Exercise-induced asthma (EIA) and bronchial hyper-responsiveness (BHR) are frequently reported among elite athletes of outdoor endurance winter sports, particularly in cross-country and biathlon skiers. The pathogenesis of EIA is related to water loss and heat-loss through the increased respiration during exercise, leading to mediator release, airways inflammation and increased parasympathetic nervous activity in the airways, causing bronchial constriction and BHR. In the competing elite athlete this is presently considered to be due to the frequently repeated increased ventilation during training and competitions in combination with the repeated environmental exposure to cold air in outdoor winter sports. It is important that athletes at risk of asthma and BHR are monitored through regular medical control with assessment of lung function and BHR, and when BHR or asthma is diagnosed, optimal controlling treatment through anti-inflammatory treatment by inhaled steroids should be started and relieving treatment (inhaled ipratropium bromide and inhaled β2-agonists) should be used to relieve bronchial constriction if present.
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Outdoor cold air exposure is a frequent occurrence in many types of sports. In outdoor winter sports subzero temperatures are often preferred. In several winter sports respiratory problems have frequently been reported and in particular exercise-induced asthma (EIA) and bronchial hyper-responsiveness (BHR). However, the realisation of the frequent occurrence of asthma among athletes came gradually. From the early 1990s several reports on asthma and BHR were made in cold weather athletes. The first report on increased occurrence of BHR was made by Larsson in 1993 on Swedish cross-country skiers,1 and then by Heir in Norwegian competitive cross-country skiers, who reported increasing prevalence with increasing age and increasing number of competitive years.2 From studies on young military skiers Heir also reported that respiratory virus infections led to prolonged periods of increased bronchial responsiveness.3 Of particular interest was the finding that exercise intensity level above 90% of maximum heart level accounted for 54% of the variation in BHR, and that the level of BHR was highest after the coldest part of the year.4 On the contrary, Sue-Chu et al found somewhat contradictory a high frequency of asthma and BHR in Swedish skiers, but not so high in young Norwegian skiers: a clinical diagnosis of asthma was made in 42% of Swedish cross-country skiers compared to 12% in young Norwegian cross-country skiers, whereas the frequency of BHR to metacholine was 43% in Swedish skiers compared to 14% in the Norwegian skiers.5
Among all athletes in the American winter Olympic team, 1998, Wilber et al found exercise-induced bronchoconstriction (EIB=reduction in forced expiratory volume in one second (FEV1) ≥10%) after a field exercise test (competition or simulated competition) in 23%, compared to 50% in the cross-country skiers,6 later confirmed by Rundell et al.7 Investigating applications for the use of inhaled asthma drugs, Ken Fitch found that endurance winter sports and especially cross-country skiing (17.2%) accounted for most of the approved applications in the 2002, 2006 and 2010 winter Olympic games.8 EIA and EIB have been used interchangeably, but a Task Force of European Respiratory Society and European Academy of Allergy and Clinical Immunology defined EIA as symptoms and signs of asthma developing after physical exercise, whereas EIB was defined as the reduction in lung function (FEV1) occurring after a standardised exercise test.9
Also for other winter sports than cross-country skiing reports on increased prevalence of asthma and BHR has been made, including biathlon and Nordic combined,10 figure skaters,11 ,12 ice hockey players13 ,14 and speed skaters.10 Of all illnesses reported during the Winter Olympics 2010, 62.8% were respiratory illnesses.15
On the basis of these reports, how can we explain the reported increased prevalence of asthma and BHR among elite winter endurance athletes, and can we use available scientific evidence to take primary and secondary preventive measures to prevent asthma development in elite endurance athletes and in those athletes affected, how to treat to reduce symptoms and to prevent long-lasting harm from the sports activities?
The aim of the present review article is to assess available evidence of pathogenic mechanisms and cold air inhalation and to present measures and treatment, which may prevent the development of asthma and BHR in these susceptible athletes.
