Sodium, Water Balance, and Dysnatremias — Intermediate

Hyperglycaemia causes a translocational hyponatraemia by osmotic movement of water from intracellular to extracellular space. The commonly used Adrogue-Madias correction adds approximately 1.6 mmol/L to measured sodium for every 5.6 mmol/L rise in glucose above 5.6 mmol/L. Here glucose excess = 30 − 5.6 = 24.4 mmol/L; 24.4/5.6 ≈ 4.36 × 1.6 ≈ 7 mmol/L. Corrected Na ≈ 122 + 7 = 129 mmol/L.

Why the others are less appropriate:
- 116 mmol/L would imply further lowering of sodium by glucose, which is incorrect mechanistically.
- 122 mmol/L ignores the effect of hyperglycaemia.
- 135 and 142 mmol/L are too high and inconsistent with the correction calculation.

Note: Multiple correction formulas exist (some use 2.4 mmol/L per 5.6 mmol/L for severe hyperglycaemia), but the Adrogue-Madias 1.6/mmol per 5.6 mmol/L is widely used for bedside estimation.

This patient has severe, acute, symptomatic hyponatraemia with neurological compromise (seizures). Immediate therapy is to raise serum sodium promptly and stabilise the brain; a bolus of hypertonic saline (concentrated sodium chloride) is the recommended initial treatment to produce a controlled acute rise in sodium and reduce cerebral oedema and seizures. The goal is a small, urgent increase (typically aiming for an initial 4–6 mmol/L rise or enough to stop seizures), not overly rapid correction overall.

Why the others are less appropriate:
- Fluid restriction and oral urea are measures for chronic or less severe hyponatraemia and are too slow in this emergency.
- Loop diuretic plus normal saline is not first-line for a patient with severe cerebral symptoms and may be inappropriate depending on volume status.
- Tolvaptan is not for acute emergency correction and is not appropriate to start during seizures.
- Isotonic 0.9% saline may be ineffective or even harmful in some causes of hyponatraemia (e.g. SIADH) and will not raise sodium quickly enough for severe symptomatic cases.

Careful monitoring of plasma sodium and neurological state is essential after hypertonic saline to avoid overcorrection.

Chronic hyponatraemia (developed over >48 hours or of unknown duration) carries a risk of osmotic demyelination if plasma sodium is corrected too quickly. Best practice is to limit correction (commonly recommended ≤8 mmol/L in the first 24 hours) and to monitor frequently. If overcorrection occurs, re-lowering sodium (e.g. with dextrose 5% infusion and desmopressin) can be considered.

Why the others are less appropriate:
- Rapid correction to normal within 12 hours is dangerous in chronic hyponatraemia and increases the risk of osmotic demyelination.
- Continuous saline infusion without attention to rate may lead to overcorrection and is unsafe.
- Tolvaptan is not universally appropriate and may cause overly rapid aquaresis; it is not a universal safety strategy.
- potassium management is important but restricting potassium is not a recognised measure to reduce osmotic demyelination risk.

Frequent monitoring (serum sodium every 2–4 hours initially) and adjustment of therapy are critical.

This patient has hypotonic hyponatraemia (low serum osmolality). Examination shows euvolaemia. High urine osmolality (>100 mOsm/kg) and elevated urine sodium (>30 mmol/L) in the setting of euvolaemia are typical of SIADH, where ADH-mediated water retention produces concentrated urine and renal sodium loss.

Why the others are less appropriate:
- Primary polydipsia causes very dilute urine (urine osmolality typically <100), not concentrated urine of 520 mOsm/kg. - Hypovolaemic hyponatraemia from diuretics would produce clinical volume depletion rather than euvolaemia, though diuretics can mimic SIADH biochemically. - Pseudohyponatraemia from hyperlipidaemia/artifact occurs with normal serum osmolality (isotonic) and is typically excluded by measuring osmolality. - Beer potomania typically presents with low urine sodium and low solute intake leading to impaired free water excretion; urine sodium would usually be low. Context and volume assessment are essential when evaluating hyponatraemia.

A low urine osmolality with hypernatraemia indicates inability to concentrate urine. A marked rise in urine osmolality after administration of desmopressin indicates that the collecting duct is responsive to ADH, implying a deficiency of ADH secretion — central diabetes insipidus.

Why the others are less appropriate:
- Primary (or psychogenic) polydipsia usually causes hyponatraemia or near-normal sodium due to excessive intake and causes low urine osmolality that may not respond significantly to desmopressin.
- Nephrogenic DI would not show a marked rise in urine osmolality after desmopressin because the kidney is unresponsive.
- SIADH would present with hyponatraemia and concentrated urine, not hypernatraemia with low urine osmolality.
- Psychogenic hypernatraemia is not a diagnosis here; the desmopressin response classically distinguishes central DI.

