The causes and consequences of seasonal variation in COPD exacerbations

Abstract

The time of year when patients experience exacerbations of chronic obstructive pulmonary disease is a much-overlooked feature of the disease. The higher incidence of exacerbations in winter has important consequences for patients in terms of increased morbidity and mortality. The seasonality also imposes a considerable burden on already-overloaded health care services, with both primary care consultations and hospital admissions increasing in number. The seasonality of exacerbations varies with latitude, and is greater in more temperate climates, where there may be less protection from outdoor and indoor cold exposure. The precise causes of the seasonality are unknown, but thought to be partly due to the increased prevalence of respiratory viral infections circulating in cold, damp conditions. Increased susceptibility to viral infection may also be a mechanism mediated through increased airway inflammation or possibly reduced vitamin D levels. The seasonality of exacerbations informs us about the triggers of exacerbations and suggests possible strategies to reduce their number.

Keywords: exacerbations of COPD; seasonality; winter morbidity; winter mortality.

Figure 1

Proportion of patients reporting an exacerbation in the (A) northern or southern regions, and in (B) the tropics, averaged over 1 calendar year. Notes: Reproduced with permission of the European Respiratory Society. Eur Respir J January 2012; 39:38–45; published ahead of print July 7, 2011, doi:10.1183/09031936.00194610 © European Respiratory Society.

Figure 2

Diary-card data collected from the London COPD cohort between November 1995 and November 2012. Notes: Points are the percentage of patient-reporting activity, worsening of respiratory symptoms or onset of exacerbation on days within 1°C, or average peak expiratory flow rate on those days. Abbreviations: COPD, chronic obstructive pulmonary disease; PEFR, peak expiratory flow rate.

Figure 3

Seasonality of COPD mortality, by age. Notes: Mean daily deaths per million population (mortality) for men and women aged 65–74, 75–84, and 85+ years for each month between 1976 and 2012 in England and Wales from COPD (coded with the ninth and tenth International Classification of Diseases [ICD] as ICD-9 490–492, 496 for years before 2001, and ICD-10 J40–J44 thereafter) were extracted from death-registration data supplied by the Office of National Statistics (ONS). To be compatible with deaths before 1983 and after 1993, daily deaths from COPD were divided by 0.940 to adjust for differences in mortality-coding instructions in the intervening years. Deaths classified using the ICD-10 (2001–2012) were divided by 0.966 to allow for differences between the ICD-9 and ICD-10. Deaths per day per million population were obtained after dividing daily deaths by daily population estimates obtained by fitting a fifth-order polynomial to yearly estimates of the populations from the ONS from census data. Abbreviation: COPD, chronic obstructive pulmonary disease.

Figure 4

Time-series analysis showing the time courses of respiratory mortality following a cold day. Notes: M RES describes the increase in mortality per 1°C fall in temperature. The y-axis is the regression coefficients of mortality on temperature on days before and after day 0. The horizontal line is the means of values at lag −30 to −16 days. *Peak value and significantly different from zero, P<0.001. Reproduced from J Epidemiol Community Health, Early increases in ischaemic heart disease mortality dissociated from and later changes associated with respira tory mortality after cold weather in south east England, Donaldson GC, Keatinge WR, 51(6),643–648, copyright 1997, with permission from BMJ Publishing Group Ltd. Abbreviation: M RES, respiratory disease mortality.

Figure 5

Weekly data on laboratory reports of respiratory virus isolation by the Health Protection Agency/Public Health England and National Health Service hospital laboratories in England and Wales. Notes: Data points are the average for that week over years 1980–2013. The y-axes show the number of viruses isolated per week. Adapted from Respiratory infections: laboratory reports 2014 [homepage on the internet]. Public Health England; 2014 [updated September 5, 2014]. Available from: https://www.gov.uk/government/publications/respiratory-infections-laboratory-reports-2014. Accessed September 24, 2014. Contains public sector information licensed under the Open Government Licence v2.0.

Figure 6

Percentage of patients going outside the home against the daily mean temperature throughout the year. Notes: Points are at the center of each 1°C interval; vertical lines are at 2.5°C and 20.5°C. Reproduced with permission from the American College of Chest Physicians from Donaldson et al.