-70% of patients
• Modest sodium rise (3-5 mmol/L over weeks)

Advantages:

• Oral
• Inexpensive
• Once established, stable effect

Disadvantages:

• Slow onset (not suitable for acute management)
• Nephrotoxic: Can cause AKI, particularly in cirrhosis or elderly
• Photosensitivity
• Tooth discolouration (avoid in children/pregnancy)
• Drug interactions (dairy products, antacids reduce absorption)

Monitoring:

• Renal function (U&E weekly initially)
• LFTs (hepatotoxicity risk)
• Serum sodium twice weekly initially

Current Role: Second-line agent; largely superseded by tolvaptan but still used where cost is prohibitive or tolvaptan contraindicated.

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Urea Sachets

Mechanism: Oral urea creates an osmotic diuresis. Urea is filtered at glomerulus and partially reabsorbed; remaining urea in tubule obligates water excretion (osmotic diuretic effect). Bypasses ADH action.

Dosing:

• 15-30 g/day dissolved in water or juice, in divided doses
• Typically 15 g BD

Efficacy:

• Effective in small studies (sodium rise 4-6 mmol/L over days)
• Predictable, dose-dependent response

Advantages:

• Safe (no risk of rapid overcorrection)
• Inexpensive
• No nephrotoxicity

Disadvantages:

• Poor palatability (bitter, salty taste) → compliance issues
• Not widely available in all countries
• Nausea, GI upset

Role: Underutilised but useful option, particularly in chronic SIADH where long-term therapy needed and tolvaptan too expensive.

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Sodium Chloride Tablets

Mechanism: Increase oral sodium intake → increase filtered sodium load → obligate water excretion; also directly raise serum Na.

Dosing:

• Slow Sodium tablets (600 mg NaCl = 10 mmol Na each)
• Typical: 6-12 tablets/day (60-120 mmol Na)

Efficacy:

• Modest; often ineffective as monotherapy in SIADH
• Reason: Increased Na is simply excreted (ANP-mediated natriuresis in SIADH)

Role: Adjunct only; not recommended as sole therapy.

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Fludrocortisone

Mechanism: Mineralocorticoid → renal sodium retention.

Efficacy in SIADH: Poor

• Retained sodium triggers further natriuresis via ANP
• Not routinely recommended

Role: May have marginal benefit in cerebral salt wasting (not SIADH).

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Treatment Algorithm

SIADH CONFIRMED
       ↓
   SYMPTOMS?
       ↓
  ┌────┴────┐
SEVERE     MILD/NONE
(Seizure,    ↓
 Coma)    CHRONIC?
  ↓         ↓
3% SALINE  YES: Treat Cause
BOLUS        + Fluid Restriction
↓            ↓
ICU      INEFFECTIVE?
         (UOsm > 500)
            ↓
         PHARMACOLOGY
         ┌──┴──┐
      TOLVAPTAN DEMECLOCYCLINE
      (fast)     (slow, cheap)
         │         │
         └────┬────┘
              ↓
          MONITOR Na
          (Target less than 10 mmol/L
           rise per 24h)

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Treating the Underlying Cause (Essential)

SIADH often resolves when the precipitant is addressed:

• Drug-induced: Stop offending agent (SSRIs, carbamazepine). Resolution typically within 1-2 weeks but can take months.
• Infection: Antibiotics for pneumonia/meningitis. SIADH resolves with infection clearance.
• Malignancy: Chemotherapy/radiotherapy for SCLC. SIADH improves with tumour response. If refractory → chronic therapy.
• CNS pathology: Neurosurgical intervention if indicated. Often self-limiting post-SAH (resolves after 2-3 weeks).
• Idiopathic: May require long-term management.

