Characterization of blink reflex interneurons by activation of diffuse noxious inhibitory controls in man

Trigeminal cervical complex: A neural network affecting the head and neck

OBJECTIVES

To introduce the trigeminal cervical complex (TCC) as a comprehensive framework for understanding the anatomical and functional scope of the afferences and efferences of the trigeminal nerve and explaining common otolaryngologic symptoms, including head and neck myofascial pain syndrome. Additionally, it explores how the bidirectional transmission of neurotransmitters contributes to the sensitization of the TCC and motor nuclei.

METHODS

The study was conducted as a narrative review. The authors performed a comprehensive search of multiple databases, including Medline/OVID, Embase, Scopus, and PubMed, covering publications from inception until August 2023. Both keywords and medical subject headings related to the TCC were utilized in the search. Information from 66 studies was extracted based on predetermined inclusion and exclusion criteria.

RESULTS

This review discusses the multiple afferent connections from cranial nerves, specifically VII, IX, X, and XI, to the TCC, and their respective efferent pathways. These connections may explain various clinical manifestations in the head and neck that cannot be attributed to other medical conditions. Additionally, the review highlights the dual sensory and motor nature of cranial nerves, emphasizing the bidirectional transmission of neurotransmitters in head and neck areas, which leads to the sensitization of both the TCC and motor nuclei innervating the cervicofacial muscles.

CONCLUSION

The authors hypothesize that the central and peripheral sensitization and the intricate connections of the TCC can elucidate the pathophysiology of conditions such as otalgia, tinnitus, hearing loss, vertigo, headache, cervicogenic dizziness, bruxism, and other symptoms affecting the head and neck.

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

Blink reflex: A practical test to evaluate the trigeminal nerve injury following percutaneous balloon compression for the treatment of trigeminal neuralgia

OBJECTIVE

To evaluate the blink reflex (BR) in estimating the potential injury of trigeminal nerve following percutaneous balloon compression (PBC) surgery, and to determine the association between BR alterations and early surgical outcomes.

METHODS

In this single-center, prospective before-and-after study, a total of 74 patients who had primary trigeminal neuralgia and scheduled for PBC between October 2020 and June 2021 were prospectively included. BR testing and facial sensory assessment were performed pre- and post-PBC. The latency and the area under the curve (AUC) of pre- and postoperative R1 (R1_pre /R1_post ) and R2 (R2_pre /R2_post ) were measured.

RESULTS

The BR components were noticeably delayed or diminished following PBC. R1_post was elicited in only 26 patients, and absent in 48 patients. The residual R1_post had markedly reduced AUC (median difference [Hodges-Lehmann]: -59.5, 95% confidence interval [CI]: -217.5 to -6.9, p = 0.023). Compared with R2_pre , the latency of R2_post was considerably delayed (mean difference: 4.3, 95% CI: 2.9 to 5.7, p < 0.001) and the AUC was greatly suppressed (median difference [Hodges-Lehmann]: -388.4, 95% CI: -548.4 to -259.5, p < 0.001). After PBC, 58 patients had immediate total pain relief, and 16 had partial relief. The absence of R1_post was found in 46 of 58 (79.3%) patients with complete remission, whereas in only 2 of 16 (12.5%) patients with partial relief. Association analysis showed that the absence of R1_post was strongly associated with total pain relief (46/58 [79.3%] vs. 2/16 [12.5%], odds ratio [OR]: 26.8, 95% CI: 5.4 to 134.5, Cramér's V: 0.6, p < 0.001). The latency of R2_post in patients with total relief was significantly delayed (mean difference: 2.5, 95% CI: 0.3 to 4.6, p = 0.028). Patients experienced graded facial numbness after PBC, of whom 31 reported mild numbness (Grades I-II) and 43 reported more severe numbness (Grades III-IV). The absence of R1_post was significantly associated with facial numbness severity, 33/43 (76.7%) in Grades III-IV vs. 15/31 (48.4%) in Grades I-II (OR: 0.284, 95% CI: 0.105 to 0.771, Cramér's V: 0.3, p = 0.012). In patients with more severe numbness, the latency of R2_post was significantly delayed (mean difference: 2.7, 95% CI: 0.1 to 5.3, p = 0.043), and the reduction of AUC was much greater (median difference [Hodges-Lehmann]: 17.2, 95% CI: 0.5 to 35.4, p = 0.041).

CONCLUSION

Both R1 and R2 were significantly diminished after PBC and these alterations were associated with early surgical outcomes, suggesting that the BR is useful in evaluating trigeminal injury following PBC and could provide objective information about early prognosis.

The N13 spinal component of somatosensory evoked potentials is modulated by heterotopic noxious conditioning stimulation suggesting an involvement of spinal wide dynamic range neurons

OBJECTIVES

Although somatosensory evoked potentials (SEPs) after median nerve stimulation are widely used in clinical practice, the dorsal horn generator of the N13 SEP spinal component is not clearly understood. To verify whether wide dynamic range neurons in the dorsal horn of the spinal cord are involved in the generation of the N13 SEP, we tested the effect of heterotopic noxious conditioning stimulation, which modulates wide dynamic range neurons, on N13 SEP in healthy humans.

METHODS

In 12 healthy subjects, we performed the cold pressor test on the left foot as a heterotopic noxious conditioning stimulus to modulate wide dynamic range neurons. To verify the effectiveness of heterotopic noxious conditioning stimulation, we tested the pressure pain threshold at the thenar muscles of the right hand and recorded SEPs after right median nerve stimulation before, during and after the cold pressor test.

RESULTS

The cold pressor test increased pressure pain threshold by 15% (p = 0.04). During the cold pressor test, the amplitude of the N13 component was significantly lower than that recorded at baseline (by 25%, p = 0.04).

DISCUSSION

In this neurophysiological study in healthy humans, we showed that a heterotopic noxious conditioning stimulus significantly reduced N13 SEP amplitude. This finding suggests that the N13 SEP might be generated by the segmental postsynaptic response of dorsal horn wide dynamic range neurons.

Conditioned Pain Modulation: Comparison of the Effects on Nociceptive and Non-nociceptive Blink Reflex

Although conditioned pain modulation (CPM) is considered to represent descending pain inhibitory mechanisms triggered by noxious stimuli applied to a remote area, there have been no previous studies comparing CPM between pain and tactile systems. In this study, we compared CPM between the two systems objectively using blink reflexes. Intra-epidermal electrical stimulation (IES) and transcutaneous electrical stimulation (TS) were applied to the right skin area over the supraorbital foramen to evoke a nociceptive or a non-nociceptive blink reflex, respectively, in 15 healthy males. In the test session, IES or TS were applied six times and subjects reported the intensity of each stimulus on a numerical rating scale (NRS). Blink reflexes were measured using electromyography (R2). The first and second sessions were control sessions, while in the third session, the left hand was immersed in cold water at 10 °C as a conditioning stimulus. The magnitude of the R2 blink and NRS scores were compared among the sessions by 2-way ANOVA. Both the NRS score and nociceptive R2 were significantly decreased in the third session for IES, with a significant correlation between the two variables; whereas, TS-induced non-nociceptive R2 did not change among the sessions. Although the conditioning stimulus decreased the NRS score for TS, the CPM effect was significantly smaller than that for IES (p = 0.002). The present findings suggest the presence of a pain-specific CPM effect to a heterotopic noxious stimulus.

