The cortical silent period is shortened in migraine with aura

Electrophysiological findings in migraine may reflect abnormal synaptic plasticity mechanisms: A narrative review

BACKGROUND

The cyclical brain disorder of sensory processing accompanying migraine phases lacks an explanatory unified theory.

METHODS

We searched Pubmed for non-invasive neurophysiological studies on migraine and related conditions using transcranial magnetic stimulation, electroencephalography, visual and somatosensory evoked potentials. We summarized the literature, reviewed methods, and proposed a unified theory for the pathophysiology of electrophysiological abnormalities underlying migraine recurrence.

RESULTS

All electrophysiological modalities have determined specific changes in brain dynamics across the different phases of the migraine cycle. Transcranial magnetic stimulation studies show unbalanced recruitment of inhibitory and excitatory circuits, more consistently in aura, which ultimately results in a substantially distorted response to neuromodulation protocols. Electroencephalography investigations highlight a steady pattern of reduced alpha and increased slow rhythms, largely located in posterior brain regions, which tends to normalize closer to the attacks. Finally, non-painful evoked potentials suggest dysfunctions in habituation mechanisms of sensory cortices that revert during ictal phases.

CONCLUSION

Electrophysiology shows dynamic and recurrent functional alterations within the brainstem-thalamus-cortex loop varies continuously and recurrently in migraineurs. Given the central role of these structures in the selection, elaboration, and learning of sensory information, these functional alterations suggest chronic, probably genetically determined dysfunctions of the synaptic short- and long-term learning mechanisms.

Attenuated alpha oscillation and hyperresponsiveness reveals impaired perceptual learning in migraineurs

Background

Anomalous phantom visual perceptions coupled to an aversion and discomfort to some visual patterns (especially grating in mid-range spatial frequency) have been associated with the hyperresponsiveness in migraine patients. Previous literature has found fluctuations of alpha oscillation (8-14 Hz) over the visual cortex to be associated with the gating of the visual stream. In the current study, we examined whether alpha activity was differentially modulated in migraineurs in anticipation of an upcoming stimulus as well as post-stimulus periods.

Methods

We used EEG to examine the brain activity in a group of 28 migraineurs (17 with aura /11 without) and 29 non-migraineurs and compared their alpha power in the pre/post-stimulus period relative to the onset of stripped gratings.

Results

Overall, we found that migraineurs had significantly less alpha power prior to the onset of the stimulus relative to controls. Moreover, migraineurs had significantly greater post-stimulus alpha suppression (i.e event-related desynchronization) induced by the grating in 3 cycles per degree at the 2nd half of the experiment.

Conclusions

These findings, taken together, provide strong support for the presence of the hyperresponsiveness of the visual cortex of migraine sufferers. We speculate that it could be the consequence of impaired perceptual learning driven by the dysfunction of GABAergic inhibitory mechanism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01410-2.

Noninvasive Neuromodulation in Headache: An Update

Background

Migraine is a common disabling primary headache condition. Although strives have been made in treatment, there remains an unmet need for safe, effective acute, and preventative treatments. The promising concept of neuromodulation of relevant neuronal targets in a noninvasive fashion for the treatment of primary headache disorders has led to the trial of numerous devices over the years.

Objective

We aimed to review the evidence on current neuromodulation treatments available for the management of primary headache disorders.

Methods

Randomized controlled trial as well as open-label and real-world studies on central and peripheral cephalic and noncephalic neuromodulation modalities in primary headaches were critically reviewed.

Results

The current evidence suggests a role of single-pulse transcranial magnetic stimulation, supraorbital nerve stimulation, and remote noncephalic electrical stimulation as migraine abortive treatments, with stronger evidence in episodic rather than in chronic migraine. Single-pulse transcranial magnetic stimulation and supraorbital nerve stimulation also hold promising evidence in episodic migraine prevention and initial positive evidence in chronic migraine prevention. More evidence should clarify the therapeutic role of the external vagus nerve stimulation and transcranial direct current stimulation in migraine. However, external vagus nerve stimulation may be effective in the acute treatment of episodic but not chronic cluster headache, in the prevention of hemicrania continua and paroxysmal hemicrania but not of short-lasting neuralgiform headache attacks. The difficulty in setting up sham-controlled studies has thus far prevented the publication of robust trials. This limitation along with the cost of these therapies has meant that their use is limited in most countries.

Conclusion

Neuromodulation is a promising nonpharmacological treatment approach for primary headaches. More studies with appropriate blinding strategies and reduction of device cost may allow more widespread approval of these treatments and in turn increase clinician's experience in neuromodulation.

