While classically described in patients with an endobronchial spread of tuberculosis, in fact, tree-in-bud opacities can be identified in virtually any type of infectious bronchiolitis. This includes Mycobacterium tuberculosis, Mycobacterium avium-intra-cellulare, bacterial, viral, and fungal infections, and allergic bronchopulmonary mycosis. This pattern is also frequently encountered in patients with AIDS in whom recurrent episodes of bronchial infection are frequent. Differential diagnosis also includes follicular bronchiolitis, an entity that is characterized by the presence of hyperplastic lymphoid follicles and germinal centers occurring along the bronchovascu-lar bundles. Most often, infectious bronchiolitis results in clusters of tree-in-bud opacities. When they are widespread and diffuse, the differential diagnosis includes “Asiatic panbronchiolitis.”, This entity has a well-established predilection in Japanese, Chinese, and Korean populations, appears to show a genetic predisposition, and is usually seen in association with chronic sinusitis. Diffuse tree-in-bud opacities are also frequently encountered in patients with cystic fibrosis and viral bronchiolitis. read more
It cannot be overemphasized that in the vast majority of cases the finding of tree-in-bud opacities should be taken as being indicative of bronchiolar infection. While tree-in-bud opacities have been described as occurring in patients with pulmonary vascular tumor emboli, in our experience this entity is exceedingly rare. As noted in one retrospective study of 141 patients with a variety of airway diseases, including bronchiolitis obliterans, bronchiolitis obliterans-organizing pneumonia, HP, respiratory bronchiolitis (RB), and pneumonia, among others, the finding of tree-in-bud opacities was identified in association only with pneumonia and/or bronchiectasis in 17% and 25% of cases, respectively. Even in patients with panbronchiolitis, while no consistent infectious agent has been associated with this disease, interestingly, most individuals respond, at least initially, to low-dose erythromycin therapy., Some authors have suggested that the therapeutic efficacy of macrolide agents may emanate from their inhibition of proinflammatory cytokines, and from mucus and water secretion from airway epithelial cells.