Bilateral somatosensory cortex disinhibition in complex regional pain syndrome type I

Bilateral Hypersensitivity to Capsaicin, Thermal, and Mechanical Stimuli in Unilateral Complex Regional Pain Syndrome

Background:

Complex regional pain syndrome is multifactorial. Exaggerated inflammatory responses to limb injury may be involved. The authors hypothesized that capsaicin-induced pain and neurogenic inflammation (skin perfusion and flare area) are increased in patients with complex regional pain syndrome compared with that in controls.

Methods:

Twenty patients with unilateral upper-limb complex regional pain syndrome and 20 age-, sex-, and body mass index–matched controls participated. Topical capsaicin 5% was applied to the back of both hands for 30 min, and pain intensity was assessed on a visual analogue scale. A laser Doppler perfusion imager scanner estimated capsaicin-induced skin perfusion and flare area. Autonomic and small-fiber function was assessed by sensory testing, quantitative sudomotor axon reflex test, and vasoconstrictor responses.

Results:

The authors found bilateral hypersensitivity to capsaicin (P ≤ 0.02), skin fold (P = 0.001), joint pressure (P < 0.0001), cold (P ≤ 0.01), and heat pain (P ≤ 0.04) in patients compared with that in controls and thermal and mechanical hyperalgesia in the complex regional pain syndrome–affected hand compared with that in the unaffected hand (P ≤ 0.001). The patients had normal capsaicin-induced flare areas, thermal detection thresholds, quantitative sudomotor axon reflex test, and vasoconstrictor responses.

Conclusions:

The main finding is bilaterally increased capsaicin-induced pain in patients compared with controls. The flare response to capsaicin was normal, suggesting that the increased pain response was not due to increased neurogenic inflammation. The bilateral hypersensitivity to painful chemical, thermal, and mechanical stimuli not confined to the innervation area of a peripheral nerve or root cannot be explained by a regional change and may partly be due to central sensitization.

Nerve injury-induced neuropathic pain causes disinhibition of the anterior cingulate cortex

Neuropathic pain caused by peripheral nerve injury is a debilitating neurological condition of high clinical relevance. On the cellular level, the elevated pain sensitivity is induced by plasticity of neuronal function along the pain pathway. Changes in cortical areas involved in pain processing contribute to the development of neuropathic pain. Yet, it remains elusive which plasticity mechanisms occur in cortical circuits. We investigated the properties of neural networks in the anterior cingulate cortex (ACC), a brain region mediating affective responses to noxious stimuli. We performed multiple whole-cell recordings from neurons in layer 5 (L5) of the ACC of adult mice after chronic constriction injury of the sciatic nerve of the left hindpaw and observed a striking loss of connections between excitatory and inhibitory neurons in both directions. In contrast, no significant changes in synaptic efficacy in the remaining connected pairs were found. These changes were reflected on the network level by a decrease in the mEPSC and mIPSC frequency. Additionally, nerve injury resulted in a potentiation of the intrinsic excitability of pyramidal neurons, whereas the cellular properties of interneurons were unchanged. Our set of experimental parameters allowed constructing a neuronal network model of L5 in the ACC, revealing that the modification of inhibitory connectivity had the most profound effect on increased network activity. Thus, our combined experimental and modeling approach suggests that cortical disinhibition is a fundamental pathological modification associated with peripheral nerve damage. These changes at the cortical network level might therefore contribute to the neuropathic pain condition.

Seeing the body produces limb-specific modulation of skin temperature

Vision of the body, even when non-informative about stimulation, affects somatosensory processing. We investigated whether seeing the body also modulates autonomic control in the periphery by measuring skin temperature while manipulating vision. Using a mirror box, the skin temperature was measured from left hand dorsum while participants: (i) had the illusion of seeing their left hand, (ii) had the illusion of seeing an object at the same location or (iii) looked directly at their contralateral right hand. Skin temperature of the left hand increased when participants had the illusion of directly seeing that hand but not in the other two view conditions. In experiment 2, participants viewed directly their left or right hand, or the box while we recorded both hand dorsum temperatures. Temperature increased in the viewed hand but not the contralateral hand. These results show that seeing the body produces limb-specific modulation of thermal regulation.