Increased Cortical Excitability in Female Migraineurs: A Transcranial Magnetic Stimulation Study Conducted in the Preovulatory Phase

BACKGROUND AND PURPOSE

The cerebral cortex has been the focus of investigations of the pathogenesis of migraine for a long time. Transcranial magnetic stimulation (TMS) is a safe and effective technique for evaluating cortex excitability. Previous studies of the duration of the cortical silent period (CSP)-a measure of intracortical inhibition-in migraine patients have yielded conflicting results. We aimed to characterize cortical excitability by applying TMS to female migraineurs during the preovulatory phase of the menstrual cycle, in order to eliminate the effects of variations in sex hormones.

METHODS

We enrolled 70 female subjects: 20 migraine with aura (MA) patients, 20 migraine without aura (MO) patients, and 30 healthy controls. We measured the CSP, resting motor threshold (rMT), and motor evoked potential (MEP) induced by TMS to evaluate cortical excitability during the preovulatory phase of the menstrual cycle.

RESULTS

The CSP was shorter in MA patients (88.93±3.82 ms, mean±SEM) and MO patients (86.98±2.72 ms) than in the control group (109.06±2.85 ms) (both p=0.001), and did not differ significantly between the MA and MO groups (p=0.925). The rMT did not differ significantly among the groups (p=0.088). MEPmax was higher in MA patients than in healthy controls (p=0.014), while that in MO patients did not differ from those in MA patients and healthy controls (p=0.079 and p=0.068).

CONCLUSIONS

We detected a shorter CSP in both MA and MO patients. This finding may indicate the presence of motor cortex hyperexcitability, which is probably due to reduced GABAergic neuronal inhibition in migraine.

Understanding intracortical excitability in phantom limb pain: A multivariate analysis from a multicenter randomized clinical trial

OBJECTIVES

To explore associations of intracortical excitability with clinical characteristics in a large sample of subjects with phantom limb pain (PLP).

METHODS

Ancillary study using baseline and longitudinal data from a large multicenter randomized trial that investigated the effects of non-invasive brain stimulation combined with sensorimotor training on PLP. Multivariate regression modeling analyses were used to investigate the association of intracortical excitability, measured by percentages of intracortical inhibition (ICI) and facilitation (ICF) with clinical variables.

RESULTS

Ninety-eight subjects were included. Phantom sensation of itching was positively associated with ICI changes and at baseline in the affected hemisphere (contralateral to PLP). However, in the non-affected hemisphere (ipsilateral to PLP), the phantom sensation of warmth and PLP intensity were negatively associated with ICI (both models). For the ICF, PLP intensity (baseline model only) and age (longitudinal model) were negatively associated, while time since amputation and amputation level (both for longitudinal model only) were positively associated in the affected hemisphere. Additionally, use of antidepressants led to lower ICF in the non-affected hemisphere for the baseline model while higher amputation level also led to less changes in the ICF.

CONCLUSION

Results revealed clear associations of clinical variables and cortical excitability in a large chronic pain sample. ICI and ICF changes appear not to be mainly explained by PLP intensity. Instead, other variables associated with duration of neuroplasticity changes (such as age and duration of amputation) and compensatory mechanisms (such as itching and phantom limb sensation) seem to be more important in explaining these variables.

Could cathodal transcranial direct current stimulation modulate the power spectral density of alpha-band in migrainous occipital lobe?

OBJECTIVE

To identify the correlation between cathodal transcranial direct current stimulation (tDCS) and the power spectral density (PSD) of alpha-band on the occipital lobe of migraineurs.

METHODS

Firstly, a cross-sectional study was performed to compare the PSD of alpha-band in the occipital cortex of 25 migraineurs versus 10 healthy volunteers in resting state and during repetitive light stimuli (RLS). Secondly, the patients participated in 12 sessions of cathodal (n = 11) or sham tDCS (n = 10) over the primary visual cortex (V1) to investigate the alpha-band PSD.

RESULTS

The alpha-band PSD on the occipital cortex was higher in migraineurs than healthy subjects in resting state and lower during the first train of RLS. Cathodal tDCS over the V1 reduced the alpha-band occipital activity in resting state but did not interfere with the functional responses to RLS when light stimulation was turned on.

CONCLUSIONS

Our findings suggest that the occipital cortex of migraineurs is hypoactive in the baseline condition, but becomes hyperactive when stimulated by light. Cathodal tDCS over the V1 decreases baseline alpha PSD in patients, possibly modulating the involved neuronal circuitries, but it cannot interfere once photic stimulation starts.

