Spinal and supraspinal processing of thermal stimuli: an fMRI study

Systematic review and co-ordinate based meta-analysis to summarize the utilization of functional brain imaging in conjunction with human models of peripheral and central sensitization

BACKGROUND AND OBJECTIVE

Functional magnetic resonance imaging, in conjunction with models of peripheral and/or central sensitization, has been used to assess analgesic efficacy in healthy humans. This review aims to summarize the use of these techniques to characterize brain mechanisms of hyperalgesia/allodynia and to evaluate the efficacy of analgesics.

DATABASES AND DATA TREATMENT

Searches were performed (PubMed-Medline, Cochrane, Web of Science and Clinicaltrials.gov) to identify and review studies. A co-ordinate based meta-analysis (CBMA) was conducted to quantify neural activity that was reported across multiple independent studies in the hyperalgesic condition compared to control, using GingerALE software.

RESULTS

Of 217 publications, 30 studies met the inclusion criteria. They studied nine different models of hyperalgesia/allodynia assessed in the primary (14) or secondary hyperalgesia zone (16). Twenty-three studies focused on neural correlates of hyperalgesic conditions and showed consistent changes in the somatosensory cortex, prefrontal cortices, insular cortex, anterior cingulate cortex, thalamus and brainstem. The CBMA on 12 studies that reported activation coordinates for a contrast comparing the hyperalgesic state to control produced six activation clusters (significant at false discovery rate of 0.05) with more peaks for secondary (17.7) than primary zones (7.3). Seven studies showed modulation of brain activity by analgesics in five of the clusters but also in four additional regions.

CONCLUSIONS

This meta-analysis revealed substantial but incomplete overlap between brain areas related to neural mechanisms of hyperalgesia and those reflecting the efficacy of analgesic drugs. Studies testing in the secondary zone were more sensitive to evaluate analgesic efficacy on central sensitization at brainstem or thalamocortical levels.

SIGNIFICANCE

Experimental pain models that provide a surrogate for features of pathological pain conditions in healthy humans and functional imaging techniques are both highly valuable research tools. This review shows that when used together, they provide a wealth of information about brain activity during pain states and analgesia. These tools are promising candidates to help bridge the gap between animal and human studies, to improve translatability and provide opportunities for identification of new targets for back-translation to animal studies.

Reliability of task-based fMRI in the dorsal horn of the human spinal cord

The application of functional magnetic resonance imaging (fMRI) to the human spinal cord is still a relatively small field of research and faces many challenges. Here we aimed to probe the limitations of task-based spinal fMRI at 3T by investigating the reliability of spinal cord blood oxygen level dependent (BOLD) responses to repeated nociceptive stimulation across two consecutive days in 40 healthy volunteers. We assessed the test-retest reliability of subjective ratings, autonomic responses, and spinal cord BOLD responses to short heat pain stimuli (1s duration) using the intraclass correlation coefficient (ICC). At the group level, we observed robust autonomic responses as well as spatially specific spinal cord BOLD responses at the expected location, but no spatial overlap in BOLD response patterns across days. While autonomic indicators of pain processing showed good-to-excellent reliability, both β-estimates and z-scores of task-related BOLD responses showed poor reliability across days in the target region (gray matter of the ipsilateral dorsal horn). When taking into account the sensitivity of gradient-echo echo planar imaging (GE-EPI) to draining vein signals by including the venous plexus in the analysis, we observed BOLD responses with fair reliability across days. Taken together, these results demonstrate that heat pain stimuli as short as one second are able to evoke a robust and spatially specific BOLD response, which is however strongly variable within participants across time, resulting in low reliability in the dorsal horn gray matter. Further improvements in data acquisition and analysis techniques are thus necessary before event-related spinal cord fMRI as used here can be reliably employed in longitudinal designs or clinical settings.

Comparison of LED- and LASER-based fNIRS technologies to record the human peri‑spinal cord neurovascular response

Recently, functional Near-Infrared Spectroscopy (fNIRS) was applied to obtain, non-invasively, the human peri‑spinal Neuro-Vascular Response (NVR) under a non-noxious electrical stimulation of a peripheral nerve. This method allowed the measurements of changes in the concentration of oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb) from the peri‑spinal vascular network. However, there is a lack of clarity about the potential differences in perispinal NVR recorded by the different fNIRS technologies currently available. In this work, the two main noninvasive fNIRS technologies were compared, i.e., LED and LASER-based. The recording of the human peri‑spinal NVR induced by non-noxious electrical stimulation of a peripheral nerve was recorded simultaneously at C7 and T10 vertebral levels. The amplitude, rise time, and full width at half maximum duration of the perispinal NVRs were characterized in healthy volunteers and compared between both systems. The main difference was that the LED-based system shows about one order of magnitude higher values of amplitude than the LASER-based system. No statistical differences were found for rise time and for duration parameters (at thoracic level). The comparison of point-to-point wave patterns did not show significant differences between both systems. In conclusion, the peri‑spinal NRV response obtained by different fNIRS technologies was reproducible, and only the amplitude showed differences, probably due to the power of the system which should be considered when assessing the human peri‑spinal vascular network.

