Текущие данные о связи языковой прочности с когнитивным снижением у пожилых людей и известными факторами риска, связанными со слабостью, саркопией и питанием: протокол аналитического обзора.

STRENGTHS AND LIMITATIONS OF THIS STUDY

The review will be based on articles retrieved from six large databases and a broad range of grey literature, including clinical trials and government reports.

The review will be reported in compliance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews guidelines for scoping reviews to ensure completeness and transparency.

Healthcare providers and experts working in the field, particularly at the Wicking Centre’s ISLAND Cognitive Clinic, ISLAND Project (Island Study Linking Ageing and Neurodegenerative Disease) and the Royal Hobart Hospital, will be involved throughout the review process to identify evidence gaps and ensure completeness.

Critical appraisal of the quality of the included studies will be done.

A limitation is that only literature published in English will be included.

Introduction

Current evidence suggests that the process of neuropathological change underlying cognitive decline leading to dementia occurs 15–20 years before the symptoms of dementia, particularly those associated with Alzheimer’s disease (AD), become apparent. 1 2 A period of 15–20 years is known as the preclinical stage of dementia. In this preclinical stage, abnormal accumulation of proteins such as amyloid-beta (Aβ) and phosphorylated tau (p-tau) accumulate in the brain. 3 Developing accessible biomarkers to facilitate early identification of dementia risk is critically important to prevent up to 40% of dementia cases and delay the progression of the condition. 4

There is now a protocol of blood-based and brain imaging biomarkers to detect preclinical AD. While valuable, these biomarkers are invasive and/or expensive, and frequently difficult to access. There is increasing urgency to identify non-invasive, affordable and readily accessible measures to assess people’s risk for dementia. Identified non-invasive biomarkers include changes in movement (speed, rhythm, accuracy) and strength, specifically in walking (gait), 5 hand grip strength 6 and finger tapping variability. 7 Changes in movement and strength have also been documented in the tongue, affecting swallowing and speech production. 8 9 Results suggest that such changes in the tongue could be further indicators of cognitive decline, with neurological changes in sensory, motor and language systems impairing cognitive processing and task performance. 9 10 Therefore, assessing tongue strength could play a significant role in predicting cognitive decline.

Measurement of tongue strength is important for multiple reasons. Changes in the muscle mass and strength of the tongue are recognised as an oral component of sarcopenia. 8 11 Sarcopenia, the physical manifestation of frailty, is evident in slower gait speed and decreased grip strength, known biomarkers of preclinical AD. 5 6 Sarcopenia, including oral sarcopenia, appears to predict cognitive decline and incident AD dementia, thought due to cellular inflammation, and decreased muscle function. 12 13 The frailty underlying sarcopenia is commonly identified through five criteria: unintentional weight loss, exhaustion, low physical activity, slowness and weakness, with combinations of at least three of these criteria defining a ‘frailty phenotype’. 14 Limited education, adverse socioeconomic circumstances and malnutrition can increase the risk of frailty. 15 16 Early detection and reduction of frailty and sarcopenia can prevent many of the significant adverse health outcomes experienced by older people, hence potentially contributing to the preservation of cognitive function.

Decreased tongue strength also may reflect reduced levels of brain-derived neurotrophic factor (BDNF), a protein important in regulating and repairing neurons and their connections in both the Central and Peripheral Nervous Systems. 17 Evidence suggests that BDNF is expressed and released by skeletal muscle, including the tongue, and plays an important role in neuronal growth and synaptic plasticity within neural circuits involved in cognitive function. 17 Maintaining muscle strength and function plays a central role in the release of BDNF, providing a potential link between muscle function and brain function. In addition, decreased tongue strength may be associated with cerebral blood flow in areas of the brain responsible for cognitive functions. 18

Further, decreased tongue strength can adversely affect the management of food, including swallowing, with consequences on nutritional health compounding sarcopenia and frailty. 19 20 Muscles of the tongue drive the swallowing process. The loss of mass and strength in tongue muscles can make ingestion of food more difficult, which in turn can result in less food being consumed, thereby contributing to malnutrition over time. 21 The coordination of swallowing also may be affected, resulting in aspiration of food, liquid and saliva, and increasing vulnerability to aspiration pneumonia. Even if subtle, chronic aspiration may result in respiratory insufficiency and hypoxia, potentially affecting attention, executive function and processing speed. 22 Measuring tongue strength is important in differentiating slowness and incoordination in swallowing thought due to ageing (presbyphagia) compared with swallowing that is disordered (dysphagia), 21 22 with potential adverse effects on cognitive function.

