Hyponatraemia — Viva

Opening (30 s)

"Hyponatraemia — one-line definition and the two questions that decide every action." ^[1]

Model: Serum Na under 135 mmol/L — a water problem, almost always non-osmotic ADH. Two questions: volume status (hypo / eu / hyper) and onset plus symptoms (acute under 48 h vs chronic or unknown; severe vs moderately severe vs asymptomatic). Unknown onset is treated as chronic. ^[1]

Station 1 — Pathophysiology (2 min)

"Why does acute Na 118 seize, and why does chronic Na 118 demyelinate if you correct it fast?" ^[1]

• Acute (under 48 h): brain has not extruded idiogenic osmoles → water moves in → cerebral oedema → seizure, herniation.
• Chronic: cells have already extruded osmoles to match the low Posm. A rapid rise in extracellular tonicity shrinks those adapted oligodendrocytes → osmotic demyelination.
• Verbalis 2013: untreated acute disease → cerebral oedema; rapid correction of chronic disease → ODS. ^[1]

Station 2 — Clinical diagnosis (2 min)

"How do you grade symptoms, and what is not a mild symptom?" ^[1]

European 2014 Table 5:

• Moderately severe: nausea without vomiting, confusion, headache.
• Severe: vomiting, cardiorespiratory distress, deep somnolence, seizures, coma (GCS 8 or under). Do not call nausea-without-vomiting "mild" and do not dump vomiting into the moderately severe bucket. ^[1]

Red flag: severe symptoms → 150 mL 3% over 20 minutes, repeated until a 5 mmol/L first-hour rise. That is not the 100 mL EAH finish-line bolus. ^[1]

Station 3 — Investigations (2 min)

"Give the essential SIAD criteria and the supplemental cut-offs people misremember." ^[1]

Essential (all required): Posm under 275; Uosm over 100 at a decreased effective osmolality; clinical euvolaemia; UNa over 30 with normal intake; no adrenal, thyroid, pituitary or renal failure; no recent diuretics. ^[1]

Supplemental (not required): uric acid under 0.24 mmol/L (not 0.30); urea under 3.6 mmol/L; FEurate over 12 percent (not under 4 or 9 percent); failure of 0.9% saline; response to fluid restriction. ^[1]

First test is always serum osmolality (true hypo-osmolar vs pseudo vs translocational). Hillier: measured Na falls about 2.4 mmol/L per 100 mg/dL (5.5 mmol/L) glucose; 1.6 works up to 400 mg/dL, 4.0 above 400. ^[1]

Station 4 — Emergency management (3 min)

"She is seizing. Dose, route, first-hour target, then the 24-hour cap." ^[1]

• European 2014: 150 mL 3% saline over 20 minutes; check Na; repeat 150 mL over the next 20 minutes, up to twice more, until +5 mmol/L in the first hour.
• After improvement: stop 3%; cap +10 mmol/L first 24 h then +8 mmol/L each 24 h thereafter until 130.
• Moderately severe, not seizing: a single 150 mL over 20 minutes; aim 5 mmol/L per 24 h.
• Ayus 2019: 100 to 150 mL boluses, 5 to 6 mEq/L in 1 to 2 hours, peripheral line acceptable; they explicitly accept the European 150 mL.
• Children with hyponatraemic seizures (Sarnaik): 4 to 6 mL/kg of 3%, rise of 3 to 5 mmol/L.
• EAH (Hew-Butler 2015): 100 mL 3% IV, repeat twice if no improvement — do not import this into the ward SIAD protocol. ^[1]

Station 5 — Definitive / long-term care (2 min)

"SIAD that is not an emergency — first-line, second-line, and what European 2014 says about vaptans." ^[1]

• First-line: fluid restriction.
• Second-line: urea 0.25 to 0.50 g/kg/day, or a loop diuretic plus oral NaCl.
• Against vaptans in profound SIAD (1C); do not recommend vaptans in moderate SIAD (1C); against lithium or demeclocycline (1D).
• SALT-1/2: oral tolvaptan 15 mg, titrate 30 then 60 mg; Na up at day 4 and day 30; hyponatraemia recurs after stopping — evidence of efficacy, not a European first-line recommendation.
• Stable hypovolaemia: 0.5 to 1.0 mL/kg/h 0.9% saline or balanced crystalloid; shock overrides. Urine output jump over 100 mL/h = over-correction risk. If over 10 mmol/L first 24 h or over 8 thereafter: stop active treatment; discuss 10 mL/kg electrolyte-free water over 1 h and i.v. desmopressin 2 micrograms (µg), not more often than every 8 h. ^[1]

Station 6 — Special populations (2 min)

• Thiazide-induced: commonest drug cause needing admission; mean age 75; 78 percent women; risk also old age, women, reduced body mass. Do not quote "check within 14 days" or unsourced SSRI/carbamazepine percentages.
• MDMA: hyponatraemia in 31 percent of a trial analysis; 0/15 if fluid-restricted vs 37 percent unrestricted; associated with oxytocin, not copeptin.
• Small-cell lung cancer is the European example of a frequent SIAD cancer — not "70 percent of malignancy-related SIADH".
• Pregnancy / labour: oxytocin can cause iatrogenic hyponatraemia; restrict free water in labour. Do not quote an unsourced 5 mmol/L gestational fall. ^[1]

Station 7 — Evidence and pitfalls (2 min)

• European 2014 (Spasovski, Intensive Care Med / EJE / NDT): 150 mL / 20 min / +5 mmol first hour; 10 then 8; against vaptans in profound SIAD.
• SALSA (Baek 2021): rapid intermittent bolus vs slow continuous infusion — overcorrection 17.2 vs 24.2 percent (NS); both effective.
• Corona 2013: overall mortality RR 2.60 (2.31 to 2.93) across 81 studies. Do not quote an unsourced in-hospital RR 2.48.
• Adrogué JAMA 2022: about 5 percent of adults / 35 percent hospitalised; even mild disease → longer stay and mortality; falls 23.8 vs 16.4 percent; fractures 23.3 vs 17.3 percent over 7.4 years. Hypertonic saline reserved for severely symptomatic patients.
• Trap: using the Adrogué–Madias formula as if it scaled linearly — it is exact only for adding 1 L. ^[1]

Station 8 — Rapid-fire pearls (1 min)

1. Na under 135; unknown onset = chronic.
2. Severe symptoms: 150 mL 3% over 20 min → +5 mmol in the first hour.
3. Cap: 10 mmol first 24 h, then 8 mmol each 24 h thereafter.
4. SIAD uric acid under 0.24 mmol/L; FEurate over 12 percent (supplemental).
5. EAH 100 mL 3% IV, not the hospital European bolus. ^[1]

Examiner pass criteria

• Speaks in mechanisms and numbers, not vague lists
• Gives the European 150 mL / 5 mmol first-hour sequence, not a generic 100 mL bolus
• Names the 10 then 8 cap and the relowering trigger
• Does not invent SSRI/SCLC/ODS-mortality percentages ^[1]

References1ShowHide

1. [1]Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia Intensive Care Med, 2014.PMID 24562549