Blepharospasm, Oromandibular Dystonia, and Meige Syndrome: Clinical and Genetic Update

Meige syndrome (MS) is cranial dystonia characterized by the combination of upper and lower cranial involvement and including binocular eyelid spasms (blepharospasm; BSP) and involuntary movements of the jaw muscles (oromandibular dystonia; OMD). The etiology and pathogenesis of this disorder of the extrapyramidal system are not well-understood. Neurologic and ophthalmic examinations often reveal no abnormalities, making diagnosis difficult and often resulting in misdiagnosis. A small proportion of patients have a family history of the disease, but to date no causative genes have been identified to date and no cure is available, although botulinum toxin A therapy effectively mitigates the symptoms and deep brain stimulation is gaining increasing attention as a viable alternative treatment option. Here we review the history and progress of research on MS, BSP, and OMD, as well as the etiology, pathology, diagnosis, and treatment.

Nociceptive blink reflex habituation biofeedback in migraine

Reduced habituation of the nociceptive blink reflex (NBR) is considered a trait marker for genetic predisposition to migraine. In this open-label randomized controlled study, we aimed to test the efficacy of a biofeedback training based on learning of habituation of the NBR (NBR biofeedback) compared with pharmacological (topiramate) treatment and NBR biofeedback plus topiramate treatment in a cohort of migraine without aura patients eligible for prophylaxis. Thirty-three migraine patients were randomly assigned to three months of treatment with: 1) NBR biofeedback, 2) NBR biofeedback plus topiramate 50 mg (b.i.d.), or 3) topiramate 50 mg (b.i.d.). Frequency of headache and disability changes were the main study outcomes. Anxiety, depression, sleep, fatigue, quality of life, allodynia and pericranial tenderness were also evaluated. NBR biofeedback reduced the R2 area, without improving R2 habituation. However, it reduced the frequency of headache and disability, similarly to the combined treatment and topiramate alone. Reduced habituation of the NBR is a stable neurophysiological pattern, scarcely modifiable by learning procedures. Training methods able to act on stress-related responses may modulate cortical mechanisms inducing migraine onset and trigeminal activation under stressful trigger factors.

Experimental evidence for weaker endogenous inhibition of trigeminal pain than extra-trigeminal pain in healthy individuals

Background and objectives The prevalence of pain syndromes that affect the territories innervated by the trigeminal nerve, such as headaches, is one of the highest and ranks second only to low back pain. A potential mechanism underlying this high prevalence may be a relatively weak endogenous pain modulation of trigeminal pain. Here, we sought to systematically compare endogenous pain modulation capabilities in the trigeminal region to those of extra-trigeminal regions in healthy subjects. Methods Healthy, pain free subjects (n = 17) underwent a battery of quantitative sensory testing to assess endogenous pain inhibition and pain enhancement efficiencies within and outside the trigeminal innervated region. Measurements included conditioned pain modulation (CPM), temporal summation of pain (TSP) and spatial summation of pain (SSP). Results Testing configurations that included trigeminal-innervated body regions displayed significantly weaker CPM when compared to extra-trigeminal innervated areas. SSP magnitude was smaller in the ophthalmic trigeminal innervation when compared to other body regions. TSP magnitude was not different between the different body regions tested. Conclusions Our findings point to regional differences in endogenous pain inhibition and suggest that in otherwise healthy individuals, the trigeminal innervation is subjected to a weaker inhibitory pain control than other body regions. Such weaker endogenous pain control could play, at least in part, a role in mediating the high prevalence of trigeminal-related pain syndromes, including primary headaches and TMD pain.

Painful stimulation of a sensitized site in the forearm inhibits ipsilateral trigeminal nociceptive blink reflexes

Exposure to moderate levels of ultraviolet B radiation (UVB) is painless but nevertheless induces an inflammatory response that sensitizes primary afferent nociceptors. Subsequently, heating the UVB-treated site can sensitize spinal nociceptors. We used a repeated-measures design to determine whether heating the UVB-treated site also triggers ipsilateral inhibitory controls. Specifically, a 2-cm diameter site on the forearm of 20 participants was exposed to UVB at twice the minimum erythema dose. 48 h later mechanical and thermal sensitivity had increased at the UVB-treated site, indicating primary hyperalgesia. In addition, sensitivity to blunt pressure had increased in the ipsilateral forehead, implying activation of an ipsilateral supra-spinal pro-nociceptive mechanism. Despite this, the area under the curve of the ipsilateral nociceptive blink reflex decreased when the UVB-treated site was heated to induce moderate pain. Together, these findings suggest that the UVB treatment sensitized primary nociceptive afferents and generated an ipsilateral supra-spinal pro-nociceptive mechanism. In addition, sensitization to heat induced by the UVB treatment strengthened an ipsilateral anti-nociceptive process elicited by heat-pain. Infrequent but enduring discharge of sensitized primary nociceptive afferents, driven by inflammation after UVB exposure, might initiate a lateralized supra-spinal pro-nociceptive influence that heightens awareness of impending harm on the sensitized side of the body. In addition, a lateralized anti-nociceptive response triggered by an intense barrage of nociceptive signals may help to differentiate stronger from weaker sources of pain.

Effect of combined opioid receptor and α2-adrenoceptor blockade on anxiety and electrically evoked startle responses

The R3 component of the electrically evoked blink reflex may form part of a startle reaction. Acoustic startle responses are augmented by yohimbine, an α₂-adrenoceptor antagonist that blocks α₂-autoreceptors, and are potentiated by opioid receptor blockade. To investigate these influences on electrically evoked startle responses, 16 mg yohimbine, with (16 participants) or without 50 mg naltrexone (23 participants), was administered in separate double-blind placebo-controlled cross-over experiments. In each experiment, R3 (a probable component of the startle response) was examined before and after high-frequency electrical stimulation of the forearm, a procedure that initiates inhibitory pain controls. Anxiety and somatic symptoms were greater after yohimbine than placebo, and were potentiated by naltrexone. Pain ratings for the electrically evoked startle stimuli decreased after high-frequency electrical stimulation in the placebo session but remained stable after drug administration. Yohimbine with naltrexone, but not yohimbine alone, also blocked an inhibitory effect of high-frequency electrical stimulation on electrically evoked sharp sensations and R3. Together, the findings suggest that adding naltrexone to yohimbine potentiated anxiety and blocked inhibitory influences of high-frequency electrical stimulation on electrically evoked sensations and startle responses. Thus, opioid peptides could reduce activity in nociceptive and startle-reflex pathways, or inhibit crosstalk between these pathways. Failure of this inhibitory opioid influence might be important in chronically painful conditions that are aggravated by startle stimuli.

