Quantitative Ultrasound Response 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.

Capsaicin Application and Injection Capsaicin will be applied directly to the region of the innervation zone at the muscle belly to sensitize the neurons within the region of taut band development. Topical and intramuscular capsaicin will be used. The capsaicin formula will be compounded by a registered pharmacist. Topical capsaicin will be delivered in a cream and injectable capsaicin will be intermixed with saline prior to injection. The control group in the topical capsaicin arm will be treated with the cream base used during the experiment without added capsaicin and the injection arm will be injected with saline. A trained medical professional on the research team will apply the topical capsaicin or topical placebo treatments. The region of topical application will measure 5cm squared in the dermatome zone location to cover an area of approximately 25cm2. A trained physician in physical medicine and rehabilitation will deliver injectable capsaicin using a 27-gauge needle at the location of the superior fascia of the upper trapezius muscle with ultrasound guidance. Intramuscular capsaicin will also be injected using ultrasound guidance to avoid the superior or inferior fascia.

To confirm the presence of central sensitization, brush allodynia will be used to detect mechanical hyperalgesia outside the region of primary nociception. The size of the region of secondary hyperalgesia will be measured to characterize the extent of central sensitization. This will be accomplished using a tape measure and the perpendicular dimensions of the region of secondary hyperalgesia will be recorded in square centimeters.

Experimental Protocol The anatomical location for ultrasound probe and electrode placement will be identified for each participant. The ultrasound probe will be placed on 3 muscles: the trapezius, the supraspinatus, and the infraspinatus. Two ultrasonic pictures will be taken of each. The area will then be cleaned with alcohol and water. The Delsys Galileo sensor, and the intramuscular needle electrode will be placed directly on the trapezius' identified area. Participants will be asked to gently contract their trapezius muscle. They will be instructed to perform a gradually increasing contraction in isometric condition, in a controlled manner. They will hold this contraction at 30% of their maximal voluntary contraction, and then perform a gradually decreasing contraction to rest. This will be performed four times for each type of study intervention and before as well as after each intervention, namely topical control cream, topical capsaicin at 50ug and 100 ug, injection of the superior aspect of the superior fascia followed by intramuscular injection with these two concentrations of capsaicin. The placement of these latter will be verified by ultrasound guidance. The intramuscular needle will then be removed and participants will be bandaged and cared for appropriately by the expert physician performing the experiment. Participants will be re-examined to determine if there were any adverse effects from the experimental procedures. If any occurred, then these will be carefully managed by the medical members of the research team and recorded. Participants will be asked to remain at the lab for an additional 30 minutes to ensure they are well prior to leaving.

Ultrasound Analysis Texture analysis will be performed using MATLAB and the image and signal processing toolbox. Analyses will include first order parameters such as mean and standard deviation of the pixel level values, as well as second order parameters, which provide details on the spatial distribution of pixel values. These include co-occurance and run-length matrices, local binary pattern and blob analysis.

EMG Analysis The following parameters will be analyzed: the firing times and frequency of individual motor units, motor unit action potential amplitude and shapes, the root mean square value (RMS) of each channel and of the entire signal, the coefficient of variation for force steadiness, and the centroid of the EMG signal in the cranial-caudal and the medial-lateral directions. These procedures will be repeated for each contraction.

The results of this study should provide insight into the structural changes and motor changes at the region of sensitization, as well as in adjacent muscles.This will provide further insight as to whether structural changes are accompanied by motor changes within the same region, and inform further investigations into the pathophysiology of myofascial pain.

The full study protocol and statistical analysis plan is submitted in the documents section.