Giant subcortical high-frequency SEPs in idiopathic generalized epilepsy: a protective mechanism against seizures?

The effects of antiepileptic drugs on high-frequency oscillations in somatosensory evoked potentials

OBJECTIVES

High frequency oscillations (HFOs) of Somatosensory evoked potentials (SEPs) reflect the activity of thalamo-cortical and cortical neurons from the sensory pathway. Antiepileptic-drugs (AEDs) reduce seizures acting on the balance between excitation and inhibition. We aimed to study the effect of AED mono and polytherapy on SEP-HFO's components.

METHODS

Twenty-five patients with focal epilepsy were enrolled for the purpose of this study. Patients were divided in 3 groups according to the number of AEDs (1, 2 or 3 AEDs). Patients in group 1 underwent SEP-HFOs recording in drug naïve condition and at 1 month after AED titration. HFOs were compared in duration, amplitude and latency among the three groups.

RESULTS

The amplitude and duration of late HFOs of the affected hemisphere (AH) are different between groups and inversely correlated with the number of AEDs. In naïve patients monotherapy reverts the asymmetry in totHFOs (total HFOs) duration.

CONCLUSION

Our results demonstrate that SEP-HFOs are sensitive to the action of AEDs on cortical excitability. This effect seems to affect mainly the cortical component of HFOs in the AH and it is related to the number of AEDs taken.

SIGNIFICANCE

SEP-HFOs might be a viable tool to probe cortical excitability changes induced by AEDs.

Thalamic and cortical hyperexcitability in juvenile myoclonic epilepsy

OBJECTIVES

Juvenile myoclonic epilepsy (JME) is a genetic generalized epilepsy marked by cortical hyperexcitability. Recent neuroimaging data suggested also a thalamic role in sustaining epileptic propensity in JME. However, thalamic hyperexcitability was not demonstrated so far. Low-frequency (LF-SEPs) and high-frequency somatosensory evoked potentials (HF-SEPs) are very sensitive to thalamic (early HF-SEPs burst, eHFO) and cortical (late HF-SEPs burst, lHFO) excitability. The aim of our experiment was to explore and discern the role of thalamic and cortical excitability in epileptic susceptibility of JME through a LF-SEPs and HF-SEPs study.

METHODS

Twenty-three subjects with JME (11 females, 30.2 ± 9.8-year-old) and 23 healthy control subjects (12 females, age: 34.7 ± 7.7-year-old) underwent right median LF-SEPs scalp recordings. Cp3'-Fz traces were filtered (400-800 Hz) to reveal HF-SEPs. All JME patients were on drug treatment and seizure free, except for sporadic myoclonus.

RESULTS

N20 LF-SEPs amplitude (p < 0.009), areas of totHFO, eHFO and lHFO (all p < 0.005) and totHFO duration (p = 0.013) were increased in JME respect to healthy subjects. totHFO area was negatively correlated with the number of antiepileptic drugs (rho = -0.505, sig.: 0.027), while eHFO area was positively correlated with the myoclonus frequency (rho = 0.555, sig = 0.014).

CONCLUSIONS

We demonstrated that in JME the thalamic hyperexcitability assists the cortical one in sustaining epileptic susceptibility.

SIGNIFICANCE

Our results support the concept of JME as a network and genetic disorder.

Thalamo-cortical network dysfunction in temporal lobe epilepsy

OBJECTIVES

Imaging and neurophysiological data shows that the cortical disfunction caused by focal epilepsy is not limited to the epileptic focus, thus raising the modern vision of focal epilepsy as a network disorder. The involvement of deep thalamo-cortical projections in temporal lobe epilepsy is a clear example. We aimed at demonstrating the interictal functional impairment of thalamo-cortical network in drug-naïve TLE patients through the study of high frequency oscillations of somatosensory evoked potentials (HF-SEP).

METHODS

Twelve healthy controls (HC; 8 females, 52.2 ± 17.3 years-old) and 12 drug-naïve TLE patients (8 females, 55.5 ± 21.5 years-old) underwent bilateral median HF-SEP, recorded by scalp electrodes. Cp3'-Fz and Cp4'-Fz traces were filtered (400-800 Hz) to evidence HF-SEP.

RESULTS

HF-SEP duration in the affected hemisphere was significantly longer when compared to that of both the unaffected hemisphere and HC hemispheres. No significant inter-hemispheric differences were found in areas, powers and latencies of HF-SEP wavelets.

CONCLUSION

Our results demonstrate that TLE induces early interictal functional impairments of the thalamo-cortical network.

SIGNIFICANCE

Our data strongly corroborates the vision of focal epilepsy as a network disorder and offers a new neurophysiological tool to test pharmacological, surgical and neuromodulatory therapies.

Cortical inhibitory dysfunction in epilepsia partialis continua: A high frequency oscillation somatosensory evoked potential study

OBJECTIVE

The pathophysiology of epilepsia partialis continua (EPC) is still unclear, a thalamo-cortical circuit dysfunction has been hypothesized. The aim of present study is the functional evaluation of the thalamo-cortical network in EPC by means of the study of low- and high-frequency somatosensory evoked potentials (LF-SEP and HF-SEP).

