Sodium Disorders Viva

Long Case Viva Defence

Candidate's opening statement (model answer)

"Mrs Chen is a 78-year-old woman who presents to the emergency department with confusion and a fall, found to have a serum sodium of 116 mmol/L. She was started on hydrochlorothiazide 25 milligrams daily by her general practitioner three weeks ago for hypertension, and her family report five days of progressive drowsiness and confusion before the fall. She has a background of hypertension, osteoporosis, and mild cognitive impairment. [1]

On examination she is drowsy with a Glasgow Coma Scale of 13, has dry mucous membranes, a postural blood pressure drop from 142 over 78 lying to 118 over 66 standing, a heart rate of 92, a low jugular venous pressure, and no oedema. Her bloods show sodium 116, potassium 2.9, urea 11, creatinine 95, serum osmolality 248, urine osmolality 380, and urine sodium 48. Her TSH is 3.1 and her morning cortisol is 480. [1]

Her main problems are:

1. Severe hypotonic hyponatraemia — sodium 116 — with cerebral symptoms and competing risks of cerebral oedema versus osmotic demyelination
2. Thiazide-induced hyponatraemia — the drug is the cause, confirmed by the renal salt-wasting pattern in a volume-depleted patient
3. Hypovolaemia with a postural drop
4. Hypokalaemia — 2.9 — which itself contributes to the hyponatraemia and raises the osmotic demyelination risk
5. A fall on a background of cognitive impairment — assess for injury, delirium, and the contribution of postural hypotension
6. Polypharmacy and the need for an alternative antihypertensive [1]

Her dominant clinical threats are, in order: cerebral oedema if her conscious state deteriorates, osmotic demyelination if I correct too fast, and the consequences of the fall. I will manage her by stopping the thiazide permanently, correcting her potassium and volume cautiously, monitoring her sodium every two to four hours with desmopressin available to relower if she overcorrects, and targeting a maximum rise of eight in the first twenty-four hours. I will then address her falls risk, her cognitive state, and her antihypertensive regimen in an integrated plan." [1]

Examiner probing questions and model answers

Q1: "Her sodium is 116 and she is drowsy. Walk me through exactly what you would do in the first hour." [1]

"First, I assess her airway, breathing, and circulation, and I put her on cardiac monitoring. I secure intravenous access. I confirm the sodium is real and not pseudohyponatraemia — her serum osmolality of 248 confirms true hypotonic hyponatraemia, and her glucose is normal so there is no hypertonic component. I review the prescription chart and stop the hydrochlorothiazide immediately and permanently. I begin cautious volume repletion with 0.9 percent saline — she is hypovolaemic, so isotonic saline will suppress her non-osmotic vasopressin and allow free water excretion. I start intravenous potassium replacement to bring her potassium above four. I admit her to high dependency. I set my sodium targets: if her conscious state deteriorates — a seizure, a further fall in GCS — I will give 3 percent hypertonic saline, 100 millilitres over 10 minutes, repeated up to a maximum of three boluses, aiming for a 4 to 5 millimole per litre rise in the first four hours. My 24-hour ceiling is a rise of 8, the lower end of the range, because her hypokalaemia and her age raise her osmotic demyelination risk. I will check her sodium every two to four hours for the first day, and I will have desmopressin drawn up and ready to relower her if she overcorrects. I do not start a vaptan — they are contraindicated in hypovolaemic hyponatraemia and must never be combined with hypertonic saline." [1]

Q2: "Why is her hypokalaemia relevant to the hyponatraemia and to your correction strategy?" [1]

"Hypokalaemia is relevant in three ways. First, it contributes to the hyponatraemia itself — when intracellular potassium is depleted, sodium moves into cells to maintain electroneutrality, lowering the measured serum sodium. Correcting the potassium therefore raises the sodium independently of any water shift. Second, hypokalaemia is an independent risk factor for osmotic demyelination syndrome — the mechanism is thought to involve impaired astrocyte osmolyte reaccumulation in the potassium-depleted state. So when I correct her sodium I must aim for the lower end of the safe range — 8 rather than 10 millimoles per litre in 24 hours. Third, the hypokalaemia is a clue to the cause — profound potassium loss with hyponatraemia in an elderly woman on a new diuretic is the classic thiazide signature, and it points me to the drug as the culprit. Replacing the potassium is part of the treatment of the hyponatraemia, not a separate problem." [1]

Q3: "Her urine sodium is 48. Why is that high in a volume-depleted patient?" [1]

"In a volume-depleted patient the kidney should be avidly retaining sodium, so the urine sodium should be under 20. A urine sodium of 48 in this setting tells me the kidney is losing sodium — which points to a renal salt-wasting mechanism. The two renal causes in this context are thiazide diuretics and adrenal insufficiency. The thiazide blocks sodium reabsorption in the distal tubule, the diluting segment, so sodium is lost in the urine and free water is retained because the diluting mechanism is also impaired. Adrenal insufficiency — specifically mineralocorticoid deficiency in Addison's disease — causes renal sodium loss, but it also causes hyperkalaemia, which she does not have, and her morning cortisol of 480 excludes it. So the high urine sodium with the low potassium and the new thiazide together confirm thiazide-induced hyponatraemia. The other renal salt-wasting syndrome, cerebral salt wasting, is seen after subarachnoid haemorrhage and traumatic brain injury — not relevant here." [1]

