A New Definition for Emphysema

Aaron B. Holley, MD


September 27, 2018

Chronic bronchitis is an established phenotype with a clinical definition, associated outcomes, and targeted treatments.[1] The same cannot be said for emphysema, but a recently published letter[2] attempts to expand what we know about this condition.

The authors of the letter performed a post hoc analysis of data from two large, National Institutes of Health-sponsored observational cohort studies designed to collect data on lung testing, radiology, biomarkers, and clinical outcomes to refine our understanding of chronic obstructive pulmonary disease (COPD) phenotypes. These studies, SPIROMICS (Subpopulations and Intermediate Outcomes in COPD Study)[3] and COPDGene (COPD Genetic Epidemiology),[4] have already added to what we know about COPD outcomes. The authors sought to help create a clinically relevant definition for emphysema.

All patients in both studies had CT performed. For their post hoc analysis, the authors used software to calculate the percentage emphysema for each patient (defined as any area less than -950 Hounsfield units). They ran percentage emphysema against important clinical outcomes.

Across outcomes, 5% emphysema on CT scan was the magic number. That is to say, at greater than 5%, patients had more exacerbations, increases in the St George's Respiratory Questionnaire, and greater mortality. The association was only relevant for patients with an FEV1/FVC < 70% (obstruction by GOLD criteria[5]). For those who were not obstructed, the relationship between percent emphysema and clinical outcomes was not present.[2]

The authors want us to consider using percent emphysema on CT when making treatment decisions for patients with COPD. Of note, they are not advocating that we order CT specifically for this purpose, but rather we use information from CT scans that were obtained for other purposes (eg, lung cancer screening). This seems reasonable, and anecdotally, I would say the majority of my patients with COPD end up undergoing CT for one reason or another.

The authors also state that the imaging software used to quantify emphysema is readily available. Unfortunately, that has not been my experience. I work at a tertiary, academic medical center with board-certified thoracic radiologists. I am told we have the software, but I have never seen it used, and I would not say that it is readily available.

In summary, SPIROMICS and COPDGene provide excellent opportunities for correlating available testing with clinical outcomes. Starting in medical school, we are taught the importance of identifying emphysema, but a precise definition is not provided. Now we have a threshold, 5%, above which we see an increase in clinically important outcomes (among patients who are obstructed on spirometry).

The next step to establishing relevance is to identify treatment targeted at patients with emphysema by this definition. In addition, if the software required to obtain the percent emphysema measurement is truly "readily available," it would be great to see it used routinely and the findings added to existing reports.

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