Bronchiolitis obliterans is a type of irreversible obstructive lung disease that results from airway injury caused by various factors, including lower respiratory tract infections, alloimmune reactions, autoimmune diseases, and inhalation of toxic chemicals. In children, the most common preceding cause of bronchiolitis obliterans is a lower respiratory tract infection, although hematopoietic stem cell transplantation or lung transplantation are also important causes.
Types of Bronchiolitis Obliterans
1. Post-infectious bronchiolitis obliterans (PIBO)
- Relatively more common among Asians
- Lower respiratory tract infection caused by adenovirus is known to be an independent risk factor
- Other pathogens: Respiratory syncytial virus (RSV), influenza virus, measles virus, human metapneumovirus, and Mycoplasma pneumoniae
- Typically presents with characteristic histopathological findings of constrictive bronchiolitis.
2. Post-transplantation bronchiolitis obliterans syndrome (BOS)
2-1. Bronchiolitis obliterans after hematopoietic stem cell transplantation (HSCT)
- A form of bronchiolitis obliterans that occurs after hematopoietic stem cell transplantation (HSCT), typically as a pulmonary manifestation of systemic chronic graft-versus-host disease.
- Risk factors: Impaired pulmonary function before or after transplantation, the use of myeloablative or busulfan-containing regimens, cytomegalovirus (CMV) infection, pre-existing lung disease, female donors, unrelated donor transplantation, and prior acute graft-versus-host disease.
2-2. Bronchiolitis Obliterans after lung transplantation
- Chronic lung allograft dysfunction (CLAD): After lung transplantation, occurring in approximately 65–70% of cases.
- Inflammatory responses, triggered by chemokines generated due to infections or acute cellular rejection, lead to the recruitment of recipient leukocytes into the transplanted lung and subsequent airway fibrosis
- Key risk factors: Inadequate immunosuppression, acute cellular rejection, community-acquired respiratory infections, donor-specific antibodies, air pollution, gastroesophageal reflux disease, and infections with Pseudomonas aeruginosa or CMV.