Epidemiology

Hyponatraemia is the most common electrolyte disorder encountered in clinical practice.[1][6] It is estimated to occur in 5% to 35% of all hospital inpatients; however, the prevalence varies among studies.[2][3]​​ It should be noted that the serum sodium concentration used to define hyponatraemia varies among studies.

In a large, US-based cohort study that assessed over 50,000 hospital admissions, the prevalence of hyponatraemia on admission was found to be 37.9%.[7] An additional 38% of patients in this study with a normal initial sodium level developed hyponatraemia during admission. Most cases (79%) of hyponatraemia in the study were considered to be very mild (i.e., 133-137 mmol/L), 15.2% were mild (128-132 mmol/L), 3.7% moderate (123-127 mmol/L), and 1.7% severe (<122 mmol/L).

In another large, US-based cohort study that assessed over 198,000 hospital admissions, the prevalence of hyponatraemia on admission was found to be 5.5%.[8] Other smaller studies have found the prevalence of hyponatraemia to range between 7% and 11% in ambulatory outpatients.[9] A retrospective cohort study demonstrated that moderate hyponatraemia (125-130 mmol/L) and severe hyponatraemia (<125 mmol/L) were present in 2.7% and 1.2% of patients, respectively, on admission to intensive care units.[10] 

Hyponatraemia is common in elderly people, especially those who are hospitalised or living in nursing homes.[11]

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