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Risk factors [22].
| Unidentified exacerbating factors |
| Unidentified allergens |
| Gastroesophageal reflux |
| Systemic diseases |
| Thyreotoxicosis |
| Carcinoid syndrome |
| Churg-Strauss syndrome and others vasculides |
| Drugs |
| β-blockers |
| Nonsteriodal anti-inflammatory drugs |
| Chronic infections |
| Mycoplasma |
| Chlamydia |
| Psycological factors |