The visual system as target of non-invasive brain stimulation for migraine treatment: Current insights and future challenges

The visual network is crucially implicated in the pathophysiology of migraine. Several lines of evidence indicate that migraine is characterized by an altered visual cortex excitability both during and between attacks. Visual symptoms, the most common clinical manifestation of migraine aura, are likely the result of cortical spreading depression originating from the extrastriate area V3A. Photophobia, a clinical hallmark of migraine, is linked to an abnormal sensory processing of the thalamus which is converged with the non-image forming visual pathway. Finally, visual snow is an increasingly recognized persistent visual phenomenon in migraine, possibly caused by increased perception of subthreshold visual stimuli. Emerging research in non-invasive brain stimulation (NIBS) has vastly developed into a diversity of areas with promising potential. One of its clinical applications is the single-pulse transcranial magnetic stimulation (sTMS) applied over the occipital cortex which has been approved for treating migraine with aura, albeit limited evidence. Studies have also investigated other NIBS techniques, such as repetitive TMS (rTMS) and transcranial direct current stimulation (tDCS), for migraine prophylaxis but with conflicting results. As a dynamic brain disorder with widespread pathophysiology, targeting migraine with NIBS is challenging. Furthermore, unlike the motor cortex, evidence suggests that the visual cortex may be less plastic. Controversy exists as to whether the same fundamental principles of NIBS, based mainly on findings in the motor cortex, can be applied to the visual cortex. This review aims to explore existing literature surrounding NIBS studies on the visual system of migraine. We will first provide an overview highlighting the direct implication of the visual network in migraine. Next, we will focus on the rationale behind using NIBS for migraine treatment, including its effects on the visual cortex, and the shortcomings of currently available evidence. Finally, we propose a broader perspective of how novel approaches, the concept of brain networks and the integration of multimodal imaging with computational modeling, can help refine current NIBS methods, with the ultimate goal of optimizing a more individualized treatment for migraine.

Multisensorial Perception in Chronic Migraine and the Role of Medication Overuse

Multisensory processing can be assessed by measuring susceptibility to crossmodal illusions such as the Sound-Induced Flash Illusion (SIFI). When a single flash is accompanied by 2 or more beeps, it is perceived as multiple flashes (fission illusion); conversely, a fusion illusion is experienced when more flashes are matched with a single beep, leading to the perception of a single flash. Such illusory perceptions are associated to crossmodal changes in visual cortical excitability. Indeed, increasing occipital cortical excitability, by means of transcranial electrical currents, disrupts the SIFI (ie, fission illusion). Similarly, a reduced fission illusion was shown in patients with episodic migraine, especially during the attack, in agreement with the pathophysiological model of cortical hyperexcitability of this disease. If episodic migraine patients present with reduced SIFI especially during the attack, we hypothesize that chronic migraine (CM) patients should consistently report less illusory effects than healthy controls; drugs intake could also affect SIFI. On such a basis, we studied the proneness to SIFI in CM patients (n = 63), including 52 patients with Medication Overuse Headache (MOH), compared to 24 healthy controls. All migraine patients showed reduced fission phenomena than controls (P < .0001). Triptan MOH patients (n = 23) presented significantly less fission effects than other CM groups (P = .008). This exploratory study suggests that CM - both with and without medication overuse - is associated to a higher visual cortical responsiveness which causes deficit of multisensorial processing, as assessed by the SIFI. PERSPECTIVE: This observational study shows reduced susceptibility to the SIFI in CM, confirming and extending previous results in episodic migraine. MOH contributes to this phenomenon, especially in case of triptans.

Migrainous Infarction And Cerebral Vasospasm: Case Report And Literature Review

Migrainous infarction (MI) is a rare complication of migraines that accounts for 0.5-1.5% of all ischemic strokes. Although the pathogenesis of MI is still debated, cortical spreading depression and the consequent biochemical cascade and hemodynamic changes are presumed to play an important role. Here we describe a case of MI and systematically review the literature on the complex and possibly bidirectional relationship between migraine and stroke. A 44-year-old female with history of migraine with visual aura presented at the Emergency Department due to a sudden onset of left limb paresis and hypoesthesia. Brain magnetic resonance imaging revealed right fronto-parietal ischemic stroke. Two days after hospitalization, the patient experienced a prolonged visual aura and showed ultrasound evidence of intracranial artery vasospasm. To date, there have been 33 published articles on a total 119 patients with MI, although intracranial vasospasm has rarely been reported. Sustained hyperexcitability of cortical neurons, impairment of γ-aminobutyric acid inhibitory circuitry, altered serotonergic transmission, release of vasoconstrictive molecules, and cerebral blood flow changes have been proposed as pathogenic mechanisms of MI. The present case provides insight into the pathophysiological link between stroke and migraine, thus aiding clinicians in therapeutic decision-making although additional studies are needed to clarify the clinical, neuroradiological, and ultrasound findings that link MI and stroke-related migraine.

