The role of CT scan in reversible airflow obstruction.

Computed tomography has had a dramatic impact on the diagnosis and management of respiratory disease. (1) Traditionally used to characterize diffuse parenchymal lung disease, CT scaning also provides insights into airway diseases. (1-5) In particular, the diagnostic sensitivity of high resolution CT scanning for bronchiectasis has established it as the gold standard for this disease. (2) Using this modality, bronchiectasis is far more common than previously recognized and can be present with minimal or absent symptoms of lung disease. (2,4) However, the clinical significance of asymptomatic bronchiectasis is not clear.

In this issue of the Southern Medical Journal, Oguzulgen et al (5) demonstrate that a small percentage of "asthmatics" have underlying bronchiectasis. They suggest that compared with age- and sex-matched controls, asthmatic patients with bronchiectasis have more severe disease. There are obvious limitations in the retrospective methods of this study, but these results raise interesting questions regarding reversible obstructive lung disease. Specifically, does the group of patients described by Oguzulgen et al primarily have bronchiectasis and present with symptoms suggestive of asthma, or are these asthmatic patients who by nature of disease severity develop bronchiectasis?

Asthma is a chronic inflammatory airways disease characterized by type 2 lymphocytic and eosinophilic bronchitis, mucous hypersecretion, and reversible airflow obstruction. It primarily affects small bronchi and bronchioles. Bronchiectasis is defined as a permanent pathologic dilation of airways that in their normal state are more than 2 mm in diameter. (2) Reversible airflow obstruction is not specific to asthma and is commonly found in other diseases such as chronic obstructive pulmonary disease, fibrosis related to prior tuberculosis, sarcoidosis, and, of course, bronchiectasis. Therefore, in the group of patients described in this study, the demonstration of reversible airflow obstruction on pulmonary function testing is as likely to reflect the presence of bronchiectasis as asthma. Furthermore, neither the symptoms nor the presence of atopy, which affects 30% of the population, would reliably differentiate asthma from bronchiectasis. These findings suggest that underlying bronchiectasis is quite likely to be the primary airways disease in these patients.

Bronchiectasis can occur as a consequence of numerous respiratory conditions ranging from recurrent or severe infections to airway obstructing lesions. (2) In asthma, prior studies using HRCT have commonly demonstrated small airway abnormalities such as prominent centrilobular shadows, which reflect mucous plugging and inflammation, or remodeling in the respiratory bronchioles. (4,6,7) Larger airway abnormalities are also common, with over 50% of asthmatics having bronchial wall thickening or at least one dilated bronchus. (4,6,7) While considered to be consistent with bronchiectasis, the clinical relevance of an isolated single bronchus dilation is highly debatable. Some studies have suggested a role for atopy as a cause of bronchiectasis, but with the exception of allergic bronchopulmonary aspergillosis (ABPA), larger studies do not support this association. In reality, significant bronchiectasis is rarely detected in asthma. (2) As ABPA was ruled out in this study, this suggests that the presence of bronchiectasis in these patients is likely to reflect alternative etiologies.

The presence of bronchiectasis has not been shown to correlate with disease severity in asthmatics. In fact, bronchial wall thickening and small airway abnormalities as described above appear to correlate better with lung function and to be associated with more severe disease, including near-fatal asthma. (7) This is not surprising since autopsy studies have also demonstrated widespread small airway plugging, inflammation and remodeling in fatal asthma. (8) Bronchiectasis is not a common finding in these patients. This is further evidence that while asthmatic patients may have associated bronchiectasis, this is unlikely to be a consequence or marker of disease severity.

In contrast, it is very likely that the patients described by Oguzulgen et al presented with symptoms and pulmonary function suggestive of asthma, but in fact had underlying bronchiectasis. International and national guidelines for assessing asthma severity rely heavily on symptom frequency and inhaler use with persistent disease defined by the presence of symptoms more than twice a week. (9) Bronchiectasis is associated with persistent, often daily symptoms, and if misdiagnosed as asthma, these patients would tend to be classified in the severe spectrum of asthmatic disease. In this retrospective study, it seems clear that the patients diagnosed with bronchiectasis were selected for HRCT due to poor disease control. Therefore, one would presume that the value of HRCT in this study was to allow proper diagnosis of bronchiectasis, rather than assess asthma severity.

This study highlights present clinical practice in patients with reversible obstructive airways disease. In a patient with a diagnosis of poorly controlled asthma, one needs to consider alternative etiologies of reversible airflow obstruction, including COPD and bronchiectasis. HRCT is the gold standard for diagnosing bronchiectasis and such a diagnosis alters the intensity and modality of therapy for this population. Unfortunately, asthma severity cannot yet be gauged radio-logically but as the use of CT scan in airways disease continues to evolve, it may yet play a role providing prognostic information in the future.

References

1. Chooi WK, Morcos SK. High resolution volume imaging of airways and lung parenchyma with multislice CT. Brit J Radiol 2004;77:S98-S105.

2. Barker AF. Bronchiectasis. N Engl J Med 2002;346:1383-1393.

3. Fenlon HM, Doran M, Sant SM. High-resolution chest CT in systemic lupus erythematosus. AJR Am J Roentgenol 1996;166:301-307.

4. Mitsunobu F, Tanizaki Y. The use of computed tomography to assess asthma severity. Curr Opin Allergy Clin Immunol 2005;5:85-90.

5. Oguzulgen KI, Kervan F, Ozis T, et al. The impact of bronchiectasis in clinical presentation of asthma. South Med J 2007;100:468-471.

6. Silva CI, Colby TV, Muller NL. Asthma and associated conditions: high-resolution CT and pathologic findings. AJR Am J Roentgenol 2004;183:817-824.

7. Lee YM, Park JS. Hwang JH, et al. High-resolution CT findings in patients with near-fatal asthma: comparison of patients with mild-to-severe asthma and normal control subjects and changes in airway abnormalities following steroid treatment. Chest 2004;126:1840-1848.

8. Carroll N, Carello S, Cooke C, et al. Airway structure and inflammatory cells in fatal attacks of asthma. Eur Respir J 1996;9:709-715.

9. Global initiative for asthma (GINA). Global strategy for asthma management and prevention. 2002 National Institutes of Health Publication No. 02-5639.

Anthony O'Regan, MD, FRCPI

Department of Respiratory Medicine, University College Hospital Galway, Galway University Hospitals, National University of Ireland Galway; Galway, Ireland.

Reprint requests to Anthony O'Regan, MD, FRCPI, Department of Respiratory Medicine, University College Hospital Galway, Galway University Hospitals, National University of Ireland Galway; Galway, Ireland. Email: [e-mail]

Accepted November 9, 2006.