Reliability of the nociceptive blink reflex evoked by electrical stimulation of the trigeminal nerve in humans

OBJECTIVE

The nociceptive blink reflex (nBR) can be useful to investigate trigeminal nociceptive function. The aim of this study was to estimate the reliability of the nBR evoked by electrical stimulation of the three branches of the trigeminal nerve under the following conditions: over time (test-retest and intrarater reliability) and by two examiners (interrater reliability).

MATERIALS AND METHODS

Twenty-one healthy participants were evaluated in two sessions (24 h apart). The nBR was elicited by a so-called "nociceptive-specific" electrode placed over the entry zone of the right supraorbital (V1R), infraorbital (V2R), mental (V3R), and left infraorbital (V2L) nerve. The outcomes were individual electrical sensory (I ₀) and pain thresholds (I _P); root mean square (RMS), area-under-the-curve (AUC), and onset latencies of R2 responses (determined twice after a recalibration session); and stimulus-evoked pain on a 0-10 numerical rating scale. Intraclass correlation coefficients (ICCs) and Kappa statistics were computed (α = 5%).

RESULTS

ICCs were fair to excellent in 82% of the psychophysical measures (fair 21%, good 31%, excellent 30%) and in 86% of V1R, V2R, and V2L nBR parameters, whereas 52% of V3R showed poor reliability. ICCs for intrarater reliability were fair to good in 70% of measurements (fair 20%, good 50%) and in 75% of interrater measurements after the recalibration (fair 55%, good 20%). All kappa values showed at least fair agreement and the majority of the nBR measures (93%) presented moderate to excellent reliability.

CONCLUSION

The nBR and its associated psychophysical measures can be considered a sufficiently reliable test.

CLINICAL SIGNIFICANCE

The nBR can be recommended as an electrophysiological technique to assess trigeminal nociceptive function.

Abnormal Modulatory Influence of Diffuse Noxious Inhibitory Controls in Migraine and Chronic Tension-Type Headache Patients

The aim of this study was to evaluate the function of pain modulating systems subserving diffuse noxious inhibitory controls (DNICs) in primary headaches. DNICs were examined in 24 migraineurs, 17 patients with chronic tension-type headache (CTTH) and 20 healthy subjects by means of nociceptive flexion RIII reflex and the cold pressor test (CPT) as heterotopic noxious conditioning stimulation (HNCS). The subjective pain thresholds (Tp) and the RIII reflex threshold (Tr) were significantly lower in CTTH vs. controls. In controls a significant inhibition of the RIII reflex was observed during the CPT (-30±, P < 0.05). Conversely, migraine and CTTH patients showed facilitation (+31±, P < 0.05 and +40±, P < 0.01, respectively) of the RIII reflex during the HNCS. This study demonstrates a dysfunction in systems subserving DNICs in both migraine and CTTH. Impairment of endogenous supraspinal pain modulation systems may contribute to the development and/or maintenance of central sensitization in primary headaches.

Occipital Nerve Blockade in Chronic Cluster Headache Patients and Functional Connectivity Between Trigeminal and Occipital Nerves

Headache syndromes often involve occipital and neck symptoms, suggesting a functional connectivity between nociceptive trigeminal and cervical afferents. Although reports regarding effective occipital nerve blockades in cluster headache exist, the reason for the improvement of the clinical symptoms is not known. Using occipital nerve blockade and nociceptive blink reflexes, we were able to demonstrate functional connectivity between trigeminal and occipital nerves in healthy volunteers. The R2 components of the nociceptive blink reflex and the clinical outcome in 15 chronic cluster headache patients were examined before and after unilateral nerve blockade of the greater occipital nerve with 5 ml prilocain (1%) on the headache side. In contrast to recent placebo-controlled studies, only nine of the 15 cluster patients reported some minor improvement in their headache. Six patients did not report any clinical change. Exclusively on the injection side, the R2 response areas decreased and R2 latencies increased significantly after the nerve blockade. These neurophysiological and clinical data provide further evidence for functional connectivity between cervical and trigeminal nerves in humans. The trigeminocervical complex does not seem to be primarily facilitated in cluster headache, suggesting a more centrally located pathology of the disease. However, the significant changes of trigeminal function as a consequence of inhibition of the greater occipital nerve were not mirrored by a significant clinical effect, suggesting that the clinical improvement of occipital nerve blockades is not due to a direct inhibitory effect on trigeminal transmission.

Suppression of Pain and the R2 Component of the Blink Reflex During Optokinetic Stimulation

The effect of exposure to a rotating optokinetic drum on the electrically evoked blink reflex was investigated in 20 healthy volunteers. Pain ratings and the area under the curve of the R2 component of the blink reflex to innocuous and nociceptive trigeminal stimulation decreased substantially during and after optokinetic stimulation. At low shock intensities, R2 decreased most during optokinetic stimulation in subjects who did not develop symptoms of motion sickness. In contrast, during the recovery period after optokinetic stimulation, suppression of R2 to moderate and intense stimuli was greatest in the most nauseated subjects. These Findings suggest that a mechanism that suppresses symptoms of motion sickness during sensory conflict also inhibits activity in wide dynamic range neurones in the trigeminal nucleus caudalis. Nausea in the absence of sensory conflict may inhibit R2 to intense electrical stimulation by provoking diffuse noxious inhibitory controls.

The Role of the Trigemino Cervical Complex in Chronic Whiplash Associated Headache: A Cross Sectional Study

OBJECTIVE

To investigate signs of central sensitization in a cohort of patients with chronic whiplash associated headache (CWAH).

BACKGROUND

Central sensitization is one of the mechanisms leading to chronicity of primary headache, and thus might contribute to CWAH. However, the pathophysiological mechanism of CWAH is poorly understood and whether it is simply an expression of the primary headache or has a distinct pathogenesis remains unclear. Thus, the factors involved in the genesis of CWAH require further investigation.

METHODS

Twenty-two patients with CWAH (20 females, 2 males; age 25-50 years, mean age 36.3 years) and 25 asymptomatic participants (13 females, 12 males; age 18-50 years, mean age 35.6 years) rated glare and light-induced discomfort in response to light from an ophthalmoscope. Hyperalgesia evoked by a pressure algometer was assessed bilaterally on the forehead, temples, occipital base, and the middle phalanx of the third finger. The number, latency, area under the curve, and recovery cycle of nociceptive blink reflexes elicited by a supraorbital electrical stimulus were also recorded.

RESULTS

Eight and 6 CWAH patients had migrainous and tension-type headache (TTH) profiles, respectively; the remainder had features attributable to both migraine and TTH. Patients in the whiplash group reported significantly greater light-induced pain than controls (8.48 ± .35 vs 6.66 ± .43 on a 0-10 scale; P = .001). The CWAH patients reported significantly lower pressure pain thresholds at all sites. For stimuli delivered at 20 second intervals, whiplash patients were more responsive than controls (4.8 ± .6 blinks vs 3.0 ± .6 blinks in a block of 10 stimuli; P = .036). While R2 latencies and the area under the curve for the 20 second interval trials were comparable in both groups, there was a significant reduction of the area under the curve from the first to the second of the 2-second interval trials only in controls (99 ± 8% of baseline in whiplash patients vs 68 ± 7% in controls; P = .009). The recovery cycle was comparable for both groups.

