Mild chronic hyponatremia is associated with falls, unsteadiness, and attention deficits.

OBJECTIVE: The study objective was to determine the eventual consequences (falls, unsteadiness, and cognitive impairment) of mild chronic hyponatremia, which is generally considered as asymptomatic.

METHODS: In a case-control study, we focused on the incidence of falls among 122 patients (mean age 72+/-13 years) with asymptomatic chronic hyponatremia (mean serum sodium concentration [SNa] 126+/-5 mEq/L), who were admitted to the medical emergency department, compared with 244 matched controls. To explore the mechanisms of the excess of falls, we prospectively asked 16 comparable patients (mean age 63+/-15 years; SNa+/-2 mEq/L) to perform 8 attention tests and a gait test consisting of 3 steps "in tandem," in which we measured the "total traveled way" by the center of pressure or total traveled way. Thereafter, the patients were treated and tested again (50% of the patients were tested first with normal SNa to avoid learning biases).

RESULTS: Epidemiology of falls: Twenty-six patients (21.3%) of 122 were admitted for falls, compared with only 5.3% of the control patients (adjusted odds ratio: 67; 95% confidence: 7.5-607; P <.001). The frequency of falls was the same regardless of the level of hyponatremia. Gait: The total traveled way by the center of pressure significantly increased in hyponatremia (1336+/-320 mm vs 1047+/-172 mm with normal SNa; P=.003). Attention tests: The mean response time was 673+/-182 milliseconds in hyponatremia and 615+/-184 milliseconds in patients with normal SNa (difference: 58 milliseconds, P <.001). The total error number in hyponatremia increased 1.2-fold (P=.001). These modifications were comparable to those observed after alcohol intake in 10 volunteers.

CONCLUSIONS: Mild chronic hyponatremia induces a high incidence of falls possibly as the result of marked gait and attention impairments. Treating these patients might prevent a considerable number of hospitalizations.