Reliable estimation of nociceptive withdrawal reflex thresholds.

BACKGROUND: Assessment of the nociceptive withdrawal reflex (NWR) is frequently applied to probe the excitability level of the spinal nociceptive circuitry. In humans, the NWR threshold (NWR-T) is often estimated by applying electrical stimulation over the sural nerve at the lateral malleolus. Such stimulation may be associated with substantial pain and discomfort rendering completion of the assessment infeasible. NEW METHOD: As an alternative to sural nerve stimulation, NWR-Ts were also estimated by electrical stimulation at the arch of the foot. Failure-rates and test-retest reliability of these two procedures were evaluated. A fully-automated interleaved up-down staircase procedure was used to estimate the NWR-T for both stimulation sites. NWRs were detected from EMG measured over the biceps femoris and tibialis anterior muscles, respectively. A total of three repeated measures were performed in two different sessions to evaluate the test-retest reliability of the two methods using Bland-Altman agreement analysis. RESULTS: The failure rate of NWR-T estimation based on electrical stimulation of the sural nerve (29%) was substantially higher than when the NWR was elicited by stimulation at the arch of the foot (5%). COMPARISON WITH EXISTING METHOD: The analysis of test-retest reliability indicated that the two methods for NWR-T estimation were equally reliable for within-session comparisons, but stimulation at the arch of the foot enabled NWR-T estimation with superior between-session reliability CONCLUSIONS: These results support a paradigm shift within NWR-T estimation favoring stimulation at the arch of the foot.

A new objective method for acquisition and quantification of reflex receptive fields.

The nociceptive withdrawal reflex (NWR) is a polysynaptic spinal reflex correlated with pain perception. Assessment of this objective physiological measure constitutes the core of existing methods for quantification of reflex receptive fields (RRFs), which however still suffer from a certain degree of subjective involvement. This article proposes a strictly objective methodology for RRF quantification based on automated identification of NWR thresholds (NWR-Ts). Nociceptive withdrawal reflex thresholds were determined for 10 individual stimulation sites using an interleaved up-down staircase method. Reflexes were detected from electromyography by evaluation of interval peak z scores and application of conduction velocity analysis. Reflex receptive field areas were quantified from interpolated mappings of NWR-Ts and compared with existing RRF quantifications. A total of 3 repeated measures were performed in 2 different sessions to evaluate the test-retest reliability of the various quantifications, using coefficients of repeatability (CRs) and hypothetical sample sizes. The novel quantifications based on identification of NWR-Ts showed a similar level of reliability within and between sessions, whereas existing quantifications all demonstrated worse between-session than within-session reliability. The NWR-T-based quantifications required a smaller sample size than any of the existing RRF measures to detect a clinically relevant effect in a crossover study design involving more than 1 session. Of all measures, quantification from mapping of inversed NWR-Ts demonstrated superior reliability both within (CR, 0.25) and between sessions (CR, 0.28). The study presents a more reliable and robust quantification of the RRF to be used as biomarker of pain hypersensitivity in clinical and experimental research.

Analysis of muscle fiber conduction velocity enables reliable detection of surface EMG crosstalk during detection of nociceptive withdrawal reflexes.

BACKGROUND: The nociceptive withdrawal reflex (NWR) is a polysynaptic spinal reflex that induces complex muscle synergies to withdraw a limb from a potential noxious stimulus. Several studies indicate that assessment of the NWR is a valuable objective tool in relation to investigation of various pain conditions. However, existing methodologies for NWR assessment evaluate standard surface electromyography (sEMG) measured over just one muscle and do not consider the possible interference of crosstalk originating from adjacent active muscles. The present study had two aims: firstly, to investigate to which extent the presence of crosstalk may affect NWR detection using a standardized scoring criterion (interval peak z-score) that has been validated without taking crosstalk into consideration. Secondly, to investigate whether estimation of muscle fiber conduction velocity can help identifying the propagating and non-propagating nature of genuine reflexes and crosstalk respectively, thus allowing a more valid assessment of the NWR. RESULTS: Evaluation of interval peak z-score did apparently allow reflex detection with high sensitivity and specificity (0.96), but only if the influence of crosstalk was ignored. Distinction between genuine reflexes and crosstalk revealed that evaluation of interval peak z-score incorporating a z-score threshold of 12 was associated with poor reflex detection specificity (0.26-0.62) due to the presence of crosstalk. Two different standardized methods for estimation of muscle fiber conduction velocity were employed to demonstrate that significantly different muscle fiber conduction velocities may be estimated during genuine reflexes and crosstalk, respectively. This discriminative feature was used to develop and evaluate a novel methodology for reflex detection from sEMG that is robust with respect to crosstalk. Application of this conduction velocity analysis (CVA) entailed reflex detection with excellent sensitivity (1.00 and 1.00) and specificity (1.00 and 0.96) for the tibialis anterior and soleus muscles. CONCLUSION: This study investigated the negative effect of electrical crosstalk during reflex detection and revealed that the use of a previously validated scoring criterion may result in poor specificity due to crosstalk. The excellent performance of the developed methodology in the presence of crosstalk shows that assessment of muscle fiber conduction velocity allows reliable detection of EMG crosstalk during reflex detection.