TMS, cortical excitability and epilepsy: The clinical impact

'Remote inhibition' of motor cortex in Epileptic encephalopathy with spike-wave activation in sleep (EE-SWAS): A TMS based cortical excitability study

OBJECTIVES

The study compared real-time motor cortex excitability using transcranial magnetic stimulation (TMS)-derived parameters between children with epileptic encephalopathy with spike-wave activation in sleep (EE-SWAS) and age-matched neurotypical controls. The EE-SWAS group received steroids as standard of care and were longitudinally followed for three months.

MATERIALS & METHODS

Children aged 5-12 years with immunotherapy-naive EE-SWAS (spike-wave-index≥50 %) and neurotypical controls were enrolled. Cognitive and behavioral assessments were performed using valid psychometric tools. Real-time motor cortex excitability was assessed by measuring resting motor threshold (RMT), short intra-cortical inhibition (SICI) and long intra-cortical inhibition (LICI) in both groups. In EE-SWAS group, a follow up evaluation with TMS at 4- and 12-week intervals, EEG, and neurobehavioral assessments at 12-weeks were performed to assess the effect of steroids on cortical excitability and to determine electroclinical outcome.

RESULTS

Forty-eight children with suspected EE-SWAS and 26 neurotypical controls were screened; 20 were enrolled in each group. Children with EE-SWAS (mean age: 8.05 ± 1.76 years) had cognitive and behavioral problems (20/20), and ongoing seizures (12/20). At baseline, the dominant motor cortex was significantly inhibited in the EE-SWAS group compared to neurotypical children{RMT(%)[86.3 ± 6.96 vs 58.05 ± 4.71(p < 0.0001)]; LICI(%)[55.05 ± 4.39 vs 73.9 ± 3.75(p < 0.0001)]; SICI(%)[39.2 ± 4.36 vs 55.45 ± 4.78(p < 0.0001)]}. Reversal of motor cortex inhibition was sequentially observed in EE-SWAS group at 4- and 12-week follow-ups{(RMT[4, 12 weeks]: 71.45 ± 9.83, 63.45 ± 8.48); (LICI[4, 12 weeks]: 66.00 ± 6.26, 74.50 ± 5.36); (SICI[4, 12 weeks]: 49.35 ± 6.24, 56.05 ± 5.57)}[repeated-measures ANOVA: p < 0.0001].

CONCLUSION

Motor cortex is remotely inhibited in EE-SWAS, which may contribute to neurobehavioral impairment. Steroids can disinhibit/reverse the epilepsy-induced motor cortex inhibition leading to improvement in neurobehavior.

Drug-resistant juvenile myoclonic epilepsy: A literature review

The ILAE's Task Force on Nosology and Definitions revised in 2022 its definition of juvenile myoclonic epilepsy (JME), the most common idiopathic generalized epilepsy disorder, but this definition may well change again in the future. Although good drug response could almost be a diagnostic criterion for JME, drug resistance (DR) is observed in up to a third of patients. It is important to distinguish this from pseudoresistance, which is often linked to psychosocial problems or psychiatric comorbidities. After summarizing these aspects and the various definitions applied to JME, the present review lists the risk factors for DR-JME that have been identified in numerous studies and meta-analyses. The factors most often cited are absence seizures, young age at onset, and catamenial seizures. By contrast, photosensitivity seems to favor good treatment response, at least in female patients. Current hypotheses on DR mechanisms in JME are based on studies of either simple (e.g., cortical excitability) or more complex (e.g., anatomical and functional connectivity) neurophysiological markers, bearing in mind that JME is regarded as a neural network disease. This research has revealed correlations between the intensity of some markers and DR, and above all shed light on the role of these markers in associated neurocognitive and neuropsychiatric disorders in both patients and their siblings. Studies of neurotransmission have mainly pointed to impaired GABAergic inhibition. Genetic studies have generally been inconclusive. Increasing restrictions have been placed on the use of valproate, the standard antiseizure medication for this syndrome, owing to its teratogenic and developmental risks. Levetiracetam and lamotrigine are prescribed as alternatives, as is vagal nerve stimulation, and there are several other promising antiseizure drugs and neuromodulation methods. The development of better alternative treatments is continuing to take place alongside advances in our knowledge of JME, as we still have much to learn and understand.

Circuits in the motor cortex explain oscillatory responses to transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a popular method used to investigate brain function. Stimulation over the motor cortex evokes muscle contractions known as motor evoked potentials (MEPs) and also high-frequency volleys of electrical activity measured in the cervical spinal cord. The physiological mechanisms of these experimentally derived responses remain unclear, but it is thought that the connections between circuits of excitatory and inhibitory neurons play a vital role. Using a spiking neural network model of the motor cortex, we explained the generation of waves of activity, so called 'I-waves', following cortical stimulation. The model reproduces a number of experimentally known responses including direction of TMS, increased inhibition, and changes in strength. Using populations of thousands of neurons in a model of cortical circuitry we showed that the cortex generated transient oscillatory responses without any tuning, and that neuron parameters such as refractory period and delays influenced the pattern and timing of those oscillations. By comparing our network with simpler, previously proposed circuits, we explored the contributions of specific connections and found that recurrent inhibitory connections are vital in producing later waves that significantly impact the production of motor evoked potentials in downstream muscles (Thickbroom, 2011). This model builds on previous work to increase our understanding of how complex circuitry of the cortex is involved in the generation of I-waves.

