Tracheobronchomalacia and Excessive Dynamic Airway Collapse
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Blackwell Publishing AsiaMelbourne, AustraliaRESRespirology1323-77992006 Blackwell Publishing Asia Pty Ltd? 2006114388406Review ArticleTBM and EDACSD Murgu and HG Colt Respirology (2006) 11, 388–406 doi: 10.1111/j.1400-1843.2006.00862.x REVIEW ARTICLE Tracheobronchomalacia and excessive dynamic airway collapse Septimiu D. MURGU AND Henri G. COLT Pulmonary and Critical Care Medicine, Department of Medicine, University of California School of Medicine, Irvine, CA, USA Tracheobronchomalacia and excessive dynamic airway collapse MURGU SD, COLT HG. Respirology 2006; 11: 388–406 Abstract: Tracheobronchomalacia and excessive dynamic airway collapse are two separate forms of dynamic central airway obstruction that may or may not coexist. These entities are increasingly recognized as asthma and COPD imitators. The understanding of these disease processes, however, has been compromised over the years because of uncertainties regarding their definitions, patho- genesis and aetiology. To date, there is no standardized classification, diagnosis or management algorithm. In this article we comprehensively review the aetiology, morphopathology, physiology, diagnosis and treatment of these entities. Key words: airflow dynamics, bronchomalacia, excessive dynamic airway collapse, tracheobron- chomalacia, tracheomalacia. INTRODUCTION first is that most published studies are case series and retrospective descriptions, many of which report a The purpose of this systematic review is to clarify con- single investigator’s experience with diagnosis and founding issues pertaining to the definition, patho- management. In fact, it is puzzling that despite the physiology, histopathology, aetiology, diagnosis, relative frequency with which TBM and EDAC are pre- classification and treatment of acquired and idio- sumably encountered, multi-institutional or prospec- pathic forms of adult tracheobronchomalacia (TBM) tive studies have not been published. The second is and excessive dynamic airway collapse (EDAC). that investigators rarely define and differentiate TBM Based on a thorough review of existing English lan- and EDAC as two separate entities, often using words guage literature since the mid-1960s, we submit that such as malacia and collapse interchangeably, TBM, although characterized by weakness of airway describing EDAC findings while calling it TBM or vice- cartilage, may occur either with or without an exces- versa. Recently available radiographic and broncho- sive dynamic invagination of the posterior membra- scopic imaging techniques however, enhance the cli- nous portion of the tracheobronchial tree. This latter nician’s ability to differentiate these two processes. A process, referred to as EDAC, appears to be a distinct third issue is the absence of a universally accepted clinicopathologic entity which may or may not result nomenclature and classification. Indeed, few investi- from hypotonia of myoelastic elements of the poste- gators classify TBM or EDAC using more than one rior membrane. We submit that TBM and EDAC can radiographic, bronchoscopic, or histopathologic occur independently from each other or together, and criterion. may be part of the same pathological process or result from very different mechanisms. Understanding the distinctions between these two METHODS entities has been challenging for several reasons. The All published literature pertaining to acquired or idio- pathic TBM was collected by searching PubMed (MEDLINE) from the mid-1960s to April 2005 using Correspondence: Henri G. Colt, UCI Medical Center, the following key words: tracheomalacia, broncho- 101 the City Drive South, Bldg 53, Rm 119, Rt 81, Orange, malacia, tracheobronchomalacia, adult, acquired, CA 92868, USA. Email: [email protected] dynamic airway collapse, choke point, tracheal col- Received 23 July 2005; invited to revise 6 October 2005; lapse, bronchial collapse and tracheobronchial col- revised 21 December 2005; accepted 31 December 2005 lapse. We also performed specific disease searches for (Associate Editor: YC Gary Lee). relapsing polychondritis, congenital tracheobron- Respirology (2006) © 2006 Asian Pacific Society of Respirology TBM and EDAC 389 chomegaly and thyroid disease because these condi- Distinguishing TBM from EDAC tions are known to be associated with TBM. Abstracts and case reports were included. When articles were In the literature, there is no clear distinction between published in a foreign language, however, only the EDAC and TBM. In fact, defining TBM as a narrowing English language abstracts were reviewed. Searches of the lumen by 50% or more while coughing as were augmented by manually reviewing the reference documented by fluoroscopic observations11 or by lists of all original