Ethical approval is approved by the University Health Network (UHN) Research Ethics Board.
This study is registered under the National Institutes of Health ClinicalTrials.gov.
This proposed study is a single centered, factorial, randomized placebo-controlled trial with two independent variables, depth of capsaicin application and dose of capsaicin, for a total of nine treatment arms. The first between groups variable will be topical capsaicin application and injectable capsaicin – including an intramuscular injection as well as intrafascial injection. Participants will be split between these three groups. Within each partition there will be three treatments: control, 50 micrograms, 100 micrograms. One-hundred micrograms reported to be an effective high dose of capsaicin. The control group will receive a topical skin lotion which is inert and has no sensitization effect, or an inert injection (for the intramuscular or intrafascial groups). An equal number of participants will be allocated to each of the nine treatment groups using an electronic randomization generator. Block randomization will be used to ensure equal allotment into each group. An alternate member of the research team will conduct the randomization schedule a priori. Participant allocation will be concealed by placing their assignment in an envelope and delivered to participants by the same member of the research team uninvolved in the measurement or randomization protocols. Participants and investigators will be blinded to the delivered dose; however, the type of capsaicin delivery cannot be blinded from either participants or investigators. The member of the research team conducting the randomization schedule and concealing allocation will have knowledge of and keep track of the doses contained in the containers and vials of the topical and injectable capsaicin, respectively.
They will deliver the appropriate dose to the team member implementing the experimental protocol to ensure there is blinding with respect to dose. This individual will not be involved in participant recruitment.
Participants will be recruited from UHN and the University of Toronto clinics, employees, volunteers, visitors, students, and external participants. UHN is a tertiary healthcare centre with eight hospitals located in Toronto, Canada and the University of Toronto is an academic institution with a main campus located in close proximity to the main UHN hospitals in Toronto. Advertisements, flyers, and in person recruitment will be employed to collect the necessary sample size for the study. The recruitment procedures will run in line with the approved guidelines from the UHN research ethics boards. Participants will not be coerced into participating and will be informed that they have the right to withdraw at any time during the experimental procedures. They will also be informed they have the right to withdraw their data prior to publication. Participants will be briefed and consented on recruitment prior to commencing the study procedures. Participant treatment allocation will be recorded on a separate document until all of the data collection is complete and the data is analyzed.
Female or male participants who meet the following inclusion criteria will be included in the study: (1) aged, 20-60; (2) participants are healthy with no past musculoskeletal and neurological medical history (3) a visual analogue score as a measure of current pain below 3 indicating low pain severity, however, ideally who experiences no pain (4) body mass index >19<25, (5) have sufficient command of the English language to provide informed consent and to understand the study protocols, (6) participants agree to sign a consent to volunteer for the research.
Participants who meet one or more of the following criteria will be excluded: (1) absence of pain,(i.e. headaches/migraine, toothaches, pain from sports injuries) at the time of recruitment (2) physical examination detection of myofascial trigger points, (3) participants present with a history of pain related disturbances such as poor sleep, cognitive disturbances, psychiatric disorders, (4) history of general medical disorder that may affect the outcome of the study such as diabetes mellitus, (5) and a history of cervical radiculopathies or (6) history of inflammatory arthropathy.
This experimental protocol will be carried out at the Kumbhare Lab, Toronto
Rehabilitation Institute to ensure any medical emergencies can be promptly cared for. Following the completion of the preliminary intake forms and answering any potential participant’s question(s), we will explain the study protocol and then, informed consent will be obtained. Each participant will be seated upright with their hands comfortably on their lap in a chair that has a high supportive back. They will be asked to relax their neck and shoulder muscles. A member of the research team will then apply the inclusion and exclusion criteria. The presence of a MTrP will be assessed by palpation of the upper trapezius muscle since this is the current method utilized in clinical practice. If there is a MTrP, then they will be asked to withdraw from the study since they do not satisfy the inclusion and exclusion criteria.
Participants will sit on a chair with one arm extended downward, and with the other arm on an armrest of the chair. The extended arm will be fixed by a wristcuff and a chain down to the floor, with a loadcell between the floor and the end of chain. The loadcell will measure the exerted force. 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. Care will be taken to image the region of the muscle that was not mechanically impacted by the insertion of the needle or injection of the substance. The area will then be cleaned with alcohol and water.
The Delsys Galelio surface 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 with a monitor showing the exerted force as well as the target force. 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 participant before as well as after the intervention. The placement of the injection needles 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. Following this, ultrasound pictures will be taken again from the same muscles mentioned previously. 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 (Figure 2).
Central sensitization and measurement techniques
Inducing central sensitization
Central sensitization will be induced using capsaicin according to the established technique [16,31,32]. It induces increased pain severity at high doses relative to heat and elicits a dose dependent pain responses, allowing for more easily quantifiable measurements .
Capsaicin delivery induces both primary and secondary nociception suggesting that central mechanisms are involved in the nociceptive response, also termed as central sensitization [16,17].
Previous evidence has confirmed that capsaicin can sensitize nociceptive and mechanoheat receptors outside of the region of primary hyperalgesia, with speculation that it can also sensitize
chemonociceptive receptors. Therefore, capsaicin can effectively be used to induce central sensitization.
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 (2cc). 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 5x5 cm square 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. The fascial layer was chosen as it is the region with the lowest pressure pain threshold relative to skin and muscle, suggesting a high density of nociceptors within the layer, and it possesses connections to the spinal cord . Changes in muscle fascia properties and nociception at muscle fascia, compared to nociception at muscle or subcutaneous tissue, has also been implicated in the development of pain [35,36]. Furthermore, 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—region of topical placement
or injection—which is the region of secondary hyperalgesia[16-18]. 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.
