Transcranial magnetic stimulation in child neurology: current and future directions

Clinical research with transcranial direct current stimulation (tDCS): challenges and future directions

BACKGROUND

Transcranial direct current stimulation (tDCS) is a neuromodulatory technique that delivers low-intensity, direct current to cortical areas facilitating or inhibiting spontaneous neuronal activity. In the past 10 years, tDCS physiologic mechanisms of action have been intensively investigated giving support for the investigation of its applications in clinical neuropsychiatry and rehabilitation. However, new methodologic, ethical, and regulatory issues emerge when translating the findings of preclinical and phase I studies into phase II and III clinical studies. The aim of this comprehensive review is to discuss the key challenges of this process and possible methods to address them.

METHODS

We convened a workgroup of researchers in the field to review, discuss, and provide updates and key challenges of tDCS use in clinical research.

MAIN FINDINGS/DISCUSSION

We reviewed several basic and clinical studies in the field and identified potential limitations, taking into account the particularities of the technique. We review and discuss the findings into four topics: (1) mechanisms of action of tDCS, parameters of use and computer-based human brain modeling investigating electric current fields and magnitude induced by tDCS; (2) methodologic aspects related to the clinical research of tDCS as divided according to study phase (ie, preclinical, phase I, phase II, and phase III studies); (3) ethical and regulatory concerns; and (4) future directions regarding novel approaches, novel devices, and future studies involving tDCS. Finally, we propose some alternative methods to facilitate clinical research on tDCS.

Diagnostic value of Hoover sign and motor-evoked potentials in acute somatoform unilateral weakness and sensory impairment mimicking vascular stroke

Acute unilateral weakness along with sensory impairment is commonly caused by obstruction of major cortical arteries in either adults or children. A somatoform presentation mimicking acute vascular stroke is very rare, especially in the pediatric age group. Here we report three adolescents presenting with acute unilateral weakness and sensory impairment along with diminished tendon reflexes who were suspected to have an acute stroke but who had developed a somatoform psychogenic disorder. Two adolescents had complete hemiplegia and one had weakness of the left leg - two had moved the alleged paralytic limbs during sleep. A normal Hoover sign was suggestive of a somatoform psychogenic etiology rather than true vascular stroke. Cortical and spinal MRI, motor-evoked potentials (MEP) and somatosensory-evoked potentials were normal. All adolescents recovered completely. Therefore, a somatoform conversion reaction should be considered in children presenting with acute unilateral weakness and sensory alterations, which is corroborated by a normal Hoover sign and intact MEP.

Motor evoked potentials of the upper extremities in healthy children

OBJECTIVE

To evaluate and compare the organization of descending motor pathways to upper extremity muscles among healthy children.

METHOD

The healthy children were 16 males and 7 females aged 1-19 years (average, 9 years), and eight healthy adults were enrolled as the control group. Transcranial magnetic stimulation was applied to bilateral motor cortices, and motor evoked potentials (MEPs) were recorded using surface electrodes from the first dorsal interossei (FDI), the biceps brachii (BIC), and the deltoid (DEL) muscles. The onset latency, central motor conduction time (CMCT), and amplitude were obtained during a relaxed state.

RESULTS

MEPs of FDI were obtained from subjects aged 13 months. The frequency of obtaining MEPs in proximal and distal muscles increased with age, although there was a less frequent incidence of obtaining MEPs in the proximal BIC and DEL muscles compared with those in the distal FDI muscle. MEP amplitudes increased with age, whereas latencies were relatively constant. CMCTs showed a similar pattern of maturation, and adult values were obtained by 13-years-of-age.

CONCLUSION

These results suggest that the proximal and distal muscles of the upper extremities show different maturation and organization patterns.

Pain during transcranial magnetic stimulation in youth

Pain or discomfort at the site of stimulation is a common side effect of transcranial magnetic stimulation. Relevant physiology and predisposing factors have not been adequately described. Literature regarding work with minors is even more limited. The authors present two cases from a child and adolescent neurophysiology transcranial magnetic stimulation protocol and one case from a therapeutic study of repetitive transcranial magnetic stimulation in adolescents with treatment-resistant major depressive disorder. Relevant literature is reviewed. Potential subjects, parents, and study teams should be well aware of this potential side effect in child and adolescent populations. Subjects with anxiety disorders may be prone to pain during these procedures. Further work could assist in identifying predisposed individuals, refining the informed consent process, and implementing procedures to minimize discomfort.