Thermal stimulus task fMRI in the cervical spinal cord at 7 Tesla

Although functional magnetic resonance imaging (fMRI) is widely applied in the brain, fMRI of the spinal cord is more technically demanding. Proximity to the vertebral column and lungs results in strong spatial inhomogeneity and temporal fluctuations in B0 . Increasing field strength enables higher spatial resolution and improved sensitivity to blood oxygenation level-dependent (BOLD) signal, but amplifies the effects of B0 inhomogeneity. In this work, we present the first task fMRI in the spinal cord at 7 T. Further, we compare the performance of single-shot and multi-shot 2D echo-planar imaging (EPI) protocols, which differ in sensitivity to spatial and temporal B0 inhomogeneity. The cervical spinal cords of 11 healthy volunteers were scanned at 7 T using single-shot 2D EPI at 0.75 mm in-plane resolution and multi-shot 2D EPI at 0.75 and 0.6 mm in-plane resolutions. All protocols used 3 mm slice thickness. For each protocol, the BOLD response to 13 10-s noxious thermal stimuli applied to the right thumb was acquired in a 10-min fMRI run. Image quality, temporal signal to noise ratio (SNR), and BOLD activation (percent signal change and z-stat) at both individual- and group-level were evaluated between the protocols. Temporal SNR was highest in single-shot and multi-shot 0.75 mm protocols. In group-level analyses, activation clusters appeared in all protocols in the ipsilateral dorsal quadrant at the expected C6 neurological level. In individual-level analyses, activation clusters at the expected level were detected in some, but not all subjects and protocols. Single-shot 0.75 mm generally produced the highest mean z-statistic, while multi-shot 0.60 mm produced the best-localized activation clusters and the least geometric distortion. Larger than expected within-subject segmental variation of BOLD activation along the cord was observed. Group-level sensory task fMRI of the cervical spinal cord is feasible at 7 T with single-shot or multi-shot EPI. The best choice of protocol will likely depend on the relative importance of sensitivity to activation versus spatial localization of activation for a given experiment. PRACTITIONER POINTS: First stimulus task fMRI results in the spinal cord at 7 T. Single-shot 0.75 mm 2D EPI produced the highest mean z-statistic. Multi-shot 0.60 mm 2D EPI provided the best-localized activation and least distortion.

Recent developments and future avenues for human corticospinal neuroimaging

Non-invasive neuroimaging serves as a valuable tool for investigating the mechanisms within the central nervous system (CNS) related to somatosensory and motor processing, emotions, memory, cognition, and other functions. Despite the extensive use of brain imaging, spinal cord imaging has received relatively less attention, regardless of its potential to study peripheral communications with the brain and the descending corticospinal systems. To comprehensively understand the neural mechanisms underlying human sensory and motor functions, particularly in pathological conditions, simultaneous examination of neuronal activity in both the brain and spinal cord becomes imperative. Although technically demanding in terms of data acquisition and analysis, a growing but limited number of studies have successfully utilized specialized acquisition protocols for corticospinal imaging. These studies have effectively assessed sensorimotor, autonomic, and interneuronal signaling within the spinal cord, revealing interactions with cortical processes in the brain. In this mini-review, we aim to examine the expanding body of literature that employs cutting-edge corticospinal imaging to investigate the flow of sensorimotor information between the brain and spinal cord. Additionally, we will provide a concise overview of recent advancements in functional magnetic resonance imaging (fMRI) techniques. Furthermore, we will discuss potential future perspectives aimed at enhancing our comprehension of large-scale neuronal networks in the CNS and their disruptions in clinical disorders. This collective knowledge will aid in refining combined corticospinal fMRI methodologies, leading to the development of clinically relevant biomarkers for conditions affecting sensorimotor processing in the CNS.

The current state of spinal cord functional magnetic resonance imaging and its application in clinical research

Since its development, spinal cord functional magnetic resonance imaging (fMRI) has utilized various methodologies and stimulation protocols to develop a deeper understanding of a healthy human spinal cord that lays a foundation for its use in clinical research and practice. In this review, we conducted a comprehensive literature search on spinal cord fMRI studies and summarized the recent advancements and resulting scientific achievements of spinal cord fMRI in the following three aspects: the current state of spinal cord fMRI methodologies and stimulation protocols, knowledge about the healthy spinal cord's functions obtained via spinal cord fMRI, and fMRI's exemplary usage in spinal cord diseases and injuries. We conclude with a discussion that, while technical challenges exist, novel fMRI technologies for and new knowledge about the healthy human spinal cord have been established. Empowered by these developments, investigations of pathological and injury states within the spinal cord have become the next important direction of spinal cord fMRI. Recent clinical investigations into spinal cord pathologies, for example, fibromyalgia, multiple sclerosis, spinal cord injury, and cervical spondylotic myelopathy, have already provided deep insights into spinal cord impairments and the time course of impairment-caused changes. We expect that future spinal cord fMRI advancement and research development will further enhance our understanding of various spinal cord diseases and provide the foundation for evaluating existing and developing new treatment plans.

