Hypernatraemia and Water Deficit Correction
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Direct answer
A serum sodium above 145 mmol/L defines hypernatraemia, which almost always reflects a free-water deficit rather than sodium excess. The deficit equals total body water multiplied by serum sodium over 140, minus one, and it is replaced orally or with 5 per cent dextrose at a rate that lowers sodium by no more than about 10 mmol/L in 24 hours. Inadequate water intake, gastrointestinal and insensible losses, osmotic diuresis and diabetes insipidus are the usual causes.
What you must remember
- Definition: serum sodium above 145 mmol/L; the level reflects water balance relative to body sodium, not the total amount of sodium.
- Free water deficit: total body water multiplied by (serum sodium divided by 140, minus one); total body water is about 0.6 of body weight in adult men and 0.5 in women and older adults.
- Central diabetes insipidus: low ADH with dilute urine, which responds to desmopressin.
- Nephrogenic diabetes insipidus: the kidney does not respond to ADH, so desmopressin is ineffective; lithium is a classic cause.
- Urine osmolality: dilute urine suggests diabetes insipidus or water-driven losses, while concentrated urine points to extrarenal water loss or sodium gain.
- Correction limit: no more than about 10 mmol/L in 24 hours, since rapid lowering of sodium risks cerebral oedema, especially in chronic hypernatraemia.
- Route: oral water when possible, otherwise 5 per cent dextrose or hypotonic saline; avoid sodium bicarbonate, a common iatrogenic sodium load.
A 70 kg man with a sodium of 160
Consider a 70 kg man with a serum sodium of 160 mmol/L, confused and drinking little, after a week of vomiting and diarrhoea.
- Estimate body water: Total body water is about 0.6 of body weight, which gives 42 litres.
- Calculate the deficit: 42 multiplied by (160 divided by 140, minus one) is about 6 litres.
- Choose the route: Oral water is preferred if he can swallow safely; otherwise intravenous 5 per cent dextrose is started with careful fluid charting.
- Set the correction limit: For a chronic process, correction over 48 hours or longer is standard, with the fall capped at about 10 mmol/L in the first 24 hours.
- Monitor sodium: Serum sodium is checked every few hours during the early phase.
- Review on warning signs: A sudden rise in urine output or a fall in sodium beyond the limit prompts a review of the fluid rate.
The correction must be slow, because the brain has adapted to the high osmolality by generating idiogenic osmoles. Falling sodium too quickly draws water into the cells and causes cerebral oedema.
Where the examiner tries to catch you
Four traps recur on this topic.
- Trap: thinking sodium measures total body sodium. Correct: it measures concentration, so a patient with a sodium of 160 may have lost water rather than salt.
- Trap: applying a fixed body-water fraction to everyone. Correct: body water is lower in obese people and older women, so the deficit is overestimated if lean weight is not used.
- Trap: confusing central and nephrogenic diabetes insipidus. Correct: a water deprivation test followed by desmopressin separates them, and lithium is the classic drug cause of nephrogenic disease.
- Trap: applying the correction limit to the volume of water given. Correct: the limit applies to the fall in sodium, not to the volume infused.
Frequently asked questions
How is the free water deficit calculated in hypernatraemia?
The free water deficit in hypernatraemia is total body water multiplied by serum sodium divided by 140, minus one. Total body water is about 0.6 of body weight in adult men and about 0.5 in women and older adults, so a 70 kg man with a sodium of 160 mmol/L has a deficit of roughly 6 litres.
What is the safe rate of correction for hypernatraemia?
The safe rate of correction for hypernatraemia is a fall in serum sodium of no more than about 10 mmol/L in 24 hours, since rapid lowering risks cerebral oedema. Chronic hypernatraemia is corrected more slowly, over 48 hours or longer, and serum sodium should be checked every few hours during the early phase.
How is central diabetes insipidus distinguished from nephrogenic diabetes insipidus?
Central diabetes insipidus is distinguished from nephrogenic diabetes insipidus by the response to desmopressin, which corrects the polyuria in the central form but not in the nephrogenic form. A water deprivation test showing failure to concentrate urine, followed by desmopressin, separates the two in the investigation of polyuria.
Why can rapid correction of hypernatraemia be dangerous?
Rapid correction of hypernatraemia can cause cerebral oedema because the brain has generated idiogenic osmoles to adapt to the high sodium. When plasma osmolality falls quickly, water moves into brain cells that still hold those solutes, and the swelling can cause seizures and neurological injury.
Which fluid is used to correct hypernatraemia in adults?
Oral water is the preferred fluid for correcting hypernatraemia in adults who can swallow safely, and 5 per cent dextrose is the usual intravenous option for free water replacement. Hypotonic saline may be used in selected cases, and sodium bicarbonate should be avoided because it adds sodium.