Early gamma-oscillations as correlate of localized nociceptive processing in primary sensorimotor cortex

Recent studies put forward the idea that stimulus-evoked gamma-band oscillations (GBOs; 30-100 Hz) play a specific role in nociception. So far, evidence for the specificity of GBOs for nociception, their possible involvement in nociceptive sensory discriminatory abilities, and knowledge regarding their cortical sources is just starting to grow. To address these questions, we used electroencephalography (EEG) to record brain activity evoked by phasic nociceptive laser stimuli and tactile stimuli applied at different intensities to the right hand and foot of 12 healthy volunteers. The EEG was analyzed in the time domain to extract phase-locked event-related brain potentials (ERPs) and in three regions of interest in the time-frequency domain (delta/theta, 40-Hz gamma, 70-Hz gamma) to extract stimulus-evoked changes in the magnitude of non-phase-locked brain oscillations. Both nociceptive and tactile stimuli, matched with respect to subjective intensity, elicited phase locked ERPs of increasing amplitude with increasing stimulus intensity. In contrast, only nociceptive stimuli elicited a significant enhancement of GBOs (65-85 Hz, 150-230 ms after stimulus onset), whose magnitude encoded stimulus intensity, whereas tactile stimuli led to a GBO decrease. Following nociceptive hand stimulation, the topographical distribution of GBOs was maximal at contralateral electrode C3, whereas maximum activity following foot stimulation was recorded at the midline electrode Cz, compatible with generation of GBOs in the representations of the hand and foot of the primary sensorimotor cortex, respectively. The differential behavior of high-frequency GBOs and low-frequency 40-Hz GBOs is indicating different functional roles and regions in sensory processing.NEW & NOTEWORTHY Gamma-band oscillations show hand-foot somatotopy compatible with generation in primary sensorimotor cortex and are present following nociceptive but not tactile stimulation of the hand and foot in humans.

Laser-evoked potentials mediated by mechano-insensitive nociceptors in human skin

OBJECTIVES

Laser-evoked potentials (LEP) were assessed after peripheral nerve block of the lateral femoral cutaneous nerve (LFCN) in healthy volunteers from partially anesthetized skin areas to differentially stimulate mechano-insensitive nociceptors.

METHODS

An ultrasound-guided nerve block of the LFCN was performed in 12 healthy male subjects with Ropivacain 1%. After 30 min, the nerve block induced significantly larger anesthetic areas to mechanical stimuli than to electrical stimuli revealing an area of differential sensitivity. LEPs, reaction times and pain ratings were recorded in response to the laser stimuli of (1) completely anesthetic skin, (2) mechano-insensitive, but electrically excitable skin ('differential sensitivity'), (3) normal skin.

RESULTS

LEP latencies in the area of differential sensitivity were increased compared to unaffected skin (228 ± 8.5 ms, vs. 181 ± 3.6 ms, p < 0.01) and LEP amplitudes were reduced (14.8 ± 1.2 μV vs. 24.6 ± 1.7 μV, p < 0.01). Correspondingly, psychophysically assessed response latencies in the differentially anesthetic skin were increased (649 ms vs. 427 ms, p < 0.01) and pain ratings reduced (1.5/10 vs. 5/10 NRS, p < 0.01).

CONCLUSION

The increase in LEP latency suggests that mechano-insensitive heat-sensitive Aδ nociceptors (MIA, type II) have a slower conduction velocity or higher utilization time than mechano-sensitive type II Aδ nociceptors. Moreover, widely branched, slowly conducting and mechano-insensitive branches of Aδ nociceptors can explain our finding. LEPs in the differentially anesthetized skin provide specific information about a mechanically insensitive but heat-sensitive subpopulation of Aδ nociceptors. These findings support the concept that A-fibre nociceptors exhibit a similar degree of modality specificity as C-fibre nociceptors.

Effect of sleep deprivation on the electrophysiological signature of habituation to noxious laser stimuli

BACKGROUND

Sleep deprivation induces hyperalgesia. However, this pro-nociceptive effect is not reflected at the electrophysiological level, since sleep restricted subjects show amplitude reduction of Laser-evoked Potentials (LEP). We aimed to explore the contribution of habituation to this paradoxical LEP amplitude decline.