A Comparison of Functional Connectivity in the Human Brainstem and Spinal Cord Associated with Noxious and Innocuous Thermal Stimulation Identified by Means of Functional MRI

The somatosensory system is multidimensional and processes important information for survival, including the experience of pain. The brainstem and spinal cord serve pivotal roles in both transmitting and modulating pain signals from the periphery; although, they are studied less frequently with neuroimaging when compared to the brain. In addition, imaging studies of pain often lack a sensory control condition, failing to differentiate the neural processes associated with pain versus innocuous sensations. The purpose of this study was to investigate neural connectivity between key regions involved in descending modulation of pain in response to a hot, noxious stimulus as compared to a warm, innocuous stimulus. This was achieved with functional magnetic resonance imaging (fMRI) of the brainstem and spinal cord in 20 healthy men and women. Functional connectivity was observed to vary between specific regions across painful and innocuous conditions. However, the same variations were not observed in the period of anticipation prior to the onset of stimulation. Specific connections varied with individual pain scores only during the noxious stimulation condition, indicating a significant role of individual differences in the experience of pain which are distinct from that of innocuous sensation. The results also illustrate significant differences in descending modulation before and during stimulation in both conditions. These findings contribute to a deeper understanding of the mechanisms underlying pain processing at the level of the brainstem and spinal cord, and how pain is modulated.

Bulbospinal nociceptive ON and OFF cells related neural circuits and transmitters

The rostral ventromedial medulla (RVM) is a bulbospinal nuclei in the descending pain modulation system, and directly affects spinal nociceptive transmission through pronociceptive ON cells and antinociceptive OFF cells in this area. The functional status of ON and OFF neurons play a pivotal role in pain chronification. As distinct pain modulative information converges in the RVM and affects ON and OFF cell excitability, neural circuits and transmitters correlated to RVM need to be defined for an in-depth understanding of central-mediated pain sensitivity. In this review, neural circuits including the role of the periaqueductal gray, locus coeruleus, parabrachial complex, hypothalamus, amygdala input to the RVM, and RVM output to the spinal dorsal horn are discussed. Meanwhile, the role of neurotransmitters is concluded, including serotonin, opioids, amino acids, cannabinoids, TRPV1, substance P and cholecystokinin, and their dynamic impact on both ON and OFF cell activities in modulating pain transmission. Via clarifying potential specific receptors of ON and OFF cells, more targeted therapies can be raised to generate pain relief for patients who suffer from chronic pain.

Beyond pain in the brain: A clinician's guide to interpreting the spinal cord's role in the pain experience

INTRODUCTION

Physical therapy practice has greatly improved in providing a biopsychosocial approach when considering persistent pain. However, the spinal cord is often overlooked as a structure with an important role in modulating nociceptive information.

PURPOSE

This article highlights the role of the dorsal horn (DH) in nociceptive processing and its impact on persistent pain conditions as they appear clinically. Key processes occurring in the spinal cord are described, including cellular changes and local spinal network responses to nociceptive stimuli. Additionally, associated clinical symptoms are discussed and some aspects of physical therapy evaluation are challenged based on the mechanisms of nociceptive processing presented in this commentary.

IMPLICATIONS

The spinal cord is an active participant in nociceptive processing, directly impacting the intensity, spread, and recurrence of pain, including within the context of central sensitization. Changes in the behavior of DH neurons are possible with sufficient stimulation and may occur after injury. Additionally, spinal cord activation patterns may lead to bilateral symptoms given adequate strength and duration despite a single peripheral driver. Viewing the spinal cord as a dynamic structure capable of up or down regulating its response to stimuli gives the clinician a better understanding of the nervous system's complex response to prolonged nociceptive input.

When cooling of the skin is perceived as warmth: Enhanced paradoxical heat sensation by pre-cooling of the skin in healthy individuals

A paradoxical heat sensation (PHS) is the misperception of warmth when the skin is cooled. PHS is uncommon in healthy individuals but common in patients with neuropathy and is associated with reduced thermal sensitivity. Identifying conditions that contribute to PHS may indirectly help us understand why some patients experience PHS. We hypothesized that pre-warming increased the number of PHS and that pre-cooling had minimal effect on PHS. We tested 100 healthy participants' thermal sensitivity on the dorsum of their feet by measuring detection and pain thresholds to cold and warm stimuli and PHS. PHS was measured using the thermal sensory limen (TSL) procedure from the quantitative sensory testing protocol of the German Research Network on Neuropathic Pain and by using a modified TSL protocol (mTSL). In the mTSL we examined the participants' thermal detection and PHS after pre-warming of 38°C and 44°C and pre-cooling of 26°C and 20°C. Compared to a baseline condition, the number of PHS responders was significantly increased after pre-cooling (20°C: RR = 1.9 (1.1; 3.3), p = 0.023 and 26°C: RR = 1.9 (1.2; 3.2), p = 0.017), but not significantly after pre-warming (38°C: RR = 1.5 (0.86; 2.8), p = 0.21 and 44°C: RR = 1.7 (.995; 2.9), p = 0.078). Pre-warming and pre-cooling increased the detection threshold of both cold and warm temperatures. We discussed these findings in relation to thermal sensory mechanisms and possible PHS mechanisms. In conclusion, PHS and thermosensation are closely related and pre-cooling can induce PHS responses in healthy individuals.

Spinal cord fMRI with MB-SWIFT for assessing epidural spinal cord stimulation in rats

Purpose

Electrical epidural spinal cord stimulation (SCS) is used as a treatment for chronic pain as well as to partially restore motor function after a spinal cord injury. Monitoring the spinal cord activity during SCS with fMRI could provide important and objective measures of integrative responses to treatment. Unfortunately, spinal cord fMRI is severely challenged by motion and susceptibility artifacts induced by the implanted electrode and bones. This pilot study introduces multi‐band sweep imaging with Fourier transformation (MB‐SWIFT) technique for spinal cord fMRI during SCS in rats. Given the close to zero acquisition delay and high bandwidth in 3 dimensions, MB‐SWIFT is demonstrated to be highly tolerant to motion and susceptibility‐induced artifacts and thus holds promise for fMRI during SCS.