[French guidelines on the use of repetitive transcranial magnetic stimulation (rTMS): safety and therapeutic indications]

During the past decade, a large amount of work on transcranial magnetic stimulation (TMS) has been performed, including the development of new paradigms of stimulation, the integration of imaging data, and the coupling of TMS techniques with electroencephalography or neuroimaging. These accumulating data being difficult to synthesize, several French scientific societies commissioned a group of experts to conduct a comprehensive review of the literature on TMS. This text contains all the consensual findings of the expert group on the mechanisms of action, safety rules and indications of TMS, including repetitive TMS (rTMS). TMS sessions have been conducted in thousands of healthy subjects or patients with various neurological or psychiatric diseases, allowing a better assessment of risks associated with this technique. The number of reported side effects is extremely low, the most serious complication being the occurrence of seizures. In most reported seizures, the stimulation parameters did not follow the previously published recommendations (Wassermann, 1998) [430] and rTMS was associated to medication that could lower the seizure threshold. Recommendations on the safe use of TMS / rTMS were recently updated (Rossi et al., 2009) [348], establishing new limits for stimulation parameters and fixing the contraindications. The recommendations we propose regarding safety are largely based on this previous report with some modifications. By contrast, the issue of therapeutic indications of rTMS has never been addressed before, the present work being the first attempt of a synthesis and expert consensus on this topic. The use of TMS/rTMS is discussed in the context of chronic pain, movement disorders, stroke, epilepsy, tinnitus and psychiatric disorders. There is already a sufficient level of evidence of published data to retain a therapeutic indication of rTMS in clinical practice (grade A) in chronic neuropathic pain, major depressive episodes, and auditory hallucinations. The number of therapeutic indications of rTMS is expected to increase in coming years, in parallel with the optimisation of stimulation parameters.

Applications of transcranial magnetic stimulation (TMS) in child and adolescent psychiatry

Transcranial magnetic stimulation (TMS) is emerging as a new treatment and neurophysiological research tool for psychiatric disorders. Recent publications suggest that this modality will also serve as a treatment and research tool in child and adolescent psychiatry. Current reports on therapeutic trials of repetitive transcranial magnetic stimulation (rTMS) in adolescents have primarily focused on depression. However, other pilot work involves the treatment of attention-deficit/hyperactivity disorder (ADHD), autism and schizophrenia. Neurophysiological studies typically utilize single and paired-pulse TMS paradigms which index cortical excitability and inhibition. Initial studies have focused on ADHD, autism, and depression. General knowledge regarding TMS among child and adolescent psychiatrists is lacking. The aim of this review is to provide an overview of TMS in the context of child and adolescent psychiatry, discuss recent therapeutic and neurophysiological studies, and examine relevant ethical considerations.

The developmental trajectory of brain-scalp distance from birth through childhood: implications for functional neuroimaging

Measurements of human brain function in children are of increasing interest in cognitive neuroscience. Many techniques for brain mapping used in children, including functional near-infrared spectroscopy (fNIRS), electroencephalography (EEG), magnetoencephalography (MEG) and transcranial magnetic stimulation (TMS), use probes placed on or near the scalp. The distance between the scalp and the brain is a key variable for these techniques because optical, electrical and magnetic signals are attenuated by distance. However, little is known about how scalp-brain distance differs between different cortical regions in children or how it changes with development. We investigated scalp-brain distance in 71 children, from newborn to age 12 years, using structural T1-weighted MRI scans of the whole head. Three-dimensional reconstructions were created from the scalp surface to allow for accurate calculation of brain-scalp distance. Nine brain landmarks in different cortical regions were manually selected in each subject based on the published fNIRS literature. Significant effects were found for age, cortical region and hemisphere. Brain-scalp distances were lowest in young children, and increased with age to up to double the newborn distance. There were also dramatic differences between brain regions, with up to 50% differences between landmarks. In frontal and temporal regions, scalp-brain distances were significantly greater in the right hemisphere than in the left hemisphere. The largest contributors to developmental changes in brain-scalp distance were increases in the corticospinal fluid (CSF) and inner table of the cranium. These results have important implications for functional imaging studies of children: age and brain-region related differences in fNIRS signals could be due to the confounding factor of brain-scalp distance and not true differences in brain activity.

