Habituation of laser-evoked potentials by migraine phase: a blinded longitudinal study

EEG Changes in Migraine-Can EEG Help to Monitor Attack Susceptibility?

Migraine is a highly prevalent brain condition with paroxysmal changes in brain excitability believed to contribute to the initiation of an attack. The attacks and their unpredictability have a major impact on the lives of patients. Clinical management is hampered by a lack of reliable predictors for upcoming attacks, which may help in understanding pathophysiological mechanisms to identify new treatment targets that may be positioned between the acute and preventive possibilities that are currently available. So far, a large range of studies using conventional hospital-based EEG recordings have provided contradictory results, with indications of both cortical hyper- as well as hypo-excitability. These heterogeneous findings may largely be because most studies were cross-sectional in design, providing only a snapshot in time of a patient's brain state without capturing day-to-day fluctuations. The scope of this narrative review is to (i) reflect on current knowledge on EEG changes in the context of migraine, the attack cycle, and underlying pathophysiology; (ii) consider the effects of migraine treatment on EEG features; (iii) outline challenges and opportunities in using EEG for monitoring attack susceptibility; and (iv) discuss future applications of EEG in home-based settings.

Habituation to Pain in Patients with Chronic Pain: Clinical Implications and Future Directions

In this review, the latest insights into habituation to pain in chronic pain are summarized. Using a systematic search, results of studies on the evidence of habituation to (experimental) pain in migraine, chronic low back pain, fibromyalgia, and a variety of chronic pain indications are presented. In migraine, reduced habituation based on self-report and the EEG-based N1 and N2-P2 amplitude is reported, but the presence of contradictory results demands further replication in larger, well-designed studies. Habituation to pain in chronic low back pain seems not to differ from controls, with the exception of EEG measures. In fibromyalgia patients, there is some evidence for reduced habituation of the N2-P2 amplitude. Our analysis shows that the variability between outcomes of studies on habituation to pain is high. As the mechanisms underlying habituation to pain are still not fully understood and likely involve several pathways, it is now too early to conclude that habituation to pain is related to clinical outcomes and can be used as a diagnostic marker. The review ends with a discussion on future directions for research including the use of standard outcome measures to improve comparisons of habituation to pain in patients and controls, as well as a focus on individual differences.

Cyclic changes of sensory parameters in migraine patients

BACKGROUND

Migraine shows a cyclic pattern with an inter-ictal-, a pre-ictal, an ictal- and a post-ictal phase. We aimed to examine changes in psychophysical parameters during the migraine cycle.

METHODS

The perception of nociceptive and non-nociceptive stimuli and an electrically induced axon-reflex-erythema were assessed in 20 healthy controls and 14 migraine patients on five consecutive days according to different phases of the migraine cycle. Pain was rated three times during a 10-second electrical stimulus. The size of the axon-reflex-erythema was determined using laser-Doppler-imaging. Intensity and hedonic estimates of odours presented by Sniffin' Sticks were rated.

RESULTS

In healthy controls, no significant changes over the test days were observed. In migraine patients pain thresholds at the head decreased with an ictal minimum. Less habituation after five seconds of stimulation at the head was found pre-ictally, whereas reduced habituation to 10-second electrical stimulation was present in all phases. The axon-reflex-erythema size showed an inter-ictal-specific minimum at the head. odours were perceived ictally as more unpleasant and intense.

CONCLUSIONS

Somatosensory functions, pain thresholds and habituation as predominantly central parameters, axon-reflex-erythema as a peripheral function of trigeminal neurons and odour perception as a predominantly extra-thalamic sensation change specifically over the migraine cycle indicating complex variations of neuronal signal processing.

Laser-Evoked Potentials to Pudendal Stimulation in Healthy Subjects: A Pilot Study

PURPOSE

Laser-evoked potentials (LEPs) are useful neurophysiological tools for investigating the A-delta sensory peripheral fibers and the central nociceptive pathway. The current investigation aims to obtain normative values of LEPs via pudendal nerve stimulation in healthy adult volunteers.

METHODS

Laser-evoked potentials were recorded in 16 men and 22 women, 22 to 75 years of age, using neodymium and yttrium and aluminum and perovskite laser bilateral stimulation to the pudendal nerve-supplied skin and the dorsal surface of the hands and feet. We assessed the perceptive threshold, latency, and amplitude of the N1 component and main vertex N2-P2 complex. The relationship between gender, age, height, and site of stimulation was statistically analyzed.

RESULTS

Both in men and in women, laser perceptive threshold increased from genitalia to foot and from hand to foot (P ≤ 0.001). N1 and N2-P2 latencies progressively increased from pudendal area to hand to foot (P ≤ 0.008). N1 and N2-P2 complex LEP amplitudes progressively decreased from hand to genitalia to foot (P ≤ 0.04). The latencies of N1 component and N2-P2 complex of LEPs correlated with body height, whereas the amplitude of the N2-P2 complex correlated negatively with age; no correlations were observed between the latencies and amplitudes with gender.