Linking cortex and contraction-Integrating models along the corticomuscular pathway

Computational models of the neuromusculoskeletal system provide a deterministic approach to investigate input-output relationships in the human motor system. Neuromusculoskeletal models are typically used to estimate muscle activations and forces that are consistent with observed motion under healthy and pathological conditions. However, many movement pathologies originate in the brain, including stroke, cerebral palsy, and Parkinson's disease, while most neuromusculoskeletal models deal exclusively with the peripheral nervous system and do not incorporate models of the motor cortex, cerebellum, or spinal cord. An integrated understanding of motor control is necessary to reveal underlying neural-input and motor-output relationships. To facilitate the development of integrated corticomuscular motor pathway models, we provide an overview of the neuromusculoskeletal modelling landscape with a focus on integrating computational models of the motor cortex, spinal cord circuitry, α-motoneurons  and skeletal muscle in regard to their role in generating voluntary muscle contraction. Further, we highlight the challenges and opportunities associated with an integrated corticomuscular pathway model, such as challenges in defining neuron connectivities, modelling standardisation, and opportunities in applying models to study emergent behaviour. Integrated corticomuscular pathway models have applications in brain-machine-interaction, education, and our understanding of neurological disease.

Drug resistance in idiopathic generalized epilepsies: Evidence and concepts

Although approximately 10%-15% of patients with idiopathic generalized epilepsy (IGE)/genetic generalized epilepsy remain drug-resistant, there is no consensus or established concept regarding the underlying mechanisms and prevalence. This review summarizes the recent data and the current hypotheses on mechanisms that may contribute to drug-resistant IGE. A literature search was conducted in PubMed and Embase for studies on mechanisms of drug resistance published since 1980. The literature shows neither consensus on the definition nor a widely accepted model to explain drug resistance in IGE or one of its subsyndromes. Large-scale genetic studies have failed to identify distinct genetic causes or affected genes involved in pharmacokinetics. We found clinical and experimental evidence in support of four hypotheses: (1) "network hypothesis"-the degree of drug resistance in IGE reflects the severity of cortical network alterations, (2) "minor focal lesion in a predisposed brain hypothesis"-minor cortical lesions are important for drug resistance, (3) "interneuron hypothesis"-impaired functioning of γ-aminobutyric acidergic interneurons contributes to drug resistance, and (4) "changes in drug kinetics"-genetically impaired kinetics of antiseizure medication (ASM) reduce the effectiveness of available ASMs. In summary, the exact definition and cause of drug resistance in IGE is unknown. However, published evidence suggests four different mechanisms that may warrant further investigation.

The Relation between Induced Electric Field and TMS-Evoked Potentials: A Deep TMS-EEG Study

Transcranial magnetic stimulation (TMS) in humans induces electric fields (E-fields, EF) that perturb and modulate the brain’s endogenous neuronal activity and result in the generation of TMS-evoked potentials (TEPs). The exact relation of the characteristics of the induced E-field and the intensity of the brains’ response, as measured by electroencephalography (EEG), is presently unclear. In this pilot study, conducted on three healthy subjects and two patients with generalized epilepsy (total: 3 males, 2 females, mean age of 26 years; healthy: 2 males, 1 female, mean age of 25.7 years; patients: 1 male, 1 female, mean age of 26.5 years), we investigated the temporal and spatial relations of the E-field, induced by single-pulse stimuli, and the brain’s response to TMS. Brain stimulation was performed with a deep TMS device (BrainsWay Ltd., Jerusalem, Israel) and an H7 coil placed over the central area. The induced EF was computed on personalized anatomical models of the subjects through magneto quasi-static simulations. We identified specific time instances and brain regions that exhibit high positive or negative associations of the E-field with brain activity. In addition, we identified significant correlations of the brain’s response intensity with the strength of the induced E-field and finally prove that TEPs are better correlated with E-field characteristics than with the stimulator’s output. These observations provide further insight in the relation between E-field and the ensuing cortical activation, validate in a clinically relevant manner the results of E-field modeling and reinforce the view that personalized approaches should be adopted in the field of non-invasive brain stimulation.

Perspectives on Understanding Aberrant Brain Networks in Epilepsy

Epilepsy is a neurological disorder affecting approximately 70 million people worldwide. It is characterized by seizures that are complex aberrant dynamical events typically treated with drugs and surgery. Unfortunately, not all patients become seizure-free, and there is an opportunity for novel approaches to treat epilepsy using a network view of the brain. The traditional seizure focus theory presumed that seizures originated within a discrete cortical area with subsequent recruitment of adjacent cortices with seizure progression. However, a more recent view challenges this concept, suggesting that epilepsy is a network disease, and both focal and generalized seizures arise from aberrant activity in a distributed network. Changes in the anatomical configuration or widespread neural activities spanning lobes and hemispheres could make the brain more susceptible to seizures. In this perspective paper, we summarize the current state of knowledge, address several important challenges that could further improve our understanding of the human brain in epilepsy, and invite novel studies addressing these challenges.

EEG Evoked Potentials to Repetitive Transcranial Magnetic Stimulation in Normal Volunteers: Inhibitory TMS EEG Evoked Potentials

The impact of repetitive magnetic stimulation (rTMS) on cortex varies with stimulation parameters, so it would be useful to develop a biomarker to rapidly judge effects on cortical activity, including regions other than motor cortex. This study evaluated rTMS-evoked EEG potentials (TEP) after 1 Hz of motor cortex stimulation. New features are controls for baseline amplitude and comparison to control groups of sham stimulation. We delivered 200 test pulses at 0.20 Hz before and after 1500 treatment pulses at 1 Hz. Sequences comprised AAA = active stimulation with the same coil for test–treat–test phases (n = 22); PPP = realistic placebo coil stimulation for all three phases (n = 10); and APA = active coil stimulation for tests and placebo coil stimulation for treatment (n = 15). Signal processing displayed the evoked EEG waveforms, and peaks were measured by software. ANCOVA was used to measure differences in TEP peak amplitudes in post-rTMS trials while controlling for pre-rTMS TEP peak amplitude. Post hoc analysis showed reduced P60 amplitude in the active (AAA) rTMS group versus the placebo (APA) group. The N100 peak showed a treatment effect compared to the placebo groups, but no pairwise post hoc differences. N40 showed a trend toward increase. Changes were seen in widespread EEG leads, mostly ipsilaterally. TMS-evoked EEG potentials showed reduction of the P60 peak and increase of the N100 peak, both possibly reflecting increased slow inhibition after 1 Hz of rTMS. TMS-EEG may be a useful biomarker to assay brain excitability at a seizure focus and elsewhere, but individual responses are highly variable, and the difficulty of distinguishing merged peaks complicates interpretation.