Methods

MB‐SWIFT with 0.78 × 0.78 × 1.50 mm³ spatial resolution and 3‐s temporal resolution was used at 9.4 Tesla in rats undergoing epidural SCS at different frequencies. Its performance was compared with spin echo EPI. The origin of the functional contrast was also explored using suppression bands.

Results

MB‐SWIFT was tolerant to electrode‐induced artifacts and respiratory motion, leading to substantially higher fMRI sensitivity than spin echo fMRI. Clear stimulation frequency‐dependent responses to SCS were detected in the rat spinal cord close to the stimulation site. The origin of MB‐SWIFT fMRI signals was consistent with dominant inflow effects.

Conclusion

fMRI of the rat spinal cord during SCS can be consistently achieved with MB‐SWIFT, thus providing a valuable experimental framework for assessing the effects of SCS on the central nervous system.

Large-scale functional ultrasound imaging of the spinal cord reveals in-depth spatiotemporal responses of spinal nociceptive circuits in both normal and inflammatory states

Despite a century of research on the physiology/pathophysiology of the spinal cord in chronic pain condition, the properties of the spinal cord were rarely studied at the large-scale level from a neurovascular point of view. This is mostly due to the limited spatial and/or temporal resolution of the available techniques. Functional ultrasound imaging (fUS) is an emerging neuroimaging approach that allows, through the measurement of cerebral blood volume, the study of brain functional connectivity or functional activations with excellent spatial (100 μm) and temporal (1 msec) resolutions and a high sensitivity. The aim of this study was to increase our understanding of the spinal cord physiology through the study of the properties of spinal hemodynamic response to the natural or electrical stimulation of afferent fibers. Using a combination of fUS and ultrasound localization microscopy, the first step of this study was the fine description of the vascular structures in the rat spinal cord. Then, using either natural or electrical stimulations of different categories of afferent fibers (Aβ, Aδ, and C fibers), we could define the characteristics of the typical hemodynamic response of the rat spinal cord experimentally. We showed that the responses are fiber-specific, located ipsilaterally in the dorsal horn, and that they follow the somatotopy of afferent fiber entries in the dorsal horn and that the C-fiber response is an N-methyl-D-aspartate receptor-dependent mechanism. Finally, fUS imaging of the mesoscopic hemodynamic response induced by natural tactile stimulations revealed a potentiated response in inflammatory condition, suggesting an enhanced response to allodynic stimulations.

Cortico-spinal imaging to study pain

Functional magnetic resonance imaging of the brain has helped to reveal mechanisms of pain perception in health and disease. Recently, imaging approaches have been developed that allow recording neural activity simultaneously in the brain and in the spinal cord. These approaches offer the possibility to examine pain perception in the entire central pain system and in addition, to investigate cortico-spinal interactions during pain processing. Although cortico-spinal imaging is a promising technique, it bears challenges concerning data acquisition and data analysis strategies. In this review, we discuss studies that applied simultaneous imaging of the brain and spinal cord to explore central pain processing. Furthermore, we describe different MR-related acquisition techniques, summarize advantages and disadvantages of approaches that have been implemented so far and present software that has been specifically developed for the analysis of spinal fMRI data to address challenges of spinal data analysis.

Determinants of Treatment Response in Painful Diabetic Peripheral Neuropathy: A Combined Deep Sensory Phenotyping and Multimodal Brain MRI Study

Painful diabetic peripheral neuropathy (DPN) is difficult to manage, as treatment response is often varied. The primary aim of this study was to examine differences in pain phenotypes between responders and nonresponders to intravenous lidocaine treatment using quantitative sensory testing. The secondary aim was to explore differences in brain structure and functional connectivity with treatment response. Forty-five consecutive patients who received intravenous lidocaine treatment for painful DPN were screened. Twenty-nine patients who met the eligibility criteria (responders, n = 14, and nonresponders, n = 15) and 26 healthy control subjects underwent detailed sensory profiling. Subjects also underwent multimodal brain MRI. A greater proportion of patients with the irritable (IR) nociceptor phenotype were responders to intravenous lidocaine treatment compared with nonresponders. The odds ratio of responding to intravenous lidocaine was 8.67 times greater (95% CI 1.4-53.8) for the IR nociceptor phenotype. Responders to intravenous lidocaine also had significantly greater mean primary somatosensory cortex cortical volume and functional connectivity between the insula cortex and the corticolimbic circuitry. This study provides preliminary evidence for a mechanism-based approach for individualizing therapy in patients with painful DPN.

