Severe Hyponatraemia on a Thiazide: Correction and Overcorrection
Mrs B is a 74-year-old woman with hypertension, osteoarthritis and depression. Her general practitioner doubled hydrochlorothiazide from 25 mg to 50 mg daily 12 days ago for poorly controlled blood pressure. She presents with 3 days of nausea, unsteadiness and new confusion; her daughter reports she has been drinking about 1.5 L of water daily and had a mild viral upper respiratory illness last week. She is afebrile, pulse 84, blood pressure 118/70 mmHg with no postural drop, JVP not raised, no oedema, mucous membranes moist, and weight stable. She takes sertraline and ibuprofen as needed. Serum sodium 116 mmol/L, osmolality 246 mOsm/kg, glucose 6.1 mmol/L, urine osmolality 480 mOsm/kg, urine sodium 62 mmol/L, and TSH and 9 am cortisol are normal. She is drowsy but rousable, obeys commands, has no seizure, and is not vomiting.
1. How do you interpret the urine osmolality and urine sodium in this patient, and what is your working diagnosis?
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Urine indices must be read in context.
- Urine osmolality 480 mOsm/kg indicates that antidiuretic hormone is active and the kidney is concentrating. That is expected with thiazides, nausea, pain and any effective circulating volume depletion, so it does not by itself prove SIADH.
- Urine sodium 62 mmol/L is unhelpful for classifying the cause while she is taking a thiazide, because the drug itself causes natriuresis. A urine sodium above 30–40 mmol/L therefore cannot be used to label this as SIADH.
- The working diagnosis is thiazide-induced hyponatraemia, precipitated by the recent dose increase, with possible contributions from sertraline, nausea and low solute intake. Her euvolemic examination, normal TSH and cortisol, and normal glucose make hypothyroidism, adrenal insufficiency and hyperglycaemic dilution unlikely.
The correct next step is to stop hydrochlorothiazide and review sertraline and ibuprofen, not to order more urine tests. The trap is applying the classic SIADH criteria - euvolemia, high urine osmolality, urine sodium above 40 - while the patient is still on the causative drug.
2. She is drowsy but not seizing. Outline your correction plan, including target rate and how you would prevent overcorrection.
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She is symptomatic but not having a seizure, so the plan differs from catastrophic hyponatraemia.
- If drowsiness is judged severe (vomiting, seizures, coma, cardiorespiratory distress), give 150 mL of 3% sodium chloride over 20 minutes and repeat once if needed, aiming for a 5 mmol/L rise to relieve symptoms.
- If not severe, stop the thiazide, restrict fluid to 800–1000 mL/day, and monitor sodium every 4–6 hours. Hypertonic saline is not mandatory for confusion alone.
- Correction ceiling: no more than 8–10 mmol/L in the first 24 hours and 18 mmol/L in 48 hours for chronic hyponatraemia, because osmotic demyelination is the main iatrogenic danger.
- Preventing overcorrection: thiazide withdrawal can produce a sudden water diuresis and a rapid rise. Monitor urine output and urine osmolality every 2 hours; if sodium rises above the ceiling, give desmopressin 1–2 micrograms subcutaneously every 6–8 hours plus 5% dextrose to re-lower it, and involve nephrology or ICU.
Document the target, the monitoring interval and the rescue plan before leaving the bedside.
3. She has a generalised tonic-clonic seizure 40 minutes after arrival. What changes?
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A generalised seizure converts this into a neurological emergency.
- Give 150 mL of 3% sodium chloride over 20 minutes immediately, and repeat up to three times (total 450 mL) until the seizure stops or sodium has risen by about 5 mmol/L. Do not wait for a repeat laboratory result.
- Protect the airway, give oxygen, check bedside glucose, and treat the seizure with a benzodiazepine if it continues after the sodium bolus.
- After the acute rise, stop active correction and revert to the chronic ceiling of 8–10 mmol/L in 24 hours. The acute 5 mmol/L is a one-off rescue, not a licence to correct aggressively.
- Admit to a high-dependency or ICU setting, monitor sodium every 2 hours, and have desmopressin ready if the rise overshoots.
- Look for a precipitant: the thiazide has been stopped, but check for vomiting, poor intake, and any new medication. Imaging is only needed if the seizure is atypical or focal.
The key judgement is that the symptomatic rescue target and the 24-hour correction ceiling are different numbers, and confusing them is how osmotic demyelination occurs.