## Distinguishing Exercise-Induced Bronchoconstriction from Allergic Asthma ### Defining Characteristics **Key Point:** Exercise-induced bronchoconstriction (EIB) is characterized by **symptom onset during or immediately after high-intensity exercise** with **spontaneous recovery within 30–60 minutes** without treatment — a temporal and recovery pattern that is pathognomonic and distinguishes it from allergic asthma, which may be triggered by exercise but typically has a different temporal relationship and allergen dependence. ### Comparative Table | Feature | Exercise-Induced Bronchoconstriction (EIB) | Allergic Asthma | |---------|---------------------------------------------|------------------| | **Trigger** | High-intensity exercise, cold/dry air | Allergen exposure (pollen, dust, animal dander) | | **Onset** | During or immediately after exercise (within 5–15 min) | Variable; may be delayed (minutes to hours after allergen exposure) | | **Peak symptoms** | 5–10 minutes post-exercise | Variable timing | | **Spontaneous recovery** | Yes, within 30–60 minutes | No; requires bronchodilator therapy | | **Skin prick test** | Usually negative | Often positive to specific allergens | | **Baseline lung function** | Normal between episodes | May show baseline obstruction or hyperresponsiveness | | **Allergen history** | Not required | Clear temporal relationship to allergen | | **Refractory period** | Present (repeat exercise within 1–4 hours causes less bronchoconstriction) | Absent | ### Pathophysiology **High-Yield:** EIB is thought to result from **osmotic stress on airway epithelium** due to rapid cooling and drying of airways during high-intensity exercise (hyperventilation with cold, dry air). This leads to mast cell degranulation and bronchospasm. The **refractory period** (reduced response to repeat exercise within 1–4 hours) is unique to EIB and reflects depletion of mast cell mediators. Allergic asthma is mediated by **IgE-dependent mast cell activation** triggered by specific allergen exposure, with variable temporal onset and duration depending on allergen load and individual sensitivity. ### Clinical Pearl **Clinical Pearl:** A teenager with dyspnea only during intense sports practice, with complete symptom resolution within 30–60 minutes and no symptoms at rest or during low-intensity activity, is highly suggestive of EIB. If the same athlete also has symptoms triggered by dust mites or pet exposure at home, **both EIB and allergic asthma may coexist** — EIB is not mutually exclusive with allergic asthma. ### Diagnostic Approach **Tip:** Confirm EIB with **exercise challenge test** (treadmill or cycle ergometer) or **eucapnic voluntary hyperventilation (EVH) test**, which reproduces the osmotic stimulus. A ≥10% drop in FEV~1~ is diagnostic. [cite:Harrison 21e Ch 297] 
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