Lumbar Spinal Cord Activity and Blood Biochemical Changes in Individuals With Diabetic Peripheral Neuropathy During Electrical Stimulation

It is difficult to perform an in vivo evaluation of the nerve conduction mechanism in a patient with diabetic peripheral neuropathy (DPN). We aim to explore possible activation differences to enable a further understanding of the nerve conduction mechanisms of diabetic neuropathy and to present a novel clinical method to evaluate nerve injury and recovery. DPN patients (n = 20) and healthy volunteers (n = 20) were included in this study to detect the functional activation of the lumbar spinal cord via electric stimulation. Spinal fMRI data sets were acquired via a single-shot fast spin echo (SSFSE) sequence. A task-related fMRI was performed via low-frequency electrical stimulation. After post-processing, the active voxels and the percentage of signal changes were calculated for the DPN evaluation and the correlations between the blood biochemical indexes, such as glucose, total cholesterol, and hemoglobin A1c were explored. Activation in the DPN patients was primarily observed in the T12 (10/13) vertebral level. The percentage of signal changes in DPN patients was higher than that in the control group (Z = -2.757, P < 0.05). Positive correlation between the percentage of signal changes and the total cholesterol/glucose in the DNP group was found (P < 0.05). Lumbar spinal cord fMRI, based on the SEEP effect, was determined to be feasible. The repetitive activation distribution was primarily located at the T12 vertebral level. Lumbar spinal cord fMRI might be used as a potential tool to assess and reveal the nerve conduction mechanisms in DPN.

Ten Key Insights into the Use of Spinal Cord fMRI

A comprehensive review of the literature-to-date on functional magnetic resonance imaging (fMRI) of the spinal cord is presented. Spinal fMRI has been shown, over more than two decades of work, to be a reliable tool for detecting neural activity. We discuss 10 key points regarding the history, development, methods, and applications of spinal fMRI. Animal models have served a key purpose for the development of spinal fMRI protocols and for experimental spinal cord injury studies. Applications of spinal fMRI span from animal models across healthy and patient populations in humans using both task-based and resting-state paradigms. The literature also demonstrates clear trends in study design and acquisition methods, as the majority of studies follow a task-based, block design paradigm, and utilize variations of single-shot fast spin-echo imaging methods. We, therefore, discuss the similarities and differences of these to resting-state fMRI and gradient-echo EPI protocols. Although it is newly emerging, complex connectivity and network analysis is not only possible, but has also been shown to be reliable and reproducible in the spinal cord for both task-based and resting-state studies. Despite the technical challenges associated with spinal fMRI, this review identifies reliable solutions that have been developed to overcome these challenges.

Evidence for a spinal involvement in temporal pain contrast enhancement

Spatiotemporal filtering and amplification of sensory information at multiple levels during the generation of perceptual representations is a fundamental processing principle of the nervous system. While for the visual and auditory system temporal filtering of sensory signals has been noticed for a long time, respective contrast mechanisms within the nociceptive system became only recently subject of investigations, mainly in the context of offset analgesia (OA) subsequent to noxious stimulus decreases. In the present study we corroborate in a first experiment the assumption that offset analgesia involves a central component by showing that an OA-like effect accounting for 74% of a corresponding OA reference can be evoked by decomposing the stimulus offset into two separate box-car stimuli applied within the same dermatome but to separate populations of primary afferent neurons. In order to draw conclusions about the levels of the CNS at which temporal filtering of nociceptive information takes place during OA we investigate in a second experiment neuronal activity in the spinal cord during a painful thermal stimulus offset employing high-resolution fMRI in healthy volunteers. Pain-related BOLD responses in the spinal cord were significantly reduced during OA and their time course followed widely behavioral hypoalgesia, but not the thermal stimulation profile. In summary, the results suggest that temporal pain contrast enhancement during OA comprises a central mechanism and this mechanism becomes already effective at the level of the spinal cord.

Functional Neuroimaging of Nociceptive and Pain-Related Activity in the Spinal Cord and Brain: Insights From Neurovascular Coupling Studies

Spinal cord and brain processes underlie pain perception, which produces systemic cardiovascular changes. In turn, the autonomic nervous system regulates vascular function in the spinal cord and brain in order to adapt to these systemic changes, while neuronal activity induces local vascular changes. Thus, autonomic regulation and pain processes in the brain and spinal cord are tightly linked and interrelated. The objective of this topical review is to discuss work on neurovascular coupling during nociceptive processing in order to highlight supporting evidence and limitations for the use of cerebral and spinal fMRI to investigate pain mechanisms and spinal nociceptive processes. Work on functional neuroimaging of pain is presented and discussed in relation to available neurovascular coupling studies and related issues. Perspectives on future work are also discussed with an emphasis on differences between the brain and the spinal cord and on different approaches that may be useful to improve current methods, data analyses and interpretation. In summary, this review highlights the lack of data on neurovascular coupling during nociceptive stimulation and indicates that hemodynamic and BOLD responses measured with fMRI may be biased by nonspecific vascular changes. Future neuroimaging studies on nociceptive and pain-related processes would gain further understanding of neurovascular coupling in the brain and spinal cord and should take into account the effects of systemic vascular changes that may affect hemodynamic responses. Anat Rec, 301:1585-1595, 2018. © 2018 Wiley Periodicals, Inc.

Advances in Spinal Functional Magnetic Resonance Imaging in the Healthy and Injured Spinal Cords

Purpose of Review

This review provides an overview of the current spinal functional magnetic resonance imaging (fMRI) studies that investigate the healthy and injured spinal cords.

Recent Findings

Spinal fMRI-derived outcome measures have previously been suggested to be sensitive to changes in neurological function in the spinal cord. A body of recent task-activated fMRI studies seems to confirm that detecting neural activity in the spinal cord using spinal fMRI may be feasible as well as reliable. Furthermore, a growing number of studies has shown that resting state fMRI in the spinal cord is also feasible, demonstrating that the investigation of changes in neural activity can also be performed in the absence of explicit tasks.