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Special Situations

Post-Operative SIADH

• Common after TURP, spinal surgery
• Often transient (resolves within days)
• Avoid hypotonic IV fluids postoperatively
• Use 0.9% saline judiciously

SIADH in Malignancy

• Often refractory and recurrent
• May require chronic tolvaptan/demeclocycline
• Discuss prognosis and goals of care

Chronic Asymptomatic SIADH (Na 125-135)

• If truly asymptomatic and stable, no acute intervention needed
• Address underlying cause
• Consider chronic mild fluid restriction (1200-1500 ml/day)
• Monitor for falls risk, osteoporosis

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9. Complications

Complications of Hyponatraemia Itself

Cerebral Oedema and Herniation

• Acute severe hyponatraemia → brain swelling → ↑ICP
• Transtentorial or tonsillar herniation
• Respiratory arrest, death
• Prevention: Recognise and treat urgently

Seizures

• Generalised tonic-clonic seizures
• Can occur at Na less than 120-125 mmol/L if acute
• Management: Hypertonic saline (treat cause, not just symptom with anticonvulsants alone)

Chronic Sequelae

• Falls and fractures: 1.67-fold increased risk even with mild chronic hyponatraemia (125-135 mmol/L) [11]
• Osteoporosis: Hyponatraemia directly affects osteoblast/osteoclast activity
• Cognitive impairment: Reversible with correction
• Gait instability: "Senile gait" in elderly often due to unrecognised hyponatraemia

Death

• Mortality in untreated severe symptomatic hyponatraemia: 5-10%
• Higher in elderly, comorbid patients

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Complications of Treatment

Osmotic Demyelination Syndrome (ODS)

Formerly "Central Pontine Myelinolysis" (CPM), now recognised to occur extra-pontine also.

Mechanism:

• Rapid correction of chronic hyponatraemia → rapid rise in plasma osmolality
• Brain cells adapted to low osmolality (osmolytes extruded)
• Sudden osmotic shift → water moves out of brain cells → cell shrinkage
• Oligodendrocytes particularly vulnerable → myelin sheath disruption in pons and other areas
• Axons preserved but demyelinated → neurological devastation

Risk Factors: [13]

• Overcorrection: > 10 mmol/L rise in 24 hours, > 18 mmol/L in 48 hours
• Chronic hyponatraemia (> 48 hours, especially >weeks)
• Severe baseline hyponatraemia (less than 120 mmol/L)
• Alcoholism, malnutrition
• Liver disease
• Hypokalaemia
• Burns

Time Course:

• Initial improvement in neurological symptoms (Na rises, cerebral oedema resolves)
• Biphasic: After 2-6 days, new neurological deterioration appears

Clinical Features:

• Dysarthria, dysphagia (pseudobulbar palsy)
• Quadriparesis or quadriplegia (corticospinal tract damage)
• Locked-in syndrome (aware but unable to move/speak; preserved vertical eye movements and blinking)
• Behavioural changes, confusion, coma
• Seizures
• Movement disorders (parkinsonism, dystonia)

Diagnosis:

• MRI brain (T2/FLAIR): Hyperintense signal in pons ("bat wing" appearance) and/or extrapontine sites (basal ganglia, thalamus, cerebellum)
• Changes may not appear until 2-4 weeks post-injury (initial MRI may be normal)

Prognosis:

• Variable: Some recover partially over months; many left with permanent severe disability
• No specific treatment (supportive care only)
• Prevention is critical

Prevention:

• Limit correction rate: less than 10 mmol/L in 24 hours, less than 18 mmol/L in 48 hours
• Check sodium frequently during treatment (every 2-4 hours in acute phase)
• If overcorrection occurs: Re-lower sodium with hypotonic fluids (5% dextrose) or desmopressin (2-4 mcg IV/SC)

Hypertonic Saline Complications

• Volume overload: Particularly in elderly, heart failure patients
• Hypernatraemia: Overshoot if not monitored
• Central line complications: If giving via CVC (infection, thrombosis)
• Local phlebitis: If giving peripherally (3% saline is hypertonic)

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10. Prognosis

Short-Term Prognosis

• With treatment: Excellent in uncomplicated SIADH if underlying cause addressed
• Severe symptomatic hyponatraemia: 5-10% mortality if untreated; less than 1-2% with appropriate treatment
• Osmotic demyelination: Major cause of poor outcome; permanent disability common if occurs

