Fibromyalgia (FM) is a relatively well-known musculoskeletal disorder characterized by widespread pain, fatigue, and sleep issues. However, few are aware of its comorbidities. The frequent pain and other symptoms normally associated with FM can be further exacerbated when occurring along with ankylosing spondylitis, axial spondyloarthritis (axSpA),psoriatic arthritis (PsA) or rheumatoid arthritis (RA). A study, published in Rheumatology sought to assess both the prevalence of FM in patients with RA, AS, axSpA or PsA as well as the extent to which comorbid FM (FM that exists simultaneously with another medical condition) affects disease activity by analyzing data obtained from various publications and research articles.
The study reviewed articles from Cochrane library, MEDLINE, Psychinfo, PubMed, Scopus, and Web of Science utilizing a specific algorithm of keywords to search for relevant content. To be eligible for inclusion, an article needed to be published in English, include patients with a pre-existing diagnosis of RA, axSpA or PsA, state the number or percentage of patients in their study diagnosed with FM, or report the impact of comorbid FM upon disease activity, and be available in full text. A total of 810 articles were pulled from the initial search, but after comprehensive filtering, only 40 of those articles met the inclusion criteria. The results were as follows:
Rheumatoid Arthritis (RA)
Out of the 40 articles, 29 of them mentioned the prevalence of FM in RA patients. However, prevalence numbers varied greatly, ranging from 4.9% to 52.4%, with a proportional average of 21%. Interestingly, when the included research articles were refined to only include studies with larger sample populations (>150 patients), the prevalence number dropped to 14%.
Furthermore, 19 of those 29 articles mentioned the impact of FM on the Disease Activity Score (DAS28)  among RA patients. Higher DAS28 scores were reported in all but one article, with 16 reporting considerable increases in DAS28 scores in those with comorbid FM compared to those with RA alone. Despite the higher overall DAS28, objective and laboratory measures in the test, such as swollen joint count and Erythrocyte Sedimentation Rate (ESR)  remained largely inconsistent between all studies, with some showing significant increases and others showing insignificant differences.
Ankylosing Spondylitis (AS)
Comorbid FM prevalence numbers in patients with AS were moderately varied, ranging from 4.11% to 25%, with a proportional average of 13%. In AS patients, the effects of comorbid FM on disease activity, measured by both DAS28 and the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) , were significant in all studies that included these statistics. Additionally, heterogeneity was very low, indicating that the studies were very consistent with regards to populations and results.
As with the results from RA, there were no significant changes in C-Reactive Protein (CRP)  or ESR levels in AS patients with comorbid FM.
Axial Spondyloarthritis (AxSpA)
Four articles reported the prevalence of FM in axSpA, with the range of 9.5% to 25.2%, however, a proportional average was not calculated due to “to fundamental differences in the classifications of the index disease groups.”
Three of the four articles mentioned the impact of comorbid FM on the disease activity in axSpA patients. Similar to the results obtained in the AS study, BASDAI scores were higher in all of these, but only one study reported a statistically significant increase.
Psoriatic Arthritis (PsA)
A total of six papers discussed the prevalence of FM in PsA, with a range of 9.6% to 27.2%, with a proportional average of 18%. Two of those six articles further discussed its effects on disease activity, but only one reported significantly increased DAS28 scores.
Though no statistically significant differences in CRP or swollen joints were found in those with comorbid FM, those without it were found to have a much higher possibility of remission.
In conclusion, FM is much more common in patients with RA, AS, axSpA or PsA than those without. FM also leads to higher disease activity, with substantial symptom worsening occurring in most cases. However, due to the lack of statistically significant changes in objective clinical and laboratory measures (ESR, swollen joints, and CRP), this study also reveals the potential fallibilities of using the DAS28 as a guideline for treatment and disease management as it relies heavily on patient-provided subjective data that may vary by patient. As such, this must be taken into account when establishing a treatment routine.
 Disease Activity Score (DAS28) – An assessment of 28 joints that a rheumatologist or specialized nurse may use to measure disease activity. The measure counts the number of swollen joints (objective measure), the number of tender joints (subjective measure), the erythrocyte sedimentation rate (ESR) or C reactive protein (CRP), and a subjective measure of health decided by the patient. The measures are then put into an algorithm and the resulting scores can be compared. A DAS28 score >5.1 indicates an active disease; <3.2 indicates low disease activity; <2.6 indicates remission.
 Erythrocyte Sedimentation Rate (ESR) – A blood test used to detect inflammatory activity in the body. Since inflammation causes cells to lump together, they are denser than normal cells. The ESR measures the distance the cells descend in a test tube in one hour – the greater the distance they have descended, the more inflammatory activity.
 Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) ) – A quick and subjective measure in which patients classify their discomfort level on a scale of 1-10 in 6 different categories, such as fatigue and stiffness, with the numbers then being averaged.
 C-reactive Protein (CRP) – A blood test similar to the ESR that identifies the amount of CRP in a patients blood. High levels of CRP indicate high rates of inflammation and is frequent in patients with painful axSpA or AS symptoms
Sources Used and Further Reading
Concomitant fibromyalgia complicating chronic inflammatory arthritis: a systematic review and meta-analysis
Fibromyalgia Common in Inflammatory Arthritis