The Prevalence and Nature of Eating and Swallowing Problems in Adults with Fibromyalgia: A Systematic Review

Fibromyalgia is a complex chronic pain condition characterized by widespread pain, fatigue, cognitive dysfunction, and sleep disturbances. People with fibromyalgia can experience both autonomic and somatic disturbances, cognitive and mental health symptoms, and hypersensitivity to external stimuli. Fibromyalgia often co-occurs with a range of well-researched comorbidities (e.g., temporomandibular disorders, migraine, and irritable bowel syndrome). However, emerging research suggests that individuals with fibromyalgia also often experience eating, drinking, and swallowing problems (e.g., odynophagia, glossodynia, etc.). However, there is very little known about these issues, their psychosocial impact, or the best means of managing them clinically. As such, the aim of this research was to examine the epidemiology, prevalence and nature of eating and swallowing problems in adults with fibromyalgia as reported within previous research. A systematic search of electronic databases, selected conference proceedings, and reference lists was completed in March 2021, with no date or language restrictions. Studies reporting the presence and nature of eating and drinking problems in this cohort were included. Eligibility was assessed by two independent reviewers who also critically appraised the included studies using the Joanna Briggs Tool. This literature search yielded a total of 38 potentially eligible studies, with 6 studies included in analysis. Studies were highly heterogeneous in methodology and design, with meta-analysis showing that dysphagia and GERD are prevalent in fibromyalgia patients (51.9% and 25.9%, respectively), among other issues. From review of existing literature, eating and swallowing problems appear to be common among adults with fibromyalgia, with potential additional repercussions for activity, participation, and quality of life. Further research is required to prospectively investigate these issues, with patient and public involvement necessary to guide impactful research planning.


Introduction
Fibromyalgia (FM) is a chronic pain disorder where the hallmark symptom is widespread and diffuse pain, with common associated symptoms including sleep disturbances, cognitive dysfunction, fatigue, headaches, depression and anxiety, and other co-occurring pain syndromes [1].The global prevalence of fibromyalgia ranges from 2 to 8% within the general population [1][2][3][4][5].There are a range of diagnostic criteria used globally, with subsequent varied nomenclature, essential criteria for diagnosis, and means of assessment.This has led to difficulties in standardized diagnosis and treatment which can complicate the provision of effective management.Following newer diagnostic criteria that was released in 2017, the disease has a female:male ratio of 2:1, similar to other pain conditions [6].The prevalence is similar in different countries, cultures, and ethnic groups and there is no evidence that FM has a higher prevalence in industrialized countries and cultures [1,5].
The current understanding of the pathophysiology of FM is that it is thought of as a "centralized pain" state, where amplification of pain perception is largely driven by changes within the central nervous system (central sensitization) [7,8].Central sensitization is defined as "an amplification of neural signaling within the central nervous system that elicits pain hypersensitivity" [8].It is hallmarked by the presence of allodynia, a painful response to a stimulus that is usually considered "non-noxious", and/or hyperalgesia, which is an increased pain response to a noxious stimulus.The mechanisms underlying central sensitization are not fully established, but it is likely that there are both peripheral and central components that play a role in this phenomenon that is seen in centralized pain disorders.Central sensitization could potentially originate in the periphery and through a variety of mechanisms that could eventually lead to longterm potentiation in the spinal cord as well as structural changes in the brain.These central nervous system changes are then the key to the maintenance of increased pain perception, with structural and functional MRI brain scans of patients with fibromyalgia demonstrating associated changes in the volume of gray matter, reduced connectivity in descending pain-modulating pathways, and increased activity and connectivity within "pro-pain" regions of the brain [9,10].Beyond pain, co-existing somatic symptoms including memory difficulties, fatigue, and sleep disturbances as well as cognitive/affective symptoms (e.g., catastrophizing, anxiety, depression) are frequently observed [11,12].Given this broad range of difficulties within both the CNS and its peripheral components, authors here hypothesize that it is possible that the complex neurological control of the swallow mechanism may also potentially be disrupted, leading to overall eating and drinking problems, including motility difficulties, pain on mastication or swallowing, or other sensorimotor abnormalities.
The prognosis for recovery in FM using traditional medicine is generally poor [13].Traditional therapies include pharmacologic interventions including anti-depressants, non-steroidal anti-inflammatory drugs, muscle relaxants, and anti-epileptics [14].The results of these interventions are only modestly effective as pharmacologic interventions have been reported to achieve symptom relief in approximately only a third of patients in the FM population [15,16].
Patients with FM commonly have lifelong histories of chronic pain throughout their body and many chronic pain syndromes tend to co-occur with FM including migraine, temporomandibular joint disorder (TMD), irritable bowel syndrome, endometriosis, and urologic chronic pelvic pain syndrome (UCPPS) [17].The pain associated with FM may lead to decreased physical strength and functioning, job loss, and in some, difficulties in meeting basic routine daily needs [18], such as the need to hydrate and nourish oneself adequately.Abnormalities with swallowing, known in medical terminology as dysphagia, are not officially classified as a feature of FM nor is it part of the current diagnostic criteria for FM, however, has been reported as a common patient experience within some recent studies [19][20][21][22][23][24].In addition, there are significant amounts of anecdotal patient reports available in online forums which reference a myriad of eating and drinking difficulties (e.g., pain on swallowing, the sensation of a bolus sticking in the throat, impaired mastication; altered taste, intra-oral burning, or dry mouth).These anecdotal accounts tentatively suggest wider repercussions beyond the sensorimotor process of swallowing, such as unintentional weight changes and alterations in oral intake and dietary habits, in line with reports from other clinical cohorts.However, despite these patient reports, there is very little known academically to fully describe and quantify these eating and drinking issues, their potential repercussions, their psychosocial impact, or the best means of managing them clinically.
This lack of research information results in working clinicians being unsure of the existence or significance of dysphagia among adults with fibromyalgia, which can lead to a lack of validation of the patients' experience or priorities, inappropriate management of such issues, and poor patient outcomes and experience of care.As such, the following primary research question guided the conduct of this study: "what is the epidemiology of eating and swallowing problems in adult patients presenting with fibromyalgia?".From this main question, 2 sub-questions emerged: "What is the prevalence of eating and swallowing problems in adult patients presenting with Fibromyalgia?" and "what is the nature of eating and swallowing problems in adult patients presenting with Fibromyalgia?".
The aim of this research was therefore to examine the epidemiology, prevalence and nature of eating and swallowing problems in adults presenting with fibromyalgia as reported within previous research.
Studies were included if they provided data on primary or secondary outcomes.It was intended that this study would act as the first step in understanding these issues, paving the way for future prospective, patient-led research on the topic, with the ultimate goal of improved patient care delivery and lived experience.

