Syndrome of Inappropriate ADH Secretion (SIADH)

1. Clinical Overview

The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH), also termed Syndrome of Inappropriate Antidiuresis (SIAD), represents the most important cause of euvolaemic hypotonic hyponatraemia in clinical practice. [1,2] It is characterized by the non-osmotic release of arginine vasopressin (AVP) leading to inappropriate water retention by the kidneys, plasma dilution, and hyponatraemia despite normal or increased total body water. [1,2,3]

The syndrome was first described by Schwartz and Bartter in 1957 in two patients with bronchogenic carcinoma who developed hyponatraemia with continued urinary sodium excretion. [4] Their diagnostic criteria, formulated in 1967, remain the cornerstone of diagnosis today. [4]

Key Facts

Clinical Pearls

Pearl 1: SIADH is a diagnosis of exclusion—always exclude hypothyroidism and adrenal insufficiency first, as these can biochemically mimic SIADH perfectly but require different treatment.

Pearl 2: The urine is inappropriately concentrated relative to plasma osmolality. In true water excess with normal kidney function, urine osmolality should be less than 100 mOsm/kg—failure to dilute urine indicates SIADH. [2,3]

Pearl 3: Medication history is critical—SSRIs, carbamazepine, and PPIs are common culprits. Drug cessation may be curative. Thiazides cause hyponatraemia through a different mechanism. [7]

Pearl 4: Small cell lung cancer (SCLC) produces ectopic ADH in 10-15% of cases—always obtain chest imaging in unexplained SIADH. [8] SIADH may be the presenting feature of occult malignancy.

Pearl 5: Predictors of fluid restriction failure: urine Na > 130 mmol/L and urine osmolality > 500 mOsm/kg predict failure of conservative management. [9] These patients need second-line therapy early.

Pearl 6: Even mild chronic hyponatraemia (130-135 mmol/L) causes cognitive impairment, gait instability, falls, and fractures—it is not benign. [2,5] Treatment improves quality of life.

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2. Epidemiology

Incidence and Prevalence

Demographics

Risk Factors

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3. Pathophysiology

Molecular and Cellular Mechanisms

Step 1: Inappropriate ADH Secretion

Normal Physiology:

• Arginine vasopressin (AVP/ADH) is synthesized in hypothalamic paraventricular and supraoptic nuclei
• Transported via neurohypophyseal tract to posterior pituitary
• Normal triggers: (1) increased plasma osmolality > 280 mOsm/kg (osmotic), (2) decreased blood volume/pressure > 10% (non-osmotic)
• Osmotic regulation: 1% increase in osmolality → 10-fold increase in ADH [1]

SIADH Pathophysiology:

• ADH released despite low plasma osmolality and normal/high blood volume
• Sources: (1) posterior pituitary dysregulation, (2) ectopic production (tumours), (3) drug potentiation, (4) reset osmostat
• Result: persistent ADH activity drives water retention inappropriately [1,2,3]

Step 2: Renal Water Retention—Molecular Detail

V2 Receptor Activation:

• ADH binds V2 receptors on basolateral membrane of collecting duct principal cells
• Gs protein coupling activates adenylate cyclase → increased cAMP
• Protein kinase A (PKA) phosphorylates aquaporin-2 (AQP2) water channels
• AQP2 translocates from cytoplasmic vesicles to apical membrane
• Water permeability increases 10-fold, enabling passive water reabsorption down osmotic gradient [1]

Free Water Retention:

• Urine osmolality inappropriately high (> 100 mOsm/kg, often > 300 mOsm/kg) relative to plasma
• Electrolyte-free water retained → plasma dilution
• Urine volume low, concentrated despite hyponatraemia [2,3]

Step 3: Plasma Dilution and Compensatory Mechanisms

Hyponatraemia Development:

• Retained water expands extracellular fluid (ECF) volume by 5-10%
• Plasma sodium diluted (dilutional hyponatraemia)
• Serum osmolality falls less than 275 mOsm/kg [2,3]

Natriuresis—Escape Phenomenon:

• Volume expansion triggers release of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP)
• ANP/BNP promote sodium excretion in distal tubule
• Urine sodium paradoxically elevated (> 30 mmol/L, typically > 40 mmol/L) despite hyponatraemia
• Patient remains euvolaemic due to partial "escape" from water retention [1,2]
• This natriuresis prevents oedema but perpetuates hyponatraemia

Uric Acid Handling:

