Laser-evoked Potentials Research Articles

Temporal and spatial temperature distribution in the glabrous skin of rats induced by short-pulse CO2 laser

Pain is a natural alarm that aids the body in avoiding potential danger and can also present as an important indicator in clinics. Infrared laser-evoked potentials can be used as an objective index to evaluate nociception. In animal studies, a short-pulse laser is crucial because it completes the stimulation before escape behavior. The objective of the present study was to obtain the temporal and spatial temperature distributions in the skin caused by the irradiation of a short-pulse laser. A fast speed infrared camera was used to measure the surface temperature caused by a CO2 laser of different durations (25 and 35 ms) and power. The measured results were subsequently implemented with a three-layer finite element model to predict the subsurface temperature. We found that stratum corneum was crucial in the modeling of fast temperature response, and escape behaviors correlated with predictions of temperature at subsurface. Results indicated that the onset latency and duration of activated nociceptors must be carefully considered when interpreting physiological responses evoked by infrared irradiation.

Peripheral nociceptor sensitization mediates allodynia in patients with distal symmetric polyneuropathy

Patients with painful neuropathy frequently complain of pain in response to normally non-painful brushing, namely dynamic mechanical allodynia. Despite many animal studies suggesting that allodynia arises when the spontaneous firing in damaged nociceptive afferents sensitise second-order nociceptive neurons to Aβ-fibre input, no studies have sought to confirm this mechanism by investigating Aβ-fibre sparing in human patients with allodynia. In this study we compared data from Aβ-fibre-mediated nerve conduction studies and nociceptive-fibre-mediated laser-evoked potentials (LEPs) in 200 patients with distal symmetric polyneuropathy (114 with neuropathic pain, 86 without). Of the 114 patients with painful neuropathy studied, 44 suffered from allodynia. Whereas no statistical difference was found in nerve conduction study data between patients with and without allodynia, LEP amplitudes were larger in patients with allodynia than in those without (P<0.01 by Mann-Whitney U test). The lack of difference in NCS data between patients with and without allodynia suggest that this type of pain, rather than arising through second-order nociceptive neuron sensitization to Aβ-fibre input, might reflect a reduced mechanical threshold in sensitised intraepidermal nociceptive nerve terminals.

Modulation of pain by emotional sounds: A laser‐evoked potential study

Previous studies have shown increases in experimental pain during induction of a negative emotion with visual stimuli, verbal statements or unpleasant odours. The goal of the present study was to analyse the spatio-temporal activation patterns underlying pain augmentation during negative emotional sounds. Negative (e.g., crying), positive (e.g., laughter) and neutral (e.g., distant traffic) sound samples of 4 s duration were presented while noxious laser stimuli were administered to the dorsum of the right hand in 16 healthy participants. The electroencephalographic laser-evoked potentials (LEPs) were modelled using six equivalent source dipoles located in the left and right fronto-opercular/anterior-insular cortex, left parietal operculum, left primary somatosensory cortex, medial parietal cortex and left medial temporal cortex. Negative emotional sounds were associated with stronger pain than neutral or positive sounds. The source activity in the left medial temporal cortex, purportedly involving hippocampal formation, in the epoch 294-330 ms was greater during negative than neutral or positive sounds. In the left fronto-opercular/anterior-insular cortex, negative sounds failed to show the positive potential component at around 260 ms that was observed during positive and especially during neutral sounds. Results suggest increased input of pain-related information into the hippocampal formation when listening to negative emotional sounds, which may in turn facilitate temporal binding between representations of noxious and other behaviourally relevant stimuli, and perhaps associative learning. Absence of the positive potential component fronto-opercular/anterior-insular cortex during negative sounds points to a slow attentional disengagement from pain and increased awareness of the painful stimulus.

