Diagnosis
The "Gold standard" diagnostic test is dynamic awake bronchoscopy, during which the airways can be observed in different phases of breathing, cough etc.
Lung function tests are not diagnostic or specific and can even be normal.
There are findings indicative of ECAC in CT Chest and CT Pulmonary Angiograms (CTPA), commonly performed in the investigation of dyspnoea; it is important that the Radiologists recognise these CT features and suggest the diagnosis. A dedicate biphasic CT scan can help support a confident diagnosis.
Imaging
- Standard chest CT
The diagnosis of ECAC can be suggested by the morphology of the trachea and Tracheal index (TI) on expiration images. This is even more pertinent for CTPA, commonly the first test performed for investigation of acute or exacerbated dyspnoea. CTPA is usually in expiration to improve the opacification of the Pulmonary arteries.
Review of the shape of the trachea, posterior membrane and visual assessment of the cross-sectional luminal area reduction can suggest ECAC.
- Dedicated Biphasic CT
Biphasic CT compares well with Bronchoscopy with accuracy reported 93- 97%.
The dedicated protocol includes paired end-inspiratory and dynamic expiratory CT to assess the tracheal collapsibility.
Technique:
Images are acquired during the end inspiration and during forced expiration. The whole chest is scanned during inspiration, the expiratory phase can be limited to the airways.
The results are better if the patients are coached to follow the radiographers’ instructions, and the procedure is clearly explained to them beforehand.
Analysis:
On the scans acquired the cross-sectional luminal area of the trachea is calculated on lung window to allow better delineation of the inner wall of the airway in both phases.
Diagnosis of ECAC has been defined as >50% airway narrowing in expiration compared to inspiration but some studies showed that a cut off >70% is more specific in the adult population, as the physiological airway narrowing in exhalation in normal individulas is variable and often >50%.
CT quantifies the collapse that correlates with the severity of ECAC.
- Grades of expiratory collapse - severity:
Mild: 70-80%, Moderate :81%- 90, Severe: > 90%
In addition, CT demonstrates the airways morphology and offers information about other concurrent lung pathology, and particularly the extent of commonly coexistent air trapping.