Intracortical inhibition of the motor cortex is normal in chorea

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.

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.

Evaluation of Cognition and Cortical Excitability in Huntington's Disease

BACKGROUND

Recent advances in neurophysiological techniques have contributed to our understanding of the pathophysiology of Huntington's disease (HD). Studies of the motor cortical excitability and central motor pathways have shown variable results.

OBJECTIVES

Our aims were to evaluate the cortical excitability changes in HD using transcranial magnetic stimulation (TMS) and correlate the changes with cognitive impairment.

METHODS

The study included 32 HD patients and 30 age- and gender-matched controls. The demographic and clinical profiles of the patients were recorded. All subjects were evaluated by TMS and resting motor threshold (RMT), central motor conduction time (CMCT), silent period (SP), short-interval intracortical inhibition (SICI), and intracortical facilitation were determined. A battery of neuropsychological tests was administered to all subjects.

RESULTS

The mean age of the patients was 42.1±14.1 years, and that of controls 39.4±12.4 years (p=0.61). There was no significant difference in RMT and CMCT between the two groups. There was a mild prolongation of the contralateral SP in HD, but it was not significant. SICI was significantly reduced in HD (p<0.0001). A significant impairment in attention, verbal fluency, executive function, visuospatial function, learning, and memory was observed in HD patients. However, there was no correlation between cortical excitability changes and cognitive impairment.

CONCLUSIONS

TMS is a valuable method of evaluating cortical excitability changes in HD. These patients have reduced SICI and significant impairment of cognition in multiple domains.

Brain stimulation in Huntington's disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder which is associated with severe disturbances of motor function, especially choreatic movements, cognitive decline and psychiatric symptoms. Various brain stimulation methods have been used to study brain function in patients with HD. Moreover, brain stimulation has evolved as an alternative or additive treatment option, besides current symptomatic medical treatment. This article summarizes the results of brain stimulation to better understand the characteristics of cortical excitability and plasticity in HD and gives a perspective on the therapeutic role for noninvasive and invasive neuromodulatory brain stimulation methods.

Transcranial magnetic stimulation to understand pathophysiology and as potential treatment for neurodegenerative diseases

Common neurodegenerative diseases include Parkinson's disease (PD), Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS) and Huntington's disease (HD). Transcranial magnetic stimulation (TMS) is a noninvasive and painless method to stimulate the human brain. Single- and paired-pulse TMS paradigms are powerful ways to study the pathophysiological mechanisms of neurodegenerative diseases. Motor evoked potential studied with single-pulse TMS is increased in PD, AD and ALS, but is decreased in HD. Changes in motor cortical excitability in neurodegenerative diseases may be related to functional deficits in cortical circuits or to compensatory mechanisms. Reduction or even absence of short interval intracortical inhibition induced by paired-pulse TMS is common in neurodegenerative diseases, suggesting that there are functional impairments of inhibitory cortical circuits. Decreased short latency afferent inhibition in AD, PD and HD may be related to the cortical cholinergic deficits in these conditions. Cortical plasticity tested by paired associative stimulation or theta burst stimulation is impaired in PD, AD and HD. Repetitive TMS (rTMS) refers to the application of trains of regularly repeating TMS pulses. High-frequency facilitatory rTMS may improve motor symptoms in PD patients whereas low-frequency inhibitory stimulation is a potential treatment for levodopa induced dyskinesia. rTMS delivered both to the left and right dorsolateral prefrontal cortex improves memory in AD patients. Supplementary motor cortical stimulation in low frequency may be useful for HD patients. However, the effects of treatment with multiple sessions of rTMS for neurodegenerative diseases need to be tested in large, sham-controlled studies in the future before they can be adopted for routine clinical practice.