Contact heat and cold, mechanical, electrical and chemical stimuli to elicit small fiber-evoked potentials: Merits and limitations for basic science and clinical use

Laser-evoked potentials are the most extensively validated method to objectively assess nociceptive pathway function in humans. Here, we review merits and shortcomings of alternative techniques using different principles of stimulus generation to stimulate Aδ- or C-fibers. Fast ramp contact heat stimuli yield reproducible responses; however, stimulus location needs to be changed to reduce peripheral habituation, and the limited steepness of temperature ramps may result in response jitter and absence of averaged responses even in some healthy subjects. Inverse temperature ramps can serve to evoke cool evoked potentials to specifically test the cold pathway; the clinical impact of such findings is promising but uncertain to date, and availability of devices optimized for this purpose is currently limited. Mechanical stimuli excite low- or high-threshold mechanoreceptors depending on both the probe surface and the applied force. Electrical stimuli can be used to excite nerve fibers directly in the epidermis, the mucosa of the gut, or the tooth pulp. Principle limitation of the applicability of mechanical and electrical stimuli is the inevitable co-excitation of tactile (Aβ-) fibers. The nasal mucosa can be stimulated using pulsed-CO(2) air streams, which excite chemo-nociceptors; although these stimuli are specific to excite thin trigeminal afferents, their use is limited as it is restricted to a relatively small region. Current data do not allow a comparative analysis on their respective diagnostic values. Quantification of analgesic efficacy in healthy subjects has been established and may be useful in phase I and IIa clinical trials.

Dose-response relationship after single oral dose administrations of morphine and oxycodone using laser-evoked potentials on UVB- and capsaicin-irritated skin in healthy male subjects

The aim of the study was to evaluate the analgesic/antihyperalgesic efficacy and to establish the dose-response relationship of morphine immediate release (IR) and oxycodone IR in a human experimental algesimetric model. Calculated effect ratios for peak-to-peak (PtP) amplitudes of laser-evoked potentials (LEPs) and visual analog scales (VAS) postlaser pain on UVB-irradiated skin (main target variables) were 1.68 and 1.18 respectively for oxycodone 10mg/morphine 20mg, 3.00 and 1.63 respectively for oxycodone 15mg/morphine 30mg, and 1.12 and 1.25 respectively for oxycodone 20mg/morphine 40mg. The effect on the laser-PtP amplitude of morphine at the highest dose (40mg) and of oxycodone at all doses (10, 15, 20mg) was considered to be clinically relevant based on a difference from placebo of ≥2.5μV. For both compounds, a statistically significant linear trend was observed between dose groups in at least 1 of the 2 main target variables (adjusted P value for both end points <.001 at all doses). Hyperalgesia developed over time vs baseline due to acute exposure to UVB irradiation and to topical/occlusive 1% capsaicin solution. For both compounds, the principal onset of analgesic/antihyperalgesic drug effects was around 0.5hours with an average peak at about 1 to 2hours and the effect lasting for more than 3hours (morphine 20 and 30mg) or 6hours (morphine 40mg and oxycodone all doses). In conclusion, the study demonstrated a solid outcome of a mixed objective/subjective human experimental algesimetric model to approach dose-response relationships and analgesic/antihyperalgesic effects of 2 opioids.

Automated single-trial assessment of laser-evoked potentials as an objective functional diagnostic tool for the nociceptive system

ObjectiveTo assess the clinical usefulness of an automated analysis of event-related potentials (ERPs). MethodsNociceptive laser-evoked potentials (LEPs) and non-nociceptive somatosensory electrically-evoked potentials (SEPs) were recorded in 37 patients with syringomyelia and 21 controls. LEP and SEP peak amplitudes and latencies were estimated using a single-trial automated approach based on time-frequency wavelet filtering and multiple linear regression, as well as a conventional approach based on visual inspection. ResultsThe amplitudes and latencies of normal and abnormal LEP and SEP peaks were identified reliably using both approaches, with similar sensitivity and specificity. Because the automated approach provided an unbiased solution to account for average waveforms where no ERP could be identified visually, it revealed significant differences between patients and controls that were not revealed using the visual approach. ConclusionThe automated analysis of ERPs characterized reliably and objectively LEP and SEP waveforms in patients. SignificanceThe automated single-trial analysis can be used to characterize normal and abnormal ERPs with a similar sensitivity and specificity as visual inspection. While this does not justify its use in a routine clinical setting, the technique could be useful to avoid observer-dependent biases in clinical research.

