Automated Nociceptive Withdrawal Reflex Measurements Reveal Normal Reflex Thresholds and Augmented Pain Ratings in Patients with Fibromyalgia

Abstract

The nociceptive withdrawal reflex (NWR) is used to probe spinal cord excitability in chronic pain states. Here, we used an automated and unbiased procedure for determining the NWR threshold and compared the reflex thresholds and corresponding pain ratings in a well-characterized cohort of fibromyalgia (n = 29) and matched healthy controls (n = 21). Surface electrical stimuli were delivered to the foot in a stepwise incremental and decremental manner. The surface electromyographic activity was recorded from the ipsilateral tibialis anterior muscle. Fibromyalgia patients reported significantly higher scores for psychological distress and pain-related disability and a significantly lower score for perceived state of health compared to the matched controls. The subjective pain ratings were significantly higher in patients. The NWR thresholds were similar to the controls. In the patients, but not in controls, the NWR thresholds and subjective pain ratings were significantly correlated. Our results showed an increased subjective pain sensitivity in fibromyalgia, but we found no evidence for spinal sensitization based on the reflex measures.

Keywords: chronic pain; electromyography; fibromyalgia; nociceptive withdrawal reflex; sensitization.

Figure 1

Subjective pain ratings in the fibromyalgia patients (n = 29) and healthy controls (n = 21). The figure shows individual data points for each participant (ratings across limbs were averaged) and median with interquartile range. Immediately after receiving each stimulus (in the 8 to 12 s interstimulus interval), the subjects were asked to rate how it felt on a graded scale from 0 to 10. Four was the liminal value for pain. The pain ratings were significantly higher in the patient group compared to the healthy controls (Mann–Whitney U-test: p < 0.001). HC, healthy controls. FM, fibromyalgia.

Figure 2

Subjective pain ratings plotted against the NWR thresholds in the fibromyalgia patients (n = 29) and healthy controls (n = 21). In subjects where NWR thresholds were obtained from both limbs, these and the corresponding pain ratings were averaged. A positive linear correlation was seen in the patient group (Spearman’s Rho = 0.460; p = 0.012).