Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines

Low intensity transcranial electrical stimulation (TES) in humans, encompassing transcranial direct current (tDCS), transcutaneous spinal Direct Current Stimulation (tsDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation or their combinations, appears to be safe. No serious adverse events (SAEs) have been reported so far in over 18,000 sessions administered to healthy subjects, neurological and psychiatric patients, as summarized here. Moderate adverse events (AEs), as defined by the necessity to intervene, are rare, and include skin burns with tDCS due to suboptimal electrode-skin contact. Very rarely mania or hypomania was induced in patients with depression (11 documented cases), yet a causal relationship is difficult to prove because of the low incidence rate and limited numbers of subjects in controlled trials. Mild AEs (MAEs) include headache and fatigue following stimulation as well as prickling and burning sensations occurring during tDCS at peak-to-baseline intensities of 1-2mA and during tACS at higher peak-to-peak intensities above 2mA. The prevalence of published AEs is different in studies specifically assessing AEs vs. those not assessing them, being higher in the former. AEs are frequently reported by individuals receiving placebo stimulation. The profile of AEs in terms of frequency, magnitude and type is comparable in healthy and clinical populations, and this is also the case for more vulnerable populations, such as children, elderly persons, or pregnant women. Combined interventions (e.g., co-application of drugs, electrophysiological measurements, neuroimaging) were not associated with further safety issues. Safety is established for low-intensity 'conventional' TES defined as <4mA, up to 60min duration per day. Animal studies and modeling evidence indicate that brain injury could occur at predicted current densities in the brain of 6.3-13A/m2 that are over an order of magnitude above those produced by tDCS in humans. Using AC stimulation fewer AEs were reported compared to DC. In specific paradigms with amplitudes of up to 10mA, frequencies in the kHz range appear to be safe. In this paper we provide structured interviews and recommend their use in future controlled studies, in particular when trying to extend the parameters applied. We also discuss recent regulatory issues, reporting practices and ethical issues. These recommendations achieved consensus in a meeting, which took place in Göttingen, Germany, on September 6-7, 2016 and were refined thereafter by email correspondence.

Brainstem and Cervical Spinal Cord Fos Immunoreactivity Evoked by Nerve Growth Factor Injection into Neck Muscles in Mice

Although myofascial tenderness is thought to play a key role in the pathophysiology of tension-type headache, very few studies have addressed neck muscle nociception. The neuronal activation pattern following local nerve growth factor (NGF) administration into semispinal neck muscles in anaesthetized mice was investigated using Fos protein immunohistochemistry. In order to differentiate between the effects of NGF administration on c-fos expression and the effects of surgical preparation, needle insertion and intramuscular injection, the experiments were conducted in three groups. In the sham group ( n = 7) cannula needles were only inserted without any injection. In the saline ( n = 7) and NGF groups ( n = 7) 0.9% physiological saline solution or 0.8 µM NGF solution were injected in both muscles, respectively. In comparison with sham and saline conditions, NGF administration induced significantly stronger Fos immunoreactivity in the mesencephalic periaqueductal grey (PAG), the medullary lateral reticular nucleus (LRN), and superficial layers I and II of cervical spinal dorsal horns C1, C2 and C3. This activation pattern corresponds very well to central nervous system processing of deep noxious input. A knowledge of the central anatomical representation of neck muscle pain is an essential prerequisite for the investigation of neck muscle nociception in order to develop a future model of tension-type headache.

Laser-Evoked Cortical Potentials in Cluster Headache

Craniofacial nociceptive processing in patients ( n = 25) suffering from unilateral cluster headache was assessed by laser-evoked cortical potentials (LEPs). Latencies and amplitudes of late (N2, P2) and middle-latency (N1) LEPs were measured in chronic (CCH, n = 9) and episodic cluster headache (ECH, n = 17). In CCH patients on headache side N1c occurred later and P2 amplitude was smaller than on the healthy control side. In active periods of ECH patients P2 latency was shorter on the headache side. In remission periods of ECH patients the N2P2 ratio was lower on the headache side. In 19 out of 26 examinations in 25 headache patients LEP deviated from normative data in healthy controls ( n = 10) without any specific pattern of altered parameters. LEPs document pathological changes in craniofacial nociception in cluster headache. However, there seems to be no pathognomonic deviation pattern that enables reliable diagnosis of cluster headache and application of LEPs in further studies of pathophysiological mechanisms.