Clinical usefulness of laser evoked potentials

Several papers were published since the first clinical applications of laser evoked potentials (LEPs) in disorders of the nociceptive system. While studies produced until five years ago were mostly addressed at identifying lesions of the nociceptive system, more recent papers used LEPs as an instrumental tool for the diagnosis of neuropathic pain. LEPs have also proved useful in demonstrating the pathophysiological mechanisms underlying different types of neuropathic pain (e.g., paroxystic and ongoing neuropathic pain). This is of utmost importance, as pain treatment should take physiopathological mechanisms, rather than etiology into account. Although there are still some limits for the routine use of LEPs as a diagnostic tool, this review of the literature demonstrates that LEP recording has become mandatory for the functional assessment of patients with hypoalgesia or neuropathic pain.

Cutaneous innervation and trigeminal pathway function in a patient with facial pain associated with Parry-Romberg syndrome

Parry-Romberg syndrome (PRS) is a rare condition manifesting with progressive hemifacial atrophy. Although reported PRS clinical disturbances include facial pain and recent studies raised the possibility that PRS-related pain is a neuropathic pain condition due to the trigeminal nerve damage, no studies have directly investigated cutaneous innervation and trigeminal pathway function in patients with this rare condition. In a 50-year-old woman presenting with a 10-year history of slowly progressive hemifacial atrophy and facial pain, we investigated large myelinated fibres with masticatory muscle electromyography and trigeminal reflexes, and tested small myelinated and unmyelinated fibres with laser-evoked potentials. We also investigated cutaneous innervation by measuring the intraepidermal nerve fibre (IENF) density after skin biopsy of the supraorbital regions. We found that neurophysiological data and IENF density came within normal ranges, with no differences between normal and affected side. Our study showing that the standard reference techniques for assessing cutaneous innervation and trigeminal pathway function disclosed no abnormalities in this patient with PRS suggest that this rare and disabling condition is not associated with trigeminal system damage. These findings indicate that in this patient PRS-related pain is not a neuropathic pain condition, rather it probably arises from the musculoskeletal abnormalities.

New Useful Tools for Small-Fiber Peripheral Neuropathy Clinical Assessment (P03.208)

Objective: Our objective is to call attention to the new laboratory tolls for small-fiber peripheral neuropathies (SFPN)/neuropathic pain (NP) clinical assessment. Background SFPN are very frequent in clinical practice. Usually, these neuropathies are painful. The best tools to assess SFPN/NP are a comprehensive patient medical history and a well performed neurological examination. Secondly, appear the electrophysiological methods, and thirdly the imaging and the morphological ones. The 2009 revised guidelines on NP assessment proposed by the European Federation of Neurological Societies (EFNS), reinforce the previous recommendations (Cruccu G et al., 2010). They considered that amongst laboratory tests, laser evoked potentials (LEPs) are the best tool for assessing SFPN/NP, and skin biopsy for assessing distal loss of unmyelinated nerve fibers. Design/Methods: Standard techniques of contact heat evoked potentials (CHEPs), corneal confocal microscopy (CCM), and Meissner corpuscles (MCs) confocal microscopy were evaluated in the clinical assessment of patients presenting SFPN/NP by our different peripheral nerve centers. Results: CHEPs provided an objective approach to investigate cerebral responses to thermal stimuli mediated by Adelta fibers. In patients with skin denervation amplitudes of CHEPs were reduced. CCM is a non-invasive and easily reproducible clinical technique that disclosed early or severe damage of small-nerve fibers of the cornea in several disorders, including diabetic neuropathy. MCs are touch-pressure sensation receptors in glabrous skin. CM of MCs showed to be a potential for non-invasive detection and monitoring of SNF denervation or reinnervation. Normal data collections are in course for these new tools by ours groups. Conclusions: We have had the opportunity to assess all of the referred methods, including those proposed in the 2009 revised guidelines on NP assessment and we conclude that CHEPS, CCM and MCsCM are highly effective, non invasive and efficient tools in this field. Supported by: CNPq. Disclosure: Dr. Nascimento has nothing to disclose. Dr. De Freitas has nothing to disclose. Dr. Pupe has nothing to disclose. Dr. Dieckmann has nothing to disclose. Dr. Smith has nothing to disclose. Dr. Valls-Sole has nothing to disclose. Dr. Schestatsky has nothing to disclose. Dr. Matta has nothing to disclose.

