The contribution of TMS to frontotemporal dementia variants

Transcranial magnetic stimulation neurophysiology in patients with non-Alzheimer's neurodegenerative diseases: A systematic review and meta-analysis

Non-Alzheimer's dementia (NAD) accounts for 30% of all neurodegenerative conditions and is characterized by cognitive decline beyond mere memory dysfunction. Diagnosing NAD remains challenging due to the lack of established biomarkers. Transcranial magnetic stimulation (TMS) is a non-invasive neurophysiological tool that enables the investigation of cortical excitability in the human brain. Paired-pulse TMS paradigms include short- and long-interval intracortical inhibition (SICI/LICI), intracortical facilitation (ICF), and short-latency afferent inhibition (SAI), which can assess neurophysiological functions of GABAergic, glutamatergic, and cholinergic neural circuits, respectively. We conducted the first systematic review and meta-analysis to compare these TMS indices among patients with NAD and healthy controls. Our meta-analyses indicated that TMS neurophysiological examinations revealed decreased glutamatergic function in patients with frontotemporal dementia (FTD) and decreased GABAergic function in patients with FTD, progressive supranuclear palsy, Huntington's disease, cortico-basal syndrome, and multiple system atrophy-parkinsonian type. In addition, decreased cholinergic function was found in dementia with Lewy body and vascular dementia. These results suggest the potential of TMS as an additional diagnostic tool to differentiate NAD.

Frontotemporal Dementia, Where Do We Stand? A Narrative Review

Frontotemporal dementia (FTD) is a neurodegenerative disease of growing interest, since it accounts for up to 10% of middle-age-onset dementias and entails a social, economic, and emotional burden for the patients and caregivers. It is characterised by a (at least initially) selective degeneration of the frontal and/or temporal lobe, generally leading to behavioural alterations, speech disorders, and psychiatric symptoms. Despite the recent advances, given its extreme heterogeneity, an overview that can bring together all the data currently available is still lacking. Here, we aim to provide a state of the art on the pathogenesis of this disease, starting with established findings and integrating them with more recent ones. In particular, advances in the genetics field will be examined, assessing them in relation to both the clinical manifestations and histopathological findings, as well as considering the link with other diseases, such as amyotrophic lateral sclerosis (ALS). Furthermore, the current diagnostic criteria will be explored, including neuroimaging methods, nuclear medicine investigations, and biomarkers on biological fluids. Of note, the promising information provided by neurophysiological investigations, i.e., electroencephalography and non-invasive brain stimulation techniques, concerning the alterations in brain networks and neurotransmitter systems will be reviewed. Finally, current and experimental therapies will be considered.

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Cortical hyperexcitability and plasticity in Alzheimer's disease: developments in understanding and management

INTRODUCTION

Transcranial magnetic stimulation (TMS) is a non-invasive neurophysiological tool that provides important insights into Alzheimer's Disease (AD). A significant body of work utilizing TMS techniques has explored the pathophysiological relevance of cortical hyperexcitability and plasticity in AD and their modulation in novel therapies.

AREAS COVERED

This review examines the technique of TMS, the use of TMS to examine specific features of cortical excitability and the use of TMS techniques to modulate cortical function. A search was performed utilizing the PubMed database to identify key studies utilizing TMS to examine cortical hyperexcitability and plasticity in Alzheimer's dementia. We then translate this understanding to the study of Alzheimer's disease pathophysiology, examining the underlying neurophysiologic links contributing to these twin signatures, cortical hyperexcitability and abnormal plasticity, in the cortical dysfunction characterizing AD. Finally, we examine utilization of TMS excitability to guide targeted therapies and, through the use of repetitive TMS (rTMS), modulate cortical plasticity.

EXPERT OPINION

The examination of cortical hyperexcitability and plasticity with TMS has potential to optimize and expand the window of therapeutic interventions in AD, though remains at relatively early stage of development.

Synaptic dysfunction in ALS and FTD: anatomical and molecular changes provide insights into mechanisms of disease

Synaptic loss is a pathological feature of all neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). ALS is a disease of the cortical and spinal motor neurons resulting in fatal paralysis due to denervation of muscles. FTD is a form of dementia that primarily affects brain regions controlling cognition, language and behavior. Once classified as two distinct diseases, ALS and FTD are now considered as part of a common disease spectrum based on overlapping clinical, pathological and genetic evidence. At the cellular level, aggregation of common proteins and overlapping gene susceptibilities are shared in both ALS and FTD. Despite the convergence of these two fields of research, the underlying disease mechanisms remain elusive. However, recent discovers from ALS and FTD patient studies and models of ALS/FTD strongly suggests that synaptic dysfunction is an early event in the disease process and a unifying hallmark of these diseases. This review provides a summary of the reported anatomical and cellular changes that occur in cortical and spinal motor neurons in ALS and FTD tissues and models of disease. We also highlight studies that identify changes in the proteome and transcriptome of ALS and FTD models and provide a conceptual overview of the processes that contribute to synaptic dysfunction in these diseases. Due to space limitations and the vast number of publications in the ALS and FTD fields, many articles have not been discussed in this review. As such, this review focuses on the three most common shared mutations in ALS and FTD, the hexanucleuotide repeat expansion within intron 1 of chromosome 9 open reading frame 72 (C9ORF72), transactive response DNA binding protein 43 (TARDBP or TDP-43) and fused in sarcoma (FUS), with the intention of highlighting common pathways that promote synaptic dysfunction in the ALS-FTD disease spectrum.