Modulation of vagal tone enhances gastroduodenal motility and reduces somatic pain sensitivity

BACKGROUND

The parasympathetic nervous system, whose main neural substrate is the vagus nerve, exerts a fundamental antinociceptive role and influences gastrointestinal sensori-motor function. Our research question was to whether combined electrical and physiological modulation of vagal tone, using transcutaneous electrical vagal nerve stimulation (t-VNS) and deep slow breathing (DSB) respectively, could increase musculoskeletal pain thresholds and enhance gastroduodenal motility in healthy subjects.

METHODS

Eighteen healthy subjects were randomized to a subject-blinded, sham-controlled, cross-over study with an active protocol including stimulation of auricular branch of the vagus nerve, and breathing at full inspiratory capacity and forced full expiration. Recording of cardiac derived parameters including cardiac vagal tone, moderate pain thresholds to muscle, and bone pressure algometry, conditioned pain modulation using a cold pressor test and a liquid meal ultrasonographic gastroduodenal motility test were performed.

KEY RESULTS

Cardiac vagal tone increased during active treatment with t-VNS and DSB compared to sham (p = 0.009). In comparison to sham, thresholds to bone pain increased (p = 0.001), frequency of antral contractions increased (p = 0.004) and gastroduodenal motility index increased (p = 0.016) with active treatment. However, no effect on muscle pain thresholds and conditioned pain modulation was seen.

CONCLUSIONS & INFERENCES

This experimental study suggests that this noninvasive approach with combined electrical and physiological modulation of vagal tone enhances gastroduodenal motility and reduces somatic pain sensitivity. These findings warrant further investigation in patients with disorders characterized with chronic pain and gastrointestinal dysmotility such as functional dyspepsia and irritable bowel syndrome.

Treatment of chronic migraine with transcutaneous stimulation of the auricular branch of the vagal nerve (auricular t-VNS): a randomized, monocentric clinical trial

Background

Aim of the study was assessment of efficacy and safety of transcutaneous stimulation of the auricular branch of the vagal nerve (t-VNS) in the treatment of chronic migraine.

Methods

A monocentric, randomized, controlled, double-blind study was conducted. After one month of baseline, chronic migraine patients were randomized to receive 25 Hz or 1 Hz stimulation of the sensory vagal area at the left ear by a handhold battery driven stimulator for 4 h/day during 3 months. Headache days per 28 days were compared between baseline and the last month of treatment and the number of days with acute medication was recorded The Headache Impact Test (HIT-6) and the Migraine Disability Assessment (MIDAS) questionnaires were used to assess headache-related disability.

Results

Of 46 randomized patients, 40 finished the study (per protocol). In the per protocol analysis, patients in the 1 Hz group had a significantly larger reduction in headache days per 28 days than patients in the 25 Hz group (−7.0 ± 4.6 vs. −3.3 ± 5.4 days, p = 0.035). 29.4 % of the patients in the 1 Hz group had a ≥50 % reduction in headache days vs. 13.3 % in the 25 Hz group. HIT-6 and MIDAS scores were significantly improved in both groups, without group differences. There were no serious treatment-related adverse events.

Conclusion

Treatment of chronic migraine by t-VNS at 1 Hz was safe and effective. The mean reduction of headache days after 12 weeks of treatment exceeded that reported for other nerve stimulating procedures.

Electrical low-frequency stimulation induces central neuroplastic changes of pain processing in man

Electrical low-frequency stimulation (LFS) inhibits pain perception and nociceptive processing as shown by psychophysical and electrophysiological means (long-term depression, LTD). Information regarding central mechanisms involved in LTD induction and maintenance are still missing. This study hypothesizes that electrical LFS induces changes in activation pattern of pain-related brain areas. Thirty-two electrophysiological and psychophysical experiments were performed in 16 healthy volunteers. Painful electrical test stimulation (0.125 Hz, 60 pulses) and conditioning LFS (1 Hz, 1200 pulses) were applied by a concentric electrode to the right hand. Test stimulation series were performed before (Pre) and after LFS (Post) or no stimulation period (Control). Volunteers rated pain perception according to a verbal rating scale (0-100). Somatosensory evoked cortical potentials were recorded with 64-channel electroencephalography. Individual dipole source modeling using CURRY software (Compumedics, Hamburg, Germany) yielded information about dipole location and strength. The strongest decrease in LFS-induced pain perception was shown after LFS (p < 0.01). Topographic distribution of cortical potentials revealed reproducible negative (N1, N2) and positive (P2) components. Dipole magnitude analysis showed a significant difference between Post LFS and Post Control for P2 (p < 0.01). P2 dipole location analysis yielded a significant posterior (p < 0.05) shift following LTD induction. Thus, data reveal central changes of pain processing after LTD induction. These experiments may help judging the potency of LTD as model for electrostimulation in future analgesic therapy.

