Introduction

Myalgic encephalomyelitis chronic fatigue syndrome (MECFS) is a condition that has major symptoms including self-reported fatigue, post-exertional malaise, and unexplained pain across the body.1 The widespread pain is measured in a systematic way and is referred to as fibromyalgia, but fibromyalgia can be diagnosed as a separate condition.2 An early definition for MECFS developed by the Centers for Disease Control (CDC) is known as the "Fukuda definition," and was frequently referenced in the literature as the mode of classification/diagnosis.3 Fukuda describes fatigue that endures for at least six months, and for some it may last for decades. To meet diagnostic criterion, the person must also report post-exertional malaise and at least four other symptoms; for example, impaired memory or concentration; sore throat; tender cervical or axillary lymph nodes; muscle pain; multi-joint pain; new headaches; and unrefreshing sleep. The most recent series of assessment tools and support for diagnostic guidelines was produced as a set of guidance tools for practitioners known as the Canadian Consensus Criteria (CCC), and these have been embraced by the MECFS community.1 The shortened term "chronic fatigue syndrome" is controversial, and the use of this term is often questioned as to whether it best describes the disease phenomena.4 The heterogenous nature of MECFS leads to speculation that there may be subgroups within the syndrome.

A major symptom of MECFS is fibromyalgia, and there is considerable overlap with both syndromes. Patients may be given just one diagnosis, such as fibromyalgia or MECFS; however, many people are misdiagnosed and given the wrong classification.2 Whilst fibromyalgia is an independent syndrome (ie, unexplained widespread pain across the body), it is also a symptom of MECFS. A universal diagnostic criterion was first established in 1990 for fibromyalgia as a combination of widespread pain across four quadrants of the body with tenderness in a minimum of 11 of 18 tender points that the person has experienced for at least three months.5 Research has reported associations between leptin and fibromyalgia, and may be used to corroborate the work and progress in the MECFS field. Fibromyalgia is considered to be a disease of a rheumatic and inflammatory nature.5 Higher levels of leptin in both MECFS and fibromyalgia cases versus controls has been reported.6,7 This protocol will examine the combined evidence of both MECFS and fibromyalgia.

Observational studies have focused on the presence or absence of MECFS and/or fibromyalgia (eg, in case vs controls studies) using blood serum or plasma biomarker differences to compare across participant groups.8 Other differences include symptom severity (often through self-report) and duration of illness.9 Another experimental design is to measure blood serum/plasma biomarkers in a cohort following an intervention (eg, exercise therapy, vibration therapy, or after dispensing medication like hydrocortisone vs a placebo).7,10 These cohort studies may be completed as part of a time series over weeks or months. Other research in MECFS and/or fibromyalgia has looked at smokers versus non-smokers and symptom severity in relation to levels of peripheral circulatory inflammatory biomarkers.11

Both MECFS and fibromyalgia have clinical features that resemble inflammatory disorders and are thought to be affected by immunological mediators such as pro-inflammatory cytokines.12 The two disorders have many similarities, but their association with leptin has been reported from unique perspectives: MECFS through immunological sciences, and fibromyalgia through rheumatology or pain studies.11,13 The primary research reports on issues such as symptom severity, pain levels, fatigue scales, and post-exertional malaise as nuances in each syndrome. A preliminary search has shown an inconsistency in study design in how differences are measured and reported across the literature. Much of the published work on leptin has been combined with other cytokines, so a targeted review that examines leptin levels alone as an outcome will provide some clarity about the adipokine's specific association with MECFS and fibromyalgia.

Leptin is an inflammatory adipokine, which is a hormone produced by fat cells that influence the body's appetite by stimulating leptin receptors in the hypothalamus and other areas of the brain.14 Leptin is derived from the Greek word "thin" and the 176 amino acid protein was only identified as recently as 1994.15 The effects of leptin are autocrine (affecting the cell producing it), paracrine (affecting neighboring cells), as well as endocrine (affecting distant cells), but it also promotes the production of pro-inflammatory cytokines, for example Interleukin 6.16 Leptin's association with MECFS was reported in 2001, linking the potential etiology to mild hypocortilism and other disorders.7 Stringer et al.9 completed a study of 10 MECFS participants and 10 healthy controls, tracking their serum blood samples over a 25-day period. They reported that there was a significant correlation of serum leptin with those participants with MECFS and their self-report of fatigue. Using a computer algorithm to correlate serum leptin profiles, the team was able to predict which days the person would feel more fatigued, demonstrating a relationship between leptin and symptom severity.

