Functional tracheal narrowing: What does it mean on CT?.

Abstract

Learning Objectives: To review different types of expiratory functional tracheal narrowing and demonstrates the utility of 4-dimensional CT (4D-CT) in diagnosis, quantification, and treatment of these important conditions. Background(s): Functional expiratory tracheal narrowing occurs in 23% of unselected adults during bronchoscopy and is not uncommonly encountered on routine CT of chest. It has been poorly understood and often terms such as tracheomalacia, tracheobronchomalacia (TBM), dynamic airway collapse (DAC) and excessive dynamic airway collapse (EDAC) have been used indiscriminately. The first two describe luminal reduction from cartilage softening and the latter refers to luminal reduction from exaggerated posterior tracheal membrane movement. These disorders are often under-diagnosed clinically and can cause severe symptomatic airflow limitation with dypnoea, wheeze, and exercise intolerance. They can be misconstrued and mistreated as other diseases, such as difficult-to-treat asthma or chronic obstructive pulmonary disease. The diagnosis and quantification of expiratory tracheal narrowing has crucial aetiological, therapeutic and prognostic implications. The gold standard for diagnosis has traditionally been bronchoscopy. 320-slice dynamic volume computed tomography permits assessment of movement of an organ and structure over time rendering 4-dimensional CT images. This 4D-CT becomes an emerging noninvasive method enables detection and quantification of airway conditions. Imaging Findings: DAC is a physiological process and is characterized by a lack of tracheal cartilage deformation and less than 50% luminal narrowing. EDAC occurs when tracheal area is reduced by greater than 50% on expiration with 'frown-like' exaggerated posterior membrane movement which may be difficult to judge without dynamic CT. It may be a transient phenomenon during acute dyspnea and responds to positive pressure ventilation. TBM indicates abnormality of the cartilage rings that may be associated with relapsing polychondritis and can be focal, segmental or diffuse. They could be of 'saber', 'crescentic', 'lunate' and 'circumferential' shapes. TBM and EDAC can be progressive. Conclusion(s): 4D-CT allows the non-invasive diagnosis of expiratory tracheal disorders and is a great utility in the assessment of disease progression and therapy guidance. 4D-CT will be a useful tool for future research in functional airway disorders and may give rise to better understanding of the pathophysiology, classification, prognosis, and treatment.