The Sonosite X-Porte Ultrasound machine located at the Toronto Rehabilitation Institute clinic will be used. A linear wave 13-5Hz probe will be placed onto the trapezius before and after the sensitization is induced (approximately 20 minutes) and an ultrasonic image will be captured. Texture feature analysis will be performed using MATLAB and the 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 cooccurance and run-length matrices, local binary pattern and blob analysis .
Measurement of motor unit activity using EMG
The Delsys Trigno surface EMG system will be used in this experiment. The system offers the benefit of having a 4-pin mini-grid that can extract individual motor unit data, without causing participants discomfort. The analysis will also be complemented by the use of intramuscular EMG, since it offers high location specificity when measuring EMG activity, while only a local region is analyzed. The intramuscular EMG data will be analyzed by a Cadwell
Sierra Wave device.
The anatomical location for electrode placement, along the C7-acromion line, will be identified for each participant. Baseline EMG measurements, as well as the ultrasonic images will be taken from the upper trapezius muscle to obtain the baseline motor unit recruitment curve. The trapezius muscle was chosen for study since it is a common site for MTrPs, given the high load cervical muscles carry as well as the general increase sedentariness that perpetuates poor posture and cervical muscle strain, leading to trigger point presentation. Furthermore, the trapezius muscle has been used frequently in previously published work, which analyze motor unit parameters, power spectrum and amplitude measurements [2,38,39].
After the identification for electrode placement and the captured baseline ultrasonic images, the area will be cleaned with alcohol and water. The Delsys Galelio sensor will be placed directly on the identified area. The EMG sensor (bandwidth of 20-450Hz ) consists of 4 metal contacts for detecting the signal at the skin surface. Both the right and left sides of the trapezius will be measured.
The Delsys Trigno system, which includes sensors, the Trigno base station, and the
EMGworks software will be used to decompose the acquired EMG signals into individual motor units. The following parameters will be analyzed: the firing times and frequency of individual motor units, motor unit action potential amplitude and shapes [40-42], 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 . This will be repeated for each contraction.
Intramuscular EMG analysis
A 27g monopolar electromyography needle will be placed into the midbelly of the upper fibers of the trapezius muscle using ultrasound guidance. The intramuscular EMG signals will be measured using Cadwell Sierra Wave. Since this system does not offer motor unit decomposition, we will use the intramuscular EMG analysis to provide us with more details of the gross EMG measurements such as the overall RMS value, the coefficient of variation for force steadiness, and the centroid measurements.
Determination of Central Sensitization:
Before measuring outcomes, approximately 20 minutes after capsaicin application we will analyze the area of secondary hyperalgesia for each study subject, then determine whether central sensitization has in fact been induced. The presence of central sensitization is confirmed by expansion of the receptive field and area of secondary hyperalgesia beyond the 25 cm2 initial application area. If it has, the study participant will undergo the rest of the experimental methodology. If it has not, the capsaicin will be applied once more and the region of secondary hyperalgesia will be re-measured in 20 minutes later. The above described process for confirming the presence of central sensitization will take place again. If the study participant has not had sensitization at this stage they will be withdrawn from the study.
The primary outcome measures:
- Recruitment and Amplitude changes before and after capsaicin application
These will be recorded for each arm of the study, please see figure 1.
- Capsaicin induced sensitization affecting rate of recruitment of motor units comparison before and after of capsaicin application.
- Ultrasound textural regional changes before and after capsaicin application
- Capsaicin induced sensitization recording electrical recording at the MTrP with comparison before and after.
- A record of adverse events
- Safety evaluation
These will be recorded for each arm of the study, please see figure 1.
The investigating physician monitors the patient clinically for signs of distress. Any signs of distress noted by investigating physician will be assessed clinically and appropriately managed. The information will be recorded on the patient’s data form. The study team will discuss all incidents and any potential causal link to the study interventions. Participants will be provided with a contact number for the principal investigator in order to report any changes that occur after the study. Our publication will include a list of any adverse events or intercurrent illness encountered. Any patient who experiences an adverse event will be triaged to their family physician or local emergency department as necessary. They will be contacted after to obtain information on the course of their treatment and the outcome of the event.
Sample size calculation.
Using GPower V3.0.10 (Dusseldorf, Germany), considering a medium effect size
(Hedge’s G) of 0.5 and a moderate correlation coefficients (0.5) among repeated measures, for electrodiagnostic recordings sample size calculation determined that a minimum of 78 participants would be required to detect differences via repeated measures ANOVA ( two measurement as before and after among 9 groups) with 80% power and an alpha of 0.05. We aim to recruit 94 participants to account for potential attrition (~20%) of participants given the study duration. We plan to recruit equal number of men and women. To date, there are no reported sex differences in induction of Central Sensitization using Capsaicin among males and females with neurological impairments.
Statistical analysis plan
Baseline participants’ characteristics including demographics will be analyzed using appropriate descriptive statistics. Mean and standard deviation will be calculated for continuous variables. Categorical variables will be presented as numbers and percentages. This is exploratory research and we are unsure of the characteristic of the dataset that will be obtained, we plan to focus on estimating effect sizes and confidence intervals with the assistance of a biostatistician. The biostatistician will create statistical models to assist in answering our research questions. We plan to evaluate the difference between the forms of delivery (topical, intrafasical, intramuscular) using a 2-way model with consultation by biostatistician.
All Statistical analyses will be conducted with SAS for Windows (version 9.3; SAS
Institute, Inc, Cary, NC). Using a two-sided test a P-value ≤ 0.05 will be considered as statistically significant.