Cognitive functions in nonhypoxemic chronic obstructive pulmonary disease (COPD).

Comment on: A comparison of cognitive functions in non-hypoxemic chronic obstructive pulmonary disease (COPD) patients and age-matched healthy volunteers using mini-mental state examination questionnaire and event-related potential, P300 analysis

Sir,

This is with regard to the article entitled “A comparison of cognitive functions in non-hypoxemic chronic obstructive pulmonary disease (COPD) patients and age-matched healthy volunteers using mini-mental state examination questionnaire and event-related potential, P300 analysis” published in your journal.[1] At the outset, I must congratulate the authors for working on a relatively less-discussed topic in India-cognition. But I have some basic reservation with the design of the study, which makes the interpretation of results as flawed. The authors have chosen a case-control study design for the purpose of conducting the study, which is adequately capable to deal with the requirements of the study. But they have left lacunae in their inadequate attention to the “matching” they have performed in the study. All cases as pointed by the authors are current or ex-smokers in comparison to all nonsmokers from the control group. As we are aware, smoking in itself is a risk factor for cognitive impairment; therefore, matching needed to take smoking into account before finalizing the design of the study. It is seen that in addition to the increased risk of cerebral atherosclerosis, certain particles in cigarette smoke are thought to have a direct neurotoxic effect with heavy-metal constituents of smoke being linked to an increased risk of Alzheimer′s disease.[23] A multicenter European cohort showed that yearly decline in Mini Mental State Examination (MMSE) was associated with smoking status, after correcting for baseline MMSE, education, and vascular events.[4] Also, smoking is a risk factor for preclinical changes detected on brain computed tomography, and longitudinal studies have found associations with middle-age smoking and cognitive dysfunction in males over a 20-year period.[5] It is seen that the cognitive deficits associated with smoking include reduced processing speed, verbal memory, and MMSE.[5]

A better way to conduct this study would be to have a case-control study with patients of COPD (if all smokers) against controls without COPD (but all smokers). If this was not possible, then adjustment for smoking as a confounder should have been made at the analysis level after the study was conducted by, maybe, using a regression model for statistics.