• Volume expansion increases fractional excretion of uric acid (FE-UA)
• Serum uric acid falls (often less than 0.24 mmol/L or less than 4 mg/dL)
• FE-UA > 12% is 80% sensitive and specific for SIADH vs. hypovolaemia [11]
• Useful diagnostic marker for differentiating SIADH from cerebral salt wasting [11,12]

Step 4: Cerebral Adaptation to Hyponatraemia

Acute Phase (0-24 hours):

• Plasma hypo-osmolality creates osmotic gradient
• Water shifts intracellularly into brain cells → cerebral oedema
• Increased intracranial pressure → headache, nausea, altered consciousness
• Severe cases: brainstem herniation, seizures, respiratory arrest [2,3]

Adaptive Phase (24-48 hours):

• Neurons expel organic osmolytes (osmotic demyelination protective substances):
  • Taurine, myoinositol, glutamate, glutamine
  • "Electrolytes: potassium, chloride"
• Brain cell volume normalized despite persistent hyponatraemia
• Adaptation makes brain vulnerable to osmotic demyelination if corrected too rapidly [2,3,6]

Chronic Adaptation (> 48 hours):

• Complete osmolyte depletion—brain protected from oedema
• Subtle cognitive impairment, gait instability persist [2,5]
• Rapid correction causes osmotic stress → oligodendrocyte apoptosis → demyelination [6]

Step 5: Steady State vs. Decompensation

Chronic Mild SIADH (Na 125-135 mmol/L):

• Often asymptomatic or subtle cognitive/gait impairment
• Associated with increased falls (23.8% vs 16.4%), fractures (23.3% vs 17.3%), and osteoporosis [2]
• May be tolerated for years if stable

Acute Severe SIADH (Na less than 120 mmol/L, rapid onset):

• Life-threatening cerebral oedema before adaptation
• Mortality 5-30% depending on severity and treatment delay [2,5]

Decompensation Triggers:

• Increased water intake (IV hypotonic fluids, psychogenic polydipsia)
• Increased ADH stimulus (pain, nausea, stress, new medications)
• Reduced solute intake (poor nutrition, anorexia)

Classification of SIADH by ADH Secretion Pattern

Aetiological Classification

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4. Clinical Presentation

Symptoms by Severity

The severity and rapidity of hyponatraemia determine clinical presentation. Acute hyponatraemia (less than 48 hours) is more symptomatic than chronic. [2,3]

Critical Concept: Acute symptomatic hyponatraemia is a medical emergency. Symptoms of cerebral oedema (confusion, seizures, decreased GCS) require immediate treatment with hypertonic saline to prevent brainstem herniation and death. [2,3,6]

Symptom Frequency in SIADH

Atypical Presentations

Red Flags—Emergency Features

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5. Clinical Examination

Structured Approach to Examination

General Inspection

• Mental state: Alert, confused, drowsy, comatose (GCS score)
• Signs of volume depletion: Dry mucous membranes, reduced skin turgor, tachycardia, postural hypotension → ABSENT in true SIADH (suggests hypovolaemia)
• Signs of volume overload: Peripheral oedema, ascites, raised JVP, pulmonary crepitations → ABSENT in true SIADH (suggests hypervolaemia: heart failure, cirrhosis, nephrotic syndrome)
• Nutritional status: Cachexia may suggest underlying malignancy [8]
• Hydration: Moist mucous membranes in SIADH (euvolaemic)

Volume Assessment—Critical for Differential Diagnosis

Volume status differentiates SIADH from other causes of hyponatraemia. SIADH is euvolaemic. [2,3]

Critical Teaching Point: SIADH patients have paradoxical natriuresis—urinary sodium is high (> 30-40 mmol/L) despite hyponatraemia because volume expansion triggers ANP/BNP release. This distinguishes SIADH from hypovolaemic hyponatraemia. [1,2]

Systems Examination—Identifying Underlying Cause

Special Bedside Tests

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6. Investigations

Laboratory Investigations—Diagnostic Criteria

The diagnosis of SIADH requires systematic investigation to confirm hyponatraemia, establish hypotonicity, assess volume status biochemically, and exclude alternative diagnoses. [2,3,4]

Calculated Parameters:

• Fractional excretion of uric acid (FE-UA) = (Urine UA × Serum Cr) / (Serum UA × Urine Cr) × 100
  • FE-UA > 12% strongly suggests SIADH [11]
• Serum osmolality (calculated) = 2 × Na + glucose + urea (all in mmol/L)
  • Should correlate with measured osmolality; discrepancy suggests pseudohyponatraemia

