Asymmetry of cortical excitability revealed by transcranial stimulation in a patient with focal motor epilepsy and cortical myoclonus

Rethinking the neurophysiological concept of cortical myoclonus

Cortical myoclonus is thought to result from abnormal electrical discharges arising in the sensorimotor cortex. Given the ease of recording of cortical discharges, electrophysiological features of cortical myoclonus have been better characterized than those of subcortical forms, and electrophysiological criteria for cortical myoclonus have been proposed. These include the presence of giant somatosensory evoked potentials, enhanced long-latency reflexes, electroencephalographic discharges time-locked to individual myoclonic jerks and significant cortico-muscular connectivity. Other features that are assumed to support the cortical origin of myoclonus are short-duration electromyographic bursts, the presence of both positive and negative myoclonus and cranial-caudal progression of the jerks. While these criteria are widely used in clinical practice and research settings, their application can be difficult in practice and, as a result, they are fulfilled only by a minority of patients. In this review we reappraise the evidence that led to the definition of the electrophysiological criteria of cortical myoclonus, highlighting possible methodological incongruencies and misconceptions. We believe that, at present, the diagnostic accuracy of cortical myoclonus can be increased only by combining observations from multiple tests, according to their pathophysiological rationale; nevertheless, larger studies are needed to standardise the methods, to resolve methodological issues, to establish the diagnostic criteria sensitivity and specificity and to develop further methods that might be useful to clarify the pathophysiology of myoclonus.

Biomarkers Obtained by Transcranial Magnetic Stimulation in Neurodevelopmental Disorders

SUMMARY

Transcranial magnetic stimulation (TMS) is a method for focal brain stimulation that is based on the principle of electromagnetic induction where small intracranial electric currents are generated by a powerful fluctuating magnetic field. Over the past three decades, TMS has shown promise in the diagnosis, monitoring, and treatment of neurological and psychiatric disorders in adults. However, the use of TMS in children has been more limited. We provide a brief introduction to the TMS technique; common TMS protocols including single-pulse TMS, paired-pulse TMS, paired associative stimulation, and repetitive TMS; and relevant TMS-derived neurophysiological measurements including resting and active motor threshold, cortical silent period, paired-pulse TMS measures of intracortical inhibition and facilitation, and plasticity metrics after repetitive TMS. We then discuss the biomarker applications of TMS in a few representative neurodevelopmental disorders including autism spectrum disorder, fragile X syndrome, attention-deficit hyperactivity disorder, Tourette syndrome, and developmental stuttering.

The Effect of Epilepsy and Antiepileptic Drugs on Cortical Motor Excitability in Patients With Temporal Lobe Epilepsy

OBJECTIVES

Transcranial magnetic stimulation (TMS) has been used to assess cortical disinhibition/excitation with epilepsy and determine the degree of patients' response to antiepileptic drugs (AEDs). However, the results of studies are variable and conflicting. We assessed cortical motor excitability in adults with temporal lobe epilepsy (TLE).

METHODS

The TMS parameters used for assessment were: resting (RMT) and active (AMT) motor thresholds, cortical silent period (CSP), and central motor conduction time (CMCT).

RESULTS AND CONCLUSIONS

This study included 40 adults (males, 22; females, 18) with TLE with impaired awareness or to bilateral tonic clonic seizures (mean age, 32.50 ± 3.38 years; duration of illness, 6.15 ± 2.02 years) and on treatment with AEDs (valproate, 15; carbamazepine, 15; levetiracetam, 10]. The majority (62.5%) were seizure-free for ≥1 year on AEDs before TMS testing. All had normal brain magnetic resonance imaging except two, who had mesial temporal sclerosis. Comparing the entire patients with controls, patients had significantly bihemispheric higher RMT and AMT particularly over the epileptic hemisphere and shorter CSP and CMCT in the epileptic hemisphere. Shorter CSP and CMCT were observed in patients on valproate or carbamazepine and those who were uncontrolled on medications but not with levetiracetam. Significant correlations were identified between RMT and AMT (P = 0.01) and between CSP and CMCT (P = 0.001). We conclude that chronic TLE had increased cortical disinhibition in the epileptic hemisphere which can spread outside the epileptogenic zone despite the apparent control on AEDs. The TMS studies using CSP and CMCT may help future prediction of pharmacoresistance and, therefore, the need of combined AEDs with multiple mechanisms of action.