Habituation to Pain in “Medication Overuse Headache”: A CO2 Laser‐Evoked Potential Study

Our aim was to investigate CO(2) laser-evoked potential (LEP) habituation to experimental pain in a group of patients affected by medication-overuse headache, with a history of episodic migraine becoming chronic, before and after treatment, consisting in acute medication withdrawal and a preventive treatment cycle. One of the main features of LEPs in migraineurs is a lower habituation to repetitive noxious stimuli during the interictal phase. LEPs were recorded to stimulation of both the right hand and the right perioral region in 14 patients and in 14 healthy subjects. The habituation of both the N1 and the vertex N2/P2 components was assessed by measuring the LEP amplitude changes across 3 consecutive repetitions of 30 trials each. In the 8 patients who had clinically improved after treatment, the N2/P2 amplitude habituation was significantly higher after treatment than before treatment following both hand (F = 43.2, P < .0001) and face stimulation (F = 6.9, P = .01). In these patients, the N2/P2 amplitude habituation after treatment was not different from that obtained in healthy controls (P = .18 and P = .73 for hand and face stimulation, respectively). On the contrary, in the patients who did not improve, the N2/P2 amplitude still showed reduced habituation after both hand (F = 3.1, P = .08) and face (F = 0.7, P = .4) stimulation. The deficient habituation of the vertex N2/P2 complex was partly restored after successful treatment of medication-overuse headache, reflecting a modification in pain-processing pathways.

Do we activate specifically somatosensory thin fibres with the concentric planar electrode? A scalp and intracranial EEG study

Laser-evoked potentials (LEPs) are acknowledged as the most reliable laboratory tool for assessing thermal and pain pathways. Electrical stimulation with a newly developed planar concentric electrode, delivering stimuli limited to the superficial skin layers, has been suggested to provide selective activation of Aδ fibres without the inconveniences linked to laser stimulation. The aim of our study was to compare the scalp and intracranial responses to planar concentric electrode stimulation (CE-SEPs) with those of LEPs and standard somatosensory-evoked potentials (SEPs). Sixteen healthy subjects, 6 patients with intracortical electrodes, and 2 patients with selective lesions of the spinothalamic pathway were submitted to Neodymium:Yttrium-Aluminium-Perovskite laser stimulations, and electrical stimulations using standard electrodes or planar concentric electrodes (CE). In both healthy controls and epileptic implanted patients, CE- and standard SEPs showed significantly shorter latencies than LEPs. This is consistent with Aβ-fibre activation, peripheral activation time being unable to account for longer LEP latencies. In the patients with spinothalamic lesions, LEPs were absent after stimulation of the affected territory, while CE-SEPs were still present. For these 2 reasons, we conclude that the planar CE does not selectively activate the Aδ and C fibers, but coexcites a significant proportion of large myelinated Aβ fibres that dominate the ensuing cortical response. The use of CE-SEPs for the detection of spinothalamic system lesions is therefore not warranted; the planar electrode can, however, represent a useful tool to study nociceptive reflexes, which can be reliably elicited even in the presence of Aβ coactivation.