Methods
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement (PRISMA) [25] informed the conduct of this research.The protocol for this review was prospectively published on the Prospero database [CRD42021240139] [26].

Outcomes of Interest
The outcomes of interest investigated here were based on both specific signs of eating and swallowing difficulties commonly discussed within the general dysphagia literature and also issues which are frequently reported within patient accounts: Primary outcomes of interest included those discussed as most common and most impactful on overall health and well-being, such as:

Studies
Eligible studies included: • Published and unpublished research in using a randomized and non-randomized, retrospective, cross-sectional, or observational design investigating the prevalence and nature of eating and drinking problems in adults diagnosed with fibromyalgia (as diagnosed clinically or reported by patient themselves) were eligible for inclusion.• There were no language, geographic, study design, or date limitations applied.

Participants
Eligible participants included: • Adults diagnosed with FM (as diagnosed clinically or reported by patient themselves) attending any healthcare setting There were no other sex/severity/illness duration limitations imposed.Studies were excluded if participants were not adults diagnosed with FM or if they had a history of head and neck cancer, head and neck surgery, and/or neurological conditions affecting this area.

Information Sources
A systematic search which encompassed search filters, medical subject headings, and key-text terms was developed for use here (see Supplemental Materials for full replicable search string).The databases which were independently searched by both authors from inception to March 2021 were: PubMed, CINAHL, Web of Science, and The Cochrane Database of Systematic Reviews.No search limits regarding publication date, language, or location of publication were applied here.
Independent hand-searches of the proceedings of the annual scientific meetings of the European Society for Swallowing Disorders (2011-2021) and the Dysphagia Research Society (both published in Dysphagia) (1992-2021) were conducted.Hand-searches of the bibliographies of studies which were ultimately included were also conducted to identify records not indexed in the directories initially searched or not identified by the initial search.
Authors of primary studies were contacted if researchers were unable to access potentially eligible articles or if there was missing data within articles which were published within the last 10 years.Initial contact was conducted via email using a standardized email template, with a reminder email after 2 weeks if there was no response, and subsequent exclusion of the article if no response was received to 2 emails.