Cortical excitability in epilepsy and the impact of antiepileptic drugs: transcranial magnetic stimulation applications

INTRODUCTION

Epileptic conditions are characterized by impaired cortical excitation/inhibition balance and interneuronal disinhibition. Transcranial magnetic stimulation (TMS) is a neurophysiological method that assesses brain excitation/inhibition.

AREA COVERED

This review was written after a detailed search in PubMed, EMBASE, ISI web of science, SciELO, Scopus, and Cochrane Controlled Trials databases from 1990 to 2020. It summarizes TMS applications for diagnostic and therapeutic purposes in epilepsy. TMS studies help to distinguish different epilepsy conditions and explore the antiepileptic drugs' (AEDs') effects on neuronal microcircuits and plasticity mechanisms. Repetitive TMS studies showed that low-frequency rTMS (0.33-1 Hz) can reduce seizures' frequency in refractory epilepsy or pause ongoing seizures; however, there is no current approval for its use in such patients as adjunctive treatment to AEDs.

EXPERT OPINION

There are variable and conflicting TMS results which reflect the distinct pathogenic mechanisms of each epilepsy condition, the dynamic epileptogenic process over the long disease course resulting in the development of recurrent spontaneous seizures and/or progression of epilepsy after it is established, and the differential effect of AEDs on cortical excitability. Future epilepsy research should focus on combined TMS/functional connectivity studies that explore the complex cortical excitability circuits and networks using different TMS parameters and techniques.

A Meta-analysis of the Cortical Silent Period in Epilepsies

OBJECTIVE

The cortical silent period (CSP) following transcranial magnetic stimulation reflects GABAB-mediated inhibition in the primary motor cortex (M1) and could contribute to understand the pathophysiological substrates of epileptic conditions. Increased CSP duration has been reported in idiopathic generalized epilepsy (IGE) and in partial epilepsy (PE) involving the M1, although other studies yielded discordant findings. We used meta-analysis to systematically assess the consistency of CSP changes in untreated patients with epilepsies.

METHODS

Databases were searched for controlled studies evaluating the CSP in drug-naïve or drug-free patients with IGE or PE. For each study, the mean difference with 95% confidence intervals (CIs) between CSP duration obtained in patients and controls was calculated. The effect of motor threshold (MT) on the CSP duration has also been explored by meta-analysis and meta-regression.

RESULTS

Fourteen studies (267 patients and 234 controls) were included. A significant mean difference (14.16 ms, 95% CI, 1.20, 27.11 ms) was found, with longer CSP in patients than in controls. The mean difference was still greater (18.05 ms) if only the 202 IGE patients were analyzed. No MT difference emerged between patients and controls. Meta-regression showed no relationship between MT and CSP duration.

CONCLUSION

Meta-analysis confirms CSP modifications in epilepsies, with enhancement of this cortical inhibitory measure at least in most IGE patients. This provides rationale for further investigations aiming to verify the hypotheses that increased CSP reflects compensatory neural phenomena counteracting transition from the interictal to ictal state and that CSP variability reflects the pathophysiological heterogeneity of epileptic syndromes.