Cortical and subcortical changes in resting-state neuronal activity and connectivity in early symptomatic ALS and advanced frontotemporal dementia

The objective of this study was to examine if patterns of resting-state brain activity and functional connectivity in cortical and subcortical regions in patients with early symptomatic amyotrophic lateral sclerosis (ALS) resemble those of behavioural variant frontotemporal dementia (bvFTD). In a cross-sectional design, eyes-closed resting-state magnetoencephalography (MEG) data of 34 ALS patients, 18 bvFTD patients and 18 age- and gender-matched healthy controls (HCs) were projected to source-space using an atlas-based beamformer. Group differences in peak frequency, band-specific oscillatory activity and functional connectivity (corrected amplitude envelope correlation) in 78 cortical regions and 12 subcortical regions were determined. False discovery rate was used to correct for multiple comparisons. BvFTD patients, as compared to ALS and HCs, showed lower relative beta power in parietal, occipital, temporal and nearly all subcortical regions. Compared to HCs, patients with ALS and patients with bvFTD had a higher delta (0.5-4 Hz) and gamma (30-48 Hz) band resting-state functional connectivity in a high number of overlapping regions in the frontal lobe and in limbic and subcortical regions. Higher delta band connectivity was widespread in the bvFTD patients compared to HCs. ALS showed a more widespread higher gamma band functional connectivity compared to bvFTD. In conclusion, MEG in early symptomatic ALS patients shows resting-state functional connectivity changes in frontal, limbic and subcortical regions that overlap considerably with bvFTD. The findings show the potential of MEG to detect brain changes in early symptomatic phases of ALS and contribute to our understanding of the disease spectrum, with ALS and bvFTD at the two extreme ends.

Diagnostic contribution and therapeutic perspectives of transcranial magnetic stimulation in dementia

Transcranial magnetic stimulation (TMS) is a powerful tool to probe in vivo brain circuits, as it allows to assess several cortical properties such asexcitability, plasticity and connectivity in humans. In the last 20 years, TMS has been applied to patients with dementia, enabling the identification of potential markers of thepathophysiology and predictors of cognitive decline; moreover, applied repetitively, TMS holds promise as a potential therapeutic intervention. The objective of this paper is to present a comprehensive review of studies that have employed TMS in dementia and to discuss potential clinical applications, from the diagnosis to the treatment. To provide a technical and theoretical framework, we first present an overview of the basic physiological mechanisms of the application of TMS to assess cortical excitability, excitation and inhibition balance, mechanisms of plasticity and cortico-cortical connectivity in the human brain. We then review the insights gained by TMS techniques into the pathophysiology and predictors of progression and response to treatment in dementias, including Alzheimer's disease (AD)-related dementias and secondary dementias. We show that while a single TMS measure offers low specificity, the use of a panel of measures and/or neurophysiological index can support the clinical diagnosis and predict progression. In the last part of the article, we discuss the therapeutic uses of TMS. So far, only repetitive TMS (rTMS) over the left dorsolateral prefrontal cortex and multisite rTMS associated with cognitive training have been shown to be, respectively, possibly (Level C of evidence) and probably (Level B of evidence) effective to improve cognition, apathy, memory, and language in AD patients, especially at a mild/early stage of the disease. The clinical use of this type of treatment warrants the combination of brain imaging techniques and/or electrophysiological tools to elucidate neurobiological effects of neurostimulation and to optimally tailor rTMS treatment protocols in individual patients or specific patient subgroups with dementia or mild cognitive impairment.

Low-Frequency Transcranial Magnetic Stimulation (LF-TMS) in Treating Depression in Patients With Impaired Cognitive Functioning

OBJECTIVE

Common methodologies for treating depressive symptoms have demonstrated decreased efficacy among individuals with impaired cognitive functioning. While transcranial magnetic stimulation (TMS) has been approved to treat major depressive disorder, few studies have analyzed the ability of TMS to treat depressive symptoms among individuals with cognitive impairments. The present study had two objectives: to determine whether low-frequency TMS (LF-TMS) might demonstrate efficacy in treating depressive symptoms among individuals with impaired cognitive functioning; and to determine whether LF-TMS might improve neurocognitive functioning above and beyond depressive symptom improvements.

METHODS

Data were derived from a pre-existing database at Eastern Virginia Medical School. Fifty-three (N=53) participants completed LF-TMS treatment. The Beck Depression Inventory II (BDI-II) and CNS Vital Signs (CNS-VS) neurocognitive assessment were administered at multiple time points throughout treatment. Participants were classified as impaired cognitive functioning or average cognitive functioning based on baseline CNS-VS scores. Data were analyzed using restricted maximum likelihood (REML) measures-within-persons longitudinal hierarchical linear modeling (HLM) with time-varying covariates.