Long-Term Prognosis

Determined primarily by the underlying cause:

Recurrence

• Drug-related: High recurrence if drug re-started
• Malignancy: Recurrent/persistent until tumour controlled
• Idiopathic: Chronic/relapsing

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11. Prevention and Screening

Primary Prevention

• Medication vigilance: Caution when prescribing SIADH-inducing drugs (SSRIs, carbamazepine) in elderly; monitor sodium at baseline and 1-2 weeks post-initiation
• Avoid hypotonic fluids postoperatively: Use 0.9% saline, avoid 5% dextrose or 0.45% saline in at-risk patients
• Hydration protocols in endurance events: Educate marathon runners to avoid excessive hypotonic fluid intake

Screening

High-Risk Groups (check U&E):

• All hospitalised patients (routine admission bloods)
• Newly started on SSRIs, carbamazepine (check at 2 weeks, 1 month)
• Post-SAH patients (days 3-10)
• Small cell lung cancer patients (at diagnosis and during chemo)
• Post-operative (day 1-3)

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12. Key Guidelines and Evidence

Guidelines

Landmark Evidence

1. Schwartz WB, et al. (1957): Original description of SIADH in lung cancer patients. Am J Med.

2. Schrier RW, et al. SALT-1 and SALT-2 Trials (2006): [14]

   • Tolvaptan vs placebo in hyponatraemia (n=448)
   • Significant Na increase, improved QoL
   • N Engl J Med. 2006;355(20):2099-112. doi:10.1056/NEJMoa065181

3. Verbalis JG, et al. (2013): Osmotic demyelination risk factors and correction rate limits. Am J Med. doi:10.1016/j.amjmed.2013.09.013

4. Spasovski G, et al. (2014): [12] ESE Clinical Practice Guideline on hyponatraemia diagnosis and treatment. Eur J Endocrinol. doi:10.1530/EJE-13-1020

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13. Examination Focus

Common MRCP Exam Questions

1. Diagnosis:

• "Low Na, low serum osmolality, high urine osmolality, high urine Na, euvolaemic. Diagnosis?"
  • "Answer: SIADH (after excluding hypothyroidism and Addison's)"

2. Causes:

• "Most common malignancy causing SIADH?"
  • "Answer: Small cell lung cancer"
• "Antibiotic associated with SIADH?"
  • "Answer: Legionella (pneumonia causing SIADH, not the antibiotic)"
• "Antidepressant class causing SIADH?"
  • "Answer: SSRIs"

3. Management:

• "First-line treatment for asymptomatic SIADH?"
  • "Answer: Fluid restriction (500-1000 ml/day)"
• "Maximum safe sodium correction rate?"
  • "Answer: less than 10 mmol/L in first 24 hours (prevents osmotic demyelination)"
• "Emergency treatment for SIADH with seizures?"
  • "Answer: 100 ml 3% hypertonic saline IV bolus over 10 minutes"

4. Complications:

• "Complication of rapid sodium correction?"
  • "Answer: Osmotic demyelination syndrome (central pontine myelinolysis)"
• "MRI findings in osmotic demyelination?"
  • Answer: T2 hyperintensity in pons ("bat wing" sign)

5. Differentials:

• "Hyponatraemia post-SAH with low urine sodium and hypotension?"
  • "Answer: Cerebral salt wasting (not SIADH; CSW is hypovolaemic)"
• "Hyponatraemia with high TSH?"
  • "Answer: Hypothyroidism (mimic, not true SIADH)"

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Viva Voce Points

Viva Point: Opening Statement: "SIADH is the syndrome of inappropriate antidiuretic hormone secretion, characterised by euvolaemic hyponatraemia due to impaired free water excretion despite low plasma osmolality. It is the most common cause of euvolaemic hyponatraemia, accounting for 30-40% of hyponatraemia in hospitalised patients."