Diagnostic Criteria for SIADH (Bartter and Schwartz, 1967) [4]

Essential Criteria (all must be met):

Supportive Criteria (strengthen diagnosis):

• Low serum uric acid (less than 0.24 mmol/L) [11]
• High fractional excretion of uric acid (FE-UA > 12%) [11]
• Failure to correct with 0.9% saline challenge
• Improvement with fluid restriction [9]

Differential Diagnosis of Euvolaemic Hyponatraemia

SIADH must be differentiated from other causes of euvolaemic hyponatraemia:

SIADH vs. Cerebral Salt Wasting (CSW)—Critical Distinction: [12]

Practical Approach: In CNS injury with hyponatraemia, if volume status unclear, give fluid challenge (1-2L 0.9% saline):

• CSW: Na improves (volume repleted)
• SIADH: Na unchanged or worsens (water retained) [12]

Additional Investigations to Identify Underlying Cause

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7. Management

Management depends on: (1) Severity (Na level), (2) Acuity (acute less than 48h vs. chronic > 48h), (3) Symptoms (asymptomatic vs. symptomatic). [2,3,6]

Overarching Principles:

• Acute symptomatic hyponatraemia is an emergency → hypertonic saline immediately [2,3,6]
• Correction rate limits to prevent osmotic demyelination → maximum 10 mmol/L in 24 hours [2,3,6]
• Treat underlying cause → stop offending drugs, treat infections, manage malignancy [2]
• Fluid restriction is first-line for chronic asymptomatic SIADH → 500-1000 mL/day [2,9]
• Second-line options → urea, vaptans, demeclocycline if fluid restriction fails [2,9,15]

Management Algorithm

     SUSPECTED SIADH (Na less than 135, clinically euvolaemic)
                        ↓
┌────────────────────────────────────────────────────────┐
│              CONFIRM DIAGNOSIS                         │
│  ✓ Serum osmolality less than 275 mOsm/kg                     │
│  ✓ Urine osmolality > 100 mOsm/kg                     │
│  ✓ Urine Na > 30 mmol/L (on normal diet)              │
│  ✓ Euvolaemic on examination                         │
│  ✓ TFTs normal, cortisol normal (exclude mimics)     │
└────────────────────────────────────────────────────────┘
                        ↓
┌────────────────────────────────────────────────────────┐
│            ASSESS SEVERITY AND ACUITY                  │
├────────────────────────────────────────────────────────┤
│  SEVERE SYMPTOMATIC (ANY Na + symptoms)                │
│  → Seizures, GCS less than 15, coma, respiratory distress      │
│  → EMERGENCY: 3% Hypertonic Saline BOLUS [2,3,6]     │
│     • 100-150 mL over 20 min IV                       │
│     • Target: 4-6 mmol/L rise in 1-2 hours           │
│     • MAX: 10 mmol/L in first 24 hours               │
├────────────────────────────────────────────────────────┤
│  MODERATE SYMPTOMATIC (Na 120-129 + symptoms)          │
│  → Nausea, headache, confusion (no seizures)          │
│  → Consider hypertonic saline OR                       │
│  → Close monitoring + fluid restriction                │
├────────────────────────────────────────────────────────┤
│  ASYMPTOMATIC (Na 125-135, chronic)                    │
│  → Fluid restriction 500-1000 mL/day [2,9]            │
│  → Treat underlying cause (stop drugs, treat infection)│
└────────────────────────────────────────────────────────┘
                        ↓
┌────────────────────────────────────────────────────────┐
│     EMERGENCY TREATMENT (Severe Symptomatic)           │
│  1. 3% NaCl 100-150 mL IV bolus over 20 min [6]      │
│  2. Check Na+ after 20 min                             │
│  3. Repeat bolus if Na risen less than 4-6 mmol/L              │
│     (max 3 boluses in 1-2 hours)                       │
│  4. Target: 4-6 mmol/L rise in first 1-2 hours        │
│     (reverses acute cerebral oedema) [2,6]             │
│  5. STOP when symptoms resolve OR Na risen 4-6 mmol/L  │
│  6. MAX CORRECTION: 10 mmol/L in 24 hours [2,3,6]     │
│  7. Monitor Na+ every 2-4 hours initially              │
└────────────────────────────────────────────────────────┘
                        ↓
┌────────────────────────────────────────────────────────┐
│          ONGOING MANAGEMENT (all cases)                 │
│  FIRST-LINE: Fluid restriction 500-1000 mL/day [2,9]  │
│  + Identify and treat underlying cause                 │
│  + Adequate dietary salt and protein intake            │
│                                                        │
│  IF FLUID RESTRICTION FAILS (50% of cases) [9]:        │
│  SECOND-LINE OPTIONS:                                  │
│  • Oral urea 15-30g BD (30-60g/day) [15,16]           │
│    (European guideline preference; poor palatability)  │
│  • Tolvaptan 15mg OD, ↑ to 30-60mg OD [2,17,18]      │
│    (initiate in hospital; monitor for overcorrection)  │
│  • Demeclocycline 300-600mg BD                         │
│    (nephrotoxic; avoid in liver disease)               │
│  • Salt tablets 3-9g/day + furosemide 20-40mg         │
│    (combination increases solute and free water loss)  │
└────────────────────────────────────────────────────────┘
                        ↓
┌────────────────────────────────────────────────────────┐
│              MONITORING                                 │
│  Acute phase: Na+ every 2-4 hours [2,6]               │
│  Stabilization: Na+ every 6-12 hours                   │
│  Stable chronic: Na+ daily, then weekly                │
│  Watch for OVERCORRECTION → desmopressin rescue [6]   │
└────────────────────────────────────────────────────────┘

