Multimorbidity and quality of life: epidemiological perspective from the European Region

Life expectancy has been increasing for decades due to societal and economic progress and advancements in both clinical medicine and public health. This trend will continue, along with the advantages and challenges it involves. Increased longevity is associated with an accumulation of diseases; hence, the longer one lives the higher the likelihood of living with more than one illness. The current single-disease oriented organisation of health care systems is not optimal for patients with multiple health conditions. And while better care is warranted, more evidence is required to support the most impactful decisions. Patient quality of life (QoL) is an important parameter of quality of care. People with multimorbidity are often considered to be at the highest risk of decreased QoL, which further impedes their health and functioning. Quality of life has therefore emerged as one of the major research goals in the field, thus representing the main focus of this dissertation.
Chapter 1 presents the background, aims and research questions of the thesis. The main aim is to enhance knowledge of multimorbidity, in particular the association between multimorbidity and QoL. Specifically, this thesis focuses on answering the following research questions:
1. What is the operational definition of multimorbidity as compared to comorbidity?
2. What is the strength of the association between disease count and QoL, and what are the relevant elements in the explanation; does the association evolve over time and how does it present across nations?
3. What are the most common disease patterns and how do they relate to QoL?
Chapter 2 highlights a common mistake made in research, the interchangeable use of the terms comorbidity and multimorbidity, and clarifies the difference. Comorbidity is a term which relates to conditions appearing in addition to an index disease, while multimorbidity does not prioritize any of the co-occurring conditions. This distinction is necessary to ensure the clear operationalisation of multimorbidity and its prevalence, as well as for enhancing health care models which may differ for patients with multimorbidity vs. comorbidity. This clarification is also timely considering the ever increasing literature on multiple diseases.
Although several definitions of multimorbidity circulate in the literature, the definition which is consistently used in this thesis is the co-existence of two or more chronic conditions within an individual.
Chapter 3 summarises existing evidence on the association between multimorbidity and QoL with a systematic review, and quantifies for the first time the strength of this association with a meta-analysis. This study explored 25,890 titles on the subject, included 74 studies in the final qualitative synthesis, and subsequently performed 6 case meta-analysis models using 39 studies. The review not only corroborated and reckoned the negative relationship between number of diseases and QoL which progresses with each added condition, but also uncovered additional relevant findings. For example, most of the studies were of a cross-sectional design and disease count was the predominant multimorbidity instrument used to measure the association, assuming a linear trend. However, the review identified a number of other multimorbidity assessment tools. Participant health status was most commonly self-reported, usually from predetermined lists of diseases. The number and types of diseases varied significantly across studies, and researchers applied a broad range of QoL and health related QoL (HRQoL) scales to investigate the association. Therefore, due to clinical and methodological differences, the studies presented high statistical heterogeneity. Meta-analyses demonstrated that the mean reduction in HRQoL for each added disease ranged from 1.6% to 4.4%, depending on the scale (QoL decline expressed in percentages based on the total scales range of 0-100); the decline was stronger for physical health