Sodium Disorders — Written Clinical Reasoning

SAQ 1 — Severe Thiazide-Induced Hyponatraemia with Hypokalaemia (20 marks, 30 minutes)

Prompt: Outline your immediate and integrated management of this patient, including the diagnostic reasoning, the correction strategy with exact sodium targets and monitoring, the management of the hypokalaemia, and the long-term plan. Justify each decision with reference to the competing risks. [1]

Model Answer

Step 1 — Diagnostic synthesis (3 marks): [1]

This is moderately severe to severe hypotonic hyponatraemia in a hypovolaemic elderly woman, almost certainly thiazide-induced, with coexistent hypokalaemia (a risk factor for osmotic demyelination syndrome) and cerebral symptoms (drowsiness, confusion, a fall). The evidence: serum osmolality 248 confirms true hypotonic hyponatraemia (rules out pseudo- and hypertonic causes); urine osmolality 380 confirms vasopressin is active (rules out primary polydipsia and low-solute intake); urine sodium 48 in a clinically volume-depleted patient indicates a renal sodium-wasting mechanism — the thiazide — rather than extra-renal loss (which would give urine sodium under 20); the postural drop, low JVP, and raised urea confirm hypovolaemia. TSH and cortisol are normal, excluding the SIADH mimics. The thiazide was started only 3 weeks ago, fitting the typical onset window. The hypokalaemia compounds the sodium problem: intracellular potassium depletion drives sodium into cells, worsening the measured hyponatraemia, and hypokalaemia independently raises the risk of osmotic demyelination when the sodium is corrected. [1]

Problem list:

1. Severe hypotonic hyponatraemia (Na 116) with cerebral symptoms — cerebral oedema risk vs ODS risk
2. Thiazide-induced — drug is the cause
3. Hypovolaemia with renal salt wasting
4. Hypokalaemia (K 2.9) — a correctable contributor to the hyponatraemia and an ODS risk factor
5. Underlying mild cognitive impairment with a fall — assess for injury, delirium, and reversible contributors
6. Polypharmacy and the need for an alternative antihypertensive
7. Osteoporosis with a fall risk [1]

Step 2 — Immediate management: stop the cause, correct potassium, replete volume cautiously (5 marks): [1]

• Stop the hydrochlorothiazide immediately and permanently. Rechallenge is dangerous; she must never take a thiazide again.
• Admit to a monitored bed (HDU given her GCS and the correction risk).
• Replace potassium with intravenous potassium chloride, aiming to bring K above 4.0 — this both corrects the deficit and will itself raise the serum sodium as potassium moves back into cells and sodium moves out.
• Replete volume cautiously with 0.9% saline. Because she is hypovolaemic, isotonic saline will suppress the non-osmotic vasopressin drive and permit free water excretion. The danger is that once vasopressin is suppressed, an autonomous water diuresis can overshoot — the sodium may rise too fast. [1]

Step 3 — The correction strategy: targets, monitoring, and the overcorrection safeguard (6 marks): [1]

The competing risks: the cerebral symptoms (GCS 13, confusion, a fall) argue for prompt correction to relieve cerebral oedema, but the chronicity is uncertain (5 days of symptoms, so likely chronic with an acute component) and the hypokalaemia raises the ODS risk, arguing for caution. My targets: [1]

• First 4 hours: if her conscious state deteriorates (seizure, GCS falls further), give 3% hypertonic saline 100 mL over 10 minutes, repeat up to a maximum of three boluses, aiming for a 4 to 5 mmol/L rise.
• 24-hour ceiling: maximum rise 8 mmol/L (I will target the lower end — 8 rather than 10 — because of the hypokalaemia and her age). So her 24-hour target is a sodium of no more than 124.
• 48-hour ceiling: no more than 18 mmol/L from baseline.
• Monitoring: serum sodium every 2 to 4 hours for the first 24 hours, strict fluid balance, daily weights.
• Overcorrection safeguard: have desmopressin (DDAVP) 1 to 2 micrograms subcutaneously drawn up and available. If the sodium rises above the ceiling, give DDAVP with 5% dextrose to relower it — this recreates a controlled hyponatraemia and is the standard salvage manoeuvre. [1]

The key teaching point: thiazide hyponatraemia overcorrects most often because stopping the drug triggers a brisk water diuresis. Anticipate it, monitor frequently, and relower if it happens [2][6].