Excitatory and inhibitory purinergic control of neck muscle nociception in anaesthetized mice

Tension-type headache is associated with noxious input from neck muscles. Due to the importance of purinergic mechanisms in muscle nociception, experimental studies typically inject alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-meATP). In contrast to native adenosine 5'-triphosphate (ATP), alpha,beta-meATP has a narrow receptor profile and remains stable in tissue. The present study administered alpha,beta-meATP or ATP in semi-spinal neck muscles in anaesthetized mice (n = 65) in order to address different effects in neck muscle nociception. The jaw-opening reflex monitored the impact of neck muscle noxious input on brainstem processing. Injection of alpha,beta-meATP induced reflex facilitation in a dose-dependent manner. In contrast, only the lowest ATP dosage evoked facilitation. Preceding P2Y(1) receptor blockade revealed facilitation even under high-dosage ATP. Ongoing facilitation after alpha,beta-meATP injection neutralized under subsequent activation of P2Y(1) receptors. Results demonstrate opposing excitatory P2X and inhibitory P2Y effects of ATP in neck muscle nociception. These mechanisms may be involved in the pathophysiology of neck muscle pain in man.

Nerve growth factor and ATP excite different neck muscle nociceptors in anaesthetized mice

Neck muscle nociception probably plays a major role in the pathophysiology of tension-type headache. Recent studies have demonstrated sustained facilitation of brainstem nociception due to noxious neck muscle input evoked by nerve growth factor (NGF) or alpha,beta-methylene ATP (ATP) in mice. Hypothesized different afferent pathways in NGF and ATP models were addressed by local application of tetrodotoxin (TTX) in neck muscles. Brainstem nociception was monitored in 55 anaesthetized mice by the jaw-opening reflex elicited by electrical tongue stimulation. Sole administration of 100 nmol/l ATP or 0.8 micromol/l NGF evoked sustained reflex facilitation for at least 95 min. Preceding TTX administration prevented ATP-induced facilitation, but was without effect on NGF. Subsequent administration of 100 nmol/l TTX reversed ATP-evoked facilitation, but was ineffective on NGF. Divergent effects of TTX suggest preferential excitation of group III muscle afferents by ATP and group IV by NGF. Thus, both models address different pathways in pericranial pain.

ATP induces sustained facilitation of craniofacial nociception through P2X receptors on neck muscle nociceptors in mice

Noxious input from neck muscles probably plays a key role in tension-type headache pathophysiology. ATP selectively excites group III and IV muscle afferents in vitro. Accordingly, ATP infusion into trapezius muscle induces strong pain and local tenderness in healthy man. The present study addresses the impact of ATP on neck muscle nociception in anaesthetized mice. Craniofacial nociceptive processing was tested by the jaw-opening reflex via noxious electrical tongue stimulation. Within 2 h after injection of 100 nmol/l or 1 micromol/l ATP into semispinal neck muscles, reflex integrals significantly increased by 114% or 328%, respectively. Preceding intramuscular administration of the P2X receptor antagonist PPADS (3-100 nmol/l) suppressed the ATP effect. Subsequent application of PPADS (100 nmol/l) caused a total recovery of facilitated reflex to baseline values. ATP induces sustained facilitation of craniofacial nociception by prolonged excitation of P2X receptors in neck muscles.

Long-term potentiation of orofacial sensorimotor processing by noxious input from the semispinal neck muscle in mice

Tension-type headache is the most common type of primary headaches but no conclusive concept of pathophysiology exists. This may be due to a lack of an appropriate animal model. This study addressed the hypothesis that noxious neck muscle input induces central sensitization of orofacial sensorimotor processing. The effect of hypertonic saline injection into the semispinal neck muscle on the jaw-opening reflex (JOR) was investigated in anaesthetized mice (n = 11). Hypertonic saline injection into the neck muscle facilitated the JOR for at least one hour: integral (+94.5%) and duration (+18.7%) increased, latency decreased (-7.5%). The reflex threshold decreased to 61% after injection. Isotonic saline injection into the neck muscle (n = 11) or hypertonic saline injection into a hindpaw muscle (n = 10) did neither change the reflex integral nor the threshold. Long-term potentiation of the JOR by noxious neck muscle input may be an appropriate model to investigate tension-type headache pathophysiology.