Dynamic responses of neurons in different states under magnetic field stimulation

Transcranial magnetic stimulation (TMS) is an effective method to treat neurophysiological disorders by modulating the electrical activities of neurons. Neurons can exhibit complex nonlinear behaviors underlying the external stimuli. Currently, we do not know how stimulation interacts with endogenous neural activity. In this paper, the effects of magnetic field on spiking neuron, bursting neuron and bistable neuron are studied based on the Hodgkin-Huxley (HH) neuron model. The results show that the neurons in three different states can exhibit different dynamic responses under magnetic field stimulation. The magnetic field stimulation could increase or decrease the firing frequencies of spiking neuron, bursting neuron and bistable neuron. The transitions between different firing patterns of neurons can be promoted by changing the parameters of the magnetic field. Magnetic field stimulation has a minimal impact on the firing temporal sequence sequences in bursting neuron than that in spiking neuron and bistable neuron. These results provided an insight into the impact of neuronal states on neuronal dynamic responses under brain stimulation and show that subtle changes in external conditions and stimuli can cause complex neuronal responses. This study can help us understand the state-dependent coding mechanism of neurons under electromagnetic stimulation.

Magnetic evoked potential polyphasia in idiopathic/genetic generalized epilepsy: An endophenotype not associated with treatment response

OBJECTIVE

Increased Motor Evoked Potential (MEP) polyphasia was recently described in idiopathic/genetic generalized epilepsy (IGE). Here, we studied the association of MEP polyphasia with treatment response and other clinical characteristics in patients with IGE.

METHODS

MEPs were recorded from the biceps brachii, flexor carpi radialis and interosseus dorsalis muscles bilaterally during tonic contraction in IGE patients (n = 72) and historical controls (n = 54) after single pulse transcranial magnetic stimulation. Detailed clinical data was available for all IGE patients; predefined endpoint was the association of MEP polyphasia with treatment response.

RESULTS

The mean number of phases was higher in the interosseus dorsalis muscle (2.33 vs. 2.13, p = 0.002) in IGE patients as compared to normal controls, as was the proportion of MEPs with more than two phases in at least one test (59.4% vs. 30%, p < 0.002). MEP polyphasia did not differ between IGE patients and controls in the biceps brachii or the flexor carpi radialis muscles and was not associated with treatment response. Extensive exploratory analyses unveiled fewer phases under valproic acid treatment (p = 0.04) but no additional associations of MEP polyphasia in the interosseous muscle with other clinical characteristics.

CONCLUSION

MEP polyphasia is a subclinical symptom of IGE patients but is not associated with treatment response or other routinely assessed clinical characteristics.

SIGNIFICANCE

MEP polyphasia is a fixed feature of IGE not modified by clinical variables.

Surround inhibition in patients with juvenile myoclonic epilepsy

OBJECTIVE

In healthy subjects, there is a reduction in the amplitudes of somatosensory-evoked potentials (SEPs) after the simultaneous stimulation of two nerves compared to the sum of separate stimulations. This reduction is due to the inhibition of one area in the cortex after stimulation of the neighboring area, which results from the surround inhibition (SI) phenomenon. In this study, we aimed to investigate whether there was a decrease in SI of SEP in patients with juvenile myoclonic epilepsy (JME).

METHODS

We included 17 patients with JME and 18 healthy subjects. Groups were similar in terms of age and gender. We recorded SEPs after stimulating (i) median nerve (mSEP), (ii) ulnar nerve (uSEP), (iii) median and ulnar nerves simultaneously (muSEP) at wrist. The arithmetic sum (aSEP) of amplitudes of mSEP and uSEP was compared with the amplitudes of muSEP. We also calculated SI%.

RESULTS

The amplitudes of SEPs were significantly higher in the JME group than in the healthy subjects (mSEP, p = 0.005; uSEP, p = 0.032; muSEP, p = 0.014). In healthy subjects and the JME group, the amplitude of muSEP was significantly lower than the aSEP (p = 0.014; p = 0.001, respectively). However, SI% was significantly higher in the JME group (p = 0.010).

SIGNIFICANCE

Although the SI is maintained in JME patients, the higher SI% indicates an impairment relative to healthy subjects.

Lacosamide-Induced Dyskinesia in Children With Intractable Epilepsy

Lacosamide, an antiepileptic drug prescribed for children with refractory focal epilepsy, is generally well tolerated, with dose-dependent adverse effects. We describe 4 children who developed a movement disorder in conjunction with the initiation and/or uptitration of lacosamide. Three patients developed dyskinesias involving the face or upper extremity whereas the fourth had substantial worsening of chronic facial tics. The patients all had histories suggestive of opercular dysfunction: 3 had seizure semiologies including hypersalivation, facial and upper extremity clonus while the fourth underwent resection of polymicrogyria involving the opercula. Onset, severity, and resolution of dyskinesias correlated with lacosamide dosing. These cases suggest that pediatric patients with dysfunction of the opercular cortex are at increased risk for developing drug-induced dyskinesias on high-dose lacosamide therapy. Practitioners should be aware of this potential side effect and consider weaning lacosamide or video electroencephalography (EEG) for differential diagnosis, particularly in pediatric patients with underlying opercular dysfunction.

