Accuracy in the Evaluation of Brain Response to Mechanical and Radiofrequency Stimuli in Humans

Study Overview
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Brief Summary
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Under normal conditions, pain arises as a consequence of the activation of nociceptive afferents (small fibers) by an external stimulus with sufficient intensity to potentially cause tissue damage. This peripheral activation is processed as perception of pain by the central nervous system. In order to reliably evaluate the state of the nociceptive system in both clinical and experimental settings, standardized tests are essential. Quantitative sensory testing (QST) is a set of tests used to measure the intensity of a stimulus that produces a specific sensory perception in a subject. For example, if we gradually apply pressure, the point where the sensation changes from pressure to pain is called the pressure pain threshold. This type of test can be performed with different types of stimuli, including hot and cold stimuli or mechanical stimuli. Although these tests have been shown as reliable in healthy volunteers and pain patients, they are subjective in their nature, since they are based on a conscious evaluation of tested subjects. Likewise, these measures show substantial variability due to differences in the application of the tests by individual examinators. In short, even though the method is quantitative, its methodological characteristics make it subjective and dependent on both the operator and the subject under study. Moreover, contrasting results have been recently found regarding the measurement variability when repeating the QST at intervals of days. Thus, it is essential to investigate and develop new QST alternatives to obtain objective markers that may potentially contribute to the understanding of the mechanisms behind chronic pain conditions.

Detailed Description
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Under normal conditions, pain arises as a consequence of the activation of nociceptive afferents (small fibers) by an external stimulus with sufficient intensity to potentially cause tissue damage. This peripheral activation is processed as perception of pain by the central nervous system. In order to reliably evaluate the state of the nociceptive system in both clinical and experimental settings, standardized tests are essential. Quantitative sensory testing (QST) is a set of tests used to measure the intensity of a stimulus that produces a specific sensory perception in a subject. For example, if we gradually apply pressure, the point where the sensation changes from pressure to pain is called the pressure pain threshold. This type of test can be performed with different types of stimuli, including hot and cold stimuli or mechanical stimuli. Although these tests have been shown as reliable in healthy volunteers and pain patients, they are subjective in their nature, since they are based on a conscious evaluation of tested subjects. Likewise, these measures show substantial variability due to differences in the application of the tests by individual examinators. In short, even though the method is quantitative, its methodological characteristics make it subjective and dependent on both the operator and the subject under study. Moreover, contrasting results have been recently found regarding the measurement variability when repeating the QST at intervals of days. Thus, it is essential to investigate and develop new QST alternatives to obtain objective markers that may potentially contribute to the understanding of the mechanisms behind chronic pain conditions. In this regard, evoked potentials (EP) measured by electroencephalography (EEG) are the most commonly used objective alternative for the functional evaluation of small fibers and the spinothalamic tract. Nociceptive EPs can be induced with various stimulation modalities, including lasers (LEP), contact heat (CHEP) and cold (CCEP), intradermal electrical stimulation (IEEP), and mechanical needling (PEP), and each modality has its own advantages and disadvantages. Recently, a novel type of EP has been proposed that is evoked by electrical stimulation in the range of 200 kHz to 3.3 MHz, that is, in the radio frequency (RF) spectrum. At such high frequencies, the nerves and muscles can no longer be electrically excited, and the physiological effects are generated exclusively due to the heating of the tissue. In strictly physiological terms, RF electrical stimuli are similar to those generated by contact heat. Importantly, non-ablative RF technology is safe, relatively inexpensive, and in widespread use in clinics (Beasley & Weiss, 2014; Lolis & Goldberg, 2012)- Therefore, the use of RF stimulation could significantly increase the accessibility of EPs as a reference electrophysiological tool for the evaluation of the state of the nociceptive system. Another attractive alternative to subjective evaluation is the EP elicited by sharp mechanical stimuli (pinprick). A device that applies this type of stimulus has recently been developed. It uses a stimulator with a tip similar to that of a blunt needle, which allows obtaining brain responses synchronized with the stimulus and evaluating the state of the spino-thalamic-cortical mechanical sensory conduction pathways. At the Faculty of Engineering of the National University of Entre Ríos (FI-UNER), a prototype was developed that allows it to be carried out in an automated manner, which allows to reduce the uncertainty derived from human subjectivity. In the proposed protocol, the precision of the brain response to RF and mechanical sharp stimuli will be evaluated in two experimental settings. We plan to assess the effect of stimulation intensity on signal parameters, such as EP latency and amplitude. Furthermore, the relationship of these parameters with psychophysical results (questionnaires) and heat thresholds will be explored to investigate the relationship between these variables.