METHODS

We compared LEP's of 12 healthy students (23.2 ± 1.1 years) after habitual sleep (HS) and a night of total sleep deprivation (TSD). Twelve repetitive laser stimulus blocks (each comprising twenty stimuli) were applied under three attention conditions ('focusing' - 'neutral' - 'distraction' condition). Stimulus blocks were split in part 1 (stimulus 1-10) and part 2 (stimulus 11-20). The contribution of habituation to the TSD-induced LEP amplitude decline was studied by calculating the percentage amplitude reduction of part 2 as compared to part 1. Individual sleepiness levels were correlated with (1) averaged LEP's and (2) the degree of habituation.

RESULTS

TSD induced hyperalgesia to laser stimuli (p < 0.001). In contrast, depending on the attention condition, the P2 amplitude of the N2P2-complex was significantly reduced ('focusing': p = 0.004; 'neutral': p = 0.017; distraction: p = 0.71). Habituation of the P2 amplitude to radiant heat was increased after TSD ('focusing': p = 0.04; 'neutral': p < 0.001; distraction: p = 0.88). TSD had no significant effect on N1 amplitudes (p > 0.05). Individual sleepiness correlated negatively with averaged P2 amplitudes (p = 0.02), but not with the degree of habituation (p = 0.14).

CONCLUSION

TSD induces hyperalgesia and results in attention-dependent enhanced habituation of the P2 component. Increased habituation may--to a substantial degree--explain the TSD-induced LEP-amplitude decline. For this article, a commentary is available at the Wiley Online Library.

Pinprick-evoked brain potentials: a novel tool to assess central sensitization of nociceptive pathways in humans

Although hyperalgesia to mechanical stimuli is a frequent sign in patients with inflammation or neuropathic pain, there is to date no objective electrophysiological measure for its evaluation in the clinical routine. Here we describe a technique for recording the electroencephalographic (EEG) responses elicited by mechanical stimulation with a flat-tip probe (diameter 0.25 mm, force 128 mN). Such probes activate Aδ nociceptors and are widely used to assess the presence of secondary hyperalgesia, a psychophysical correlate of sensitization in the nociceptive system. The corresponding pinprick-evoked potentials (PEPs) were recorded in 10 subjects during stimulation of the right and left hand dorsum before and after intradermal injection of capsaicin into the right hand and in 1 patient with a selective lesion of the right spinothalamic tract. PEPs in response to stimulation of normal skin were characterized by a vertex negative-positive (NP) complex, with N/P latencies and amplitudes of 111/245 ms and 3.5/11 μV, respectively. All subjects developed a robust capsaicin-induced increase in the pain elicited by pinprick stimulation of the secondary hyperalgesic area (+91.5%, P < 0.005). Such stimulation also resulted in a significant increase of the N-wave amplitude (+92.9%, P < 0.005), but not of the P wave (+6.6%, P = 0.61). In the patient, PEPs during stimulation of the hypoalgesic side were reduced. These results indicate that PEPs 1) reflect cortical activities triggered by somatosensory input transmitted in Aδ primary sensory afferents and spinothalamic projection neurons, 2) allow quantification of experimentally induced secondary mechanical hyperalgesia, and 3) have the potential to become a diagnostic tool to substantiate mechanical hyperalgesia in patients with presumed central sensitization.

Interoceptive and multimodal functions of the operculo-insular cortex: tactile, nociceptive and vestibular representations

The operculo-insular cortex has been termed the 'homeostatic control center' or 'general magnitude estimator' of the human mind. In this study, somatosensory, nociceptive and caloric vestibular stimuli were applied to reveal, whether there are mainly common, or possibly specific regions activated by one modality alone and whether lateralization effects, time pattern differences or influences of the aversive nature of the stimuli could be observed. Activation of the dorsal posterior insula was caused by all stimuli alike thus terming this area multimodal. Early phases of the noxious heat and caloric vestibular stimulation led to responses in the anterior insula. Using conjunction analyses we found that left- and right-sided tactile stimulation, but not nociceptive stimulation, caused a joint activation of the cytoarchitectonic area OP1 and nociceptive but not tactile stimulation of the anterior insula bilaterally. Tactile activation in the parietal operculum (SII, OP1) was distinct from nociceptive activation (OP3 and frontal operculum). The joint activation by all three stimuli located in the dorsal posterior insula argues for the presence of multisensory structures. The distinct activation of the anterior insula by aversive stimuli and the posterior insula by multisensory signals supports the concept of a partitioned insular cortex recently introduced based on connectivity studies and meta-analyses.