Diagnostic Criteria (Bartter-Schwartz):

• Hyponatraemia (less than 135 mmol/L)
• Low plasma osmolality (less than 275 mOsm/kg)
• Inappropriately concentrated urine (> 100 mOsm/kg)
• High urine sodium (> 30 mmol/L)
• Clinical euvolaemia
• Normal thyroid and adrenal function

Major Causes (SIADH mnemonic: Malignancy, Meds, CNS, Chest):

• Malignancy: Small cell lung cancer (classic)
• Drugs: SSRIs, carbamazepine, opiates, cyclophosphamide
• CNS: Stroke, SAH, meningitis, TBI
• Pulmonary: Pneumonia (esp. Legionella), TB

Pathophysiology (Molecular Level): "ADH binds V2 receptors on collecting duct principal cells, activating adenylyl cyclase and raising cAMP. This phosphorylates aquaporin-2 water channels, causing their insertion into the apical membrane. Water is reabsorbed, causing dilutional hyponatraemia. The resulting volume expansion triggers ANP release, leading to natriuresis—hence the paradoxical high urine sodium despite low serum sodium."

Management Principles:

• Acute symptomatic (seizures/coma): 100 ml 3% saline bolus IV
• Chronic asymptomatic: Fluid restriction (500-1000 ml/day)
• Refractory: Tolvaptan (V2 antagonist, SALT trials showed efficacy)
• Critical safety limit: Correct less than 10 mmol/L in 24 hours to prevent osmotic demyelination

Complications: "The major complication of treatment is osmotic demyelination syndrome, caused by overcorrection. Rapid rise in osmolality causes brain cell shrinkage and myelin shearing, particularly in the pons. Patients develop locked-in syndrome 2-6 days post-correction. Prevention is key: limit correction to less than 10 mmol/L per 24 hours and check sodium every 2-4 hours during treatment."

Key Evidence:

• SALT-1 and SALT-2 trials (NEJM 2006): Tolvaptan significantly increased serum sodium vs placebo in euvolaemic hyponatraemia
• ESE Guidelines 2014: Comprehensive diagnostic algorithm and correction rate recommendations

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Common Exam Mistakes (Avoid to Pass)

❌ Mistake 1: Diagnosing SIADH without checking TSH and cortisol

• Why it fails: Hypothyroidism and Addison's mimic SIADH exactly
• ✅ Correct: Always exclude before labelling as SIADH

❌ Mistake 2: Rapid correction with 0.9% saline or aggressive hypertonic saline

• Why it fails: Causes osmotic demyelination → permanent disability
• ✅ Correct: Limit to less than 10 mmol/L rise per 24 hours; check Na every 2-4 hours

❌ Mistake 3: Fluid restricting cerebral salt wasting (mistaken for SIADH)

• Why it fails: CSW is hypovolaemic; restriction worsens cerebral perfusion → stroke risk
• ✅ Correct: Assess volume status carefully; CSW needs fluids, SIADH needs restriction

❌ Mistake 4: Using 5% dextrose or hypotonic fluids in SIADH

• Why it fails: Adds free water → worsens hyponatraemia
• ✅ Correct: Use 0.9% saline if fluids needed (or hypertonic if severe)

❌ Mistake 5: Treating asymptomatic chronic hyponatraemia as an emergency

• Why it fails: Unnecessary hypertonic saline → overcorrection risk
• ✅ Correct: Chronic mild hyponatraemia tolerates slow correction (fluid restriction adequate)

❌ Mistake 6: Forgetting to search for underlying cause

• Why it fails: Misses treatable malignancy or infection
• ✅ Correct: CXR, drug review, TSH, cortisol mandatory

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Model Answer: Clinical Scenario

Question: "A 68-year-old man presents with confusion. Sodium 118 mmol/L. How would you manage?"

Model Answer:

"This is severe hyponatraemia. I would approach this systematically:

Immediate Assessment:

• ABCDE: Assess airway, breathing, circulation. GCS score. Check for seizure activity.
• History: Speed of onset (acute less than 48h vs chronic), symptoms (headache, nausea, seizures), drug history (SSRIs, diuretics), medical history (malignancy, lung disease).
• Examination: Volume status (hypo/eu/hypervolaemic), respiratory signs (pneumonia, malignancy), neurological signs.

