Differential Diagnosis
Contemporary practice holds that when the 1990 ACR RCC or the 2010 ACR FDC are met, it is appropriate to indicate that the affected patient has FMS irrespective of, or in addition to, any other legitimate medical diagnoses. Since the main complaints of FMS patients pertain to body pain, the differential diagnosis must consider a wide variety of other painful conditions.

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As examples of secondary FMS, nearly 30% of patients with RA, 40% of systemic lupus erythematosus (SLE) patients, and 50% of Sjögren’s syndrome patients have concomitant FMS. Patients with a rheumatic disease and FMS seem to experience articular pain out of proportion to their synovitis. This must be considered in treating the rheumatic condition because increasing the dosage of anti-rheumatic medications in the absence of active inflammation may have little effect on the pain amplified by FMS. The best results are obtained by treating each of the conditions separately.

Rheumatic disease patients with concomitant FMS should be warned that a transient increase in the FMS symptoms may occur with each decrease in glucocorticoid dosage (steroid-withdrawal FMS), so the usual FMS therapy may need to be increased transiently. This is a surprising phenomenon because glucocorticoid is not helpful in treating primary FMS. To avoid interference with a steroid taper, it is best to decrease the dosage in graduated steps at about 2-week intervals. The rate of the taper depends on the current dosage: For prednisone equivalent to 60mg/day, step down directly to 30mg/d, then by 5 mg/dose steps from 30mg/day to 15mg/day, then by 2.5 mg/dose steps to 5 mg/day, then by 1mg/dose steps until off.

Infectious/Inflammatory conditions that seem to be associated with FMS include hepatitis C, tuberculosis, syphilis, and Lyme disease. The prevalence of overlap may depend upon the community prevalence of the infectious disease. A group of academic clinicians in a Lyme endemic area evaluated 788 patients with apparent infection for a mean of 2.5 years.¹⁹ Twenty percent of Lyme infected individuals met criteria for FMS. The symptoms of FMS developed within 1-4 months after infection, often in association with Lyme arthritis. The signs of Lyme disease have generally resolved with antibiotic therapy, but the FMS symptoms have often persisted. The largest subgroup of the 788 patients did not actually have Lyme disease but met criteria for FMS or chronic fatigue syndrome.

An association between subacute bacterial endocarditis and FMS has not been formally explored, but the characteristic somatic symptoms with endocarditis (arthralgias, myalgias) suggest that diagnostic confusion could occur.

Conclusion
FMS is a criteria-based diagnosis which identifies a moderately severe chronic pain condition affecting women more commonly than men. It is not a diagnosis of exclusion. The research classification of FMS depends on a history of widespread pain and prominent allodynic tenderness to palpation at 11 or more of 18 anatomically-defined tender points. Newer diagnostic criteria for this condition have been validated for the setting of clinical practice and emphasize the pain of fibromyalgia less while focusing more on its comorbidities. Health care professionals are encouraged to use the newer criteria in making the diagnosis of fibromyalgia and to follow the clinical outcomes of evidence-based management, with serial assessment using the revised Fibromyalgia Impact Questionnaire that has been validated for this application. Fibromyalgia can overlap with other clinical conditions from which it can be clinically distinguished, so the diagnostician must remain alert to other conditions.