Summary

Current task-activated and resting state fMRI studies suggest that spinal fMRI has a strong potential to provide novel imaging biomarkers that can be used to investigate plastic changes in the injured spinal cord.

Reticular Formation and Pain: The Past and the Future

The involvement of the reticular formation (RF) in the transmission and modulation of nociceptive information has been extensively studied. The brainstem RF contains several areas which are targeted by spinal cord afferents conveying nociceptive input. The arrival of nociceptive input to the RF may trigger alert reactions which generate a protective/defense reaction to pain. RF neurons located at the medulla oblongata and targeted by ascending nociceptive information are also involved in the control of vital functions that can be affected by pain, namely cardiovascular control. The RF contains centers that belong to the pain modulatory system, namely areas involved in bidirectional balance (decrease or enhancement) of pain responses. It is currently accepted that the imbalance of pain modulation towards pain facilitation accounts for chronic pain. The medullary RF has the peculiarity of harboring areas involved in bidirectional pain control namely by the existence of specific neuronal populations involved in antinociceptive or pronociceptive behavioral responses, namely at the rostroventromedial medulla (RVM) and the caudal ventrolateral medulla (VLM). Furthermore the dorsal reticular nucleus (also known as subnucleus reticularis dorsalis; DRt) may enhance nociceptive responses, through a reverberative circuit established with spinal lamina I neurons and inhibit wide-dynamic range (WDR) neurons of the deep dorsal horn. The components of the triad RVM-VLM-DRt are reciprocally connected and represent a key gateway for top-down pain modulation. The RVM-VLM-DRt triad also represents the neurobiological substrate for the emotional and cognitive modulation of pain, through pathways that involve the periaqueductal gray (PAG)-RVM connection. Collectively, we propose that the RVM-VLM-DRt triad represents a key component of the “dynamic pain connectome” with special features to provide integrated and rapid responses in situations which are life-threatening and involve pain. The new available techniques in neurobiological studies both in animal and human studies are producing new and fascinating data which allow to understand the complex role of the RF in pain modulation and its integration with several body functions and also how the RF accounts for chronic pain.

Imaging Acute and Chronic Pain in the Human Brainstem and Spinal Cord

While acute pain serves as a protective mechanism designed to warn an individual of potential or actual damaging stimuli, chronic pain provides no benefit and is now considered a disease in its own right. Since the advent of human brain imaging techniques, many investigations that have explored the central representation of acute and chronic pain have focused on changes in higher order brain regions. In contrast, far fewer have explored brainstem and spinal cord function, mainly due to significant technical difficulties. In this review, we present some of the recent human brain imaging studies that have specifically explored brainstem and spinal cord function during acute noxious stimuli and in individuals with chronic pain. We focus particularly on investigations that explore changes in areas that receive nociceptor afferents and compare humans and experimental animal data in an attempt to describe both microscopic and macroscopic changes associated with acute and chronic pain.

Cervical spinal functional magnetic resonance imaging of the spinal cord injured patient during electrical stimulation

PURPOSE

To evaluate the spatial distribution and signal intensity changes following spinal cord activation in patients with spinal cord injury.

METHODS

This study used spinal functional magnetic resonance imaging (fMRI) based on signal enhancement by extra-vascular water protons (SEEP) to assess elicited responses during subcutaneous electrical stimulation at the right elbow and right thumb in the cervical spinal cord.

RESULTS

Seven healthy volunteers and seven patients with cervical spinal cord injury (SCI) were included in this study. Significant functional activation was observed mainly in the right side of the spinal cord at the level of the C5-C6 cervical vertebra in both the axial and sagittal planes. A higher percentage of signal changes (4.66 ± 2.08 % in injured subjects vs. 2.78 ± 1.66 % in normal) and more average activation voxels (4.69 ± 2.59 in injured subjects vs. 2.56 ± 1.13 in normal subject) in axial plane at the C5-C6 cervical vertebra with a statistically significant difference. The same trends were observed in the sagittal plane with higher percentage of signal changes and more average activation voxels, though no statistically significant difference compared with the control group.

CONCLUSIONS

Spinal SEEP fMRI is a powerful noninvasive method for the study of local neuronal activation in the human spinal cord, which may be of clinical value for evaluating the effectiveness of interventions aimed at promoting recovery of function using electrical stimulation.

Neuroimaging of Central Sensitivity Syndromes: Key Insights from the Scientific Literature

Central sensitivity syndromes are characterized by distressing symptoms, such as pain and fatigue, in the absence of clinically obvious pathology. The scientific underpinnings of these disorders are not currently known. Modern neuroimaging techniques promise new insights into mechanisms mediating these postulated syndromes. We review the results of neuroimaging applied to five central sensitivity syndromes: fibromyalgia, chronic fatigue syndrome, irritable bowel syndrome, temporomandibular joint disorder, and vulvodynia syndrome. Neuroimaging studies of basal metabolism, anatomic constitution, molecular constituents, evoked neural activity, and treatment effect are compared across all of these syndromes. Evoked sensory paradigms reveal sensory augmentation to both painful and nonpainful stimulation. This is a transformative observation for these syndromes, which were historically considered to be completely of hysterical or feigned in origin. However, whether sensory augmentation represents the cause of these syndromes, a predisposing factor, an endophenotype, or an epiphenomenon cannot be discerned from the current literature. Further, the result from cross-sectional neuroimaging studies of basal activity, anatomy, and molecular constituency are extremely heterogeneous within and between the syndromes. A defining neuroimaging "signature" cannot be discerned for any of the particular syndromes or for an over-arching central sensitization mechanism common to all of the syndromes. Several issues confound initial attempts to meaningfully measure treatment effects in these syndromes. At this time, the existence of "central sensitivity syndromes" is based more soundly on clinical and epidemiological evidence. A coherent picture of a "central sensitization" mechanism that bridges across all of these syndromes does not emerge from the existing scientific evidence.