Recent Advances in Pharmacotherapy for Migraine Prevention: From Pathophysiology to New Drugs

Migraine is a common and disabling neurological disorder, with a significant socioeconomic burden. Its pathophysiology involves abnormalities in complex neuronal networks, interacting at different levels of the central and peripheral nervous system, resulting in the constellation of symptoms characteristic of a migraine attack. Management of migraine is individualised and often necessitates the commencement of preventive medication. Recent advancements in the understanding of the neurobiology of migraine have begun to account for some parts of the symptomatology, which has led to the development of novel target-based therapies that may revolutionise how migraine is treated in the future. This review will explore recent advances in the understanding of migraine pathophysiology, and pharmacotherapeutic developments for migraine prevention, with particular emphasis on novel treatments targeted at the calcitonin gene-related peptide (CGRP) pathway.

Phase clustering in transcranial magnetic stimulation-evoked EEG responses in genetic generalized epilepsy and migraine

BACKGROUND

Epilepsy and migraine are paroxysmal neurological conditions associated with disturbances of cortical excitability. No useful biomarkers to monitor disease activity in these conditions are available. Phase clustering was previously described in electroencephalographic (EEG) responses to photic stimulation and may be a potential epilepsy biomarker.

OBJECTIVE

The objective of this study was to investigate EEG phase clustering in response to transcranial magnetic stimulation (TMS), compare it with photic stimulation in controls, and explore its potential as a biomarker of genetic generalized epilepsy or migraine with aura.

METHODS

People with (possible) juvenile myoclonic epilepsy (JME), migraine with aura, and healthy controls underwent single-pulse TMS with concomitant EEG recording during the interictal period. We compared phase clustering after TMS with photic stimulation across the groups using permutation-based testing.

RESULTS

We included eight people with (possible) JME (five off medication, three on), 10 with migraine with aura, and 37 controls. The TMS and photic phase clustering spectra showed significant differences between those with epilepsy without medication and controls. Two phase clustering-based indices successfully captured these differences between groups. One participant was tested multiple times. In this case, the phase clustering-based indices were inversely correlated with the dose of antiepileptic medication. Phase clustering did not differ between people with migraine and controls.

CONCLUSION

We present methods to quantify phase clustering using TMS-EEG and show its potential value as a measure of brain network activity in genetic generalized epilepsy. Our results suggest that the higher propensity to phase clustering is not shared between genetic generalized epilepsy and migraine.

Migrainous Infarction And Cerebral Vasospasm: Case Report And Literature Review

Migrainous infarction (MI) is a rare complication of migraines that accounts for 0.5-1.5% of all ischemic strokes. Although the pathogenesis of MI is still debated, cortical spreading depression and the consequent biochemical cascade and hemodynamic changes are presumed to play an important role. Here we describe a case of MI and systematically review the literature on the complex and possibly bidirectional relationship between migraine and stroke. A 44-year-old female with history of migraine with visual aura presented at the Emergency Department due to a sudden onset of left limb paresis and hypoesthesia. Brain magnetic resonance imaging revealed right fronto-parietal ischemic stroke. Two days after hospitalization, the patient experienced a prolonged visual aura and showed ultrasound evidence of intracranial artery vasospasm. To date, there have been 33 published articles on a total 119 patients with MI, although intracranial vasospasm has rarely been reported. Sustained hyperexcitability of cortical neurons, impairment of γ-aminobutyric acid inhibitory circuitry, altered serotonergic transmission, release of vasoconstrictive molecules, and cerebral blood flow changes have been proposed as pathogenic mechanisms of MI. The present case provides insight into the pathophysiological link between stroke and migraine, thus aiding clinicians in therapeutic decision-making although additional studies are needed to clarify the clinical, neuroradiological, and ultrasound findings that link MI and stroke-related migraine.

Intracortical facilitation within the migraine motor cortex depends on the stimulation intensity. A paired-pulse TMS study

Introduction

Connectivity within the primary motor cortex can be measured using the paired-pulse transcranial magnetic stimulation (TMS) paradigm. This evaluates the effect of a first conditioning stimulus on the motor evoked potential (MEP) elicited by a second test stimulus when different interstimulus intervals are used. Aim of the present study was to provide, in patients suffering from migraine without aura (MwoA), additional information on intracortical facilitation (ICF), short intracortical inhibition (SICI), and long intracortical inhibition (LICI), using different intensities of the test stimulus (TS).

Methods

We enrolled 24 patients with episodic MwoA and 24 age- and sex-matched healthy volunteers. Both patients and controls were randomly assigned to two different experimental groups: the first group underwent evaluation of ICF, while in the second group we assessed SICI and LICI. All these measures were assessed by using three different suprathreshold intensities of the TS (110%, 130% and 150% of the resting motor threshold, RMT). Interstimulus intervals (ISIs) of 10 ms were used for testing ICF, while SICI and LICI were carried out by using 2 ms and 100 ms ISIs respectively. All migraine patients underwent the experimental protocol while in the interictal pain-free state.

