Causes and outcomes of hyponatraemia at Mater Dei Hospital, Malta

Abstract

CONTEXT: Hyponatraemia is the most common electrolyte balance disorder in clinical practice, amounting to 15–20% of casualty visits. While there is general agreement that associated mortality rates are high, most studies are uncontrolled.

OBJECTIVE: To determine the characteristics, causes and outcome of severe hyponatraemia (<125 mmol/l) in hospitalised patients and to indentify mortality predictors.

DESIGN: This is a retrospective case-controlled study of all medical admissions in the months of February, June and November, who at any point during the index admission developed a serum sodium <125 mmol/l. For each case, an age- and gender-matched control was identified.

RESULTS: A total of 5195 medical admissions were reviewed. Of these, 193 patients had a sodium level <125 mmol/. 26 patients were excluded from the case group leaving a total of 167 cases and 193 controls. Length of hospital stay was more prolonged in the case group (12 vs 8 days, P<0.001). There was a highly significant excess mortality, both during the index admission (25% in cases vs 7% in controls (P < 0.001)) as well as till the end of the follow-up period (52% in cases vs 22% in controls (P<0.001)). Mortality was unrelated to severity of hyponatraemia. Patients who developed the lowest serum sodium later on during their admission (ie sodium levels continued falling during the admission or fell de novo), had a higher rate of mortality than patients whose lowest serum sodium was on the day of admission (64.3% vs 45%, P 0.019). A cox regression analysis showed that hyponatraemia (P<0.001), male gender (P 0.033), age (P 0.021), and serum creatinine level (P 0.008) were independent risk factors for mortality. There was no statistically significant difference between the rates of ITU admission at different levels of hyponatraemia <125 mmol/l (P=0.497). Thus, serum sodium levels of <125 mmol/l should be used to identify patients who need more intensive monitoring and therapy irrespective of the degree of hyponatraemia. Only 41% of cases developed neurological symptoms, of these, confusion and altered level of consciousness were the more prevalent at 12% each, followed by falls (9%), unsteady gait (4%) and seizures (4%). The cause for hyponatraemia was frequently poorly evaluated and in 23% of cases no definite diagnosis was made.

CONCLUSION: Data on assessment, investigation and management of hyponatraemia illustrates variability and shortcomings in clinical practice. The question remains whether the relationship between hyponatraemia and increased mortality is causal or associative.