CONCLUSIONS

Our results corroborate previous findings of mechanical hypersensitivity and photophobia in CWAH patients. The neurophysiological data provide further evidence for hyperexcitability in central nociceptive pathways, and endorse the hypothesis that CWAH may be driven by central sensitization.

Differences in central facilitation between episodic and chronic migraineurs in nociceptive-specific trigeminal pathways

Background

The trigeminal nociceptive system plays a pivotal role in the pathophysiology of migraines. The present study investigated whether there are differences between patients with episodic migraine (EM) and patients with chronic migraine (CM) in trigeminal pain processing at the brainstem and cortical levels using the nociceptive blink reflex (nBR) and pain-related evoked potentials (PREP).

Methods

This study assessed 68 female migraineurs (38 EM patients and 30 CM patients) and 40 age-matched controls using simultaneous recordings of nBR and PREP during the interictal period.

Results

In terms of the nBR, EM patients displayed significantly decreased latencies and larger amplitudes and area-under-the-curve (AUC) values for the R2 component, whereas CM patients showed significantly prolonged latencies and smaller amplitudes and AUC values for the R2 component (p < 0.05). In terms of PREP, both the EM and CM patients had decreased latencies (N1, P1), with larger amplitude compared with the controls (p < 0.05), which indicates facilitation at the cortical level. Additionally, the amplitude and AUC values of the R2 component exhibited a negative correlation, whereas the latency of the R2 component for the nBR showed a positive correlation, with the frequency of headaches in migraineurs (p < 0.01).

Conclusions

In the present study, the facilitation in the trigeminal nociceptive pathway of the EM group suggests the occurrence of migraine-specific hyperexcitability. Additionally, the suppression of R2 at the brainstem level in the CM group may relate to impaired or dysfunctional descending pain modulation. These findings suggest that there are adaptive or maladaptive responses due to the chronification of migraine attacks.

Involvement of α2-adrenoceptors in inhibitory and facilitatory pain modulation processes

BACKGROUND

In healthy humans, high-frequency electrical stimulation (HFS) of the forearm not only produces hyperalgesia at the site of stimulation but also reduces sensitivity to pressure-pain on the ipsilateral side of the forehead. In addition, HFS augments the ipsilateral trigeminal nociceptive blink reflex and intensifies the ipsilateral component of conditioned pain modulation. The aim of this study was to determine whether α2-adrenoceptors mediate these ipsilateral nociceptive influences.

METHODS

The α2-adrenoceptor antagonist yohimbine was administered to 22 participants in a double-blind, placebo-controlled crossover study. In each session, thermal and mechanical sensitivity in the forearms and forehead was assessed before and after HFS. In addition, the combined effect of HFS and yohimbine on the nociceptive blink reflex and on conditioned pain modulation was explored. In this paradigm, the conditioning stimulus was cold pain in the ipsilateral or contralateral temple, and the test stimulus was electrically evoked pain in the forearm.

RESULTS

Blood pressure and electrodermal activity increased for several hours after yohimbine administration, consistent with blockade of central α2-adrenoceptors. Yohimbine not only augmented the nociceptive blink reflex ipsilateral to HFS but also intensified the inhibitory influence of ipsilateral temple cooling on electrically evoked pain at the HFS-treated site in the forearm. Yohimbine had no consistent effect on primary or secondary hyperalgesia in the forearm or on pressure-pain in the ipsilateral forehead.

CONCLUSIONS

These findings imply involvement of α2-adrenoceptors both in ipsilateral antinociceptive and pronociceptive pain modulation processes. However, a mechanism not involving α2-adrenoceptors appears to mediate analgesia in the ipsilateral forehead after HFS.

Lateralized nociceptive blink reflex habituation deficit in episodic cluster headache: Correlations with clinical features

BACKGROUND

We previously observed impaired habituation mechanisms of the conventional blink reflex (BR) in patients with episodic cluster headache (ECH) during the bout, studying only the affected side. Here, we have studied the nociceptive-specific BR (nBR) both on the affected and non-affected sides, and in relation to clinical features.

PARTICIPANTS AND METHODS

We recorded nBR in 18 ECH patients during the bout, and in 18 healthy volunteers (HVs). We compared pain threshold, area, and habituation of the nBR, recorded both for the affected and non-affected sides.

RESULTS

In patients, the pain threshold on the affected side was lower than that of the non-affected side (p = 0.009), and lower than in HVs (p = 0.038). Reflex area was decreased on both sides (p < 0.05) compared with HVs, whereas habituation was significantly impaired only on the affected side (p = 0.025 vs. HVs; p = 0.003 vs. non-affected). The habituation slope was positively correlated with the number of days since the onset of the bout and the daily attack frequency.

CONCLUSIONS

Our data reflect lateralized pathological variations in craniofacial nociception in ECH patients over the course of the cluster period. We hypothesized that this is due to malfunctioning of mechanisms that regulate hypothalamic activity and descending aminergic controls.

Effects of visual cortex activation on the nociceptive blink reflex in healthy subjects

Bright light can cause excessive visual discomfort, referred to as photophobia. The precise mechanisms linking luminance to the trigeminal nociceptive system supposed to mediate this discomfort are not known. To address this issue in healthy human subjects we modulated differentially visual cortex activity by repetitive transcranial magnetic stimulation (rTMS) or flash light stimulation, and studied the effect on supraorbital pain thresholds and the nociceptive-specific blink reflex (nBR). Low frequency rTMS that inhibits the underlying cortex, significantly decreased pain thresholds, increased the 1st nBR block ipsi- and contralaterally and potentiated habituation contralaterally. After high frequency or sham rTMS over the visual cortex, and rMS over the right greater occipital nerve we found no significant change. By contrast, excitatory flash light stimulation increased pain thresholds, decreased the 1st nBR block of ipsi- and contralaterally and increased habituation contralaterally. Our data demonstrate in healthy subjects a functional relation between the visual cortex and the trigeminal nociceptive system, as assessed by the nociceptive blink reflex. The results argue in favour of a top-down inhibitory pathway from the visual areas to trigemino-cervical nociceptors. We postulate that in normal conditions this visuo-trigeminal inhibitory pathway may avoid disturbance of vision by too frequent blinking and that hypoactivity of the visual cortex for pathological reasons may promote headache and photophobia.

