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Hypernatraemia and dehydration in older adults
Hypernatraemia (sodium above 145 mmol/L) in an older adult is almost always a water deficit — the patient is not thirsty or cannot drink unaided; correct it slowly, about up to 10 mmol/L in 24 hours in chronic hypernatraemia, and treat the cause: acute illness, fever, diuretics, high glucose or dependence on help.
Why it matters in the exam
The exam presents a care-home resident with dementia, fever and confusion, a sodium of 158 and little drinking, and asks about treatment: if there are signs of shock — isotonic saline first to stabilise, and then hypotonic fluids (5% glucose or half-strength saline) at a rate that lowers sodium gradually. Candidates are also asked to calculate the free water deficit: total body water times (sodium divided by 140 minus 1), where total body water in an older adult is about 0.5 of body weight in men and about 0.45 in women.
Recurring points: correcting chronic hypernatraemia too quickly may cause cerebral oedema; reduced thirst and urine-concentrating capacity with age, dementia and dependence on help are the main risk factors; a BUN-to-creatinine ratio above 20 supports dehydration; and in mild dehydration in an older adult who does not drink enough, subcutaneous fluids can be given. The European guideline recommends about 1.6 litres of fluid a day for women and about 2.0 litres for men, unless there is a medical restriction.
The exam sometimes also presents hypernatraemia due to water loss through the kidneys — diabetes insipidus or heavy urine output after relief of an obstruction — so urine volume and osmolality are checked.
Key points
- Sodium above 145 = water deficit.
- Risk factors: reduced thirst, dementia, dependence, fever, diuretics.
- Water deficit = total body water × (sodium/140 − 1).
- Chronic — up to about 10 mmol/L in 24 hours.
- Shock — saline first; then hypotonic fluids.
Summary table
| Issue | Key point | Values |
|---|---|---|
| Definition | High sodium = relative water deficit | Above 145 mmol/L |
| Risk factors | Reduced thirst, dementia, dependence for drinking, fever, diuretics, high glucose, lactulose | Care-home resident at high risk |
| Symptoms | Drowsiness, confusion, weakness, seizures | By severity and speed of rise |
| Water deficit | Total body water × (sodium/140 − 1) | Older adult: 0.5 in men, 0.45 in women |
| Correction rate | Chronic — up to about 10 mmol/L in 24 hours | Acute (less than 48 hours) — faster |
| Fluids | Shock — isotonic saline first; then 5% glucose or half-strength saline | Subcutaneous fluids in mild dehydration |
A free exam question
בת 70 ., עם רקע של סכיזופרניה, מגיעה למיון עם ירידה במצב הכרה, בלבול וכאבי ראש בבדיקתה: ללא בצקות, ללא רושם לסימני התייבשות. בבדיקות מעבדה היפונתרמיה של 122 mEq/L . בבדיקת השתן אוסמולארית של 100 mOsm/kg . ריכוז הנתרן בשתן 10 mM . איזה מהמצבים הבאים יכול להסביר את ההיפונתרמיה? (the question as examined, in Hebrew)
- א. פולידיפסיה
- ב. היפותירואידיזם
- ג. טיפול בתיאזיד
- ד. גידול ריאתי
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Sources
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Fluid and electrolyte disturbances — hypernatraemia
- ESPEN guideline on clinical nutrition and hydration in geriatrics, 2022 — Hydration
- Geriatrics Review Syllabus (American Geriatrics Society), 11th edition (2022) — Fluid and electrolyte disorders in older adults
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.