MBBS OSCE · nephrology
OSCE — Hyponatraemia
Eight-minute OSCE: a 72-year-old woman on a thiazide with Na 118 mmol/L, confusion without seizure — volume assessment, SIAD exclusions, European 2014 correction limits, and when 3% saline is indicated.
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Brief (to candidate)
You have 8 minutes with a 72-year-old woman brought in after a fall. She is confused. Medications: hydrochlorothiazide, ramipril, sertraline. Serum Na 118 mmol/L. You must take a focused history, outline examination and investigations, give immediate management, and state correction limits. [1]
Clinical context (examiner only)
She is postural, dry mucosae, no oedema. Na 118, glucose 5.4 mmol/L, Posm 246 mOsm/kg, Uosm 380 mOsm/kg, UNa 52 mmol/L. No seizure, no vomiting, GCS 13. Onset is unknown (likely days to weeks). This is hypovolaemic thiazide-associated hyponatraemia, not SIAD, and not a severe-symptom 3% emergency. The traps are labelling her SIAD because UNa is over 30 (she is on a diuretic), giving a 100 mL 3% bolus "because Na is 118", and failing to watch for auto-correction once volume is restored. [1]
Candidate tasks
- Clarify onset (when last well), symptoms (vomiting, seizure, somnolence vs headache/confusion), fluid intake, thiazide start, SSRI, alcohol, heart-failure/cirrhosis clues, and endurance-exercise/MDMA if relevant.
- Examine volume status first: pulse, postural BP, JVP, mucosae, oedema, GCS, respiratory distress.
- First-line tests: serum osmolality, paired urine osmolality and urine Na, glucose, cortisol, TSH, potassium, renal function, uric acid / FEurate if SIAD is still on the table.
- Immediate plan matching symptoms and volume, not the absolute Na alone.
- State the European 2014 correction cap and the over-correction response.
- Name ODS risk and disposition (HDU if active 3% or a large expected water diuresis).
- One modifier: older women with lower body mass are the thiazide-admission phenotype (mean age 75, 78 percent women). [1]
Examiner checklist
| Domain | Pass behaviours |
|---|---|
| Definition | Na under 135 mmol/L; classify by volume and by onset/symptoms |
| Assessment | Volume first; does not call this SIAD while a thiazide is still in play |
| Investigations | Posm, Uosm, UNa, cortisol, TSH, glucose; uric acid under 0.24 / FEurate over 12 percent only as supplemental SIAD |
| Emergency care | No seizure/vomiting/coma → no 150 mL 3% bolus; stop the thiazide |
| Definitive care | Stable hypovolaemia: 0.5 to 1.0 mL/kg/h 0.9% saline or balanced crystalloid; shock overrides |
| Safety | Cap 10 mmol/L first 24 h then 8 mmol/L each 24 h thereafter; urine output jump over 100 mL/h is an over-correction warning |
| Safety | If she does seize: 150 mL 3% over 20 minutes, repeat to +5 mmol/L in the first hour (European 2014), not 100 mL as the ward default |
| Communication | Explains auto-correction after volume restore; relower if over 10 mmol first 24 h or over 8 thereafter |
Model outline
Working diagnosis: hypovolaemic hyponatraemia, thiazide-associated, unknown onset so treat as chronic, moderately severe symptoms at most (confusion without vomiting). Stop the thiazide. Restore volume with 0.5 to 1.0 mL/kg/h isotonic crystalloid if stable. Recheck Na; watch urine output. Do not fluid-restrict (that is SIAD, and she is dry). Do not start a vaptan (European 2014 recommends against vaptans in profound SIAD and does not recommend them in moderate SIAD). If over-correction starts, stop active treatment and discuss 10 mL/kg electrolyte-free water over 1 h plus i.v. desmopressin 2 micrograms (µg), not more often than every 8 h. If a seizure develops during the station, switch immediately to the 150 mL / 20-minute / +5 mmol first-hour European sequence. [1]
References1ShowHide
- [1]Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia Intensive Care Med, 2014.PMID 24562549