Endobronchial OCT Useful for Assessing Airway Remodeling and Function in Asthma
Key findings
- Endobronchial optical coherence tomography (OCT) was used to measure mucus and airway microstructure in allergic individuals with and without asthma
- Evidence of airway remodeling was detected in individuals with mild asthma, and there was a difference in reaction to allergen challenge between mildly asthmatic and nonasthmatic individuals
- A relationship was observed between baseline remodeling and baseline airway obstruction, highlighting the physiological relevance of even mild airway remodeling
- As a minimally invasive technique, endobronchial OCT should prove useful for asthma research as well as for clinical monitoring
For asthma, anti-inflammatory treatments fail to control the symptoms sufficiently in some patients, emphasizing the need to better understand the pathogenesis of airway remodeling in the disease. Optical coherence tomography (OCT), which offers high-resolution volumetric images of cellular microstructure, is compatible with bronchoscopy, but it isn't often applied to the study of asthma.
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By using endobronchial OCT, David Adams, PhD, instructor in medicine, Melissa J. Suter, PhD, associate professor of medicine at Harvard Medical School, working in the Division of Pulmonary and Critical Care Medicine at Massachusetts General Hospital, and colleagues have demonstrated both structural and functional differences between mildly asthmatic and nonasthmatic individuals, which they describe in Respirology.
Study Methods
The researchers recruited 20 allergic, mildly asthmatic individuals (AA), 22 nonasthmatic allergic controls (AC) and three healthy controls (HC) to undergo endobronchial OCT. They imaged a 3 cm airway segment in the right middle and right upper lobe of each subject immediately before and 24 hours after a segmental allergen challenge to the right middle lobe.
From a single scan, the researchers were able to quantify four metrics:
- Epithelial thickness
- Mucosal thickness
- Mucosal buckling, a measure of bronchoconstriction
- Mucus contrast, a measure of mucus content
Baseline Results
Baseline differences between groups were observed for each of the four features:
- The AA group had significantly increased epithelial and mucosal thickness and mucosal buckling compared with the AC and HC groups
- Mucus contrast was significantly higher in the AA group than in the AC group
OCT and Spirometry
At baseline, the researchers measured FEV1/FVC (forced expiratory volume in 1 second/forced vital capacity) and FEV1 percent predicted for the AA and AC groups, then compared the results with the four baseline OCT metrics:
- No correlation was observed between any of the metrics and FEV1 percent predicted
- In the AA group, both epithelial thickness and mucosal buckling significantly correlated with FEV1/FVC
Response to Allergen Challenge
- Compared with baseline values, epithelial thickness and mucosal thickness significantly increased in response to an allergen in the AA and AC groups
- In the AA group, mucosal buckling significantly increased in response to an allergen, indicating resistance to relaxation
- In the AA group, mucosal buckling also increased significantly in response to the diluting agent used in the allergen challenge, indicating hyperresponsiveness of the airway
- Mucus contrast significantly increased in the AA group compared with both the baseline of the AA group and the post-allergen result in the AC group
Toward the Future
These findings suggest that abnormalities of airway smooth muscle have a crucial role in the pathogenesis of asthma. The relationship between bronchoconstriction and airway smooth muscle deserves further investigation, as does continued development of technology that can directly assess airway smooth muscle without a biopsy. Such advances could facilitate the development of bronchial thermoplasty and other asthma therapies that target airway smooth muscle.
The study results also demonstrate the utility of endobronchial OCT for both asthma research and clinical monitoring of airway remodeling, as it offers a minimally invasive approach for measuring multiple important features of remodeling simultaneously.
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