At no stage has the college ever asked about hypernatremia specifically, but it appears as a garnish to the following SAQs:

Oh's Manual coverage of hypernatremia is limited to half of page 954 from Finfer and Delaney's fluid and electrolyte chapter, which references only one paper (Adrogué and Madias, 2000). Of this half-page, the majority is taken up by a Blue Box, which classifies causes of hypernatremia into water loss and salt gain. Unfortunately, in the "salt gain" category there is only one entry: "Hypertonic, saline or sodium bicarbonate". The other category is sufficiently diverse to merit a tabulated listing:

Causes of  Hypernatremia

Extrarenal water loss

  • Dehydration by exposure
  • Burns
  • Gastric losses
  • Diarrhoea

Salt gain

  • Infusion of sodium-rich fluids of some sort (eg. hypertonic saline)
  • Ingestion of sea water
  • Salt pica

Nephrogenic DI

  • Hypercalcemia
  • hypokalemia
  • Lithium
  • Pyelonephritis
  • Medullary sponge kidney
  • Multiple myeloma
  • Amyloid
  • Sarcoid

Central DI

  • Traumatic brain injury
  • Pituitary tumour
  • Meningitis
  • Encephalitis
  • Tuberculosis
  • Sarcoidosis
  • Idiopathic

Renal losses

  • Glucosuria
  • Mannitol
  • Urea therapy
  • Loop diuretics
  • Post obstructive diuresis
  • Hyperaldosteronism

 

 

       

Generally, adverse effects of hypernatremia develop at sodium concentrations in excess of 155-160 mmol/L

  • Increased temperature
  • Restlessness
  • Irritability
  • Confusion
  • Drowsyness
  • Coma
  • Seizures
  • Subarachnoid haemorrhage (due to brain shrinkage and vascular rupture) 

Rapid rehydration may give rise to cerebral oedema, and this is the main risk in starting any sort of corrective therapy. If the hypernatremia developed over hours, reducing the sodium concentration by 1mmol/hr is appropriate. Just as in the correction of hyponatremia, a daily decrease of 10mmol/L of sodium is a sensible goal.

At some stage, the college may expect their candidates to calculate the water deficit.

A conventional equation is as follows:

Water deficit = total body water × (1- [140 ÷ serum sodium])

Adrogué and Madias (2000) recommend a refinement to this:

 

\(Change\space in \space sodium= {(Infusate \space Na^+ + Infusate \space K^+ ) - serum \space Na^+ \over (body\space weight \times 0.6) + 1 }\)

 

Thus, a 70kg male with a serum sodium of 160 will enjoy a 3mmol/L decrease in his serum sodium after receiving 1000ml of 5% dextrose with 30mmol of KCl in it.

References

Adrogué, Horacio J., and Nicolaos E. Madias. "Hypernatremia." New England Journal of Medicine 342.20 (2000): 1493-1499.