Sodium Disorders, Kidney Disease in the Elderly

Aging increases the risk of many disease states and impacts organ function. Of these, sodium disorders are common in the elderly, and the aged kidney may play a role in their development. Hyponatremia is more common than hypernatremia in the elderly. Defined as a serum sodium level of less than 135 mmol/L, hyponatremia is commonly associated with symptoms of nausea, vomiting, and altered mentation. Despite varied age cut-offs in studies to define “elderly,” various epidemiologic studies consistently show an increased prevalence of hyponatremia in patients older than 50–65 years of age compared to younger populations. Older kidneys possess an impaired ability to excrete free water, and older patients may also have underlying comorbidities or medication use which may increase their risk for hyponatremia. The aged kidney also has diluting defects, increasing the risk for hyponatremia, especially in settings of high free water intake. Hypernatremia defined by a serum sodium level of more than 145 mmol/L is less common, but it tends to develop in the elderly and infirmed population due to reduced intake of free water and increased free water excretion. The aged kidney acquires concentrating defects, which impair its ability to conserve free water, and underlying comorbidities such as dementia or neurologic deficits place patients at higher risk for reduced free water intake. Both hypernatremia and hyponatremia are associated with increased morbidity and mortality in older patients.