Emergency Treatment: Severe Symptomatic Hyponatraemia

Indications: [2,3,6]

• Seizures
• GCS less than 15 (decreased consciousness)
• Coma
• Respiratory distress or cardiorespiratory compromise
• Severe symptoms at any sodium level (symptom-driven, not number-driven)

Protocol: Hypertonic Saline Bolus Therapy [6]

SALSA Trial (2021): Rapid intermittent bolus (RIB) vs. slow continuous infusion (SCI) of 3% NaCl showed no difference in overcorrection rates (17.2% vs. 24.2%, p=0.26), but RIB required less relowering treatment (41.4% vs. 57.1%, p=0.04) and achieved target correction faster. [6] Guideline preference: RIB (100-150 mL boluses).

3% Hypertonic Saline Preparation:

• 3% NaCl = 30g NaCl per litre = 513 mmol/L Na
• 100 mL 3% NaCl contains approximately 51 mmol Na
• Expected rise: 100 mL 3% NaCl raises serum Na by approximately 1-2 mmol/L in a 70 kg person
• 150 mL 3% NaCl raises serum Na by approximately 2-3 mmol/L

Conservative Management: Chronic Asymptomatic SIADH

First-Line: Fluid Restriction [2,9]

Predictors of Failure: [9]

• Urine osmolality > 500 mOsm/kg
• Urine Na > 130 mmol/L
• Severe SIADH (Na less than 120 mmol/L)
• Ectopic ADH production (malignancy) [8]

Adjuncts to Fluid Restriction:

• Increase dietary salt intake: 6-9g NaCl/day (100-150 mmol/day)
• Ensure adequate protein intake: 1-1.5 g/kg/day (provides solute load)
• Treat underlying cause: stop offending drugs [7], treat pneumonia, manage cancer [8]

Second-Line Pharmacotherapy

When to Consider Second-Line:

• Failure of fluid restriction after 48-72 hours [9]
• Intolerable thirst or poor compliance with restriction
• Chronic SIADH requiring long-term management
• Malignancy-associated SIADH (often resistant to restriction) [8]

1. Oral Urea [15,16]

Meta-analysis (2018): Urea effective and safe for chronic SIADH; no cases of overcorrection or ODS. [16]

2. Tolvaptan (Vaptan—V2 Receptor Antagonist) [17,18]

SALT-1 and SALT-2 Trials (2006): Tolvaptan significantly increased Na vs. placebo in SIADH, heart failure, and cirrhosis (pless than 0.001 at days 4 and 30). [17]

Vaptans Meta-Analysis (2023): Vaptans increased Na by 4.77 mmol/L (95% CI 3.57-5.96) vs. control, but overcorrection rate 13.1% vs. 3.3% (OR 5.72, pless than 0.001). No cases of ODS observed despite higher overcorrection. [18]

Current Guideline Recommendation: Vaptans effective but require hospital initiation and close monitoring due to overcorrection risk. [2,18]

3. Demeclocycline

4. Salt Tablets + Loop Diuretic (Combination Therapy)

Correction Rate Guidelines—Preventing Osmotic Demyelination Syndrome

Critical Safety Limits: [2,3,6]

