We evaluated in 110 consecutive elderly hyponatremic patients the value of traditional clinical and biochemical data and the place of a test infusion of 2 liters isotonic saline over 24 hours, in establishing the etiology of the
hyponatremia. The causes of
hyponatremia were as follows: 31%
SIADH patients, 23% patients with
hyponatremia due to
diuretics, 18% potomania patients, 15%
salt depleted patients, 5%
salt depleted
SIADH patients, 5% patients with a
salt loosing syndrome and 3% patients with
hyponatremia of unknown origin. Several
salt depleted (SD) and
SIADH patients could be confounded. Usually, adults with
SIADH show plasma
uric acid values <4 mg/dL. In our elderly population, 41% of SD patients presented plasma
uric acid <4 mg/dL, while 27% of
SIADH patients showed plasma
uric acid >4 mg/dL. Eighty-two percent of SD patients appeared to have plasma
urea levels >30 mg/dL, but this was also the case in 21% of
SIADH patients. Twenty-nine of the SD patients presented a urinary
sodium >30 mEq/L, but all had fractional
sodium excretion (FENa) lower than 0.5%. However, in
SIADH, 42% of the patients presented also FENa <0.5%. Fractional excretion of
urea (FE
urea) below 50% was encountered in 82% of SD patients and FE
urea above 50% in only 52% of the
SIADH patients. Plasma
renin and
aldosterone values were poorly discriminative. A test infusion with 2 liters isotonic saline over 24 hours allowed a correct classification of all the patients. In about 2/3 of the population, administration of isotonic saline could be considered as useful (SD, most
diuretic patients, potomania patients,
salt loosing syndrome patients and some SD
SIADH patients). A plasma
sodium (PNa) increase of at least 5 mEq/L 24 hours after saline infusion has been suggested as highly suggestive of SD. Nevertheless, 29% of our SD patients did not increase their PNa level by 5 mEq/L or more, while 30% of our
SIADH patients did. PNa improved after 2 liters isotonic saline over 24 hours in 90 patients (85%) as opposed to 12 others (9
SIADH and 3
diuretic patients), decreasing their plasma
sodium. The isotonic saline infusion test, only allows a reliable classification of
hyponatremia, as far as both PNa and
sodium excretion were taken into account. In the
SIADH group, 6 patients (5%) presented initially manifest solute depletion and retained the 2 liters isotonic saline before developping inappropriate natriuresis. Six patients showed a transient
salt loosing syndrome with high fractional
potassium excretion (FEK) and high calciuria, which differentiates them from
thiazide patients presenting also high FEK, but low calciuria. These patients were also polyuric at admission. The saline infusion was well tolerated in all but 2 patients, developing mild pulmonary congestion at the end of the test infusion.