RESULTS

LF-TMS produced significant reductions in depressive symptoms for individuals in both cognitive functioning groups; however, a significant group-by-time interaction indicates differential effects between these two groups. Low-frequency TMS produced significant improvements in three neurocognitive domains above and beyond improvements in depressive symptoms; however, the reliability of these changes may be questionable.

CONCLUSIONS

This study adds to the growing body of empirical findings for LF-TMS treatment in improving neurocognitive functioning above and beyond other treatment-related effects.

Quantitative Assessment of Cortical Excitability in Alzheimer's Dementia and Its Association with Clinical Symptoms: A Systematic Review and Meta-Analyses

BACKGROUND

Alzheimer's disease (AD) is characterized by cognitive and neuropsychiatric symptoms (NPS) due to underlying neurodegenerative pathology. Some studies using electroencephalography (EEG) have shown increased epileptiform and epileptic activity in AD.

OBJECTIVE

This review and meta-analyses aims to synthesize the existing evidence for quantitative abnormalities of cortical excitability in AD and their relationship with clinical symptoms.

METHODS

We systematically searched and reviewed publications that quantitatively assessed cortical excitability, using transcranial magnetic stimulation (TMS) resting motor threshold (rMT), active motor threshold (aMT), motor evoked potential (MEP) or directly from the cortex using TMS-EEG via TMS-evoked potential (TEP). We meta-analyzed studies that assessed rMT and aMT using random effects model.

RESULTS

We identified 895 publications out of which 37 were included in the qualitative review and 30 studies using rMT or aMT were included in the meta-analyses. The AD group had reduced rMT (Hedges' g = -0.99, 95%CI [-1.29, -0.68], p <  0.00001) and aMT (Hedges' g = -0.87, 95%CI [-1.50, -0.24], p <  0.00001) as compared with control groups, indicative of higher cortical excitability. Qualitative review found some evidence of increased MEP amplitude, whereas findings related to TEP were inconsistent. There was some evidence supporting an inverse association between cortical excitability and global cognition. No publications reported on the relationship between cortical excitability and NPS.

CONCLUSION

There is strong evidence of increased motor cortex excitability in AD and some evidence of an inverse association between excitability and cognition. Future studies should assess cortical excitability from non-motor areas using TMS-EEG and examine its relationship with cognition and NPS.

Transcranial Magnetic Stimulation as a Diagnostic and Therapeutic Tool in Various Types of Dementia

Dementia is recognized as a healthcare and social burden and remains challenging in terms of proper diagnosis and treatment. Transcranial magnetic stimulation (TMS) is a diagnostic and therapeutic tool in various neurological diseases that noninvasively investigates cortical excitability and connectivity and can induce brain plasticity. This article reviews findings on TMS in common dementia types as well as therapeutic results. Alzheimer’s disease (AD) is characterized by increased cortical excitability and reduced cortical inhibition, especially as mediated by cholinergic neurons and as documented by impairment of short latency inhibition (SAI). In vascular dementia, excitability is also increased. SAI may have various outcomes, which probably reflects its frequent overlap with AD. Dementia with Lewy bodies (DLB) is associated with SAI decrease. Motor cortical excitability is usually normal, reflecting the lack of corticospinal tract involvement. DLB and other dementia types are also characterized by impairment of short interval intracortical inhibition. In frontotemporal dementia, cortical excitability is increased, but SAI is normal. Repetitive transcranial magnetic stimulation has the potential to improve cognitive function. It has been extensively studied in AD, showing promising results after multisite stimulation. TMS with electroencephalography recording opens new possibilities for improving diagnostic accuracy; however, more studies are needed to support the existing data.

On the Use of TMS to Investigate the Pathophysiology of Neurodegenerative Diseases

Neurodegenerative diseases are a collection of disorders that result in the progressive degeneration and death of neurons. They are clinically heterogenous and can present as deficits in movement, cognition, executive function, memory, visuospatial awareness and language. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation tool that allows for the assessment of cortical function in vivo. We review how TMS has been used for the investigation of three neurodegenerative diseases that differ in their neuroanatomical axes: (1) Motor cortex-corticospinal tract (motor neuron diseases), (2) Non-motor cortical areas (dementias), and (3) Subcortical structures (parkinsonisms). We also make four recommendations that we hope will benefit the use of TMS in neurodegenerative diseases. Firstly, TMS has traditionally been limited by the lack of an objective output and so has been confined to stimulation of the motor cortex; this limitation can be overcome by the use of concurrent neuroimaging methods such as EEG. Given that neurodegenerative diseases progress over time, TMS measures should aim to track longitudinal changes, especially when the aim of the study is to look at disease progression and symptomatology. The lack of gold-standard diagnostic confirmation undermines the validity of findings in clinical populations. Consequently, diagnostic certainty should be maximized through a variety of methods including multiple, independent clinical assessments, imaging and fluids biomarkers, and post-mortem pathological confirmation where possible. There is great interest in understanding the mechanisms by which symptoms arise in neurodegenerative disorders. However, TMS assessments in patients are usually carried out during resting conditions, when the brain network engaged during these symptoms is not expressed. Rather, a context-appropriate form of TMS would be more suitable in probing the physiology driving clinical symptoms. In all, we hope that the recommendations made here will help to further understand the pathophysiology of neurodegenerative diseases.