CONCLUSIONS

This study provides normative data on pudendal LEPs versus hand and foot LEPs. Incorporation of pudendal LEPs into clinical practice could provide a valuable neurophysiological tool for the study of pelvic pain syndromes.

Migraine understood as a sensory threshold disease

Migraine encompasses a broader spectrum of sensory symptoms than just headache. These "other" symptoms, eg, sensory phobias, cognitive and mood changes, allodynia, and many others indicate an altered sensitivity to sensory input which can be measured, in principle, by quantifying sensory threshold changes longitudinally over time. Photophobia, for example, can be quantified by investigating the discomfort thresholds towards the luminance of light. The aim of this review is to look into how thresholds change in patients with migraine. We performed a PubMed search up to June 2018 targeting all peer-reviewed articles evaluating the changes in threshold, sensory phobia, or sensitivity in patients with migraine. Migraineurs, in general, exhibit lower sensory thresholds compared with healthy controls. These threshold changes seem to follow the different phases during a migraine cycle. In general, thresholds reach a nadir when the headache starts (the ictal phase), rise after the headache ends, and then gradually descend towards the next attack. The sensory modality of measurement-mechanical, thermal, or nociceptive-and the location of measurement-trigeminal vs somatic dermatome-also influence the sensory threshold. Functional imaging studies provide evidence that the hypothalamo-thalamo-brainstem network may be the driving force behind the periodic threshold changes. In summary, there is evidence in the literature that migraine could be understood as a periodic sensory dysregulation originating from the brain. Nevertheless, the interstudy discrepancy is still high due to different study designs and a lack of focus on distinct migraine phases. Further well-designed and harmonized studies with an emphasis on the cyclic changes still need to be conducted.

C-tactile touch perception in migraineurs – a case-control study

Background

Migraine is characterized by sensory hypersensitivity and habituation deficits. Slow brushing over the skin activates C-tactile nerve fibers, which mediate pleasant touch and analgesic effects in healthy subjects. As this function is altered in painful conditions, we aimed to examine whether the C-tactile processing is disrupted in migraines.

Methods

To psychophysically assess C-tactile function, we applied optimal and suboptimal C-tactile stroking stimuli on the dorsal forearm (body reference area) and the trigeminally innervated skin of 52 interictal migraineurs and 52 matched healthy controls. For habituation testing, 60 repeated C-tactile optimal stimuli were presented in both test areas. The participants rated each stimulus on a visual analogue scale by intensity, pleasantness, and painfulness.

Results

Regarding C-tactile function, migraineurs showed unphysiological rating patterns but no significantly different pleasantness ratings than controls. During repeated stimulation, controls showed stable pleasantness ratings while migraineurs’ ratings decreased, especially in those experiencing tactile allodynia during headaches. Migraineurs taking triptans responded like controls.

Conclusion

The C-tactile function of migraineurs is subclinically altered. Repeated C-tactile stimulation leads to altered habituation but differs from previous work by the direction of the changes. Although the pathophysiology remains unknown, causative mechanisms could include central and peripheral neuronal sensitization, tactile allodynia and hedonic stimulus attributions.

Increased pain sensitivity but normal pain modulation in adolescents with migraine

Inhibitory pain modulation has been reported to be deficient in adults across different types of chronic pain, including migraine. To determine whether a similar phenomenon occurs in youth, we performed a quantitative sensory testing investigation in adolescents with migraine (N = 19). These patients were compared to healthy adolescents with (Fam-His; N = 20) or without (Healthy; N = 29) a family history of migraine (eg, first-degree relative with migraine). Subjects were first familiarized with the stimuli and visual analogue rating scales using graded noxious stimuli (0°C, 43-49°C range). These data were used to explore potential pain sensitivity differences between the groups. Pain inhibition was assessed by conditioned pain modulation (CPM), which used both suprathreshold heat pain (heat CPM) and pressure pain thresholds (pressure CPM) as the test stimuli before and during cold-water immersion (8°C). In response to the graded heat stimuli, Fam-His participants reported higher pain intensity ratings compared with patients with migraine, who in turn reported higher pain intensity ratings than the healthy controls (F = 3.6, [df = 2, 459], P = 0.027). For heat and pressure CPM, there was no significant group difference in the magnitude of CPM responses. Thus, adolescents with migraine and healthy adolescents have similar inhibitory pain modulation capability, despite having marked differences in pain sensitivity. Although Fam-His participants are asymptomatic, they demonstrate alterations in pain processing, which may serve as markers for prediction of migraine development.

Current understanding of cortical structure and function in migraine

Objective

To review and discuss the literature on the role of cortical structure and function in migraine.

