Introduction

The prevalence and complexity of multiple long-term conditions (MLTC or multimorbidity) are increasing rapidly, especially among older people and those who are socio-economically deprived [1–4]. MLTC challenge health and social care systems as their presence is associated with increased dependency and complexity of care needs, higher health and care use and costs, poorer quality of life and mental health problems [5,6]. MLTC and frailty (decline in physiological reserves, contributing to increased vulnerability to adverse health outcomes) are interrelated, exacerbating the onset and severity of the other [7,8]. Over 16% of people with MLTC are frail, and over 70% of people who are frail live with MLTC [7]. Most people with MLTC and frailty live in community settings, where many rely on informal care and support from immediate family, relatives and friends to maximise quality of life and maintain independence. In 2022−23, an estimated 8% of the United Kingdom (UK) population (5.2 million people) were providing informal care, with a third of them aged ≥55 years [9]. Such care, which commonly includes support with activities of daily living, e.g., eating, dressing, grooming, bathing, shopping, attending appointments, managing and administering medications, often stems from love, respect, or duty, bringing positive benefits such as strengthened bonds and personal fulfilment. However, it frequently comes at a heavy cost to carers, especially to older spouse or partner carers who often live with long-term conditions themselves and have unmet support needs [10].

Care is a dynamic and mostly reciprocal relationship where the care recipient and informal carer both influence and are reliant on one another in different support domains [11–13]. Dyadic theoretical models, such as the Actor-Partner Interdependence Model [14] and the Theory of Dyadic Illness Management [15], provide a theoretical basis for understanding the reciprocal dynamics within care relationships by positing that an individual’s traits and behaviours not only shape their own experiences and outcomes but also have effects on their care partner and vice versa. This bidirectional and mutual influence underpins the rationale for dyadic or pair-based interventions, actively engaging carers and care recipients in interventions simultaneously to improve outcomes for one or both members of the dyad [16]. Given that poor experience of providing care for adults with MLTC and/or frailty may negatively affect care experiences and outcomes for both informal carers and care recipient adults with MLTC and/or frailty, there is a potential for dyad-tailored support interventions to achieve more meaningful and sustained improvements in dyadic outcomes. Previous reviews examining the effects of dyadic interventions on health outcomes have focused predominantly on informal carers and care recipient adults with a single disease (e.g., cancer, dementia) [17,18] or a mode of delivery of dyadic interventions (e.g., eHealth) [19]. The evidence of the benefits of dyadic interventions in the context of MLTC and frailty is lacking. It is also unclear if the effects of dyadic interventions differ according to informal carer characteristics, such as between co-resident older carers (spouses or partners), who typically provide higher-intensity care while managing their own long-term conditions [10] and adult family members, relatives or friends, who may or may not be co-resident and often provide lower-intensity support.

The aim of this review is to systematically synthesise current evidence on the effectiveness of dyadic interventions in improving outcomes for adults with MLTC and/or frailty (aged ≥55 years) and their informal carers. Specific research questions sought by the review are:

• What is the effectiveness of community dyadic interventions in improving outcomes for adults MLTC and/or frailty aged ≥55 years and their co-resident older informal carers (spouses or partners) aged ≥55 years?
• What is the effectiveness of community dyadic interventions in improving outcomes for adults MLTC and/or frailty (aged ≥55 years) and their informal carers (adult family members, relatives, or friends)?

Materials and methods

The review protocol followed the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) guidelines [20]. The protocol was registered with PROSPERO (CRD420251144604).

Outcomes

We will include studies reporting on quality of life (QoL), health and social care usage, including primary care usage (≥1 during follow up), emergency care attendance (≥1 during follow-up), hospital admission (≥1 during follow-up), social care usage (≥1 during follow up), caregiving burden and care competence, social participation or exclusion, mortality and other frequently reported outcomes measured by validated instruments or any clinically meaningful metrics. QoL could be measured by validated self-reported outcome instruments, such as the Short Form (SF)-12, SF-36, EQ-5D-3L, EQ-5D-5L. Caregiving or carer burden and care competence could be measured using tools capturing different aspects of carers’ experience (e.g., emotional, physical, social, and financial impacts) and care competence (knowledge, skills, and attitudes) as covered in the Zarit Burden Interview, Caregiver Strain Index, Caregiver Reaction Assessment, Relative Stress Scale, Family Caregiver Task Inventory or Caregiver Self-Assessment Questionnaire. Social participation or exclusion may be measured using self-reported items describing frequency, quality, and diversity of involvement in social activities and community, such as Social Participation Questionnaire (SPQ), Late-Life Function and Disability Instrument (LLFDI), UCLA Loneliness Scale, Lubben Social Network Scale (LSNS)-6. This list is not exhaustive, and other validated measures of outcomes will also be considered. The review selection criteria are detailed in Table 1.

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Table 1. Study selection criteria.

https://doi.org/10.1371/journal.pone.0333728.t001

Discussion

The growing prevalence and complexity of MLTC present an urgent challenge for health and social care systems worldwide. While care and support provided to community-dwelling adults with MLTC and/or frailty is often delivered within reciprocal dyadic relationships, existing evidence has largely overlooked this relational dimension, focusing instead on single-disease contexts or specific modes of intervention delivery. To achieve more meaningful and sustained improvements in dyadic outcomes, it is important to provide a systematic synthesis of the effects of various dyadic interventions targeting adults with MLTC and/or frailty and their informal carers, and map the features of community settings in which these interventions were developed and evaluated in relation to various outcomes. By applying a dyadic lens, this systematic review will address this evidence gap and advance understanding of how community-based care and support services could be re-designed, tailored and implemented to improve outcomes for both adults MLTC and/or frailty and their informal carers, supporting them to live well and longer in their communities.

Findings will have implications for research, policy, and practice. For researchers, it will provide clear evidence of benefit of dyadic interventions in context of MLTC and frailty and identify gaps in implementation research in later life, such as the types of dyadic interventions that have been most rigorously evaluated, outcomes frequently measured and those overlooked, and the methodological limitations that require further exploration. For service providers and policymakers, the review will provide evidence-based insights into the types of community-based dyadic interventions that have the potential to reduce unmet support needs and enhance care experiences and outcomes for people with MLTC and/or frailty and their carers. To help bridge the gap between research and practice, the review findings will be disseminated not only through academic publications but also, in collaboration with key stakeholders, co-produced as an “effective action policy brief” that translates evidence into practical, actionable guidance. This guidance can inform the co-development and implementation of dyadic interventions that are effective, acceptable, feasible, and tailored to local needs and contexts.

Conclusion

Review findings will address the evidence gap in dyadic implementation research in later life and help inform clinical decision-making, policy development and program planning for adults with MLTC and/or frailty and their informal carers, particularly in primary care and other community health settings.

Supporting information