The epidemiology and risk factors of asthma-COPD overlap in low- and middle-income countries
Published online: October 3, 2018
Asthma-COPD Overlap (ACO) refers to a chronic respiratory condition which features clinical and biological markers of both asthma and chronic obstructive pulmonary disease (COPD). COPD and asthma have been well-defined: the former characterized by poorly reversible airflow obstruction via abnormal inflammatory responses secondary to noxious exposures and the latter characterized by airway hyperresponsiveness and inflammation leading to reversible obstruction. However, the mechanism of injury of ACO is yet unknown. Currently ACO is not considered to be a standalone condition, but rather a spectrum of phenotypes on a continuum of eosinophilic and neutrophilic inflammatory illness, which manifests in increased airflow variability with incompletely reversible obstruction. Clinical research to date has largely focused on well-defined groups of asthma or COPD patients, often excluding those who fail to fall into those strict sets. Further, population-based analyses have been conducted in high-income settings or large metropolitan cities in low- and middle-income countries (LMICs) and there is a gap in knowledge regarding ACO in other resource-poor settings. As residents of LMIC are often exposed to unique risk factors such as unplanned urbanization, biomass fuel smoke exposure, and increasing tobacco use, there exists the potential for a significant population of undiagnosed ACO in these settings.
In a recently published article in The Journal of Allergy and Clinical Immunology (JACI), Morgan and colleagues presented an analysis of data from four population-based studies conducted in LMICs: the CRONICAS Cohort Study in Peru, the Pulmonary Risk in South America (PRISA) study in Argentina, Chile, and Uruguay; a longitudinal study in Bangladesh, and the Lung Function in Nakaseke and Kampala (LiNK) study in Uganda. Broadly, each study developed a population-based sample and included questionnaires covering health histories and exposures as well as lung testing via spirometry. The final analytical sample was comprised of 11,923 adults across 12 settings with varying urbanization, geography, and socioeconomic status. Asthma was defined as a self-report of: wheeze in one year, medication use for asthma in one year, or having a physician’s diagnosis of asthma in the past. COPD was defined as a post-bronchodilator FEV1/FVC ratio below the lower limit of normal of the GLI mixed-ethnic reference population. ACO was defined as the presence of both. Participants were classified as non-obstructed non-asthma, asthma-only, COPD-only, or ACO. Important risk factors included tobacco exposure, quantified by pack years, and biomass fuel smoke exposure, defined as belonging to a household that burned natural solid fuels for heating or cooking. Risk factors and health outcomes were determined for ACO compared to other health groups by the use of multivariable mixed effects modeling with random intercepts to account for correlation within sites.
The authors found a prevalence of ACO of 3.8% across all sites, which varied from 0% to 7.8% across settings. The odds of having ACO compared to non-obstructed non-asthma individuals were higher with household exposure to biomass fuel smoke, smoking, and having primary or less education. A total of 31.6% of those with ACO had an FEV1 percent of predicted below 50% compared to 10.9% of those with COPD-only and 3.5% of those with asthma-only. In multivariable modeling, ACO was associated with reduced FEV1 and FVC compared to those in the asthma- or COPD-only groups.
In the first large-scale, population-based analysis of ACO in LMICs, a high prevalence of severe respiratory disease was found, including a sizable proportion of those showing features consistent with Asthma-COPD Overlap. ACO was associated with severely obstructed breathing and both biomass fuel smoke exposure and tobacco smoking appear to be risk factors. The authors call for increased attention on this illness in LMIC areas and for exposures common to LMICs, such as biomass fuel smoke, to be considered in future consensus documents on ACO.
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.