Objective pain diagnostics: Clinical neurophysiology

Neurophysiological techniques help in diagnosis, prognosis and treatment of chronic pain, and are particularly useful to determine its neuropathic origin. According to current standards, the diagnosis of definite neuropathic pain (NP) needs objective confirmation of a lesion or disease of somatosensory systems, which can be provided by neurophysiological testing. Lesions causing NP mostly concern the pain-temperature pathways, and therefore neurophysiological procedures allowing the specific testing of these pathways (i.e., A-delta and C-fibres, spino-thalamo-cortical tracts) are essential for objective diagnosis. Different techniques to stimulate selectively pain-temperature pathways are discussed. Of these, laser-evoked potentials (LEPs) appear as the easiest and most reliable neurophysiological method of assessing nociceptive function, and their coupling with autonomic responses (e.g., galvanic skin response) and psychophysics (quantitative sensory testing - QST) can still enhance their diagnostic yield. Neurophysiological techniques not exploring specifically nociception, such as standard nerve conduction velocities (NCV) and SEPs to non-noxious stimulation, should be associated to the exploration of nociceptive systems, not only because both may be simultaneously affected to different degrees, but also because some specific painful symptoms, such as paroxysmal discharges, may depend on specific alteration of highly myelinated A-beta fibres. The choice of techniques is determined after anamnesis and clinical exam, and tries to answer a number of questions: (a) is the pain-related to injury of somatosensory pathways?; (b) to what extent are different subsystems affected?; (c) are mechanisms and lesion site in accordance with imaging data?; (d) are results of use for diagnostic or therapeutic follow-up? Neuropathic pain (NP) affects more than 15 million people in Western countries, and its belated diagnosis leads to insufficient or delayed therapy. The use of neurofunctional approaches to obtain a "physiological photograph" of somatosensory function is therefore highly relevant, as it yields significant clues about the type and mechanisms of pain, thus prompting rapid and optimised therapy.

Nociceptive pathway function is normal in cervical dystonia: a study using laser-evoked potentials

Cervical dystonia (CD) is often associated with pain in the neck muscles, though the mechanisms underlying pain in this condition are still largely unknown. The aim of this study was to assess laser pain rating and CO(2) laser-evoked potentials (LEPs) in CD patients with pain in the posterior neck region. We assessed the N2/P2 LEP complex and laser pain rating in a group of 20 CD patients and in 21 normal subjects. In 11 of the 20 CD patients (group I), the N2/P2 complex was recorded after stimulation of the skin overlying the right and left deltoid muscles (painless and non-dystonic). In the remaining nine CD patients (group II), the N2/P2 complex was recorded after stimulation of the skin over the splenius capitis muscle (painful and dystonic) and after stimulation of the skin overlying the contralateral splenius muscle (painless and non-dystonic). In group I patients, the N2/P2 LEP amplitude and laser pain rating after stimulation of both shoulders did not differ significantly from those obtained in normal subjects. Similarly, in group II patients, the N2/P2 LEP amplitude and laser pain rating after stimulation of the painful and dystonic splenius capitis muscle did not differ significantly from those obtained from either the contralateral painless, non-dystonic splenius capitis or normal subjects. The results of this study demonstrate that cutaneous nociceptive pathway function in CD patients is normal, thereby indicating that muscle pain in CD is not associated with any central sensitization of nociceptive inputs in either painful (dystonic) or non-painful (non-dystonic) body areas.

