Original article
Causes and consequences of comorbidity: A review

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Abstract

A literature search was carried out to identify and summarize the existing information on causes and consequences of comorbidity of chronic somatic diseases. A selection of 82 articles met our inclusion criteria. Very little work has been done on the causes of comorbidity. On the other hand, much work has been done on consequences of comorbidity, although comorbidity is seldom the main subject of study. We found comorbidity in general to be associated with mortality, quality of life, and health care. The consequences of specific disease combinations, however, depended on many factors. We recommend more etiological studies on shared risk factors, especially for those comorbidities that occur at a higher rate than expected. New insights in this field can lead to better prevention strategies. Health care workers need to take comorbid diseases into account in monitoring and treating patients. Future studies on consequences of comorbidity should investigate specific disease combinations.

Introduction

The aging of populations is a worldwide phenomenon, especially in Western societies. In 1990 the proportion of those 65 years or over ranged from 11% to 18% in Japan, Western Europe, North America, and Australia. This percentage is expected to increase to approximately 19–26% in 2025 [1]. Because many health problems are known to increase with age, this demographic trend may lead to an increase in the absolute number of chronic conditions in the population. In addition, because there is a growing body of evidence that older people are at risk for multiple, comorbid conditions, the prevalence of comorbidity in the general population, as well as among those seeking health care, will probably also increase and become a common phenomenon.

Comorbidity refers to one or more other diseases among people with an index-disease (e.g., cardiovascular disease) 2, 3. In elderly populations, comorbidity occurs frequently, as can be illustrated by three examples. In a Dutch General Practice population, 79% of the elderly with a chronic health condition had one or more comorbid diseases [4]. A Dutch cancer registry found that the prevalence of comorbidity among incident cancer patients ranged from 12% among patients younger than 45 years to 60% among patients of 75 years or older [5]. Finally, the Duke Established Populations for Epidemiologic Studies of the Elderly showed that older persons with hypertension, coronary artery disease, cerebrovascular disease, diabetes, and cancer reported substantial comorbidity [6]. This ranged from 47% among those with hypertension to 88% among those with cerebrovascular disease.

An international expert meeting on comorbidity and chronic diseases, organized by the Netherlands Organization for Scientific Research (NWO) in 1996, concluded that the importance of comorbidity is clear, due to its high prevalence in older populations and its impact on health and health care. However, despite the growing number of studies, it also was concluded that comorbidity has been rarely studied as an autonomous phenomenon [7], and more research is needed into the etiology and consequences. The present review was conducted to accumulate and summarize the available information in the recent literature on causes and consequences of comorbidity of a wide range of chronic somatic diseases in order to give more specific recommendations for future research, public health and health care practice. Outcomes to be studied were mortality, functional status or quality of life, and health care (health care utilization, treatment strategy, complications of treatment and discharge destination or readmission to hospital).

Section snippets

Methods

To organize our review findings about the causes and consequences of comorbidity, we used an extension of the general, integrative public health model, commonly applied to study the epidemiology of diseases (Fig. 1). The selection of the index-diseases was based on the burden of disease for patients and society in terms of mortality and/or morbidity (prevalence and severity), excluding mental diseases 8, 9. In case of a broad disease category, a selection of the most important diseases (in

Methodological aspects

The most important methodological characteristics of the selected articles are presented in Table 1, Table 2. More extensive tables can be found on the internet pages of the journal and of the National Institute of Public Health and the Environment (RIVM) (internet address of the Journal of Clinical Epidemiology: www.sciencedirect.com/web editions; internet address of the RIVM: www.rivm.nl/public health/phsf/download/comorb2001pdf ). Comorbid diseases were assessed by different methods,

Discussion

In this article, we gave an overview of the comorbidity studies concerning either causes or consequences of comorbidity, published between 1993 and 1997. The concept of comorbidity has been receiving increasing attention, as evidenced by the large and increasing number of studies each year; in our literature search the number of studies increased from 53 in 1993 to 141 in 1997. As such, we recommend that reviews of this kind be conducted from to time to time to monitor the trends of research in

Acknowledgements

This research was supported by the Netherlands Organization for Scientific Research (NWO). William Satariano's sabbatical leave at the National Institute for Public Health and the Environment (02/99–06/99) was supported in part by a Fulbright Senior Scholarship. The authors thank Henriëtte F. Treurniet and Alice H. de Boer (National Institute of Public Health and the Environment) and Judella A. Daal (Slotervaart Hospital, Department of Clinical Geriatry) for their helpful comments.

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