Medicine – Causes of Hypernatraemia

Hypernatraemia is an elevation of the serum sodium concentration, usually defined as:

Serum Na⁺ >145 mmol/L.

The most important concept is that hypernatraemia is usually a disorder of water balance rather than simply an excess of sodium. In most patients, the problem is:

Too little water relative to total-body sodium and potassium.

Therefore, the major mechanisms are:

Free-water loss.

Inadequate water intake/replacement.

Excess sodium administration.

Hypernatraemia produces hypertonicity, causing water to move out of cells. The brain is particularly vulnerable to this cellular dehydration.

1. Basic Mechanism

Serum sodium concentration rises when:

Water loss > sodium loss

or when:

Sodium gain > water gain.

Therefore:

FREE-WATER DEFICIT → ↑ SERUM Na⁺.

This is why hypernatraemia is particularly common when a patient cannot respond normally to thirst.

2. Role of Thirst

An intact thirst mechanism is an extremely powerful defence against hypernatraemia.

When plasma osmolality rises:

Hypothalamic osmoreceptors stimulated

↓

Thirst increases

• ●

ADH secretion increases

↓

Patient drinks water

• ●

Kidneys conserve water

↓

Serum osmolality moves toward normal.

Therefore persistent severe hypernatraemia usually develops when there is:

Impaired access to water

or

Water loss so large that intake cannot compensate.

3. High-Risk Patients

Hypernatraemia is particularly likely in patients who cannot independently obtain water.

Examples include:

Infants.

Frail older adults.

Unconscious patients.

Intubated patients.

Patients with neurological impairment.

Patients dependent on others for fluid intake.

Therefore hypernatraemia in hospitalised or dependent patients should always prompt careful assessment of:

Fluid intake and ongoing losses.

4. Major Classification

The causes can be organised into:

Water loss.

Sodium gain.

Water loss through osmotic diuresis.

Diabetes insipidus.

This is more useful than memorising individual diseases without understanding their mechanisms.

5. Water Loss Without Adequate Replacement

The original notes correctly identify:

Fluid loss without water replacement

as a major cause.

Water can be lost through:

Skin.

Respiratory tract.

Gastrointestinal tract.

Kidneys.

If the lost water is not adequately replaced, serum sodium rises.

6. Insensible Water Loss

Water is continuously lost through:

Skin

and

Respiration.

These are called:

Insensible losses.

Normally they are replaced by drinking water.

However, insensible losses increase substantially with:

Fever.

Tachypnoea.

High environmental temperature.

Mechanical ventilation in some circumstances.

If replacement is inadequate:

Free-water deficit → hypernatraemia.

7. Burns

The original notes correctly include:

Burns.

Extensive burns damage the normal skin barrier and can cause substantial:

Water and electrolyte loss.

Increased evaporative water loss from damaged skin can contribute to:

Hypernatraemia, particularly if replacement does not adequately match free-water losses.

However, the exact sodium abnormality depends on the composition of losses and replacement fluids.

8. Vomiting

The original notes include:

Vomiting.

Vomiting causes loss of both:

Water

and

Electrolytes.

Hypernatraemia can occur when the water deficit becomes proportionally greater and the patient cannot replace the lost water.

Therefore:

VOMITING + POOR WATER INTAKE → POSSIBLE HYPERNATRAEMIC DEHYDRATION.

However, vomiting does not automatically cause hypernatraemia; depending on replacement and physiology, sodium may be normal or low.

9. Diarrhoea

An important additional gastrointestinal cause is:

Diarrhoea.

Severe watery diarrhoea can cause substantial water loss.

If:

Water loss exceeds sodium loss

and replacement is inadequate:

Hypernatraemia develops.

This is particularly important in:

Infants and dependent older adults.

10. Excessive Sweating

Profuse sweating can also produce hypernatraemia.

Sweat is generally:

Hypotonic relative to plasma.

Therefore the body loses proportionally more water than sodium.

With prolonged sweating and inadequate water replacement:

Serum sodium rises.

11. Sodium Gain

Hypernatraemia can also occur because of:

Excessive sodium administration.

This is less common than water-loss hypernatraemia but is particularly relevant in:

Hospital settings.

12. Excessive Saline Administration

The original notes state:

“Excessive fluid replacement with saline.”

This requires some qualification.

Hypernatraemia is most likely when a patient receives excessive amounts of:

Hypertonic sodium-containing fluid, such as hypertonic saline.

