CASE REPORT 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.