The role of the trigeminal sensory nuclear complex in the pathophysiology of craniocervical dystonia

Isolated focal dystonia is a neurological disorder that manifests as repetitive involuntary spasms and/or aberrant postures of the affected body part. Craniocervical dystonia involves muscles of the eye, jaw, larynx, or neck. The pathophysiology is unclear, and effective therapies are limited. One mechanism for increased muscle activity in craniocervical dystonia is loss of inhibition involving the trigeminal sensory nuclear complex (TSNC). The TSNC is tightly integrated into functionally connected regions subserving sensorimotor control of the neck and face. It mediates both excitatory and inhibitory reflexes of the jaw, face, and neck. These reflexes are often aberrant in craniocervical dystonia, leading to our hypothesis that the TSNC may play a central role in these particular focal dystonias. In this review, we present a hypothetical extended brain network model that includes the TSNC in describing the pathophysiology of craniocervical dystonia. Our model suggests the TSNC may become hyperexcitable due to loss of tonic inhibition by functionally connected motor nuclei such as the motor cortex, basal ganglia, and cerebellum. Disordered sensory input from trigeminal nerve afferents, such as aberrant feedback from dystonic muscles, may continue to potentiate brainstem circuits subserving craniocervical muscle control. We suggest that potentiation of the TSNC may also contribute to disordered sensorimotor control of face and neck muscles via ascending and cortical descending projections. Better understanding of the role of the TSNC within the extended neural network contributing to the pathophysiology of craniocervical dystonia may facilitate the development of new therapies such as noninvasive brain stimulation.

Coexistence of ipsilateral pain-inhibitory and facilitatory processes after high-frequency electrical stimulation

BACKGROUND

High-frequency electrical stimulation (HFS) of the human forearm evokes analgesia to blunt pressure in the ipsilateral forehead, consistent with descending ipsilateral inhibitory pain modulation. The aim of the current study was to further delineate pain modulation processes evoked by HFS by examining sensory changes in the arm and forehead; investigating the effects of HFS on nociceptive blink reflexes elicited by supraorbital electrical stimulation; and assessing effects of counter-irritation (electrically evoked pain at the HFS-conditioned site in the forearm) on nociceptive blink reflexes before and after HFS.

METHODS

Before and after HFS conditioning, sensitivity to heat and to blunt and sharp stimuli was assessed at and adjacent to the conditioned site in the forearm and on each side of the forehead. Nociceptive blink reflexes were also assessed before and after HFS with and without counter-irritation of the forearm.

RESULTS

HFS triggered secondary hyperalgesia in the forearm (a sign of central sensitization) and analgesia to blunt pressure in the ipsilateral forehead. Under most conditions, both HFS conditioning and counter-irritation of the forearm suppressed electrically evoked pain in the forehead, and the amplitude of the blink reflex to supraorbital stimuli decreased. Importantly, however, in the absence of forearm counter-irritation, HFS conditioning facilitated ipsilateral blink reflex amplitude to supraorbital stimuli delivered ipsilateral to the HFS-conditioned site.

CONCLUSIONS

These findings suggest that HFS concurrently triggers hemilateral inhibitory and facilitatory influences on nociceptive processing over and above more general effects of counter-irritation. The inhibitory influence may help limit the spread of sensitization in central nociceptive pathways.

Assessment of the wind-up phenomenon in the equine nociceptive trigeminal system

Repeated sub-threshold nociceptive electrical stimulation resulting in temporal summation of the limb nociceptive withdrawal reflex is a well-established non-invasive model to investigate the wind-up phenomenon in horses. Due to structural similarities of the trigeminal sensory nucleus to the dorsal horn of the spinal cord, temporal summation should be evoked by repeated transcutaneous electrical stimulation of trigeminal afferents. To evaluate this hypothesis repeated transcutaneous electrical stimulation was applied to the supraorbital and infraorbital nerves of 10 horses. Stimulation intensities varied between 0.5 and 1.3 times the trigemino-cervical reflex threshold defined for single stimulation. Evoked electromyographic activity of the orbicularis oculi, splenius and cleidomastoideus muscles was recorded and the signals analysed in the previously established epochs typical to the early and late component of the blink reflex and to the trigemino-cervical reflex. Behavioural reactions were evaluated with the aid of numerical rating scale. The nociceptive late component and the trigemino-cervical reflex were not elicited by sub-threshold intensity repeated transcutaneous electrical stimulation. Furthermore, the median reflex amplitude for the 10 horses showed a tendency to decline over the stimulation train so temporal summation of afferent trigeminal inputs could not be observed. Therefore, the modulation of trigeminal nociceptive processing attributable to repeated Aδ fibre stimulations seems to differ from spinal processing of similar inputs as it seems to have an inhibitory rather than facilitatory effect. Further evaluation is necessary to highlight the underlying mechanism.

Differences between episodic and chronic tension-type headaches in nociceptive-specific trigeminal pathways

BACKGROUND

The trigeminal nociceptive system plays a pivotal role in the pathophysiology of tension-type headaches (TTH).

OBJECTIVE

This study investigated and compared nociceptive-specific trigeminal pathways in patients with episodic and chronic TTH (ETTH and CTTH, respectively) using the nociceptive blink reflex (nBR) and nociceptive trigeminocervical reflex (nTCR).

METHODS

We recorded nBR and nTCR in patients with ETTH and CTTH, and healthy controls using concentric electrodes and subsequently compared the threshold (i.e. sensory, pain) and parameters of reflex (i.e. the R2 component of the nBR and the late responses of the nTCR).

RESULTS

Women with ETTH ( N  = 40) and CTTH ( N  = 32) and age-matched controls ( N  = 40) were recruited. CTTH patients displayed significantly lower amplitude and area under the curve (AUC) values of the R2 component for the nBR compared with those displayed by ETTH patients and controls ( P  < 0.05). Moreover, the amplitude and AUC of the R2 component was negatively correlated with the frequency of headaches, whereas the latency of the R2 component for the nBR was positively correlated with the frequency and duration of headaches in the TTH groups ( P  < 0.05). However, no significant differences in the late response parameters (i.e. latency, duration, amplitude, or AUC) were noted between the groups in terms of the nTCR.

CONCLUSIONS

R2 suppression associated with CTTH suggests decreased brainstem excitability. This may be the result of excessive descending inhibitory influences.

Facial nerve palsy and hemifacial spasm

Facial nerve lesions are usually benign conditions even though patients may present with emotional distress. Facial palsy usually resolves in 3-6 weeks, but if axonal degeneration takes place, it is likely that the patient will end up with a postparalytic facial syndrome featuring synkinesis, myokymic discharges, and hemifacial mass contractions after abnormal reinnervation. Essential hemifacial spasm is one form of facial hyperactivity that must be distinguished from synkinesis after facial palsy and also from other forms of facial dyskinesias. In this condition, there can be ectopic discharges, ephaptic transmission, and lateral spread of excitation among nerve fibers, giving rise to involuntary muscle twitching and spasms. Electrodiagnostic assessment is of relevance for the diagnosis and prognosis of peripheral facial palsy and hemifacial spasm. In this chapter the most relevant clinical and electrodiagnostic aspects of the two disorders are reviewed, with emphasis on the various stages of facial palsy after axonal degeneration, the pathophysiological mechanisms underlying the various features of hemifacial spasm, and the cues for differential diagnosis between the two entities.