Results

A main finding of the study was that an increased ICF could be seen in migraineurs as compared to the healthy subjects only by using a 110% intensity of the TS. Instead, no significant differences were observed between patients and controls as regards both measures of intracortical inhibition.

Conclusion

We show that hyperresponsivity of the glutamatergic intracortical circuits can be detected in the migraine motor cortex only by applying a low suprathreshold intensity of stimulation. Our results strengthen the notion that, to be reliable, the assessment of cortical excitability in migraine should always include evaluation of the cortical response to different stimulation intensities.

Current and novel insights into the neurophysiology of migraine and its implications for therapeutics

Migraine headache and its associated symptoms have plagued humans for two millennia. It is manifest throughout the world, and affects more than 1/6 of the global population. It is the most common brain disorder, and is characterized by moderate to severe unilateral headache that is accompanied by vomiting, nausea, photophobia, phonophobia, and other hypersensitive symptoms of the senses. While there is still a clear lack of understanding of its neurophysiology, it is beginning to be understood, and it seems to suggest migraine is a disorder of brain sensory processing, characterized by a generalized neuronal hyperexcitability. The complex symptomatology of migraine indicates that multiple neuronal systems are involved, including brainstem and diencephalic systems, which function abnormally, resulting in premonitory symptoms, ultimately evolving to affect the dural trigeminovascular system, and the pain phase of migraine. The migraineur also seems to be particularly sensitive to fluctuations in homeostasis, such as sleep, feeding and stress, reflecting the abnormality of functioning in these brainstem and diencephalic systems. Implications for therapeutic development have grown out of our understanding of migraine neurophysiology, leading to major drug classes, such as triptans, calcitonin gene-related peptide receptor antagonists, and 5-HT1F receptor agonists, as well as neuromodulatory approaches, with the promise of more to come. The present review will discuss the current understanding of the neurophysiology of migraine, particularly migraine headache, and novel insights into the complex neural networks responsible for associated neurological symptoms, and how interaction of these networks with migraine pain pathways has implications for the development of novel therapeutics.

Pathophysiology of Migraine: A Disorder of Sensory Processing

Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the "humors" through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.

Critical Flicker Frequency in Migraine. A Controlled Study in Patients without Prophylactic Therapy

The critical flicker frequencies (CFF) of individuals with migraine with and without aura were determined and compared to those of normal controls. Twenty-six migraine patients, 12 with aura and 14 without aura and 30 healthy controls were included. Migraineurs were tested during a migraine-free period, through both the continuous flicker method (CFM) and the forced choice method (FCM). Migraineurs presented a mean flickering fusion threshold lower than healthy controls with the CFM (40.45 vs. 44.33, respectively; P = 0.019) and with the FCM (34.16 Hz vs. 38.5 Hz, respectively, P = 0.019). Both groups of migraineurs had significantly lower thresholds as compared to controls, migraineurs with aura presenting the lowest thresholds for the fusion of flickering ( P = 0.008 and P = 0.0001 with the CFM and the FCM, respectively). Results confirmed and extended previous observations of abnormal flicker fusion thresholds in migraineurs. We hypothesize that this finding might be related to a shorter cortical stimulation silent period.

Motor and Occipital Cortex Excitability in Migraine Patients

OBJECTIVE

We evaluated motor and occipital cortex excitability in migraine patients using transcranial magnetic stimulation.

METHODS

In this study, we included 15 migraine patients with aura (MwA), 15 patients without aura (MwoA) between attacks, and 31 normal healthy controls. Motor thresholds at rest, amplitudes of motor evoked potentials, central motor conduction time and cortical silent period were measured by stimulation of the motor cortex by using 13.5 cm circular coil and recording from abductor digiti minimi muscle. Additionally, phosphene production and the threshold of phosphene production was determined by stimulation of the visual cortex with the same coil.

RESULTS

No significant differences were observed between the groups with respect to the motor thresholds, Motor evoked potential max/compound muscle action potential max (MEPmax/Mmax) amplitudes, central motor conduction times and duration of cortical silent period. Although not statistically significant, the proportion of the migraineurs with phosphene generation (90%) was found to be higher than that of normal controls (71%). Phosphene threshold levels in migraine patients, however, were significantly lower than those of the controls with MwA patients having the lowest levels.

CONCLUSION

Our findings indicate that the occipital cortex, but not the motor cortex, is hyperexcitable in migraine patients.

