Endocrine Abstracts

The risk of death with severe hyponatraemia is well known. What is less clear is the mortality risk according to the pattern of the developing hyponatraemia.

Methods: From our laboratory database we retrospectively collected data of all adult patients with severe hyponatraemia (<120 mmol/l) over 6 months. Of 49 155 samples, 101 (0.2%) were <120 mmol/l, obtained from n=54 patients. Two paediatric cases were excluded leaving n=52. Normonatraemic controls (n=52) were identified by plasma sodium of 135 mmol/l over the same study period, and whose nadir during hospitalisation was ≥130 mmol/l. Results are mean±S.D. Duration of hospitalisation, expressed as median (range), was confirmed from discharge summaries. Unpaired t-test, χ² and Mann–Whitney tests were used as necessary.

Results: Hyponatraemic patients (age 63±19 years, M25, F27) did not differ from controls (age 74±13years, M21, F31). Medical admissions were more prevalent than surgical in both groups (n=45 vs n=47, NS). Admission sodium in hyponatraemic patients was 120.2±9.3 vs 136.5±3.4 mmol/l in controls (P<0.001), with nadir during hospitalisation of 115.3±3.9 mmol/l vs 134.1±2.2 mmol/l in controls (P<0.001). Hyponatraemic patients had higher mortality (n=17 vs n=4, P=0.002) and longer hospitalisation: 12.5 (1–58) days vs 6.5 (1–32) days, than controls (P<0.001). In 26 patients (50%), severe hyponatraemia occurred only after admission. This subgroup comprised a higher proportion of surgical patients (23.1% vs 4%, P=0.04) than those whose nadir was on admission. Furthermore, both mortality (N=13 vs N=4, P=0.01) and duration of hospitalisation, 17 (3–58) days vs 9 (1–57) days, were greater (P=0.05). Only 3 of the 13 patients (23%) who died in this subgroup had assessment of adrenal function.

Conclusions: Severe hyponatraemia in hospitalised patients is associated with prolonged admission and increased mortality than normonatraemic patients. Progressive hyponatraemia following admission incurs a higher risk of death. This may represent illness-severity, sick-cell syndrome, iatrogenic disease or undiagnosed adrenal dysfunction.