Seasonality of Hypoosmolar Hyponatremia in Medical Inpatients - Data from a Nationwide Cohort Study.

CONTEXT: Hyponatremia is the most prevalent electrolyte disturbance in hospitalized patients. Previous studies have shown a seasonal variation of profound hyponatremia with higher prevalence during warmer months.
OBJECTIVE: This study aimed at analyzing the seasonal prevalence and sex- and age-specific differences of hyponatremia in medical inpatients.
DESIGN: Nationwide cohort study from January 2009 and December 2015 using prospective administrative data.
SETTING: Medical inpatients. PATIENTS: Diagnosis of hypoosmolar hyponatremia. MAIN OUTCOME MEASURES: The primary outcome was the monthly alteration in hyponatremia prevalence. Secondary outcomes were the association of outdoor temperature with hyponatremia prevalence and differences among sex and age groups.
RESULTS: Of 2 426 722 medical inpatients, 84 210 were diagnosed with hypoosmolar hyponatremia, of whom 61% (n = 51 262) were female. The highest overall prevalence of hyponatremia was observed in July (4.5%, n = 8976); the lowest in December (2.7%, n = 6530). The overall prevalence of hyponatremia in women compared with men was higher by 58% (odds ratio [OR], 1.58; 95% confidence interval [CI], 1.56-1.60). The sex-specific difference was most pronounced in the warmest month of July (mean temperature 20.1°C (OR, 1.76; 95% CI, 1.68-1.84). We observed the strongest association between seasonality and hyponatremia in elderly (>80 years) female inpatients admitted during the month of July (OR, 2.40; 95% CI, 2.20-2.62]).
CONCLUSION: The prevalence of diagnosed hypoosmolar hyponatremia in medical inpatients increases during summer months with higher outdoor temperature. Elderly female inpatients were most susceptible to the seasonal rise in hyponatremia prevalence. © Endocrine Society 2020. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.