Effects of transcranial magnetic stimulation on the performance of the activities of daily living and attention function after stroke: a randomized controlled trial

OBJECTIVE

We aimed to interrogate the effects of transcranial magnetic stimulation (TMS) on the performance in activities of daily living (ADL) and attention function after stroke.

DESIGN

Randomized controlled trial.

SETTING

Inpatient rehabilitation hospital.

SUBJECTS

We randomized 62 stroke patients with attention dysfunction who were randomly assigned into two groups, and two dropped out from each group. The TMS group (n = 29) and a sham group (n = 29), whose mean (SD) was 58.12 (6.72) years. A total of 33 (56.9%) patients had right hemisphere lesion while the rest 25 (43.1%) patients had left hemisphere lesion.

INTERVENTIONS

Patients in the TMS group received 10 Hz, 700 pulses of TMS, while those in the sham group received sham TMS for four weeks. All the participants underwent comprehensive cognitive training.

MAIN MEASURES

At baseline, and end of the four-week treatment, the performance in the activities of daily living was assessed by Functional Independence Measure (FIM). On the other side, attention dysfunction was screened by Mini-Mental State Examination (MMSE), while the attention function was assessed by the Trail Making Test-A (TMT-A), Digit Symbol Test (DST) and Digital Span Test (DS).

RESULTS

Our data showed a significant difference in the post-treatment gains in motor of Functional Independence Measure (13.00 SD 1.69 vs 4.21 SD 2.96), cognition of Functional Independence Measure (4.69 SD 1.56 vs 1.52 SD 1.02), total of Functional Independence Measure (17.69 SD 2.36 vs 5.72 SD 3.12), Mini-Mental State Examination (3.07 SD 1.36 vs 1.21 SD 0.62), time taken in Trail Making Test-A (96.67 SD 25.18 vs 44.28 SD 19.45), errors number in Trail Making Test-A (2.72 SD 1.03 vs 0.86 SD 1.03), Digit Symbol Test (3.76 SD 1.09 vs 0.76 SD 0.87) or Digital Span Test (1.69 SD 0.54 vs 0.90 SD 0.72) between the TMS group and the sham group (P < 0.05).

CONCLUSIONS

Taken together, we demonstrate that TMS improves the performance in the activities of daily living and attention function in patients with stroke.

Cortical excitability measured with transcranial magnetic stimulation in children with epilepsy before and after antiepileptic drugs

AIM

To evaluate cortical excitability with transcranial magnetic stimulation (TMS) in children with new-onset epilepsy before and after antiepileptic drugs (AEDs).

METHOD

Fifty-five drug-naïve patients (29 females, 26 males; 3-18y), with new-onset epilepsy were recruited from 1st May 2014 to 31st October 2017 at the Child Neurology Department, Queen Silvia's Children's Hospital, Gothenburg, Sweden. We performed TMS in 48 children (23 females, 25 males; mean [SD] age 10y [3y], range 4-15y) with epilepsy (27 generalized and 21 focal) before and after the introduction of AEDs. We used single- and paired-pulse TMS. We used single-pulse TMS to record resting motor thresholds (RMTs), stimulus-response curves, and cortical silent periods (CSPs). We used paired-pulse TMS to record intracortical inhibition and facilitation at short, long, and intermediate intervals.

RESULTS

There were no differences in cortical excitability between children with generalized and focal epilepsy at baseline. After AED treatment, RMTs increased (p=0.001), especially in children receiving sodium valproate (p=0.005). CSPs decreased after sodium valproate was administered (p=0.050). As in previous studies, we noted a negative correlation between RMT and age in our study cohort. Paired-pulse TMS could not be performed in most children because high RMTs made suprathreshold stimulation impossible.

INTERPRETATION

Cortical excitability as measured with RMT decreased after the introduction of AEDs. This was seen in children with both generalized and focal epilepsy who were treated with sodium valproate, although it was most prominent in children with generalized epilepsy. We suggest that TMS might be used as a prognostic tool to predict AED efficacy.

WHAT THIS PAPER ADDS

Resting motor threshold (RMT) correlated negatively with age in children with epilepsy. No differences in cortical excitability were noted between patients with generalized and focal epilepsy. Treatment with antiepileptic drugs decreased cortical excitability as measured with transcranial magnetic stimulation (TMS). Decreased cortical excitability with increased RMT was recorded, especially after sodium valproate treatment. Paired-pulse TMS was difficult to perform because of high RMTs in children.

Biomarkers Obtained by Transcranial Magnetic Stimulation of the Motor Cortex in Epilepsy

Epilepsy is associated with numerous neurodevelopmental disorders. Transcranial magnetic stimulation (TMS) of the motor cortex coupled with electromyography (EMG) enables biomarkers that provide measures of cortical excitation and inhibition that are particularly relevant to epilepsy and related disorders. The motor threshold (MT), cortical silent period (CSP), short interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long interval intracortical inhibition (LICI) are among TMS-derived metrics that are modulated by antiepileptic drugs. TMS may have a practical role in optimization of antiepileptic medication regimens, as studies demonstrate dose-dependent relationships between TMS metrics and acute medication administration. A close association between seizure freedom and normalization of cortical excitability with long-term antiepileptic drug use highlights a plausible utility of TMS in measures of anti-epileptic drug efficacy. Finally, TMS-derived biomarkers distinguish patients with various epilepsies from healthy controls and thus may enable development of disorder-specific biomarkers and therapies both within and outside of the epilepsy realm.

Are the days of counting seizures numbered?