The Use of Biomarkers and Genetic Screening to Diagnose Frontotemporal Dementia: Evidence and Clinical Implications

Within the wide range of neurodegenerative brain diseases, the differential diagnosis of frontotemporal dementia (FTD) frequently poses a challenge. Often, signs and symptoms are not characteristic of the disease and may instead reflect atypical presentations. Consequently, the use of disease biomarkers is of importance to correctly identify the patients. Here, we describe how neuropsychological characteristics, neuroimaging and neurochemical biomarkers and screening for causal gene mutations can be used to differentiate FTD from other neurodegenerative diseases as well as to distinguish between FTD subtypes. Summarizing current evidence, we propose a stepwise approach in the diagnostic evaluation. Clinical consensus criteria that take into account a full neuropsychological examination have relatively good accuracy (sensitivity [se] 75–95%, specificity [sp] 82–95%) to diagnose FTD, although misdiagnosis (mostly AD) is common. Structural brain MRI (se 70–94%, sp 89–99%) and FDG PET (se 47–90%, sp 68–98%) or SPECT (se 36–100%, sp 41–100%) brain scans greatly increase diagnostic accuracy, showing greater involvement of frontal and anterior temporal lobes, with sparing of hippocampi and medial temporal lobes. If these results are inconclusive, we suggest detecting amyloid and tau cerebrospinal fluid (CSF) biomarkers that can indicate the presence of AD with good accuracy (se 74–100%, sp 82–97%). The use of P-tau₁₈₁ and the Aβ_1–42/Aβ_1–40 ratio significantly increases the accuracy of correctly identifying FTD vs. AD. Alternatively, an amyloid brain PET scan can be performed to differentiate FTD from AD. When autosomal dominant inheritance is suspected, or in early onset dementia, mutation screening of causal genes is indicated and may also be offered to at-risk family members. We have summarized genotype–phenotype correlations for several genes that are known to cause familial frontotemporal lobar degeneration, which is the neuropathological substrate of FTD. The genes most commonly associated with this disease (C9orf72, MAPT, GRN, TBK1) are discussed, as well as some less frequent ones (CHMP2B, VCP). Several other techniques, such as diffusion tensor imaging, tau PET imaging and measuring serum neurofilament levels, show promise for future implementation as diagnostic biomarkers.

Biological, Neuroimaging, and Neurophysiological Markers in Frontotemporal Dementia: Three Faces of the Same Coin

Frontotemporal dementia (FTD) is a heterogeneous clinical, genetic, and neuropathological disorder. Clinical diagnosis and prediction of neuropathological substrates are hampered by heterogeneous pictures. Diagnostic markers are key in clinical trials to differentiate FTD from other neurodegenerative dementias. In the same view, identifying the neuropathological hallmarks of the disease is key in light of future disease-modifying treatments. The aim of the present review is to unravel the progress in biomarker discovery, discussing the potential applications of available biological, imaging, and neurophysiological markers.

Transcranial Magnetic Stimulation for the Assessment of Neurodegenerative Disease

Transcranial magnetic stimulation (TMS) is a noninvasive technique that has provided important information about cortical function across an array of neurodegenerative disorders, including Alzheimer's disease, frontotemporal dementia, Parkinson's disease, and related extrapyramidal disorders. Application of TMS techniques in neurodegenerative diseases has provided important pathophysiological insights, leading to the development of pathogenic and diagnostic biomarkers that could be used in the clinical setting and therapeutic trials. Abnormalities of TMS outcome measures heralding cortical hyperexcitability, as evidenced by a reduction of short-interval intracortical inhibition and increased in motor-evoked potential amplitude, have been consistently identified as early and intrinsic features of amyotrophic lateral sclerosis (ALS), preceding and correlating with the ensuing neurodegeneration. Cortical hyperexcitability appears to form the pathogenic basis of ALS, mediated by trans-synaptic glutamate-mediated excitotoxic mechanisms. As a consequence of these research findings, TMS has been developed as a potential diagnostic biomarker, capable of identifying upper motor neuronal pathology, at earlier stages of the disease process, and thereby aiding in ALS diagnosis. Of further relevance, marked TMS abnormalities have been reported in other neurodegenerative diseases, which have varied from findings in ALS. With time and greater utilization by clinicians, TMS outcome measures may prove to be of utility in future therapeutic trial settings across the neurodegenerative disease spectrum, including the monitoring of neuroprotective, stem-cell, and genetic-based strategies, thereby enabling assessment of biological effectiveness at early stages of drug development.

Cortical function in Alzheimer's disease and frontotemporal dementia

Objectives

Alzheimer’s disease (AD) and the behavioral variant of frontotemporal dementia (bvFTD) are the most common causes of dementia; however, their overlapping clinical syndromes and involved brain regions make a differential diagnosis difficult. We aimed to identify the differences in the cognition and motor cortex excitability between AD and bvFTD patients.