Safety and tolerability of repetitive transcranial magnetic stimulation in patients with pathologic positive sensory phenomena: a review of literature

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is emerging as a valuable therapeutic and diagnostic tool. rTMS appears particularly promising for disorders characterized by positive sensory phenomena that are attributable to alterations in sensory cortical excitability. Among these are tinnitus, auditory and visual hallucinations, and pain syndromes.

OBJECTIVE

Despite studies addressing rTMS efficacy in suppression of positive sensory symptoms, the safety of stimulation of potentially hyperexcitable cortex has not been fully addressed. We performed a systematic literature review and metaanalysis to describe the rTMS safety profile in these disorders.

METHODS

Using the PubMed database, we performed an English-language literature search from January 1985 to April 2011 to review all pertinent publications. Per study, we noted and listed pertinent details. From these data we also calculated a crude per-subject risk for each adverse event.

RESULTS

One hundred six publications (n = 1815) were identified with patients undergoing rTMS for pathologic positive sensory phenomena. Adverse events associated with rTMS were generally mild and occurred in 16.7% of subjects. Seizure was the most serious adverse event, and occurred in three patients with a 0.16% crude per-subject risk. The second most severe adverse event involved aggravation of sensory phenomena, occurring in 1.54%.

CONCLUSIONS

The published data suggest rTMS for the treatment or diagnosis of pathologic positive sensory phenomena appears to be a relatively safe and well-tolerated procedure. However, published data are lacking in systematic reporting of adverse events, and safety risks of rTMS in these patient populations will have to be addressed in future prospective trials.

Low-frequency repetitive transcranial magnetic stimulation of the anterior temporal lobes does not dissociate social versus nonsocial semantic knowledge

Social conceptual knowledge is imperative to communicate with, interact with, and interpret human society; however, little is known about the neural basis of social concepts. Previous research has predominantly suggested that the right anterior temporal lobe (ATL) may specifically represent social conceptual knowledge, whereas the left ATL is necessary for general semantic processing. However, this view has not always been supported by empirical studies. Employing a lateralized design and two different semantic tasks and a nonsemantic control task, we aimed to clarify some of these ambiguities by potentially dissociating left from right functionality and social from nonsocial concepts, using inhibitory repetitive transcranial magnetic stimulation (rTMS) coupled with a sham and control site stimulation ( N = 56). The results showed that stimulation of the left ATL led to overall faster processing times without affecting accuracy, whilst the right ATL and control groups did not significantly change in reaction times or accuracy. No difference occurred between social and nonsocial concepts after stimulation. This study is the first to show that inhibition of the left temporal lobe may improve performance on a semantic task and provides evidence that the ATLs may be lateralized in conceptual processing. The results do not confirm that the right temporal lobe is crucial for social conceptual processing, as inhibition did not significantly affect performance for social concepts.

Seizure in a nonpredisposed individual induced by single-pulse transcranial magnetic stimulation

Seizure induction is a rare, but serious adverse effect of the otherwise very safe method of transcranial magnetic stimulation (TMS). There are only very few single case reports concerning seizure in single-pulse TMS. All of these reports describe individuals with neurological disorders or epileptogenic medication. To our knowledge, we are the first to describe a healthy subject who developed symptoms of a seizure after single-pulse TMS during motor threshold estimation. This case report provides evidence that single-pulse TMS may provoke a seizure even in the absence of neurological risk factors. Differential diagnoses of a classic neurological seizure, that is, convulsive syncope and psychogenic seizure, are discussed. Neurogenic seizure after TMS and convulsive syncope are the most probable hypotheses, although clear specification of this singular incident remains impossible. Therefore, to minimize the risk for such rare adverse effects, existing and new suggestions are combined to provide reasonable precautions to be taken before and during TMS application.