Focal dystonia and the Sensory-Motor Integrative Loop for Enacting (SMILE)

Performing accurate movements requires preparation, execution, and monitoring mechanisms. The first two are coded by the motor system, the latter by the sensory system. To provide an adaptive neural basis to overt behaviors, motor and sensory information has to be properly integrated in a reciprocal feedback loop. Abnormalities in this sensory-motor loop are involved in movement disorders such as focal dystonia, a hyperkinetic alteration affecting only a specific body part and characterized by sensory and motor deficits in the absence of basic motor impairments. Despite the fundamental impact of sensory-motor integration mechanisms on daily life, the general principles of healthy and pathological anatomic–functional organization of sensory-motor integration remain to be clarified. Based on the available data from experimental psychology, neurophysiology, and neuroimaging, we propose a bio-computational model of sensory-motor integration: the Sensory-Motor Integrative Loop for Enacting (SMILE). Aiming at direct therapeutic implementations and with the final target of implementing novel intervention protocols for motor rehabilitation, our main goal is to provide the information necessary for further validating the SMILE model. By translating neuroscientific hypotheses into empirical investigations and clinically relevant questions, the prediction based on the SMILE model can be further extended to other pathological conditions characterized by impaired sensory-motor integration.

Brain stimulation in migraine

Migraine is a very prevalent disease with great individual disability and socioeconomic burden. Despite intensive research effort in recent years, the etiopathogenesis of the disease remains to be elucidated. Recently, much importance has been given to mechanisms underlying the cortical excitability that has been suggested to be dysfunctional in migraine. In recent years, noninvasive brain stimulation techniques based on magnetic fields (transcranial magnetic stimulation, TMS) and on direct electrical currents (transcranial direct current stimulation, tDCS) have been shown to be safe and effective tools to explore the issue of cortical excitability, activation, and plasticity in migraine. Moreover, TMS, repetitive TMS (rTMS), and tDCS, thanks to their ability to interfere with and/or modulate cortical activity inducing plastic, persistent effects, have been also explored as potential therapeutic approaches, opening an interesting perspective for noninvasive neurostimulation for both symptomatic and preventive treatment of migraine and other types of headache. In this chapter we critically review evidence regarding the role of noninvasive brain stimulation in the pathophysiology and treatment of migraine, delineating the advantages and limits of these techniques together with potential development and future application.

Neuromagnetic abnormality of motor cortical activation and phases of headache attacks in childhood migraine

The cerebral cortex serves a primary role in the pathogenesis of migraine. This aberrant brain activation in migraine can be noninvasively detected with magnetoencephalography (MEG). The objective of this study was to investigate the differences in motor cortical activation between attacks (ictal) and pain free intervals (interictal) in children and adolescents with migraine using both low- and high-frequency neuromagnetic signals. Thirty subjects with an acute migraine and 30 subjects with a history of migraine, while pain free, were compared to age- and gender-matched controls using MEG. Motor cortical activation was elicited by a standardized, validated finger-tapping task. Low-frequency brain activation (1∼50 Hz) was analyzed with waveform measurements and high-frequency oscillations (65–150 Hz) were analyzed with wavelet-based beamforming. MEG waveforms showed that the ictal latency of low-frequency brain activation was significantly delayed as compared with controls, while the interictal latency of brain activation was similar to that of controls. The ictal amplitude of low-frequency brain activation was significantly increased as compared with controls, while the interictal amplitude of brain activation was similar to that of controls. The ictal source power of high-frequency oscillations was significantly stronger than that of the controls, while the interictal source power of high-frequency oscillations was significantly weaker than that of controls. The results suggest that aberrant low-frequency brain activation in migraine during a headache attack returned to normal interictally. However, high-frequency oscillations changed from ictal hyper-activation to interictal hypo-activation. Noninvasive assessment of cortical abnormality in migraine with MEG opens a new window for developing novel therapeutic strategies for childhood migraine by maintaining a balanced cortical excitability.

Patterns of habituation and clinical fluctuations in migraine

Background

Habituation deficit, suggesting a deregulation of cortical excitability, represents a typical hallmark of interictal stages of migraine. We previously demonstrated that several neurophysiological markers of altered cortical excitability are significantly correlated to spontaneous clinical fluctuations of migraine. We therefore aimed at verifying whether clinical fluctuations are correlated to specific patterns of somatosensory evoked potential (SEP) habituation.

Methods

We analyzed habituation after median nerve stimulation of both high-frequency oscillations (HFOs) and N20 SEP in 25 migraine patients and 18 healthy volunteers. Subjects underwent six consecutive series of 500 stimuli.

Results

Migraine patients as a whole showed a significant habituation deficit of the N20 response. Moreover, spontaneously worsening patients show a clear potentiation of this wave in the last block of stimuli, whereas in spontaneously improving patients the N20 amplitude remained stable. Presynaptic HFOs were smaller in worsening patients and larger in improving ones, but they did not undergo habituation in patients as well as in healthy subjects.