Similarly, measuring tongue strength is an important component when assessing the degree of neuromuscular involvement to differentiate types of dysarthria (a motor speech disorder). 23 In addition, difficulties with accurate speech movements related to tongue strength may result in decreased social engagement and cognitive decline. 24–26 Decreases in tongue pressure in older adults have been found to correlate with lower Mini-Mental State Examination scores and decreases in ability to make rapid movements of the tongue on request. 26 Although relationships appear complex, investigators have speculated that the link between tongue strength and cognitive function may centre in social engagement where decreased tongue pressure and dexterity affect intelligibility and ease of speech, with consequent decreased social engagement and cognitive stimulation, resulting in cognitive decline. 26

For these multiple reasons, tongue strength may be an important and overlooked factor in predicting cognitive decline. This paper describes an a priori protocol to map current evidence for the association of tongue strength with cognitive decline leading to dementia in older adults and known risk factors for cognitive decline: frailty, sarcopenia and nutritional health. Findings from the scoping review will provide additional perspectives to direct subsequent research and inform policymakers, ultimately benefiting older adults primarily and the broader community at large. Figure 1 depicts the proposed conceptual framework for tongue strength and its association with cognitive decline and the known risk factors of frailty, sarcopenia and nutritional health.

Research questions

Three research questions will be addressed:

What is the current evidence on the association of tongue strength with cognitive decline in adults aged ≥50 years?

How does tongue strength associate with frailty and sarcopenia in adults aged ≥50 years?

What is the association of tongue strength with nutritional health in adults aged ≥50 years?

Methods and analysis

Protocol design

The proposed review will be conducted based on the methodological framework introduced by Arksey and O’Malley 27 and in accordance with the Joanna Briggs Institute (JBI) methodology for scoping reviews. 28 This protocol and the corresponding scoping review will be reported according to the standards for Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA). 29 The protocol is registered at the Open Science Framework ( https://osf.io/2fq69 ).

Review criteria

The criteria for the review will be based on JBI’s Participants, Context and Concept approach. The following inclusion criteria will be employed to select studies relating to older adults (Participants) in relation to their cognitive function, the factors that are known to increase the risk of cognitive decline: frailty, sarcopenia and nutritional health (Context), and the association between these factors and tongue strength (Concept).

Inclusion criteria

Studies will be eligible if they satisfy these inclusion criteria: (1) human studies, (2) participants ≥50 years of age and (3) studies with tongue pressure values measured in relation to at least one of the following: frailty, sarcopenia, nutritional health, cognitive function, cognitive decline and dementia (Alzheimer’s, vascular, frontotemporal and Lewy body and (4) peer-reviewed and published in English.

Exclusion criteria

Studies will be excluded if they meet the following criteria: (1) animal studies, (2) participants younger than 50 years or age not reported, (3) articles, clinical trials, government reports, commentaries, conference presentations, abstracts, letters, reviews and editorials with no information on tongue strength measurement, (4) tongue strength measured in relation to cachexia, or other progressive neurological motor conditions, that is, multiple sclerosis, Parkinson’s disease, amyotrophic lateral sclerosis and (5) publications not written in English.

Information sources

Six major databases will be searched from inception and no publication date restrictions will be applied. The databases are PubMed, PsycINFO (Ovid), Scopus, Embase (Ovid), CINAHL and Web of Science. Grey literature also will be searched to identify additional studies, clinical trials and policy papers appropriate for inclusion. Reference lists of key articles will be searched to identify additional studies appropriate for inclusion. Handsearching of literature not available on electronic databases (websites of key organisations and related professional associations) also will be included.

