Neuropathic pain features suggestive of small fiber neuropathy in fibromyalgia syndrome: a clinical and ultrasonographic study on female patients

Objectives: To evaluate the performance of two screening tools, respectively Pain Detect Questionnaire (PDQ) and Douleur Neuropathique 4 questions (DN4), and the optimal cut-off point of the sural nerve cross-sectional area (CSA), in identifying the neuropathic pain features suggestive of a small fiber neuropathy (SFN), in patients with fibromyalgia syndrome (FM). Methods: Consecutive adult female FM patients fulfilling the American College of Rheumatology (ACR) 2016 criteria have been enrolled. Patients underwent a clinical assessment and ultrasound examination of the sural nerve CSA. In each patient was established the presence of neuropathic pain features suggestive of the presence of SFN. The performance of PDQ versus DN4 was compared to the clinical judgment of SFN as external criterion analysing the area under the receiver operating characteristic curve (AUCROC). The optimal sural nerve CSA cut-off was established with the ROC curve analysis versus the clinical judgment of SFN. Results: The study involved 80 patients (mean age 49.5±10.5 years, mean disease duration 5.2±4.9 years, mean revised FIQR score 60.9±19.6). Comparing the AUC-ROCs of the screening tools with clinical judgment of SFN, a better AUC was documented, although not significantly (p=0.715), for DN4 (0.875) compared to PDQ (0.857). A sural nerve CSA of 3 mm2 identifies neuropathic pain features with a sensitivity of 70% and a specificity of 90%. Conclusions: Screening tools have a good concordance in identifying neuropathic pain features suggestive of SFN in FM patients, although a better performance is provided by DN4. Determining the CSA sural nerve with an ultrasound examination may provide some information about the possible presence of SFN.