The main concern is brain swelling and resultant herniation.
Treatment should be prompt and aim at reducing ICF volume using hypertonic saline for the symptomatic patient with a plasma sodium < 125 mmol/l. Aim to raise plasma sodium to 130 mmol/l.
When calculating sodium deficit assume that the volume behaves as if the sodium is dissolved in total body water, as the cell membrane is permeable to water and not sodium.
How much 5% saline (856 mmol Na+/L) should be given to a 35kg patient to raise the plasma sodium by 5 mmol/l? (Click here for the answer)