Search strategy

The search strategy will be developed in an iterative process by the research team. An academic librarian at the University will be consulted to advise on a stepwise searching process. The plan is to search with a combination of the following terms: (“Oral function” OR “tongue strength” OR “tongue pressure” OR “tongue weakness” OR “lingual pressure” OR “oral motor function” OR “oro-motor” OR “oral hypofunction”) AND (dysphagia OR presbyphagia, OR “deglutition disorder” OR “nutrition disorder” OR undernutrition, OR “weight loss” OR frailty OR “pre-frail” OR sarcopenia OR “muscle atrophy” OR “muscle weakness” OR “muscle mass” OR “muscle function” OR “intramuscular fat” OR dementia OR “cognitive dysfunction” OR “cognitive decline” OR cognition) AND (ag?ing OR “older adults” OR aged OR frail elderly OR elderly) ( table 1 ). A pilot search to test the appropriateness of key terms is presented in table 2 .

Study records

Data management

JBI methodology for scoping reviews will be followed to ensure that the review is conducted systematically, transparently and rigorously. Searching will be made in different databases and will be exported to both EndNote and Covidence software to remove duplicates and find full text articles. Covidence software will be used to screen the articles. Data will be collected in a structured form and entered into a Microsoft Excel database.

Selection process

The results of the database searches will be integrated, and duplicates removed by the first author. Two members of the research team will independently screen the titles and abstracts of the identified papers to exclude those that do not satisfy the inclusion criteria. Titles and abstracts of the articles with questionable eligibility will be reviewed by the research team. Disagreements about eligibility will be discussed between the reviewers until a consensus is reached; an independent reviewer will arbitrate if necessary.

Full-text articles will be retrieved for the remaining articles (or articles not excluded during title and abstract screening) and reviewed independently by two members of the research team to ensure they meet eligibility. Disagreements will be resolved via consensus or arbitration with a third reviewer. Articles continuing to meet the eligibility criteria will be included in the review. A Kappa score will be calculated to identify the level of agreement between reviewers while selecting the title/abstract and full-text article. 30 A moderate level of agreement between reviewers will be considered the minimal requirement. If the agreement is less, the eligibility criteria will be revised and used consistently. The process of study selection will be reported using a PRISMA flow chart.

Data collection process

A data extraction form will be created by the team based on the preliminary scoping phase. The form will include all the variables identified to answer the research questions ( table 3 ). The data will be charted in Microsoft Excel.

All reviewers will complete a pilot test of the extraction form on a sample of the included studies. Discrepancies in extracted data will be discussed between reviewers until consensus is reached or by arbitration of another independent reviewer, if required. Consistent use of the form between reviewers will be checked. The extraction form will be revised as needed.

Data items

Alongside standard bibliographical information (ie, age group, mean age, authors, title, journal and year of publication), objectives of each included article, outcomes assessed and conclusions and recommendations in relation to tongue strength will be extracted. Information on tongue strength, the device used to measure tongue strength, characteristics of the study populations, definitions and measurements of frailty, sarcopenia and nutritional health, measurement of muscle mass, muscle strength and any factors associated with tongue strength status will be documented. Tools used to assess cognitive function, including any neuropsychological battery, and their relationship with tongue strength status will be documented. The level of tongue strength according to cognitive function: normal cognitive function, preclinical stage, subjective cognitive impairment, mild cognitive impairment and dementia, will also be documented.

Authors or coauthors of the retrieved articles will be contacted if the information sought is missing in the full-text paper.

Outcomes and prioritisation

Analysis of the extracted data will provide information on the body of research that has been conducted on tongue strength in relation to cognitive decline and the factors that are known to increase the risk of cognitive decline: frailty, sarcopenia and nutritional health. Analysis also will identify areas that require further investigation. Documentation of the limitations of each of the reported studies will guide and direct the next research steps.

Risk of bias in individual studies

A formal quality assessment will be conducted for each of the included articles. The quality of evidence will be documented using the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) framework. 31 The JBI critical appraisal tool for randomised controlled trial and observational studies 28 will be used for risk of bias assessment of each included article.

Confidence in cumulative evidence

Three reviewers will be involved to assess quality of evidence and resolve any disagreement. For each included article, each reviewer will follow the four levels of evidence defined in the GRADE protocol 31 (high, moderate, low, very low). Evaluation of the type of study design will determine an initial ‘high’ (randomised controlled trial) or ‘low’ (observational) quality rating. Evaluation of risk of bias, inconsistency, indirectness, imprecision and publication bias may lower the quality rating. Evaluation of reported effect size, dose response and control of confounding variables may increase the quality rating. The GRADE framework, applied to each included article, will ensure confidence that a careful and systematic approach has been undertaken to document the quality of current evidence on the association of tongue strength with cognitive decline in older adults in relation to frailty, sarcopenia and nutritional health.

