Confusion in an Elderly Man on a New Antidepressant
A 76-year-old man is brought in by his daughter with three days of worsening confusion and unsteadiness on his feet. He has hypertension treated with hydrochlorothiazide 25 mg daily and started sertraline 50 mg three weeks ago for low mood. He is afebrile, blood pressure 138/82 mmHg, pulse 88/min, no orthostatic drop, mucous membranes moist, no oedema, chest clear. He is drowsy but rousable and answers simple questions slowly; there is no focal neurology and no neck stiffness. Serum sodium 118 mmol/L, potassium 3.6 mmol/L, glucose 6.1 mmol/L, serum osmolality 246 mOsm/kg, urine osmolality 480 mOsm/kg, urine sodium 62 mmol/L. Creatinine 88 µmol/L, TSH and morning cortisol normal.
1. How do you classify this hyponatraemia, what is the likely mechanism, and what is your management in the first hour?
Read the worked answer
Classification. Serum osmolality is low (246) with an inappropriately concentrated urine (480) and high urine sodium (62) in a clinically euvolemic man - this is hypotonic, euvolemic hyponatraemia. Hyperglycaemia, hyperlipidaemia and pseudohyponatraemia are excluded; hypothyroidism and adrenal insufficiency are excluded by normal TSH and cortisol.
Mechanism. A thiazide plus an SSRI is a classic synergistic combination: thiazides impair free-water excretion and cause mild volume depletion that drives ADH release, while the SSRI increases ADH secretion. His confusion is far more likely dilutional hyponatraemia than a primary dementia or a new stroke - the tempo (three days) fits.
First hour.
- Stop the hydrochlorothiazide and sertraline now.
- Fluid restriction to about 800–1000 mL/day; no intravenous isotonic saline boluses (they can worsen hyponatraemia in SIADH by being excreted and worsening the urine-to-serum tonicity gradient).
- Do not give hypertonic saline to an asymptomatic patient at 118 - he is confused but not seizing, vomiting, or comatose.
- Check sodium every 4–6 hours initially; nurse with seizure precautions; look for and treat precipitants (pain, nausea, infection) that raise ADH.
- Set an explicit ceiling: no more than 8–10 mmol/L rise in the first 24 hours.
2. Two hours later he has a generalised tonic-clonic seizure and the repeat sodium is 116 mmol/L. What do you give, and what are your targets over 24 and 48 hours?
Read the worked answer
This is now severe symptomatic hyponatraemia and the risk calculus changes.
- Give a bolus of 3% hypertonic saline 100–150 mL over 10–20 minutes; repeat up to two or three times until the seizure stops or the sodium has risen by 4–6 mmol/L. The aim of the acute phase is to get out of the danger zone, not to normalise the value.
- Once symptoms have resolved, stop acute correction and reassess. Do not chase a 'normal' sodium.
- Limits: total rise ≤ 8–10 mmol/L in the first 24 hours and ≤ 18 mmol/L in 48 hours, to avoid osmotic demyelination - he has additional risk factors (age, possible malnutrition, chronic hyponatraemia of at least several days' duration).
- If you overshoot, give desmopressin 1–2 µg with free water (glucose 5%) to re-lower the sodium under close monitoring rather than simply observing.
- Move him to a high-dependency area, monitor sodium every 2–4 hours during active correction, and document the ceiling in the notes so the next shift does not over-correct.
3. Which medications will you stop and restart, and what safety-netting does this man and his daughter need at discharge?
Read the worked answer
Stop permanently: hydrochlorothiazide - thiazides are the commonest drug cause of severe hyponatraemia in older adults and rechallenge is unsafe. Switch his hypertension to a drug that does not impair water handling (e.g. amlodipine), and review whether his blood pressure even needs treating to target given his age and frailty.
Sertraline: stop it for now. If his mood still needs treating after the sodium has normalised, a lower-risk agent (e.g. mirtazapine) with a baseline and one-week sodium check is preferable to restarting an SSRI, and talking therapy should be offered. Document the reaction prominently.
Also review: NSAIDs (increase ADH effect and worsen renal handling), any desmopressin, and 'sick-day rules' - to seek review during vomiting, diarrhoea or acute illness.
Counselling points for the daughter:
- This was a drug interaction, not dementia, but confusion may take days to fully resolve.
- Recheck sodium at 1 week and after any medication change.
- Fluid restriction advice if a future episode occurs.
- Return immediately for vomiting, worsening confusion, drowsiness or a fit.
- Enquire who supervises his medicines, since the sertraline was presumably started without anyone reviewing the diuretic.