Corticospinal activity evoked and modulated by non-invasive stimulation of the intact human motor cortex

A number of methods have been developed recently that stimulate the human brain non-invasively through the intact scalp. The most common are transcranial magnetic stimulation (TMS), transcranial electric stimulation (TES) and transcranial direct current stimulation (TDCS). They are widely used to probe function and connectivity of brain areas as well as therapeutically in a variety of conditions such as depression or stroke. They are much less focal than conventional invasive methods which use small electrodes placed on or in the brain and are often thought to activate all classes of neurones in the stimulated area. However, this is not true. A large body of evidence from experiments on the motor cortex shows that non-invasive methods of brain stimulation can be surprisingly selective and that adjusting the intensity and direction of stimulation can activate different classes of inhibitory and excitatory inputs to the corticospinal output cells. Here we review data that have elucidated the action of TMS and TES, concentrating mainly on the most direct evidence available from spinal epidural recordings of the descending corticospinal volleys. The results show that it is potentially possible to test and condition specific neural circuits in motor cortex that could be affected differentially by disease, or be used in different forms of natural behaviour. However, there is substantial interindividual variability in the specificity of these protocols. Perhaps in the future it will be possible, with the advances currently being made to model the electrical fields induced in individual brains, to develop forms of stimulation that can reliably target more specific populations of neurones, and open up the internal circuitry of the motor cortex for study in behaving humans.

Changes in motor cortical excitability in patients with Sydenham's chorea

BACKGROUND AND PURPOSE

The neurophysiological characteristics of motor cortex have been well characterized in patients with Huntington's disease. We present the first data on cortical excitability in patients with Sydenham's chorea.

METHODS

Motor cortex excitability was examined using transcranial magnetic stimulation in 16 patients in the early clinical stages of Sydenham's chorea and in 17 age- and sex-matched control subjects. Investigations included resting and active motor threshold, motor evoked potential, input-output curves, contralateral silent period, and transcallosal inhibition.

RESULTS

Resting and active motor threshold were significantly higher and motor evoked potentials were significantly smaller in patients in comparison with controls. The input-output curves were shallower in both hemispheres of patients with chorea compared with controls. No significant differences were seen in silent period or transcallosal inhibition duration.

CONCLUSION

Sydenham's chorea is characterized by reduced excitability of corticospinal output similar to that observed in Huntington's disease.

Movement disorders in systemic lupus erythematosus and the antiphospholipid syndrome

Movement disorders (MDs), particularly chorea, may be the presenting neurological complication of systemic lupus erythematosus (SLE) and the antiphospholipid syndrome (APS), but the association is not often initially recognized. Current evidence suggests an autoimmune mechanism related to antiphospholipid antibodies in these two conditions, although the antigenic target within the central nervous system has not yet been identified. Based on a comprehensive review of the literature, this article summarizes the current knowledge on MDs in SLE and APS. A high index of suspicion is required to make an early diagnosis and initiate appropriate treatment to provide symptomatic relief and to prevent other systemic complications related to the autoimmune process.

Abnormal motor cortex excitability in preclinical and very early Huntington's disease

BACKGROUND

In Huntington's disease (HD), the cerebral cortex is involved early in the disease process. The study of cortical excitability can therefore contribute to understanding HD pathophysiology.

METHODS

With transcranial magnetic stimulation (TMS) we examined motor cortex excitability in 8 premanifest HD gene carriers, 8 very early symptomatic HD patients, and 22 healthy control subjects. Electrophysiological measures were correlated with the clinical stage of HD to identify motor cortical dysfunction before overt clinical disease onset.

RESULTS

Premanifest and early manifest HD patients had higher resting and active motor cortex thresholds than control subjects (p = .024). At rest, recruitment of motor evoked potentials was more gradual in both patient groups than in control subjects (p = .001). When active, recruitment and the duration of the cortical silent period were similar in all groups. There was a tendency for short interval intra-cortical inhibition to have a higher threshold in all patients taken together but not in each group separately. Short latency afferent inhibition (SAI) was reduced in early manifest patients compared with control subjects and premanifest patients (p < .001) and in contrast to all other measures was inversely associated with estimated time to onset (p = .013, adjusted R(2) = .32) and the Unified Huntington's Disease Rating Scale motor score (p = .001, adjusted R(2) = .5). A combination of age, CAG repeat length, and SAI strongly predicted the Unified Huntington's Disease Rating Scale motor score (p = .001, adjusted R(2) = .68).