Complex regional pain syndrome type I affects brain structure in prefrontal and motor cortex

The complex regional pain syndrome (CRPS) is a rare but debilitating pain disorder that mostly occurs after injuries to the upper limb. A number of studies indicated altered brain function in CRPS, whereas possible influences on brain structure remain poorly investigated. We acquired structural magnetic resonance imaging data from CRPS type I patients and applied voxel-by-voxel statistics to compare white and gray matter brain segments of CRPS patients with matched controls. Patients and controls were statistically compared in two different ways: First, we applied a 2-sample ttest to compare whole brain white and gray matter structure between patients and controls. Second, we aimed to assess structural alterations specifically of the primary somatosensory (S1) and motor cortex (M1) contralateral to the CRPS affected side. To this end, MRI scans of patients with left-sided CRPS (and matched controls) were horizontally flipped before preprocessing and region-of-interest-based group comparison. The unpaired ttest of the "non-flipped" data revealed that CRPS patients presented increased gray matter density in the dorsomedial prefrontal cortex. The same test applied to the "flipped" data showed further increases in gray matter density, not in the S1, but in the M1 contralateral to the CRPS-affected limb which were inversely related to decreased white matter density of the internal capsule within the ipsilateral brain hemisphere. The gray-white matter interaction between motor cortex and internal capsule suggests compensatory mechanisms within the central motor system possibly due to motor dysfunction. Altered gray matter structure in dorsomedial prefrontal cortex may occur in response to emotional processes such as pain-related suffering or elevated analgesic top-down control.

Complex regional pain syndrome: more than a peripheral disease

SUMMARY At early stages, complex regional pain syndrome (CRPS) is clinically characterized by damage of peripheral tissues and nerves (edema, activation of osteoblasts, hyperalgesia to blunt pressure). These signs are the result of a dysbalance of pro- and anti-inflammatory cytokines, which normalizes approximately 6 months after the beginning of the disease, independent from clinical outcome. At the same time, evolving clinical signs such as allodynia, cold hyperalgesia, reduced tactile acuity or symptoms of disrupted body representation (e.g., neglect-like syndrome, impaired hand laterality recognition or shift of the body midline) suggest a crucial role of the CNS in the pathophysiology of this pain syndrome. Imaging studies have found a severe but reversible reduction of the cortical hand representation (primary and secondary somatosensory cortices and primary motor cortices). Interestingly however, complex multisensory integration in central association areas are unaffected in CRPS, as patients are capable of integrating artificial body parts or recognize 2D forms despite tactile dysfunction. Furthermore, despite its unilateral clinical manifestation, it has been shown that in CRPS but not in other unilateral neuropathic pain syndromes, alterations in cortical excitability occur bilaterally, both in sensory and motor regions. In conclusion, a more widespread and bilateral pattern of CNS reorganization appears to characterize CRPS, which might be related to dysfunctions in the basal ganglia or in thalamo–cortical structures. Consequently, CRPS treatment should involve not only anti-inflammatory measures and pain therapy, but also the integration of neurorehabilitative training programs.

Absent median somatosensory evoked potential is a predictor of type I complex regional pain syndrome after stroke

PURPOSE

The objective was to determine whether the abnormal finding of somatosensory evoked potentials (SEPs) associated with the development of type I complex regional pain syndrome (CRPS) after stroke.

METHODS

This was a retrospective study conducted from January, 2003, to December, 2007. Seventy patients were confirmed as CRPS type I, and one hundred and eighty-two patients were assigned to the control group. The initial clinical data were reviewed including age, gender, main type of stroke, lateralization and location of the lesion, presence of glenohumeral subluxation, and the development of CRPS. Somatosensory evoked potentials tests (SEP) in median nerve (N20) and posterior tibial nerve (P37) were performed.