PURPOSE OF REVIEW

The estimation of seizure frequency is a cornerstone of clinical management of epilepsy and the evaluation of new therapies. Current estimation approaches are significantly limited by several factors. Comparing patient diaries and objective estimates (through both inpatient video-EEG monitoring of and long-term ambulatory EEG studies) reveal that patients document seizures inaccurately. So far, few practical alternative methods of estimation have been available.

RECENT FINDINGS

We review the systems of counting currently utilized and their limitations, as well as the limitations imposed by problems defining clinical events. Alternative methodologies that permit the volatility of seizure rates to be accommodated, and possible alternative measures of brain excitability will be outlined. Recent developments in technologies around data capture, such as wearable and implantable devices, as well as significant advances in the ability to analyse the large data-sets supplied by these systems have provided a wealth of information.

SUMMARY

There are now unprecedented opportunities to utilize and apply these insights in routine clinical management and assessment of therapies. The rapid adoption of long-term, wearable monitoring systems will permit major advances in our understanding of the natural history of epilepsy, and lead to more effective therapies and improved patient safety.

Therapeutic effect of repetitive transcranial magnetic stimulation on non-lesional focal refractory epilepsy

The effects of repetitive transcranial magnetic stimulation (rTMS) on epilepsy remain unclear. This study was performed to investigate the effects of low-frequency rTMS on non-lesional focal epilepsy. This was a prospective open-label longitudinal study with four patients with multi-drug-resistant non-lesional focal epilepsy and no control group. The patients received daily doses of 900 pulses of 0.5-Hz stimulation for 10 days over the epileptic foci in neocortical areas determined by electrical source analysis. The outcomes were measured in terms of seizure reduction. The incidences of seizures were measured at baseline (4 weeks), intervention (2 weeks), and follow up (8 weeks). Seizure reduction was observed in three of four subjects. The effects of rTMS persisted over a follow-up period of 8 weeks. One of the four patients did not respond to rTMS and showed no seizure reduction. The low frequency rTMS would be an effective treatment for non-lesional focal refractory epilepsy, may be an adjunctive treatment with conventional medical treatment for refractory epilepsy. These results are therefore useful for planning treatment strategies for patients with refractory epilepsy, as well as for treatment of epilepsy.

Closed-loop Neuropharmacology for Epilepsy: Distant Dream or Future Reality?

Epilepsy is considered the most frequent severe neurological condition but most patients treated with medication become seizure free. The management of treatment, however, is highly empirical, mainly relying on observation. A closed-loop therapy for epilepsy would be very valuable for more efficient treatment regimens. Here we discuss monitoring treatment (therapeutic drug monitoring) and the potential developments in this field, as well as providing a review of potential biomarkers that could be used to monitor the disease activity. Finally, we consider the pharmacogenetic input in epilepsy treatment.

Trajectory of Parvalbumin Cell Impairment and Loss of Cortical Inhibition in Traumatic Brain Injury

Many neuropsychiatric symptoms that follow traumatic brain injury (TBI), including mood disorders, sleep disturbance, chronic pain, and posttraumatic epilepsy (PTE) are attributable to compromised cortical inhibition. However, the temporal trajectory of cortical inhibition loss and its underlying mechanisms are not known. Using paired-pulse transcranial magnetic stimulation (ppTMS) and immunohistochemistry, we tracked functional and cellular changes of cortical inhibitory network elements after fluid-percussion injury (FPI) in rats. ppTMS revealed a progressive loss of cortical inhibition as early as 2 weeks after FPI. This profile paralleled the increasing levels of cortical oxidative stress, which was accompanied by a gradual loss of parvalbumin (PV) immunoreactivity in perilesional cortex. Preceding the PV loss, we identified a degradation of the perineuronal net (PNN)-a specialized extracellular structure enwrapping cortical PV-positive (PV+) inhibitory interneurons which binds the PV+ cell maintenance factor, Otx2. The trajectory of these impairments underlies the reduced inhibitory tone, which can contribute to posttraumatic neurological conditions, such as PTE. Taken together, our results highlight the use of ppTMS as a biomarker to track the course of cortical inhibitory dysfunction post-TBI. Moreover, the neuroprotective role of PNNs on PV+ cell function suggests antioxidant treatment or Otx2 enhancement as a promising prophylaxis for post-TBI symptoms.

Effect of repetitive transcranial magnetic stimulation on action myoclonus: A pilot study in patients with EPM1

OBJECTIVE

The objective of this study was to explore the short-term effects of repetitive transcranial magnetic stimulation (rTMS) on action myoclonus.

METHODS

Nine patients with Unverricht-Lundborg (EPM1) progressive myoclonus epilepsy type underwent two series of 500 stimuli at 0.3Hz through round coil twice a day for five consecutive days. Clinical and neurophysiological examinations were performed two hours before starting the first rTMS session and two hours after the end of the last rTMS session.

RESULTS

Eight patients completed the protocol; one discontinued because of a transient increase in spontaneous jerks. The unified myoclonus rating scale indicated a 25% reduction in posttreatment myoclonus with action score associated with an increase in the cortical motor threshold and lengthening of the cortical silent period (CSP). The decrease in the myoclonus with action scores correlated with the prolongation of CSP.

CONCLUSIONS

Repetitive transcranial magnetic stimulation can be safely used in patients with EPM1, improves action myoclonus, and partially restores deficient cortical inhibition.

Motor cortex excitability in seizure-free STX1B mutation carriers with a history of epilepsy and febrile seizures

OBJECTIVE

Mutations in STX1B encoding the presynaptic protein syntaxin-1B are associated with febrile seizures with or without epilepsy. It is unclear to what extent these mutations are linked to abnormalities of cortical glutamatergic or GABAergic neurotransmission. We explored this question using single- and paired-pulse transcranial magnetic stimulation (TMS) excitability markers.

