Drowsiness after a fall in an older woman with profound hyponatraemia
A 74-year-old woman is brought to the emergency department after her daughter found her drowsy on the bedroom floor; she had fallen and could not get up for an unknown period. She has hypertension, depression and osteoarthritis. Regular medicines are hydrochlorothiazide 25 mg daily, started six weeks ago; citalopram 20 mg daily, started three weeks ago; ramipril 5 mg daily; and paracetamol as required. She is confused but rousable and has no focal neurological deficit. Blood pressure 118/68 mmHg, pulse 84 beats/min regular, temperature 36.4 °C, oxygen saturation 96% on air. Mucous membranes are dry but there is no postural drop; jugular venous pressure is not raised and there is no peripheral oedema. Investigations: sodium 116 mmol/L, potassium 3.9 mmol/L, urea 3.2 mmol/L, creatinine 72 µmol/L, glucose 6.1 mmol/L, serum osmolality 246 mOsm/kg, urine osmolality 512 mOsm/kg, urine sodium 68 mmol/L. Non-contrast CT of the head is normal.
1. How would you classify the hyponatraemia and what is the most likely cause? Which additional tests would you request?
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Classification
- Serum osmolality 246 mOsm/kg (<275) with a normal glucose makes this hypotonic hyponatraemia; the measured osmolality excludes pseudohyponatraemia and hyperglycaemic dilution.
- Urine osmolality 512 mOsm/kg (>100) means ADH is active - the kidney is not simply excreting free water.
- Urine sodium 68 mmol/L (>30) with a clinically euvolemic patient (no oedema, no postural drop, JVP not raised, urea low at 3.2) points to SIADH rather than hypovolaemia or hypervolaemia.
Most likely cause
Two drugs started close together are the obvious culprits: thiazide diuretic and citalopram, both classic causes of SIADH, often within the first weeks of therapy. The low urea and normal creatinine fit a euvolemic, vasopressin-driven picture rather than dehydration alone.
Tests to confirm and exclude alternatives
- TSH and free T4 (hypothyroidism).
- 9 am cortisol, with short synacthen if low or if there is any hypotension, hyperkalaemia or hypoglycaemia (adrenal insufficiency).
- Repeat sodium and osmolality, plus urine studies, to confirm persistence.
- Only if drug causes are excluded and the picture does not resolve: consider chest imaging and malignancy screen - do not launch an unfocused tumour hunt before withdrawing the likely precipitants.
2. What are your targets for correcting the sodium and how would you achieve them over the first 24 hours? What would you do if the sodium had risen by 12 mmol/L after 12 hours?
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Assess for emergency indications first. She is drowsy and confused but is not seizing and is not comatose. That is moderate, not severe, symptomatology, so hypertonic saline is not automatically indicated. Reserve 100–150 mL of 3% saline over 10–20 minutes (repeated up to three times) for seizures, coma or clear deterioration.
Usual approach
- Stop hydrochlorothiazide and citalopram immediately.
- Fluid restriction of roughly 800–1000 mL/day, with daily weights and strict fluid balance.
- Treat any pain, nausea or sepsis that is stimulating ADH.
- Check sodium every 2 hours initially, then 4–6 hourly once stable.
Correction targets
- If symptomatic, a rise of 4–6 mmol/L in the first 6 hours is acceptable.
- Maximum rise of 8–10 mmol/L in 24 hours in a chronic, likely malnourished older patient; some guidelines are even more cautious (≤8 mmol/L).
- She is at modest-to-high risk of osmotic demyelination given chronicity, age and likely poor intake.
If sodium has risen 12 mmol/L by 12 hours
This is overcorrection. Stop any active correction, give 5% dextrose with or without desmopressin 1–2 µg to re-lower the sodium to within the safe target, involve a senior physician or nephrologist, and continue frequent monitoring for at least 24–48 hours. Watch for delayed dysarthria, ataxia, quadriparesis or reduced consciousness.
3. What issues would you address before discharge, particularly in relation to her fall and future risk?
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Medication review - the core intervention
- Do not restart hydrochlorothiazide; document thiazide-induced hyponatraemia clearly.
- Reassess citalopram: it is both a hyponatraemia precipitant and a falls/fracture risk factor. Depending on the indication and mood, consider stopping, or switching to an agent with a lower SIADH signal while monitoring sodium.
- Review ramipril and any antihypertensives against lying/standing blood pressures, aiming to avoid orthostatic hypotension.
- Avoid sedatives, benzodiazepines, Z-drugs and anticholinergics.
Falls and bone health
- A multifactorial falls assessment: vision, gait and balance, footwear, home hazards, syncope screen and ECG.
- She has already fallen and has a fragility fracture risk profile - arrange DEXA, calcium and vitamin D, and consider bone protection.
- Refer to physiotherapy and occupational therapy; consider a falls-prevention programme.
Safety-netting
Explain the diagnosis to her and her daughter, advise reducing free fluid intake if instructed, and arrange a repeat sodium within a week of any medication change. Communicate the drug-related diagnosis to the GP and flag it prominently in the discharge summary so it is not re-prescribed.