This patient has hypernatraemia with clinical hypovolaemia (hypotension and volume depletion). The immediate priority is haemodynamic resuscitation and restoration of intravascular volume; isotonic 0.9% saline is appropriate initial fluid to restore circulating volume. Once intravascular volume is restored and the patient is haemodynamically stable, then hypotonic fluids (for example, 5% dextrose) can be used to gradually lower plasma sodium, taking care to avoid overly rapid correction.

Why the others are less appropriate:
- Hypertonic saline is used to raise sodium and would worsen hypernatraemia; inappropriate here.
- Slow infusion of 5% dextrose as first-line in a hypotensive, volume-depleted patient risks further intravascular depletion.
- Tolvaptan is not indicated for acute hypovolaemic hypernatraemia and is contraindicated in hypovolaemia.
- Oral free water may be useful if the patient is safe to swallow and absorption appropriate, but in a hypotensive, collapsed elderly patient IV volume resuscitation is the priority.

After stabilisation, calculate free water deficit and plan safe gradual lowering of sodium (e.g. target ≤10 mmol/L per 24 hours).

Pseudohyponatraemia occurs when sodium measurement by indirect ion-selective electrodes is falsely low because a significant proportion of plasma volume is occupied by lipids or proteins, leading to normal plasma water sodium concentration with an artifactual low reported sodium. In such cases measured serum osmolality is normal (isotonic), supporting pseudohyponatraemia.

Why the others are less appropriate:
- True hypotonic hyponatraemia would be associated with low serum osmolality.
- Translocational hyponatraemia from hyperglycaemia is associated with elevated measured osmolality and high glucose; here osmolality is normal and lipids/proteins are high.
- SIADH and hypovolaemic hyponatraemia are both hypotonic disorders with low measured serum osmolality, which is not the case here.

If pseudohyponatraemia is suspected, check osmolality or repeat sodium using a direct electrode method to confirm.

Vasopressin V2 receptor antagonists (vaptans) block vasopressin action at the renal V2 receptor in the collecting duct, reducing aquaporin insertion and thereby increasing free-water excretion (aquaresis) without major natriuresis. This raises serum sodium.

Why the others are less appropriate:
- They do not primarily promote natriuresis via distal tubule sodium inhibition.
- They are antagonists, not agonists — they cause aquaresis, not urine concentration.
- They are not first-line for all acute symptomatic hyponatraemias (they are not appropriate in emergency neurologic compromise and have specific indications/contraindications).
- They do not affect plasma albumin.

Use of vaptans requires careful patient selection and monitoring because of risk of overly rapid sodium rise and other adverse effects.

Thiazides impair the kidney's ability to dilute urine by inhibiting sodium reabsorption in the distal tubule, which reduces free-water clearance. They also cause mild volume depletion and natriuresis; the volume depletion stimulates ADH release and enhances water reabsorption, leading to hyponatraemia. Elderly patients are particularly susceptible.

Why the others are less appropriate:
- Thiazides do not directly stimulate pituitary ADH secretion as their primary action; the ADH increase is secondary to volume changes.
- Thiazides do not cause an osmotic diuresis; rather they reduce diluting capacity.
- Pseudohyponatraemia is due to lab artefact from lipids/proteins, unrelated to thiazides.
- Thiazides do not reduce urea production to a clinically relevant extent causing hyponatraemia.

Stopping the thiazide and carefully managing volume and sodium usually corrects the problem; monitor sodium during correction.

Post-neurological injury hyponatraemia can be due to cerebral salt wasting (CSW) where there is renal sodium loss leading to hypovolaemia. This patient's clinical hypovolaemia and high urine sodium support CSW. Management for CSW is volume repletion with isotonic saline (and often replacement of ongoing sodium losses), not fluid restriction.

Why the others are less appropriate:
- Fluid restriction and demeclocycline are treatments for SIADH (euvolaemic hyponatraemia) and would worsen hypovolaemia in CSW.
- Immediate hypertonic saline bolus is reserved for severe symptomatic hyponatraemia (e.g. seizures); routine normalisation with hypertonic saline is not first-line for CSW.
- Tolvaptan induces free-water loss and is inappropriate in hypovolaemic hyponatraemia.
- Loop diuretics would worsen volume depletion and are contraindicated in hypovolaemic states.

Accurate clinical assessment of volume status is essential because treatment differs markedly between SIADH and cerebral salt wasting.