Seizure and Severe Hyponatremia After Starting a Thiazide
A 72-year-old woman is brought to the emergency department after a witnessed generalized tonic-clonic seizure lasting about 60 seconds. She was well until 10 days ago, when hydrochlorothiazide 25 mg daily was started for hypertension. For the past 3 days she has had nausea and poor oral intake but no diarrhea. She is confused (GCS 14), afebrile, pulse 88/min, BP 138/76 mmHg, with no jugular venous distension, no edema and moist mucous membranes. Serum sodium is 116 mmol/L, potassium 3.4 mmol/L, chloride 84 mmol/L and bicarbonate 22 mmol/L; glucose 6.1 mmol/L, urea 3.8 mmol/L, creatinine 0.8 mg/dL. Serum osmolality is 248 mOsm/kg, urine osmolality 520 mOsm/kg and urine sodium 62 mmol/L. TSH and morning cortisol are normal. She takes no other drugs and has no history of seizures.
1. How would you classify the hyponatremia and identify its cause in this patient?
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The serum osmolality of 248 mOsm/kg confirms true hypo-osmolar hyponatremia; hyperglycemia and pseudohyponatremia are excluded. Clinically she is euvolemic - no edema, no raised JVP, moist mucosae and a normal urea - which argues against heart failure, cirrhosis or marked volume depletion. A urine osmolality of 520 mOsm/kg shows that ADH is active and free water is not being excreted, and a urine sodium of 62 mmol/L shows ongoing renal sodium loss. Normal TSH and cortisol exclude hypothyroidism and adrenal insufficiency. The likely diagnosis is thiazide-induced hyponatremia with SIADH-like physiology: thiazides cause sodium wasting and impair dilution, while nausea and poor intake provide a potent ADH stimulus. The seizure makes this severe symptomatic hyponatremia and a medical emergency, not an outpatient problem.
2. Describe the immediate treatment, including the fluid, the target rise and the monitoring plan.
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Give 3% hypertonic saline 100–150 mL IV over 10–20 minutes as a bolus, aiming to raise the sodium by 4–6 mmol/L to abort the seizure and reverse cerebral edema. The bolus can be repeated up to three times if symptoms persist, checking the sodium between boluses. If seizures continue, treat with a benzodiazepine and protect the airway.
Once symptoms improve, stop the bolus strategy and limit correction to ≤8 mmol/L in 24 hours and ≤18 mmol/L in 48 hours. Check the sodium every 2–4 hours initially. Stop hydrochlorothiazide. Treat the hypokalemia, because potassium replacement itself raises the sodium and can contribute to overcorrection. If the sodium rises faster than planned, give desmopressin 1–2 micrograms and 5% dextrose with free water to bring it back down. The goal is a controlled rise, not a normal sodium.
3. Why is rapid correction dangerous, and what would you do if the sodium overcorrected?
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This hyponatremia is probably chronic - it has evolved over at least several days and possibly longer - so brain cells have adapted by extruding osmolytes. A rapid rise in extracellular tonicity shrinks them and can cause osmotic demyelination, presenting days later with dysarthria, quadriparesis, pseudobulbar palsy or "locked-in" syndrome. Risk is higher with malnutrition, alcohol use, liver disease and hypokalemia.
If the sodium rises more than 8 mmol/L in 24 hours or faster than intended, intervene promptly: give desmopressin 1–2 micrograms IV/subcutaneously to stop free water excretion, and give 5% dextrose with or without free water to re-lower the sodium to the planned trajectory. Recheck frequently. If osmotic demyelination is suspected later, urgent relowering may still help. After stabilization, review the cause, avoid thiazides permanently, and document the reaction; fluid restriction is appropriate only if SIADH persists after the drug is withdrawn.