Discussion

Structural and functional findings suggest that changes in cortical morphology and function contribute to migraine susceptibility by modulating dynamic interactions across cortical and subcortical networks. The involvement of the cortex in migraine is well established for the aura phase with the underlying phenomenon of cortical spreading depolarization, while increasing evidence suggests an important role for the cortex in perception of head pain and associated sensations. As part of trigeminovascular pain and sensory processing networks, cortical dysfunction is likely to also affect initiation of attacks.

Conclusion

Morphological and functional changes identified across cortical regions are likely to contribute to initiation, cyclic recurrence and chronification of migraine. Future studies are needed to address underlying mechanisms, including interactions between cortical and subcortical regions and effects of internal (e.g. genetics, gender) and external (e.g. sensory inputs, stress) modifying factors, as well as possible clinical and therapeutic implications.

Clinical neurophysiology of pain

Clinical neurophysiologic investigation of pain pathways in humans is based on specific techniques and approaches, since conventional methods of nerve conduction studies and somatosensory evoked potentials do not explore these pathways. The proposed techniques use various types of painful stimuli (thermal, laser, mechanical, or electrical) and various types of assessments (measurement of sensory thresholds, study of nerve fiber excitability, or recording of electromyographic reflexes or cortical potentials). The two main tests used in clinical practice are quantitative sensory testing and pain-related evoked potentials (PREPs). In particular, PREPs offer the possibility of an objective assessment of nociceptive pathways. Three types of PREPs can be distinguished depending on the type of stimulation used to evoke pain: laser-evoked potentials, contact heat evoked potentials, and intraepidermal electrical stimulation evoked potentials (IEEPs). These three techniques investigate both small-diameter peripheral nociceptive afferents (mainly Aδ nerve fibers) and spinothalamic tracts without theoretically being able to differentiate the level of lesion in the case of abnormal results. In routine clinical practice, PREP recording is a reliable method of investigation for objectifying the existence of a peripheral or central lesion or loss of function concerning the nociceptive pathways, but not the existence of pain. Other methods, such as nerve fiber excitability studies using microneurography, more directly reflect the activities of nociceptive axons in response to provoked pain, but without detecting or quantifying the presence of spontaneous pain. These methods are more often used in research or experimental study design. Thus, it should be kept in mind that most of the results of neurophysiologic investigation performed in clinical practice assess small fiber or spinothalamic tract lesions rather than the neuronal mechanisms directly at the origin of pain and they do not provide objective quantification of pain.

Cyclic changes in sensations to painful stimuli in migraine patients

Introduction

Migraine is characterized by cycling phases (interictal, preictal, ictal and postictal) with differing symptoms, while in chronic tension type headache pain phases are fluctuating. The question we asked is whether these phases are associated with changes in parameters of somatosensation and axon-reflex erythema.

Methods

Patients with episodic migraine and chronic tension type headache were examined psychophysically in the interictal, preictal and ictal phase and healthy subjects on five different test days. Thresholds and suprathreshold ratings of pressure and electrical pain were assessed on three different regions of the head. In migraine patients and in healthy controls, electrically induced axon-reflex erythema was measured in the area of the first trigeminal branch. All migraine patients filled out questionnaires about prodromal symptoms at every visit.

Results

The axon-reflex erythema was always larger in patients with migraine in contrast to healthy subjects. The pressure pain threshold was lower in migraine patients and chronic tension type headache in comparison to healthy subjects. Electrical pain thresholds did not differ between headache patients and healthy subjects and showed no changes between the phases. However, suprathreshold pain ratings showed less habituation solely in the preictal phase of migraine. The number of prodromal symptoms in migraine patients was increased in the preictal and ictal phase.

Discussion

Reduced habituation was the unique sign of the preictal phase in migraine patients, independently of prodromal symptoms, whereas a larger axon-reflex erythema and higher pressure pain sensitivity are constitutional and non-phase dependent properties of migraine. Reduced inhibitory mechanisms in the preictal phase may contribute to trigger headache attacks in migraine.

Electrophysiological Characteristics of the Migraine Brain: Current Knowledge and Perspectives

BACKGROUND

Despite pain being its most prominent feature, migraine is primarily a disorder of sensory processing. Electrophysiology-based research in the field has consistently developed over the last fifty years.

OBJECTIVE

To summarize the current knowledge on the electrophysiological characteristics of the migraine brain, and discuss perspectives.

METHODS

We critically reviewed the literature on the topic to present and discuss articles selected on the basis of their significance and/or novelty.

RESULTS

Physiologic fluctuations within time, between-subject differences, and methodological issues account as major limitations of electrophysiological research in migraine. Nonetheless, several abnormalities revealed through different approaches have been described in the literature. Altogether, these results are compatible with an abnormal state of sensory processing.

PERSPECTIVES

The greatest contribution of electrophysiological testing in the future will most probably be the characterization of sub-groups of migraine patients sharing specific electrophysiological traits. This should serve as strategy towards personalized migraine treatment. Incorporation of novel methods of analysis would be worthwhile.