Methods

Twenty-seven AD patients and 30 bvFTD patients were included in the study. Each participant received a neurological evaluation. Cognitive event-related potentials (P300) were recorded during an auditory oddball task. Next, the excitability of the motor cortex, including the resting, facilitated motor threshold (RMT and FMT) and cortical silent period (CSP), were assessed during transcranial magnetic stimulation (TMS).

Results

The bvFTD patients exhibited significantly longer P300 latencies compared with AD patients. There was a significant negative correlation between cognition and P300 latency in the bvFTD group. The AD patients showed significantly reduced RMT and FMT values compared to the bvFTD group; however, no significant correlation was found between AD severity and the excitability of the motor cortex.

Conclusions

Cognition and motor cortical functions are different between AD and bvFTD patients. Noninvasive electrophysiological examinations have the potential to identify unique pathophysiological features that can be used to differentially diagnose AD and bvFTD patients.

Cortical hyperexcitability in patients with C9ORF72 mutations: Relationship to phenotype

INTRODUCTION

Patients with mutations in C9orf72 can have amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), or ALS-FTD. The goals were to establish whether cortical hyperexcitability occurs in C9orf72 patients with different clinical presentations.

METHODS

Cortical thresholds and silent periods were measured in thenar muscles in 19 participants with C9orf72 expansions and 21 healthy controls using transcranial magnetic stimulation (TMS). El Escorial and Rascovsky criteria were used to diagnose ALS and FTD. Fourteen participants with C9orf72 expansions were re-tested 6 months later. Correlations with finger-tapping speed, timed peg test, the ALS functional rating scale, and Dementia Rating Scale were examined.

RESULTS

Most participants with C9orf72 expansions had normal or low cortical thresholds. Among them, ALS patients had the lowest thresholds and significantly shorter silent periods. Thresholds correlated with timed peg-test scores. TMS did not correlate with the Dementia Rating Scale.

CONCLUSIONS

TMS measures of cortical excitability may serve as noninvasive biomarkers of ALS disease activity. Muscle Nerve, 2016 Muscle Nerve 54: 264-269, 2016.

Altered Cortical Synaptic Plasticity in Response to 5-Hz Repetitive Transcranial Magnetic Stimulation as a New Electrophysiological Finding in Amnestic Mild Cognitive Impairment Converting to Alzheimer's Disease: Results from a 4-year Prospective Cohort Study

INTRODUCTION

To investigate cortical excitability and synaptic plasticity in amnestic mild cognitive impairment (aMCI) using 5 Hz repetitive transcranial magnetic stimulation (5 Hz-rTMS) and to assess whether specific TMS parameters predict conversion time to Alzheimer's disease (AD).

MATERIALS AND METHODS

Forty aMCI patients (single- and multi-domain) and 20 healthy controls underwent, at baseline, a neuropsychological examination and 5 Hz-rTMS delivered in trains of 10 stimuli and 120% of resting motor threshold (rMT) intensity over the dominant motor area. The rMT and the ratio between amplitude of the 1st and the 10th motor-evoked potential elicited by the train (X/I-MEP ratio) were calculated as measures of cortical excitability and synaptic plasticity, respectively. Patients were followed up annually over a period of 48 months. Analysis of variance for repeated measures was used to compare TMS parameters in patients with those in controls. Spearman's correlation was performed by considering demographic variables, aMCI subtype, neuropsychological test scores, TMS parameters, and conversion time.

RESULTS

Thirty-five aMCI subjects completed the study; 60% of these converted to AD. The baseline rMT and X/I-MEP ratio were significantly lower in patients than in controls (p = 0.04 and p = 0.01). Spearman's analysis showed that conversion time correlated with the rMT (0.40) and X/I-MEP ratio (0.51).

DISCUSSION

aMCI patients displayed cortical hyperexcitability and altered synaptic plasticity to 5 Hz-rTMS when compared with healthy subjects. The extent of these changes correlated with conversion time. These alterations, which have previously been observed in AD, are thus present in the early stages of disease and may be considered as potential neurophysiological markers of conversion from aMCI to AD.

Transcranial magnetic stimulation (TMS)/repetitive TMS in mild cognitive impairment and Alzheimer's disease

Several Transcranial Magnetic Stimulation (TMS) techniques can be applied to noninvasively measure cortical excitability and brain plasticity in humans. TMS has been used to assess neuroplastic changes in Alzheimer's disease (AD), corroborating findings that cortical physiology is altered in AD due to the underlying neurodegenerative process. In fact, many TMS studies have provided physiological evidence of abnormalities in cortical excitability, connectivity, and plasticity in patients with AD. Moreover, the combination of TMS with other neurophysiological techniques, such as high-density electroencephalography (EEG), makes it possible to study local and network cortical plasticity directly. Interestingly, several TMS studies revealed abnormalities in patients with early AD and even with mild cognitive impairment (MCI), thus enabling early identification of subjects in whom the cholinergic degeneration has occurred. Furthermore, TMS can influence brain function if delivered repetitively; repetitive TMS (rTMS) is capable of modulating cortical excitability and inducing long-lasting neuroplastic changes. Preliminary findings have suggested that rTMS can enhance performances on several cognitive functions impaired in AD and MCI. However, further well-controlled studies with appropriate methodology in larger patient cohorts are needed to replicate and extend the initial findings. The purpose of this paper was to provide an updated and comprehensive systematic review of the studies that have employed TMS/rTMS in patients with MCI and AD.