Repeated forced swim stress affects the expression of pCREB and ΔFosB and the acetylation of histone H3 in the rostral ventromedial medulla and locus coeruleus

The rostral ventromedial medulla (RVM) and locus coeruleus (LC) play crucial roles in descending pain modulation system. In the present study we examined the expression of phospho-cAMP response element-binding protein (pCREB) and ΔFosB and the acetylation of histone H3 in the RVM and LC after forced swim stress (FS) and complete Freund's adjuvant (CFA) injection to clarify changes in descending pain modulatory system in a rat model of stress-induced hyperalgesia. FS (day 1, 10min; days 2-3, 20min) induced a significant increase in the expression of pCREB and ΔFosB and the acetylation of histone H3 in the RVM, whereas the FS induced a significant increase only in the acetylation of histone H3 in the LC. CFA injection into the hindpaw did not induce a significant change in those expression and acetylation. Quantitative image analysis demonstrated that the numbers of pCREB-, acetylated histone H3- and ΔFosB-IR cells in the RVM were significantly higher in the FS group than those in the naive group. The CFA injection after the FS did not affect the FS-induced increases in the expression of pCREB and ΔFosB and the acetylation of histone H3 in the RVM even though nullified the increase in the acetylation of histone H3 in the LC. These findings suggest different neuroplasticities between the RVM and LC after the FS, which may be involved in activity change of descending pain modulatory system after the CFA injection.

Distinct BOLD fMRI Responses of Capsaicin-Induced Thermal Sensation Reveal Pain-Related Brain Activation in Nonhuman Primates

Background

Approximately 20% of the adult population suffer from chronic pain that is not adequately treated by current therapies, highlighting a great need for improved treatment options. To develop effective analgesics, experimental human and animal models of pain are critical. Topically/intra-dermally applied capsaicin induces hyperalgesia and allodynia to thermal and tactile stimuli that mimics chronic pain and is a useful translation from preclinical research to clinical investigation. Many behavioral and self-report studies of pain have exploited the use of the capsaicin pain model, but objective biomarker correlates of the capsaicin augmented nociceptive response in nonhuman primates remains to be explored.

Methodology

Here we establish an aversive capsaicin-induced fMRI model using non-noxious heat stimuli in Cynomolgus monkeys (n = 8). BOLD fMRI data were collected during thermal challenge (ON:20 s/42°C; OFF:40 s/35°C, 4-cycle) at baseline and 30 min post-capsaicin (0.1 mg, topical, forearm) application. Tail withdrawal behavioral studies were also conducted in the same animals using 42°C or 48°C water bath pre- and post- capsaicin application (0.1 mg, subcutaneous, tail).

Principal Findings

Group comparisons between pre- and post-capsaicin application revealed significant BOLD signal increases in brain regions associated with the ‘pain matrix’, including somatosensory, frontal, and cingulate cortices, as well as the cerebellum (paired t-test, p<0.02, n = 8), while no significant change was found after the vehicle application. The tail withdrawal behavioral study demonstrated a significant main effect of temperature and a trend towards capsaicin induced reduction of latency at both temperatures.

Conclusions

These findings provide insights into the specific brain regions involved with aversive, ‘pain-like’, responses in a nonhuman primate model. Future studies may employ both behavioral and fMRI measures as translational biomarkers to gain deeper understanding of pain processing and evaluate the preclinical efficacy of novel analgesics.

Noradrenergic Locus Coeruleus pathways in pain modulation

The noradrenergic system is crucial for several activities in the body, including the modulation of pain. As the major producer of noradrenaline (NA) in the central nervous system (CNS), the Locus Coeruleus (LC) is a nucleus that has been studied in several pain conditions, mostly due to its strategic location. Indeed, apart from a well-known descending LC-spinal pathway that is important for pain control, an ascending pathway passing through this nucleus may be responsible for the noradrenergic inputs to higher centers of the pain processing, such as the limbic system and frontal cortices. Thus, the noradrenergic system appears to modulate different components of the pain experience and accordingly, its manipulation has distinct behavioral outcomes. The main goal of this review is to bring together the data available regarding the noradrenergic system in relation to pain, particularly focusing on the ascending and descending LC projections in different conditions. How such findings influence our understanding of these conditions is also discussed.

Sensitization of the Nociceptive System in Complex Regional Pain Syndrome

BACKGROUND

Complex regional pain syndrome type I (CRPS-I) is characterized by sensory, motor and autonomic abnormalities without electrophysiological evidence of a nerve lesion.

OBJECTIVE

Aims were to investigate how sensory, autonomic and motor function change in the course of the disease.