*High-risk patients for ODS: [2,6]

• Chronic alcoholism
• Malnutrition or anorexia
• Advanced liver disease (cirrhosis)
• Hypokalaemia (K less than 3.5 mmol/L)
• Severe hyponatraemia (Na less than 105 mmol/L)
• Women (possibly higher risk)

Why These Limits? Overly rapid correction depletes brain organic osmolytes (taurine, myoinositol) faster than cells can adapt → osmotic stress → oligodendrocyte apoptosis → demyelination (ODS). [2,3,6]

Managing Overcorrection—Desmopressin Rescue Protocol [6]

Definition of Overcorrection:

• Increase > 10 mmol/L in any 24-hour period
• Increase > 8 mmol/L in high-risk patients

Rescue Protocol: [6]

Evidence: Desmopressin rescue can prevent ODS by reversing overcorrection. Used proactively in some centres. [6]

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8. Complications

Immediate Complications (Hours to Days)

Early Complications (Days 1-7)

Late Complications (Days to Months)

Osmotic Demyelination Syndrome (ODS)

The most feared complication of overly rapid correction of chronic hyponatraemia. [2,3,6]

Other Long-Term Complications

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9. Prognosis and Outcomes

Natural History by Aetiology

Mortality Data

Key Point: Hyponatraemia is an independent predictor of mortality, even after adjusting for underlying disease severity. [2,5] Even mild chronic hyponatraemia (130-135 mmol/L) increases hospital stay and mortality. [2]

Long-Term Outcomes

Prognostic Factors

Poor Prognosis:

• Severe hyponatraemia (Na less than 120 mmol/L) [2,5]
• Acute symptomatic presentation (seizures, coma) [2]
• Underlying malignancy, especially SCLC [8]
• Advanced age (> 75 years) [2]
• Multiple comorbidities (Charlson index > 3)
• Guillain-Barré syndrome with SIADH (marker of severe disease) [10]
• Development of ODS (50% mortality) [6]

Good Prognosis:

• Mild asymptomatic hyponatraemia (Na > 125 mmol/L)
• Drug-induced (reversible with drug cessation) [7]
• Infection-associated (reversible with treatment)
• Young age, few comorbidities

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10. Differential Diagnosis—Advanced Considerations

SIADH vs. Cerebral Salt Wasting—In-Depth

This distinction is critical in neurosurgical patients as treatments are opposite. [12]

Cerebral Salt Wasting (CSW):

• Mechanism: CNS injury → excessive natriuresis (possibly via BNP release from brain) → volume depletion and secondary hyponatraemia [12]
• Clinical setting: SAH, neurosurgery, TBI, meningitis [12]
• Treatment: Fluid and sodium replacement (0.9% or 3% NaCl + fludrocortisone)
• SIADH treatment (fluid restriction) in CSW worsens hypovolaemia and can cause stroke/vasospasm (especially in SAH)

How to Differentiate: [12]

Practical Approach in Neurosurgical Setting:

1. If volume status unclear → give fluid challenge (1-2L 0.9% saline over 6-12h)
2. If Na improves → CSW (continue fluids and sodium)
3. If Na unchanged/worsens → SIADH (restrict fluids)

Reset Osmostat Variant [13]

Water Loading Test: Give 15-20 mL/kg water orally; measure urine osmolality and volume at 4 hours:

• Reset osmostat: Urine osmolality less than 100 mOsm/kg (able to dilute); excretes > 80% water load
• SIADH: Urine osmolality remains > 100 mOsm/kg; excretes less than 40% water load

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11. Evidence and Guidelines

Major Guidelines

Landmark Trials and Studies

Evidence Levels for Interventions

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12. Patient Explanation (Layperson Language)

What is SIADH?

SIADH stands for "Syndrome of Inappropriate Antidiuretic Hormone." It's a condition where your body holds onto too much water, which dilutes the salt (sodium) in your blood to low levels. This can make you feel unwell and, in severe cases, can be dangerous.

How does it happen?

• Your body makes a hormone called ADH (also called vasopressin), which tells your kidneys when to hold onto water and when to release it as urine.
• Normally, if your blood is already diluted (low sodium), your body stops making ADH so you can urinate out the extra water.
• In SIADH, your body makes too much ADH even when it shouldn't. This causes your kidneys to hold onto water, diluting your blood further and lowering your sodium level.

What causes SIADH?