Transfer of cortical motor representation after a perinatal cerebral insult

In a 16-year-old boy with hemiplegia and severe, intractable epilepsy after a neonatal cerebral ischemic insult, cortical motor control was only equivocally assessed by functional magnetic resonance imaging. Therefore, high-precision navigated transcranial magnetic stimulation was performed, which demonstrated that cortical control of muscles on the paretic side was selectively affected. Leg muscle control was located in the contralateral hemisphere, as expected in healthy individuals, whereas forearm muscles were controlled from both hemispheres, and hand muscles were controlled only from the hemisphere ipsilateral to the paresis.

The emerging role for repetitive transcranial magnetic stimulation in optimizing the treatment of adolescent depression

Major depressive disorder (MDD) in adolescents is a common illness and significant public health problem. Treatment is challenging because of recurrences and limited modalities. Selective serotonin reuptake inhibitors and cognitive behavioral therapy are considered the standard of care in severe or treatment-resistant MDD in this age group. However, responses to these interventions are often suboptimal. A growing body of research supports the efficacy of repetitive transcranial magnetic stimulation (rTMS) for the treatment of MDD in adults. Induced seizures are a primary safety concern, although this is rare with appropriate precautions. There is, however, limited experience with rTMS as a therapeutic intervention for adolescent psychiatric disturbances. This review will summarize the rTMS efficacy and safety data in adults and describe all published experience with adolescent MDD. Applications in other adolescent psychiatric illnesses such as schizophrenia and attention-deficit/hyperactivity disorder are reviewed. Safety and ethical issues are paramount with investigational treatments in adolescent psychiatric illnesses. However, further research with rTMS in adolescent MDD is imperative to establish standards for optimal stimulation site, treatment parameters, and its role in treatment algorithms. These may diverge from adult data. Early intervention with neuromodulation could also hold the promise of addressing the developmental course of dysfunctional neurocircuitry.

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.

Lateralization of forelimb motor evoked potentials by transcranial magnetic stimulation in rats

OBJECTIVES

To approximate methods for human transcranial magnetic stimulation (TMS) in rats, we tested whether lateralized cortical stimulation resulting in selective activation of one forelimb contralateral to the site of stimulation could be achieved by TMS in the rat.

METHODS

Motor evoked potentials (MEP) were recorded from the brachioradialis muscle bilaterally in adult male anesthetized rats (n=13). A figure-of-eight TMS coil was positioned lateral to midline. TMS intensity was increased stepwise from subthreshold intensities to maximal machine output in order to generate input-output curves and to determine the motor threshold (MT) for brachioradialis activation.

RESULTS

In 100% of the animals, selective activation of the contralateral brachioradialis, in the absence of ipsilateral brachioradialis activation was achieved, and the ipsilateral brachioradialis was activated only at TMS intensities exceeding contralateral forelimb MT. With increasing TMS intensity, the amplitudes of both the ipsilateral and contralateral signals increased in proportion to TMS strength. However, the input-output curves for the contralateral and ipsilateral brachioradialis were significantly different (p<0.001) such that amplitude of the ipsilateral MEP was reliably lower than the contralateral signal.

CONCLUSIONS

We demonstrate that lateralized TMS leading to asymmetric brachioradialis activation is feasible with conventional TMS equipment in anesthetized rats.

SIGNIFICANCE

These data show that TMS can be used to assess the unilateral excitability of the forelimb descending motor pathway in the rat, and suggest that rat TMS protocols analogous to human TMS may be applied in future translational research.

Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research

This article is based on a consensus conference, which took place in Certosa di Pontignano, Siena (Italy) on March 7-9, 2008, intended to update the previous safety guidelines for the application of transcranial magnetic stimulation (TMS) in research and clinical settings. Over the past decade the scientific and medical community has had the opportunity to evaluate the safety record of research studies and clinical applications of TMS and repetitive TMS (rTMS). In these years the number of applications of conventional TMS has grown impressively, new paradigms of stimulation have been developed (e.g., patterned repetitive TMS) and technical advances have led to new device designs and to the real-time integration of TMS with electroencephalography (EEG), positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Thousands of healthy subjects and patients with various neurological and psychiatric diseases have undergone TMS allowing a better assessment of relative risks. The occurrence of seizures (i.e., the most serious TMS-related acute adverse effect) has been extremely rare, with most of the few new cases receiving rTMS exceeding previous guidelines, often in patients under treatment with drugs which potentially lower the seizure threshold. The present updated guidelines review issues of risk and safety of conventional TMS protocols, address the undesired effects and risks of emerging TMS interventions, the applications of TMS in patients with implanted electrodes in the central nervous system, and safety aspects of TMS in neuroimaging environments. We cover recommended limits of stimulation parameters and other important precautions, monitoring of subjects, expertise of the rTMS team, and ethical issues. While all the recommendations here are expert based, they utilize published data to the extent possible.