Urgent Investigations:

• Paired serum and urine osmolality, urine sodium
• U&Es, glucose, serum osmolality
• 9am cortisol, TSH (exclude Addison's and hypothyroidism)
• CXR (pneumonia, malignancy)

Management:

• If severe symptoms (seizures, GCS ≤12): 3% hypertonic saline 100 ml IV bolus over 10 minutes. Repeat up to 3 times until symptoms resolve or Na risen by 5 mmol/L. Transfer to HDU/ICU.
• If moderate symptoms, confusion only: Admit, cautious fluid restriction or low-dose hypertonic saline, frequent Na monitoring.
• If asymptomatic: Fluid restriction 1000 ml/day, treat underlying cause.

Safety:

• Monitor sodium closely: Every 2-4 hours initially.
• Correction limit: Do not exceed 10 mmol/L rise in first 24 hours (prevents osmotic demyelination).
• If overcorrection: Give 5% dextrose or desmopressin to re-lower sodium.

Treat Underlying Cause:

• If SIADH confirmed (euvolaemic, UOsm > 100, UNa > 30, normal cortisol/TSH), identify cause: malignancy screen, infection, drugs.
• Stop culprit drugs, treat pneumonia/infection, oncology referral if cancer.

Follow-up:

• If refractory to fluid restriction: Consider tolvaptan (V2 antagonist) or demeclocycline.
• Long-term: Address underlying pathology; monitor for recurrence."

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14. Advanced Topics

Reset Osmostat (Detailed)

In reset osmostat, the hypothalamic osmoreceptor "set point" is lowered. The osmoregulatory system functions normally but defends a lower-than-normal serum osmolality.

Characteristics:

• Chronic stable hyponatraemia (e.g., Na 125-130 mmol/L)
• ADH appropriately suppressed if osmolality falls below new set point
• Water load can be excreted (unlike classic SIADH)
• Volume regulation intact

Causes:

• Pregnancy (physiological, set point ↓ ~10 mOsm/kg)
• Malnutrition, chronic illness
• Quadriplegia
• Tuberculosis

Diagnosis:

• Water loading test: Able to excrete > 80% of water load within 4 hours (SIADH cannot)

Management:

• Do not treat: Attempts to normalise sodium are futile; the new set point is defended
• Sodium will not rise with fluid restriction
• Accept chronic mild hyponatraemia if asymptomatic

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SIADH vs Cerebral Salt Wasting (CSW): Definitive Differentiation

This is a common exam topic and real clinical dilemma, especially post-SAH.

Clinical Context: CSW almost exclusively occurs in neurosurgical patients (post-SAH, post-craniotomy). If hyponatraemia in this setting, safer to assume CSW and give fluids than restrict and risk cerebral ischaemia.

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Urea Therapy: Why It Works

Mechanism often misunderstood. Detailed explanation:

Osmotic Load Principle:

• Oral urea is absorbed, enters circulation, filtered at glomerulus
• ~50% is reabsorbed in proximal tubule; remainder stays in tubular fluid
• Urea in distal tubule/collecting duct creates osmotic gradient
• Even with ADH action and AQP2 insertion, water cannot be fully reabsorbed (osmotic force opposes)
• Result: Obligate water excretion (osmotic diuresis)
• Bypasses ADH block

Why Urea, Not Other Osmoles?:

• Mannitol (IV): Effective but requires IV access, hospital administration
• Glucose: Would worsen hyperglycaemia, limited in diabetics
• Urea: Oral, safe, dose-dependent, no metabolic consequences (excreted unchanged)

Evidence: Small trials show efficacy (4-6 mmol/L rise over 1-2 weeks). Underutilised due to palatability.

