ORIGINAL RESEARCH

Exercise-associated hyponatraemia on theKokoda Trailemm_1470 712..716

Sean P Rothwell, David J Rosengren, Amanda M Rojek, Julian M Williams, William G Lukin andJaimi GreensladeDepartment of Emergency Medicine, Royal Brisbane and Women’s Hospital, Herston, Queensland,Australia

Abstract

Objective: The objective of the present study was to determine the prevalence of exercise-associatedhyponatraemia in hikers/trekkers along the Kokoda Trail.

Methods: This was a cross-sectional study of 191 trekkers on the Kokoda Trail, Papua New Guinea.Blood was taken and analysed immediately using point-of-care technology 2 days walkfrom each end of the Trail.

Results: The main outcome measure was hyponatraemia defined as serum sodium level less than135 mmol/L. Three participants (1.6%, 95% CI 0.5–4.5%) were found to have mildhyponatraemia. The hyponatraemic group had a median estimated fluid intake onthe day of testing that was almost double that of the normal sodium group (6 Lvs 3.3 L).

Conclusion: Exercise-associated hyponatraemia occurs in trekkers on the Kokoda Trail. Strategies forprevention of exercise-associated hyponatraemia should be delivered to trekkers via thetrekking companies, chiefly focussing on only drinking in response to thirst.

Key words: exercise-associated hyponatraemia, hyponatraemia, Kokoda.

Introduction

The Kokoda Trail is a 96 km jungle trail in the highlandsof Papua New Guinea, which was the location of heroicfighting by Australian soldiers in World War II. Inaddition to the historical significance, hiking the trekhas become a popular endurance challenge, whichhas attracted a heterogeneous population of trekkers.Consequently, there has been an increased incidence ofinjury and illness. For example, there have been sixunexplained deaths on the Trail since 2006 and aero-

medical evacuations of hikers have become increasinglycommon (private communication Kokoda Track Author-ity). There have been two published cases of severe(Na < 125 mEq/L) exercise-associated hyponatraemia(EAH) in Kokoda Trail trekkers, one in 20061 and theother in 2008.2 Whether these cases are unusual or rep-resent substantial underreporting of EAH has neverbeen investigated.

In a wilderness setting, appropriate identification andmanagement of EAH present a clinical challenge asthe definitive diagnosis of EAH relies on biochemical

Correspondence: Dr Sean P Rothwell, Emergency Department, Royal Brisbane and Women’s Hospital, Butterfield Street, Herston Qld,4006, Australia. Email: sean_rothwell@health.qld.gov.au

Sean P Rothwell, MBBS, FACEM, FAWM, Emergency Physician; David J Rosengren, MBBS, FACEM, FAWM, Emergency Physician; AmandaM Rojek, B App Sci (Hons), Medical Student; Julian M Williams, BSc, MBBS, FACEM, Emergency Physician; William G Lukin, MBBS, FACEM,Emergency Physician; Jaimi Greenslade, PhD, B Psych (Hons), Research Officer.

doi: 10.1111/j.1742-6723.2011.01470.xEmergency Medicine Australasia (2011) 23, 712–716

© 2011 The AuthorsEMA © 2011 Australasian College for Emergency Medicine and Australasian Society for Emergency Medicine

analysis, rarely available to medical first responders tothe Kokoda Trail. Although it is vital to differentiatebetween dehydration and EAH, these presentations areoften similar. Namely, both conditions might be charac-terized by headache, nausea and lethargy. If EAH iserroneously diagnosed as dehydration and treated withfluid resuscitation, the consequences are potentiallyfatal. A high index of suspicion is the key to prevention.Knowledge of EAH remains poor among recreationalathletes and hiking companies where the ‘just addwater’ approach to hydration remains prominent.3,4 Thepresent study was aimed at determining the prevalenceof EAH in those hiking the Kokoda Trail.

Methods

Study design and setting

This was a cross-sectional, observational study of trek-kers on the Kokoda Trail. Data were collected over a4 day period in April 2010. These 4 days aimed torecruit all the trekkers who were on the Kokoda Trailfor ANZAC Day celebrations on 25 April and representsa popular time to complete the trek. The project wasapproved by the Royal Brisbane and Women’s HospitalHuman Research Ethics Committee. All individualstrekking the Kokoda Trail during the study period wereeligible for inclusion in the study and there were noexclusion criteria.

Study protocol

Two teams, consisting of three investigators each, setup camp at Ioribaiwa Village and the Isurava battle site,respectively. These villages are located 2 days walkfrom each end of the trail.

