Hyponatraemic encephalopathy despite a modest rate of fluid intake during a 109 km cycle race.

OBJECTIVE To report a case of exertional hyponatraemic encephalopathy that occurred despite a modest rate of fluid intake during a 109 km cycling race. METHODS Men and women cyclists were weighed before and after the race. All subjects were interviewed and their water bottles measured to quantify fluid ingestion. A blood sample was drawn after the race for the measurement of serum Na(+) concentration. RESULTS From the full set of data (n = 196), one athlete was found to have hyponatraemic encephalopathy (serum [Na(+)] 129 mmol/l). She was studied subsequently in the laboratory for measurement of sweat [Na(+)] and sweat rate. CONCLUSIONS Despite a modest rate of fluid intake (735 ml/h) and minimal predicted sweat Na(+) losses, this female athlete developed hyponatraemic encephalopathy. The rate of fluid intake is well below the rate currently prescribed as optimum. Drinking to thirst and not to a set hourly rate would appear to be the more appropriate behaviour.