[Nociceptive system : Nociceptors, fiber types, spinal pathways, and projection areas]

In order to transform a nociceptive stimulus into a painful perception, a highly specialized chain of structural and functional elements is necessary. This system comprises nociceptors in the periphery with specific molecular properties for differential coding of noxious submodalities, ascending and descending tracts that can control the input into the dorsal horn of the spinal cord as well as supraspinal processing that regulates the integration of nociceptive information with other sensory modalities and autonomic function. In this article, structures involved in nociceptive signal processing starting from the periphery up to spinal and cerebral structures are discussed in the order of their spatio-temporal activation sequence - as far as these are known. Already from the basis of the different receptor subtypes found on the thinly or unmyelinated nerve fibers in the periphery, the predominant principle of polymodality is demonstrated. Different input to the dorsal horn of the spinal cord by different nerve fiber populations to superficial and deep layers is explained, ascending tracts as well as descending systems capable of either facilitating or inhibiting the upstream flow of nociceptive information, together with their known transmitters. Finally, thalamic relay nuclei for sensory and nociceptive signals, as well as subcortical and cortical projection targets are discussed. To complete the current view of the nociceptive system, information from molecular biology and anatomic tracing studies as well as data from functional electrophysiologic cell recordings in animals and imaging studies in humans are assembled.

A cross-sectional investigation of discontinuation of self-injury and normalizing pain perception in patients with borderline personality disorder

OBJECTIVE

Several studies have shown reduced pain perception in patients with borderline personality disorder (BPD) and current self-injurious behavior (SIB). The aim of the present study was to test whether pain perception in patients with current SIB is different from that of patients who had stopped SIB, and whether pain perception of the latter group differs from healthy controls (HC).

METHOD

We investigated 24 borderline patients and 24 HC. Thirteen patients showed current SIB (BPD-SIB) and 11 patients did not exhibit SIB anymore (BPD-non-SIB). Pain thresholds were assessed using thermal stimuli and laser radiant heat pulses.

RESULTS

We found significant linear trends for all pain measures. The BPD-SIB group was less sensitive than the BPD-non-SIB group and the latter were less sensitive than HC. The pain sensitivity negatively correlated with borderline symptom severity.

CONCLUSION

The results suggest an association between the termination of SIB, decline of psychopathology and normalization of pain perception in borderline patients.

Explicit episodic memory for sensory-discriminative components of capsaicin-induced pain: immediate and delayed ratings

Pain memory is thought to affect future pain sensitivity and thus contribute to clinical pain conditions. Systematic investigations of the human capacity to remember sensory features of experimental pain are sparse. In order to address long-term pain memory, nine healthy male volunteers received intradermal injections of three doses of capsaicin (0.05, 1 and 20 microg, separated by 15 min breaks), each given three times in a balanced design across three sessions at one week intervals. Pain rating was performed using a computerized visual analogue scale (0-100) digitized at 1/s, either immediately online or one hour or one day after injection. Subjects also recalled their pains one week later. Capsaicin injection reliably induced a dose-dependent flare (p<0.001) without any difference within or across sessions. The strong burning pain decayed exponentially within a few minutes. Subjects were able to reliably discriminate pain magnitude and duration across capsaicin doses (both p<0.001), regardless of whether first-time ratings were requested immediately, after one hour or after one day. Pain recall after one week was similarly precise (magnitude: p<0.01, duration: p<0.05). Correlation with rating recall after one week was best when first-time ratings were requested as late as one day after injection (R(2)=0.79) indicating that both rating retrievals utilized similar memory traces. These results indicate a reliable memory for magnitude and duration of experimentally induced pain. The data further suggest that the consolidation of this memory is an important interim stage, and may take up to one day.

