Classification of bronchiolitis obliterans syndrome-taxonomic realism or skepticism.

Pulmonary failure after lung or heart/lung transplantation occurs for many reasons. Primary graft failure, acute pulmonary rejection, reimplantation response, pulmonary infection, and airway complications are examples of discrete causes of pulmonary failure that may occur at various times after transplantation. Although there may be considerable overlap in their clinical and radiologic presentations, each has a reasonably standard investigatory pathway and therapeutic approach. However, there is an enigmatic process that may be evident within the first 6 months after lung transplantation and is characterized by loss of airflow with a radiologically clear graft and is currently termed bronchiolitis obliterans syndrome (BOS). This phenomenon results in considerable controversy in terms of definition, classification, natural history, and therapy. The pathologic entity of obliterative bronchiolitis (OB) can be conceptualized as the biologic process of “wound healing”—that is repair of epithelial injury as the final common pathway of a number of processes involving the terminal and respiratory bronchioles of the allograft lung. The prevailing concept is that the clinical manifestation of OB, that is, BOS, does not require the presence of histologically proven OB and is characterized by progressive small airway obstruction. Unless stabilization occurs, this process ultimately leads to death. The accepted risk factors for BOS are frequent episodes of acute rejection and lymphocytic bronchiolitis, and possible risk factors include cytomegalovirus (CMV) pneumonitis, diffuse alveolar damage associated with ischemia–reperfusion injury, and some nonCMV pulmonary infections. In 1993, a staging system was formulated for BOS based on the degree of airflow obstruction as measured by the forced expiratory volume in 1 second (FEV1). 1 The purpose was to provide a means of quantifying the degree of airflow limitation in individual patients, as a way of objectively assessing the impact of therapy, and to provide a “common currency” for describing the severity of this condition for the purposes of scientific interchange. Recently, a modification to this staging system was proposed, which now includes a potential-BOS stage (BOS 0-p) that refers to patients with a 10% to 19% fall in FEV1 or a 25% or greater decrease from baseline in midexpiratory flow rate (FEF25–75). The purpose of this additional stage, as outlined by the committee, is to signal the possible onset of BOS and to prompt closer surveillance. Staging the severity of airflow obstruction in BOS has a useful role. However, the proposal to increase the complexity of this staging system does serve as a focal point for considering the broader issue of the nosology of BOS. Are there other ways to classify BOS that may be more useful therapeutically? Traditionally, the classification of disease in medical practice embodies the principles of taxonomic realism, implying that diseases are distinct entities, and hence the identification of a new disease depends on the discovery of an underlying scientific truth. The alternative principle to taxonomic realism is taxoFrom the Division of Cardiothoracic Surgery, University of Alabama, Birmingham, Alabama. Submitted May 14, 2002; accepted June 4, 2002. Reprint requests: Dr. D.C. McGiffin, University of Alabama at Birmingham, Division of Cardiothoracic Surgery, 1530 3rd Avenue South, 780 Lyons-Harrison Research Bldg., Birmingham, AL 35294-0007. Telephone: 1-205-934-6580; Fax: 1205-975-2526. E-mail: david.mcgiffin@ccc.uab.edu J Heart Lung Transplant 2002;21:941–944. Copyright © 2002 by the International Society for Heart and Lung Transplantation. 1053-2498/02/$–see front matter PII S1053-2498(02)00469-2