The contribution of transcranial magnetic stimulation in the diagnosis and in the management of dementia

Transcranial magnetic stimulation (TMS) is emerging as a promising tool to non-invasively assess specific cortical circuits in neurological diseases. A number of studies have reported the abnormalities in TMS assays of cortical function in dementias. A PubMed-based literature review on TMS studies targeting primary and secondary dementia has been conducted using the key words "transcranial magnetic stimulation" or "motor cortex excitability" and "dementia" or "cognitive impairment" or "memory impairment" or "memory decline". Cortical excitability is increased in Alzheimer's disease (AD) and in vascular dementia (VaD), generally reduced in secondary dementias. Short-latency afferent inhibition (SAI), a measure of central cholinergic circuitry, is normal in VaD and in frontotemporal dementia (FTD), but suppressed in AD. In mild cognitive impairment, abnormal SAI may predict the progression to AD. No change in cortical excitability has been observed in FTD, in Parkinson's dementia and in dementia with Lewy bodies. Short-interval intracortical inhibition and controlateral silent period (cSP), two measures of gabaergic cortical inhibition, are abnormal in most dementias associated with parkinsonian symptoms. Ipsilateral silent period (iSP), which is dependent on integrity of the corpus callosum is abnormal in AD. While single TMS measure owns low specificity, a panel of measures can support the clinical diagnosis, predict progression and possibly identify earlier the "brain at risk". In dementias, TMS can be also exploited to select and evaluate the responders to specific drugs and, it might become a rehabilitative tool, in the attempt to restore impaired brain plasticity.

Apraxia and motor dysfunction in corticobasal syndrome

Background

Corticobasal syndrome (CBS) is characterized by multifaceted motor system dysfunction and cognitive disturbance; distinctive clinical features include limb apraxia and visuospatial dysfunction. Transcranial magnetic stimulation (TMS) has been used to study motor system dysfunction in CBS, but the relationship of TMS parameters to clinical features has not been studied. The present study explored several hypotheses; firstly, that limb apraxia may be partly due to visuospatial impairment in CBS. Secondly, that motor system dysfunction can be demonstrated in CBS, using threshold-tracking TMS, and is linked to limb apraxia. Finally, that atrophy of the primary motor cortex, studied using voxel-based morphometry analysis (VBM), is associated with motor system dysfunction and limb apraxia in CBS.

Methods

Imitation of meaningful and meaningless hand gestures was graded to assess limb apraxia, while cognitive performance was assessed using the Addenbrooke's Cognitive Examination – Revised (ACE-R), with particular emphasis placed on the visuospatial subtask. Patients underwent TMS, to assess cortical function, and VBM.

Results

In total, 17 patients with CBS (7 male, 10 female; mean age 64.4+/− 6.6 years) were studied and compared to 17 matched control subjects. Of the CBS patients, 23.5% had a relatively inexcitable motor cortex, with evidence of cortical dysfunction in the remaining 76.5% patients. Reduced resting motor threshold, and visuospatial performance, correlated with limb apraxia. Patients with a resting motor threshold <50% performed significantly worse on the visuospatial sub-task of the ACE-R than other CBS patients. Cortical function correlated with atrophy of the primary and pre-motor cortices, and the thalamus, while apraxia correlated with atrophy of the pre-motor and parietal cortices.

Conclusions

Cortical dysfunction appears to underlie the core clinical features of CBS, and is associated with atrophy of the primary motor and pre-motor cortices, as well as the thalamus, while apraxia correlates with pre-motor and parietal atrophy.

Transcranial magnetic stimulation in patients with early cortical dementia: A pilot study

Context:

The diagnostic accuracy of the currently available tools carries poor sensitivity resulting in significant delay in specific diagnosis of cortical dementias. Considering the properties of default mode networking of the brain it is highly probable that specific changes may be seen in frontotemporal dementias (FTDs) and Alzheimer's disease sufficiently early.

Aim:

The aim of this study is to look for changes in Transcranial Magnetic Stimulation (TMS) in cortical dementia.

Materials and Methods:

Evaluated with a single pulse TMS with the figure of eight coil and recorded from right first dorsal interossei (FDI). Resting Motor Threshold (RMT) was estimated on the opposite motor cortex (T1). Second site of stimulation was cervical spine at C7-T2. Central motor conduction time (CMCT) is equal toT1-T2. Silent Period (SP) identified by applying TMS pulse to contracting FDI.

Conclusions:

RMT was reduced in seven out of eight Alzheimer's dementias. CMCT was in the upper limit of normal in both patients with FTD. The most consistent observation was that SP was reduced and there were escape discharges noticed during the SP suggesting increased cortical excitability and decreased cortical inhibition. This suggests probable early asymptomatic changes in the gamma-aminobutyric acid (GABA) nergic and cholinergic system is taking place. This if confirmed may give some insight into early diagnosis and therapeutic role of GABA agonists in these disorders.

