Question 24

With respect to hyponatraemia:

a) Outline the classification and underlying causes. (50% marks)

a) Outline the specific treatment of severe hyponatraemia (i.e. sodium level < 120 mmol/L and/or associated with significant adverse symptoms). (50% marks)

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College Answer


Classify hyponatraemia:

  • Hypertonic
  • Isotonic
  • Hypotonic

Further subdivide hypotonic:

  • Hypervolaemic
  • Euvolaemic
  • Hypovolaemic

Serum Na+ < 135 mmol/L

What is serum osmolality?

Low (< 285 mOsm/kg)

Normal (285 – 295 mOsm/kg)

High (> 295 mOsm/kg)





Hypertonic infusions



What is volume status?

What is urinary [Na+]?

< 20 mmol/L

> 20 mmol/L




Skin losses 

excess sweating 3rd space losses  Burns




Renal tubular acidosis

Adrenal insufficiency


Water intoxication

Decreased solute intake

Renal failure


Adrenal insufficiency


Cerebral salt wasting



Heart failure

Nephrotic syndrome

Acute renal failure Chronic renal failure


Management is divided into 

  • Emergency and short term sodium elevation
  • Specific treatment of the underlying cause

Emergency treatment of sodium level

If the patient is symptomatic, then the serum sodium level needs to be urgently elevated by approximately 2- 4% e.g. 2.5 – 5 mmol/L (these are rough figures).

This is done by giving a specific sodium dose, which is usually in the form of hypertonic saline (e.g. 3%) to avoid any more excess water, over a brief period e.g. 30 minutes.

It is calculated by the following formula.

Sodium dose = Total body water x desired change in sodium level

For example in a 70 kg man, a total of 200 ml of 3% saline will raise the serum sodium by 2.5 mmol/L

Short-term sodium management

Once symptoms have resolved, the aim is to correct the sodium level by roughly 0.5 – 1.0 mmol/L per hour over the next 24 hours. And how this is done depends on the underlying cause.

Specific treatment of underlying cause:

  • Fluid restriction for water intoxication / SIADH
  • Fluid restriction for cardiac / liver failure
  • Fluid rehydration with appropriate fluids for vomiting / diarrhoea
  • Aggressive fluid resuscitation for significant fluid losses e.g. pancreatitis or burns
  • Stopping of offending medications e.g. diuretics
  • Thyroid replacement for hypothyroidism
  • Steroids for adrenal insufficiency
  • Arginine vasopressin receptor antagonists (e.g. conivaptan)



Mindlessly regurgitate the hyponatremia algorithm? Don't mind if I do. This is the "classical" approach:

The "classical" diagnostic algorithm for hyponatremia

In word form:

  • Causes of hyper-osmolar hyponatremia
  • Causes of iso-osmolar hyponatremia
  • Causes of hypo-osmolar hyponatremia:
    • With hypervolemia:
      • Congestive heart failure
      • Cirrhosis
      • Nephrotic syndrome
      • Acute renal failure
      • Chronic renal failure
    • With hypovolemia:
      • Hypovolemia
      • Diarrhoea
      • Vomiting
      • Sweating
      • Blood loss
      • Burns
      • Pancreatitis
      • Diuretics, eg. thiazides
      • Hypoaldosteronism (and spironolactone)
      • Renal tubular acidosis
      • Cerebral salt wasting
      • Osmotic diuresis
      • Ketonuria
      • Bicarbonate wasting in metabolic alkalosis
    • With euvolemia:
      • Psychogenic polydipsia
      • Beer potomania
      • Hypothyroidism
      • Hypoadrenalism
      • Glucocorticoid deficiency
      • SIADH


In the management of severe (symptomatic) hyponatremia, the college were clearly after some sort of hypertonic saline protocol. The fact that the hyponatremia is being described as "severe" and "symptomatic" suggests that simple fluid restriction was not going to cut it. However, the alternative smust be mentioned. Thus:

  • Hypervolemic and euvolemic hyponatremia should be managed with fluid restriction
  • Hypovolemic hyponatremia must be managed with isotonic saline replacement
  • Hypertonic saline:
    • calculate the deficit:
      Sodium deficit = 0.6 ×body weight × (desired Na+ - current Na+)
    • Correct the deficit at a rate of change no more than 0.5mmol/L/hr.
  • Other strategies:
    • Loop diuretics
    • Hydrocortisone or fludrocortisone
    • SIADH-specific therapy:
      • Demeclocycline (oral)
      • Tolvaptan or satavaptan (oral)
      • Conivaptan (IV)
      • Lithium (oral)
      • Urea (oral)

Where it comes to a discussion of hyptertonic saline, the college recommend rapid corection of symptomatic hyponatremia, which is followed by slow correction of asymptomatic hyponatremia. This is consistent with the recent European guidelines (Spasovski et al, 2014). The guideline development group felt that the risk of brain oedema outweighs the risk of osmotic demyelination syndrome. Specifically, they recommend the infusion of 150ml of 3% saline over 20 minutes, then checking the sodium, and then repeating the infusion.


Spasovski, Goce, et al. "Clinical practice guideline on diagnosis and treatment of hyponatraemia." European Journal of Endocrinology 170.3 (2014): G1-G47.

David M., Arnold S. Berns, and Anthony D. Ivankovich. "Isotonic hyponatremia following transurethral prostate resection." Journal of clinical anesthesia 2.1 (1990): 48-53.