CASE REPORT

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

J P Dugas¹, T D Noakes¹

¹ UTC/MRC Research Unit for Exercise Science and Sports Medicine, University of Cape Town, Sports Science Institute of South Africa, Cape Town, South Africa

Correspondence to:
Correspondence to:
J P Dugas
UTC/MRC Research Unit for Exercise Science and Sports Medicine, University of Cape Town, Sports Science Institute of South Africa, PO Box 115, Newlands 7725, Cape Town, South Africa; jdugas{at}sports.uct.ac.za

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.

Keywords: hyponatraemia; cycling; exercise; fluid balance; sodium

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Commentary

P B Laursen²

² Edith Cowan University, School of Biomedical and Sports Science, Perth, WA, Australia; p.laursen{at}ecu.edu.au

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