METHODS

19 CRPS-I patients (17 with acute, 2 with chronic CRPS, mean duration of disease 5.7±8.3, range 1-33 months) were examined with questionnaires (LANSS, NPS, MPI, Quick DASH, multiple choice list of descriptors for sensory, motor, autonomic symptoms), motor and autonomic tests as well as quantitative sensory testing according to the German Research Network on Neuropathic Pain at two visits (baseline and 36±10.6, range 16-53 months later).

RESULTS

CRPS-I patients had an improvement of sudomotor and vasomotor function, but still a great impairment of sensory and motor function upon follow-up. Although pain and mechanical detection improved upon follow-up, thermal and mechanical pain sensitivity increased, including the contralateral side. Increase in mechanical pain sensitivity and loss of mechanical detection were associated with presence of ongoing pain.

CONCLUSIONS

The results demonstrate that patients with CRPS-I show a sensitization of the nociceptive system in the course of the disease, for which ongoing pain seems to be the most important trigger. They further suggest that measured loss of function in CRPS-I is due to pain-induced hypoesthesia rather than a minimal nerve lesion. In conclusion, this article gives evidence for a pronociceptive pain modulation profile developing in the course of CRPS and thus helps to assess underlying mechanisms of CRPS that contribute to the maintenance of patients' pain and disability.

Assessing Nociception by fMRI of the Human Spinal Cord: A Systematic Review

OBJECTIVE

To assess the use of fMRI of the spinal cord in measuring noxious stimulation.

METHODS

The Scopus, Medline, EMBASE, and Web of Science databases were searched, along with the reference lists of included articles. Two independent reviewers screened abstracts, full-text articles, and extracted data. Original research was included if fMRI of the human spinal cord was used to measure responses to noxious stimulation.

RESULTS

Of the 192 abstracts screened, 19 met the search criteria and were divided according to their focus: investigating pain responses (n = 6), methodology (n = 6), spinal cord injury (n = 2), or cognition–pain interactions (n = 5). All but one study appear to have observed activity in ipsilateral and dorsal gray matter regions in response to noxious stimuli, although contralateral or ventral activity was also widely observed.

CONCLUSIONS

Although nociception can be investigated using spinal fMRI, establishing reliability, standardizing methodology, and reporting of results will greatly advance this field.

Neural correlates of temporal summation of second pain in the human brainstem and spinal cord

Temporal summation of second pain (TSSP) occurs when painful stimuli are presented repetitively (≥ 0.33 Hz) and results from a C-fibre evoked enhancement (or "wind-up") of the dorsal horn neurons. Based on electrophysiological studies in intact animals, windup is considered a purely central phenomenon. With advancements in functional MRI (fMRI), we can now probe the central mechanisms of this pain response in humans. The aim of this study is to characterize the fMRI responses in the healthy human brainstem and spinal cord that correspond to TSSP. Functional MRI of healthy female adults (N = 15) was conducted while brief, repetitive heat pain stimuli were applied to the right thenar eminence (C6 dermatome), and TSSP (0.33 Hz) and control (0.17 Hz) heat pain paradigms were employed. The stimulus intensity was adjusted to each participant's heat pain sensitivity. Data were analyzed by means of a general linear model, and region-of-interest analyses. As predicted, participants demonstrated significant behavioural summation of pain in the TSSP condition. FMRI results identified enhanced activity in the spinal cord dorsal horn at C6 in response to the TSSP condition. Additionally, multiple areas of the brainstem (RVM and PAG) showed greater responses with the TSSP condition. These results suggest that, in humans, increased pain perception in the TSSP condition is reflected by greater responses in the dorsal horn and in regions known to play a role in the descending modulation of pain, which may modulate the spinal cord response.

Spinal fMRI reveals decreased descending inhibition during secondary mechanical hyperalgesia

Mechanical hyperalgesia is one distressing symptom of neuropathic pain which is explained by central sensitization of the nociceptive system. This sensitization can be induced experimentally with the heat/capsaicin sensitization model. The aim was to investigate and compare spinal and supraspinal activation patterns of identical mechanical stimulation before and after sensitization using functional spinal magnetic resonance imaging (spinal fMRI). Sixteen healthy subjects (6 female, 10 male, mean age 27.2 ± 4.0 years) were investigated with mechanical stimulation of the C6 dermatome of the right forearm during spinal fMRI. Testing was always performed in the area outside of capsaicin application (i.e. area of secondary mechanical hyperalgesia). During slightly noxious mechanical stimulation before sensitization, activity was observed in ipsilateral dorsolateral pontine tegmentum (DLPT) which correlated with activity in ipsilateral spinal cord dorsal gray matter (dGM) suggesting activation of descending nociceptive inhibition. During secondary mechanical hyperalgesia, decreased activity was observed in bilateral DLPT, ipsilateral/midline rostral ventromedial medulla (RVM), and contralateral subnucleus reticularis dorsalis, which correlated with activity in ipsilateral dGM. Comparison of voxel-based activation patterns during mechanical stimulation before/after sensitization showed deactivations in RVM and activations in superficial ipsilateral dGM. This study revealed increased spinal activity and decreased activity in supraspinal centers involved in pain modulation (SRD, RVM, DLPT) during secondary mechanical hyperalgesia suggesting facilitation of nociception via decreased endogenous inhibition. Results should help prioritize approaches for further in vivo studies on pain processing and modulation in humans.