RESULTS

CRPS groups revealed significantly higher incidence of the absent and abnormal hemiplegic median SEP, hemorrhagic stroke, and glenohumeral subluxation (GHS). Binary logistic regression analysis indicated that GHS (exp.(B)=4.083, p<0.01) with the absent median SEP (exp.(B)=3.246, p<0.01) were significant independent predictors of CRPS onset.

CONCLUSIONS

In conclusion, GHS and the absent median SEP at sub-acute phase of stroke were primary predictors of the onset of post-stoke CRPS. Implications for Rehabilitation Recent investigations have suggested that autonomic, motor and somatosensory abnormalities of CRPS are impairments involving the central nervous system (CNS) as well as the peripheral neurogenic inflammatory process. However, the understanding of the pathophysiology of CRPS is still far from complete. The absence of SEP at the sub-acute stage of stroke correlated with the onset of post-stroke CRPS type I. The SEP evaluation at the sub-acute period after stroke might be generally used for predicting the concomitant development of post-stroke CRPS type I as well as functional recovery after stroke.

[The rubber hand illusion in patients with complex regional pain syndrome. Successful illusion induction shows multisensory integration]

Next to neurogenic inflammation and pathological sympathetic-afferent coupling, functional imaging studies have shown the crucial role of maladaptive cortical reorgansation in the pathophysiology of CRPS. Bilateral neuroplastic alterations in the somatosensory cortex seem to play a substantial role in the dysfunctional sensory processing of stimuli. The aim was to investigate the multimodal integration of sensory and visual stimuli into the body scheme and the influence of higher cognitive body representation in the integration of multimodal schema, body relevant stimuli in patients with CRPS. The investigated sample included 24 patients suffering from CRPS of the upper extremities, 21 patients with chronic hand pain of other origins and 24 healthy probands. The rubber hand illusion was carried out for the first time in patients with complex regional pain syndrome (CRPS). The reprentations show that the patients can integrate a rubber hand in their body representation to the same degree as healthy patients. The intact experience of the rubber hand illusion by CRPS patients indicates that the integration of congruent visual and tactile stimuli in CRPS is intact.

Primary somatosensory cortex function in complex regional pain syndrome: a systematic review and meta-analysis

That complex regional pain syndrome (CRPS) is associated with functional reorganization in the primary somatosensory cortex (S1) is widely accepted and seldom questioned. Despite more than a decade of research, there has been no systematic review of the CRPS literature concerning the changes in S1 function, and therefore the extent of these changes is unclear. Here we conduct a systematic review and meta-analysis to quantify the spatial and temporal aspects of S1 function in CRPS. A comprehensive search strategy identified functional neuroimaging studies of S1 in CRPS. We adhered to a rigorous systematic review protocol when extracting data and appraising risk of bias. Outcomes were grouped into spatial representation; activation levels, including disinhibition; peak latency of activation; and glucose metabolism. Meta-analysis was conducted where possible. Fifteen studies were included, all investigating upper-extremity CRPS. In patients with CRPS, the S1 spatial representation of the affected hand is smaller than that of the unaffected hand and that of non-CRPS controls; however, this evidence comes from only a few studies. There is no difference in activation, disinhibition, or latency of peripherally evoked S1 responses in CRPS. The risk of bias was high across studies, mainly from unclear sampling methods and unblinded analysis of outcomes.

PERSPECTIVE

The evidence for a difference in function of the primary somatosensory cortex in CRPS compared with controls is clouded by high risk of bias and conflicting results, but reduced representation size seems consistent.

