Quantitative Response of Healthy Muscle Following the Induction of Capsaicin

Myofascial pain syndrome (MPS) is a prevalent chronic pain disorder primarily characterized by myofascial trigger points (MTrP). There is limited knowledge on the pathophysiology and mechanisms underlying MTrP and its development. Ultrasound investigations that have been performed on trigger points can differentiate between active and latent trigger points, but have not characterized the central sensitization aspect of them. The electromyography (EMG) investigations of central sensitization that have been performed mostly use hypertonic saline to induce hyperalgesia, and have demonstrated conflicting results. Central sensitization has been proposed as the primary mechanism underlying MTrP development. Central sensitization is associated with hyperexcitability of neuronal responses to normal or noxious stimuli. There is a need for a study that quantifies regional central sensitization using ultrasound to measure the changing structure, and electromyography to measure specific motor unit activity responses in the muscle.

Purpose The purpose of this study is to determine whether sensitizing healthy muscle using capsaicin induces a change in the structural integrity of the affected muscle, and determine if the motor unit frequency and amplitudes change within that region. Central sensitization will be induced using topical capsaicin and injectable capsaicin at three different concentrations. This is an exploratory trial that aims to provide preliminary evidence on whether central sensitization is a direct cause of taut band and MTrP development.

Our specific research questions are followings:

1. Does sensitization cause a significant regional change in the texture features of the targeted muscle or surrounding muscles?
2. Does capsaicin induced sensitization elicit a change in motor unit action potential(MUAP) amplitude by approximately 20 to 25% within the region of experimentally induced central sensitization or in the region of texture feature changes? Our hypothesis for this portion of the study is that sensitization modifies the anterior horn cell activity which will be measured by the amplitude of the potentials.
3. Does capsaicin induced sensitization elicit continuous electrical activity as observed in MTrPs? This tests the hypothesis that sensitization creates the presence of continuous low amplitude action potentials.
4. Does capsaicin induced sensitization influence the rate of recruitment of motor units in the muscle within the region induced? Does this occur within the regions identified by the changes in texture features? The recruitment of motor units is normally expected to follow the Henneman size principle.[20-23] Our hypothesis for this portion of the study is that sensitization will cause an aberration in recruitment. If this is demonstrated, then there is modification of the normal processing at both dorsal and ventral horns of the spinal cord.
5. Is there a location dependent EMG response to capsaicin induced sensitization? In other words, do the texture features or EMG responses from muscles that lie anatomically distant from the location of the site of intramuscular capsaicin injection have changes from their baseline calculations? This would test the hypothesis that type III and IV afferents have an influence on the anterior horn cells that lie distant to the ones that are supplying muscle fibers in the vicinity of the stimulated sensory afferents.
6. Is there a dose dependent EMG response at motor units to capsaicin induced sensitization? Here, the hypothesis of causality will be assessed in a preliminary manner. If there is a relationship then further experiments that more thoroughly assess causality will be needed.

This proposed study is a single centered, factorial, randomized placebo-controlled trial with two independent variables, depth of capsaicin application(3 levels) and dose of capsaicin(3 levels), for a total of 6 treatment arms and three control groups.The first between groups variable will be topical capsaicin application, injectable(intrafascial) and injectable(intramuscular) capsaicin injection. Within each partition, there will be three treatments: control, 50 micrograms, 100 micrograms. The control group will receive a topical skin lotion that is inert and has no sensitization effect. An equal number of participants will be allocated to each of the six treatment groups using an electronic randomization generator. Block randomization will be used to ensure equal allotment into each group.