METHODS

We studied nine currently asymptomatic adult STX1B mutation carriers with history of epilepsy and febrile seizures, who had been seizure-free for at least eight years without antiepileptic drug treatment, and ten healthy age-matched controls. Resting motor threshold (RMT), and input-output curves of motor evoked potential (MEP) amplitude, short-interval intracortical inhibition (SICI, marker of GABA_Aergic excitability) and intracortical facilitation (ICF, marker of glutamatergic excitability) were tested.

RESULTS

RMT, and input-output curves of MEP amplitude, SICI and ICF revealed no significant differences between STX1B mutation carriers and healthy controls.

CONCLUSIONS

Findings suggest normal motor cortical GABA_Aergic and glutamatergic excitability in currently asymptomatic STX1B mutation carriers.

SIGNIFICANCE

TMS measures of motor cortical excitability show utility in demonstrating normal excitability in adult STX1B mutation carriers with history of seizures.

Neuropharmacological and neuroprotective activities of some metabolites produced by cell suspension culture of Waltheria americana Linn

Waltheria americana is a plant used in Mexican traditional medicine to treat some nervous system disorders. The aims of the present study were to isolate and determine the neuropharmacological and neurprotective activities of metabolites produced by a cell suspension culture of Waltheria americana. Submerged cultivation of W. americana cells provided biomass. A methanol-soluble extract (WAsc) was obtained from biomass. WAsc was fractionated yielding the chromatographic fractions 4WAsc-H₂O and WAsc-CH₂Cl₂. For the determination of anticonvulsant activity in vivo, seizures were induced in mice by pentylenetetrazol (PTZ). Neuropharmacological activities (release of gamma amino butyric acid (GABA) and neuroprotection) of chromatographic fractions were determined by in vitro histological analysis of brain sections of mice post mortem. Fraction 4WAsc-H₂O (containing saccharides) did not produce neuronal damage, neurodegeneration, interstitial tissue edema, astrocytic activation, nor cell death. Pretreatment of animals with 4WAsc-H₂O and WAsc-CH₂Cl₂ from W. americana cell suspensions induced an increase in: GABA release, seizure latency, survival time, neuroprotection, and a decrease in the degree of severity of tonic/tonic-clonic convulsions, preventing PTZ-induced death of up to 100% of animals of study. Bioactive compounds produced in suspension cell culture of W. americana produce neuroprotective and neuropharmacological activities associated with the GABAergic neurotransmission system.

Comparison between adaptive and fixed stimulus paired-pulse transcranial magnetic stimulation (ppTMS) in normal subjects

•

Excitability indices from adaptive paired-pulse TMS correlated to those of fixed-stimulus ppTMS.

•

Floor/ceiling effects in fixed-stimulus ppTMS excitability data did not occur with adaptive ppTMS.

•

Adaptive ppTMS seems to be more sensitive in detecting changes in cortical inhibition.

Objectives

Paired-pulse TMS (ppTMS) examines cortical excitability but may require lengthy test procedures and fine tuning of stimulus parameters due to the inherent variability of the elicited motor evoked potentials (MEPs) and their tendency to exhibit a ‘ceiling/floor effects’ in inhibition trials. Aiming to overcome some of these limitations, we implemented an ‘adaptive’ ppTMS protocol and compared the obtained excitability indices with those from ‘conventional’ fixed-stimulus ppTMS.

Methods

Short- and long interval intracortical inhibition (SICI and LICI) as well as intracortical facilitation (ICF) were examined in 20 healthy subjects by adaptive ppTMS and fixed-stimulus ppTMS. The test stimulus intensity was either adapted to produce 500 μV MEPs (by a maximum likelihood strategy in combination with parameter estimation by sequential testing) or fixed to 120% of resting motor threshold (rMT). The conditioning stimulus was 80% rMT for SICI and ICF and 120% MT for LICI in both tests.

Results

There were significant (p < 0.05) intraindividual correlations between the two methods for all excitability measures. There was a clustering of SICI and LICI indices near maximal inhibition (‘ceiling effect’) in fixed-stimulus ppTMS which was not observed for adaptive SICI and LICI.

Conclusions

Adaptive ppTMS excitability data correlates to those acquired from fixed-stimulus ppTMS.

Significance

Adaptive ppTMS is easy to implement and may serve as a more sensitive method to detect changes in cortical inhibition than fixed stimulus ppTMS. Whether equally confident data are produced by less stimuli with our adaptive approach (as already confirmed for motor threshold estimation) remains to be explored.

Defective interhemispheric inhibition in drug-treated focal epilepsies

BACKGROUND

Focal epilepsies (FEs) arise from a lateralized network, while in generalized epilepsies (GEs) there is a bilateral involvement from the outset. Intuitively, the corpus callosum is the anatomical substrate for interhemispheric spread.

OBJECTIVE

We used transcranial magnetic stimulation (TMS) to explore whether there are any physiological differences in the corpus callosum of drug-treated patients with FE and those with genetic GE (GGE), compared to healthy subjects (HS).

METHODS

TMS was used to measure the interhemispheric inhibition (IHI) from right-to-left primary motor cortex (M1) and viceversa in 16 patients with FE, 17 patients with GGE and 17 HS. A conditioning stimulus (CS) was given to one M1 10 and 50 ms before a test stimulus delivered to the contralateral M1. Motor evoked potentials (MEPs) were analysed both as a function of the side of stimulation and of the epileptic focus (left-right).

RESULTS

In HS, IHI was reproducible with suppression of MEPs at ISIs of 10 and 50 ms. Similar effects occurred in GGE patients. FE patients behaved differently, since IHI was significantly reduced bilaterally. When FE patients were stratified according to the side of their epileptic focus, the long-ISI IHI (=50 ms) appeared to be defective only when the CS was applied over the "focal" hemisphere.