Blink reflex in patients with postparalytic facial syndrome and blepharospasm: trigeminal and auditory stimulation

OBJECTIVE

The enhancement of blink reflex (BR) excitability was shown in patients with postparalytic facial syndrome (PFS) and essential blepharospasm (EB). We prospectively investigated patients with PFS and EB whether BR alterations demonstrated by trigeminal stimulation will similarly be observed upon auditory stimulation.

METHODS

Fifteen patients with PFS, 15 patients with EB, and 30 healthy volunteers were involved. Electrically stimulated trigeminal BR and auditory BR were studied bilaterally.

RESULTS

The mean R2 amplitude and duration values were highest in EB patients, being significantly higher than PFS patients (p < 0.05) and control group (p < 0.01). The mean R2 duration in PFS patients were also significantly longer in compared to control group (p = 0.025). EB patients showed a higher mean R (auditory) amplitude and duration than PFS patients (p < 0.05) and controls (p < 0.04). The mean R (auditory) duration was longer on symptomatic side of PFS patients in compared to controls (p = 0.05).

CONCLUSIONS

We observed that there is an enhanced excitability of BR circuit in postparalytic facial syndrome and essential blepharospasm, which could be evoked by auditory stimulation in addition to trigeminal stimulation.

SIGNIFICANCE

The enhanced excitability in patients with EB and PFS probably originates from the final common pathway of BR circuit, namely facial motor or premotor neurons.

Heterotopic ischemic pain attenuates somatosensory evoked potentials induced by electrical tooth stimulation: Diffuse noxious inhibitory controls in the trigeminal nerve territory

The purpose of this study was to determine whether the late component of somatosensory evoked potentials (SEP) induced by electrical tooth stimulation and pain intensity are inhibited by heterotopic ischemic stimulation. The tourniquet pressure with 50 mmHg greater than the individual's systolic pressure was applied to the left upper arm for 10 min as ischemic conditioning stimulation. The late component of SEP and visual analogue scale (VAS) were recorded at 4 times and both were significantly decreased when ischemic conditioning stimulation was applied. The maximum reductions in SEP amplitude and the VAS value were 26.1% and 21.2%, respectively, during ischemic conditioning stimulation. After-effect was observed 5 min after removal of the conditioning stimulation. The present study revealed that heterotopic ischemic stimulation attenuated the late component of SEP induced by electrical tooth stimulation, triggering diffuse noxious inhibitory controls (DNIC) and after-effects in the trigeminal nerve territory. It was also suggested that the DNIC effect differs, depending on the intensity, kind, and quality of the test and conditioning stimuli.

The inter- and intra-individual variance in descending pain modulation evoked by different conditioning stimuli in healthy men

Background and purpose

Conditioned pain modulation (CPM) is a phenomenon in which pain is inhibited by heterotopic noxious stimulation. It is not known how the experimental condition affects the magnitude of the CPM response and the inter-and intra-individual variations. It is important to get the information of the test–retest reliability and inter–individual variations of CPM to apply CPM as a diagnostic tool or for screening analgesic compounds. This study evaluated (1) the magnitude of CPM, (2) the inter-individual coefficient of variation (inter-CV) and (3) the intra-individual coefficient of variation (intra-CV) to (A) different stimulus modalities to evoke CPM and (B) different assessment sites.

Methods

Twelve healthy men (age 19–38 years) participated in this study. Cold pressor pain (CPP) (immersing the hand into cold water), tourniquet pain (cuff around the upper arm) and mechanical pressure pain (craniofacial region) were used in randomized order as conditioning stimuli (CS). The test stimulus (TS) was pressure pain applied to the right masseter muscle, left forearm and leg (bilateral tibialis anterior: TA). The responses were pressure pain thresholds (PPT), pressure pain tolerance (PPTol) thresholds and the pain intensity which was assessed on a visual analogue scale (VAS, 0–10 cm) following 1.4 and 1.6 × PPT applied to TA. The TS was applied before, during and 10 min after the CS. The intra-individual CV was estimated between different days.

Results

CPP induced the most powerful CPM on PPT (66.3 ± 10.0% increase), VAS ratings (41.5 ± 5.3% reduction) and PPTol (32.6±4.6% increase), especially at TA, and resulted in the smallest inter-CV (41.4–60.1%). Independently of the CS, the inter-CV in general showed that the recordings from the orofacial region and the forearm had smaller values than from the leg. The smallest intra-CV value was obtained in pain ratings with CPP (27.0%).

Conclusions

This study suggests that (1) the CPP evokes the largest CPM, (2) the leg as the assessment site results in the largest CPM responses and (3) the CPP causes the smallest inter- and intra-CV.

Implication

The present investigation implicates that the CPP is the most efficient conditioning stimulus to induce CPM when assessed by pressure pain thresholds.

Nociceptive blink reflex and pain-related evoked potentials in hypnic headache

Background: Central facilitation of trigeminal pain processing and deficient habituation was observed in different headache and facial pain disorders. This overactivation seems to be primarily associated with chronic pain states.

Objective: To investigate the function of the trigeminal nociceptive system in patients with hypnic headache (HH).

Methods: Fifteen HH patients according International Classification of Headache Disorders II criteria and 15 age- and gender-matched healthy controls were investigated using the nociceptive blink reflex (nBR) and trigeminal pain-related evoked potentials (PREP).

Results: nBR and PREP responses showed no significant differences comparing HH patients and healthy controls. Moreover, no habituation deficit was detected in HH patients.

Conclusion: Central facilitation and change in habituation do not seem to be a crucial part in the pathophysiology of HH despite the chronic nature of this disease. Facilitation or habituation deficit does not seem to be exclusively related to chronic pain disorders in general. Further research is needed to illuminate the pathophysiology of HH.

Cutaneous limb inflammation produces analgesia to pressure pain in the ipsilateral forehead of healthy volunteers

To investigate the pain-modulatory effects of a local inflammatory stimulus on pain elsewhere in the body, capsaicin was applied topically to the forearm of 14 healthy female volunteers. Pressure-pain thresholds and sensitivity to sharpness were assessed on each side of the forehead twice per day during 48 hours of capsaicin treatment, and in the treated and contralateral forearm before and at the end of treatment. Heat was applied to the treated area to rekindle pain at times of forehead assessment. Hyperalgesia to sharpness, but not pressure pain, developed in the treated area whereas sensations remained stable in the contralateral forearm. Sharpness ratings decreased bilaterally in the forehead after 6 hours of treatment, and ipsilateral analgesia to pressure pain developed in the forehead when the capsaicin site was heated after 48 hours of treatment. These findings suggest that pain modulation involves unilateral regulatory mechanisms in addition to local and generalized pain control. The dissociated changes to sharpness and pressure pain indicate distinct cutaneous and deep central pain pathways.

