Hypernatraemia

What is hypernatraemia?

Hypernatraemia is defined as a serum sodium concentration exceeding 145 mmol/L. It is caused by either water deficit and excess solute.¹ Severe symptoms of hypernatraemia are usually only found with acute and large rises in sodium plasma concentration above 160 mmol/L.¹

Serum sodium concentration, and hence osmolality, is normally kept from rising significantly by the release of antidiuretic hormone (ADH) which results in concentration of urine, and the stimulation of thirst which increases water intake.¹

Epidemiology

Incidence

Hypernatraemia is relatively rare in primary care and more common in hospital where the homeostatic mechanisms are more likely to be impaired or subverted by intravenous fluids. Studies suggest that hypernatraemia is present in up to 3% of patients in hospital and up to 9% of patients in ITUs.² Other studies suggest that between 9% and 47% of patients had hypernatraemia during an ICU admission.³⁴

The incidence of hypernatraemia has increased significantly over the past 2 decades, increasing from 13% to 24%.⁴

Risk factors for hypernatraemia¹

Sustained hypernatraemia generally occurs when the thirst mechanism or independent access to water is impaired. Consequently, those most at risk include:

• Elderly patients - usually associated with infirmity or febrile illness, particularly with complete dependency on others for fluids.

• Infants - at risk with diarrhoea and inadequate breastfeeding, where there is a poor milk supply or an inexperienced mother. Hypernatraemic dehydration secondary to exclusive breastfeeding is an increasingly recognised problem.^{5 67} Estimates of prevalence range from 1-2% in higher income countries to up to 20% in lower income countries.

• Patients with altered mental status.

• Those with hypothalamic lesions affecting sense of thirst (adipsia).

• Those with critical illness causing acute kidney injury and consequent inability to concentrate urine, or dependency on intravenous fluids.

Hypernatraemia causes⁴

• Pure free water loss (dehydration):

  • Inadequate water intake.

  • Diabetes insipidus (DI) - either cranial or nephrogenic in origin.

  • Thirst impairment:

    • Dementia.

    • Hypothalamic lesions impairing osmoreceptor function or the thirst response.

• Hypotonic fluid loss (dehydration + hypovolaemia):

  • Dermal losses:

    • Burns.

    • Excessive sweating - endurance sportsmen and women, particularly under heat stress, are vulnerable. One study suggested that 25% of collapsed marathon runners were hypernatraemic, compared with 9% of asymptomatic marathon runners.⁸

  • Gastrointestinal losses:

    • Diarrhoea, laxative abuse.

    • Vomiting.

    • Nasogastric drains.

    • Fistulas.

  • Urinary losses:

    • Loop diuretics.

    • Osmotic diuresis - eg, hyperglycaemic states.

    • Acute tubular necrosis (polyuric early stage).

• Hypertonic sodium gain (may cause hypervolaemia):

  • Iatrogenic:

    • Use of hypertonic saline.

    • Tube feeding.

    • Intravenous antibiotics containing sodium.

    • Intravenous sodium bicarbonate.

    • Hypertonic dialysis.

    • Use of isotonic saline to replace losses in osmotic diuresis.

  • Excess salt ingestion:

    • Inadvertent - for example, infant formula error.

    • Poisoning - more common in children, and a type of non-accidental injury. This is thought to be very rare in the UK. The minimum amount of salt that could be fatal to an infant or child is likely to be in the range of 0.75 to 3 g/kg (approximately 13 to 51 mmol) body weight.

  • Hyperaldosteronism (usually only a mildly elevated sodium).

• Intracellular shift of water (rare):

  • Very strenuous exercise or electroshock-induced seizure causes a transient rise in cell osmolality and thus water moves into cells.

Hypernatraemia symptoms¹

Most patients with hypernatraemia present with evidence of dehydration and hypovolaemia (dry mouth, abnormal skin turgor, oliguria, tachycardia, or orthostatic hypotension).

Symptoms and signs specifically of hypernatraemia tend to occur when the level rises above 160 mmol/L. This causes symptoms related to CNS dysfunction (lethargy, weakness, confusion, irritability, myoclonic jerks and seizures).

In diabetes insipidus there is typically thirst, polydipsia, and polyuria.

In older people, symptoms and signs may not always be classical. Studies in the elderly have shown the most typical signs associated with hypernatraemia in the elderly to be abnormal subclavicular and thigh skin turgor, dry oral mucosa, and recent change in consciousness.⁹

Investigations

• Note that immediate admission to an emergency department is required if significant hypernatraemia is suspected clinically.

• Check serum sodium, potassium, urea, creatinine, calcium, and plasma glucose.

• Request lithium levels where appropriate. Lithium intake is associated with polyuria and diabetes insipidus therefore increasing the risks of hypernatraemia.¹⁰

• Request urine and serum osmolality if diabetes insipidus is suspected; in this case there would be a high serum osmolality (>300 mOsm/kg) combined with an inappropriately dilute urine (less than serum osmolality).

• Neuroimaging where indicated clinically.

Hypernatraemia treatment and management

Aims are to:

• Treat any underlying disorder if possible.

• Correct dehydration by replacing free water losses.

• Correct hypovolaemia, if present, by giving electrolytes in addition to free water.

Assess severity

• Repeat the blood test to confirm the true result, and exclude pseudohypernatraemia which may occur with hypoproteinaemic states and some methods of serum sodium measurement.

• Try to establish whether this is an acute and rapidly changing or chronic and stable picture; this and the patient's clinical condition are generally more important than the absolute serum sodium value in determining action.

• Seek specialist advice if a clinical cause is not apparent, oral rehydration is not possible, or where serum sodium is 155 mmol/l or more.

Correction of hypernatraemia¹¹

Address the underlying cause where possible - for example, stop gastrointestinal fluid losses, control pyrexia, correct hyperglycaemia, and withhold lactulose and diuretics. This may be sufficient to reverse the hypernatraemia. Where active correction of hypernatraemia is to be undertaken, fluids should be administered orally or enterally and intravenous therapy used only as a last resort. The key to hypernatraemia management is regular monitoring of the patient and the serum sodium, and then adjusting the hypotonic (relative to the patient's serum sodium) infusion accordingly.

Complications

• Cerebral bleeding, subarachnoid haemorrhage, subdural haemorrhage, permanent brain damage, and death secondary to the rupture of bridging veins with acute hypernatraemia.

• Cerebral oedema with overfast correction of chronic hypernatraemia. Also convulsions and permanent brain injury may occur. However recent studies suggest that faster correction than currently recommended may result in better outcomes.²¹²

Prognosis

The mortality rate depends on the severity of the condition and the rapidity of its onset:

• Severe hypernatraemia carries a mortality rate of approximately 37-60%.¹³

• Studies on critically ill patients in ICU suggest that hypernatraemia is an independent risk factor for mortality.¹⁴ Most cases appear to arise after admission to ICU and, therefore, may be at least partially iatrogenic in origin.^{15 16}

• Severe hypernatraemia occurs in up to 0.2% of children in hospital with a mortality rate of up to 15%.¹²

Prevention

Health professionals need to be alert to the risk of medical care itself precipitating hypernatraemia and other disorders of sodium and water balance in frail elderly or critically unwell patients.

To prevent hypernatraemic dehydration in breastfed infants, early weighing and lactation support are suggested in order to detect correctable problems swiftly.¹⁷ Daily weights and a 'rule of thumb' of referring infants that lose more than 10% of body weight in the first postnatal week are widely used, although some argue that the use of charts for relative weight change is a better screening strategy.¹⁸