Prevalence and current findings

The National Health and Medical Research Council provided an Australian prevalence estimate of 48,000 to 240,000 people living with MECFS.17 Estimates vary: an analysis of general practitioner (GP) visits in the UK found 14.8 cases (95% CI 14.5, 15.1) per 100,000 people.18 A meta-analysis of 14 studies of clinical diagnosis in the general population found a pooled prevalence estimate of 0.76% (95% CI: 0.23-1.29).19 In America, the CDC indicates there are around 836,000 to 2.5 million people with MECFS, costing the economy about $24 billion annually.20 The worldwide prevalence of fibromyalgia is estimated at 2.7% with a female to male ratio of 3:1.21 Again, estimates vary between 0.2% and 6.6% in the general population.22 There are no definitive biological tests to confirm the presence of MECFS or to what degree the person is affected by the illness. There is an apparent stigma associated with MECFS and fibromyalgia because there are no accepted biological markers that indicate a diagnosis or cause.2 Other viral conditions such as Epstein Barr virus or glandular fever can be identified by testing for the specific viral reaction (titer or antibody test).23 Both syndromes are at a higher frequency in women than in men with a ratio of approximately 3:1 respectively.2

A systematic review comparing 77 serum cytokines in case versus control studies was completed by Blundell et al. in 2015, and referenced only one study showing an association of leptin with MECFS.8 The Hornig et al. study showed that leptin was not only higher in MECFS participants compared to controls, but suggested there was a potential inter-cytokine correlation with other regulatory networks.13

Conversely, a meta-analysis that reviewed 64 cytokines, including leptin, used statistical significance defined by p-values to compare studies and found the use of cytokines as a biomarker in MECFS was inconclusive; however, the review did not include evidence from fibromyalgia.24 Other published research has reported a potential association between leptin and MECFS and fibromyalgia, indicating that leptin may affect the role of pro-inflammatory cytokines and that symptom severity may have cytokine signatures.6 Cytokines have previously been linked with measures of fatigue, pain, inflammation, depression, and autoimmune responses.1 At this time, no systematic review has been identified that compared leptin in both MECFS and fibromyalgia as a combined group, but previous researchers have combined the two syndromes to investigate other cytokines such as C-Reactive Protein (CRP).12 A systematic review of observational studies that combines the measurement of leptin in both disorders would be of value to determine whether there is a difference in leptin levels between cases and controls. Combining these syndromes will provide a larger cohort for analysis.

A preliminary search of PROSPERO, MEDLINE, the Cochrane Library and the JBI Database of Systematic Reviews and Implementation Reports was conducted to identify similar systematic reviews. One systematic review compares the difference between cases and controls, focusing on multiple cytokines, but it did not include leptin. This proposed protocol differs in that it will be reviewing studies that focus on the analysis of leptin only and will combine the search across both syndromes described earlier.

Review question

Is there a difference in circulating leptin levels in MECFS and/or fibromyalgia that can be used as an outcome measure in cases and controls? (Leptin is measured in ng/mL and is assessed through the analysis of blood plasma, blood serum, or cerebrospinal fluid levels [ng/mL].)

Inclusion criteria

Population

The review will consider studies that include people of any age diagnosed with MECFS, and/or fibromyalgia. Reference to diagnostic criteria must be detailed; for example, diagnosis by a GP, Fukuda definition, Canadian Consensus Criterion, or other descriptions indicating the method of diagnosis. Attention to the diagnostic method for both MECFS and fibromyalgia is an important factor for heterogeneity. Where no reference or details are provided about diagnostic method, attempts will be made to contact the authors where appropriate. Other language terms for MECFS or fibromyalgia will be included; for example, if it states that there is "chronic fatigue" that lasts for at least six months (see Appendix I for other terms to be searched).

Exposure of interest

The exposure is a diagnosis of MECFS and/or fibromyalgia. The diagnosis will be described by operational criteria such as a diagnosis from a GP or those described by the Fukuda definition, Canadian Consensus Criterion, or American College of Rheumatology (ACR).1

Comparator

The comparator will be healthy participants without other physical disorders, such as cancer or medical illness, that would cause symptoms of pain and fatigue. Different methods of recruitment have been described, where some studies will recruit specifically for the study, while others make comparisons to another healthy cohort that has previously been studied.

Outcomes

The outcome is circulating leptin levels, and whether there is a difference between comparators and those with the condition. Included studies will measure levels of plasma leptin or other methods of measuring leptin such as serum levels or cerebrospinal fluid levels. When comparing study outcomes, issues such as the way leptin was measured, the frequency of when samples were taken (hourly, daily, or once only), and confounders such as body mass index (BMI) will be considered. Where BMI is included in the article, these will be recorded and compared across studies in both cases and controls. Leptin levels will be used as a comparative outcome, however, the differences in study design may prove difficult to compare across studies. This will be managed by focusing on the Leptin levels being translated into nanograms per mL and discrepancies between study methods identified. An example will include whether they measured serum or plasma, and whether there is an indication of the time samples were taken.