Combined EEG and MEG analysis of early somatosensory evoked activity in children and adolescents with focal epilepsies

OBJECTIVE

The study aimed to evaluate differences between EEG and MEG analysis of early somatosensory evoked activity in patients with focal epilepsies in localizing eloquent areas of the somatosensory cortex.

METHODS

Twenty-five patients (12 male, 13 female; age 4-25 years, mean 11.7 years) were included. Syndromes were classified as symptomatic in 17, idiopathic in 2 and cryptogenic in 6 cases. 10 patients presented with malformations of cortical development (MCD). 122 channel MEG and simultaneous 33-channel EEG were recorded during tactile stimulation of the thumb (sampling rate 769 Hz, band-pass 0.3-260 Hz). Forty-four hemispheres were analyzed. Hemispheres were classified as type I: normal (15), II: central structural lesion (16), III: no lesion, but central epileptic discharges (ED, 8), IV: lesion or ED outside the central region (5). Analysis of both sides including one normal and one type II or III hemisphere was possible in 15 patients. Recordings were repeated in 18 hemispheres overall. Averaged data segments were filtered (10-250 Hz) and analyzed off-line with BESA. Latencies and amplitudes of N20 and P30 were analyzed. A regional source was fitted for localizing S1 by MRI co-registration. Orientation of EEG N20 was calculated from a single dipole model.

RESULTS

EEG and MEG lead to comparable good results in all normal hemispheres. Only EEG detected N20/P30 in 3 hemispheres of types II/III while MEG showed no signal. N20 dipoles had a more radial orientation in these cases. MEG added information in one hemisphere, when EEG source analysis of a clear N20 was not possible because of a low signal-to-noise ratio. Overall N20 dipoles had a more radial orientation in type II when compared to type I hemispheres (p=0.01). Further N20/P30 parameters (amplitudes, latencies, localization related to central sulcus) showed no significant differences between affected and normal hemispheres. Early somatosensory evoked activity was preserved within the visible lesion in 5 of the 10 patients with MCD.

CONCLUSIONS

MEG should be combined with EEG when analyzing tactile evoked activities in hemispheres with a central structural lesion or ED focus.

SIGNIFICANCE

At time, MEG analysis is frequently applied without simultaneous EEG. Our results clearly show that EEG may be superior under specific circumstances and combination is necessary when analyzing activity from anatomically altered cortex.

Mechanisms and predictors of chronic facial pain in lateral medullary infarction

The purpose of this study was to identify clinical predictors and anatomical structures involved in patients with pain after dorsolateral medullary infarction. Eight out of 12 patients (67%) developed poststroke pain within 12 days to 24 months after infarction. The pain occurred in the ipsilateral face (6 patients) and/or the contralateral limbs and trunk (5 patients, 3 of whom also had facial pain). Ipsilateral facial pain was significantly correlated with lower medullary lesions, including those of the spinal trigeminal tract and/or nucleus, as documented by magnetic resonance imaging. The R2 blink reflex component was abnormal only in patients with facial pain. Likewise, pain and temperature sensation in the ipsilateral face was decreased in all patients with facial pain but not in patients without pain. Ipsilateral touch sensation in the face was also decreased in all patients with facial pain, but the lesions revealed on magnetic resonance imaging did not involve the principal sensory nucleus of the fifth cranial nerve, and the R1 blink reflex latencies were normal. Although facial pain was correlated with lesions of the spinal trigeminal tract and/or nucleus, none of the lesions involved the subnucleus caudalis, which contains most nociceptive neurons. These findings suggest that facial pain after medullary infarction is due to lesions of the lower spinal trigeminal tract (axons of primary afferent neurons), leading to deafferentation of spinal trigeminal nucleus neurons.