Mechanisms of exercise induced asthma and cold air
R S Jones was the first to publish scientific works on EIA exactly 50 and 51 years ago.16 ,17 Two main theories have been presented and debated as to what causes EIA.18–22 The first theory launched related to the loss of heating and cooling of the airways due to the increased ventilation rate during exercise.23 Through respiration the inhaled air is warmed up to 37°C and is fully saturated with vapour. The respiratory heat loss increases with increasing exercise intensity due to increased ventilation. The respiratory heat loss and cooling of the airways will further be enhanced if the inhaled air is cold.24 ,25 The cooled airways result in reflex parasympathetic nerve stimulation causing bronchoconstriction, and at first reflex vasoconstriction of bronchial venules to conserve heat. At cessation of exercise, the increased ventilation ceases, and so do the cooling stimulus, causing a rebound vasodilatation. The resulting smooth muscle constriction due to nerve stimulation and mucosal oedema due to vasodilatation in susceptible individuals,26 reduces the size of the bronchial lumen with increased airways resistance.27
The other theory is based on that the enhanced water loss due to the increased minute ventilation increases the osmolality in the extracellular fluid of the bronchial mucosal membranes. This further induces an efflux of intracellular water to the extracellular space28 causing an intracellular increase in ion concentration.29 This process result in release of mediators; both newly formed eicosanoids30 and preformed mediators like histamine from intracellular granules are released and cause bronchoconstriction. In addition to its cooling effect, cold air contains much less water than air at higher temperatures such as room temperatures (figure 1). The exhaled air warmed up to 37°C will contain much more water than inhaled cold air of −20°C, thus greatly enhancing the water loss also.28
Mechanisms of athlete's asthma and cold air
That EIA might represent a specific problem in sports medicine was not realised until 1989.31 The first report that BHR increased after heavy exercise was made on adolescent swimmers (12–18 years of age) swimming 3000 m. The increase in bronchial responsiveness correlated with the increase in exercise load (increase in blood lactate) in both asthmatic and healthy swimmers.31 Several reports followed on cold weather types of endurance sports like cross-country skiing and the frequent occurrence of asthma and BHR among the elite athletes,1 ,2 and when seasonal variation in BHR was examined in young military ski recruit troop, it was found that BHR to metacholine was most pronounced just after the coldest period of the year.4 Later Sue Chu et al showed that adolescent cross-country skiers (ski-gymnasts) during one competitive winter season developed signs of inflammation (lymphoid follicles and deposition of tenascin) in their bronchial biopsies independent of being asthmatics or not.32
Also in exercising animals inflammatory changes in the airways have been found. In particular, epithelial damage is a repeated finding. Mice, exercised by running, developed inflammation and epithelial damage in their airways as compared with sedentary mice.33 Also in Alaskan sledge dogs exposed both to cold air and to strenuous exercise over several days and examined by bronchoscopy with bronchoalveolar lavage (BAL) before and after a sledge race across Alaska epitheial debris and inflammatory changes were found in the BAL fluid.34 The epithelial damage and reduced repair were also found to be of major importance in experimental studies,35 suggesting this to be the primary lesion in asthma and EIB. The primary lesion may be epithelial damage due to the heavily increased ventilation during the intensive exercise with the exposure to cold air increasing water-loss and causing increased parasympathetic nervous activity, increased bronchial smooth muscle contraction, increased bronchial mucus production and increased airways inflammation leading to BHR and asthma symptoms (figure 2). During normal tidal breathing, the nose functions like a rebreathing organ with warming up and humidifying the inspired air. As the exercise increases, the demands for ventilation increases, and the athletes will stop breathing trough the nose and instead have oral ventilation. This will further increase the heat and water loss through the expired air.
This hypothesis is supported by the following findings.