Large sodium loads can raise extracellular sodium concentration, particularly if renal sodium excretion is impaired or insufficient water is available.

13. Normal Saline – Important Clarification

Ordinary:

0.9% sodium chloride

contains approximately:

154 mmol/L of sodium.

It is isotonic saline and is not usually a major direct cause of severe hypernatraemia when appropriately administered.

The more classic iatrogenic sodium-loading causes are:

Hypertonic saline.

Excess sodium bicarbonate.

Incorrectly prepared/high-sodium feeds or solutions.

Therefore the original phrase “excessive saline” is better remembered as:

EXCESSIVE HYPERTONIC SODIUM ADMINISTRATION.

14. Sodium Bicarbonate

Large amounts of:

Sodium bicarbonate

can provide a substantial sodium load.

Therefore excessive administration can contribute to:

Hypernatraemia.

This is another important example of:

Sodium-gain hypernatraemia.

15. Hyperosmolar Hyperglycaemic State

The original term:

Hyperosmolar non-ketotic state – HONK

is now more commonly called:

Hyperosmolar hyperglycaemic state – HHS.

HHS is characterised by:

Severe hyperglycaemia.

Marked hyperosmolality.

Profound dehydration.

Usually relatively little ketoacidosis compared with DKA, although overlap can occur.

16. Why HHS Causes Severe Water Loss

Very high blood glucose exceeds the renal threshold.

↓

Glucose enters urine.

↓

Glucose acts as an osmotic agent.

↓

Osmotic diuresis.

↓

Large urinary losses of:

Water + sodium + potassium.

However, the water deficit can become proportionally very large.

Therefore:

HHS → PROFOUND FREE-WATER DEFICIT.

17. Sodium in HHS Can Be Misleading

Early in severe hyperglycaemia, extracellular glucose draws water:

Out of cells.

This may initially lower the measured sodium through:

Translocational dilution.

However, ongoing osmotic diuresis causes enormous free-water losses.

Therefore the measured sodium may become:

Normal or high, and the corrected sodium may reveal substantial hypernatraemia even when the initial measured sodium is not elevated.

18. Diabetes Insipidus

The original notes correctly identify:

Diabetes insipidus – DI

as one of the most important causes of hypernatraemia.

DI causes inability to conserve free water.

There are two major forms:

Central DI.

Nephrogenic DI.

19. Central Diabetes Insipidus

Central DI results from:

Insufficient ADH/vasopressin secretion.

Therefore:

↓ ADH

↓

↓ Collecting-duct water reabsorption

↓

Large quantities of:

Dilute urine

↓

Free-water loss

↓

Hypernatraemia if water intake cannot keep pace.

20. Causes of Central DI

Important causes include:

Pituitary/hypothalamic surgery.

Head trauma.

Craniopharyngioma and other hypothalamic/pituitary lesions.

Sarcoidosis.

Langerhans cell histiocytosis.

Idiopathic/autoimmune disease.

Genetic disorders such as Wolfram syndrome.

21. Nephrogenic Diabetes Insipidus

In nephrogenic DI:

ADH is present

but:

The kidney does not respond adequately.

Therefore the collecting ducts cannot appropriately conserve water.

The result is:

Large-volume dilute urine → free-water loss → possible hypernatraemia.

22. Causes of Nephrogenic DI

Important causes include:

Lithium.

Hypercalcaemia.

Hypokalaemia.

Chronic renal tubular/interstitial disease.

Inherited AVPR2 or AQP2 abnormalities.

For examination purposes:

LITHIUM = CLASSIC ACQUIRED NEPHROGENIC DI CAUSE.

23. Why DI Does Not Always Cause Hypernatraemia

This is an important clinical point.

A patient with DI and an intact thirst mechanism may drink enough water to replace urinary losses.

Therefore:

DI does not automatically mean hypernatraemia.

Hypernatraemia develops particularly when:

Water intake cannot match urinary water loss.

This is especially dangerous in unconscious, confused or dependent patients.

24. Conn Syndrome

The original notes include:

Conn syndrome – primary hyperaldosteronism.

Aldosterone promotes:

Na⁺ reabsorption

and

K⁺ and H⁺ secretion.

Therefore primary aldosteronism classically produces:

Hypertension.

Hypokalaemia.

Metabolic alkalosis.

25. Does Conn Syndrome Usually Cause Hypernatraemia?

This requires an important correction.

