With an occurrence of up to 30%, hyponatremia, defined as a serum sodium concentration of less than 135 mmol/L, is the most commonly encountered electrolyte disorder in hospitalized patients today. The highest incidence of hyponatremia is found in the elderly.

Retrospective analyses clearly show an association between hyponatremia and mortality as well as morbidity and this association is not confined only to severely decreased sodium levels. Hyponatremia can be an acute and life-threatening condition, which is usually reflected by severe signs and symptoms, e.g. coma or seizures,but more often it is of chronic nature, and symptoms may be subtle or even apparently absent due to cerebral adaptation processes. Signs and symptoms include, among others, nausea, drowsiness, cognitive impairment, memory deficits, depression, muscle cramps, or gait instability. However, our knowledge of the highly diverse clinical picture of hyponatremia stems from anecdotic reports and experiments that were performed by the investigators on themselves in the 1930ies. Thus, no accurate and thorough description of the symptomatology of hyponatremiain disease has been carried out so far, except maybe for the extreme conditions, i.e. in patients suffering from seizures or coma. This is why especially patients with chronic mild-to-moderate hyponatremia (sometimes defined as a sodium concentration \> 125 mmol/L) who may well have subtle or unspecific symptoms are often perceived as being asymptomatic. Recent investigations have demonstrated that such patients have a significantly increased risk for falls and bone fractures and reduced cognitive capacities. Moreover, anecdotic reports have shown that some patients that were believed to suffer from depression or dementia improved markedly after correction of their concomitanthyponatremia. There is also mounting evidence that hyponatremia induces osteoporosis which adds to the risk of bone fractures. Furthermore, in the SALT-trials, the mental component of the SF-12 questionnaire (Short Form-12) improved significantly after hyponatremia had been corrected. Interestingly, the mean sodium concentration prior to treatment was 129 mmol/L indicating mild or moderate hyponatremia. Given the high prevalence of mild-to-moderate hyponatremiain the elderly, unspecific symptoms like mood instability, cognitive deficits or others, thatare often attributed to old age could simply be a consequence of their low sodium level.

To this end, it might well be that by treating hyponatremia the morbidity/mortality in this population could be reduced and quality of life improved. In light of the growing medico-economic challenges we are facing secondary to the steadily increasing life expectancy and the demographic evolution in the western world and the medical problems that come along with this development the influence of successful correction of hyponatremia on mental and physical health should be evaluated. To our knowledge this has not been investigated so far.

The syndrome of inappropriate ADH secretion (SIADH) accounts for more than a third of all cases of hyponatremia and probably is by far the most frequent aetiology of chronic mild to moderate hyponatremia. While newly developed SIADH is often induced by malignancies, pulmonary or cerebral disease in the general population, longstanding chronic hyponatremia in elderly is usually due to idiopathic or drug-associated SIADH. Apart from SIADH, hypovolemia or reduction of the effective arterial blood volume, e.g. in cardiac failure or liver cirrhosis, are common causes of hyponatremia. Although hyponatremia might have an impact on the mental and physical status as well as on the patient's prognosis regardless of the underlying cause, most aetiologies except for SIADH might be confounding factors, since they often induce a "hyponatremia-like" clinical picture by themselves, e.g. hepatic encephalopathy in liver disease etc. Another possible confounder may be hemoglobin. Recent work shows that hyponatremia is associated with anaemia, which is a highly prevalent symptom in elderly patients and an acknowledged risk factor for poor outcome in this population. The underlying pathogenesis of this association is still unknown. Regarding the fact that anaemia and hyponatremia are both risk factors for increased morbidity and mortality in elderly patients that are potentially curable, analysis of influence on the multidimensional geriatric assessment (MGA) before and after correction is challenging. To our knowledge, these possible confounders have not been accounted for in the existing literature. Moreover, no accurate evaluation of the origin of hyponatremia has been reported.