Types of studies

This review will consider observational studies of varying study designs including prospective and retrospective cohort studies, case-control studies, time-series, and analytical cross-sectional studies that include both cases and healthy comparators.

Methods

This protocol will be reported based on the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) checklist and will follow the JBI methodology for systematic reviews of etiology and risk.25 By including a broad, inclusive review approach assessing any difference of leptin between MECFS and/or fibromyalgia and controls, the results of this review may assist in the development of future research, treatments, diagnostic tests, or prognostic models for people with MECFS and/or fibromyalgia.

Search strategy

The search strategy will aim to locate both published and unpublished studies. An initial limited search of Scopus (Elsevier), PubMed (NIH National Library of Medicine), and Embase (Elsevier) was undertaken to identify articles on the topic. The text words contained in the titles and abstracts of relevant articles, and the index terms used to describe the articles were used to develop a full search strategy for PubMed (see Appendix I), Embase, Scopus, Science Direct (Elsevier), and PsycINFO (Ovid). The search strategy will include all identified keywords and index terms, and the format will be adapted for each included information source, including unpublished studies such as theses, conference abstracts, and government reports found using Google Scholar. The reference list of all articles selected for critical appraisal will be screened for additional studies. Studies will not be date limited, but the human equivalent of leptin was not discovered until 1994.26 Only studies published in English will be included due to limited resources for translation.

Study selection and management

Following the search, all identified citations will be collated and uploaded into EndNote v.X9 (Clarivate Analytics, PA, USA) and duplicates removed. Titles and abstracts will be independently screened by two reviewers for assessment against the inclusion criteria. This process will be completed using Covidence (Veritas Health Innovation, Melbourne, Australia) and a third reviewer will be used to arbitrate discrepancies. Potential, relevant studies will be retrieved in full and their citation details imported into the JBI System for the Unified Management, Assessment and Review of Information (JBI SUMARI; JBI, Adelaide, Australia). The full text of selected citations will be assessed in detail against the inclusion criteria by two independent reviewers. Reasons for exclusion of full text studies that do not meet the inclusion criteria will be recorded and reported in the systematic review.

Assessment of methodological quality

Eligible studies will be critically appraised by two independent reviewers at the study level for methodological quality in the review. The quality assessment will use the relevant study type assessment tool identified by JBI, including the JBI critical appraisal checklist for cohort studies, case-control studies, case series, case reports, and cross sectional studies, where these studies are identified for inclusion.25 Any disagreements that arise will be resolved through discussion or with a third reviewer. All studies, irrespective of methodological quality, will be included in the review, except those that do not provide leptin as an outcome measure.

Data extraction

Data will be extracted using a standardized data extraction tool informed by JBI SUMARI.27 As well as the outcome of interest (leptin), other information like populations, study methods, dependent variables of significance to the review question, and specific objectives of each study will be extracted. Demographics and participant type will be extracted such as sex, age group, and any contextual factors provided about the study. Authors of papers will be contacted to request missing or additional data where required.

Data synthesis

The authors will complete a quantitative analysis that synthesizes findings across studies using pooled effect sizes and confidence intervals of the measures provided. Dekkers et al.28 recommend that where the experimental design is not exact and it is unclear whether the same phenomenon is being measured, a random effects model should be considered.28 Standard meta-analysis for continuous outcomes will be pooled using JBI SUMARI. Heterogeneity will be assessed using an appropriate method (such as I²). An overall effects score that provides a risk ratio will be calculated as per the JBI Manual for Evidence Synthesis.25 A potential limitation of the study is that details at the level of measurement (eg, nanograms per mL [or equivalent]) will not be provided. Where this is the case, a tabular or narrative synthesis will be completed that will provide a useful summary of information.29

Assessing certainty of findings

The Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach for grading the certainty of evidence will be followed and a Summary of Findings (SoF) will be created using GRADEPro GDT (McMaster University, ON, Canada).30 The SoF will present the following information where appropriate: absolute risks for the treatment and control, estimates of relative risk, and a ranking of the quality of the evidence based on the risk of bias, directness, heterogeneity, precision, and risk of publication bias of the review results. The outcomes reported in the SoF will be the level of heterogeneity, and risk of bias.

Acknowledgments

Liaison librarian Vikki Langton, The University of Adelaide, for her support with the database search.

This protocol contributes to a Master of Clinical Science degree at The University of Adelaide for MM.

Appendix I: Search strategy

Table 1: Logic grid

Table 2: Example search strategy using PubMed (NIH)

Search conducted on July 27, 2020

References