research and all review articles. bronchoscopy12 may give rise to a missed diagnosis of Because the purpose of this systematic review was EDAC and to false positive cases of TBM in the neither to perform a meta-analysis nor to grade the absence of cartilaginous involvement. In one fre- evidence of the published literature, this methodol- quently quoted review article, the term TBM is used ogy is limited but not flawed by the fact that only a interchangeably to describe EDAC and malacia.1 single database for locating articles was used and that EDAC has been also referred to as TBM in several other electronic databases such as EMBASE, LILACS reports using bronchoscopy for diagnosis.2,12–14 In the or Best Evidence were excluded. radiology literature, TBM has also been classically defined as a reduction in airway CSA greater than 50% on expiratory images.6–8 It appears that prior investi- DEFINITIONS gators have often used the term TBM while referring to EDAC and vice versa.6–9 Tracheobronchomalacia The term malacia derives from the Greek word Morphologies of TBM and EDAC (Fig. 1) ‘malakia’, which means softness. TBM has been defined as a condition in which there is weakness of The radiology and bronchoscopy literature refer to the tracheal and bronchial walls due to softening of several morphological types of TBM.1,2,7,9,12–17 The cres- the supporting cartilage and hypotonia of myoelastic cent or membranous type is due to an apparent weak- elements.1,2 As a result, the trachea and main bronchi ness of the membranous part of the airway and felt to lose their usual degree of stiffness and the airway represent an atrophy of the longitudinal elastic fibres walls come closer together. This results in a reduction of the posterior wall.1,2,7,9,12–14 The cartilaginous type of of airway lumen and causes a disease state encom- TBM is caused by a weakness of the lateral and ante- passing intermittent or continuous dyspnoea, diffi- rior cartilaginous walls of the airways.7,15–17 This type culty clearing secretions, cough, wheezing, recurrent can have a crescent or saber-sheath appearance on bronchitis or pneumonia that has traditionally been bronchoscopic or radiographic examination depend- called TBM. ing on whether the anterior or lateral walls of the air- way are weakened. This is a dynamic process and is different from saber-sheath trachea which is a fixed Excessive dynamic airway collapse narrowing described in up to 5% of older males with COPD.18–20 A combined or circumferential type refers TBM is also referred to in the literature, however, as to a combination of crescent and saber-sheath type16 tracheobronchial collapse,3,4 expiratory tracheobron- or when a disease state, such as polychondritis, chial collapse, expiratory tracheobronchial stenosis,2 results in circumferential narrowing of the airway tracheobronchial dyskinesia,5 or described as lumen.21 dynamic airway collapse (DAC).1,6–9 This contributes to some confusion regarding these distinct entities. Dynamic CT measurements of the normal trachea PREVALENCE during forced expiration show a mean decrease of 35% (range 11–61%) in the cross sectional area (CSA) The reported prevalence of TBM and EDAC varies of the trachea between inspiration and expiration. with the study population, the diagnostic methodol- Thus, a certain degree of DAC characterized by invag- ogies employed, and the criteria used to define airway ination of the posterior membrane of the tracheo- collapse. Previous investigators often referred to TBM bronchial tree is physiological and probably as a narrowing of the anteroposterior diameter of enhances expectoration and secretion clearance. In the airways without specifying whether collapse was healthy individuals, cine-bronchography studies secondary to a cartilaginous process.2,11,13,22 This show, in fact, that the tracheobronchial lumen during describes a crescent-type abnormality, forcing the coughing is 18–39% narrower than the maximal reader to presume that saber-type malacia was not inspiratory lumen observed during restful respira- noted. In many of these papers, EDAC is described tion.10 DAC is exaggerated, however, in some patients but not identified as such.1,6–9 with obstructive pulmonary disease such as chronic In one study, expiratory collapse of the trachea bronchitis, emphysema, asthma and TBM. Excessive and main bronchi was noted to be greater than 50% collapse may also be seen as an isolated finding in during cough. This was seen during bronchoscopy patients during cough and forced expiration. A reduc- in 11 of 78 patients (14.1%) referred to a pulmonolo- tion of airway lumen by 50% or more in the sagittal gist for evaluation of chronic cough lasting an aver- diameter has been considered abnormal4,11 and, age of 72 months.23 This is a frequently quoted paper when due to invagination of the posterior