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Hyponatraemia
Hyponatraemia is almost always an excess of water relative to sodium; it is worked up in three steps (serum osmolality, urine osmolality, urine sodium with a volume assessment), treated with 3% NaCl only when symptoms are severe, and never corrected too fast.
Why it matters in the exam
This is one of the most frequently tested topics in internal medicine. A typical question presents a confused patient with a sodium of 118 mmol/L and asks for the next step: are there severe symptoms (seizures, reduced consciousness, vomiting) that call for a bolus of hypertonic saline, or is this chronic hyponatraemia to be treated slowly according to its cause? The candidate must remember the limit on the rate of correction — no more than 10 mmol/L in the first 24 hours, and 8 in a high-risk patient — because rapid correction causes the osmotic demyelination syndrome.
The other part is the differential diagnosis. SIADH is asked about in the context of small-cell lung cancer, brain disease, pneumonia and drugs (SSRIs, carbamazepine, cyclophosphamide). The diagnostic criteria are low serum osmolality, urine that is not maximally dilute, a high urine sodium, a clinically euvolaemic patient and normal thyroid and adrenal function. A known trap: giving 0.9% saline to an SIADH patient with very concentrated urine can lower the sodium further. Heavy beer drinking with poor food intake (low solute intake) and primary polydipsia also appear in questions — there the urine is very dilute.
Key points
- Three steps: serum osmolality, urine osmolality, urine sodium with a volume assessment.
- Severe symptoms: 3% NaCl 150 mL over 20 minutes until a rise of 5 mmol/L.
- Correction ceiling: 10 mmol/L in 24 hours, and 8 if high risk.
- SIADH: euvolaemia, concentrated urine, urine sodium above 30, normal thyroid and adrenal function.
- SIADH treatment: fluid restriction, then urea or tolvaptan.
Summary table
| Step | Finding | Meaning | Action |
|---|---|---|---|
| Serum osmolality | Normal or high | Pseudohyponatraemia or hyperglycaemia | Correct for glucose; do not treat the sodium |
| Urine osmolality | Below 100 mOsm/kg | Primary polydipsia or low solute intake | Restrict drinking, improve diet |
| Low urine sodium | Below 30 mmol/L | Low effective volume: dehydration, heart failure, cirrhosis | Fluids if dehydrated; treat the disease in oedematous states |
| High urine sodium, euvolaemic | Above 30 mmol/L | SIADH, adrenal or thyroid insufficiency | Fluid restriction, then urea or tolvaptan |
| Severe symptoms | Seizure, reduced consciousness, vomiting | Cerebral oedema | 3% NaCl 150 mL over 20 minutes, up to two or three times |
| Rate of correction | Up to 10 mmol/L in 24 hours (8 if high risk) | Prevent osmotic demyelination | If exceeded → water or dextrose 5% and desmopressin |
A free exam question
מטופל בן 74 מאושפז מזה מספר שבועות עקב כוויה נרחבת. בבדיקות מעבדה שגרתיות התגלה כי סובל מהיפונתרמיה. מה מהבאים נכון בהתייחס לטיפול בהפרעה אלקטרוליטרית זו? (the question as examined, in Hebrew)
- א. במקרה של הפרעה אקוטית )ב 24- שעות האחרונות(, התיקון צריך להתבצע על פני 24 שעות כדי להימנע מבצקת מוחית
- ב. קצב התיקון של הפרעה כרונית חמורה צריך להיות שווה או נמוך מ- 0.5 .Meq/hr
- ג. במקרה של פרכוסים יש להקפיד על קצב תיקון איטי יותר, כדי להימנע מהחמרת המצב
- ד. אחת מאפשרויות הטיפול הינה מתן מים חופשיים )Water Free( בלבד
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Sources
- European Clinical Practice Guideline on the Diagnosis and Treatment of Hyponatraemia (ESE/ESICM/ERBP), 2014 — Diagnostic algorithm and treatment of severe and chronic hyponatraemia
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Fluid and Electrolyte Disturbances: Sodium
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.