METHODS

Median LF-SEP and HF-SEP were recorded in 3 patients with EPC and in 2 patients with rolandic lesions without EPC (non-EPC). Recording electrodes were placed on P3, C3, F3 and P4, C4, F4 of scalp regions. HF-SEP were obtained by an offline 400-800 Hz filtering of P3-F3 and P4-F4 traces.

RESULTS

In EPC patients, we found a significant suppression of post-synaptic HF-SEP burst and an amplitude reduction of the P24 wave of the LF-SEPs. Both these components are related to cortical inhibitory interneuron activity. HF-SEP and LF-SEP were normal in non-EPC patients.

CONCLUSION

The different results obtained in patients with a rolandic lesion with and without EPC supports the hypothesis that EPC might be correlated to a dysfunction of gabaergic interneurons of a cortical sensory-motor network.

SIGNIFICANCE

Our results might contribute to the understanding of the physiological basis of the cortical dysfunction causing epilepsia partialis continua.

Oscillatory Activities in Neurological Disorders of Elderly: Biomarkers to Target for Neuromodulation

Non-invasive brain stimulation (NIBS) has been under investigation as adjunct treatment of various neurological disorders with variable success. One challenge is the limited knowledge on what would be effective neuronal targets for an intervention, combined with limited knowledge on the neuronal mechanisms of NIBS. Motivated on the one hand by recent evidence that oscillatory activities in neural systems play a role in orchestrating brain functions and dysfunctions, in particular those of neurological disorders specific of elderly patients, and on the other hand that NIBS techniques may be used to interact with these brain oscillations in a controlled way, we here explore the potential of modulating brain oscillations as an effective strategy for clinical NIBS interventions. We first review the evidence for abnormal oscillatory profiles to be associated with a range of neurological disorders of elderly (e.g., Parkinson's disease (PD), Alzheimer's disease (AD), stroke, epilepsy), and for these signals of abnormal network activity to normalize with treatment, and/or to be predictive of disease progression or recovery. We then ask the question to what extent existing NIBS protocols have been tailored to interact with these oscillations and possibly associated dysfunctions. Our review shows that, despite evidence for both reliable neurophysiological markers of specific oscillatory dis-functionalities in neurological disorders and NIBS protocols potentially able to interact with them, there are few applications of NIBS aiming to explore clinical outcomes of this interaction. Our review article aims to point out oscillatory markers of neurological, which are also suitable targets for modification by NIBS, in order to facilitate in future studies the matching of technical application to clinical targets.

Expression of praxis induction on cortical excitability in juvenile myoclonic epilepsy

OBJECTIVE

This study aimed to evaluate the effects of praxis induction on sensorimotor cortical and transcallosal excitability in juvenile myoclonic epilepsy (JME).

METHODS

A total of 36 subjects (18-62years) were included. The JME group was screened by video-electroencephalography neuropsychological protocol and divided into JME without praxis induction [JME-WI (n=12)], JME with praxis-induced seizures or epileptiform discharges [JME-PI (n=10)], and healthy controls (n=14). Motor and somatosensory cortical excitability and transcallosal pathways were evaluated through single-pulse transcranial magnetic stimulation (sTMS) and somatosensory evoked potentials (SEPs).

RESULTS

Motor and transcallosal excitabilities tested with sTMS were not different in the motor-dominant or non-dominant hemisphere among groups. Significant differences were found in cortical SEP amplitudes in the P27 component of the non-dominant hemisphere (p=0.03, Cohen's d=0.98), N35 in the dominant hemisphere (p=0.04, Cohen's d=0.96), and P27-35 interpeak amplitude in both somatosensory cortices of the JME-PI group (p=0.03, Cohen's d=0.96; p=0.02, Cohen's d=1.05) when compared with healthy controls. Giant SEPs were observed in two (16.7%) and five (50%) patients of the JME-WI and JME-PI groups, respectively. Cortical latencies did not reveal differences.

CONCLUSIONS

Praxis induction was associated with enhanced excitability in the somatosensory cortex of JME patients.

SIGNIFICANCE

These findings may help clarifying the less favorable therapeutic response in the JME-PI group and indicate identifying praxis induction as an important determinant in differentiating between JME patients.