The effect of multiple stimuli on the modulation of the ‘nociceptive’ blink reflex

The 'nociceptive' blink reflex is a method of examining human trigeminal pain pathways. We explored temporal summation of this reflex by using a train of pulses, rather than a single pulse, and remote activation of diffuse noxious inhibitory control (DNIC), to improve reliability, flexibility and nociceptive specificity of this technique. The R2 component of the nociceptive blink reflex response (nR2) was assessed in 28 healthy volunteers using between 1 and 7 pulses per stimulus train (inter-pulse interval 5 ms). The effect of DNIC on single-, double-, and triple-pulse nR2 was investigated. Compared to single pulses, double and triple pulses increased the sensation of pain, reduced the tactile and pain thresholds, and facilitated the blink reflex responses (reduced onset latency, increased magnitude and persistence of nR2). The maximal reflex facilitation was achieved using a triple pulse. Higher pulse numbers had no additional facilitatory effect. Activation of the DNIC system using heterotopic pain suppressed the nR2 evoked by double and triple stimulation by 16 and 42%, respectively, but not the nR2 from a single pulse. Stimulation with double and triple pulses may be more suitable to study influences on nociceptive pathways than single pulses and may widen the methodological flexibility of the nociceptive blink reflex technique. This technique may be useful in studying the trigeminal nociceptive system with particular reference to primary headache disorders and their neuropharmacology.

Optimized stimulation and recording parameters of human 'nociception specific' blink reflex recordings

OBJECTIVE

Few non-invasive methods are available for studying trigeminal nociception in humans. The 'nociception specific' blink reflex (nBR) is elicited by a preferential stimulation of trigeminal nociceptive fibers and is highly sensitive to changes in trigeminal nociception. The aim of this study was to establish its optimal stimulation parameters.

METHODS

The inter-individual variability of R2 responses (41 healthy subjects), symmetry of R2 responses after right vs. left sided stimulation (25 subjects), optimal stimulation parameters (3 groups, each 16 subjects) and re-test reliability over time (15 subjects) were studied.

RESULTS

The nBR had a R2 response with a latency of 44.7+/-7.3ms, but no R1 response. The mean difference of R2 latencies (right vs. left sided stimulation) was 0.4+/-2.1(SD)ms, the mean ratio of the R2 response areas (RA) was 1.05+/-0.39 (SD). Thus, lateral differences of onset latencies greater than 4.6ms (0.4+4.2 (2SD)) or >1.83-fold (1.05+0.78 (2SD)) of RA can be assumed as outside of normal range. Long inter-block intervals of 7min minimized the cumulative habituation of R2 RAs to 23%, onset latencies remained constant. Re-test reliability over 4 weeks was high both for latencies (Crohnbach's alpha=0.85) and RAs (alpha=0.94).

CONCLUSIONS

The nBR with the established stimulation parameters could be used for studying trigeminal nociception.

Sensitization of trigeminal nociception specific for migraine but not pain of sinusitis

Trigeminal pain processing was studied in 14 patients with unilateral migraine attacks and 14 age- and sex-matched patients with comparable unilateral headache from frontal sinusitis. Using a nociception-specific blink reflex method (nBR), a facilitation of nBR responses predominantly on the headache side was observed in migraine, but not in sinusitis. The facilitation of trigeminal nociception may be specific for migraine rather than a consequence of peripheral pain such as frontal sinusitis.

Acute migraine headache: possible sensitization of neurons in the spinal trigeminal nucleus?

OBJECTIVE

To investigate trigeminal sensory processing in patients with migraine using a novel "nociception-specific" blink reflex.

METHODS

Seventeen patients with unilateral migraine headache were studied within 6 hours of onset. Blink reflexes were elicited with a standard stimulating electrode (standard blink reflex) and concentric stimulating electrode (nociception-specific blink reflex) during the acute migraine attack, after treatment with IV lysine acetylsalicylate (1,000 mg) or oral zolmitriptan (5 mg) and interictally.

RESULTS

After standard stimulation, no differences were detected for the R1 and R2 onset latencies and areas under the curve (AUC) between the different time points and the headache and nonheadache side. Nociception-specific stimulation revealed a shortening of R2 onset latencies (44.3 +/- 5.4 ms for headache side vs 48.9 +/- 5.8 ms for nonheadache side) during the acute migraine attack compared with the headache-free interval (49.8 +/- 5.3 vs 49.8 +/- 4.5 ms). The AUC of the R2 increased on the headache side by 680% and on the nonheadache side by 230% compared with the headache-free interval. Drug treatment parallel to pain relief increased the onset latencies (zolmitriptan: 48.0 +/- 8.2 ms for headache side vs 52.3 +/- 7.6 ms for nonheadache side; lysine acetylsalicylate: 48.0 +/- 5.0 ms for headache side vs 51.2 +/- 5.6 ms for nonheadache side) and reduced the AUC of R2 (zolmitriptan by 45% and lysine acetylsalicylate by 48%).

CONCLUSION

The data suggest temporary sensitization of central trigeminal neurons during acute migraine attacks.

