Exploring Pain Modulation With TMS and Repeated Pain Conditioning in Healthy Individuals

Background:

Pain and Its Burden

Pain is a prevalent and complex health condition that significantly impacts individuals, families, and society as a whole. Of the 39.4 million adults experiencing pain most days and persisting for over three months, nearly two-thirds report it as "constantly present," with over half describing it as "unbearable and excruciating" . Pain can be categorized into physiological and pathological types. Nociceptive and inflammatory pain are physiological responses that serve protective and adaptive functions, whereas pathological pain is considered non-protective and maladaptive.

Central Mechanisms in Pain Modulation

Laboratory-based quantitative sensory testing (QST) evaluates sensory function using psychophysical methods, teasing out components of the nervous system that demonstrate loss or gain of sensory function contributing to pain. The conditioned pain modulation (CPM) paradigm is a dynamic QST measure of a pain inhibitory process in which pain sensitivity in one area decreased in response to a remotely applied painful stimulus and is used to assess the ability of the person to endogenously inhibit pain. CPM may be a valuable tool in directing mechanistic-based approaches for treating painful conditions. For example, individuals with painful diabetic neuropathy and poorly functioning CPM have more significant analgesia when given duloxetine, a medication that augments conditioned pain modulation. Additionally, identifying and treating the source of pain is associated with positive changes in CPM. Healthy individuals demonstrate more efficient CPM when compared with people in pain and a systematic review of adults with chronic pain revealed that CPM efficiency improves after a reduction in clinical pain. For example, total hip arthroplasty for treating painful osteoarthritis reduces joint pain and improves post-operative CPM. Collectively, these studies suggest that the CPM test has the potential to predict risk and treatment outcomes for chronic pain, therefore CPM may be considered an essential treatment moderator and mediator.

The activation of central mechanisms, particularly endogenous inhibitory systems, is proposed as a fundamental process underlying various pain management interventions. Descending pain inhibitory systems, which are part of central mechanisms, play a crucial role in mediating the analgesic effects of interventions such as manual therapy. Manual therapy stimulates afferent neuronal inputs that activate the central nervous system's descending pathways to inhibit pain. Conditioned Pain Modulation (CPM) shares a similar mechanism, inhibiting pain through descending modulation pathways. Continuous stimulation of these central mechanisms through CPM could potentially alter pain processing and enhance pain inhibition.

Role of Transcranial Magnetic Stimulation (TMS) in Pain Inhibition

TMS is a non-invasive neurophysiological technique that employs magnetic fields to modulate and measure brain activity. It is particularly valuable in studying pain inhibition mechanisms and understanding brain processing of pain stimuli. TMS measures cortical excitability by delivering magnetic pulses to specific brain regions and recording motor-evoked potentials. This technique is increasingly used to investigate neural mechanisms involved in pain perception and inhibition, especially in conditions where these processes may be disrupted, such as chronic pain disorders. By targeting regions like the primary motor cortex or prefrontal cortex, TMS helps researchers examine the roles of excitatory and inhibitory pathways in pain suppression or facilitation.

TMS and Conditioned Pain Modulation (CPM)

TMS is employed to assess the brain's natural ability to inhibit pain through CPM paradigms. In these studies, TMS is used to evaluate how effectively a person's brain can inhibit pain when a secondary, often painful, stimulus is introduced. Individuals with lower cortical excitability in response to noxious stimuli are thought to have stronger endogenous pain inhibition, whereas those with higher cortical excitability may have impaired pain inhibition mechanisms.

Integrating TMS and CPM in Pain Research

In this study, the investigators will employ CPM as both an assessment tool and an intervention method. Combining TMS and CPM provides a comprehensive approach to understanding the neurophysiological and psychosocial factors influencing pain modulation. TMS offers direct measurements of cortical excitability, while CPM assesses an individual's ability to modulate pain through endogenous pathways. As an intervention, CPM will be utilized to potentially modulate nervous system responses. This approach aims to enhance pain inhibitory mechanisms through repeated exposure to controlled painful stimuli, potentially leading to adaptive neuroplastic changes. By simultaneously examining cortical excitability via TMS and endogenous pain modulation through CPM, the investigators aim to delineate the complex interplay betthe investigatorsen central and peripheral mechanisms underlying pain processing.

Research Gaps and Future Directions

Research has demonstrated alterations in cortical excitability associated with pain experiences, predominantly focusing on chronic pain conditions. However, there is a paucity of investigations examining cortical excitability in the context of experimental pain responses and modifications in central pain processing mechanisms. Moreover, although psychological factors such as catastrophizing and hypervigilance have been associated with pain sensitivity, the role of sociocultural constructs such as ethnic identity, optimism, resilience, perceived stress, marginalization, and their relationship with pain inhibition and cortical excitability via continuous stimulation of central mechanisms remains underexplored.

