How do you diagnose and manage a patient with hyponatremia?

Diagnosis and Management of Hyponatremia: Case-Based Approach

Case 25: Patient with Na⁺ 118 mmol/L, Serum Osmolality 260 mOsm/kg, Urine Osmolality 550 mOsm/kg, Urine Na⁺ 45 mmol/L

This patient has SIADH (Syndrome of Inappropriate Antidiuretic Hormone secretion). 1

Diagnostic Reasoning

The laboratory findings definitively point to SIADH based on the following criteria 2, 1:

• Hypotonic hyponatremia: Serum osmolality <275 mOsm/kg (patient has 260) 2
• Inappropriately concentrated urine: Urine osmolality >100 mOsm/kg (patient has 550) 2, 3
• Elevated urinary sodium: >20-40 mEq/L (patient has 45) 3
• Normal thyroid and adrenal function: Rules out hypothyroidism and adrenal insufficiency 2
• Clinical euvolemia: Implied by the presentation 2

The key distinguishing feature is the inappropriately high urine osmolality (550 mOsm/kg) relative to the low serum osmolality (260 mOsm/kg), indicating the kidneys are retaining water when they should be excreting it 3. The elevated urine sodium (45 mmol/L) represents physiologic natriuresis occurring as the body attempts to maintain fluid balance despite water retention 3.

Common Pitfall

Do not confuse this with cerebral salt wasting (CSW), which would present with signs of hypovolemia (orthostatic hypotension, dry mucous membranes, decreased skin turgor) rather than euvolemia 2, 3. In neurosurgical patients, distinguishing between SIADH and CSW is critical because treatment approaches differ fundamentally 2, 1.

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Case 26: 70-Year-Old on Thiazide Diuretics with Confusion and Na⁺ 122

Immediately discontinue the thiazide diuretic and initiate treatment based on symptom severity. 1

Immediate Management Steps

For symptomatic hyponatremia with confusion 2, 1:

1. Stop the thiazide immediately 1
2. Assess volume status clinically: Look for orthostatic hypotension, dry mucous membranes, decreased skin turgor, or signs of volume overload 1, 3
3. Check urine sodium: A level <30 mmol/L suggests hypovolemia and predicts response to normal saline 1

Treatment Algorithm Based on Volume Status

If hypovolemic (most likely with thiazide-induced hyponatremia) 1:

• Administer 0.9% normal saline for volume repletion 2, 1
• Monitor sodium every 2-4 hours initially 1
• Do not exceed correction of 8 mmol/L in 24 hours 2, 1

If euvolemic or hypervolemic 1:

• Implement fluid restriction to 1 L/day 2, 1
• Consider oral sodium chloride supplementation (100 mEq three times daily) if no response to fluid restriction 1

Critical Correction Rate Guidelines

The maximum correction rate is 8 mmol/L per 24 hours 2, 1. For elderly patients with chronic hyponatremia, alcoholism, or malnutrition, use an even more cautious rate of 4-6 mmol/L per day 2, 1. Exceeding these rates risks osmotic demyelination syndrome, which can cause dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis 2.

Monitoring Protocol

• Every 2 hours during initial correction for symptomatic patients 1
• Every 4 hours after symptom resolution 1
• Watch for signs of osmotic demyelination syndrome, which typically appears 2-7 days after rapid correction 1

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Case 27: Post-Operative Patient with Na⁺ 125 and Seizures

This is a medical emergency requiring immediate administration of 3% hypertonic saline. 2, 1

Immediate Emergency Steps

Administer 3% hypertonic saline immediately 2, 1, 4:

1. Give 100 mL bolus of 3% saline over 10 minutes 1
2. Repeat up to three times at 10-minute intervals until seizures stop 1
3. Target correction of 6 mmol/L over 6 hours or until severe symptoms resolve 2, 1
4. Total correction must not exceed 8 mmol/L in 24 hours 2, 1

Calculation for Hypertonic Saline

Use the formula: Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg) to calculate sodium deficit 1. Initial infusion rate can be estimated as: body weight (kg) × desired rate of increase in sodium (mmol/L per hour) 5.

Critical Monitoring During Emergency Treatment

• Check sodium every 2 hours during active correction 1
• Admit to ICU for close monitoring 1
• Once severe symptoms resolve, slow correction rate and continue monitoring every 4 hours 1

Common Pitfall

The critical level for seizure development is approximately 120 mmol/L 2. However, do not delay treatment to pursue a complete diagnostic workup—treat the emergency first 6, 4.