Motor cortical excitability in peritoneal dialysis: a single-pulse TMS study

The aim of this paper is to investigate cortical excitability in patients with end-stage renal disease receiving peritoneal dialysis (PD) without any symptoms suggestive of uremic encephalopathy. We performed transcranial magnetic stimulation for 52 PD patients and 28 normal subjects. We compared the active motor threshold (AMT), resting motor threshold (RMT), root latency, central motor conduction time (CMCT), and cortical silent period (CSP) in PD patients to those in normal subjects. AMT, RMT, CMCT, and CSP were not significantly different between PD patients and normal subjects. However, root latency was significantly prolonged in PD patients compared to normal subjects. The root latency correlated linearly with HbA1c or duration of PD in the patients. The results suggest that the corticospinal tract and the cortical and spinal excitabilities are preserved but the peripheral nerves are disturbed in PD patients. The severity of peripheral neuropathy corresponds to the severity of DM and the duration of PD. We uncovered no evidence suggestive of any subclinical abnormality of the motor cortical excitability in PD patients.

Intraoperative neurophysiology of the motor system in children: a tailored approach

INTRODUCTION

Intraoperative neurophysiology has moved giant steps forward over the past 15 years thanks to the advent of techniques aimed to reliably assess the functional integrity of motor areas and pathways.

INTRAOPERATIVE NEUROPHYSIOLOGICAL TECHNIQUES

Motor evoked potentials recorded from the muscles and/or the spinal cord (D-wave) after transcranial electrical stimulation allow to preserve the integrity of descending pathways, especially the corticospinal tract (CT), during brain and spinal cord surgery. Mapping techniques allow to identify the motor cortex through direct cortical stimulation and to localize the CT at subcortical levels during brain and brainstem surgery. These techniques are extensively used in adult neurosurgery and, in their principles, can be applied to children. However, especially in younger children, the motor system is still under development, making both mapping and monitoring techniques more challenging. In this paper, we review intraoperative neurophysiological techniques commonly used in adult neurosurgery and discuss their application to pediatric neurosurgery, in the light of preliminary experience from our and other centers. The principles of development and maturation of the motor system, and especially of the CT, are reviewed focusing on clinical studies with transcranial magnetical stimulation.

Electrophysiological features of myoclonus-dystonia

Inherited myoclonus-dystonia (M-D) is an autosomal dominant disorder characterized by myoclonus and dystonia that often improves with alcohol. To examine the electrophysiologic characteristics of M-D, we studied 6 patients from 4 different families and 9 age-matched healthy subjects. Neurophysiological studies performed include electromyography (EMG)-electroencephalography (EEG) polygraphy, jerk-locked back-averaged EEG, somatosensory evoked potentials (SEP), long-latency reflex (LLR) to median and digital nerve stimulation, and transcranial magnetic stimulation studies with short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long-interval intracortical inhibition (LICI). All 6 patients showed myoclonus and dystonia on clinical examination and EMG testing. The EMG burst durations ranged from 30.4 to 750.6 milliseconds (mean, 101.5 milliseconds). Jerk-locked back-averaged EEG failed to reveal any preceding cortical correlates. Median nerve SEP revealed no giant potential. No patients had exaggerated LLR to median or digital nerve stimulation. There was no significant difference in SICI, ICF, and LICI between M-D patients and normal subjects. Myoclonus in inherited M-D is likely of subcortical origin. Normal intracortical inhibition and facilitation suggest that the GABAergic circuits in the motor cortex are largely intact and that the mechanisms of myoclonus and dystonia are different from those for cortical myoclonus and other dystonic disorders.

Transcranial magnetic stimulation in child neurology: current and future directions

Transcranial magnetic stimulation (TMS) is a method for focal brain stimulation based on the principle of electromagnetic induction, where small intracranial electric currents are generated by a powerful, rapidly changing extracranial magnetic field. Over the past 2 decades TMS has shown promise in the diagnosis, monitoring, and treatment of neurological and psychiatric disease in adults, but has been used on a more limited basis in children. We reviewed the literature to identify potential diagnostic and therapeutic applications of TMS in child neurology and also its safety in pediatrics. Although TMS has not been associated with any serious side effects in children and appears to be well tolerated, general safety guidelines should be established. The potential for applications of TMS in child neurology and psychiatry is significant. Given its excellent safety profile and possible therapeutic effect, this technique should develop as an important tool in pediatric neurology over the next decade.