Q4: "Twenty-four hours later her sodium is 131. What happened, and what do you do?" [1]

"She has overcorrected — her sodium rose 15 millimoles per litre in 24 hours, exceeding my 8-millimole ceiling. The mechanism is almost certainly an autonomous water diuresis after I stopped the thiazide and repleted her volume: once the non-osmotic vasopressin drive was switched off, her kidney excreted the retained free water briskly, and the sodium overshot. This is the classic overcorrection pattern in thiazide-induced hyponatraemia, and it is exactly why I was checking her sodium every two to four hours and why I had desmopressin ready. The immediate management is to relower her sodium back into a safe range. I give desmopressin 1 to 2 micrograms subcutaneously or intravenously every 8 hours, combined with free water — oral if she can drink, or 5 percent dextrose intravenously. This recreates a controlled hyponatraemic state. I target a sodium in the low 120s. The evidence suggests that prompt relowering reduces the risk of osmotic demyelination, which is the complication I am now trying to prevent. I will continue the desmopressin and free water until her sodium is stable in the target range, then taper and allow a slow, controlled rise within the ceiling going forward." [1]

Q5: "What is osmotic demyelination syndrome, and how would you recognise it?" [1]

"Osmotic demyelination syndrome, or ODS, is the catastrophic neurological complication of overcorrecting chronic hyponatraemia. The pathophysiology is that the chronically hyponatraemic brain has shed intracellular osmoles — first electrolytes, then organic osmolytes like glutamate, taurine, and myo-inositol — to protect itself from cerebral oedema. When the serum sodium is raised rapidly, extracellular tonicity rises faster than the brain can reaccumulate those osmoles, water is dragged out of astrocytes, the astrocytes die, and the oligodendrocytes that depend on them demyelinate — classically in the pons, producing central pontine myelinolysis, but also extra-pontine. Clinically it presents with a biphasic course: the patient improves initially as the sodium rises, then deteriorates days to a week later with dysarthria, dysphagia, quadriparesis, locked-in syndrome, seizures, coma, and often death. MRI shows the characteristic pontine and extra-pontine demyelination, though the imaging may lag the clinical picture by several days. The patients at highest risk are those with a sodium under 105, hypokalaemia, alcohol use disorder, malnutrition, anorexia, advanced liver disease, and the elderly — and Mrs Chen has two of those risk factors, which is why I targeted the lower correction range. Prevention is the only effective treatment; once ODS occurs, management is supportive." [1]

Q6: "If this had been SIADH from a small cell lung cancer rather than a thiazide, how would your management differ?" [1]

"The principles are the same — symptom severity drives urgency, and the correction ceiling applies — but the cause-directed treatment differs. For chronic moderate SIADH, first-line is fluid restriction to 800 to 1000 millilitres per day, which is effective in about a third of patients. If fluid restriction fails — and a useful predictor is a urine-to-serum electrolyte ratio above one, which indicates the urine is too tonic for restriction to work — I would add high solute intake: salt tablets 2 to 3 grams a day with a loop diuretic to enable excretion of the water load, or oral urea 15 to 30 grams a day, which is cheap and effective but unpalatable. A vaptan, tolvaptan, is an option for resistant SIADH — the SALT trials showed a 4 to 5 millimole per litre rise over 30 days — but I would use it cautiously: inpatient initiation, sodium checks every 6 hours, no fluid restriction on day one, never combined with hypertonic saline, and avoided in liver disease. Critically, I would not give normal saline — in SIADH the kidney retains the water and excretes the sodium, so the sodium falls further, the desalination phenomenon. And I would address the underlying cancer — SIADH from small cell lung cancer often resolves with chemotherapy, so the prognosis is tied to the oncological response." [1]

Q7: "Let's switch to hypernatraemia. A 70-kilogram intubated patient has a sodium of 160. Calculate the free water deficit and tell me how you would correct it." [1]

"The free water deficit is total body water times the quantity serum sodium over 140 minus one. For a 70-kilogram man the total body water is 70 times 0.6, which is 42 litres. So the deficit is 42 times 160 over 140 minus one, which is 42 times 0.143, which is approximately 6 litres of free water. But I do not correct all of that at once — the brain has accumulated osmoles in chronic hypernatraemia, and rapid correction causes cerebral oedema, the mirror image of osmotic demyelination. I aim to lower the sodium by no more than 10 millimoles per litre in 24 hours, over 48 to 72 hours, and I add the ongoing losses — insensible, urine, and any gastrointestinal — as they occur. The preferred route is oral or enteral water via the nasogastric tube, which is the safest and most physiological. If I need intravenous fluid, I use 5 percent dextrose, which provides free water once the glucose is metabolised — not normal saline, because at a sodium of 154 it would not lower the serum sodium and could even raise it. I monitor the sodium every 4 to 6 hours. If there is diabetes insipidus, I confirm central versus nephrogenic with a desmopressin challenge: central DI responds with a brisk rise in urine osmolality and is treated with desmopressin; nephrogenic DI does not respond and is treated with a thiazide and amiloride plus a low-sodium diet, and by treating the cause — stopping lithium, correcting hypercalcaemia. The key teaching point is that hypernatraemia is iatrogenic more often than admitted, and the single most important intervention is often to audit the fluid chart and provide free water." [1]

