A 63-year-old woman is brought in after a witnessed episode of generalized shaking involving all limbs, followed by prolonged confusion. Her husband reports urinary incontinence during the episode. She was diagnosed with small-cell lung cancer 2 months earlier. Due to low mood she recently began a new antidepressant. Since then, she has felt tired, nauseated, and “foggy.”

Examination: Pulse 88/min, BP 118/72 mmHg, mucous membranes moist, no signs of dehydration.
Key labs:

Sodium 115 mmol/L
Low serum osmolality
High urine osmolality
High urine sodium
Cortisol and thyroid function normal

She is now post-ictal, confused, and symptomatic.

Which immediate treatment is most appropriate?

A. IV isotonic saline
B. Demeclocycline
C. Oral vaptan therapy
D. Restrict fluids to 1 L/day
E. IV hypertonic (3%) saline

Table of Contents

Correct answer: IV hypertonic (3%) saline

Detailed Explanation

This woman has severe symptomatic hyponatraemia: sodium 115 mmol/L plus a witnessed seizure. Her biochemical pattern—low serum osmolality combined with high urine osmolality and high urine sodium—strongly supports SIADH, most likely triggered by her lung cancer or the recent antidepressant. SIADH causes inappropriate retention of water, diluting sodium and leading to brain swelling.

Severe hyponatraemia with neurological symptoms is a medical emergency because the swollen brain may herniate. Hypertonic (3%) saline is the only treatment that can rapidly but safely raise sodium enough to reduce cerebral oedema. The recommended initial step is a 150 mL bolus, followed by rechecking sodium after 20–30 minutes. The immediate goal is an increase of 4–6 mmol/L, with a maximum of 10 mmol/L in 24 hours to avoid osmotic demyelination.

Fluid restriction is too slow for acute symptomatic cases. Demeclocycline and vaptans are for chronic SIADH that fails conservative measures. Isotonic saline can worsen hyponatraemia in SIADH due to continued renal sodium loss.

Thus, hypertonic saline is the safest and most appropriate first-line therapy.

Cheat Sheet: Severe Hyponatraemia (SIADH)

Severity

Mild: 130–134
Moderate: 120–129
Severe: <120

Symptoms

Early: confusion, headache, nausea
Late: seizures, coma, respiratory arrest

SIADH Pattern

Low serum osmolality
High urine osmolality (>100 mOsm/kg)
High urine sodium (>30 mmol/L)
Euvolemia clinically

Common Causes

Small-cell lung cancer
SSRIs, carbamazepine
CNS disorders
Pulmonary disease

Acute Severe Symptomatic Hyponatraemia Management

Give 3% hypertonic saline bolus (150 mL)
Monitor sodium closely
Aim ↑ 4–6 mmol/L immediately
Do NOT exceed 10 mmol/L in 24 h

Avoid

Isotonic saline (may worsen SIADH)
Fluid restriction alone
Demeclocycline/vaptans in acute cases

Complication of Overcorrection

Osmotic demyelination syndrome → irreversible neurological damage

Flashcards (20)

1. Q: What sodium level defines severe hyponatraemia?
A: <120 mmol/L.

2. Q: What dangerous symptom often appears in severe hyponatraemia?
A: Seizures.

3. Q: Immediate therapy for acute severe symptomatic hyponatraemia?
A: 3% hypertonic saline.

4. Q: Target sodium rise in initial phase?
A: 4–6 mmol/L.

5. Q: Maximum rise allowed in 24 hours?
A: 10 mmol/L.

6. Q: Classic malignancy causing SIADH?
A: Small-cell lung cancer.

7. Q: Is SIADH associated with hypo-, eu-, or hypervolemia?
A: Euvolemia.

8. Q: Serum osmolality in SIADH?
A: Low.

9. Q: Urine osmolality in SIADH?
A: High.

10. Q: Urine sodium in SIADH?
A: >30 mmol/L.

11. Q: Drug class commonly causing SIADH?
A: SSRIs.

12. Q: Why not use isotonic saline in SIADH?
A: Worsens hyponatraemia.

13. Q: Drugs for chronic refractory SIADH?
A: Demeclocycline or vaptans.

14. Q: Major risk of rapid sodium correction?
A: Osmotic demyelination.

15. Q: Symptom of osmotic demyelination?
A: Quadriparesis or dysarthria.

16. Q: What is the underlying mechanism of SIADH?
A: Excess ADH → water retention.

17. Q: What happens to serum sodium in SIADH?
A: Diluted.

18. Q: Can nausea be an early symptom of hyponatraemia?
A: Yes.

19. Q: Best monitoring frequency during hypertonic saline?
A: Every 20–30 minutes initially.

20. Q: What is the risk if sodium is not corrected at all?
A: Cerebral oedema, herniation, death.

MCQs (5)

1. A patient with sodium 116 mmol/L has a seizure. Best immediate treatment?

A. Fluid restriction
B. Isotonic saline
C. Demeclocycline
D. Vaptan
E. Hypertonic saline
Answer: E

2. SIADH features which lab combination?

A. High serum osmolality, low urine osmolality
B. Low serum osmolality, high urine osmolality
C. High sodium and low urine sodium
D. Hypervolemia with oedema
Answer: B

3. An antidepressant commonly implicated in SIADH:

A. Statins
B. SSRIs
C. Beta-blockers
D. Anti-histamines
Answer: B

4. Excessively rapid correction of sodium causes:

A. SIADH
B. Diabetes insipidus
C. Osmotic demyelination syndrome
D. Respiratory alkalosis
Answer: C

5. What is the safe limit for sodium increase over 24 hours?

A. 3 mmol/L
B. 6 mmol/L
C. 10 mmol/L
D. 20 mmol/L
Answer: C

Summary

This woman has acute, severe symptomatic hyponatraemia due to SIADH, triggered by small-cell lung cancer and possibly an SSRI. Her sodium is critically low at 115 mmol/L, and she has had a generalized seizure, indicating dangerous cerebral oedema. SIADH is characterised by low serum osmolality, high urine osmolality, and high urine sodium despite a clinically normal fluid status. In this emergency, the correct and lifesaving treatment is hypertonic 3% saline, given in controlled boluses to raise sodium by 4–6 mmol/L initially. Overcorrection must be avoided because rapid sodium rise can cause osmotic demyelination syndrome, resulting in irreversible neurological injury. Fluid restriction, demeclocycline, and vaptans are not appropriate during acute symptomatic episodes, as they act too slowly. Isotonic saline can worsen hyponatraemia in SIADH due to renal sodium loss. Management requires close monitoring, typically in high-dependency settings, with frequent sodium checks and strict avoidance of rapid shifts. Treatment of the underlying cause, such as cancer or medication triggers, is also essential for long-term control.

Hyponatremia