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Hypernatremia ([Na⁺] >145 mEq/L) is associated with extravascular hypertonicity with resultant intracellular dehydration.

Clinical presentations may include altered mental status, irritability, hyperreflexia, tachycardia, orthostasis, and dry mucous membranes.^3,7,10,21 The differential diagnosis would less commonly include salt overload (iatrogenic saline infusion or tube feedings), nephrogenic diabetes insipidus (e.g. associated with lithium exposure), or central diabetes insipidus (due to central nervous system disorders). Salt overload may be diagnosed by the patient’s history. The urine osmolality will be inappropriately low for the serum osmolality (or iso‐osmolar) to the serum in cases of diabetes insipidus. Response to DDAVP or measurement of copeptin^49,50 will distinguish central from nephrogenic diabetes insipidus.

As discussed previously, the more common causes of hypernatremia in the frail elderly are due to diminished thirst, inadequate access to fluids, and superimposed water losses due to gastrointestinal and febrile illnesses.^{2,3,13,14,20,51} People with dementia may have further diminished thirst responses and inadequate ADH responses similar to those observed in partial central diabetes insipidus.¹⁷

The most clinically significant symptoms are associated with central nervous system dysfunction, ranging from confusion to coma. In extreme cases, brain dehydration may be associated with vascular rupture and subarachnoid haemorrhage. Chronic dehydration results in adaptive responses by the intracellular generation of organic osmoles. Correcting chronic hypernatremia must consider that the overly rapid correction of the hyponatremia may result in cerebral oedema. Therefore, acute management must follow the serum sodium on a regular basis (q 4 hours) to adjust the regimen.^1,2

Treatment should reverse the underlying causes, such as treating pyrexia, managing gastrointestinal fluid losses, and withholding diuretics or lactulose. Correcting the hyperosmolality should then proceed with a goal to decrease serum osmolality incrementally by approximately 10 mOsm/kg over 24 hours rather than full correction of serum sodium to normal levels.¹⁰ There are various formulas for correction, which are usually based on the correction of total body water (see below).^9,10 One must also supplement for anticipated continuing obligatory water losses (insensible, urinary, and gastrointestinal water loss). Obligatory water loss may range from 0.5 to 1.5 litres per day of free water. Replacement fluids may include 0.9% saline, 0.45% saline, and 5% dextrose in water. If the dehydration is purely due to water without solute loss, the regimen for D5% water may be the easiest to calculate. The concept is to supplement the free water losses with obligatory losses. The calculated free water should reduce the current serum sodium by 10 mOsm/kg over 24 hours:

where TBW is total body water, and

and

or

In situations where solute is lost in addition to pure water, 0.45% saline may be a more appropriate intravenous infusion. The infused sodium will stay in the intravascular space to raise the blood pressure. However, 0.45% saline corrects the osmolality by the free water component.¹⁰ Thus an infusion with 0.9% saline (154 mEq/L) will only add free water relative to the difference from the patient’s serum sodium, and 0.45% saline is only 50% free water. Because of the fear that overly rapid correction of the hypernatremia will cause hyponatremic cerebral oedema, clinicians are prone to use 0.9% saline for the correction. However, infusion of 0.9% saline, although hypotonic to the existing serum sodium, will not reduce the serum osmolality and may not supply sufficient free water to keep up with obligatory losses.¹⁰ It should be noted that there are many assumptions in the calculations for rehydration: that there are no solute losses either before or after the infusion (i.e. the water changes are due to changes in pure water), that the total body water is either 50 or 60% of body weight, and that the insensible water losses are between 500 mL and 1500 mL over 24 hours, depending on the underlying illness, such as fever or diarrhoea. Therefore, it is mandatory to maintain close clinical and laboratory monitoring to allow for the large range of assumptions to remain within the desired correction range.