Hyponatraemia is a common biochemical finding, but clinical features due to it are infrequent. They are most likely to occur when the plasma sodium concentration has fallen quickly to below 120 mmol/litre. In a study of 73 hyponatraemic individuals, it was possible to identify four categories of patient, the clinical features becoming more severe as the sodium level fell. In 25 instances there were no effects (mean plasma sodium 118·3 mmol/litre), in a further 25 cases there was confusion only (mean plasma sodium 117·1 mmol/litre), in 13 there were focal neurological signs and in 10 there were convulsions (mean plasma sodium 110·8 mmol/litre). In the group with convulsions there were six deaths, the four survivors all being young women. The 13 cases of ‘focal’ neurological signs included three instances each of hemiparesis and monoparesis, seven of extra-pyramidal disturbance and six of cerebellar ataxia. All these abnormalities resolved when the plasma sodium concentration rose to 125 mmol/litre.
Active measures to raise the plasma sodium level are only needed when there have been convulsions and the aim should be to achieve a value no higher than 120 mmol/litre. In other cases, the only treatment required is to restrict fluid intake.