Conclusions

Potentiation of the N20 response in spontaneously worsening migraineurs confirms that the reduction of the thalamocortical drive plays a major role in migraine pathogenesis. Moreover, the stable pattern we observed in spontaneously improving patients suggests that compensatory mechanisms can also play an important role. The normal response to repeated stimuli of HFOs in migraineurs might indicate that, although its initial amount depends on clinical conditions, high-frequency thalamocortical drive remains stable during the stimulation and probably reflects the activity of a buffer mechanism.

Cortical inhibition at rest and under a focused attention challenge in adults with migraine with and without aura

Objectives: We aimed to further elucidate the functional and attentional regulation of cortical excitability in migraine patients.

Methods: We investigated the cortical silent period (CSP) after transcranial magnetic stimulation as a measure of cortical inhibition under three conditions: resting condition, cortical preactivation during reaction preparation, and during the post-processing of a motor response using a visual contingent negative variation paradigm in adults with migraine with aura, migraine without aura and healthy controls.

Results: CSP was reduced in individuals with migraine with aura and unaffected in those with migraine without aura under resting conditions. Along with the intensity of transcranial magnetic stimulation, CSP increased equally in all groups (equal slopes). Furthermore, the functional challenge by a contingent negative variation task requiring focused sustained attention led to a comparable reduction of CSP duration in all groups.

Discussion: Our data provide further hints towards the conclusion that a specific cortical inhibition deficit in migraine with aura but not migraine without aura is due to a tonic imbalance and not related to increased reactions to phasic stressors. Given that CSP at rest is related to GABA-ergic inhibition whereas the CSP reduction during late contingent negative variation is thought to be related to dopaminergic disinhibition in the basal ganglia, our results point towards reduced GABA-ergic cortical inhibition related to dysfunctional thalamo-cortical loops, especially in migraine with aura.

Peri-ictal changes of cortical excitability in children suffering from migraine without aura

In adult patients with migraine, transcranial magnetic stimulation (TMS) has been used to examine cortical excitability between attacks, but there have been discrepant results. No TMS study has examined cortical excitability in children or adolescents with migraine. Here, we employed TMS to study regional excitability of the occipital (phosphene threshold [PT] and suppression of visual perception) and motor (resting motor threshold and cortical silent period) cortex in ten children suffering from migraine without aura and ten healthy age-matched controls. Patients were studied 1-2 days before and after a migraine attack as well as during the inter-migraine interval. The motion aftereffect was also investigated at each time-point as an index of cortical reactivity to moving visual stimuli. Migraineurs had lower PTs compared to healthy participants at each time-point, indicating increased occipital excitability. This increase in occipital excitability was attenuated 1-2 days before a migraine attack as indicated by a relative increase in PTs. The increase in PTs before the next attack was associated with a stronger TMS-induced suppression of visual perception and a prolongation of the motion aftereffect. Motor cortex excitability was not altered in patients and did not change during the migraine cycle. These findings show that pediatric migraine without aura is associated with a systematic shift in occipital excitability preceding the migraine attack. Similar systematic fluctuations in cortical excitability might be present in adult migraineurs and may reflect either a protective mechanism or an abnormal decrease in cortical excitability that predisposes an individual to a migraine attack.

Reduced cerebellar inhibition in migraine with aura: a TMS study

Subtle clinical cerebellar alterations have been found in migraine. Moreover, abnormalities in visual and motor cortex excitability consistent with a lack of inhibitory efficiency have been described in migraine, and it is known that cerebellum exerts an inhibitory control on cerebral cortex. Here, we investigated if impairment of cerebellar activity on motor cortex, i.e. reduced inhibitory control, can be found in migraine. Ten migraineurs with aura and seven healthy controls underwent a transcranial magnetic stimulation (TMS) protocol to investigate the cerebellar inhibitory drive on motor cortex: a conditioning pulse on right cerebellar cortex was delivered 5, 7, 10, 15 ms before a test stimulus (TS) on contralateral motor cortex. The cerebellar conditioning stimulus inhibits the size of the motor-evoked potential (MEP) produced by the TS alone by approximately 30-50%. Amplitude of MEP to TS alone showed no significant difference between patients and controls. Cerebellar conditioning TMS showed a significant deficit of cerebellar inhibition in migraine patients as compared to controls at all interstimulus intervals (5-15 ms) tested. Cerebellar inhibition is reduced in migraineurs. This could account, at least in part, for the reduced inhibitory efficiency previously showed in cerebral cortex of these patients.