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Case 28: Monitoring During Correction to Avoid Complications

Monitor serum sodium every 2 hours during initial correction, then every 4 hours after symptom resolution, with a strict limit of 8 mmol/L correction in 24 hours. 1

Monitoring Protocol by Severity

For severe symptomatic hyponatremia 1:

• Every 2 hours during initial correction
• Continue until symptoms resolve
• Then switch to every 4 hours monitoring

For moderate/asymptomatic hyponatremia 1:

• Every 4 hours initially
• Then daily once stable

Preventing Osmotic Demyelination Syndrome

Absolute correction limits 2, 1:

• Maximum 8 mmol/L in 24 hours for standard-risk patients
• Maximum 4-6 mmol/L per day for high-risk patients (advanced liver disease, alcoholism, malnutrition, prior encephalopathy)

If overcorrection occurs 1:

1. Immediately discontinue current fluids
2. Switch to D5W (5% dextrose in water) to relower sodium
3. Consider desmopressin to slow or reverse the rapid rise
4. Target bringing total 24-hour correction to no more than 8 mEq/L from starting point

Signs of Osmotic Demyelination Syndrome to Monitor

Watch for these symptoms appearing 2-7 days after rapid correction 2, 1:

• Dysarthria (difficulty speaking)
• Dysphagia (difficulty swallowing)
• Oculomotor dysfunction (eye movement problems)
• Quadriparesis (weakness in all four limbs)

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Case 29: Role of Loop Diuretics Combined with Saline

Loop diuretics combined with saline are used to manage chronic SIADH and edematous hypervolemic hyponatremia by promoting free water excretion while replacing sodium. 7, 5

Mechanism and Indications

Loop diuretics (furosemide) work by impairing urinary concentration, allowing excretion of free water while saline replaces sodium losses 7. This combination is particularly useful in:

• Chronic SIADH resistant to fluid restriction 2, 7
• Hypervolemic hyponatremia with volume overload (heart failure, cirrhosis) 7, 5
• Situations where fluid restriction alone is insufficient 7

Treatment Protocol

For chronic SIADH 2, 1:

1. Start with fluid restriction to 1 L/day as first-line
2. If inadequate response, add loop diuretic (furosemide)
3. Supplement with oral sodium chloride 100 mEq three times daily 1
4. Monitor sodium levels every 4 hours initially, then daily 1

Important Caveat

Do not use loop diuretics in hypovolemic hyponatremia until euvolemia is achieved, as they may worsen hypovolemia 1. In hypervolemic states, avoid hypertonic saline unless life-threatening symptoms are present, as it may worsen edema and ascites 1.

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Case 30: Exercise-Associated Hyponatremia and Psychogenic Polydipsia

For exercise-associated hyponatremia with severe symptoms, administer 3% hypertonic saline boluses; for psychogenic polydipsia, implement strict fluid restriction. 1, 6

Exercise-Associated Hyponatremia Management

For severe symptomatic cases 1:

• Administer 100 mL bolus of 3% saline over 10 minutes
• Repeat up to three times at 10-minute intervals until symptoms improve
• Target sodium increase of 4-6 mEq/L over 1-2 hours

For mild/asymptomatic cases 6:

• Stop fluid intake immediately
• Allow natural correction through renal water excretion
• Monitor sodium levels every 4 hours

Psychogenic Polydipsia Management

Primary treatment is strict fluid restriction 1, 5:

• Limit fluids to 1 L/day or less 2, 1
• Behavioral interventions to address compulsive water drinking
• Monitor for compliance with fluid restriction
• Check sodium levels daily initially

Key Distinguishing Features

Exercise-associated hyponatremia 6:

• Develops acutely (<48 hours) during or after prolonged exercise
• Caused by excessive free water intake during exercise combined with non-osmotic ADH release
• Common in marathon runners and endurance athletes

Psychogenic polydipsia 5:

• Chronic condition with compulsive water drinking
• Often seen in psychiatric patients
• Develops more gradually
• Urine osmolality typically <100 mOsm/kg (dilute urine)

Common Pitfall

Do not use hypertonic saline for asymptomatic exercise-associated hyponatremia—the body will correct this naturally with cessation of water intake 6. Reserve hypertonic saline only for severe symptoms (seizures, altered mental status, cardiorespiratory distress) 1, 4.