Motor potentials by magnetic stimulation in periventricular leukomalacia

Periventricular leukomalacia is characterized by damage to the brain's white matter and impairments in motor function. Motor-evoked potentials by transcranial magnetic stimulation evaluate corticospinal tract function. We analyzed alterations in motor-evoked potentials in newborns with periventricular leukomalacia. Thirty infants (aged 4.37 +/- 1.1 months mean +/- S.D.) were divided into three groups: 10 healthy, and 10 with focal and 10 with diffuse periventricular leukomalacia. Potentials recorded in the right abductor pollicis brevis of healthy infants indicated a total motor conduction time of 26.3 +/- 2.4 ms, central motor conduction time of 17.0 +/- 2.6 ms, and central motor conduction velocity of 12.3 +/- 2.2 m/s. In the tibialis anterior, total motor conduction time was 27.4 +/- 2.6 ms; central motor conduction time was 16.7 +/- 2.8 ms, and central motor conduction velocity was 25.2 +/- 3.4 m/s. In the focal periventricular leukomalacia and diffuse periventricular leukomalacia groups, an increase in central motor conduction time and a decrease in central motor conduction velocity (P < 0.05) were evident, without differences between the two groups. Motor-evoked potentials in periventricular leukomalacia revealed an increase in central motor conduction time and a decrease in central motor conduction velocity, without differences between diffuse and focal types.

Transcranial magnetic stimulation

PURPOSE OF REVIEW

To present state-of-the-art transcranial magnetic stimulation (TMS) therapy, especially when it is used in psychiatric disorders, on the basis of an exhaustive literature search from 2006 to date (June 2008) on TMS papers published in Medline and Embase. Other references and comments from our own experience started 8 years ago have also been taken into account.

RECENT FINDINGS

The mechanism of action of TMS is now better understood. There is strong evidence of the safety and tolerability of TMS when standard protocols are used. The efficacy of the stimulation of the dorsolateral prefrontal cortex in depression is well documented, and there is evidence of the utility of TMS in posttraumatic stress disorder, in persistent auditory hallucinations in schizophrenia and in attention-deficit disorder with hyperactivity.

SUMMARY

There is enough evidence of the efficacy and safety of TMS in depression to include this technique in the therapeutic protocols of major depression. However, more research is needed on the use of this technique in other psychiatric and nonpsychiatric disorders such as posttraumatic stress disorder, persistent auditory hallucinations, attention-deficit disorder with hyperactivity and tinnitus.

“Complementary and Alternative Medicine Treatments for Children with Autism Spectrum Disorders”

Complementary and alternative medical (CAM) treatments are commonly used for children with autism spectrum disorders. This review discusses the evidence supporting the most frequently used treatments, including categories of mind-body medicine, energy medicine, and biologically based, manipulative, and body-based practices, with the latter two treatments the most commonly selected by families. Clinical providers need to understand the evidence for efficacy (or lack thereof) and potential side effects. Some CAM practices have evidence to reject their use, such as secretin, whereas others have emerging evidence to support their use, such as melatonin. Most treatments have not been adequately studied and do not have evidence to support their use.

Neurocardiogenic syncope complicating pediatric transcranial magnetic stimulation

Pediatric applications of transcranial magnetic stimulation are rapidly expanding, but lack the safety data established for adults. Transcranial magnetic stimulation-induced neurocardiogenic syncope may represent an age-dependent adverse event that is essentially undescribed in the transcranial magnetic stimulation literature. We report on 2 adolescents (of 10 children studied) with transcranial magnetic stimulation-related neurocardiogenic syncope, identify modifiable risk factors, and suggest measures to improve the safety of future pediatric transcranial magnetic stimulation studies.