Positive metacholine bronchial challenges were found more often in competitive swimmers and winter sport athletes compared to healthy controls.36 Positive tests for eucapnic voluntary hyperpnoea were found in comparable numbers.37 Inflammatory markers in induced sputum with increased neutrophil cell counts in induced sputum were found both in swimmers and winter sport athletes, and the neutrophil counts correlated to number of training hours per week in both groups.36 Eosinophil counts were increased in swimmers in particular, as also was number of bronchial epithelial cells.36
Also amateur endurance runners had increased number of bronchial epithelial cells with apoptosis in induced sputum after repeated half-marathon races, in addition to increased serum levels of clara cell protein 16 (CC16) and increased supernatant interleukin 8 levels in induced sputum.38 Also after exercise testing urinary levels of CC16 increased, but the levels were reduced when the study subjects inhaled warm, humid air during running,39 whereas urinary CC16 levels were increased after performing eucapnic voluntary hyperpnoea test with dry air, whether they had a positive test with bronchial constriction or not.40 This can be taken as a sign of lung epithelial stress after exercise causing bronchial constriction. Similarly, increased levels of urinary CC16 levels were found after exercise test in swimmers, but not after mannitol test. Again increased CC16 levels was found after exercise tests both with positive and negative responses.41 Furthermore, Hallstrand found increased number of columnar epithelial cells in induced sputum of asthmatic patients with EIB, compared with asthmatic patients without EIB,42 and also reported increased expression of the gel-forming mucin, MUC5AC measured by PCR in induced sputum. Also the levels of supernatant cystienyl leukotrienes, and the ratio of cysteinyl leukotrienes to prostaglandins were higher in patients with EIB.42 Furthermore, the number of columnar epithelial cells were related to degree of EIB and associated with the supernatant sputum levels of cysteinyl leukotrienes and histamine.43 These findings indicate epithelial damage in the respiratory mucous membranes with mucin hyperproduction and a link from epithelial damage to increased airways inflammation through the presence of increased levels of cysteinyl leukotrienes and histamine in induced sputum as well as increased expression of MUC5AC.43
The link between respiratory epithelial damage and increased airways inflammation is further strengthened by the findings that epithelial damage in asthma is linked to increased secretion of IL-17 and IL-22, and that IL-17 and IL-22 production by Th17 cells plays a critical role in established asthma.44
Extracellular water movement across cell membranes is an important mechanistic part of the pathogenesis of EIA.28 Aquaporin is a channel for aqueous water transport driven by osmotic forces generated by sodium and chlorine ions and expressed in respiratory subepithelial glandular cells and alveolar type 1 cells of the lungs.45 Mice lacking the gene for the aqueous water channel aquaporin (Aqp) five exhibit methacholine-induced bronchiolar hyper-responsiveness in comparison with normal mice.46 Park et al45 found a relationship between metacholine bronchial responsiveness and diminished pilocarpine-induced sweat secretion, tearing rate and salivary flow rate in healthy athletes indicating an autonomic dysfunction with involvement of the parasympathetic nervous system.
Intensive and regularly repeated training influences autonomic regulation. Filipe et al47 demonstrated increased parasympathetic activity in athletes, especially in endurance runners by pupillometry. Also Knopfli et al in two studies reported higher parasympathetic nervous activity in top cross-country skiers and in training children using an 4 s exercise test and influence upon R–R distance in electrocardiogram.48 ,49 In addition there is increased parasympatethic tone in EIB as shown by the blocking effect of atropine inhalation on EIB induced by cold air inhalation.50
The training environment is important for the adolescent athlete. Owing to regular exercise up to twice daily with increased minute ventilation especially when training endurance, the athlete has a higher exposure to environmental air and possible pollutants and chemicals in the surrounding air.
As stated, young competitive swimmers develop BHR at an earlier age, earlier than competitive cross-country skiers.51 ,52 Exposure for swimming pools has been reported to increase asthma prevalence and EIA in Belgian children,53 even though a large English birth cohort study contradicted this finding and found cumulative swimming to be associated with increased lung function and decreased risk of asthma symptoms, especially in children with pre-existing respiratory illness.54 Several studies have reported increased neutrophils and eosinophils in induced sputum from competitive swimmers55 ,56 and frequently bronchial hyper-responsiveness.51 ,55 ,56 Recently, similar findings as previously reported in bronchial biopsies from cross-country skiers32 was found in swimmers.57
Increased leukotriene B4 (LTB)4 levels in exhaled breath condensate was reported in Italian elite swimmers.58 The environmental agent active in swimming pools is organic chlorine products, thought to cause airways inflammation and bronchial hyper-responsiveness.59
Different types of sport will have different environmental exposures. Cross-country skiers are repeatedly exposed to cold air,2 athletes training and competing in ice rinks may be exposed to NOx from the freezing machinery as well as to ultrafine particles from the resurfacing machines,60 in line with reports of high asthma prevalence among ice hockey players14 and figure skaters.11
A study from South California including 3535 children living in six areas with high pollution levels (ozone) and six with low pollution levels, reported after a 5-year follow-up that children actively participating in more than three types of sports in areas with high ozone levels had an increased risk of asthma. Participating in sports in areas with low ozone levels gave no increased risk of asthma.61 The mechanisms and factors working together in developing asthma in elite athletes have been summarised in figure 2.