Transcranial magnetic stimulation for the treatment of pharmacoresistant nondelusional auditory verbal hallucinations in dementia

Auditory verbal hallucinations (AVHs) are known as a core symptom of schizophrenia, but also occur in a number of other conditions, not least in neurodegenerative disorders such as dementia. In the last decades, Transcranial Magnetic Stimulation (TMS) emerged as a valuable therapeutic approach towards several neurological and psychiatric diseases, including AVHs. Herein we report a case of a seventy-six-years-old woman with vascular-degenerative brain disease, complaining of threatening AVHs. The patient was treated with a high-frequency temporoparietal (T3P3) rTMS protocol for fifteen days. A considerable reduction of AVHs in frequency and content (no more threatening) was observed. Although further research is needed, this seems an encouraging result.

Dementia

Transcranial magnetic stimulation (TMS) has been used extensively to characterize motor system pathophysiology in Alzheimer's disease (AD) and other forms of dementia, as well to monitor the effects of certain pharmacological agents. Among the studies focusing on motor cortical excitability measures, the most consistent finding is a significant reduction of short-latency afferent inhibition (SAI) in AD and other forms of dementia in which the cholinergic system is affected, such as dementia with Lewy bodies. SAI evaluation may thus provide a reliable biomarker of cortical cholinergic dysfunction in dementias. Moreover, most TMS studies have demonstrated cortical hyperexcitability and asymptomatic motor cortex functional reorganization in the early stages of the disease. Integrated approaches utilizing TMS together with high-density EEG have indicated impaired cortical plasticity and functional connectivity across different neural networks in AD. Paired associative stimulation-induced plasticity has also been found to be abnormal in patients with AD. The development of novel noninvasive methods of brain stimulation, in particular repetitive TMS (rTMS) and transcranial direct current stimulation (tDCS), has increased the interest in neuromodulatory techniques as potential therapeutic tools for cognitive rehabilitation in AD. Preliminary studies have revealed that rTMS and tDCS can induce beneficial effects on specific cognitive functions in AD. Future studies are warranted to replicate and extend the initial findings.

New insights into Alzheimer's disease progression: a combined TMS and structural MRI study

BACKGROUND

Combination of structural and functional data of the human brain can provide detailed information of neurodegenerative diseases and the influence of the disease on various local cortical areas.

METHODOLOGY AND PRINCIPAL FINDINGS

To examine the relationship between structure and function of the brain the cortical thickness based on structural magnetic resonance images and motor cortex excitability assessed with transcranial magnetic stimulation were correlated in Alzheimer's disease (AD) and mild cognitive impairment (MCI) patients as well as in age-matched healthy controls. Motor cortex excitability correlated negatively with cortical thickness on the sensorimotor cortex, the precuneus and the cuneus but the strength of the correlation varied between the study groups. On the sensorimotor cortex the correlation was significant only in MCI subjects. On the precuneus and cuneus the correlation was significant both in AD and MCI subjects. In healthy controls the motor cortex excitability did not correlate with the cortical thickness.

CONCLUSIONS

In healthy subjects the motor cortex excitability is not dependent on the cortical thickness, whereas in neurodegenerative diseases the cortical thinning is related to weaker cortical excitability, especially on the precuneus and cuneus. However, in AD subjects there seems to be a protective mechanism of hyperexcitability on the sensorimotor cortex counteracting the prominent loss of cortical volume since the motor cortex excitability did not correlate with the cortical thickness. Such protective mechanism was not found on the precuneus or cuneus nor in the MCI subjects. Therefore, our results indicate that the progression of the disease proceeds with different dynamics in the structure and function of neuronal circuits from normal conditions via MCI to AD.

Motor neuron dysfunction in frontotemporal dementia

Frontotemporal dementia and motor neuron disease share clinical, genetic and pathological characteristics. Motor neuron disease develops in a proportion of patients with frontotemporal dementia, but the incidence, severity and functional significance of motor system dysfunction in patients with frontotemporal dementia has not been determined. Neurophysiological biomarkers have been developed to document motor system dysfunction including: short-interval intracortical inhibition, a marker of corticospinal motor neuron dysfunction and the neurophysiological index, a marker of lower motor neuron dysfunction. The present study performed detailed clinical and neurophysiological assessments on 108 participants including 40 consecutive patients with frontotemporal dementia, 42 age- and gender-matched patients with motor neuron disease and 26 control subjects. Of the 40 patients with frontotemporal dementia, 12.5% had concomitant motor neuron disease. A further 27.3% of the patients with frontotemporal dementia had clinical evidence of minor motor system dysfunction such as occasional fasciculations, mild wasting or weakness. Biomarkers of motor system function were abnormal in frontotemporal dementia. Average short-interval intracortical inhibition was reduced in frontotemporal dementia (4.3 ± 1.7%) compared with controls (9.1 ± 1.1%, P < 0.05). Short-interval intracortical inhibition was particularly reduced in the progressive non-fluent aphasia subgroup, but was normal in patients with behavioural variant frontotemporal dementia and semantic dementia. The neurophysiological index was reduced in frontotemporal dementia (1.1) compared with controls (1.9, P < 0.001), indicating a degree of lower motor neuron dysfunction, although remained relatively preserved when compared with motor neuron disease (0.7, P < 0.05). Motor system dysfunction in frontotemporal dementia may result from pathological involvement of the primary motor cortex, with secondary degeneration of lower motor neurons in the brainstem and anterior horn of the spinal cord.