Meningeal nociception: electrophysiological studies related to headache and referred pain

Headaches, which are usually referred to characteristic sites of the skull, are believed to involve meningeal nociceptors located in the dura mater encephali. Animal experiments show that these meningeal nociceptors are polymodal and usually highly sensitive to mechanical stimulation. These properties are also characteristic for the second order neurons in the spinal trigeminal nucleus, most of which receive convergent input from facial receptive sites. Sensitization of primary and secondary neurons by chemical irritants to mechanical stimuli may be an important mechanism in the generation of headaches. The convergent input from extracranial structures, which seems to be differentially organized in rodents and man, may explain the typical features of referred headache. Targets for analgesics used in the therapy of headaches (non-steroidal antiinflammatory drugs, 5-HT(1) receptor agonists) are probably meningeal nociceptors and different sites of the central trigeminal nociceptive and antinociceptive pathways.

Is the response pattern of on- and off-cells in the rostral ventromedial medulla to noxious stimulation independent of stimulation site?

The classification of on- and off-cells in the rostral ventromedial medulla is based on the response pattern to noxious tail heat. It is generally assumed that on- and off-cells respond equally to noxious stimulation anywhere on the body surface, but so far this assumption has not been systematically examined. In the present study the effects of noxious thermal and mechanical stimuli applied to the tail, the extremities and the craniofacial region on the extracellularly recorded activity of 13 on- and 23 off-cells were investigated in lightly anesthetized rats. In 3 out of 13 on-cells and 11 out of 23 off-cells the response pattern evoked by noxious stimulation of the extremities or the craniofacial region differed from the response pattern elicited by noxious tail heat. In comparison with the response pattern to noxious tail heat, stimulation of the extremities or the craniofacial region reproducibly evoked opposite reactions in 2 on- and 9 off-cells and did not change neuronal activity in one on- and 2 off-cells. The results of the present study raise the question of whether the response pattern of on- and off-cells in the rostral ventromedial medulla can be sufficiently predicted by a classification that is exclusively based on the cellular behavior to noxious heat stimulation of the tail.

Are 'neutral cells' in the rostral ventro-medial medulla subtypes of on- and off-cells?

The classification of cells in the rostral ventromedial medulla (RVM) is based on the response pattern to noxious tail heat: on-cell activity increased, off-cell activity decreased, and activity of neutral cells is unaffected by noxious heat tail stimulation. It is generally assumed that on-, off- and neutral cells respond equally to noxious stimulation applied anywhere on the body surface, but so far this assumption has not been systematically examined. In the present study the effects of thermal and mechanical stimuli applied to the tail, the extremities and the orofacial region on the extracellularly recorded activity of 14 neutral cells were investigated in lightly anesthetized rats. Although the neutral cells did not respond to noxious tail heat, all of them responded to most of the other stimuli in an on- or off-manner. Especially cell responses to pinch stimuli applied to the skin of the ear, the forehead and the nose differed from the neutral behavior. The fact that the neutral cells in the present study responded in an off- or on-manner by applying noxious stimuli different from noxious tail heat suggests that these cells are possibly subtypes of on- and off-cells in the RVM.

Neither a general flexor nor a withdrawal pattern of nociceptive reflexes evoked from the human foot

In humans motor reactions to noxious radiant heat stimulation of the sole and the dorsum of the foot do not resemble a locally specific pattern of multiple modular withdrawal reflexes but rather a general flexion reflex pattern with a few exceptions which did neither fit a withdrawal nor a flexion reflex pattern. The partly observed excitatory feed back to foot extensors from nociceptive afferents of the foot sole is functionally discussed as a foot stabilizing mechanism under particular conditions.

A new method to increase nociception specificity of the human blink reflex

OBJECTIVE

The medullary R2 response of the blink reflex can be elicited by innocuous and noxious stimuli. The purpose of this study was to elicit a nociception specific R2 response with a new surface electrode.

METHODS

In 10 healthy subjects the blink reflex was elicited using a standard (10-15 mA) and a new concentric surface electrode type (0.6-1.6 mA) which produces a pin-prick-like pain.

RESULTS

After topical local anaesthesia with lignocaine/prilocaine R1 was unchanged, R2 was attenuated by 12% after standard stimulation but was almost abolished (-91%) with the new electrode type.

CONCLUSION

Stimulation with low stimulus intensities but electrode-dependent high current density allows preferential depolarization of superficial nociceptive A-delta fibres. This new method is less traumatic than others and is useful in the study of trigeminal nociception.