Study Selection
Records identified through the systematic search were initially exported to Zotero [27], with subsequent transference to Covidence [28] for automatic removal of duplicates.Authors then independently screened all remaining records at title/abstract level, with progression of appropriate records to the full-text screening stage if they met eligibility criteria.Any inconsistencies between authors regarding eligibility were initially flagged by Covidence and discussed, with any ongoing disputes mediated by a third independent reviewer.Inclusion/exclusion of records was tracked by automatic generation of a PRISMA flow diagram by the Covidence platform.

Data Collection Process and Data Items
Data was independently extracted by both authors using the Joanna Briggs Institute (JBI) prevalence data extraction form [29] on topics such as: methodological details, recruitment, sampling, demographics, outcome measurement, and prevalence data.Data were then cross-checked for concordance, with another independent reviewer available to mediate disputes, if required, however, as all data extraction was consistent, this option was not required.Data were extracted on the primary and secondary outcomes of interest listed above.

Critical Appraisal of Included Studies
To critically appraise individual studies, the first author independently utilized the JBI Checklist for Prevalence Studies [30].This tool is readily accessible online and facilitates the critical appraisal of the methodological quality of a study to inform the interpretation of individual study results while also providing information to guide future syntheses [30].The inclusion/exclusion of a range of methodological factors are scored (e.g., sampling frame, study size, measurement of the condition in a standard and reliable way) as per "yes" (2 points)/"no" (0 points)/"unclear" (1 point)/"not applicable".In this study, quality bands were established as follows: poor quality = 0-6 points; moderate quality = 7-12 points; strong quality = 13-19 points.Risk of bias across studies was mitigated for as follows: (1) publication bias was addressed by designing a systematic search strategy which accounted for electronic databases, grey literature, and results not indexed in traditional methods; (2) selective reporting within studies was addressed by contacting authors were data was inconsistent/missing and seeking further information.

Summary Measures and Synthesis of Results
Narrative description of included studies was conducted, with descriptive statistics used to initially explore data.
Meta-analysis was conducted using the CMA Software programme [31].Prevalence figures were presented using 95% CIs, with forest plots constructed for all estimates.Data were pictographically displayed using accessible graphs and charts to visually synthesize findings.

Results
A total of 37 results were retrieved during the initial database search (see Fig. 1 and Table 1), with 1 subsequent record identified as potentially eligible for inclusion from the Dysphagia Research Society conference proceedings in 2020.As such, 38 potentially eligible records were initially identified.Following screening, 6 studies were ultimately included (Fig. 1).

Participant Demographics
Data regarding 2927 individuals with fibromyalgia were extracted.As outlined in Table 2, the pooled age range and gender ratio were unspecified in the majority of studies.
Aspiration/penetration on food/liquids/secretions or impaired mastication or pain or fatigue on chewing were not reported in any studies Table 4.
As above, minimal reporting of the secondary outcomes was found within included studies, hindering authors in sufficiently answering research questions.Meta-analysis could not be completed for the outcomes of glossodynia or dysgeusia due to discrepancies in sample size and study design.Narratively, glossodynia was reported in 2 studies [21,23], however.Rhodus [21] reported glossodynia in 32.83% of participants, while Seccia's [23] one participant experienced this issue.Dysgeusia was also reported in 2 studies [21,23].Rhodus [21] reported dysgeusia in 34.32% of participants, while Seccia's [23] one participant experienced this issue.
Odynophagia was reported in only 1 study [23] with this prevalence reported in this study to be 100%.Xerostomia was also reported in only 1 study [21], with this prevalence reported in this study to be 70.9%.Sensation of choking/ bolus sticking in pharyngeal region was reported in 1 study  1 3 [20], with this prevalence reported in this study to be 48%.Unintentional weight loss was not reported in any of the included studies.

Critical Appraisal of Included Studies
Half of all studies were classed as moderate quality (n = 3; 50%), followed by strong (n = 2; 33.3%), and poor-quality ratings (n = 1; 16.66%) (see Table 5).The criteria which contributed to positive ratings primarily included the use of valid methods for the identification of fibromyalgia and dysphagia, description of the sample and setting in sufficient detail, and adopting an appropriate sample frame.Items contributing to lower ratings primarily included standard and reliable measurement for all participants, use of appropriate recruitment methods, and calculation of sample size.

