Excerpt from Exercise-Induced Asthma

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Background

Exercise-induced asthma (EIA) is a condition of respiratory difficulty that is triggered by aerobic exercise and lasts several minutes. Symptoms of EIA may resemble those of allergic asthma, or they may be much more vague and go unrecognized, resulting in probable underreporting of the disease. (See also the Medscape Asthma Resource Center and the eMedicine article Asthma.) 

Exercise-induced urticaria, or anaphylaxis, is often presumed to be related to EIA, even though this condition is extremely rare and unrelated. EIA is related to histamine release.^{1, 2, 3} Only 500-1000 cases of exercise-induced urticaria have been reported in the literature. In this condition, there is an early stage of exercise-related fatigue and itchiness, followed by early onset of urticaria and angioedema, which is initially mild.⁴ If progression occurs, there is choking, stridor, nausea, vomiting, and even hypotension. A late stage that is marked by headache may also occur. As implied by the alternative name of anaphylaxis, EIA can be life threatening; however, this can be prevented by exercise modification or avoidance of certain conditions (see Sport-Specific Biomechanics, below).
 
(See also the eMedicine articles Exercise-Induced Anaphylaxis [in the Pediatrics section], Urticaria [in the Allergy and Immunology section], and Angioedema [in the Emergency Medicine section].)

Frequency

United States

EIA affects 12-15% of the population. Ninety percent of asthmatic individuals and 35-45% of people with allergic rhinitis experience EIA, but even when those with rhinitis and allergic asthma are excluded, a 3-10% incidence of EIA is seen in the general population.³

EIA seems to be more prevalent in some winter or cold-weather sports.⁵ Some studies have demonstrated rates as high as 35% or even 50% in competitive-caliber figure skaters, ice hockey players, and cross-country skiers.^{6, 7}

Functional Anatomy

The problem in EIA occurs distal to the glottis, in the lower airway. Bronchoconstriction is involved that is distinguishable from laryngospasm, which can occur in other exercise-related conditions. One such example is the condition known as vocal cord dysfunction in which there is paradoxical narrowing of the vocal cords during inspiration, resulting in stridor that is often misconstrued as audible wheezing.^{8, 9} Normally, the vocal cords open with inspiration. (See also the eMedicine article Vocal Cord Dysfunction.)

Sport-Specific Biomechanics

EIA usually affects individuals who participate in sports that include an aerobic component. The condition can be seen in any sport, but EIA is much less common in predominantly anaerobic activities. This is likely due to the role of consistent and repetitive air movement through the airways (seen in aerobic sports), which affect airway humidity and temperature. EIA triggers an unknown biochemical and neurochemical pathway, resulting in the bronchospasm, which manifests as the symptoms of the disease.

Although the exact mechanism of EIA is unknown, there are 2 predominant theories as to how the symptom complex is triggered. One is the airway humidity theory, which suggests that air movement through the airway results in relative drying of the airway. This, in turn, is believed to trigger a cascade of events that results in airway edema secondary to hyperemia and increased perfusion in an attempt to combat the drying. The result is bronchospasm.

The other theory is based on airway cooling and assumes that the air movement in the bronchial tree results in a decreased temperature of the bronchi, which may also trigger a hyperemic response in an effort to heat the airway. Again, the result is a spasm in the bronchi.

Many authors think that there may be a combination of the above 2 theories that takes place, but the biochemical or physical pathways that mediate these responses are unclear. Evidence may even exist to support the idea that the resulting cascades are not the inflammatory pathways to which we attribute allergic asthma.

Likewise, certain sports and their environments predispose individuals with asthma to experience EIA. Sports played in cold and dry environments usually result in more symptom manifestation for athletes with this condition. On the other hand, when the environment is warm and humid, the incidence and severity of EIA decrease.

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