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15. References

1. Ellison DH, Berl T. Clinical practice. The syndrome of inappropriate antidiuresis. N Engl J Med. 2007;356(20):2064-72. doi:10.1056/NEJMcp066837

2. Schwartz WB, Bennett W, Curelop S, Bartter FC. A syndrome of renal sodium loss and hyponatremia probably resulting from inappropriate secretion of antidiuretic hormone. Am J Med. 1957;23(4):529-42. doi:10.1016/0002-9343(57)90224-3

3. Sterns RH, Silver SM. Cerebral salt wasting versus SIADH: what difference? J Am Soc Nephrol. 2008;19(2):194-6. doi:10.1681/ASN.2007101118

4. Upadhyay A, Jaber BL, Madias NE. Incidence and prevalence of hyponatremia. Am J Med. 2006;119(7 Suppl 1):S30-5. doi:10.1016/j.amjmed.2006.05.005

5. Corona G, Giuliani C, Parenti G, et al. Moderate hyponatremia is associated with increased risk of mortality: evidence from a meta-analysis. PLoS One. 2013;8(12):e80451. doi:10.1371/journal.pone.0080451

6. Liamis G, Milionis H, Elisaf M. A review of drug-induced hyponatremia. Am J Kidney Dis. 2008;52(1):144-53. doi:10.1053/j.ajkd.2008.03.004

7. Hansen O, Sørensen P, Hansen KH. The occurrence of hyponatremia in SCLC and the influence on prognosis: a retrospective study of 453 patients treated in a single institution. Lung Cancer. 2010;68(1):111-4. doi:10.1016/j.lungcan.2009.05.015

8. Hannon MJ, Behan LA, O'Brien MM, et al. Hyponatremia following mild/moderate subarachnoid hemorrhage is due to SIAD and glucocorticoid deficiency and not cerebral salt wasting. J Clin Endocrinol Metab. 2014;99(1):291-8. doi:10.1210/jc.2013-3032

9. Cunha BA, Chawla K, Connolly JJ. Legionnaires' disease and hyponatraemia. Clin Microbiol Infect. 2016;22(3):e1-3. doi:10.1016/j.cmi.2015.10.025

10. De Picker L, Van Den Eede F, Dumont G, et al. Antidepressants and the risk of hyponatremia: a class-by-class review of literature. Psychosomatics. 2014;55(6):536-47. doi:10.1016/j.psym.2014.01.010

11. Renneboog B, Musch W, Vandemergel X, et al. Mild chronic hyponatremia is associated with falls, unsteadiness, and attention deficits. Am J Med. 2006;119(1):71.e1-8. doi:10.1016/j.amjmed.2005.09.026

12. Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1-47. doi:10.1530/EJE-13-1020

13. Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-42. doi:10.1016/j.amjmed.2013.07.006

14. Schrier RW, Gross P, Gheorghiade M, et al. Tolvaptan, a selective oral vasopressin V2-receptor antagonist, for hyponatremia. N Engl J Med. 2006;355(20):2099-112. doi:10.1056/NEJMoa065181

15. Soupart A, Decaux G. Therapeutic recommendations for management of severe hyponatremia: current concepts on pathogenesis and prevention of neurologic complications. Clin Nephrol. 1996;46(3):149-69. PMID: 8879850

16. Furst H, Hallows KR, Post J, et al. The urine/plasma electrolyte ratio: a predictive guide to water restriction. Am J Med Sci. 2000;319(4):240-4. doi:10.1097/00000441-200004000-00007

17. Grant P, Ayuk J, Bouloux PM, et al. The diagnosis and management of inpatient hyponatraemia and SIADH. Eur J Clin Invest. 2015;45(8):888-94. doi:10.1111/eci.12465

18. Decaux G, Musch W. Clinical laboratory evaluation of the syndrome of inappropriate secretion of antidiuretic hormone. Clin J Am Soc Nephrol. 2008;3(4):1175-84. doi:10.2215/CJN.04431007

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Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists and follow local guidelines. In SIADH management, frequent sodium monitoring and adherence to correction rate limits are essential to prevent irreversible complications.