PERSPECTIVE

The findings lend support to an increasing body of research which demonstrates that pain modulation involves hemilateral mechanisms in addition to local and generalized controls. Elucidation of mechanisms that modulate ipsilateral pain processing may help to clarify the pathophysiology of complex regional pain syndrome, which is characterized by hemilateral hyperalgesia.

Extrasegmental analgesia of heterotopic electroacupuncture stimulation on visceral pain rats

Acupuncture has been applied in the clinic to treat visceral pain for a long time. However, the underlying mechanism still remains unknown. In the present study, extrasegmental analgesia of electroacupuncture (EA) at orofacial acupoints on visceral pain rats was investigated. The results revealed that nociceptive EA stimulation applied at heterotopic acupoints or nonacupoints to activate A(δ) and/or C fibers induced c-fos expression in the paratrigeminal nucleus (PTN) and significantly inhibited acetic acid-induced abdominal contractions and c-fos expression in the nucleus of the solitary tract (NTS). However, non-nociceptive EA or non-EA stimulation applied at heterotopic acupoints was totally ineffective. After infraorbital nerves transaction or pretreated by capsaicin, the EA analgesia was dramatically inhibited. Snake venom pretreatment had no influence on this analgesia. Consequently, heterotopic EA stimulation trigger the pain-inhibiting effect of diffuse noxious inhibitory controls (DNIC), in which PTN-NTS secondary neural pathway may be involved and small-diameter (A(δ) and/or C) fibers are crucial.

Associative stimulation of the supraorbital nerve fails to induce timing-specific plasticity in the human blink reflex

BACKGROUND

Associative high-frequency electrical stimulation (HFS) of the supraorbital nerve in five healthy individuals induced long-term potentiation (LTP)-like or depression (LTD)-like changes in the human blink reflex circuit according to the rules of spike timing-dependent plasticity (Mao and Evinger, 2001). HFS given at the onset of the R2 component of the blink reflex (HFS(LTP)) produced a lasting facilitation of the R2, whereas HFS given shortly before R2 (HFS(LTD)) caused a lasting suppression of the R2. In patients with benign essential blepharospasm (BEB), a focal dystonia affecting the orbicularis oculi muscles, HFS(LTP) induced excessive LTP-like associative plasticity relative to healthy controls, which was normalized after botulinum toxin (BTX) injections (Quartarone et al, 2006).

METHODOLOGY/PRINCIPAL FINDINGS

We used HFS conditioning of the supraorbital nerve to study homeostatic metaplasticity of the blink reflex circuit in healthy subjects and dystonic patients. On separate days, we tested the conditioning effects on the R2 response and paired-pulse R2 inhibition after (i) HFS(LTP), (ii) HFS(LTP) followed by HFS(LTP), and (iii) HFS(LTP) followed by HFS(LTD). Controls also received (iv) HFS(LTD) alone and (v) a non-intervention protocol. In BEB patients, HFS(LTP) followed by HFS(LTD) was given before and after BTX treatment. We were not able to replicate the bidirectional timing-dependent effects of HFS(LTP) and HFS(LTD) alone. All HFS protocols produced a non-specific reduction of the R2 response and a relative decrease in paired-pulse inhibition. These R2 changes also occurred in controls when no HFS was applied. There was also no trace of a homeostatic response pattern in BEB patients before or after BTX treatment.

CONCLUSION/SIGNIFICANCE

Our data challenge the efficacy of associative HFS to produce bidirectional plasticity in the human blink reflex circuit. The non-specific decrease of the R2 response might indicate habituation of the blink reflex following repeated electrical supraorbital stimulation. The increase of inhibition after paired pulse stimulation might reflect homeostatic behaviour to prevent further down regulation of the R2 response to preserve the protection of this adverse-effects reflex.

Tonic pain abolishes cortical habituation of visual evoked potentials in healthy subjects

We investigated changes in visual cortex excitability by analyzing visual evoked potential (VEP) habituation in healthy subjects during tonic pain evoked by the cold-pressor test (CPT). We tested VEP amplitude habituation (slope of the linear regression line for N1-P1 amplitude from the 1st to 6th block of 100 sweeps) in 19 healthy volunteers during 4 experimental conditions: baseline; no-pain (hand held in warm water, 25 degrees C); pain (hand held in cold water, 2-4 degrees C); and the after-effects of tonic pain. During baseline and no-pain sessions, VEPs habituated normally across the 6 consecutive blocks (mean slope -.28 and -.18%), whereas during pain and its after-effects they failed to decrease (0%, and -.11%). Tonic pain induced by the CPT abolishes normal VEP habituation and the lack of habituation persists after the CPT is stopped. Tonic pain probably abolishes VEP habituation by acting on brainstem neural structures which modulate thalamo-cortical activation thereby changing visual cortex excitability.

PERSPECTIVE

This study shows that tonic pain alters visual cortex excitability, a brain region unrelated to pain processing. These changes probably reflect defensive strategies against pain. Extending the study from healthy volunteers to patients with migraine between attacks would offer the opportunity to investigate visual cortical excitability under conditions when baseline habituation is absent.

Blink reflexes in chronic tension-type headache patients and healthy controls

OBJECTIVE

To use the R2 component of nociceptive-specific blink reflex (nBR) to probe the trigeminal nociceptive system and compare chronic tension-type headache (CTTH) patients and healthy controls.

METHODS

Thirty patients with CTTH and 30 age- and sex-matched healthy control subjects were included. nBR were evoked by painful electrical pulses (0.5ms duration), delivered by a concentric electrode placed on the left lower forehead close to the supraorbital foramen. The EMG activity in the orbicularis oculi muscles was recorded bilaterally. The electrical stimulus intensities to evoke individual sensory threshold (Is) and pin-prick pain sensation (Ip) were assessed. A fixed stimulation intensity of 1.5x Ip was used to evoke the nBR. The perceived pain intensity of the electrical stimulus (I(BR)) was assessed by the subjects on a 0-10cm visual analogue scale (VAS).

RESULTS

Is did not differ between CTTH patients and controls (P=0.687) but were lower in females than in males (P=0.020). CTTH patients had higher scores on I(BR) than controls (P=0.026). ANOVA showed significantly higher pre-stimulus EMG values in CTTH patients on the left (stimulated) side (P<0.001), whereas there were no differences between males and females (P>0.168). There were no significant differences in the absolute values of the nBR, however CTTH patients had significantly lower values of the normalized root mean square (RMS) (P=0.035) and area under the curve (AUC) (P=0.042) of the nBR on the left side compared with control subjects with no sex-related differences (P>0.070). The onset latencies and duration were not significantly different between CTTH and control subjects or between female and male subjects (P>0.270). There was no significant correlation between any of the BR parameters (RMS, AUC) and clinical characteristics of CTTH (headache intensity and duration) (P>0.163).

CONCLUSIONS

The results of the present study did not detect a significantly different blink reflex response in CTTH patients, but suggested that painful electrical stimulation was associated with consistent increases in eye muscle activity on the same side.

