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Primary Care Hacks

Hypernatraemia in Primary Care

Guidelines presents Primary Care Hacks, a series of clinical aide-memoires across a range of topics. Developed by Dr Kevin Fernando, Primary Care Hacks aim to provide a quick and easy resource for primary healthcare professionals and ultimately help improve patients' lives.

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Hypernatraemia in Primary Care

Abnormal sodium levels are commonly encountered in primary care. This Primary Care Hack on hyponatraemia, alongside the companion Primary Care Hack on hyponatraemia, will aid appropriate investigation of abnormal sodium levels to help elucidate possible underlying causes.

Click on the link below for a downloadable PDF of this Primary Care Hack

Hypernatraemia in Primary Care

What is Hypernatraemia?
  • Hypernatraemia is defined as a serum sodium concentration of >145 mmol/l
  • The clinical effects of hypernatraemia depend on speed of onset, severity, and underlying cause
  • Hypernatraemia occurs due to net water loss or excess sodium intake
  • The most common cause of hyponatraemia is dehydration. Even in mild cases, hypernatraemia is a potent stimulator of thirst. It can also present with confusion, headache, nausea and vomiting, lethargy, irritability, seizures, or coma
  • Individuals over 65 years of age, those with dementia, or those in residential care are at increased risk of hypernatraemia
  • Hypernatraemia is always associated with an elevated serum osmolality >295 mmol/kg.
Causes of Hypernatraemia
  • Low fluid intake and dehydration
  • GI and transdermal water loss, e.g. diarrhoea and vomiting, sweating and burns
  • Renal water loss, e.g. osmotic diuretics, diabetes (see below), polyuria of acute tubular necrosis
  • Osmotic diuresis due to undiagnosed or suboptimally controlled diabetes or HHS (formerly HONK)
    • HHS is a life-threatening diabetic emergency characterised by severe hyperglycaemia and dehydration and high serum osmolality
  • DI (rare, 1:25000)
    • central DI usually results from pituitary pathology affecting ADH production, e.g. brain tumour or head injury
    • nephrogenic DI is a renal resistance to ADH, e.g. electrolyte disturbance (hypercalacaemia or hypokalaemia), kidney disease, or drug toxicity (commonly lithium)
  • Conditions leading to hyperaldosteronism/ sodium excess such as Conn’s syndrome (primary aldosteronism), Cushing’s syndrome (persistently and inappropriately elevated levels of cortisol), chronic heart failure, nephrotic syndrome, or use of steroids
  • Excess sodium intake (rare).
Investigations for Hypernatraemia
  • Serum osmolality is a measure of the concentration of different solutes in plasma and is primarily determined by sodium, glucose, and urea. NR is usually 275–295 mmol/kg and is tightly maintained by ADH, which regulates fluid balance. An increase in serum osmolality results in secretion of ADH, which increases water reabsorption in the kidneys to return serum osmolality to baseline
  • Urine osmolality is a measure of urine concentration and whether this is appropriate for the clinical state of the individual. It provides an estimate of ADH activity. NR is usually 300–900 mmol/kg water. If osmolality ≤100 mOsm/kg (dilute urine), ADH is not acting. If osmolality is >100 mOsm/kg (concentrated urine), ADH is acting. After 12–14 hours’ fluid restriction, urinary osmolality should be >850 mmol/kg water
  • Urinary sodium is a measure of the concentration of sodium in a litre of urine. It is useful for the differential diagnosis of hyponatraemia but must be interpreted alongside volume status, and is therefore difficult to interpret in those taking diuretics
  • Serum urea is a marker of extracellular fluid volume. A raised urea may suggest dehydration.
ADH=antidiuretic hormone; CRT=capillary refill time; DH=diuretic hormone; DI=diabetes insipidus; HbA1c=haemoglobin A1c; HONK=hyperosmolar non-ketotic syndrome; HHS=hypersomolar hyperglycaemic state; U/E=urea and electrolytes JVP=jugular venous pressure; Na=sodium.
Primary Care Hacks are developed by Dr Kevin Fernando, GP Partner, North Berwick Health Centre; GP with special interest in CVRM and medical education; Content Advisor for WebMD Medscape Global and UK. This Primary Care Hack is based on the author's interpretation of relevant summaries of product characteristics. Primary Care Hacks are for information for primary healthcare professionals in the UK only. They bring together currently available recommendations and/or prescribing information and indications for therapeutics licensed within Great Britain. Licensed indications and/or prescribing information for Northern Ireland may differ. You are advised to review local licensed indications before prescribing any therapeutic. Primary Care Hacks are reviewed intermittently to ensure the information is up to date at the time of publication. Primary Care Hacks are independently produced by WebMD, LLC and have not been created in conjunction with any guideline or prescribing body.