Severe Hyponatraemia with Confusion on a Thiazide
A 78-year-old woman is brought to the emergency department after two falls and increasing confusion over 3 days. She has hypertension and osteoarthritis and takes hydrochlorothiazide 25 mg daily, amlodipine 5 mg, and occasional ibuprofen. Her daughter says she has been less interested in food but has continued drinking water. On examination, she is drowsy but oriented to person only. BP is 132/78 mmHg, pulse 84/min, respiratory rate 16/min, and she has no focal neurological deficit. Mucous membranes are moist, there is no peripheral oedema, and jugular venous pressure is not raised. Blood tests: sodium 118 mmol/L, potassium 3.6 mmol/L, chloride 88 mmol/L, bicarbonate 24 mmol/L, glucose 5.6 mmol/L, creatinine 120 µmol/L (baseline 90), serum osmolality 252 mOsm/kg, urine osmolality 420 mOsm/kg, urine sodium 48 mmol/L. TSH and cortisol are normal. She has not had a seizure.
1. How would you classify the hyponatraemia and determine the likely cause?
Read the worked answer
This is severe hypotonic hyponatraemia: sodium is 118 mmol/L with a low serum osmolality and no hyperglycaemia or obvious hyperlipidaemia to explain it. The urine is concentrated at 420 mOsm/kg, which is inappropriate for hypotonic hyponatraemia, and urine sodium is high at 48 mmol/L. That pattern suggests either a thiazide effect or SIADH rather than simple water deprivation. Thiazides cause sodium wasting and impair free-water excretion, and the recent ibuprofen may further reduce renal water clearance. She is clinically euvolemic or mildly hypovolaemic, so the cause is likely thiazide-induced hyponatraemia with a possible SIADH-like component. Check for hypothyroidism and adrenal insufficiency, which are normal here. Review all drugs, including over-the-counter analgesics. Do not assume SIADH without assessing volume status, because treating true volume depletion as SIADH can worsen the problem.
2. What is the safe correction rate, and when would you use hypertonic saline?
Read the worked answer
She has severe symptomatic hyponatraemia with confusion and falls, but no seizure or coma. Correct slowly, generally by no more than 8–10 mmol/L in the first 24 hours, and less in patients at high risk of osmotic demyelination such as older adults, those with malnutrition, alcohol use, or liver disease. Use isotonic saline cautiously if she is volume depleted, but monitor sodium every 2–4 hours initially because thiazide-induced hyponatraemia can correct unpredictably once the diuretic is stopped and water intake changes. If she develops a seizure, reduced consciousness, or other severe neurological signs, give a small bolus of 3% hypertonic saline to raise sodium by about 4–6 mmol/L rapidly, then stop and resume slow correction. Do not aim for a normal sodium in the first day. Stop or slow fluids if the sodium rises too quickly.
3. What else would you do in the first 24 hours, and what would make you reconsider the diagnosis?
Read the worked answer
Stop hydrochlorothiazide and ibuprofen, and review whether amlodipine is needed. Treat pain and falls risk, monitor neurological status, and check sodium, potassium, and creatinine frequently. If she is volume depleted, cautious isotonic saline may be appropriate; if she is euvolemic and the urine remains concentrated, fluid restriction may be needed, but only after the volume status is clear. Reconsider the diagnosis if she develops hypotension, rising creatinine, or signs of volume overload, because the initial assessment of volume status can be wrong. Also reconsider SIADH if urine osmolality remains high despite correction of volume, or if there is an underlying pulmonary or central nervous system cause. The key judgement is to correct slowly while watching for overcorrection, because the brain injury from rapid correction can be worse than the presenting symptoms.