Low frequency repetitive transcranial magnetic stimulation over premotor cortex can improve cortical tremor

OBJECTIVE

To examine the effects of a 30 min, 1 Hz subthreshold rTMS in a case of cortical tremor which is caused by hyperexcitability of sensorimotor cortex.

METHODS

Stimulation was applied over primary and, in a second time, over premotor cortex (M1 and PMC, respectively). Tremor was monitored by accelerometers placed on the index fingers of hands outstretched, before and several times after rTMS. Each rTMS session consisted of 1800 pulses delivered at 1 Hz with an intensity of 90% of resting motor threshold.

RESULTS

PMC but not M1 stimulation led to a decrease of the postural tremor (90% decrease of acceleration total spectral power). This functional benefit was associated to normalization of electrophysiologic parameters (short-interval intracortical inhibition and cortical silent period duration). Moreover, when stimulating PMC during two daily sessions, improvement of the tremor was longer than one day stimulation and this benefit was associated with functional improvement.

CONCLUSIONS

This study shows that 1 Hz rTMS over premotor cortex can improve cortical tremor.

SIGNIFICANCE

These results raise the interest of the motor cortical stimulation as a possible therapeutic target for treatment of action tremor.

Changes in cortical excitability differentiate generalized and focal epilepsy

OBJECTIVE

Different pathophysiological mechanisms related to the balance of cortical excitatory and inhibitory influences may underlie focal and generalized epilepsies. We used transcranial magnetic stimulation to search for interictal excitability differences between patients with idiopathic generalized epilepsy (IGE) and focal epilepsy.

METHODS

Sixty-two drug-naive patients with newly diagnosed epilepsy (35 IGE, 27 focal epilepsy) were studied. In the latter group, the seizure focus was not located in the motor cortex. Motor threshold at rest, cortical silent period threshold, recovery curve analysis using paired-pulse stimulation at a number of interstimulus intervals), and cortical silent period were determined. Results were compared with those of 29 control subjects.

RESULTS

Hyperexcitability was noted in the recovery curves at a number of interstimulus intervals in both hemispheres in patients with IGE and in the hemisphere ipsilateral to the seizure focus in those with focal epilepsy compared with control subjects and the contralateral hemisphere in focal epilepsy. Motor threshold and cortical silent period threshold were higher in the ipsilateral hemisphere in focal epilepsy compared with the contralateral hemisphere. No other intragroup or intergroup differences were found in the other measures.

INTERPRETATION

The disturbance of cortical excitatory/inhibitory function was found to be bilateral in IGE, whereas in focal epilepsy it spread beyond the epileptic focus but remained lateralized. This finding confirms that there are differences in cortical pathophysiology comparing the two major types of epilepsy.

Myoclonus and transcranial magnetic stimulation

The neural dysfunction at the origin of myoclonus may locate at various anatomical levels within the central nervous system, including the motor cortices. Transcranial magnetic stimulation (TMS) can be used to assess the balance between inhibitory and excitatory processes involved in the regulation of motor cortex activity and thereby, may be of value to determine the pathophysiological mechanisms of myoclonus. Using paired-pulse paradigms with various interstimulus intervals, TMS studies showed that intracortical inhibition (ICI) was reduced in progressive myoclonic epilepsy (PME). In contrast, ICI was decreased only for short interstimulus intervals in patients with juvenile myoclonic epilepsy (JME). Transcallosal inhibition and sensorimotor integration were also both altered in PME but not in JME. Actually, the loss of inhibitory regulation within the central nervous system might represent an intrinsic mechanism of myoclonus, whether of epileptic origin or not. Finally, the other TMS parameters of excitability (motor threshold, silent period, intracortical facilitation) were found normal in most cases of myoclonus. According to these observations, it was quite conceivable that the application of repetitive trains of TMS (rTMS) at inhibitory low-frequency (around 1 Hz) might be able to relieve myoclonus by restoring ICI. A few reported cases illustrate the efficacy of low-frequency rTMS to alleviate myoclonic symptoms. Therapeutic-like perspectives are opened for rTMS in these forms of myoclonus that are related to motor cortical hyperexcitability secondary to the loss of ICI.

