Confusion, Vomiting and a Sodium of 116 After Three Weeks of Thiazide
A 58-year-old woman is brought to the emergency department by her daughter with two days of vomiting, lethargy and new confusion. Her only new medication is hydrochlorothiazide 25 mg daily, started three weeks ago for hypertension; she takes nothing else and has no history of alcohol misuse, seizures or head injury. On examination she is drowsy but rousable and oriented to person only. BP 138/84 mmHg, pulse 88/min, JVP not raised, mucous membranes moist, no oedema, chest clear, no focal neurology. Serum sodium 116 mmol/L, potassium 3.4 mmol/L, chloride 88 mmol/L, bicarbonate 26 mmol/L, glucose 5.2 mmol/L, serum osmolality 248 mOsm/kg, urine osmolality 480 mOsm/kg, urine sodium 58 mmol/L. TSH and a 9 a.m. cortisol are normal. The thiazide was stopped in the department two hours ago and she has already received 500 mL of 0.9% saline. No seizure has been witnessed.
1. Classify the hyponatraemia by tonicity. Using the urine data, explain how the thiazide and the vomiting together are sufficient to explain it, and state the single most useful confirmatory manoeuvre at this point.
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Classification. Serum osmolality of 248 mOsm/kg with sodium 116 mmol/L is true hypotonic hyponatraemia; glucose is normal, so this is not translocational, and measured osmolality matches the calculated value, so there are no unmeasured osmoles (no mannitol, no alcohol).
Why the urine data localise the problem to ADH. Urine osmolality 480 mOsm/kg (well above 100) means ADH is acting and the kidney cannot excrete free water - the dilution defect is hormonal, not renal. Urine sodium 58 mmol/L (above 30) in a patient with moist mucosae and no oedema means she is not sodium-avid, so true volume depletion is not the dominant driver.
How the drug does it. Thiazides block the distal NaCl cotransporter, producing mild volume contraction that is below the threshold for visible signs but sufficient to trigger ADH release; at the same time they reduce the kidney's capacity to dilute urine. The two effects together cause water retention. Vomiting adds further ADH release and bicarbonate loss, and the resulting alkalosis is consistent with the bicarbonate of 26 with chloride 88.
Confirmation. A water-load test is neither safe nor necessary now that the drug has been stopped. The decisive manoeuvre is a supervised water restriction to under 1 L/day with paired serum and urine osmolality before and after: a brisk hypotonic diuresis with urine osmolality falling below 100 and sodium rising within 24 hours confirms thiazide-induced, ADH-driven hyponatraemia.
2. Write your correction plan for the first 24 hours: fluid choice, rate targets, monitoring interval, and the situations in which you would actively slow the correction.
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Is she severely symptomatic? Confusion counts as a severe symptom, so an immediate bolus is justified: 150 mL of 3% saline over 20 minutes, repeated up to three times until neurological improvement, aiming at a rise of 4-6 mmol/L in the first 6 hours and not more.
Once symptoms settle, stop the hypertonic saline and switch to conservative treatment: thiazide stopped (done), fluid restriction 800-1000 mL/day, oral sodium if she is not overloaded, and potassium replacement - correcting a potassium of 3.4 itself raises the sodium.
Ceiling. Total rise no more than 8-10 mmol/L in 24 hours and 18 mmol/L in 48 hours, because her hyponatraemia is chronic and she carries additional risk factors for osmotic demyelination (hypokalaemia, age, possible poor nutrition).
Monitoring. Sodium every 2 hours during active treatment, then every 4-6 hours for the first day, with strict fluid balance and daily weights.
Actively slow the correction if the sodium rises more than 6 mmol/L in any 6-hour window or approaches 10 mmol/L in 24 hours, or if a water diuresis begins - the practical tools are 5% dextrose plus desmopressin 1-2 µg to switch off the diuresis and clamp the sodium, then re-check hourly.
Do not give further 0.9% saline: it will not correct her, and it adds to the risk of overcorrection once the thiazide effect wears off.
3. Six hours later the repeat sodium is 128 mmol/L. Explain the mechanism, the specific hazard, and your management from this point.
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What went wrong. The sodium has risen 12 mmol/L in 6 hours - well beyond both the 4-6 mmol/L target and the 8-10 mmol/L daily ceiling. The mechanism is predictable: with the thiazide withdrawn, distal sodium delivery is restored, the ADH stimulus falls (vomiting has stopped, volume is being replaced), and a water diuresis begins. The 500 mL of saline contributed little; the auto-correction did the rest.
The hazard is osmotic demyelination (central pontine myelinolysis), which appears days later with dysarthria, dysphagia, spastic quadriparesis and sometimes a locked-in state. Her risk is increased by hypokalaemia, age and probable poor recent intake. A rapidly rising sodium is therefore not good news - it is the emergency.
Management. Stop all hypertonic saline. Re-lower the sodium to no more than 8 mmol/L above baseline - back towards 124 mmol/L - using 5% dextrose with desmopressin 1-2 µg IV or subcutaneous to abolish the water diuresis and clamp the value. Replace potassium. Check sodium hourly until stable for 24 hours, then 4-hourly for a day, and review neurology daily for a week; document the whole sequence. If she develops new neurological signs, urgent MRI and discussion with nephrology or critical care are warranted. Reassess the original diagnosis: if she later needs an antihypertensive, thiazides should not be restarted.