SIGNIFICANCE

These findings add further information to central nociceptive pathways in CTTH patients.

Nitroglycerin sensitises in healthy subjects CNS structures involved in migraine pathophysiology: evidence from a study of nociceptive blink reflexes and visual evoked potentials

Nitroglycerin (NTG), a NO donor, induces an attack in migraine patients approximately 4-6 h after administration. The causative mechanisms are not known, but the long delay leaves room for a central effect, such as a change in neuronal excitability and synaptic transmission of various CNS areas involved in pain and behaviour including trigeminal nucleus caudalis and monoaminergic brain stem nuclei. To explore the central action of NTG, we have studied its effects on amplitude and habituation of the nociceptive blink reflex (nBR) and the visual evoked potential (VEP) before, 1 h and 4 h after administration of NTG (1.2 mg sublingual) or placebo (vehicle sublingual) in two groups of 10 healthy volunteers. We found a significant decrease in nBR pain and reflex thresholds both 1 and 4 h post-NTG. At the 4 h time point R2 latency was shorter (p=0.04) and R2 response area increased (p<0.01) after NTG but not after placebo. Habituation tended to become more pronounced after both NTG and placebo administration. There was a significant amplitude increase in the 5th VEP block (p=0.03) at 1h after NTG and in the 1st block (p=0.04) at 4 h. VEP habituation was replaced by potentiation at both delays after NTG; the change in habituation slope was significant at 1h (p=0.02). There were no significant VEP changes in subjects who received sublingual placebo. In conclusion, we found that in healthy subjects sublingual NTG, but not its vehicle, induces changes in a trigeminal nociceptive reflex and an evoked cortical response which are comparable to those found immediately before and during an attack of migraine. These changes could be relevant for the attack-triggering effect of NTG in migraineurs.

Supraspinal modulation of trigeminal nociception and pain

OBJECTIVE

This study examined modulation of trigeminal pain/nociception by 2 supraspinal mechanisms: emotional controls of nociception and diffuse noxious inhibitory controls.

BACKGROUND

Prior research suggests emotional picture viewing (emotional controls) and tonic noxious stimuli (diffuse noxious inhibitory controls) engage supraspinal mechanisms to modulate pain and nociceptive processes. It is currently unknown, however, whether emotional controls modulate trigeminal pain and nociception. Additionally, the influences of emotional controls and diffuse noxious inhibitory controls have not been compared in the same group of participants.

METHODS

Noxious electrodermal stimuli were delivered to the trigeminal nerve using a concentric electrode designed to selectively activate nociceptive fibers. Trigeminal nociception and pain were assessed (34 participants) from the nociceptive blink reflex and pain ratings, respectively. Emotional controls were engaged by presentation of standardized picture stimuli (pleasant, neutral, and unpleasant) shown to reliably evoke pleasure-induced inhibition and displeasure-induced facilitation of pain and nociception. Diffuse noxious inhibitory controls were engaged with a forearm ischemia task.

RESULTS

Trigeminal pain (self-report ratings) and nociception (blinks) were facilitated by unpleasant pictures and inhibited by pleasant pictures. Emotion induction (as assessed from trend analysis) explained 51% of the variance in trigeminal pain and 25% of the variance in trigeminal nociception. Additionally, forearm ischemia inhibited trigeminal pain but not nociception. The baseline vs ischemia comparison explained 17% of the variance in pain report and 0.1% of the variance in blinks. Supraspinal modulation by emotional controls and diffuse noxious inhibitory controls were uncorrelated.

CONCLUSIONS

Emotional controls and diffuse noxious inhibitory controls modulated trigeminal pain and emotional controls modulated trigeminal nociception. These procedures can be used to study supraspinal modulation of nociceptive processing in disorders of the trigeminal pain system, including headache.

Diffuse noxious inhibitory controls triggered by heterotopic CO2 laser conditioning stimulation decreased the SEP amplitudes induced by electrical tooth stimulation with different intensity at an equally inhibitory rate

The purpose of this study was to investigate (1) whether selective Adelta-fiber stimulation with CO(2) laser produces a diffuse noxious inhibitory controls (DNIC) effect in the trigeminal nerve territory; and (2) whether the DNIC effect differs depending on test stimulus intensities under constant conditioning stimuli. To examine whether the CO(2) laser radiation on the dorsum of the hand selectively stimulates Adelta-fibers, laser evoked potentials (LEP) were recorded. The mean peak latency of LEP was 381.4 ms. The findings revealed that the CO(2) laser selectively stimulated Adelta-fibers. Electrical tooth stimuli with 3 levels of intensities (1.2, 1.4, 1.6 times the pain threshold) were applied to subjects as test stimulation in randomized order, with a CO(2) laser stimulus of 18 mJ/mm(2) applied to the dorsum of the hand for 4 min as the noxious conditioning stimulus. Somatosensory evoked potentials (SEP) induced by electrical tooth stimulation were recorded and tooth pain intensity was evaluated using a visual analogue scale (VAS). The amplitudes of the SEP late component and VAS values were significantly decreased only during the conditioning stimuli without aftereffect. The inhibitory rates of the amplitudes ranged from 31.3% to 34.6% and the VAS values from 29.0% to 31.2%. There were no significant differences in their inhibitory rates between the 3 test stimulus intensities. The result indicated that selective Adelta-fiber stimulation with the CO(2) laser produces a DNIC effect in the trigeminal nerve territory and suggested that the DNIC effect does not depend on the intensity of the test stimuli.

Effect of heterotopic noxious conditioning stimulation on electrical and pressure pain thresholds in two different anatomical regions

OBJECTIVES

The aims of the study were to investigate the influence of heterotopic noxious conditioning stimulation (HNCS) on pain thresholds in the orofacial and spinal regions and to find out whether there are gender differences in this respect.

MATERIAL AND METHODS

Thirty healthy subjects (15 of each sex) with a mean (SD) age of 25.1 (4.4) years participated. Pain thresholds to electrical (EPT) and pressure stimuli (PPT) were recorded in the masseter muscle and 1st upper incisor (tooth), as well as in the fingertip, before, during, and 5 and 15 min after a cold pressor task to the contralateral hand immersed in ice-cold water for a maximum of 5 min.

RESULTS

With the exception of the EPT in the orofacial region, all pain thresholds increased during the HNCS and then returned to baseline during the 15 min follow-up. The significant changes in EPT were greater in the finger than in the tooth, while the changes in PPT were greater in the masseter muscle than in the finger. Electrical stimuli in the finger induced greater significant changes of pain thresholds than pressure. In the orofacial region, pressure induced greater significant changes in pain thresholds during HNCS than electrical stimuli did. The HNCS induced pain of high intensity and unpleasantness, i.e. varying between 5 and 10 on the numeric rating scale (NRS). There were no gender differences in the response to the HNCS.

CONCLUSION

We conclude that, in general, HNCS induced by cold pressor stimulation increases pain thresholds, but the magnitude of the effect differs between the orofacial region and the finger and is influenced by the tissue and type of test stimuli.