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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.

      Hyponatremia may be associated with comorbidities of falls, bone fractures, and death from heart disease, although causality has not been proven. There are associations of chronic hyponatremia with underlying diseases (congestive heart failure or cirrhosis), and chronic hyponatremia is associated with dementia. There are also associations of acute changes in hyponatremia linked to delirium, gait disturbances, and falls. It would seem prudent to limit drugs that are associated with hyponatremia (commonly, thiazide diuretics and SSRIs) and avoid drugs that predispose individuals to orthostatic hypotension.28

      Correcting hypo‐ and hypernatremia should be guided by neurocognitive symptoms. In seriously symptomatic people or for hyponatremia ([Na+] <125 mEq/L) or hypernatremia ([Na+] >145 mEq/L), adjustments of serum over the ensuing 24 hours should be designed to increase the [Na+] by 6–8 mEq/L in hyponatremia (to avoid osmotic demyelination syndromes) or to decrease the [Na+] by approximately 10 mEq/L in hypernatremia (to avoid cerebral oedema). Vaptans are vasopressin antagonists that are shown to improve serum [Na+] in hyponatremia, allow discontinuation of fluid restriction, and have exhibited modest effects on quality of life testing. Further studies are necessary to provide guidance on the role of thirst and ADH responsiveness in cognitive disorders, what role dementia has in chronic hyponatremia, and the benefits of acute correction of hyponatremia in delirium states, parameters of gait, and neurocognitive function.

      Key points

       The elderly are at risk of hypo‐ and hypernatremia and need to be evaluated for abnormalities in thirst or mentation, untoward effects of medications, or physiological effects on arterial blood volume.

       Although hyponatremia is associated with comorbidities such as falls, bone fractures, and death, causality has not been proven.

       Cautious correction of serum sodium should be guided by neurocognitive symptoms in hyponatremia by [Na+] 6–8 mEq/L/24 hours and hypernatremia [Na+] <10 mEq/L/24 hours.

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