Medication Adherence of Older Adults with Hypertension: A Systematic Review

Introduction

Hypertension is a significant risk factor for heart disease and stroke, the leading causes of death in the United States (U.S.).¹ In 2020, approximately 670,000 deaths in the US involved hypertension as a primary cause.¹ Meanwhile, over 75% of patients with hypertension appear unable to control their blood pressure,² and the US government spends around $131 billion annually on hypertension healthcare services.³ Among older adults, undertreated hypertension is significantly high.⁴ Arterial stiffening due to aging—an essential clinical expression of hypertension in this population—is a significant health problem that can lead to heart failure, stroke, and death.^5,6 According to the US National Health and Nutrition Examination Survey (NHANES), about 70% of adults aged 65 or older have hypertension, and this is projected to increase yearly.⁴ Nevertheless, this population group has been underrepresented in clinical trials and other research due to concerns regarding frailty, poor physical function, cognitive impairment, or polypharmacy.⁴

Over 55 million Americans with hypertension are treated with antihypertensive medications,⁷ and consequently show an improvement in blood pressure control.⁸ A previous study shows that adherence to antihypertensives significantly lowers the risk of cardiovascular disease (CVD) occurrences among older adults.⁹ Furthermore, research shows a possible threshold effect in decreasing cardiovascular events for adherence at around 80%.⁹ Nevertheless, the adherence rate to antihypertensive medication has been reported to range from only 20 to 50%.¹⁰ Moreover, especially in older adults with hypertension, the medication adherence rate is notably low compared to other age ranges due to progressive cognitive decline or depression developing with age, amongst other factors.¹¹ Given these circumstances, there is a need to systematically review results from contemporary empirical investigations to comprehensively pinpoint factors influencing medication adherence among older adults with hypertension. The review presented here directly addresses this gap, answering the question: What factors influence medication adherence among older adults with hypertension?

Methods

Identify Relevant Studies

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)¹² were applied in this review to present the flow diagram of the literature’s identification, screening, exclusion, and inclusion. Three electronic databases, PubMed, MEDLINE, and Google Scholar, were systematically searched on July 15, 2022, to identify preliminary studies published between 2019 and 2022, reporting factors influencing medication adherence among older adults living with hypertension. The researcher combined the search terms: Hypertension OR High Blood Pressure* OR Essential Hypertension AND Medication Adherence OR Medication Nonadherence OR Medication Noncompliance OR Medication Persistence OR Medication Compliance OR Medication Noncompliance AND Elderly OR Older Adult* OR Frail Elder* using Boolean phrases. In addition, reference lists of the included studies were manually searched to obtain relevant studies. All references identified were stored in EndNote.

Study Selection

The researcher screened titles and abstracts of eligible studies. Subsequently, the full text was also assessed to decide whether or not it was relevant. A third anonymous researcher was required to resolve disagreements when discrepancies occurred. Inclusion criteria were implemented to guarantee that only studies considered relevant to our objective were included. Similarly, exclusion criteria were used to eliminate literature not affiliated with the review (see Box 1).

Box 1 Inclusion and Exclusion Criteria of Included Studies

Data Extraction

The summary of included studies (Table 1) developed for this review included the following data for each study: references, published year, settings, target population, study design, sample size, age, sex, problem and purpose, medication adherence (MA) measurement, main outcome (factor influencing MA), themes (factors influencing MA), and implications/suggestions.

Table 1 A Summary of Included Studies

Data Synthesis

The convergent integrated analysis framework suggested by Joanna Briggs Institute (JBI) for systematic reviews was adopted for the data synthesis of the included studies.³¹ This framework, specialized for the simultaneous analysis of qualitative and quantitative data, transforms data of different categories into the same format to facilitate data integration.³² In our review, themes were extracted from the key findings of the included studies by examining the similarities and differences between the key findings, similar to how qualitative researchers produce themes. For example, one of the key findings by Macquart de Terline et al is that “Patients with a lower wealth index were more likely to be less adherent to their antihypertensive medications (OR: 1.83, 95% CI [1.38–2.45], p < 0.001).” was coded as “Lower wealth index”, forming the theme, “Financial status.”¹³

Results

Search results

A total of 448 articles were initially identified and screened by title and abstract according to the inclusion and exclusion criteria (Box 1). This screening resulted in 16 articles remaining eligible for the full-text screening. During the full-text screening phase, three articles were excluded: two for reporting results exclusively for irrelevant populations, and one for reporting on a study with an aim beyond the interest of the review. Thirteen studies were included in the final review. The PRISMA¹² was applied to outline the retrieval process (see Figure 1).