CONCLUSIONS

FE patients had a defective inhibitory response of contralateral M1 to inputs travelling from the "focal" hemisphere that was residual to the drug action. Whilst IHI changes would not be crucial for the GGE pathophysiology, they may represent one key factor for the contralateral spread of focal discharges, and seizure generalization.

Altered visual contrast gain control is sensitive for idiopathic generalized epilepsies

OBJECTIVE

Visual hyperexcitability in the form of abnormal contrast gain control has been shown in photosensitive epilepsy and idiopathic generalized epilepsies. We assessed the accuracy and reliability of measures of visual contrast gain control in discerning individuals with idiopathic generalized epilepsies from healthy controls.

METHODS

Twenty-four adult patients with idiopathic generalized epilepsy and 32 neurotypical control subjects from two study sites participated in a prospective, cross-sectional study. We recorded steady-state visual evoked potentials to a wide range of contrasts of a flickering grating stimulus. The resultant response magnitude vs. contrast curves were fitted to a standard model of contrast response function, and the model parameters were used as input features to a linear classifier to separate patients from controls. Additionally we compared the relative contribution of model parameters towards the classification using a sparse feature-selection approach.

RESULTS

Classification accuracy was 80% or better. Sensitivity and specificity both were 80-85%. Cross validation confirmed robust classifier performance generalizable across the data from the two samples. Patients' relative lack of gain control at high contrasts was the most important information distinguishing patients from controls.

CONCLUSIONS

Individuals with idiopathic generalized epilepsy were distinguishable from the neurotypical with a high degree of accuracy and reliability by a reduction in gain control at high contrasts.

SIGNIFICANCE

Gain control is an essential neural operation that regulates neuronal sensitivity to stimuli and may represent a novel biomarker of hyperexcitability.

Cathodal transcranial direct-current stimulation for treatment of drug-resistant temporal lobe epilepsy: A pilot randomized controlled trial

Objective

To investigate the effect of cathodal transcranial direct‐current stimulation (c‐tDCS) on seizure frequency in patients with drug‐resistant temporal lobe epilepsy (TLE).

Method

Twenty‐nine patients with drug‐resistant TLE participated in this study. They were randomized to experimental or sham group. Twenty participants (experimental group) received within‐session repeated c‐tDCS intervention over the affected temporal lobe, and nine (sham group) received sham tDCS. Paired‐pulse transcranial magnetic stimulation was used to assess short interval intracortical inhibition (SICI) in primary motor cortex ipsilateral to the affected temporal lobe. SICI was measured from motor evoked potentials recorded from the contralateral first dorsal interosseous muscle. Adverse effects were monitored during and after each intervention in both groups. A seizure diary was given to each participant to complete for 4 weeks following the tDCS intervention. The mean response ratio was calculated from their seizure rates before and after the tDCS intervention.

Results

The experimental group showed a significant increase in SICI compared to the sham group (F = 10.3, p = 0.005). None of the participants reported side effects of moderate or severe degree. The mean response ratio in seizure frequency was −42.14% (standard deviation [SD] 35.93) for the experimental group and −16.98% (SD 52.41) for the sham group.

Significance

Results from this pilot study suggest that tDCS may be a safe and efficacious nonpharmacologic intervention for patients with drug‐resistant TLE. Further evaluation in larger double‐blind randomized controlled trials is warranted.

Cortico-cortical and motor evoked potentials to single and paired-pulse stimuli: An exploratory transcranial magnetic and intracranial electric brain stimulation study

BACKGROUND

Paired-pulse (PP) paradigms are commonly employed to assess in vivo cortical excitability using transcranial magnetic stimulation (TMS) to stimulate the primary motor cortex and modulate the induced motor evoked potential (MEP). Single-pulse cortical direct electrical stimulation (DES) during intracerebral EEG monitoring allows the investigation of brain connectivity by eliciting cortico-cortical evoked potentials (CCEPs). However, PP paradigm using intracerebral DES has rarely been reported and has never been previously compared with TMS.

OBJECTIVE

The work was intended (i) to verify that the well-established modulations of MEPs following PP TMS remain similar using DES in the motor cortex, and (ii) to evaluate if a similar pattern could be observed in distant cortico-cortical connections through modulations of CCEP.

METHODS

Three patients undergoing intracerebral EEG monitoring with electrodes implanted in the central region were studied. Single-pulse DES (1-3 mA, 1 ms, 0.2 Hz) and PP DES using six interstimulus intervals (5, 15, 30, 50, 100, and 200 ms) in the motor cortex with concomitant recording of CCEPs and MEPs in contralateral muscles were performed. Finally, a navigated PP TMS session targeted the intracranial stimulation site to record TMS-induced MEPs in two patients.

RESULTS

MEP modulations elicited by PP intracerebral DES proved similar among the three patients and to those obtained by PP TMS. CCEP modulations elicited by PP intracerebral DES usually showed a pattern comparable to that of MEP, although a different pattern could be observed occasionally.

CONCLUSION

PP intracerebral DES seems to involve excitatory and inhibitory mechanisms similar to PP TMS and allows the recording of intracortical inhibition and facilitation modulation on cortico-cortical connections. Hum Brain Mapp 37:3767-3778, 2016. © 2016 Wiley Periodicals, Inc.

Lateralizing Cortical Excitability in Drug Naïve Patients with Generalized or Focal Epilepsy

Background and Purpose:

Numerous transcranial magnetic stimulation (TMS) studies have defined the characteristic features of TMS in epilepsy. TME parameters were expected to classify the epilepsy syndrome or drug responses. However, the results such as cortical silent periods (CSP) are variable according to conditions of patients. Here, we investigate whether specific TMS parameters have localizing or lateralizing values in drug-naïve epilepsy patients.

