Most cases of acute bronchitis are viral infections. The most common viruses implicated in acute bronchitis are the same as those that cause upper respiratory infections and include coronavirus, rhinovirus, respiratory syncytial virus, and adenovirus. In some younger populations of military recruits and college students, other pathogens such as Chlamydia pneumoniae and Mycoplasma pneumoniae have been isolated from patients with acute bronchitis. However, these pathogens have been identified in only a minority of patients with acute bronchitis, and it is unclear if these agents are involved in causing the symptoms. The uncertainty about the role of these organisms is supported by a study that identified Mycoplasma in a subset of patients presenting with acute bronchitis but found that treatment of these patients with a macrolide resulted in no quicker recovery than patients without Mycoplasma who were treated with the same antibiotic. Rarely, Bordetella bronchiseptica may be observed, but to date has only been reported in immunocompromised individuals.
The symptoms of acute bronchitis are due to acute inflammation of the bronchial wall, which causes increased mucus production along with edema of the bronchus. This leads to the productive cough that is the hallmark of a lower respiratory tract infection. While the infection may clear in several days, repair of the bronchial wall may take several weeks. During the period of repair, patients will continue to cough. Pulmonary function studies of patients with acute bronchitis demonstrate bronchial obstruction similar to that in asthma. As the symptoms of acute bronchitis abate, pulmonary function returns to normal.
Half of all patients with acute bronchitis continue to cough for >2 weeks. In a quarter of patients, cough may last for >1 month. This is termed postbronchitis syndrome. This period probably reflects ongoing repair to the bronchial walls after the clearance of the acute infection.
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