Acetylsalicylic acid inhibits meningeal nociception in rat

Although acetylsalicylic acid (ASA) is one of the most commonly administered drugs in the treatment of acute headaches, the sites of its action and the mechanisms of its therapeutic efficacy are still unclear. In this study using extracellular recording we examined the effects of ASA on spontaneous and mechanically evoked activities of neurons within the medullary dorsal horn with input from the parietal dura mater in rat. Their dural receptive fields were identified by von Frey filaments and found to be mainly located at the medial meningeal artery. All units showed spontaneous activity and had convergent input from the face. Neuronal activities were recorded before and after intravenously applied ASA (30 mg/kg) in 13 and saline in four units. Systemic application of ASA inhibited spontaneous and mechanically evoked activity within 15 min after application. Additionally, neuronal activities were recorded before, during and after topical application of ASA (1 mg/ml) onto the parietal dura mater in 5 units. Topically applied ASA inhibited the mechanically evoked activity, whereas the spontaneous activity remained unchanged. It is concluded, that there are different effects of systemic and topical ASA on trigeminal neuronal activity, which may be due to both central and peripheral mechanisms.

Afferent input to the medullary dorsal horn from the contralateral face in rat

Sensory deficits on the contralateral face in Wallenberg's lateral medullary syndrome (WS) may be due to an involvement of the crossing contralateral trigeminothalamic tract. Alternatively, neurons within the medullary dorsal horn (MDH) get input from the contralateral face. MDH neurons supplying the supraorbital nerve area in rat were recorded by electrophysiological techniques. In this first study on contralateral projections, about 60% of the neurons received excitatory afferent input from the contralateral face as well as the ipsilateral supraorbital area. Thus, contralateral sensory deficits in WS may be due to an involvement of these neurons.

Conditioning of the masseter inhibitory reflex by homotopically applied painful heat in humans

During contraction of the jaw-closing muscles, afferent input from the intraoral and perioral region can elicit two bilateral suppression periods (SP1 and SP2, respectively) in the masseter electromyogram (EMG). Non-painful electrical stimulation 2 cm from the left labial commissure was used in the present study to evoke these trigeminal inhibitory reflexes. The subjects maintained a level of 50% of their maximum masseter EMG. The degree of suppression was quantified as the percentage suppression of the mean EMG activity in a fixed post-stimulus interval (SP2, 40-90 ms). Further, brief (200 ms) painful radiant heat conditioning stimuli were delivered to the ipsilateral cheek, in order to investigate the influence of nociceptive input on the (non-nociceptive) trigeminal masseter inhibitory reflex. Nine different conditions combining radiant heat and electrical stimuli were used. Twelve stimuli were presented for each condition. The radiant heat preceded the electrical test stimuli by fixed inter-stimulus intervals (ISI), ranging from 100 ms to 500 ms. At 250-350 ms ISIs, the bilateral SP2 suppression was significantly reduced to less than 10%, in comparison to an average suppression degree of 32.5% without conditioning stimuli. The subjects perceived the heat stimulus before the electrical stimulus for a majority of the 12 pairs of stimuli at these ISIs. No differences were found in the VAS ratings for the different conditions. For the contralateral SP1, larger suppression was seen for the 300 ms ISI compared with stimulation without conditioning heat stimuli. Onset and offset for the SP1 was, however, only detected in three subjects using a criteria of 20% suppression of the pre-stimulus activity. A pre-pulse inhibitory effect onto inter-neurons in the SP2 pathways or habituation of the same inter-neurons by the heat stimuli are suggested as possible explanations for the interaction between the non-nociceptive and nociceptive input in the present study.

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

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

Convergence of nociceptive and non-nociceptive input onto the medullary dorsal horn in man

Referred pain arising in orofacial pain states is probably due to convergence of different somatosensory input onto the medullary dorsal horn (MDH). To examine convergence between nociceptive and non-nociceptive input onto the MDH, the blink reflex (BR) was applied. R1- and R2-components can be evoked by innocuous stimuli, but only the R2 is elicited by painful heat. The BR was elicited by innocuous electrical stimuli applied to the supraorbital nerve. A conditioning painful heat pulse which did not evoke any BR was homotopically applied to the left forehead preceding the electrical stimulus by 75 ms. While R1 remained unchanged, the R2 was facilitated by about 30%. This study demonstrates a convergence of low-threshold mechanoreceptive and nociceptive inputs onto interneurons of the MDH in man.