Despite increased sodium reabsorption:

Clinically significant hypernatraemia is NOT a typical feature of primary hyperaldosteronism.

Why?

Initial sodium retention causes extracellular volume expansion.

↓

This suppresses proximal sodium reabsorption and promotes:

Pressure natriuresis.

↓

Other natriuretic mechanisms increase sodium excretion.

↓

A new sodium balance is established.

This phenomenon is sometimes called:

Aldosterone escape.

Therefore serum sodium usually remains:

Normal or only minimally increased.

26. High-Yield Conn Pattern

For examinations, Conn syndrome is much better remembered as:

HYPERTENSION + HYPOKALAEMIA + METABOLIC ALKALOSIS.

With:

↑ Aldosterone

and

↓ Renin.

Do not rely on hypernatraemia as a major diagnostic feature.

27. Renal Water Loss

Besides DI and HHS, other renal processes can cause excessive water loss.

These include:

Osmotic diuresis.

Post-obstructive diuresis.

Recovery phase of AKI.

Certain diuretics in appropriate circumstances.

If water replacement is inadequate, these conditions may produce:

Hypernatraemia.

28. Osmotic Diuresis

An osmotic substance within the renal tubules retains water in the tubular lumen.

This increases urine output.

Important examples include:

Glucose in uncontrolled diabetes mellitus.

Mannitol.

Therefore:

OSMOTIC DIURESIS → ↑ URINARY WATER LOSS → POSSIBLE HYPERNATRAEMIA.

29. Post-Obstructive Diuresis

After relief of significant urinary obstruction, some patients develop:

Marked polyuria.

The kidneys may temporarily have impaired concentrating ability and excrete large quantities of water and electrolytes.

Without adequate replacement:

Dehydration + hypernatraemia

can develop.

30. Clinical Features of Hypernatraemia

Symptoms largely result from:

Hypertonicity and cellular dehydration.

The brain is especially affected.

Patients may develop:

Intense thirst.

Weakness.

Irritability.

Lethargy.

Confusion.

Neuromuscular hyperexcitability.

31. Severe Hypernatraemia

Severe or rapidly developing hypernatraemia can cause:

Marked neurological dysfunction.

Seizures.

Reduced consciousness.

Coma.

In severe acute cases, brain-cell shrinkage can contribute to:

Intracranial vascular injury or haemorrhage.

32. Chronic Adaptation

When hypernatraemia develops gradually, brain cells adapt by accumulating intracellular osmolytes.

This reduces cellular water loss and limits brain shrinkage.

However, this adaptation creates an important treatment issue:

Chronic hypernatraemia should generally be corrected carefully rather than abruptly.

33. Investigation

The first questions should be:

Is the patient losing water?

Can the patient access water?

Is urine appropriately concentrated?

Has the patient received excessive sodium?

Useful investigations include:

Serum sodium.

Serum glucose.

Renal function.

Serum and urine osmolality.

Urine volume.

Urinary electrolytes when appropriate.

34. Urine Osmolality

Urine osmolality is particularly useful.

If a hypernatraemic patient has:

Very concentrated urine,

the kidneys are appropriately conserving water.

This suggests water loss from elsewhere or inadequate intake, such as:

GI loss, skin loss or impaired access to water.

If the patient has:

Inappropriately dilute urine despite hypernatraemia,

the kidneys are failing to conserve water.

Think particularly about:

Diabetes insipidus.

35. Hypernatraemia With Polyuria

A particularly useful clinical pattern is:

Hypernatraemia + polyuria.

Think:

Diabetes insipidus.

Osmotic diuresis from hyperglycaemia.

Post-obstructive diuresis.

Recovery phase of AKI.

Urine osmolality and glucose help distinguish these mechanisms.

36. Hypernatraemia With Low Urine Volume

If the patient has hypernatraemia but the kidneys are producing a small amount of concentrated urine, think more about:

Extrarenal water loss

or

Inadequate water intake.

Examples include:

Fever.

Sweating.

Burns.

Diarrhoea.

Vomiting with inadequate replacement.

37. Treatment Principles

The fundamental treatment is:

Replace the water deficit

and

Treat the underlying cause.

However, treatment must take into account:

Circulatory status.

Severity.

Duration of hypernatraemia.

Ongoing water losses.