The rubber hand illusion in complex regional pain syndrome: preserved ability to integrate a rubber hand indicates intact multisensory integration

In patients with complex regional pain syndrome (CRPS) type 1, processing of static tactile stimuli is impaired, whereas more complex sensory integration functions appear preserved. This study investigated higher order multisensory integration of body-relevant stimuli using the rubber hand illusion in CRPS patients. Subjective self-reports and skin conductance responses to watching the rubber hand being harmed were compared among CRPS patients (N=24), patients with upper limb pain of other origin (N=21, clinical control group), and healthy subjects (N=24). Additionally, the influence of body representation (body plasticity [Trinity Assessment of Body Plasticity], neglect-like severity symptoms), and clinical signs of illusion strength were investigated. For statistical analysis, 1-way analysis of variance, t test, Pearson correlation, with α=0.05 were used. CRPS patients did not differ from healthy subjects and the control group with regard to their illusion strength as assessed by subjective reports or skin conductance response values. Stronger left-sided rubber hand illusions were reported by healthy subjects and left-side-affected CRPS patients. Moreover, for this subgroup, illness duration and illusion strength were negatively correlated. Overall, severity of neglect-like symptoms and clinical signs were not related to illusion strength. However, patients with CRPS of the right hand reported significantly stronger neglect-like symptoms and significantly lower illusion strength of the affected hand than patients with CRPS of the left hand. The weaker illusion of CRPS patients with strong neglect-like symptoms on the affected hand supports the role of top-down processes modulating body ownership. Moreover, the intact ability to perceive illusory ownership confirms the notion that, despite impaired processing of proprioceptive or tactile input, higher order multisensory integration is unaffected in CRPS.

Surgically induced neuropathic pain: understanding the perioperative process

Nerve damage takes place during surgery. As a consequence, significant numbers (10%-40%) of patients experience chronic neuropathic pain termed surgically induced neuropathic pain (SNPP). The initiating surgery and nerve damage set off a cascade of events that includes both pain and an inflammatory response, resulting in "peripheral and central sensitization," with the latter resulting from repeated barrages of neural activity from nociceptors. In affected patients, these initial events produce chemical, structural, and functional changes in the peripheral and central nervous systems (CNS). The maladaptive changes in damaged nerves lead to peripheral manifestations of the neuropathic state-allodynia, sensory loss, shooting pains, etc, that can manifest long after the effects of the surgical injury have resolved. The CNS manifestations that occur are termed "centralization of pain" and affect sensory, emotional, and other (eg, cognitive) systems as well as contributing to some of the manifestations of the chronic pain syndrome (eg, depression). Currently there are no objective measures of nociception and pain in the perioperative period. As such, intermittent or continuous pain may take place during and after surgery. New technologies including direct measures of specific brain function of nociception and new insights into preoperative evaluation of patients including genetic predisposition, appear to provide initial opportunities for decreasing the burden of SNPP, until treatments with high efficacy and low adverse effects that either prevent or treat pain are discovered.

Influence of parameter settings on paired-pulse-suppression in somatosensory evoked potentials: a systematic analysis

OBJECTIVE

Paired-pulse somatosensory evoked potentials (SEPs) are a common tool to investigate excitability in the human somatosensory cortex. Comparing literature about paired-pulse SEP, there is no standard set of stimulation parameters, while little is known about the influence of stimulation parameters on paired-pulse suppression.

METHODS

We analyzed changes of paired-pulse ratios by varying repetition rates from 1 to 9Hz, and using stimulus intensities of 250% of the sensory threshold and 100%, 120%, and 140% of the motor threshold, which are most frequently used in studies using paired-pulse SEPs.

RESULTS

We found a significant effect of repetition rate on paired-pulse suppression with increasing paired-pulse ratios from 1 to 9Hz, which is mainly caused by a change of single pulse amplitudes. We found no difference in paired-pulse suppression at the tested stimulation intensities.

CONCLUSIONS

The extent of paired-pulse ratios across different studies should be interpreted with caution due to the high dependence on repetition rate, while the results at the commonly used stimulus intensities are comparable.

SIGNIFICANCE

For an optimized parameter setting with sufficient paired-pulse suppression, we suggest a stimulation rate of 1 or 3Hz and a stimulation intensity of 250% of sensory threshold or slightly above motor threshold.