Figure 1 PRISMA flow chart. Notes: Adapted from Moher D, Liberati A, Tetzlaff J, Altman DG, Prisma Group. Reprint--Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. Annals of internal medicine. 2009;151(4):264–269.33

Description of Included Studies

Table 2 shows that the included studies were published in the following years: 2019 (n = 3, 21.1%), 2020 (n = 4, 30.8%), 2021 (n = 5, 38.5%), and 2022 (n = 1, 7.7%). These studies were primarily conducted in the United States of America (USA; n = 3, 23.1%), Taiwan (n = 2, 15.4%), and Iran (n = 2, 15.4%). The research reported in the remaining five papers was conducted in China, Brazil, Japan, Tunisia, and Latvia, with one paper detailing data collection across multiple settings, including 12 sub-Saharan African countries: Benin, Cameroon, Congo (Brazzaville), Democratic Republic of the Congo, Gabon, Guinea, Côte d’Ivoire, Mauritania, Mozambique, Niger, Senegal, and Togo. The most commonly reported study design was cross-sectional (n = 11, 84.6%), followed by observational (n = 1, 7.7%) and retrospective cohort studies (n = 1, 7.7%). The reported sample sizes varied, with 1 to 200 (n = 1, 7.7%), more than 200 to 300 (n = 5, 38.5%), and over 300 (n = 7, 53.8%). Five studies (38.5%) included participants aged between 55 and 59 years. Popular tools for measuring medication adherence in these studies included the 8-item Morisky Medication Adherence Scale (n = 3, 23.1%) and the four-item Morisky, Green, and Levine Scale (n = 3, 23.1%).

Table 2 The Characteristics of the Included Studies

Description of Factors Influencing Medication Adherence Among Older Adults Living with Hypertension Themes

A summary of the findings from the included studies is provided in Table 1. Fifteen themes are extracted from the key findings of the included studies (Table 3). For example, the number of medications used (n = 7 studies, 53.9%), financial status (n = 5 studies, 38.5%), sex (n = 5 studies, 38.5%), age (n = 4 studies, 30.1%), duration of disease (n = 3 studies, 23.1%), comorbidity (n = 3 studies, 23.1%), and health compliance (n = 3 studies, 23.1%) are among the frequent themes extracted.

Table 3 Factors Influencing Medication Adherence Among Older Adults Living with Hypertension

Discussion

Data synthesis from the systematic review has identified fifteen key factors influencing medication adherence among older adults living with hypertension. These factors are organized into themes. The most prevalent themes include the number of medications used, financial status, sex, age, duration of the disease, comorbidities, and health compliance, and these will be the focus of our in-depth discussion.

Seven of the included studies reported a consistent trend: a higher number of prescribed hypertensive medications correlated with lower medication adherence.^{13,16,20,23,24,28,29} This aligns with robust evidence from a nationwide study conducted in the Republic of Korea, which aimed to investigate the impact of the number of medications and age on antihypertensive medication adherence. The study demonstrated a significant decline in adherence when nine or more total medications were taken, irrespective of age.³⁴ Furthermore, another study revealed that factors such as the number of medications and characteristics contributing to medication regimen complexity—such as dosage forms and dosing frequency—exert a notable influence on medication adherence, particularly among older adults.^35–37 Consequently, healthcare professionals must adopt a more strategic approach to enhance adherence to antihypertensive medications, tailoring interventions based on the total number of prescribed medications.

