Confusion and Hyponatraemia Three Weeks After Starting a Thiazide
A 74-year-old woman is brought to the emergency department after falling at home. Her daughter reports three days of worsening confusion and lethargy. Hydrochlorothiazide 25 mg daily was started three weeks ago for newly diagnosed hypertension; she takes no other diuretic, no NSAIDs, and no antidepressants. She drinks about 1.5 L of water a day and has had no vomiting or diarrhoea. Blood pressure is 106/62 mmHg supine and 94/58 mmHg standing, pulse 96/min, mucous membranes dry, no oedema, no goitre. Serum sodium 116 mmol/L, potassium 3.1 mmol/L, chloride 84 mmol/L, bicarbonate 26 mmol/L, urea 5.0 mmol/L (BUN 14 mg/dL), creatinine 64 micromol/L (0.72 mg/dL), glucose 96 mg/dL, serum osmolality 248 mOsm/kg, urine osmolality 342 mOsm/kg, spot urine sodium 48 mmol/L. TSH and free T4 are normal. She is oriented to person only and cannot complete a five-item recall.
1. The urine sodium is 48 mmol/L. Explain how this result should and should not be used to separate SIADH from thiazide-associated hyponatraemia here, and state which findings carry the most weight.
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A urine sodium above 40 mmol/L is often taught as a fingerprint of SIADH, but in this patient it is close to uninformative. A thiazide blocks the diluting segment of the distal tubule, so urine stays concentrated and sodium-rich whether the patient is euvolemic or frankly depleted; urine osmolality of 342 mOsm/kg and urine sodium of 48 mmol/L are entirely compatible with thiazide effect alone. Strictly, SIADH cannot be diagnosed while a thiazide is on board, because the drug itself causes ADH-dependent water retention and natriuresis. Weight the clinical assessment instead: orthostatic drop of 12 mmHg, dry mucous membranes, and a low-normal urea with a low creatinine suggest reduced effective circulating volume and low muscle mass rather than euvolemia. If uncertainty persisted, a fractional excretion of urate below 12% would favour volume depletion, whereas a low serum uric acid with high FE urate favours SIADH. Practically, the label changes little acutely. It changes a great deal at discharge: fluid restriction, the reflex treatment for SIADH, can worsen thiazide-induced hyponatraemia, and the drug must never be restarted.
2. Outline your immediate management, including the target rate of correction and what you would do if the sodium rises too quickly.
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Symptomatic hyponatraemia with confusion justifies hypertonic saline regardless of the underlying label. Give 3% saline 100-150 mL as an intravenous bolus over 10-20 minutes, aiming for a 4-6 mmol/L rise, and repeat once or twice (maximum about 300 mL) until confusion improves or the sodium has risen roughly 5 mmol/L; then stop active correction and reassess. Do not chase a normal sodium. Because the hyponatraemia has been present for at least three weeks and she is small and elderly, cap the rise at 8 mmol/L in 24 hours and 8 mmol/L at 48 hours to limit osmotic demyelination risk. Replace potassium concurrently, since potassium repletion itself raises the sodium. Measure sodium every 2-4 hours during active therapy and watch urine output. If the sodium overshoots, give desmopressin 1-2 micrograms intravenously with 5% dextrose and free water to re-lower it gradually. Monitor for delayed dysarthria, ataxia or quadriparesis, and stop hydrochlorothiazide permanently.
3. What would you document and arrange at discharge to prevent a recurrence, and what would you tell the patient and daughter?
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Record a clear adverse drug reaction entry for thiazides, not merely 'intolerant', because rechallenge carries a high risk of recurrent severe hyponatraemia, often within days. Choose an alternative antihypertensive such as a dihydropyridine calcium channel blocker or an ACE inhibitor, starting at low dose, and check sodium and potassium within 1-2 weeks of any new agent, particularly if she is later started on an SSRI, carbamazepine, or a second diuretic. Note that her age, female sex, low body weight and low baseline sodium are recognised risk factors, so the threshold for monitoring should stay low. Counsel her to seek review for confusion, nausea, headache or unsteadiness and to avoid large fluid boluses and low-sodium diets without advice. Document the fall and the presenting sodium so future clinicians can gauge severity. Consider a falls assessment and bone health review, since the presentation was a fall and the sodium was likely falling over several days.