Data synthesis

Data from the included studies will be summarised and results presented in tables and figures, as appropriate. Quantitative analysis will be conducted using descriptive methods, including frequencies, means and medians; qualitative data will be analysed thematically in four major categories relating to tongue strength (cognitive decline, frailty, sarcopenia and nutritional health). Gaps and limitations of the included studies also will be documented.

Consultation

The Arksey and O’Malley methodological approach includes an optional consultation with stakeholders and experts throughout the review process. 27 To ensure this scoping review is relevant, comprehensive and meaningful to those who will use or benefit from the outcomes, an interdisciplinary team of a speech pathologist, neurologist and physiotherapist will guide the review. This interdisciplinary team will seek ongoing input from community members, policy-makers, researchers and healthcare providers who are working to reduce dementia risk. These consultants will include staff working at the Wicking Centre’s ISLAND Cognitive Clinic, 32 the Wicking Centre’s ISLAND Project 33 and at the Royal Hobart Hospital. All will be invited to three meetings to discuss and provide feedback on 1) the proposed review process and study design, (2) results of the search and (3) plans for dissemination of the results.

Patient and public involvement

No patients were involved in the development of this protocol. Members of the public are involved and are included in the consultants described in the previous section.

Changes to the protocol

Due to the wide-ranging and investigative nature of a scoping review, alterations to the protocol may be necessary as new data come to light. Any discrepancies or enhancements to the protocol, along with an explanation for these adjustments, will be explicitly noted in the reporting of the results of the reviews.

Ethics and dissemination

Because the scoping review methodology will synthesise evidence from publicly available sources, this study does not require ethics approval.

The scoping review will provide a comprehensive overview of current evidence on the association of tongue strength with cognitive decline in older adults and the factors that are known to increase the risk of cognitive decline: frailty, sarcopenia and nutritional health. The review will highlight areas where evidence is debatable or lacking. Results will be disseminated in workshops and through social media for those interested in dementia risk reduction. A paper reporting the findings of the review will be submitted to a scientific journal for publication and data will be presented at relevant conferences.

Discussion

This scoping review aims to investigate the current evidence on the association of tongue strength with cognitive decline and dementia in older people and the known risk factors of frailty, sarcopenia and nutritional health. This review has substantial potential impact as evaluating tongue strength may be an important, and overlooked, factor in predicting cognitive decline leading to dementia.

This protocol is strengthened by establishing a kappa score to show the consistency between raters when screening studies from their titles to abstracts and full texts, and by using the GRADE framework to document the quality of each included study to ensure confidence that a careful and systematic approach has been undertaken to document the quality of current evidence on the association of tongue strength with cognitive decline in older adults in relation to frailty, sarcopenia and nutritional health.

A key limitation of the planned review is that it will focus solely on articles published in English. If there are any differences or deviations required in the implementation of this protocol, these amendments will be explicitly described and justified in the published paper.

Supplementary Material

Reviewer comments

Author's manuscript

Footnotes

Twitter: @janealty1, @KateLawlerPT, @LynGoldberg7

Contributors: GYY, LRG and JA conceptualised the review idea, formulated review questions, developed the search strategy, created the data extraction framework and drafted the initial version of the review protocol. LRG and JA provided general guidance to GYY and supervised the work. GYY, LRG, JA and KL critically revised the protocol. All authors made substantial intellectual contributions to the development and draft of this protocol and read and approved the final review protocol manuscript.

Funding: This work is supported by funding from J.O. and J.R. Wicking Trust (Equity Trustees).The funder has no direct involvement in the study design, data collection, data analysis and manuscript writing. The work also is supported by a scholarship for PhD study to the first author by the College of Health and Medicine, University of Tasmania.

Competing interests: The authors have no potential conflicts of interest with respect to the research, authorship, and/or publication of this protocol.

Patient and public involvement: Patients and/or the public were involved in the design, or conduct, or reporting, or dissemination plans of this research. Refer to

the Methods section for further details.

Provenance and peer review: Not commissioned; externally peer reviewed.

Ethics statements

Patient consent for publication

Not applicable.

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