Dyspnea-pain counterirritation induced by inspiratory threshold loading: a laser-evoked potentials study

experimentally induced dyspnea of the work/effort type inhibits, in a top-down manner, the spinal transmission of nociceptive inputs (dyspnea-pain counterirritation). Previous studies have demonstrated that this inhibition can be assessed by measuring the nociceptive flexion reflex (RIII). However, its clinical application is limited because of the strong discomfort associated with the electrical stimuli required to elicit the RIII reflex. we examined whether the dyspnea-pain counterirritation phenomenon can be evaluated by measuring the effect of work/effort type dyspnea on the magnitude of laser-evoked brain potentials (LEPs). 10 normal male volunteers were studied (age: 19-30 years). LEPs were elicited using a CO(2) laser stimulator delivering 10- to 15-ms stimuli of 6 ± 0.7 W over a 12.5 mm(2) area. The EEG was recorded using nine scalp channels. Non-nociceptive somatosensory-evoked potentials (SEPs) served as control. LEPs and SEPs were recorded before, during, and after 10 min of experimentally induced dyspnea [inspiratory threshold loading (ITL)]. pain caused by the nociceptive laser stimulus was mild. ITL consistently induced dyspnea, mostly of the "excessive effort" type. Amplitude of the N2-P2 wave of LEPs decreased by 37.6 ± 13.8% during ITL and was significantly correlated with the intensity of dyspnea [r = 0.66, CI 95% (0.08-0.92, P = 0.0319)]. In contrast, ITL had no effect on the magnitude of non-nociceptive SEPs. experimentally induced dyspnea of the work/effort type reduces the magnitude of LEPs. This reduction correlates with the intensity of dyspnea. The recording of LEPs could constitute a clinically applicable approach to assess the dyspnea-pain counterirritation phenomenon in patients.

Klinische Anwendung schmerzevozierter Potenziale

Pain-related evoked potentials (PREPs) represent a novel method for the evaluation of peripheral and central nociceptive pathways, e.g. in the diagnosis of small fiber neuropathy (SFN) or after therapeutic interventions for headache. Compared to contact heat-evoked and laser-evoked potentials, recording of PREPs is less stressful for the subjects and technically less demanding. The clinical usefulness of PREPs has been described for SFN associated with diabetes, HIV and hepatitis C infections as well as in headache and facial pain disorders. They have also been evaluated after interventional methods, such as direct current stimulation (tDCS). The article reviews and discusses the advantages and pitfalls of this technique in the context of recent clinical studies as compared to other paradigms of peripheral electrical stimulation and delineates perspectives and possible indications.

Palmitoylethanolamide Restores Myelinated-Fibre Function in Patients with Chemotherapy-Induced Painful Neuropathy

We assessed the effect of palmitoylethanolamide (PEA) on pain and nerve function in patients with chemotherapy-induced painful neuropathy, in 20 patients undergoing thalidomide and bortezomib treatment for multiple myeloma. All patients were evaluated before and after a two-month treatment with PEA 300 mg BID using pain and warmth thresholds; blinded examiners measured motor and sensory nerve fibre function and laser-evoked potentials. Although no variables returned to normal values, pain and all neurophysiological measures � assessing Aα, Aβ, and Aδ fibres � significantly improved (P < 0.05). In contrast, warmth thresholds, assessing unmyelinated afferents, remained unchanged (P > 0.50). Although a placebo effect might play a role in the reported pain relief, the changes in neurophysiological measures indicate that PEA exerted a positive action on myelinated fibre groups. PEA, possibly by moderating mast cell hyperactivity, relieved conduction blocks secondary to endoneural edema. In a severe condition such as painful neuropathy associated with multiple myeloma and chemotherapy, a safe substance such as PEA provides significant restoration of nerve function.