Another noteworthy finding underscores the significant role of financial status in influencing medication adherence among older adults with hypertension. For instance, one study revealed that difficulties in paying monthly bills were a key determinant of medication non-adherence and poorer self-rated health, even after accounting for age, gender, race/ethnicity, education, employment status, and income.³⁸ Similarly, five studies included in our review established a substantial association between financial status and medication adherence.^{13,16,20,21,28} Specifically, a study by Macquart de Terline et al demonstrated that older hypertensive patients with a lower wealth index were more likely to exhibit lower adherence to antihypertensive medications.¹³ Likewise, Akkara et al identified that individuals with a low socioeconomic level were seven times more likely to demonstrate poor compliance with medication use compared to those with a high socioeconomic level.¹⁶ Our review illuminates the intricate connection between financial constraints and medication adherence, emphasizing the need for socio-medical interventions. These interventions could include enhancing access to affordable medications and implementing upstream social policies to promote equity. It becomes evident that further research to advance this agenda must comprehensively measure the nuanced aspects of socioeconomic status.

Hypertension, a critical cardiovascular risk factor, poses substantial complications regardless of gender. Strong evidence supports the association between gender and medication adherence among older adults with hypertension.^39–41 However, another study in the literature reveals heterogeneity in medication adherence when comparing males and females; for instance, a cross-sectional study by Akkara et al involving 276 older adults with hypertension found that poor compliance with hypertensive medication is 2.3 times more likely in women than men.¹⁶ Conversely, three studies included in this review consistently reported that females are more likely to adhere to antihypertensive medications.^18,21,24 The conflicting findings underscore the complexity of the relationship between gender and medication adherence, warranting further research to elucidate this association. Such investigations are essential for guiding relevant public health policies and strategies.

Our current review identifies a significant relationship between age and adherence to antihypertensive medication among older adults.^15,23,24,29 In a study examining therapy, knowledge about the disease and its control, and demographic differences, adherence levels were found to correlate with the patient’s age: the older the patient, the more likely they are to exhibit higher medication adherence.¹⁵ However, our review includes studies with contrasting results. For instance, Ward et al suggested that older adults with hypertension aged between 80–84 years are more likely to be nonadherent to their antihypertensive medication.²³ Change et al found that age ≥ 65 was associated with forgetting to take medication,²⁴ and Wakia et al identified older adults with hypertension aged ≥ 71 years as being at risk for poor medication adherence.²⁹ These divergent impacts of age on medication adherence reported in the literature may stem from the studies being conducted in different countries, featuring variations in sociodemographics, healthcare systems, and broader cultural contexts. Further research across diverse geographical settings is warranted to enhance the generalizability of findings and provide a clearer understanding of how age influences medication adherence.

Three studies included in this review investigated the influence of hypertension duration on medication adherence.^15,20,26 For instance, a cross-sectional study conducted in Latvia with 187 older adults suffering from hypertension revealed that the longer the patient had experienced hypertension, the more adherent they were to their medication. Similarly, the study by Mamaghani et al aimed to assess baseline barriers to medication adherence among hypertensive patients in deprived rural areas and found a significant positive correlation between the duration of hypertension and medication adherence.²⁰ It is plausible that individuals with more extensive experience living with chronic diseases, particularly hypertension, have acquired skills in self-care, recognizing the positive impact of adherence on blood pressure control and complication prevention.¹⁰ However, research by Chu et al yielded a different perspective, indicating that older adults with hypertension for more than 9.5 years had a higher probability of lower medication adherence (83%) compared to those with a shorter duration of hypertension (≤ 9.5 years).²⁶ This study suggests that tailored health education programs, considering risk factors such as disease duration, could be instrumental in managing and enhancing medication adherence among older adults with hypertension.²⁶

Previous research aimed to assess the prevalence of medication adherence and associated factors among patients with multimorbidity, highlighting an association between multimorbidity and low medication adherence.⁴² Similarly, our current review reveals that older adults with hypertension experiencing comorbidities or complications due to hypertension are more likely to exhibit poor medication adherence.^20,24,30 The study by S.M. Chang et al reported that the presence of comorbidities such as diabetes, kidney disease, or insomnia was associated with a reduction in medication dosage.²⁴ Additionally, Wan et al found that older adults with high admission blood pressure faced a greater risk of poor medication adherence, while those without hypertension complications had a lower risk of poor medication adherence.³⁰ These findings contribute to the identification of high-risk individuals with poor adherence, particularly those with comorbidities. This knowledge allows healthcare providers to promptly recognize individuals likely to show poor adherence and pay special attention to their medication regimens.