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

The blink reflex consists of an early, pontine R1-component and a late, medullary R2-component. R1 and R2 can be evoked by innocuous stimuli, but only the R2 also by painful heat, suggesting that the R2 is mediated by wide dynamic range neurons (WDR) of the spinal trigeminal nucleus. Remote noxious stimuli suppress the activity in WDR neurons via activation of diffuse noxious inhibitory controls (DNIC), whereas low-threshold mechanoreceptive neurons (LTM) are unaffected. In order to characterize the trigeminal interneurons of R1 and R2 we investigated the modulation of the blink reflex by remote painful heat. The blink reflex was elicited in 11 healthy subjects by innocuous electrical pulses applied to the left supraorbital nerve. The remote, painful heat stimuli were applied by a Peltier type thermode to the left volar forearm. Remote painful heat of 44 to 46 degreesC significantly suppressed the R2 by 15% (p<0.01), while the R1 remained unchanged. These results provide further evidence that the R2 is mediated by medullary WDR neurons and the R1 by pontine LTM neurons.

Convergence of nociceptive and non-nociceptive inputs onto spinal reflex pathways to the tibialis anterior muscle in humans

The interaction of low-threshold mechanoreceptive and nociceptive inputs onto spinal neurones probably plays a major role in the pathophysiology of the clinical sign of allodynia. This phenomenon was investigated by modulation of the early component of the flexor reflex (FR) in the tibialis anterior (TA) muscle, elicited by electrical stimulation of the medial plantar nerve at the sole of the foot, by homotopically applied painful heat in humans. This early reflex with an electrical threshold of 2.7-fold the detection threshold and a mean afferent conduction velocity of 49 m s-1 is a non-nociceptive FR. When applying conditioning painful heat (46 degrees C) to the sole of the foot this reflex was significantly increased by a factor of 3.4 (non-painful electrical stimuli; n = 5) and 2.0 (painful electrical stimuli; n = 11). The onset latencies were significantly shortened from 74.2 to 64.0 ms and 69.6 to 63.7 ms, respectively. A late nociceptive FR was also facilitated. While the Hoffmann reflex (HR) in the TA muscle was nearly abolished by painful heat, the HR in the soleus (SO) muscle remained unchanged. These data suggest a convergence of low-threshold mechanoreceptive and nociceptive inputs onto spinal reflex pathways in humans, probably at an interneuronal level in humans.

The Hoffmann reflex of human plantar foot muscles

Electrical stimulation of the tibial nerve in the popliteal fossa evoked an M wave (10.9 ms) and a late reflex response (38.1 ms) in the plantar foot muscles of all 10 volunteers. The late response had a somewhat lower electrical threshold than the corresponding M wave (8.5 versus 9 mA), and reached a maximum of amplitude when the stimulus intensity was increased, but was strongly suppressed by further increased intensity. A more distal stimulation of the tibial nerve at the ankle shortened the onset latency of the M wave and lengthened that of the late response. The reflex was facilitated by activation of synergists and inhibited by activation of antagonists. We showed that the late response was contaminated neither by volume conducted activity from the soleus muscle, as shown by intramuscular recordings from the abductor hallucis muscle, nor by a F wave, as shown by double stimulation. In summary, we conclude that this late response in human plantar foot muscles corresponded to an H reflex, which may be used to assess alterations of distal motoneuronal excitability.

Cerebral potentials are not evoked by activation of Golgi tendon organ afferents in human abductor hallucis muscle

The earliest components of the somatosensory evoked potentials (SSEP) are generated by excitation of the fastest low-threshold, thick myelinated group I afferents. Ib afferents belonging to this group innervate Golgi tendon organs and are excited almost selectively by a muscle twitch. Contraction of the abductor hallucis muscle was evoked by intramuscularly placed teflon insulated needle electrodes in ten healthy volunteers. We failed to demonstrate any cortical activity (over Cz' vs. Fz) following isolated muscle contractions. Our results do not point to a significant projection of group Ib afferents to the human somatosensory cortex at least with the small numbers of fibers activated by intramuscular stimulation of abductor hallucis muscle.

Nociceptive masseter inhibitory reflexes evoked by laser radiant heat and electrical stimuli

Electrical stimulation of the mental nerve evokes two suppression periods SP1 and SP2 in masseter muscle activity bilaterally. In order to investigate a possible nociceptive origin of the suppression periods, we compared the reflex responses evoked by electrical stimulation and by selective activation of nociceptors in hairy skin using painful infrared laser stimuli. The SP was elicited during more than 90% maximal voluntary contraction. Thresholds for detection, pain, and SP in the mental nerve area were determined by the method of limits. A suppression period was evoked by laser stimuli in nine of ten subjects bilaterally. The mean onset latency was 46.9 ms, the mean duration 58.9 ms. The electrical threshold of SP1 (9 mA) was 7.7 x I(0), about 20% smaller than I(P), and significantly higher than I(SP2) (4.7 mA). The onset latencies and durations were 11.7 ms and 21 ms for SP1, and 45 ms and 42.7 ms for SP2 (stimulus intensity 2 x I(P)). The mean difference in onset latencies between laser SP and electrically evoked SP1 was 35.1 +/- 6.2 ms, which closely matches the nociceptor response latency to a laser heat pulse. Based on the threshold and the onset latency we conclude that at least SP1 and laser SP are nociceptive in origin and mediated by group III fibers.