Seizure incidence during single- and paired-pulse transcranial magnetic stimulation (TMS) in individuals with epilepsy

OBJECTIVE

We reviewed published data and our own data to determine a quantitative incidence of seizure in subjects with epilepsy undergoing single- and paired-pulse transcranial magnetic stimulation (spTMS and ppTMS) and to explore conditions that may increase this risk.

METHODS

A PubMed literature search was performed, and articles from this search were reviewed. Subjects from our institution also were included.

RESULTS

The crude risk of a TMS-associated seizure ranges from 0.0 to 2.8% for spTMS and 0.0-3.6% for ppTMS. Medically intractable epilepsy and lowering antiepileptic drugs were associated with increased incidence. There was significant center-to-center variability that could not be explained by differences in patient population or by differences in reported stimulation parameters. In all cases, seizures were similar to each subject's typical seizure and without long-term adverse outcome. In most cases, doubt was expressed in the original reports as to whether the seizures were induced by TMS or merely coincidental.

CONCLUSIONS

The incidence of seizure in a subject with epilepsy during spTMS and ppTMS appears to be small and not associated with long-term adverse outcome. The incidence is higher under the specific conditions mentioned above.

SIGNIFICANCE

These findings may enable researchers to more accurately inform subjects of seizure risk during TMS.

Transcranial magnetic stimulation and epilepsy

Epileptic conditions are characterized by an altered balance between excitatory and inhibitory influences at the cortical level. Transcranial magnetic stimulation (TMS) provides a noninvasive evaluation of separate excitatory and inhibitory functions of the cerebral cortex. In addition, repetitive TMS (rTMS) can modulate the excitability of cortical networks. We review the different ways that TMS has been used to investigate pathophysiological mechanisms and effects of antiepileptic drugs in patients with epilepsy and epileptic myoclonus. The safety of different TMS techniques is discussed too. Finally, we discuss the therapeutic prospects of rTMS in this field.

Excitability changes in human peripheral nerve axons in a paradigm mimicking paired-pulse transcranial magnetic stimulation

A peripheral nerve model was developed to determine whether changes in axonal excitability could affect the findings in studies of cortical processes using paired-pulse transcranial magnetic stimulation (TMS). The recovery of axonal excitability from a conditioning stimulus smaller than the test stimulus was qualitatively similar to that with suprathreshold conditioning stimuli. There was an initial decrease in excitability, equivalent to refractoriness at conditioning-test intervals < 4 ms, an increase in excitability, equivalent to supernormality, at intervals of 5-20 ms and a second phase of decreased excitability, equivalent to late subnormality at intervals > 30 ms. H reflex studies using conditioning stimuli below threshold for the H reflex established that these excitability changes could be faithfully translated across an excitatory synapse. Changing membrane potential by injecting polarising current altered axonal excitability in a predictable way, and produced results similar to those reported for many disease states using paired-pulse TMS. Specifically, axonal hyperpolarisation produced a smaller decrease in excitability followed by a greater increase in excitability. This study supports the view that changes in excitability of the stimulated axons should be considered before synaptic mechanisms are invoked in the interpretation of findings from paired-pulse TMS studies.