Q8: "What is the thiazide paradox, and why does it matter?" [1]

"The thiazide paradox is the observation that a thiazide diuretic — a drug whose job is to increase urine output in oedematous states — actually reduces urine output in nephrogenic diabetes insipidus. The mechanism is that the thiazide induces a mild volume contraction, which increases proximal tubular reabsorption of sodium and water, reducing the delivery of filtrate to the defective collecting duct. Less filtrate reaches the segment that cannot reabsorb water, so less water is lost. It is a beautiful example of using physiology to circumvent a defect. In lithium-induced nephrogenic DI specifically, amiloride is preferred alongside or instead of the thiazide, because amiloride blocks lithium entry into the principal cell through the epithelial sodium channel, partially protecting the kidney from further lithium toxicity. Combined with a low-sodium diet to reduce the distal sodium delivery, this regimen can reduce polyuria substantially in patients who cannot stop their lithium. It is a classic viva discriminator because it sounds paradoxical until you understand the mechanism." [1]

Short Case Discussion

Scenario: "Assess this hyponatraemic patient's volume status and look for signs suggesting the cause."

Candidate presentation (model): [1]

"I examined this patient's volume status and looked for signs of the underlying cause. Beginning at the hands, there is no palmar or buccal pigmentation to suggest Addison's disease, no clubbing, the pulse is 92 and regular, and the skin turgor is reduced with dry mucous membranes. In the neck the jugular venous pressure is low at 2 centimetres, with no elevation. Cardiovascular examination reveals a postural blood pressure drop from 142 over 78 lying to 118 over 66 standing, with no apex displacement and no third heart sound. The respiratory examination is clear. The abdomen is soft with no ascites, no palpable masses or ballotable kidneys, and no hepatic stigmata. There is no peripheral or sacral oedema. There is no cachexia to suggest malignancy, and there are no thyroid stigmata. The patient is drowsy with a Glasgow Coma Scale of 13. [1]

In summary, this patient is hypovolaemic, with a postural drop, a low jugular venous pressure, dry mucous membranes, and reduced skin turgor. There are no signs to suggest an oedema-forming state such as heart failure, cirrhosis, or nephrotic syndrome. The absence of hyperpigmentation, hyperkalaemia, and hypotension argues against primary adrenal insufficiency. Given the hypovolaemic picture with a renal salt-wasting pattern on the urine biochemistry, I would review the medication chart for a thiazide diuretic, which is the most likely cause. I would also check a morning cortisol and a TSH to formally exclude the SIADH mimics before attributing this to a drug." [1]

Examiner: "What signs would have changed your differential?" [1]

"If I had found a raised jugular venous pressure, peripheral oedema, a third heart sound, or crackles, I would have classified her as hypervolaemic — pointing to heart failure, cirrhosis, or advanced renal failure as the cause. If I had found buccal and palmar crease pigmentation with hyperkalaemia and hypotension, I would have diagnosed Addison's disease as the cause — a medical emergency requiring immediate glucocorticoids and mineralocorticoids. If I had found ascites, spider naevi, palmar erythema, and caput medusae, I would have pointed to cirrhosis. If I had found cachexia, clubbing, or a pleural effusion, I would have considered an underlying malignancy driving SIADH — classically small cell lung cancer. If she had appeared euvolaemic with none of these signs, I would have considered SIADH from a pulmonary or central nervous system cause, or a drug. The volume status assessment is the single most useful bedside manoeuvre because it divides the differential into three families and directs the investigation." [1]

Examiner: "When would you give 3 percent saline to a hyponatraemic patient?" [1]

"I would give 3 percent hypertonic saline only for severe symptomatic hyponatraemia — a seizure, coma, Cheyne-Stokes respiration, vomiting, or decerebrate posturing — regardless of the absolute sodium value. The regimen I use is 100 millilitres over 10 minutes, repeated up to a maximum of three boluses, aiming for a 4 to 5 millimole per litre rise in the first four hours. I would not give it for asymptomatic or mildly symptomatic hyponatraemia, where the risk of overcorrection outweighs the benefit. I would not give it to a hypervolaemic patient except in extremis. And I would always have desmopressin available to relower if the correction overshoots the 8 to 10 millimole per 24-hour ceiling. The bolus approach has replaced the older weight-based continuous infusion because it is more controllable and carries a lower overcorrection risk." [1]