Pain-evoked blink reflex

The electrically evoked blink reflex (BR) consists of an ipsilateral R1 component (R1) at 11 ms and two bilateral components R2 at 33 ms and R3 at 83 ms. It is still unclear whether the R2 is mediated by activation of tactile or nociceptive afferents. For testing the nociceptive hypothesis, nociceptors of the supraorbital nerve were selectively activated by infrared laser stimuli in 10 subjects. Only painful laser stimuli evoked a bilateral early polyphasic BR response (LR2) at 71 ms. Stimulation of infraorbital and mental nerve dermatomes was equally effective. A late bilateral reflex response at 130 ms was occasionally observed. Regarding the nociceptor activation time of about 40 ms, onset latencies were within the range of the electrically evoked R2 and R3, respectively. The good accordance of R2 and LR2 may be due to activation of identical nociceptive fibers or to convergence of electrically evoked tactile and laser-elicited nociceptive input onto common multireceptive neurons.

The R3 component of the blink reflex: normative data and application in spinal lesions

The clinical value of the R3 component (R3) of the blink reflex (BR) for differentiating between lesions at the medulla oblongata and cervical spinal cord level is still unclear. In 50 healthy volunteers (25 women, 25 men, aged 20-75 years) reproducible ipsi- and contralateral R3 responses could be evoked showing a mean onset latency of 84 ms, a duration of 32 ms and a side-to-side difference of almost 3 ms. The latency increased with age. Eleven patients with lesions of the cervical spinal cord (segments C1-C6) showed normal R3 latencies. In 4 patients with pathology of the brain-stem, however, R3 was abnormal, showing identical changes as observed with the R2 component. Our results suggest that the reflex arc of R3 does not descend to the cervical spinal cord but within the brain-stem possibly takes the same connections as the R2 component.

Electrophysiological brainstem investigations in obstructive sleep apnoea syndrome

Phasic inspiratory genioglossus activity prevents pharyngeal airway collapse in healthy subjects during sleep and is diminished or absent in obstructive sleep apnoea syndrome (OSAS), thus leading to pharyngeal obstruction. Case reports of OSAS after pontomedullary lesions indicate that impaired inspiratory genioglossal activity may result from brainstem lesions. We therefore investigated brainstem functions in 18 awake patients with OSAS using brainstem auditory evoked potentials, blink reflex, masseter reflex, masseter inhibitory reflex (in 11 of 18 patients), magnetic evoked potentials of the tongue and electrooculography with vestibular testing. Fifteen of 18 patients showed no electrophysiological abnormalities. One patient had a left pontine and two patients a bilateral pontomesencephalic lesion, although a causal connection with OSAS was not conclusively confirmed. Our results do not support the assumption of a relevant structural brainstem lesion in OSAS patients with normal neurological findings.

The pterygoid reflex in man and its clinical application

A technique for eliciting and recording the stretch reflex (R) of the medial pterygoid muscle (Pter) is described. The latency was 6.9 +/- 0.43 ms in 23 healthy volunteers (mean age 23.7 years) showing a side-to-side difference of 0.29 +/- 0.21 ms. The PterR latencies were little shorter and side-to-side differences little greater than of the masseter reflex. Observations in 5 selected patients with small brainstem lesions suggest that the neurons of the PterR afferents form a cluster within the caudal portion of the trigeminal mesencephalic nucleus. Testing the masseter and pterygoid reflexes provides a more precise localization of small ponto-mesencephalic lesions.

Cerebral potentials elicited by mechanical stimuli to the human leg: influence of artifacts

Mechanical stimulation with a reflex hammer was applied to the quadriceps muscle tendon of healthy volunteers and patients. The time-locked electrical signals were recorded from the scalp. In all cases, reproducible potentials could be recorded, with latencies in a range of 20 ms to 100 ms. The potentials recorded in patients under complete spinal anesthesia were similar to those derived before the anesthesia. In brain dead patients who showed absence of median nerve or posterior tibial nerve SEP, reproducible potentials after mechanical stimuli could be recorded as well. The results suggest that the hammer taps lead to mechanical shock waves which are propagated along the body producing time locked artifacts in the EEG record which are not eliminated by the averaging technique.