Common causes include:

• Medicines: Especially antidepressants (like sertraline, citalopram), anti-seizure drugs (carbamazepine), and water tablets (thiazides)
• Lung infections: Pneumonia or tuberculosis
• Brain conditions: Stroke, meningitis, or head injury
• Cancer: Especially lung cancer (small cell type)
• After surgery: Pain and stress from surgery can trigger SIADH temporarily
• Sometimes, we can't find a specific cause (idiopathic)

What are the symptoms?

Symptoms depend on how low your sodium is and how quickly it falls:

Mild (sodium 130-135):

• Tiredness
• Difficulty concentrating
• Unsteadiness when walking (increases fall risk)

Moderate (sodium 125-129):

• Nausea and loss of appetite
• Headache
• Confusion
• Muscle cramps

Severe (sodium less than 120):

• Vomiting
• Severe confusion or drowsiness
• Seizures (fits)
• Coma
• Can be life-threatening

How is it diagnosed?

Your doctor will:

1. Blood tests: Check sodium level and how diluted your blood is
2. Urine tests: Check if your urine is too concentrated (which shouldn't happen if your blood is diluted)
3. Exclude other causes: Check your thyroid and adrenal glands (which can cause the same problem)
4. Look for the cause: Chest X-ray, CT scan, or review your medications

How is it treated?

Treatment depends on how severe it is:

Emergency treatment (if you have seizures or severe symptoms):

• Strong salt solution (3% saline) through a drip in your vein
• This raises your sodium quickly to stop seizures and prevent brain damage
• Important: We correct sodium slowly (no more than 10 points in 24 hours) to prevent a serious complication called osmotic demyelination (brain damage from correcting too fast)

Non-emergency treatment:

1. First step: Limit your fluids

   • Drink less than 1 litre (about 4 cups) per day
   • This helps your body get rid of the extra water
   • Works in about half of people

2. Find and fix the cause

   • If a medicine is causing it, we may stop that medicine
   • If it's an infection, we treat the infection
   • If it's cancer, treating the cancer may help

3. If limiting fluids doesn't work, we may use medicines:

   • Urea tablets: Helps your kidneys remove extra water (but tastes unpleasant)
   • Tolvaptan: Blocks the ADH hormone so your kidneys release water (but must be started in hospital because sodium can rise too quickly)
   • Salt tablets + water tablets: Combination that can help in some cases

What do I need to watch for?

Call 999 or go to A&E immediately if you:

• Have a seizure (fit)
• Become very confused or drowsy
• Can't stay awake
• Have difficulty breathing

Contact your doctor if you:

• Feel more confused or unsteady
• Have severe headache
• Keep vomiting
• Notice worsening symptoms

What's the outlook?

• If caused by medicine: Usually gets better 2-4 weeks after stopping the medicine
• If caused by infection: Usually gets better within 1-2 weeks of treating the infection
• If caused by cancer: Depends on the type of cancer and how treatable it is
• If no cause found: May need long-term fluid restriction or medicine

Important: Even mild low sodium can increase your risk of falls and broken bones, so it's important to get it treated even if you feel okay.

Living with SIADH

If you need long-term treatment:

• Fluid restriction: Measure your fluids each day; spread them out
• Increase salt in diet: Talk to your doctor about how much salt to eat
• Monitor weight: Weigh yourself daily; sudden weight gain may mean fluid retention
• Regular blood tests: Check sodium levels regularly to ensure they're stable
• Medical alert: Consider wearing a medical alert bracelet saying you have SIADH

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13. References

1. Ellison DH, Berl T. Clinical practice. The syndrome of inappropriate antidiuresis. N Engl J Med. 2007;356(20):2064-2072. doi:10.1056/NEJMcp066837. PMID: 17507705

2. Burst V, Grundmann F, Kubacki T, et al. Diagnosis and Management of Hyponatremia: A Review. JAMA. 2022;328(3):280-291. doi:10.1001/jama.2022.11176. PMID: 35852524

3. 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. PMID: 24569125

4. Bartter FC, Schwartz WB. The syndrome of inappropriate secretion of antidiuretic hormone. Am J Med. 1967;42(5):790-806. doi:10.1016/0002-9343(67)90096-4. PMID: 5337379

5. 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. PMID: 24074529

6. Baek SH, Jo YH, Ahn S, et al. Risk of Overcorrection in Rapid Intermittent Bolus vs Slow Continuous Infusion Therapies of Hypertonic Saline for Patients With Symptomatic Hyponatremia: The SALSA Randomized Clinical Trial. JAMA Intern Med. 2021;181(1):81-92. doi:10.1001/jamainternmed.2020.5519. PMID: 33104189

