BIRMINGHAM, ENGLAND (FRONTLINE MEDICAL NEWS) – One in five people with ankylosing spondylitis could have comorbid fibromyalgia, according to data from the British Society for Rheumatology Biologics Register for Ankylosing Spondylitis (BSRBR-AS).

The analysis, which included more than 880 patients with axial spondyloarthritis (SpA), found that 20.7% met 2011 research criteria for the chronic pain condition.

The prevalence of fibromyalgia was similar when the modified New York ( mNY ) criteria were used to diagnose SpA, at 19.7%, but slightly higher at 25.2% when SpA patients met the Assessment of SpondyloArthritis international Society ( ASAS ) imaging criteria but not mNY criteria, and substantially lower at 9.5% when SpA patients met ASAS clinical criteria only.

“The background is that some patients with axial SpA are recognized clinically to have comorbid fibromyalgia,” Gary J. Macfarlane, MD, PhD , said at the British Society for Rheumatology annual conference.

Dr. Macfarlane, chief investigator of the BSRBR-AS and professor and chair of clinical epidemiology at the University of Aberdeen (Scotland), added that having comorbid fibromyalgia might “distort the responses of some of the key patient-reported measures and that may lead to some patients having inappropriate therapy.”

So the aim of the present analysis was to provide data on the frequency of SpA and fibromyalgia co-occurrence, characterize which patients might be more likely to have both conditions, and also provide information that would inform future studies looking at the optimal management of such patients.

“The patients most likely to meet fibromyalgia criteria were female, either HLA-B27 negative or untested, and there was a particularly strong association with higher levels of [social] deprivation,” Dr. Macfarlane reported.

Patients who had both SpA and fibromyalgia also were found to be more likely to have been treated with a biologic than those who had SpA alone (51% vs. 32%), and there also was an associated with the time missed (15.1% vs. 2.5%) or impaired (50.8% vs. 22.8%) at work.

In a comparison of the characteristics of patients with SpA who met the fibromyalgia research criteria with those who did not, Dr. Macfarlane observed that they had worse disease activity, function, metrology, and global scores as measured using Bath Ankylosing Spondylitis disease indices:

• Disease activity scores were a respective 6.7 and 3.6, giving a difference of 3.1 (95% confidence interval, 2.9-3.3).

• Function scores were a respective 6.6 and 3.7, with a difference of 2.9 (95% CI, 2.6-3.3).

• Metrology scores were a respective 4.2 and 3.6, with a difference of 0.6 (95% CI, 0.3-0.9).

• Global scores were a respective 6.9 and 3.7, with a different of 3.2 (95% CI, 2.9-3.6).

Dr. Macfarlane reported that there were “extremely large differences” on the patient-reported measures of quality of life, depression, and anxiety. Other common problems in the group meeting the fibromyalgia criteria were sleeping difficulties and high levels of fatigue, he said.

Patients with both SpA and fibromyalgia fared worse on quality of life scores measured using the disease-specific Ankylosing Spondylitis Quality of Life ( ASQoL ) questionnaire where they scored a mean of 7.1 points (95% CI, 6.4-7.7) higher than did those with SpA alone.

The mean differences in depression and anxiety, both measured using the Hospital Anxiety and Depression Scale ( HADS ), was 4.8 (95% CI, 4.3-5.2) and 4.7 (95% CI, 4.1-5.2).

The mean difference in the sleep disturbance scale was 5.3 (95% CI, 4.5-6.0), and the mean difference in Chalder Fatigue Scale scores was 4.0 (95% CI, 3.5-4.4).

In contrast, there was no difference in the proportion of patients who had levels of C-reactive protein above 1 mg/dL or in the number of proportion of patients who had extraspinal manifestations of SpA, with the exception of tender or swollen joint counts.

Dr. Macfarlane noted that the fibromyalgia research criteria had not been validated for use in patients with axial SpA but that a grant had been awarded by Arthritis Research UK to look at this and also to look into optimizing options for managing patients with both conditions.

The BSRBR-AS is currently the newest of the biologics registries and began recruiting patients with axial SpA as of December 2012 from 82 centers across the United Kingdom. The register enrolls patients who have not previously been treated with a tumor necrosis factor inhibitor drug and are then followed-up for a 5-year period. The 2011 fibromyalgia research criteria have been used as part of the baseline assessment since September 2015, and clinicians also are asked to report whether they think that patients have fibromyalgia.

The BSRBR-AS is funded by the British Society for Rheumatology, which receives funds from AbbVie, Pfizer, and UCB. Dr. Macfarlane did not report having any conflicts of interest.