A randomized double blinded placebo controlled study to evaluate motor unit abnormalities after experimentally induced sensitization using capsaicin

Abstract

Central sensitization is a condition that represents a cascade of neurological adaptations, resulting in an amplification of nociceptive responses from noxious and non-noxious stimuli. However, whether this abnormality translates into motor output and more specifically, ventral horn abnormalities, needs to be further explored. Twenty healthy participants aged 20-70 were randomly allocated to topical capsaicin or a placebo topical cream which was applied onto their left upper back to induce a transient state of sensitization. Visual analogue scale (VAS) ratings of pain intensity and brush allodynia score (BAS) were used to determine the presence of pain and secondary allodynia. Surface electromyography (sEMG) and intramuscular electromyography (iEMG) were used to record motor unit activity from the upper trapezius and infraspinatus muscles before and twenty minutes after application of capsaicin/placebo. Motor unit recruitment and variability were analyzed in the sEMG and iEMG, respectively. An independent t-test and Kruskal-Wallis H test were performed on the data. The sEMG results demonstrated a shift in the motor unit recruitment pattern in the upper trapezius muscle, while the iEMG showed a change in motor unit variability after application of capsaicin. These results suggest that capsaicin-induced central sensitization may cause changes in ventral horn excitability outside of the targeted spinal cord segment, affecting efferent pathway outputs. This preclinical evidence may provide some explanation for the influence of central sensitization on changes in movement patterns that occur in patients who have pain encouraging of further clinical investigation.Clinical Trials registration number: NCT04361149; date of registration: 24-Apr-2020.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1

PRISMA flow chart.

Figure 2

(a, b) Surface electromyography electrode placement on upper trapezius muscle and infraspinatus. (c) Intramuscular needle electrode placement on trapezius muscle.

Figure 3

Location of the area of 10 cm by 10 cm application of topical placebo cream or capsaicin onto the skin.

Figure 4

Process of creating the motor unit signature from a set of recordings. Each column of the matrix corresponds to the recordings from all electrode contacts at a single time step up to the time T. The columns are then concatenated to create a single vector corresponding to motor unit signature.

Figure 5

(a) Example of motor unit action potentials detected pre and post-recording with the capsaicin treatment. Cross correlation = 0.995. (b) Example of motor unit action potentials detected pre and post-recording with the placebo condition. Cross correlation = 0.974.

Figure 6

Pre- and post-recording motor units recruitment order in the trapezius muscle. A slope of 1 would correspond to each pre-recording motor unit being recruited in the same order in the post-recording. Shape analysis threshold = 0.8 and amplitude margin = 20%. Capsaicin: R2 Linear = 0.464, y = 0.72x + 7.88; Placebo: R2 Linear = 0.589, y = 0.93x + 1.74.

Figure 7

(a) Pre- and post-recording motor unit recruitment order in the infraspinatus muscle. Shape analysis threshold = 0.8 and amplitude margin = 20%. Capsaicin: R2 Linear = 0.402, y = 1.06x + 1.02; Placebo: R2 Linear = 0.100, y = 0.43x + 5.49. (b) Pre- and post-recording motor unit recruitment order in the infraspinatus muscle. Shape analysis threshold = 0.7 and amplitude margin = 30%. Capsaicin: R2 Linear = 0.335, y = 0.77x + 2.71; Placebo: R2 Linear = 0.090, y = 0.26x + 4.87.