Validation of the revised Symptom Impact Questionnaire with a proposed fibromyalgia phenotype using experimentally-induced pain and patient self-reports

Objective. The Symptom Impact Questionnaire (SIQR), now used for over a decade, has strong psychometric properties based on patients’ subjective questionnaire data and correlations with other general measures of severity. However, the construct validity of the SIQR in assessing the central features of fibromyalgia (FM) has not been tested specifically with more objective measures. This study examined the construct validity of the SIQR using clinical examination of prominent features of FM, as well as patient questionnaire data. Methods. We determined if SIQR severity groups (low, moderate, high severity) in 158 chronic pain patients (50 FM, 108 Pain/No FM) predicted four central features of FM tenderness and pain: digital palpation tenderness, blood pressure cuff evoked pain, widespread pain locations, and a persistent deep ache question. Results. Low, moderate, and high SIQR severity groups showed concomitant increases in tenderness in response to digital evoked palpation (F=23.5; p<0.0000; η_p²=0.23; MR=.54), blood pressure cuff evoked pain (F=17.0; p<0.0000; η_p²=0.18; MR=0.48) and number of pain location (F=38.8; p<0.0000; η_p²=0.33; MR.59). Strongest differences in SIQR severity were found in response to the question, “I have a persistent deep aching over most of my body” (F=87.5; p<0.0000; η_p²=0.53; MR=0.74). Conclusion. The SIQR strongly predicts the central features of FM tenderness and pain including its widespreadness and its multifaceted character. We propose that tenderness, both locally and over most of the body, attendant to the SIQR is the hallmark of the FM phenotype: tenderness is focal, diffuse, deep, and superficial.