Discussion
This systematic review represents the first attempt at synthesizing all available information regarding the prevalence of eating, drinking, and swallowing concerns among adults with fibromyalgia.Findings of this review demonstrate that limited research has been conducted on this topic, with only 6 eligible studies identified.In addition to this low number of reports, identified studies were heterogeneous in design, sample, and methodology, and of variable quality, precluding meaningful synthesis or meta-analysis in most cases.Insufficient evidence pertaining to both primary and secondary outcomes was found here to accurately answer the relevant research questions.With regards to primary outcomes, meta-analysis could only be conducted regarding the prevalence of -dysphagia (51.9%) and GERD (25.9%).While the high rates of occurrence of dysphagia and GERD in adults with fibromyalgia reported in these meta-analyses support existing research [39,40], the impact of the results are somewhat restricted due to the limited number of studies included in each meta-analysis.Meta-analysis could not be conducted on any included secondary outcomes, as such, highlighting the scarcity of available evidence on this topic.In addition, issues often anecdotally reported by patients to negatively impact their functioning and well-being (e.g., glossodynia or dysgeusia, odynophagia, or sensations of pharyngeal bolus sticking) were found to rarely be the primary focus of included studies here, and were given minimal attention within the published papers.Furthermore, difficulties regularly identified as core experiences of dysphagia among other clinical cohorts were often not addressed at all within studies included here (e.g., impaired, painful, or tiring mastication, aspiration/penetration, and unintentional weight loss) [41,42].Therefore, as these core patient issues were rarely investigated in detail, we have gathered only a small amount of new information on their nature, prevalence, or impact from the analysis here.However, evidence from other patient groups demonstrates the negative physical and psychosocial ramifications of living with such issues without appropriate clinical management, in addition to the potential for increased morbidity and mortality if they are not thoroughly addressed.Therefore, this raises the potential that due to a lack of research in this area, clinicians may not be aware of the full spectrum of swallowing difficulties that his cohort report, and therefore, patients may not receive the specialist care they require to live well with these issues, resulting in poor clinical outcomes and reduced satisfaction with care.Despite these limitations, the present systematic review suggests that eating, drinking, and swallowing issues are experienced by adults with fibromyalgia and clearly evidences the need for high quality and patient-centered research on this topic in the future in order to improve the delivery or evidence-based and empathetic care.
While the etiology of fibromyalgia-associated eating, drinking, and swallowing problems is complex and causation still not fully established, it is known that central sensitization of the nervous system plays a role in the development and maintenance of pain in fibromyalgia and has been associated with a chronic orofacial pain syndrome known as burning mouth syndrome.Burning mouth syndrome, also known as glossodynia is a chronic oropharyngeal pain syndrome characterized by a burning sensation of the oral mucosa in the absence of a specific lesion or abnormality [43].The condition most often affects peri-and postmenopausal women and is frequently associated with dysgeusia, dysphagia, and xerostomia.The pathophysiology of burning mouth syndrome is regarded as multifactorial and complex.It is generally characterized as neuropathic in origin and has numerous contributing factors.Peripheral and central sensitization has been reported as being present in patients with burning mouth syndrome and is felt to play a critical role in the development and maintenance of the condition [44,45].While central sensitization may be a potential contributor to eating, drinking, and swallowing problems in patients with fibromyalgia, there are other theories within the literature that deserve mention.For example, heart rate variability has been described as a reliable biomarker for autonomic nervous system (ANS) dysfunction and has been observed as being altered in patients with a variety of pain conditions including fibromyalgia, GERD [46] and IBS [47].The typical alterations in the ANS that have been reported include sympathetic hyperactivation and decreased parasympathetic function [48].Adaptation to external stressors, such as a painful stimulus, has been found to be less efficient in fibromyalgia, with patients showing a higher sympathetic response with less parasympathetic recovery [49][50][51].Recent studies have shown an altered intra-swallow heart rate variability with normal and effortful swallowing among healthy individuals [52,53].Although definitive links cannot be proven, one may hypothesize that in persons with central sensitization, associated peripheral nervous system difficulties, and an already dysregulated ANS that the autonomic changes seen during the normal swallow may be amplified and abnormal, thus leading to the experience of fibromyalgia-associated dysphagia.Further research in this area is therefore warranted to improve our understanding of this condition.
Finally, due to the complex nature of fibromyalgia, many patients are prescribed more than one medication to manage the multiple symptoms and accompanying comorbid medical and psychiatric disorders that coexist with fibromyalgia [54,55].Although this polypharmacy can be an effective clinical strategy for patients with fibromyalgia, these tablets may be difficult to take via the oral route if swallowing difficulties exist, and side effects can occur that can be detrimental to the patient's health and wellbeing.Many of the medications used to treat fibromyalgia and other centralized pain syndromes have the potential to cause xerostomia, drowsiness, and gastrointestinal upset which can possibly contribute to swallowing problems, eating difficulties, and communication problems.Effective communication between the patient and their medical providers may be the most effective way to manage the potential side effects that are seen with polypharmacy.Furthermore, understanding a patient's medical history (including their prior and current medications), regular monitoring of adverse events, and discussing treatment  goals can all be used to customize an effective pharmacologic regimen while attempting to minimize interactions and side effects.It is essential that medication usage and it's contribution to/amelioration of swallowing difficulties among this group are studied further in the future to allow for development of effective management programmes for this group.While this research represents a first step in our understanding of fibromyalgia-associated dysphagia, there are yet some tentative implications for clinical practice at this early stage.For example., it is important that clinicians become aware of the potential for dysphagia among those with fibromyalgia, and that they are prepared to listen to and value the lived experiences of their patients, advocating for onward referral to dysphagia specialists (e.g., speech and language therapists (SLTs)) were required, and contributing to and consuming future research on this topic as appropriate.In addition, as this research suggests that a cohort of people living with fibromyalgia who present to healthcare settings experience varied and currently untreated eating and drinking problems, it is essential to develop, strengthen, and advance multidisciplinary relationships among clinicians such as neurologists, rheumatologists, internists, dentists, SLTs, physiotherapists, dieticians, and other relevant professionals to improve future care delivery.