All trekkers passing through the camps wereapproached and invited to participate in the study.Written consent was obtained from participants. Ablood sample was taken and analysed immediatelyusing the Chem 8+ cartridge on an iStat analyser (AbbotPoint of Care, Princeton, NJ, USA). A short face-to-faceinterview was also conducted to obtain informationabout participant age, sex, medical history, currentmedications, presence of any symptoms, estimated fluidintake, time since last urination and previous trekkingexperience. All iStat machines were formally calibratedeach day using a simulator disc, the validity of whichwas confirmed in ideal lab conditions before departureand then again on return.

Measurements

A Chem 8+ cartridge for the iStat analyser recordedserum sodium, potassium, chloride, TCO2, anion gap,ionized calcium, glucose, urea nitrogen, creatinine, hae-matocrit and haemoglobin. Sodium levels below135 mmol/L on the iStat analyser were defined ashyponatraemia.

The iStat cartridge sodium measurement has a coef-ficient of variation of 0.4% at the lower limit and corre-lates well with laboratory-based analysers (r > 0.84)(validation data supplied by Abbott Point of Carecurrent as of 22 February 2010).

To ensure cartridge performance was not compro-mised in the tropical environment, quality control testsof both high- and low-range levels were performed oncartridges from each batch each day during sampleanalysis.

Statistical analysis

The prevalence of hyponatraemia was calculated anddescriptive statistics were used to compare baselinecharacteristics in the hyponatraemic and non-hyponatraemic trekkers. Statistics were computed usingSPSS version 15 (SPSS, Chicago, IL, USA) for Windowsand confidence interval (CI) analysis version 2, Univer-sity of Southampton.

Results

A total of 196 trekkers passed through the 2 studylocations and 191 trekkers agreed to participate in thestudy. Eighty-three per cent (n = 159) of the populationwere male and the median age was 42 years (range35–50 years). The median weight of participants was82 kg (range 73–91 kg) and median body mass indexwas 26 (range 24.1–28.3).

Three participants (1.6%, 95% CI 0.5–4.5%) werefound to have hyponatraemia. Two had serum sodiumlevels of 129 mmol/L and 128 mmol/L and were bothasymptomatic. The third participant had a serumsodium of 132 mmol/L and complained of nausea andbloating. The sodium level of all three participantsreturned to normal range after approximately 4 h of restand fluid restriction.

The three hyponatraemic individuals had estimatedfluid intakes for the day of testing of 4.5, 6 and 9.7 L.This compares with the median of 3.3 L (interquartilerange: 2.3–4.7; 95th percentile: 6.9) in the normal sodium

Exercise-associated hyponatraemia

713© 2011 The AuthorsEMA © 2011 Australasian College for Emergency Medicine and Australasian Society for Emergency Medicine

group. Similarly, for the previous day the hypona-traemic individuals reported drinking 5.7, 5.8 and 6.3 L,respectively, whereas the non-hyponatraemic groupreported drinking a median of 3.6 (interquartile range:2.6–4.8, 95%: 6.5) litres. Four participants were still ontheir first day of trekking when arriving at IoribaiwaVillage, so did not provide data for the previousday.

In the present study, 17 participants were taking salttablets, none of whom was hyponatraemic. Thehyponatraemic group were younger (30 vs 42 years) andhad a lower bodyweight than the normal sodium group(see Table 1).

Discussion

Exercise-associated hyponatraemia is defined as aplasma serum sodium concentration below the referencerange (usually 135 mmol/L) occurring in the context ofphysical activity.4 In the present study, the prevalenceof hyponatraemia in trekkers walking the Kokoda Trailwas 1.6%. Although the number of hyponatraemic par-ticipants was too small to apply most statistical tests,comparison of the median estimated fluid of these par-ticipants revealed that their fluid intake was nearlydouble that of the healthy participants, both on the dayof testing and on the previous day. This supports thefindings of previous research that the risk of EAH ishighest in those who consume fluid volumes dispropor-tionate to the requirements of the body.5

There is conflicting evidence with regards to the roleof sodium supplementation;6 however, none of the par-ticipants who reported taking salt supplements hadhyponatraemia.

When symptomatic, EAH most commonly causesnausea, vomiting, bloating and headache. However, ifleft untreated the condition might progress to pulmo-nary oedema, cerebral oedema and even death. EAH isnow widely regarded as a dilutional hyponatraemiaoccurring in the setting of excessive fluid consumption.Inappropriate secretion of anti-diuretic hormone duringexercise might also play a role. Suggestions that exces-sive excretion of sodium from the body might causeEAH have largely been dismissed.6–8

Walking the Kokoda Trail exposes the trekker to anumber of identified risk factors for EAH. These includehot environmental conditions, greater than 4 h exerciseduration, high availability of drinking fluids and exces-sive drinking behaviour, slow performance pace, eventinexperience and use of non-steroidal anti-inflammatoryagents.6,9

The prevalence of EAH in the present study of 1.6%was lower than previously reported in enduranceevents and hiking,10–12 which might in part have beenresulting from systematic bias introduced by the openor unblinded study design (discussed further below).