Different Mode of Afferents Determines the Frequency Range of High Frequency Activities in the Human Brain: Direct Electrocorticographic Comparison between Peripheral Nerve and Direct Cortical Stimulation

Physiological high frequency activities (HFA) are related to various brain functions. Factors, however, regulating its frequency have not been well elucidated in humans. To validate the hypothesis that different propagation modes (thalamo-cortical vs. cortico-coritcal projections), or different terminal layers (layer IV vs. layer II/III) affect its frequency, we, in the primary somatosensory cortex (SI), compared HFAs induced by median nerve stimulation with those induced by electrical stimulation of the cortex connecting to SI. We employed 6 patients who underwent chronic subdural electrode implantation for presurgical evaluation. We evaluated the HFA power values in reference to the baseline overriding N20 (earliest cortical response) and N80 (late response) of somatosensory evoked potentials (HFA(SEP(N20)) and HFA(SEP(N80))) and compared those overriding N1 and N2 (first and second responses) of cortico-cortical evoked potentials (HFA(CCEP(N1)) and HFA(CCEP(N2))). HFA(SEP(N20)) showed the power peak in the frequency above 200 Hz, while HFA(CCEP(N1)) had its power peak in the frequency below 200 Hz. Different propagation modes and/or different terminal layers seemed to determine HFA frequency. Since HFA(CCEP(N1)) and HFA induced during various brain functions share a similar broadband profile of the power spectrum, cortico-coritcal horizontal propagation seems to represent common mode of neural transmission for processing these functions.

Recording human cortical population spikes non-invasively--An EEG tutorial

BACKGROUND

Non-invasively recorded somatosensory high-frequency oscillations (sHFOs) evoked by electric nerve stimulation are markers of human cortical population spikes. Previously, their analysis was based on massive averaging of EEG responses. Advanced neurotechnology and optimized off-line analysis can enhance the signal-to-noise ratio of sHFOs, eventually enabling single-trial analysis.

METHODS

The rationale for developing dedicated low-noise EEG technology for sHFOs is unfolded. Detailed recording procedures and tailored analysis principles are explained step-by-step. Source codes in Matlab and Python are provided as supplementary material online.

RESULTS

Combining synergistic hardware and analysis improvements, evoked sHFOs at around 600 Hz ('σ-bursts') can be studied in single-trials. Additionally, optimized spatial filters increase the signal-to-noise ratio of components at about 1 kHz ('κ-bursts') enabling their detection in non-invasive surface EEG.

CONCLUSIONS

sHFOs offer a unique possibility to record evoked human cortical population spikes non-invasively. The experimental approaches and algorithms presented here enable also non-specialized EEG laboratories to combine measurements of conventional low-frequency EEG with the analysis of concomitant cortical population spike responses.

Different levels of cortical excitability reflect clinical fluctuations in migraine

BACKGROUND

In a previous study we demonstrated that high-frequency oscillations (HFOs) elicited by median nerve stimulation are significantly correlated to clinical fluctuations of migraine. We aimed at verifying whether clinical fluctuations and HFO changes are correlated to N20 somatosensory evoked potential (SEP) recovery cycle, which is likely to reflect the functional refractoriness of primary somatosensory cortex neurons.

METHODS

We analysed both HFOs and N20 SEP recovery cycle to paired stimulation in 21 migraine patients and 18 healthy volunteers.

RESULTS

Shortened recovery cycle correlated with low-amplitude HFOs as well as with clinical worsening. By contrast, prolonged recovery cycle correlated with enhanced HFOs, as well as with spontaneous clinical improvement.

CONCLUSIONS

In our migraine patients the strict relationship between presynaptic HFO amplitude and N20 recovery function suggests that changes of both parameters might be caused by modifications of the thalamo-cortical drive. Our findings suggest that the thalamo-cortical drive during interictal stages could fluctuate from abnormally high to abnormally low levels, depending on mechanisms which reduce cortical excitability in spontaneously improving patients, and increase cortical excitability in spontaneously worsening ones.

Somatosensory High Frequency Oscillations reflect clinical fluctuations in migraine

OBJECTIVE

It has been demonstrated that the early part of 600 Hz High Frequency Oscillations (HFOs), probably generated in the terminal part of thalamo-cortical somatosensory radiations, are abnormally reduced between attacks in migraineurs. We aimed at verifying whether spontaneous clinical fluctuations in migraine are correlated to HFO changes.

METHODS

We recorded somatosensory evoked potentials in 28 migraine patients. Clinical fluctuations (number of attacks in the 6 months preceding and following the test) were correlated to the HFOs' amplitudes. Moreover, eight out of 28 patients underwent a longer follow-up, including HFO control and clinical observation during the 12 months following the baseline recording.

RESULTS

The amplitude of early presynaptic HFOs was significantly correlated to the clinical evolution, since spontaneous worsening was associated with reduced presynaptic HFOs, whereas spontaneous improvement was associated with enhanced presynaptic HFOs (correlation test, p<0.05). No correlation was found between the amplitude of postsynaptic HFOs and clinical fluctuations. Patients undergoing longer follow-up showed substantially unchanged HFOs, accordingly with their stable clinical condition.

CONCLUSIONS

HFOs' enhancement in spontaneously improved patients can reflect the increased activity of brainstem arousal related structures, which in turn increases the thalamo-cortical drive and the cortical lateral inhibition mediated by GABAergic interneurons.

SIGNIFICANCE

HFOs' recording could represent a useful tool in the functional assessment of migraine.