Migraine with and without headache

Transient visual and neurological episodes are relatively common and can occur for the first time in middle and old age. In many cases these transient events are migraine auras. An aura is a transient, stereotypical, visual or neurological episode usually lasting 4 to 60 minutes in duration. Migraine is usually, but not always, associated with headache and can be accompanied by systemic and autonomic symptoms. Diagnosis is dependent on International Headache Society criteria. The pathophysiology is believed to involve neurovascular mechanisms. There is a hereditary component to migraine. When migraine auras occur in the absence of headache they are termed acephalgic migraines. Late onset migraine accompaniment is an acephalgic migraine that presents in middle-aged and older adults. It is usually benign. Migraines can be mimicked by other more serious conditions. Most patients with a stable migraine pattern and normal neurological evaluation do not require further testing. Some patients with atypical presentation, older age, or suspected secondary causes need further investigation.

Cortical inhibition and habituation to evoked potentials: relevance for pathophysiology of migraine

Dysfunction of neuronal cortical excitability has been supposed to play an important role in etiopathogenesis of migraine. Neurophysiological techniques like evoked potentials (EP) and in the last years non-invasive brain stimulation techniques like transcranial magnetic stimulation (TMS) and transcranial direct current stimulation gave important contribution to understanding of such issue highlighting possible mechanisms of cortical dysfunctions in migraine. EP studies showed impaired habituation to repeated sensorial stimulation and this abnormality was confirmed across all sensorial modalities, making defective habituation a neurophysiological hallmark of the disease. TMS was employed to test more directly cortical excitability in visual cortex and then also in motor cortex. Contradictory results have been reported pointing towards hyperexcitability or on the contrary to reduced preactivation of sensory cortex in migraine. Other experimental evidence speaks in favour of impairment of inhibitory circuits and analogies have been proposed between migraine and conditions of sensory deafferentation in which down-regulation of GABA circuits is considered the more relevant pathophysiological mechanism. Whatever the mechanism involved, it has been found that repeated sessions of high-frequency rTMS trains that have been shown to up-regulate inhibitory circuits could persistently normalize habituation in migraine. This could give interesting insight into pathophysiology establishing a link between cortical inhibition and habituation and opening also new treatment strategies in migraine.

Shortened cortical silent period in facial muscles of patients with migraine

Despite intensive neurophysiological research, evidence is lacking to show whether abnormal cortical excitability in migraine reflects a primary cortical disturbance or reduced control by thalamo-cortical loops. One way to contribute to the scientific discussion on this topic is to deliver transcranial magnetic stimulation (TMS) and test the cortical silent period (SP) recorded in facial muscles. The facial-muscle SP is a purely cortical phenomenon that reflects the excitability of inhibitory interneurons, and can disclose changes in cortical inhibition even in patients without documented primary lesions of the motor cortices. To test the interictal excitability of cortical motor inhibitory interneurons in migraine, we investigated the facial-SP in patients with migraine with and without aura between attacks. In 26 patients and 15 age-matched controls, high-intensity magnetic stimuli were delivered with a round coil centered at the vertex during a maximal muscle contraction. Electromyographic responses were recorded from surface electrodes placed over the subjects' perioral muscles. Facial SPs were significantly shorter in patients than in controls. The SP shortening provides neurophysiological evidence showing hypoexcitability of cortical inhibitory neurons in patients with migraine between attacks. Despite a possible primary deficit of cortical inhibitory interneurons in migraine, we favor the interpretation of a secondary disfacilitation by hypoactive thalamo-cortical loops. Based on this interpretation, the interictal reduced cortical inhibition documented by the shortened SP could be considered the motor counterpart of the reduced preactivation excitability level in the sensory cortices purported to explain why cortical evoked responses habituate poorly in patients with migraine.

Intracortical Inhibition and Facilitation in Migraine-A Transcranial Magnetic Stimulation Study

OBJECTIVE

Migraine is a disease of altered cortical excitability between attacks. However, the mechanisms of abnormal excitability in migraine are insufficiently investigated. Hence, the aim of the study was to investigate intracortical inhibition/facilitation of the motor circuit in migraine.

METHODS

Sixteen women suffering from migraine without aura and 15 healthy women were investigated using a suprathreshold transcranial magnetic stimulation (TMS) in the paired-pulse paradigm with long interstimulus intervals (ISI = 20, 60, 120 ms) and measurement of the cortical silent period.

RESULTS

We found no differences for the cortical silent period and for the long intracortical inhibition between the groups. Concerning intracortical facilitation, this ability was significantly more pronounced in patients suffering from migraine compared with healthy controls.

CONCLUSION

Migraineurs produce an increased intracortical facilitation. The results may be discussed in line of glutamatergic mechanisms in migraine, which could be related to altered facilitation.