Abolished laser-evoked potentials and normal blink reflex in midlateral medullary infarction

We investigated two patients presenting with the rare finding of almost isolated hemianalgesia with a sensory level on the contralateral side sparing the face. Clinical findings, electrophysiological studies (absent laser-evoked pain-related somatosensory potentials, normal electrically evoked somatosensory potentials, magnetically evoked potentials, and blink reflexes), and magnetic resonance imaging showed the ventrolateral medullar tegmentum containing the spinothalamic tract to be affected by lacunar infarction. The blink reflex R2 component was unimpaired in both patients.

Brain electrical source analysis of primary cortical components of the tibial nerve somatosensory evoked potential using regional sources

Tibial nerve somatosensory evoked potentials (SEPs) show higher amplitudes ipsilateral to the side of stimulation, whereas subdural recordings revealed a source in the foot area of the contralateral hemisphere. We now investigated this paradoxical lateralization by performing a brain electrical source analysis in the P40 time window (34-46 ms). The tibial nerve was stimulated behind the ankle (8 subjects). On each side, 2048 stimuli were applied twice. SEPs were recorded using 32 magnetic resonance imaging (MRI)-verified electrode positions (bandpass 0.5-500 Hz). In each case, the P40 amplitude was higher ipsilaterally (0.45 +/- 0.14 microV) than contralaterally (-0.49 +/- 0.16 microV). The best fitting regional source, however, was always located in the contralateral hemisphere with a mean distance of 8.2 +/- 4.3 mm from the midline. The positivity pointed ipsilaterally shifting from a frontal orientation (P37) to a parietal direction (P40). The P40 dipole moment was 2.5 times stronger than the dipole moment of P37, which makes P40 most prominent in EEG recordings. However, with its oblique dipole orientation compared to the tangential P37 dipole, it is systematically underestimated in MEG. Dipole orientations explained interindividual variability of scalp potential distribution. SEP amplitudes were smaller when generated in the dominant (left) hemisphere. This is explained by deeper located sources (5.4 +/- 1.6 mm) with a more tangential orientation (delta theta = 17.5 +/- 2.3 degrees) in the left hemisphere.

[Differentiation of pain-related and morphine related impairment of cognitive performance and mood in bone marrow transplantation patients]

AIMS

Severe pain of mucositis induced by cytostatic conditioning therapy in bone marrow transplantation (BMT) patients generally requires continuous parenteral opioid treatment. Cognitive and psychological disturbances are frequent complications subsequent to BMT and may result from cerebral opioid side effects. The aim of the study was to evaluate the efficiency of continuous morphine in mucositis pain and the side-effects in BMT patients. Particular emphasis was placed on the question of whether reaction times, which are usually measured to estimate opioid effects upon vigilance, are influenced by pain-induced cognitive impairments in pain patients.

METHODS

While in hospital, 10 BMT patients were examined daily with the aid of a mucositis scale, subjective visual analog scales (VAS) for pain and mood parameters, a German version of the McGill pain questionnaire (MPQ), and a mental performance task battery, and the morphine given was documented. Mental performance tests were simple auditory reaction time and choice reaction time. Seven patients also performed a Sternberg memory search test in which they had to use 'yes' and 'no' response keys to match visually presented test letters to a previously memorized set of letters. Practice and baseline data were collected within the first week before BMT.

RESULTS

The intensity and duration of mucositis differed from patient to patient, resulting in different pain intensities and MPQ scores. Prolongation of the choice reaction time averaged over the period of mucositis treatment correlated significantly with residual pain intensity (Spearman r(s) = 0.88, p < 0.01) but not with morphine dose (r(s) = 0.35, p = 0.33). For the Sternberg memory search test greater correlation coefficients resulted between reaction time and morphine dose (r(s) = 0.86, p = 0.014) than between reaction time and residual pain intensity (r(s) = 0.61, p = 0.15). In turn, pain intensity, unlike morphine dose, was significantly correlated with high scores in the mood parameters for depression, passivity, and tiredness.

CONCLUSIONS

It is concluded that both pain and morphine can impair cognitive performance, but that these mental stressors seem to differ according to qualitative criteria. Whereas pain might slow reaction time by distracting a patient's attention, particularly in low mental demand tasks, morphine could interfere with more specific cognitive processes, such as short-term memory operations, that are required in more complex tasks.