Exercise-associated hyponatraemia is arguably themost important medical condition in endurance athlet-ics.4 The prevalence of hyponatraemia has beenreported as high as 13% for marathon runners10 and18% for ultra-distance triathlon competitors,12 but isalso known to occur on shorter courses.13 Of the athletesparticipating in these events, the risk of EAH is highestin those who consume larger volumes of fluids and whosubsequently gain weight during activity.5,14

There are also some reports of EAH in non-competitive endurance activity, including those under-taking military exercises.15–17 Severe manifestations ofEAH have been documented in hikers in the Grand

Table 1. Characteristics of hyponatraemic and non-hyponatraemic patients

Variable Sodium � 135n = 188

Sodium < 135n = 3

Age, median (range) 42 (20–75) 30 (22–53)Male, n (%: 95% CI) 158 (84.0: 78.1–88.6) 1 (33.3: 6.1–79.2)Bodyweight, kg, median (range) 82.5 (52–125) 70 (56–83)Body mass index, median (range)† 26.0 (18.3–36.1) 25.1 (20.6–29.4)Salt tablets, n (%: 95% CI) 17 (9.0: 5.7–14.0) 0 (0: 0.0–56.2)Non-steroidal anti-inflammatory drugs, n (%: 95% CI) 36 (19.1: 14.2–25.4) 1 (33.3: 6.1–79.2)Total fluid today, median (range) 3.3 (0.8–10) 6 (4.5–9.7)Total fluid yesterday, median (range)‡ 3.6 (0.5–11) 5.8 (5.7–6.3)No. walks greater than 5 h over the previous 12 months§ 0 (0–100) 3 (0–3)

†n = 189. Two individuals did not provide their height. None of these individuals was hyponatraemic. ‡n = 187. Four individuals didnot provide data on total fluid intake yesterday. None of these individuals was hyponatraemic. §n = 190. One individual did not provide dataon number of walks greater than 5 h over the previous 12 months. None of these individuals was hyponatraemic.

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714 © 2011 The AuthorsEMA © 2011 Australasian College for Emergency Medicine and Australasian Society for Emergency Medicine

Canyon region of North America,11,18,19 Alaska,20 theNepalese Himalayas21 and Papua New Guinea.1,2

One of the strengths of the present study was its datacollection points. The investigators chose to perform thestudy on trekkers completing the second day of the trek,because this is the first difficult day and is the pointwhere medical emergencies reach their peak. Forinstance, it is around this time period that four of the sixdeaths and the two cases of severe EAH haveoccurred.1,2 After this point, fewer incidents have beenrecorded.

We acknowledge several limitations of the presentstudy. For example, fluid consumption was retrospec-tively estimated by participants, which might have ledto an over- or under-estimate. The participant’s weightwas also self-reported and thus subject to recall error.Because the stated weight was also a single pre-trekweight, it could not be used to determine fluid overload,as for previous studies.6

Although specific details of the study were not pro-vided to trekkers until arrival into the camp, the authorshad had previous contact with some of the major trek-king companies with regards to EAH, which might haveinfluenced trek leaders’ instructions to trekkers regard-ing drinking behaviour.

In view of the unexplained Trail deaths and emer-gency retrievals, EAH might be an important medicalcondition affecting Kokoda Trail walkers. Morbidityand mortality from EAH are potentially preventablethrough ensuring awareness in all hikers of the risk ofEAH and how to reduce their risk; and by increasingknowledge about the condition for trek leaders andmedics. The study showed that EAH is prevalent onthe Kokoda Trail. Strategies for education and preven-tion of EAH on the Kokoda Trail should be similar tothose already established in competitive enduranceevents.22,23 This message should be delivered to trek-kers via the trekking companies and the KokodaTrack licensing authority, chiefly focussing on drink-ing in response to thirst and not excessively consum-ing fluids for fear of dehydration as previouslybelieved.

Acknowledgements

We would like to thank Abbott Point-of-Care for donat-ing the iStat Chem 8+ cartridges, Abacus ALS fordonating capillary pipette blood sampling devices andQueensland Health Pathology Services for loaning theiStat analysers used in the research. We would like to

thank Professor Allan Frost, Mr David Thomae and DrJulius Plinduo for their assistance with data collection inthe field. We would also like to thank Professor TonyBrown and Dr Penelope McBride for reviewing thismanuscript. This project was funded by a grant fromthe Australian Government through the joint PNG-Australia Kokoda Track Safety Package, managed bythe Kokoda Track Authority (KTA).

Competing interests

None declared.

Accepted 14 June 2011

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