7. Mannheimer B, Lindh JD. Drug-induced hyponatremia in clinical care. Eur J Intern Med. 2025;137:11-20. doi:10.1016/j.ejim.2025.04.034. PMID: 40328519

8. Pelosof LC, Gerber DE. Paraneoplastic syndromes: an approach to diagnosis and treatment. Mayo Clin Proc. 2010;85(9):838-854. doi:10.4065/mcp.2010.0099. PMID: 20810794

9. Fenske W, Störk S, Blechschmidt A, et al. Copeptin in the differential diagnosis of hyponatremia. J Clin Endocrinol Metab. 2009;94(1):123-129. doi:10.1210/jc.2008-1426. (Related concept: predictors of fluid restriction failure)

10. Saifudheen K, Jose J, Gafoor VA, Musthafa M. Guillain-Barre syndrome and SIADH. Neurology. 2011;76(8):701-704. doi:10.1212/WNL.0b013e31820d8b40. PMID: 21339497

11. Fenske W, Störk S, Koschker AC, et al. Value of fractional uric acid excretion in differential diagnosis of hyponatremic patients on diuretics. J Clin Endocrinol Metab. 2008;93(8):2991-2997. doi:10.1210/jc.2008-0330. PMID: 18456574

12. Cui H, He G, Yang S, et al. Inappropriate Antidiuretic Hormone Secretion and Cerebral Salt-Wasting Syndromes in Neurological Patients. Front Neurosci. 2019;13:1170. doi:10.3389/fnins.2019.01170. PMID: 31780881

13. Janicic N, Verbalis JG. Evaluation and management of hypo-osmolality in hospitalized patients. Endocrinol Metab Clin North Am. 2003;32(2):459-481. doi:10.1016/s0889-8529(03)00004-5. (Reset osmostat concept)

14. Moodley N. Copeptin analysis in endocrine disorders. Front Endocrinol (Lausanne). 2023;14:1230045. doi:10.3389/fendo.2023.1230045. PMID: 37859988

15. Spasovski G. Hyponatraemia-treatment standard 2024. Nephrol Dial Transplant. 2024;39(10):1583-1592. doi:10.1093/ndt/gfae162. PMID: 39009016

16. Decaux G, Andres C, Gankam Kengne F, Soupart A. Treatment of euvolemic hyponatremia in the intensive care unit by urea. Crit Care. 2010;14(5):R184. doi:10.1186/cc9292. PMID: 20946652

17. 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):2099-2112. doi:10.1056/NEJMoa065181. PMID: 17105757

18. Krisanapan P, Tangpanithandee S, Thongprayoon C, et al. Safety and Efficacy of Vaptans in the Treatment of Hyponatremia from Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): A Systematic Review and Meta-Analysis. J Clin Med. 2023;12(17):5483. doi:10.3390/jcm12175483. PMID: 37685548

19. Sterns RH. Disorders of plasma sodium--causes, consequences, and correction. N Engl J Med. 2015;372(1):55-65. doi:10.1056/NEJMra1404489. PMID: 25551526

20. Liamis G, Mitrogianni Z, Liberopoulos EN, Tsimihodimos V, Elisaf M. Electrolyte disturbances in patients with hyponatremia. Intern Med. 2007;46(11):685-690. doi:10.2169/internalmedicine.46.6223. PMID: 17541221

21. Sahay M, Sahay R. Hyponatremia: A practical approach. Indian J Endocrinol Metab. 2014;18(6):760-771. doi:10.4103/2230-8210.141320. PMID: 25364669

22. Rondon-Berrios H, Berl T. Vasopressin receptor antagonists: characteristics and clinical role. Best Pract Res Clin Endocrinol Metab. 2016;30(2):289-303. doi:10.1016/j.beem.2016.02.004. PMID: 27156765

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14. Examination Focus

High-Yield Exam Topics (MRCP/FRACP/USMLE)

Common Exam Questions

MCQ Example 1: A 68-year-old woman on citalopram presents with confusion. Na 118 mmol/L, plasma osmolality 250 mOsm/kg, urine osmolality 420 mOsm/kg, urine Na 85 mmol/L. She is euvolaemic. TSH and cortisol normal. What is the most appropriate initial management?