Methods:

We recruited 148 consecutive untreated patients with epilepsy (idiopathic generalized epilepsy (IGE) 38, focal epilepsy (FE) 110, mean age 31.4 years) and 38 age- and gender-matched normal subjects. We obtained resting motor threshold (RMT), motor-evoked potential (MEP), CSP, short interval intracortical inhibition (SICI, inter-stimuli interval 2–5 ms), and intracortical facilitation (ICF, inter-stimuli interval 10–20 ms). TMS were performed during a seizure-free state of more than 48 h.

Results:

In IGE, no interhemispheric difference in CSP was found (p > 0.05). However, the mean CSP was longer in IGE patients than in normal controls at all stimulus intensities (p < 0.05). The mean CSP in ipsilateral hemisphere (IH) of FE was significantly longer at all stimulus intensities than that in normal controls (p < 0.001). The CSP in IH was longer than that in the contralateral hemisphere of FE. There was no significant difference in CSP between FE and IGE. SICI was significantly reduced only in the IH of FE versus normal subjects. RMT, MEP amplitudes, and ICF did not differ among IGE, FE, and normal controls.

Conclusions:

We found that prolonged CSP and reduced SICI in FE indicate asymmetrically increased cortical inhibition and excitation in the epileptic hemispheres. It suggests that CSP among TMS parameters has a crucial role to lateralize the epileptic hemisphere in FE.

Neuron matters: electric activation of neuronal tissue is dependent on the interaction between the neuron and the electric field

In laboratory research and clinical practice, externally-applied electric fields have been widely used to control neuronal activity. It is generally accepted that neuronal excitability is controlled by electric current that depolarizes or hyperpolarizes the excitable cell membrane. What determines the amount of polarization? Research on the mechanisms of electric stimulation focus on the optimal control of the field properties (frequency, amplitude, and direction of the electric currents) to improve stimulation outcomes. Emerging evidence from modeling and experimental studies support the existence of interactions between the targeted neurons and the externally-applied electric fields. With cell-field interaction, we suggest a two-way process. When a neuron is positioned inside an electric field, the electric field will induce a change in the resting membrane potential by superimposing an electrically-induced transmembrane potential (ITP). At the same time, the electric field can be perturbed and re-distributed by the cell. This cell-field interaction may play a significant role in the overall effects of stimulation. The redistributed field can cause secondary effects to neighboring cells by altering their geometrical pattern and amount of membrane polarization. Neurons excited by the externally-applied electric field can also affect neighboring cells by ephaptic interaction. Both aspects of the cell-field interaction depend on the biophysical properties of the neuronal tissue, including geometric (i.e., size, shape, orientation to the field) and electric (i.e., conductivity and dielectricity) attributes of the cells. The biophysical basis of the cell-field interaction can be explained by the electromagnetism theory. Further experimental and simulation studies on electric stimulation of neuronal tissue should consider the prospect of a cell-field interaction, and a better understanding of tissue inhomogeneity and anisotropy is needed to fully appreciate the neural basis of cell-field interaction as well as the biological effects of electric stimulation.

Transcranial Magnetic Stimulation Combined with EEG Reveals Covert States of Elevated Excitability in the Human Epileptic Brain

BACKGROUND

Transcranial magnetic stimulation combined with electroencephalogram (TMS-EEG) can be used to explore the dynamical state of neuronal networks. In patients with epilepsy, TMS can induce epileptiform discharges (EDs) with a stochastic occurrence despite constant stimulation parameters. This observation raises the possibility that the pre-stimulation period contains multiple covert states of brain excitability some of which are associated with the generation of EDs.

OBJECTIVE

To investigate whether the interictal period contains "high excitability" states that upon brain stimulation produce EDs and can be differentiated from "low excitability" states producing normal appearing TMS-EEG responses.

METHODS

In a cohort of 25 patients with Genetic Generalized Epilepsies (GGE) we identified two subjects characterized by the intermittent development of TMS-induced EDs. The high-excitability in the pre-stimulation period was assessed using multiple measures of univariate time series analysis. Measures providing optimal discrimination were identified by feature selection techniques. The "high excitability" states emerged in multiple loci (indicating diffuse cortical hyperexcitability) and were clearly differentiated on the basis of 14 measures from "low excitability" states (accuracy = 0.7).

CONCLUSION

In GGE, the interictal period contains multiple, quasi-stable covert states of excitability a class of which is associated with the generation of TMS-induced EDs. The relevance of these findings to theoretical models of ictogenesis is discussed.

TMS and drugs revisited 2014

The combination of pharmacology and transcranial magnetic stimulation to study the effects of drugs on TMS-evoked EMG responses (pharmaco-TMS-EMG) has considerably improved our understanding of the effects of TMS on the human brain. Ten years have elapsed since an influential review on this topic has been published in this journal (Ziemann, 2004). Since then, several major developments have taken place: TMS has been combined with EEG to measure TMS evoked responses directly from brain activity rather than by motor evoked potentials in a muscle, and pharmacological characterization of the TMS-evoked EEG potentials, although still in its infancy, has started (pharmaco-TMS-EEG). Furthermore, the knowledge from pharmaco-TMS-EMG that has been primarily obtained in healthy subjects is now applied to clinical settings, for instance, to monitor or even predict clinical drug responses in neurological or psychiatric patients. Finally, pharmaco-TMS-EMG has been applied to understand the effects of CNS active drugs on non-invasive brain stimulation induced long-term potentiation-like and long-term depression-like plasticity. This is a new field that may help to develop rationales of pharmacological treatment for enhancement of recovery and re-learning after CNS lesions. This up-dated review will highlight important knowledge and recent advances in the contribution of pharmaco-TMS-EMG and pharmaco-TMS-EEG to our understanding of normal and dysfunctional excitability, connectivity and plasticity of the human brain.