Effects of motor response expectancy on cortical processing of noxious laser stimuli

Previous studies have shown pain reductions during motor cortex stimulation or voluntary movements. To shed more light on cortical changes associated with decreases in pain during heightened level of motor preparedness in absence of movement, we decided to analyse the effects of motor readiness on EEG laser-evoked potentials (LEPs) by manipulating the expectancy of motor responses. Noxious laser stimuli were administered to the right hand in absence of any movements during periods associated with either high or no expectancy of motor response (HMRE or NMRE, respectively). Subjects reported greater pain intensity during NMRE than HMRE trials. The N1 component of LEPs, peaking at 141 ms and generated in the contralateral operculo-insular cortex, was larger during HMRE than NMRE periods. The amplitude of the N1 component during NMRE correlated with pain intensity. The P2 component peaked earlier during HMRE (336 ± 30 ms) than NMRE (356 ± 29 ms, P < 0.05) condition and its amplitude showed statistically significant positive correlation with subjective pain intensity. Results suggest that pain reduction during high motor expectancy may be related to summation of effects of motor readiness and nociceptive processing in operculo-insular cortex. Subjective pain intensity appears to be formed at an early, sensory stage of processing of laser stimulus in the absence of motor task and only later, during the period in which multiple behavioural challenges are evaluated, if motor readiness is heightened.

The primary somatosensory cortex largely contributes to the early part of the cortical response elicited by nociceptive stimuli

Research on the cortical sources of nociceptive laser-evoked brain potentials (LEPs) began almost two decades ago (Tarkka and Treede, 1993). Whereas there is a large consensus on the sources of the late part of the LEP waveform (N2 and P2 waves), the relative contribution of the primary somatosensory cortex (S1) to the early part of the LEP waveform (N1 wave) is still debated. To address this issue we recorded LEPs elicited by the stimulation of four limbs in a large population (n=35). Early LEP generators were estimated both at single-subject and group level, using three different approaches: distributed source analysis, dipolar source modeling, and probabilistic independent component analysis (ICA). We show that the scalp distribution of the earliest LEP response to hand stimulation was maximal over the central-parietal electrodes contralateral to the stimulated side, while that of the earliest LEP response to foot stimulation was maximal over the central-parietal midline electrodes. Crucially, all three approaches indicated hand and foot S1 areas as generators of the earliest LEP response. Altogether, these findings indicate that the earliest part of the scalp response elicited by a selective nociceptive stimulus is largely explained by activity in the contralateral S1, with negligible contribution from the secondary somatosensory cortex (S2).

P21.19 Thermal quantitative sensory testing (tQST) and laser evoked potentials (LEPs) in evaluating long-term recovery following traumatic and iatrogenic trigeminal injuries

P21.19 Thermal quantitative sensory testing (tQST) and laser evoked potentials (LEPs) in evaluating long-term recovery following traumatic and iatrogenic trigeminal injuries

The effects of total and REM sleep deprivation on laser-evoked potential threshold and pain perception

We investigated the effects of total and rapid eye movement (REM) sleep deprivation on the thermal nociceptive threshold and pain perception using the objective laser-evoked potential (LEP) and the subjective visual analogue scale (VAS). Twenty-eight male adult volunteers were assigned into Control (CTRL), Total (T-SD), and REM (REM-SD) Sleep Deprivation groups. The T-SD and REM-SD volunteers were totally or selectively deprived of sleep for 2 and 4 consecutive nights, respectively. Pain parameters were measured daily during the experimental period. Volunteers were stimulated on the back of the hand by blocks of 50 diode laser pulses. Intensities increased between successive blocks, ranging from nonnoxious to noxious levels, and the LEP threshold was identified based on the evoked-response onset. Both the LEP threshold and VAS ratings were significantly increased after the second night of T-SD. No significant variations were observed in the REM-SD group, suggesting a predominant role for slow wave sleep rather than selective REM-SD in pain perception. Also, for both sleep-deprived groups, the mean values of the LEP threshold and VAS ratings showed a gradual increase that was proportional to the SD deprivation time, followed by a decrease after 1 night of sleep restoration. These findings demonstrate a hyperalgesic modification to pain perception (as reflected by the augmented VAS) and a concomitant increase in the LEP threshold following T-SD, an apparently contradictory effect that can be explained by differences in the ways that attention affects these pain measurements.