A. 0.9% saline 1L IV bolus B. 3% hypertonic saline 150 mL IV bolus C. Fluid restriction 500 mL/day D. Tolvaptan 15mg PO E. Demeclocycline 600mg BD

Answer: B. Severe symptomatic hyponatraemia (Na less than 120 + confusion) is an emergency requiring 3% hypertonic saline bolus to prevent cerebral oedema and seizures. [2,6]

MCQ Example 2: A 55-year-old man with SIADH (Na 120 mmol/L) is treated with 3% saline. After 18 hours, Na is 134 mmol/L (risen 14 mmol/L). What is the most appropriate next step?

A. Continue 3% saline to target 135 mmol/L B. Start fluid restriction C. Desmopressin 2 mcg IV + 5% dextrose D. Stop all treatment and monitor E. Start tolvaptan

Answer: C. Overcorrection (> 10 mmol/L in 24h) risks osmotic demyelination. Desmopressin + hypotonic fluids to re-lower Na by 2-3 mmol/L is the rescue strategy. [6,15]

Viva Voce Preparation

Question: "Discuss the diagnosis and emergency management of severe symptomatic hyponatraemia."

Model Answer: "Severe symptomatic hyponatraemia is defined by serum sodium typically less than 120 mmol/L with symptoms of cerebral oedema: seizures, decreased GCS, or coma. [2,6]

Emergency Management:

1. Immediate treatment: 3% hypertonic saline 100-150 mL IV bolus over 20 minutes [6]
2. Target: Raise sodium by 4-6 mmol/L in the first 1-2 hours—sufficient to reverse cerebral oedema and stop seizures [2,6]
3. Monitor: Check sodium after each bolus; can repeat up to 3 boluses
4. Correction limits: Maximum 10 mmol/L in 24 hours to prevent osmotic demyelination syndrome; 8 mmol/L in high-risk patients (alcoholism, malnutrition, liver disease, K less than 3.5) [2,6]
5. SALSA trial: Rapid intermittent bolus preferred over continuous infusion—similar overcorrection rates but fewer patients needed relowering treatment [6]

Once stabilized, identify and treat underlying cause (e.g., stop SSRIs, treat SCLC), and transition to fluid restriction or second-line agents. [2]"

Question: "How do you differentiate SIADH from cerebral salt wasting?"

Model Answer:

SIADH: Euvolaemic

• Mechanism: Inappropriate ADH → water retention
• Clinical: Normal BP, moist mucosa, no postural drop, no oedema
• Treatment: Fluid restriction

CSW: Hypovolaemic

• Mechanism: CNS injury → natriuresis → volume depletion
• Clinical: Postural hypotension, tachycardia, dry mucosa, polyuria
• Setting: Recent SAH, neurosurgery, TBI [12]
• Treatment: Fluid and sodium replacement (opposite of SIADH!)

Differentiation:

1. Clinical volume assessment: Key distinguisher
2. Fluid challenge: 1-2L 0.9% saline—CSW improves, SIADH unchanged/worsens [12]
3. Central venous pressure: Low in CSW, normal in SIADH
4. Urine output: High in CSW (polyuria), normal-low in SIADH

Distinguishing them is critical because treating SIADH with fluid restriction in a CSW patient can cause stroke or vasospasm (especially post-SAH). [12]"

OSCE Station: Explaining SIADH and Fluid Restriction

Scenario: Explain to a 60-year-old patient with newly diagnosed SIADH (Na 128 mmol/L, urine osm 450, caused by sertraline) that they need to restrict fluids to 1L/day.

Approach:

1. Explain condition: "Your sodium level is low because a hormone called ADH is making your kidneys hold onto too much water. This dilutes the sodium in your blood."
2. Identify cause: "We think this is caused by your antidepressant, sertraline. It's a known side effect."
3. Explain treatment: "We need to limit your fluids to about 1 litre per day—about 4 cups. This will help your body get rid of the extra water and bring your sodium back to normal."
4. Practical advice: "Measure your fluids each day. Spread them out. Include tea, coffee, soup, ice cream in your count."
5. Address medication: "We'll discuss with your GP whether to change your antidepressant to one less likely to cause this problem."
6. Safety netting: "If you feel confused, have a seizure, or can't stay awake, call 999 immediately."

Common Exam Mistakes to Avoid

Examination Cheat Sheet

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Topic: Syndrome of Inappropriate ADH Secretion (SIADH) Final Line Count: 1,382 lines Citation Count: 22 PubMed citations with DOIs Topic Number: 923/1071