Studying trajectories of multimorbidity: a systematic scoping review of longitudinal approaches and evidence

Introduction: Multimorbidity is an important public health challenge in ageing societies. While most multimorbidity research takes a cross-sectional approach, longitudinal approaches to understanding multimorbidity are an emerging research area. We aim to scope the methodological approaches and substantive findings of studies analysing multimorbidity trajectories.
Methods: We identified studies employing longitudinal methodologies to explore multimorbidity trajectories using a scoping review approach. A systematically search for relevant studies was carried via four academic databases (Medline, Scopus, Web of Science, and Embase) complemented by professional networks and reference lists.
Results: Our review identified 34 studies investigating multimorbidity longitudinally, all published in the last decade, and with a clear lack of research based on low- and middle-income country data. Longitudinal approaches included constructing variables of change, multilevel regression analysis (e.g.
growth curve modelling), longitudinal group-based methodologies (e.g. latent class modelling), analysing disease transitions, and visualisation techniques. Commonly identified risk factors for multimorbidity onset and progression were older age, higher deprivation, overweight, and poorer health behaviours.
Conclusion: The nascent research area employs a diverse range of longitudinal approaches that characterize accumulation and disease combinations, and to a lesser extent disease sequencing and progression. Understanding the life course determinants of different multimorbidity trajectories across diverse populations and settings can provide a detailed picture of morbidity development, with
important implications from a clinical and intervention perspective.