Noninvasive brain stimulation in Alzheimer's disease: systematic review and perspectives for the future

BACKGROUND

A number of studies have applied transcranial magnetic stimulation (TMS) to physiologically characterize Alzheimer's disease (AD) and to monitor effects of pharmacological agents, while others have begun to therapeutically use TMS and transcranial direct current stimulation (tDCS) to improve cognitive function in AD. These applications are still very early in development, but offer the opportunity of learning from them for future development.

METHODS

We performed a systematic search of all studies using noninvasive stimulation in AD and reviewed all 29 identified articles. Twenty-four focused on measures of motor cortical reactivity and (local) plasticity and functional connectivity, with eight of these studies assessing also effects of pharmacological agents. Five studies focused on the enhancement of cognitive function in AD.

RESULTS

Short-latency afferent inhibition (SAI) and resting motor threshold are significantly reduced in AD patients as compared to healthy elders. Results on other measures of cortical reactivity, e.g. intracortical inhibition (ICI), are more divergent. Acetylcholine-esterase inhibitors and dopaminergic drugs may increase SAI and ICI in AD. Motor cortical plasticity and connectivity are impaired in AD. TMS/tDCS can induce acute and short-duration beneficial effects on cognitive function, but the therapeutic clinical significance in AD is unclear. Safety of TMS/tDCS is supported by studies to date.

CONCLUSIONS

TMS/tDCS appears safe in AD, but longer-term risks have been insufficiently considered. TMS holds promise as a physiologic biomarker in AD to identify therapeutic targets and monitor pharmacologic effects. In addition, TMS/tDCS may have therapeutic utility in AD, though the evidence is still very preliminary and cautious interpretation is warranted.

Transcranial magnetic stimulation in Alzheimer's disease: a neurophysiological marker of cortical hyperexcitability

Recently, neuropathological studies have shown an important motor cortex involvement in Alzheimer's disease (AD), even in its early stages, despite the lack of clinically evident motor deficit. Transcranial magnetic stimulation (TMS) studies have demonstrated that cortical excitability is enhanced in AD patients. This cortical hyperexcitability is believed to be a compensatory mechanism to execute voluntary movements, despite the progressive impairment of associative cortical areas. At present, it is not clear if these motor cortex excitability changes might be the expression of an involvement of intracortical excitatory glutamatergic circuits or an impairment of inhibitory cholinergic and, to a lesser extent, gabaergic activity. Although the main hypothesis for the pathogenesis of AD remains the degeneration of the basal forebrain cholinergic neurons, the development of specific TMS protocols, such as the paired-pulse TMS and the study of the short-latency afferent inhibition, points out the role of other neurotransmitters, such as gamma-amino-butyric acid, glutamate and dopamine. The potential therapeutic effect of repetitive TMS in restoring or compensating damaged cognitive functions, might become a possible rehabilitation tool in AD patients. Based on different patterns of cortical excitability, TMS may be useful in discriminating between physiological brain aging, mild cognitive impairment, AD and other dementing disorders. The present review provides a perspective of these TMS techniques by further understanding the role of different neurotransmission pathways and plastic remodelling of neuronal networks in the pathogenesis of AD.

Transcranial magnetic stimulation in Alzheimer's disease: a neurophysiological marker of cortical hyperexcitability

Recently, neuropathological studies have shown an important motor cortex involvement in Alzheimer's disease (AD), even in its early stages, despite the lack of clinically evident motor deficit. Transcranial magnetic stimulation (TMS) studies have demonstrated that cortical excitability is enhanced in AD patients. This cortical hyperexcitability is believed to be a compensatory mechanism to execute voluntary movements, despite the progressive impairment of associative cortical areas. At present, it is not clear if these motor cortex excitability changes might be the expression of an involvement of intracortical excitatory glutamatergic circuits or an impairment of inhibitory cholinergic and, to a lesser extent, gabaergic activity. Although the main hypothesis for the pathogenesis of AD remains the degeneration of the basal forebrain cholinergic neurons, the development of specific TMS protocols, such as the paired-pulse TMS and the study of the short-latency afferent inhibition, points out the role of other neurotransmitters, such as gamma-amino-butyric acid, glutamate and dopamine. The potential therapeutic effect of repetitive TMS in restoring or compensating damaged cognitive functions, might become a possible rehabilitation tool in AD patients. Based on different patterns of cortical excitability, TMS may be useful in discriminating between physiological brain aging, mild cognitive impairment, AD and other dementing disorders. The present review provides a perspective of these TMS techniques by further understanding the role of different neurotransmission pathways and plastic remodelling of neuronal networks in the pathogenesis of AD.