Whereas the initial stimulus to this process most probably is given by the markedly increased ventilation due to repeated intensive training sessions and repeated competitions, cold air contributes by stimulating the parasympathetic nervous system, thereby increasing contraction of bronchial smooth muscle and increasing inflammation through generation of mediators like cysteinyl leukotrienes and inducing mucus hypersecretion. When also other stimuli contribute to this process by causing increased epithelial damage and inflammation, such as respiratory virus infections, BHR develops and asthma symptoms may appear for the first time, as demonstrated by increased BHR for prolonged periods of time after respiratory virus infections.3
It is evident that the environmental conditions in which sports and training are practised, are important for the respiratory health of the adolescent athlete. Pollution and harmful chemicals in the environmental air increase the risk for asthma development and BHR in the competing athlete. The effective ventilation of swimming pools is important, and effort should be made to develop not harmful methods of disinfection of the water in swimming pools. Cross-country skiing competitions should not be carried out in very cold environments, and endurance sports should not be carried out in areas with high air pollution. For the allergic athletes, exposure to aeroallergens may worsen asthma symptoms, and the presence of concomitant allergic rhinitis may reduce quality of life and sports performance.62
It is thus important that consideration to environmental exposure including climatic conditions is taken when planning sports events, also including when designing arenas for sports events, such as swimming pools, ice shrinks and others.
Can preventive and therapeutic measures be taken against asthma development in elite endurance athletes?
If possible endurance and interval training should not be performed in very cold temperatures, nor in outdoor competitions. By inhaling air of −20°C the EIB of asthmatic children is doubled,63 and the effect on the athlete with much higher ventilation volumes may be even higher.
As a general advice, competitions in endurance sports should not take place in temperatures lower than −15°C.
As a primary prevention tool, protection equipment like the heat and water exchangers,64 such as the Lungplus (Hörby, Sweden)65 and the Jonas Sport mask (Suomen Jonas Oy, Helsiniki, Finland) should be used during training in cold environments. This is as important to use by athletes who may have developed asthma and BHR, as a secondary preventive measure.
Top athletes within sport disciplines prone to develop asthma, should be monitored with regard to the development of symptoms of asthma and BHR. Especially important is the measurement of bronchial responsiveness, the most sensitive methods being either metacholine bronchial provocation or eucapnic voluntary hyperpnoea.66
Secondary preventive measures will include anti-inflammatory treatment, the most important being inhaled steroids. Until now, inhaled steroids represent the only treatment, which counteract the respiratory inflammatory processes in the athlete. Inhaled steroids also improve the epithelial healing.35 The current international guidelines for treating EIA should be followed when caring for asthmatic athletes including pretreatment before exercise with inhaled β2-agonists and inhaled ipratropium bromide and for symptomatic treatment when having symptoms of asthma. Although the regulations set by the World Anti-Doping Agency (WADA) and the International Olympic Committee (IOC) now have been relieved as regard to the use of asthma therapy in sports, it has been very well documented the need of objective documentation of bronchial responsiveness as guiding the treatment of asthma in the competing athlete.8 The follow-up and optimal treatment of asthmatic athletes may have life-saving consequences.67 Early diagnosis and early start of treatment may enable the athlete to continue participation in sports and possibly have less symptoms when later on completing the athletic career.68
What this study adds
This underlines the need for early diagnosis, regular medical follow-up with regular use of anti-inflammatory treatment foremost by the use of inhaled corticosteroids when bronchial hyper-responsiveness is diagnosed.
Furthermore, the endurance athlete with asthma and bronchial hyperrepsonsiveness will probably benefit from inhaled treatment blocking increased parasymptatheitic tone like inhaled ipratropium bromide and inhaled tiotropium.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.