Limitations
Although this study presents the first attempt to quantify the prevalence of eating, drinking, and swallowing issues among adults with fibromyalgia, it is prudent to note that no research is without limitations.
To begin, there has been previous variability in the diagnosis of fibromyalgia, its associated nomenclature, and diagnostic criteria.As such, the search string may have not detected certain studies which did not conform to the reference standard diagnostic criteria.Furthermore, publication bias may also add to the search detecting fewer studies than that which potentially have been conducted.However, to mitigate this potential, researchers searched several databases, conference proceedings, and reference lists to increase the coverage of the search.
Additionally, due to the limited number of studies available on this topic, sample sizes included in the analysis were small, and study designs and populations were heterogeneous.This limited the researcher's ability to conduct large-scale meta-analysis, thus hindering researchers in adequately answering research questions.
Furthermore, FM is a diagnosis this can be observed in patients with other medical disorders that can cause eating, drinking, and swallowing problems (e.g., stroke, multiple sclerosis, rheumatologic disorders).This point is important as the present work focused only on studies done in patients with FM, however, future rigorous research is warranted to identify whether the addition of the FM diagnosis bears any additional problems when present with these co-morbid disorders.Therefore, it can co-exist in disorders that may present swallowing disorders.
Finally, many medications used to treat FM can cause side effects that may lead to eating, drinking, and swallowing problems.Most studies included in this review did not control for the use of these medications in the FM populations of interest, and as such the impact of these medications on the development and or maintenance of eating, drinking, and swallowing problems deserves further investigation.
Therefore, future studies in this area should strive to recruit larger, representative samples of people with fibromyalgia to facilitate more valid and reliable analysis of the prevalence, nature, and lived experience of fibromyalgia-associated dysphagia.It is advised that these studies investigate the potential role of co-morbid conditions or medications used for the treatment of these issues among people who are experiencing fibromyalgiarelated dysphagia, using standardized diagnostic criteria, in large representative samples.This will facilitate greater understanding of common symptom profiles in this groups, with the view to develop patient-centered and evidencebased management plans into the future.

Conclusions
This study highlighted that limited research information is available on the prevalence and nature of dysphagia among adults with fibromyalgia.However, despite the limited amount of research available, studies identified here suggest that this is an emerging field of research which requires greater attention in the future.This study also identified methodological limitations within the available literature which may impact on the generalizability of results, therefore re-emphasizing the need for future prospective and patient-led research investigating the epidemiology, nature, impact, and management of fibromylagiarelated eating and swallowing problems in order to ensure that patients receive sufficient support to live well with these issues.
Funding Open Access funding provided by the IReL Consortium.This project received no support from external funding.
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Fig. 2
Fig. 2 Forest plot regarding prevalence of dysphagia

Table 1
Results from database searches

Table 2
Participant demographics

Table 4
Results of individual studies