Parenchymal destruction on CT and impaired lung function in asthma
Published online: August 23, 2021
Current clinical management and treatment, such as inhaled corticosteroids, bronchodilators, and biologics have not fully attained to preserve the lung function in patients with asthma. There remains a subgroup of patients with asthma who show fixed airflow obstruction (FAO) and/or an accelerated decline in lung function. FAO in asthma is generally believed to be caused by airway remodeling. Meanwhile, autopsy studies have shown the destruction of alveolar walls attached to small airways, termed alveolar attachments, and centrilobular emphysema in non-smokers with asthma. Moreover, physiological study has suggested the loss of elastic recoil in older longstanding non-smokers with asthma with FAO, even after recommended treatments. However, it has not been elucidated that parenchymal destruction had a functional role in asthma.
Shimizu et.al. examine in a recently published article in The Journal of Allergy and Clinical Immunology (JACI), whether parenchymal destruction evaluated on Computed Tomography (CT) is associated with FAO, or lower forced expiratory volume in 1 sec (FEV1) at the baseline, and greater longitudinal decline, irrespective of airway remodeling in the Hokkaido-based Investigative Cohort Analysis for Refractory Asthma (Hi-CARAT), the prospective asthma cohort including smokers. Two CT indices, low attenuation area percentage (LAA%) and exponent D are adopted in this analysis to evaluate parenchymal destruction. LAA% is generally used as an emphysema index. Exponent D reflects fractal property in an object exhibiting self-similarity at various length scales. Mishima et.al demonstrated that emphysematous changes on CT in chronic obstructive lung diseases (COPD) possesses fractal property and Tanabe et al. showed in a computer simulation that when LAA% increases, a decrease in exponent D could reflect coalescence of low attenuation clusters representing emphysematous destruction rather than simple local lung expansion.
Exponent D are lower and LAA% are higher in COPD (N = 42) and asthma with FAO (N = 101) than in asthma without FAO (N = 88). The decreased D and increased LAA% are associated with FAO regardless of smoking status or asthma severity. In multivariable analysis, decreased D and increased LAA% are associated with an increased odds ratio of FAO and decreased FEV1, irrespective of airway indices (i.e. airway wall area percent (WA%), which is the ratio of airway wall area to total airway dimension at segmental airway, and airway fractal dimension (AFD), which represents fractal property in airway tree). Moreover, decreased exponent D affects a decline in FEV1 for five years in patients with severe asthma, independent of smoking status. No significant correlations are found between exponent D and blood eosinophil count or the percentage of eosinophils or neutrophils in sputum.
In conclusion, parenchymal evaluation with a combination of LAA% and exponent D on CT may suggest the presence of parenchymal destruction in patients with asthma with FAO regardless of smoking status and asthma severity. Moreover, parenchymal destruction expressed by decreased exponent D and increased LAA% are associated with lower FEV1% predicted, and decreased exponent D affects the longitudinal FEV1 decline independent of airway indices on CT, WA% and AFD in asthma. Of note, exponent D did not correlate with the eosinophil count in blood and sputum. Therefore, more attention should be paid to the possibility that both airway disease and parenchymal destruction underlie physiological impairments and may affect clinical outcomes in a subgroup of asthma, who requires personalized managements and novel interventions in the future.
The Journal of Allergy and Clinical Immunology (JACI) is an official scientific journal of the AAAAI, and is the most-cited journal in the field of allergy and clinical immunology.
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