Official Title
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Accuracy in the Evaluation of Brain Response to Mechanical and Radiofrequency Stimuli in Humans

Conditions
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To Characterize Brain Response Evoked by Radiofrequency and Pinprick Stimuli in Humans

Intervention / Treatment
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* Other: Radiofrequency application
* Other: Pinprick stimuli
* Other: Electrical stimulation


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: ● Age between 18 and 60 years. Willingness and ability to fully understand the content and scope of the experiment and comply with its instructions. Have signed the informed consent. Exclusion Criteria: ● Pregnancy. Ongoing chronic pain or neuromuscular disorder, or any Desis that effect the nociceptive system and not allowed to be evaluated in normal Condition History of addictive behavior, defined as abuse of alcohol, cannabis, opioids, or other drugs. History of heat sensitivity disorders. History of mental illness. Presence of fever, tuberculosis, malignant tumors, infectious processes, acute inflammatory processes Implantation of pacemakers or metal prostheses. Use of analgesics within 24 hours prior to participation in the experiment. Lack of sleep (< 6 hours) the night before the experiment. High alcohol intake the evening before the experiment.

Ages Eligible for Study
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Minimum Age: 18 Years
Maximum Age: 60 Years

Sexes Eligible for Study
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All

Accepts Healthy Volunteers
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Accepts Healthy Volunteers

Study Plan
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How is the study designed?
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Design Details

Primary Purpose: Basic Science
Allocation: Non-Randomized
Intervention Model: Single Group Assignment
Masking: None (Open Label)


Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| Experimental: Radiofrequency-evoked potentials (RFEPs)<br>The application of RF stimuli will be carried out using an adapted electrocoagulation device (ECD). These devices have several types of applicators, depending on the type of stimulation to be performed. In the bipolar mode, the applicator consists of a small clamp whose tips constitute the active and return electrodes, and the electric current flows only through the tissue captured between the two tips of the clamp. In the unipolar mode, the active electrode is an interchangeable tip with a variable surface (resembling for example a blade or a needle), and the return electrode is a metal plate in contact with another part of the volunteer's body. In this case, the current also flows from the active electrode to the return electrode, but over a considerably longer path. In both cases the physiological effect is similar: the stimulation elicits superficial, localized and limited heating of the tissue in the vicinity of the active electrode. | Other: Radiofrequency application<br>* Radiofrequency stimuli will be applied at pain threshold intensity. Arm and leg will be stimulated.<br>|
| Experimental: Pinprick evoked potentials<br>An automatic stimulator with a section of approximately 0.35 mm in diameter and with a blunt tip was developed to apply this type of stimulus. This stimulator has a calibrated spring, which allows force to be gradually applied, while providing a safety margin to avoid accidents during tests. This allows two types of measurements to be made depending on the speed with which the stimulus is applied. | Other: Pinprick stimuli<br>* Pinprick stimuli will be administered at varying speeds and forces through the automated device<br>Other: Electrical stimulation<br>* The electrical stimuli will be applied using a concentric pin electrode, consisting of a 1.2 mm diameter stainless steel outer ring The stimulation intensity will be increased from 50 µA (usually imperceptible), in steps of 10 µA until the electrical detection threshold (EDT) is found, which is defined as the minimum stimulation intensity that can be perceived. Once the EDT is determined, stimuli are applied in relation to this intensity, usually in the range of 2 to 5 times the EDT based on intervolunteers sensation report.<br>|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Latency and amplitude of EPs evoked by RF and pinprick stimuli | measure the Latency of the signals in Brian after stimulation from RF and pinprick | 2 hours during each experimental setting | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Changes in responses to somatosensory stimuli with Pinprick thresholds | Intensity of pinprick sensation measure using a Numerical Scale Rating ranging from 0 no sensation to 100 most painful sensation. | after each pinprick stimulation | 
| Conduction velocity in radiofrequency stimuli | To determine the conduction speed of the stimuli, a series of 20 stimuli will be applied at the intensity estimated at the pain threshold, in the regions of the dorsum of the foot, in the volar forearm, in the fifth lumbar, and in the neck at the level of the seventh cervical vertebrae. The distance from the stimulation points will be measured using a measuring tape. From the latency of the resulting evoked potentials and the measurement of the physical distance, the conduction speed of the stimulus will be estimated, and therefore the type of fibers that mostly conduct these stimuli will be estimated. | 10 minutes at the end of the RF setting |