Transcranial magnetic stimulation and epilepsy

Transcranial magnetic stimulation has been used to study generalized and focal epilepsies for more than a decade. The technique appears safe and has yielded important information about the mechanisms underlying epilepsy. Transcranial magnetic stimulation findings differ depending on the epilepsy syndrome, lending support to the concept that there are distinct pathophysiologies underlying each condition. In most studies of generalized epilepsies, transcranial magnetic stimulation has indicated a state of relative hyperexcitability of excitatory cortical interneurons and possibly inhibitory interneurons as well, which can be reversed through the actions of anticonvulsant medications. Transcranial magnetic stimulation studies in patients with a seizure focus in the motor cortex indicate increased cortical excitability and reduced inhibition, but in patients with seizure foci located elsewhere the findings are similar to those in generalized epilepsies. Transcranial magnetic stimulation has also been used to study the mode of action of anticonvulsants and may prove to be a useful means of testing the potential for new drugs to act as anticonvulsants. Repetitive transcranial magnetic stimulation may prove to have a therapeutic role by producing long-lasting cortical inhibition after a train of impulses.

Applications of transcranial magnetic stimulation in movement disorders

The author reviews the applications of transcranial magnetic stimulation (TMS) in a series of movement disorders--namely, Parkinson's disease, corticobasal degeneration, multiple system atrophy, progressive supranuclear palsy, essential tremor, dystonia, Huntington's chorea, myoclonus, the ataxias, Tourette's syndrome, restless legs syndrome, Wilson's disease, Rett syndrome, and stiff-person syndrome. Single- and paired-pulse TMS studies have been done mainly for pathophysiologic purposes. Repetitive TMS has been used largely for therapy. Many TMS abnormalities are seen in the different diseases. They concur to show that motor cortical areas and their projections are the main target of the basal ganglia dysfunction typical of movement disorders. Interpretation has not always been clear, and sometimes there were discrepancies and contradictions. Largely, this may be the result of the extreme heterogeneity of the methods used and of the patients studied. It is premature to give repetitive TMS a role in treatment. Overall, however, TMS gives rise to a new, outstanding enthusiasm in the neurophysiology of movement disorders. There is reason to predict that TMS, with its continuous technical refinement, will prove even more helpful in the near future. Then, research achievements are reasonably expected to spill over into clinical practice.

Motor cortex disinhibition in Alzheimer's disease

OBJECTIVES

To explore subclinical disturbances in the motor cortex of patients with Alzheimer's disease (AD).

METHODS

We used transcranial magnetic stimulation in a paired pulse technique to test intracortical inhibition (ICI) and intracortical facilitation in mildly to moderately demented AD patients with a normal neurological examination. Patients were studied before and during treatment with the cholinesterase inhibitor donepezil.

RESULTS

AD patients had a reduced ICI compared to an age-matched control group. The amount of disinhibition correlated with the severity of dementia. Treatment with 10 mg donepezil daily was associated with an increase of ICI.

CONCLUSIONS

The subclinical motor cortex disinhibition in AD patients indicates a functional disturbance, and is probably associated with a cholinergic deficit.

Motor cortex excitability in patients with focal epilepsy

We studied the excitability of the motor cortex using, transcranial magnetic stimulation (TMS) in patients with temporal and extratemporal epilepsy. We applied single and paired-pulse TMS to 15 patients with temporal (n = 7), extratemporal (n = 6) and focal epilepsy lateralised to one hemisphere (n = 2). Patients had no antiepileptic drugs in the last 48 h and were seizure free for 4 h prior to testing. We determined the threshold for EMG responses at rest (RMT), the cortically evoked silent period (CSSP) and intracortical inhibition (ICI, intervals of 2-4 ms) and facilitation (ICF, 7-15 ms) and compared the results to those obtained in 17 normal controls. ICI and ICF was reduced in both hemispheres (P < 0.01. ANOVA) compared to the controls. In the hemisphere of seizure origin ('abnormal') there was a reduction of ICF (P < 0.01) and normal ICI, in the 'normal' hemisphere there was a reduced ICI (P < 0.01) and a slight reduction of ICF (P < 0.05). ICF on the 'abnormal' side was reduced (P < 0.05) compared to the 'normal' hemisphere. RMT was increased in two patients, but group comparison of RMT and CSSP showed no significant differences between patients and controls. The results suggest a remote effect of epileptic activity onto the motor cortex leading to an alteration of activity in local inhibitory circuits.