Prevalence and incidence of and risk factors for asthma and exercise-induced laryngeal obstruction in elite endurance athletes

Background: Asthma is highly prevalent in endurance athletes, but we lack up-to-date information on the prevalence and incidence of asthma in cross-country skiers. Exercise-induced laryngeal obstruction (EILO) is an important differential diagnosis to exercise-induced asthma, and its symptoms can mimic asthma. The two conditions may co-exist, and misdiagnosis of EILO may result in unnecessary asthma treatment. The gold standard diagnostic test for EILO is continuous laryngoscopy during exercise (CLE). Aims: The aim of this thesis was to study the prevalence and incidence of asthma in endurance athletes, with a special focus on cross-country skiers. In addition, the thesis aimed to assess the prevalence of EILO in cross-country skiers and to study the intra-individual variability of laryngeal obstruction scores using the CLE test. Methods: Study 1 (papers I & II) comprised a cohort of elite endurance athletes who participated in an annual postal survey including questions regarding asthma, allergy, and exercise between 2011 and 2015. The invited athletes were Swedish elite skiers and orienteers, belonging to national teams, universities with elite sport contracts, Swedish National Elite Sport Schools, or national top ranking. The study population in paper I comprised adolescent skiers at Swedish National Elite Sport Schools (n=253) between 2011 and 2013, together with a reference population aged 16–20 years that was matched for school municipality (n=500) and invited in 2013. Paper II included all skiers and orienteers (n=666) participating in the prospective survey in 2011–2015. The incidence of physician- diagnosed asthma was defined as the number of incident cases of physician- diagnosed asthma divided by the summarized time at risk in person-years in the population without physician-diagnosed asthma at baseline. In study 2 (papers III & IV), elite skiers and competitive athletes (n=109) were screened for allergy, exercise-induced bronchoconstriction, and EILO at Östersund Hospital between 2015 and 2017. The participants answered a questionnaire regarding asthma, allergy, and exercise and underwent a CLE test, eucapnic voluntary hyperventilation (EVH) test, and skin prick test. Laryngeal obstruction was assessed at the glottic and supraglottic levels using a visual grade iv score (0–3 points). EILO was defined as ≥ 2 points at maximal effort exercise. Current asthma was defined as self-reported physician-diagnosed asthma and use of asthma medication in the last 12 months. All participants were invited to a follow-up examination off-season if the first examination was performed during the competitive season, or on-season if the first visit occurred during off-season. Paper III included 89 elite skiers that completed the first baseline visit. Paper IV included all 29 athletes that completed the baseline and follow-up testing regardless of diagnosis, treatment, and respiratory symptoms. Results: In paper I, the response rate was 96% in the skier population and 48% in the reference population. Skiers at Swedish National Elite Sport Schools had a higher prevalence of physician-diagnosed asthma than the reference population (27% vs. 19%, p=0.046). Median age at asthma onset was higher in skiers compared to in the reference population (12.0vs 8.0 years, p<0.001). Female sex, family history of asthma, nasal allergies, and being a skier were independent risk factors associated with physician-diagnosed asthma. In paper II, the response rate was 88.7% at baseline and decreased by year of follow-up. The population at risk at baseline consisted of 290 skiers and 159 orienteers, and the incidence rate (95% confidence interval [CI]) of physician- diagnosed asthma was 61.2 (45.7–80.3) per 1000 person-years. Risk factors for incident physician-diagnosed asthma were family history of asthma, being a skier, and wheezing without having a cold. In paper III, 24 (27%) of the 89 elite cross-country skiers fulfilled the study criterion for EILO. Current asthma was present in 34 (38%) skiers, whereof 10 (29%) of whom had concomitant EILO. A higher proportion of skiers with EILO and current asthma reported wheezing or shortness of breath following exercise compared to skiers with asthma only. In paper IV, the CLE score was mainly unchanged at follow-up after 3–23 months. In the 11 athletes with moderate supraglottic obstruction at first visit and receiving advice for breathing exercises, 3 (27%) had mild obstruction at follow- up. Among athletes without no or mild supraglottic obstruction at first visit, 3 (17%) had moderate laryngeal supraglottic obstruction at follow-up. Conclusions: Adolescent skiers have a high prevalence of self-reported physician-diagnosed asthma. The onset of asthma commonly occurs during early adolescence. Endurance athletes also have a high incidence of physician-diagnosed asthma v during their career, especially among skiers. Furthermore, skiers have a high prevalence of EILO, including nearly 30% of skiers with current asthma. In endurance athletes, the grade of laryngeal obstruction according to CLE was fairly stable when reassessed after 3–23 months, regardless of the grade of laryngeal obstruction, respiratory symptoms at baseline, and advice on breathing exercises after the first visit. Therefore, when assessing skiers with exercise-related respiratory symptoms, both EILO and asthma should be considered.