Medication literacy plays a crucial role in influencing adherence to antihypertensive drugs among patients with hypertension.^43–45 Two studies included in our review specifically identified health literacy as a predictive factor for medication adherence among older adults with hypertension.^18,28 First, the study by Heizomi et al explored gender-based associations of health literacy with self-reported medication adherence among patients with primary hypertension, revealing a significant correlation between health literacy and medication adherence.¹⁸ Similarly, a cross-sectional study by Pinhati et al reported that nonadherence was associated with low health literacy.²⁸ In light of these findings, it is recommended that the association between health literacy and medication adherence be further confirmed through interventional studies. Such confirmation would solidify health literacy as a formal mitigating strategy for medication adherence and other public health goals.

The studies included in our review also demonstrated a connection between medication adherence and health compliance among hypertensive patients.^15,21,24 For instance, the study by Gavrilova et al found that older adults with hypertension who diligently follow up on their family physician’s medication prescriptions are more likely to adhere to their medication.¹⁵ Similarly, Adinkrah et al highlighted that a higher level of adherence to antihypertensive medications correlated with a greater continuity of medical care.²¹ Moreover, compliance with a low oil, sugar, and sodium diet was identified as a protective factor against discontinuing medication, as discovered by S.M. Chang et al. Consequently, considering the health compliance model in future studies to improve medication adherence among older adults with hypertension is recommended. Developing interventions that leverage the compliance component could contribute to enhancing sustainable medication adherence within this population.²⁴

Limitations and Future Prospects of Research

The review has limitations that should be considered. Firstly, the review only includes studies published in English, potentially excluding relevant literature published in other languages. Additionally, the exclusion of studies beyond the review’s aim might have led to the omission of important research, limiting the breadth of the review. Furthermore, the variability in study designs, settings, and population characteristics among the included studies may restrict the generalizability of the findings and pose challenges in synthesizing the results effectively.

Despite these limitations, the study offers valuable insights that can guide future research. Longitudinal studies could capture changes in medication adherence over time among older adults with hypertension, providing insights into adherence trajectories and associated factors. Furthermore, randomized controlled trials evaluating the effectiveness of tailored interventions, such as medication reminders, patient education programs, and financial assistance schemes, are needed to improve medication adherence in this population.

Adopting a comprehensive approach that considers multiple factors simultaneously, such as demographic, clinical, psychosocial, and healthcare system factors, could enhance our understanding of medication adherence behavior and inform the development of more nuanced interventions. Exploring the potential of digital health interventions, such as mobile health apps and wearable devices, in improving medication adherence among older adults with hypertension could offer personalized support and real-time monitoring opportunities.

Finally, future research should address health equity considerations by investigating disparities in medication adherence and developing strategies to promote equitable access to healthcare services and medications among vulnerable populations. Overall, addressing these limitations and pursuing these research avenues can advance our understanding of medication adherence among older adults with hypertension and contribute to the development of more effective interventions and policies to support this population.

Conclusion

This systematic review provides crucial insights into the multifaceted factors influencing medication adherence among older adults with hypertension. The comprehensive evidence gathered here underscores the complexity of medication adherence in this population, highlighting the interplay of factors such as the number of medications, financial status, sex, age, disease duration, comorbidities, and health compliance. These findings are instrumental in guiding the development of innovative, evidence-based interventions tailored to improve medication adherence in hypertensive older adults. By addressing these key factors, such interventions promise to enhance the management of hypertension, potentially leading to better health outcomes and improved quality of life. Furthermore, the insights gleaned from this review are invaluable for policymakers and healthcare stakeholders. They offer a robust foundation for formulating policies and programs that are attuned to the unique needs of older adults living with hypertension and other chronic conditions. The implementation of these evidence-based strategies can significantly contribute to the well-being and healthcare management of the aging population.