Step 4 — Confirm the diagnosis and exclude mimics formally (2 marks): [1]

Although thiazide-induced hyponatraemia is the leading diagnosis, I would also send:

• A short Synacthen test if the morning cortisol is borderline (hers is 480, which is normal and excludes adrenal insufficiency in this acute setting, but a borderline result would warrant confirmation).
• A repeat TSH now that she is unwell.
• A urine and serum osmolality pair to document the response to treatment.
• A CT brain if her conscious state does not improve with correction, to exclude a structural cause for the fall and confusion. [1]

Step 5 — Long-term management (4 marks): [1]

• Hypertension: switch from the thiazide to an ACE inhibitor or a calcium channel blocker, with the SGLT2 inhibitor class now also relevant given her age and cardiovascular risk. Reassess her blood pressure targets in light of the fall (postural hypotension contributed).
• Falls workup: orthostatic blood pressures, ECG, review of all medications for sedating or hypotensive agents, vision, gait, home safety assessment.
• Cognitive assessment: distinguish delirium (reversible, from the hyponatraemia and the fall) from her underlying mild cognitive impairment; reassess once the sodium has corrected.
• Osteoporosis and the fall: check for a fracture (the fall may have caused an occult injury); optimise calcium, vitamin D, and osteoporosis therapy.
• Patient and family communication: explain that the thiazide caused the problem, that she must avoid it permanently, that the sodium will be corrected slowly to protect her brain, and provide written sick-day and medication guidance.
• GP letter documenting the thiazide adverse reaction and the alternative antihypertensive plan. [1]

SAQ 2 — Hypernatraemia in the Intubated Patient: Free Water Deficit and Diabetes Insipidus (10 marks, 20 minutes)

Prompt: A 55-year-old, 70 kg man is intubated in ICU 3 days after a traumatic brain injury with a base-of-skull fracture. His serum sodium has risen from 142 to 160 mmol/L over 24 hours. Urine output is 5 litres per day; urine osmolality is 160 mOsm/kg. Calculate his free water deficit and outline your diagnostic and management plan. [1]

Model Answer

This is hypernatraemia from renal water loss — the inappropriately dilute urine (160 mOsm/kg) in the face of a serum sodium of 160 indicates either central or nephrogenic diabetes insipidus. The traumatic brain injury makes central DI the leading diagnosis. [1]

Step 1 — Calculate the free water deficit (2 marks): [1]

Water deficit = TBW x (serum Na / 140 − 1), where TBW = weight x fraction (0.6 for an adult man). [1]

• TBW = 70 x 0.6 = 42 litres.
• Deficit = 42 x (160/140 − 1) = 42 x 0.143 = 6 litres of free water. [1]

But this is the deficit to a sodium of 140; I do NOT correct to 140 all at once. I correct slowly — no more than 10 mmol/L in 24 hours, over 48 to 72 hours — to avoid cerebral oedema. And I must add ongoing losses: he is losing 5 litres of dilute urine per day, which must be replaced as it occurs [4].

Step 2 — Confirm central versus nephrogenic DI (2 marks): [1]

• Give desmopressin (DDAVP) 1 to 2 micrograms intravenously or subcutaneously and measure urine osmolality over 2 hours. A rise to above 700 to 750 mOsm/kg confirms central DI (the kidney can concentrate when given vasopressin); no rise confirms nephrogenic DI.
• In this context (traumatic brain injury, base-of-skull fracture), central DI from posterior pituitary stalk injury is overwhelmingly likely, and the desmopressin test will confirm it.
• A plasma copeptin (if available) can be measured — a low copeptin with hypernatraemia and dilute urine supports central DI. [1]

Step 3 — Management plan (4 marks): [1]

• Replace the free water deficit slowly over 48 to 72 hours. Preferred route is entereral water via the nasogastric tube (safest, most physiological); if IV is needed, use 5% dextrose (D5W) — not normal saline, which would not lower the sodium. Aim for a sodium fall of no more than 10 mmol/L in 24 hours (0.5 mmol/L per hour).
• Replace ongoing urine losses hourly with an equivalent volume of water or D5W; set up a urine output-to-replacement protocol.
• Start desmopressin once central DI is confirmed — DDAVP 1 to 2 micrograms IV/SC every 8 to 12 hours, titrated to urine output. Use the lowest dose that controls polyuria; overshoot causes hyponatraemia.
• Monitor serum sodium every 4 to 6 hours, strict fluid balance, daily weights, urine osmolality and electrolytes.
• Anticipate the triphasic response if this is surgical or traumatic pituitary injury: an initial central DI phase (now), a SIADH phase in days 4 to 10 from unregulated release of stored vasopressin, then permanent DI. Stop DDAVP and fluid-restrict if the sodium falls in the second week. [1]

Step 4 — Communicate and document (2 marks): [1]

• Document the calculated deficit, the target correction rate, the desmopressin test result, and the monitoring plan in the chart.
• Brief the nursing team on the hourly urine-output replacement and the sodium-check frequency.
• Discuss with the family the mechanism (brain injury disrupting the vasopressin system), the reversible versus permanent nature of the DI (often temporary in traumatic injury), and the prognosis.
• Coordinate with the neurosurgical and endocrine teams for long-term follow-up and to assess for other pituitary deficits. [1]

The key teaching point: hypernatraemia in the intubated patient is almost always a failure of free water provision combined with a concentrating defect — audit the fluid chart, calculate the deficit, replace slowly, and replace ongoing losses as they occur [4][5].