Michael Brun Jensen

Introducing the reflex probability maps in the quantification of nociceptive withdrawal reflex receptive fields in humans

The aim of the present study was to improve the assessment of reflex receptive fields (RRF) in humans, using reflex sensitivity and reflex probability maps. Repeated electrical stimulation was applied to elicit the nociceptive withdrawal reflex (NWR) in fifteen healthy volunteers using two stimulation paradigms: fixed (FSI) and adjusted (ASI) stimulation intensities. Stimulation was applied on sixteen sites in the foot sole, and pain intensity ratings and EMG responses were recorded. RRF sensitivity and probability maps were derived, and RRF areas were calculated. During FSI, the stimulation intensities were constant and the pain ratings dropped significantly (p<0.01). In contrast, during ASI the pain ratings were stable, but there was a significant increase in the stimulation intensities (p<0.01). None of the paradigms altered significantly the RRF areas, but the FSI paradigm had lower estimation error (p<0.01). In all cases, the estimation error remained under 10% and 5% after five and ten repetitions, respectively. The 2nd stimulus in the train consistently rendered larger and more reliable RRF areas than the 1st stimulus. The present analysis can be useful in order to identify the most adequate stimulation parameters and quantification variables for RRF assessment in experimental and clinical pain research.

Surface EMG crosstalk during phasic involuntary muscle activation in the nociceptive withdrawal reflex

The human nociceptive withdrawal reflex is typically assessed using surface electromyography (sEMG). Based on sEMG, the reflex receptive field (RRF) can be mapped. However, EMG crosstalk can cause erroneous results in the RRF determination. Single differential (SD) vs. double differential (DD) surface EMG were evaluated. Different electrode areas and inter‐electrode‐distances (IED) were evaluated. The reflexes were elicited by electrical stimulation of the sole of the foot. EMG was obtained from both tibialis anterior (TA) and soleus (SOL) using both surface and intramuscular EMG (iEMG). The amount of crosstalk was significantly higher in SD recordings than in DD recordings (P < 0.05). Crosstalk increased when electrode measuring area increased (P < 0.05) and when IED increased (P < 0.05). Reflex detection sensitivity decreases with increasing measuring area and increasing IED. These results stress that for determination of RRF and similar tasks, DD recordings should be applied. Muscle Nerve 46: 228–236, 2012

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

BackgroundThe 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.ResultsEvaluation 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.ConclusionThis 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.

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

Abstract 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.

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.

Novel cross correlation technique allows crosstalk resistant reflex detection from surface EMG

Existing methods for withdrawal reflex detection from surface electromyography (sEMG) do not consider the potential presence of electrical crosstalk, which in practical applications may entail reduced detection accuracy. This study estimated muscle fiber conduction velocities (CV) for the tibialis anterior (TA) and soleus (SOL) muscles of both genuine reflexes and identified crosstalk, measured during antagonistic reflex responses. These estimations were used to develop and assess a novel method for reflex detection resistant to crosstalk. Cross correlations of two single differential (SD) sEMG signals recorded along the muscle fibers were performed and two features were extracted from the resulting correlograms (average CV and maximal cross correlation). Reflex detection based on evaluation of the extracted features was compared to a conventional reflex detection method (thresholding of interval peak z-scores), applied on both SD and double differential (DD) sEMG. Intramuscular electromyography (iEMG) was used as validation for reflex detection. Apparent CV due to electrical crosstalk alone were more than one order of magnitude higher than CV estimated for genuine reflexes. Conventional reflex detection showed excellent sensitivity but poor specificity (0.19-0.76) due to the presence of crosstalk. In contrast, cross correlation analysis allowed reflex detection with significantly improved specificity (0.91-0.97). The developed methodology may be readily implemented for more reliable reflex detection.

Reflex threshold assessment methodology for evaluation of central sensitisation is vulnerable to EMG crosstalk

Abstract Background/aims The nociceptive withdrawal reflex (NWR) is a robust measure of nociception highly correlated with subjective pain and several studies indicate that identification of reflex thresholds may be a valuable objective tool during diagnosis of chronic pain conditions related to central sensitisation. The NWR involves complex muscle synergies but most methodologies for assessment of human withdrawal reflexes evaluates surface electromyography (sEMG) measured over just one muscle and does not consider the possible interference of crosstalk from adjacent muscles. This study investigated if standardised reflex thresholds are robust with respect to crosstalk. Methods Reflexes were elicited by electrical stimulation at the sole of the foot and measured using sEMG and intramuscular electromyography (iEMG) from the tibialis anterior and soleus muscles in 15 healthy subjects. 489 sEMG recordings were evaluated using a standardised reflex threshold parameter (interval peak z-score) proposed by Rhudy and France [1] to detect occurring reflexes. The outcome of this standardised reflex detection were compared to two different visual evaluations: (1) assessing only sEMG and (2) assessing both sEMG and iEMG (enabling distinction between genuine reflexes and crosstalk) for the production of two respective ROC-curves. Results The first ROC-curve indicated that interval peak z-score allows reflex detection with high accuracy and has an optimal threshold around z = 12. However, the second ROC-curve based on evaluation 2 revealed that many recordings identified as reflexes were incorrectly categorised reflecting crosstalk. Crosstalk did severely reduce the accuracy of the performed reflex detection – a z-score threshold of 12 was found associated with an extremely poor specificity (0.26). Conclusion Crosstalk may severely reduce the accuracy of standardised reflex threshold assessment methodologies. Use of standard sEMG for reflex recording may cause an alarmingly low specificity of reflex detection and should be avoided whenever the presence crosstalk is suspected.