Is asymptomatic hyponatremia really asymptomatic?

Consequences of hyponatremia are generally mild and remain unnoticed by both physician and patient. When water restriction, usually prescribed to avoid water intoxication, fails to normalize serum sodium values, clinicians will tolerate mild stable hyponatremia (especially when serum sodium is >125 mEq/L [1 mEq/L = 1 mmol/L]). In a recent study, we observed that mild chronic hyponatremia contributes to an increased rate of falls, probably due to impairment of attention, posture, and gait mechanisms. Eight attention tests were used to assess 16 patients with asymptomatic hyponatremia (mean serum sodium, 128 +/- 3 mEq/L) due to syndrome of inappropriate antidiuretic hormone secretion. On attention tests for visual and auditory stimuli, there was an increase in mean response latency of 58 msec (P <0.001) and an increase in total error number of 20% (P <0.001). The results of 3 stereotyped steps on a platform "in tandem" with eyes open, expressed as total traveled way of center of pressure, was also different in patients with hyponatremia (mean serum sodium, 128 +/- 3 mEq/L) when compared with normonatremic patients (1.3 vs 1 m; P <0.003). Noteworthy, in volunteers of similar age, after mild alcohol intake (0.55 g/kg body weight), attention and gait tests were less affected. In another series of 122 consecutive patients hospitalized from the emergency room with mild hyponatremia, approximately 21% were admitted for falls. After controlling for age, sex, and other known risk factors for falls, the adjusted odds ratio for falls in patients with hyponatremia was 67, compared with a control group. These data show that the concept of asymptomatic hyponatremia does not withstand a detailed epidemiologic analysis of falls or of sensitive posture, gait, and attention tests.