In this study, we have demonstrated that the French version of the FSDC is a reliable instrument for measurement of symptom severity in patients with FM, with test-retest reliability coefficients ranging from .600 to .912, for both the individual component as well as total scores. We have also further demonstrated validity of this instrument with good correlation between the FSDC and other measures used to assess symptom complaint in FM. Therefore this study, conducted in a French speaking population of FM patients, further supports the usefulness of the FSDC in FM patients.
The FSDC was devised following the reevaluation of diagnostic criteria for FM, acknowledging that the symptomology of this syndrome extends beyond that of only body pain
]. As FM presents a spectrum of severity rather than uniquely an all or none diagnosis, the composite and severity of symptoms, rather than solely a report of pain, should be addressed. Incorporating symptoms beyond pain will provide a more meaningful assessment of the global impact of this condition in an individual patient. Following the proposal for redefining diagnostic criteria, the working group has proposed a severity scale in order to grade severity of symptoms in a patient with FM
]. The FSDC sets out to address these symptoms, with weighting equivalent to two thirds for pain, and one third for other symptoms. The maximum score for the FSDC is 31, with 13 suggested as a cutoff point to discriminate those with FM from those without FM
]. The FSDC has been validated in English and in Japanese patients with FM, as well as patients with non-FM rheumatic disease
]. This recognition of co associated symptoms with pain in FM is in line with increased neurophysiologic understanding of this condition
When the new criteria for FM were proposed in 2010, the questionnaire devised for diagnostic purposes was originally completed both by the patient and physician
]. In order to enable the questionnaire to be used for survey purposes and to introduce a severity scale, a fully patient-completed questionnaire, the FSDC, was developed
]. This questionnaire measures the overall distress of FM and was previously termed the “fibromyalgianess scale”
]. This composite of symptoms is however not unique to FM but may be applicable to patients with other chronic pain conditions. We therefore propose that the FSDC should now be renamed the “Polysymptomatic Distress Scale” (PDS). We believe that this clarification will simplify patient care. Further studies testing the validity of the FSDC now named the PDS in other rheumatic diseases and chronic pain conditions are required.
There are a number of points that require clarification. There is currently no single measurement for symptom severity in FM that is entirely comparable with the FSDC. The two measures that most closely align with the FSDC are the patient global assessment of disease severity and the FIQ
]. The FIQ has a functional as well as a symptom component, but does not address symptoms of cognition or other somatic symptoms, whereas the FSDC measures only symptoms, and does not assess functional status. It is for this reason that we assessed the comparable individual components of pain, fatigue and sleep disturbance for the FIQ and FSDC in order to make direct comparisons. It is also acknowledged that the specific questions regarding pain, sleep disturbance, mood, and fatigue are nuanced differently in the FIQ and FSDC and could be open to different interpretations. However, even with these considerations, both the total FIQ and patient global assessment of disease severity, as well as individual measures of pain, fatigue, and unrefreshing sleep, correlated well with the FSDC indicating construct validity for assessment in patients with FM. An additional consideration is that all patients in our study had an established diagnosis of FM. It could be argued that these patients might express symptom severity at the extreme end of the spectrum and therefore would be more likely to show construct validity. Our patients are however similar to those reported by other authors
We have also demonstrated validity for the pain component of the FSDC when compared to a number of other measures of pain assessment including a numerical count of pain areas marked on a body map, pain VAS, and MPQ. It is notable that the pain component of the FSDC, namely the WPI, assesses only the location of body pain, whereas the pain VAS assesses intensity, and the MPQ assesses mostly the emotional impact of pain. It is therefore interesting to note that the different pain measures correlated well with the WPI. This might suggest that the location or diffuseness of pain in FM may be associated with severity from the patient viewpoint.
An important consideration is the simplicity of the FSDC from the patient viewpoint, and the ease of scoring for the investigator. A questionnaire that requires simple numerical addition without need for any adjustments represents an attractive tool that might even become useful in clinical practice. The whole questionnaire is on a single page, allowing for a first impression as one that is less challenging than a lengthier document. Even though this questionnaire is remarkably simple and easy to administer in English, we followed a rigorous translation protocol and did not identify any important cultural adaptations that required attention. We do acknowledge the limitation of this study which was conducted at a single tertiary care centre and therefore recommend that further testing of the FSDC should be done at the primary care level.