38. Hypovolaemic Hypernatraemia

If the patient has severe intravascular volume depletion or shock, the immediate priority is restoration of:

Circulating volume.

Isotonic crystalloid may initially be required for haemodynamic resuscitation.

Once circulation is stabilised, the remaining:

Free-water deficit

can be corrected with an appropriate hypotonic strategy.

39. Free-Water Replacement

Depending on the clinical situation, free water may be replaced:

Orally.

Enterally.

or using appropriate:

Intravenous hypotonic fluid.

The exact strategy depends on the patient’s clinical condition and electrolyte status.

40. Treatment of Central DI

Central DI is generally treated with:

Desmopressin – DDAVP

when appropriate.

Desmopressin replaces the deficient antidiuretic effect.

Therefore:

DESMOPRESSIN → ↑ COLLECTING-DUCT WATER REABSORPTION → ↓ URINE OUTPUT.

41. Treatment of Nephrogenic DI

Treatment involves correcting the underlying cause where possible.

Examples include:

Correct hypercalcaemia.

Correct hypokalaemia.

Review lithium therapy.

Ensure adequate water replacement.

Selected patients may benefit from:

Thiazide therapy, and amiloride is particularly useful in lithium-associated nephrogenic DI.

42. Avoid Over-Rapid Correction

In chronic hypernatraemia, rapid reduction of extracellular osmolality can cause water to move rapidly into adapted brain cells.

This creates a risk of:

Cerebral oedema.

Therefore chronic or duration-unknown hypernatraemia is generally corrected:

Gradually with careful monitoring.

43. Causes of Hypernatraemia – Note Form

INADEQUATE WATER REPLACEMENT / EXTRARENAL WATER LOSS:

Burns.

Fever.

Sweating.

Tachypnoea.

Vomiting.

Diarrhoea.

Inability to access water.

RENAL WATER LOSS:

Central diabetes insipidus.

Nephrogenic diabetes insipidus.

Osmotic diuresis.

Post-obstructive diuresis.

Recovery phase of AKI.

HYPERGLYCAEMIC OSMOTIC DIURESIS:

Hyperosmolar hyperglycaemic state – HHS.

Severe uncontrolled diabetes.

SODIUM GAIN:

Hypertonic saline.

Excess sodium bicarbonate.

Excessive high-sodium solutions/feeds.

44. Important Corrections to the Original Notes

The original:

“HONK”

is better called:

HYPEROSMOLAR HYPERGLYCAEMIC STATE – HHS.

HHS causes profound dehydration through:

Glucose-induced osmotic diuresis.

The original:

“Excessive fluid replacement with saline”

is more accurately remembered as:

EXCESSIVE SODIUM ADMINISTRATION, PARTICULARLY HYPERTONIC SALINE OR OTHER LARGE SODIUM LOADS.

The original:

“Conn syndrome”

requires qualification.

Primary hyperaldosteronism causes sodium retention, but persistent clinically significant hypernatraemia is unusual because of aldosterone escape and intact thirst/water regulation.

Remember Conn syndrome primarily as:

HYPERTENSION + HYPOKALAEMIA + METABOLIC ALKALOSIS.

The original:

“Fluid loss without water replacement”

is the most important general principle.

Hypernatraemia most commonly reflects:

WATER DEFICIT RELATIVE TO SODIUM.

Key Clinical Pattern

For rapid recall:

HYPERNATRAEMIA = THINK TOO LITTLE WATER FIRST.

WATER LOSS:

BURNS + FEVER + SWEATING + VOMITING + DIARRHOEA.

↓

No adequate water replacement

↓

HYPERNATRAEMIA.

POLYURIA + HYPERNATRAEMIA:

Think:

DIABETES INSIPIDUS

or

OSMOTIC DIURESIS.

HHS:

SEVERE HYPERGLYCAEMIA → GLYCOSURIA → OSMOTIC DIURESIS → MASSIVE WATER LOSS.

SODIUM GAIN:

Think:

HYPERTONIC SALINE / EXCESS SODIUM BICARBONATE.

CONN SYNDROME:

Do not primarily think hypernatraemia.

Think:

HYPERTENSION + ↓ K⁺ + METABOLIC ALKALOSIS + ↑ ALDOSTERONE + ↓ RENIN.

And the central principle is:

HYPONATRAEMIA IS USUALLY RELATIVE WATER EXCESS.

HYPERNATRAEMIA IS USUALLY RELATIVE WATER DEFICIT.