Specific Aims:

Aim 1: To investigate the effect of repeated CPM exposure on cortical excitability Hypothesis 1: Repeated CPM exposure will be associated with alterations in cortical excitability.

Aim 2: To investigate the mediation effect of sociocultural factors on the relationship between repeated CPM exposure and cortical excitability.

Hypothesis 2: Sociocultural factors will mediate the relationship between repeated CPM exposure and cortical excitability, influencing the degree of cortical excitability changes.

Aim 3: To investigate the mediation effect of sociocultural factors on the relationship between repeated CPM exposure and pain sensitivity.

Hypothesis 3: Sociocultural factors will mediate the relationship between repeated CPM exposure and pain perception.

Impact and Significance

This investigation into pain inhibitory mechanisms has the potential to significantly advance our understanding of chronic pain pathophysiology and identify novel therapeutic targets. By elucidating the neurobiological processes underlying pain perception and modulation, this study addresses critical clinical questions regarding changes in cortical excitability and the influence of psychosocial factors on pain perception following nervous system training. The primary innovation of this proposal lies in its comprehensive assessment of central pain mechanisms through transcranial magnetic stimulation (TMS) in conjunction with continuous stimulation via Conditioned Pain Modulation (CPM). This approach, combined with the evaluation of sociocultural constructs such as ethnic identity, optimism, resilience, and marginalization among Hispanic and non-Hispanic populations, offers a unique perspective on pain processing.

By bridging the gap between neurophysiological mechanisms and sociocultural influences on pain, this research has the potential to significantly impact both our theoretical understanding of chronic pain and clinical practice in pain management. The insights gained could inform more targeted and effective interventions, ultimately improving the quality of life for individuals suffering from chronic pain.

Research Plan:

The study in this proposal will be based on psychophysical data and self-reported questionnaires using methods approved by the University of Texas El Paso Institutional Review Board. This prospective experimental study involves healthy participants who will undergo baseline assessments, including psychological questionnaires and psychophysical approaches to measuring individual sensitivity, endogenous pain modulation and transcranial magnetic stimulation. Immediately after an intervention period, participants will be reassessed. All assessments will be performed by an evaluator who will be blind to psychological measure data and group assignment.

Participants:

Seventy individuals will be recruited by posted fliers approved by the IRB on the University of Texas El Paso campus and the El Paso community.

Inclusion criteria: a) pain-free; b) between 18-75 years old. Exclusion criteria: Participants will not be excluded based on race or gender but will be excluded if they meet any of the following a) non-English speaking; b) systemic medical condition is known to affect sensation; c) regular use of prescription pain medication to manage pain; d) current or history of chronic pain condition; e) currently using blood thinning medication; f) any blood clotting disorder such as hemophilia; g) any contraindication to the application of ice or cold packs, such as uncontrolled hypertension, cold urticaria, cryoglobulinemia, paroxysmal cold hemoglobinuria, and circulatory compromise; h) involved in vigorous physical activities like heavy lifting, digging, aerobics or fast bicycling.

Individuals meeting the preliminary screening criteria will undergo an in-person TMS safety screen (English and Spanish language versions included in study case report forms). Briefly, individuals will pass the TMS safety screen if they have no metal in the head or eyes, no pacemakers, shunts, or lines, no history of craniotomy, no history of focal seizures. A detailed screening questionnaire is included in the submitted case report forms.

Recruitment and Informed Consent Procedure:

Potential participants for this study will be recruited through a comprehensive strategy employing multiple channels. Recruitment methods will include strategically placed flyers, targeted social media campaigns, word-of-mouth referrals, and direct recommendations from healthcare providers. Individuals expressing interest in the study will be encouraged to contact the research team directly. The primary Investigator or co-investigator will read from an IRB-approved phone script or respond through an IRB-approved e-mail script to provide an overview of basic information about the study and answer questions. Participants wishing to participate will be scheduled for the formal consent process with either the Principal Investigator or co-investigator. Informed consent will occur in the Department of Physical Therapy lab space at UTEP. At that time, the primary Investigator or co-investigator will explain the study procedure, study purpose, benefits/risks of participating, and use of protected health information. The investigators will ensure appropriate time is spent with the participant to answer any questions and ensure he or she understands the procedures and risks associated with the study. If the participant chooses to participate, informed consent will be obtained from the participant as appropriate.

Measures:

Measures will be collected by the primary investigator, co-investigator, and/or research assistant under the direct supervision of the primary or co-investigator.

Questionnaires:

Demographic and Historical Factors: Study participants will complete a standard intake information form including gender, age, employment status, marital status, educational level, and health history.

The Brief Pain Inventory - Short Form (BPI-sf): The Brief Pain Inventory - Short Form (BPI-sf) is a 9 item self-administered questionnaire used to evaluate