CONCLUSIONS

Our findings add to the evidence for early cortical involvement in HD possibly before HD signs appear.

Abnormal intracortical facilitation in early-stage Huntington's disease

OBJECTIVE

It is known from neuropathological and imaging studies that the neuronal degeneration in Huntington's disease (HD) is already quite severe when the first symptoms of the disease become clinically evident. This study was aimed at detecting neurophysiological changes, as assessed by means of transcranial magnetic stimulation (TMS), involved in the early pathogenesis of the neurodegeneration in HD.

METHODS

Motor cortex excitability was examined in 12 patients with HD in the early clinical stage of the disease and in 15 age-matched control subjects, using a range of TMS protocols. Central motor conduction time, resting and active motor threshold, duration of the cortical silent period, the short-interval paired-pulse intracortical inhibition (SICI) and the paired-pulse intracortical facilitation (ICF) were examined.

RESULTS

The early-stage HD patients showed a statistically significant reduction in ICF. The other measures did not differ significantly from the control subjects.

CONCLUSIONS

Our findings provide neurophysiological evidence that changes in motor function are present in the early HD. Since ICF is thought to depend upon the activity of intracortical glutamatergic excitatory circuits, the results of our study support the theory that altered NMDA receptor function plays an important role in the pathogenesis of HD.

SIGNIFICANCE

These findings may provide clues to the underlying pathophysiology of the disease. A more complete understanding of the changes in motor cortex excitability that occur early in the course of HD will lead to a better definition of the disease process and may allow earlier diagnosis and intervention.

Motor cortical excitability studied with repetitive transcranial magnetic stimulation in patients with Huntington's disease

OBJECTIVE

TMS techniques have provided controversial information on motor cortical function in Huntington's disease (HD). We investigated the excitability of motor cortex in patients with HD using repetitive transcranial magnetic stimulation (rTMS).

METHODS

Eleven patients with HD, and 11 age-matched healthy subjects participated in the study. The clinical features of patients with HD were evaluated with the United Huntington's Disease Rating Scale (UHDRS). rTMS was delivered with a Magstim Repetitive Magnetic Stimulator through a figure-of-8 coil placed over the motor area of the first dorsal interosseus (FDI) muscle. Trains of 10 stimuli were delivered at 5 Hz frequency and suprathreshold intensity (120% resting motor threshold) with the subjects at rest and during voluntary contraction of the target muscle.

RESULTS

In healthy subjects at rest, rTMS produced motor evoked potentials (MEPs) that increased in amplitude over the course of the trains. Conversely in patients, rTMS left the MEP size almost unchanged. In both groups, during voluntary contraction rTMS increased the silent period (SP) duration.

CONCLUSIONS

Because rTMS modulates motor cortical excitability by activating cortical excitatory and inhibitory interneurons these findings suggest that in patients with HD the excitability of facilitatory intracortical interneurones is decreased.

SIGNIFICANCE

We suggest that depressed excitability of the motor cortex in patients with HD reflects a disease-related weakening of cortical facilitatory mechanisms.

Investigating the cortical origins of motor overflow

Motor overflow refers to the involuntary movements which may accompany the production of voluntary movements. While overflow is not usually seen in the normal population, it does present in children and the elderly, as well as those suffering certain neurological dysfunctions. Advancements in methodology over the last decade have allowed for more convincing conclusions regarding the cortical origins of motor overflow. However, despite significant research, the exact mechanism underlying the production of motor overflow is still unclear. This review presents a more comprehensive conceptualization of the theories of motor overflow, which have often been only vaguely defined. Further, the major findings are explored in an attempt to differentiate the competing theories of motor overflow production. This exploration is done in the context of a range of neurological and psychiatric disorders, in order to elucidate the possible underlying mechanisms of overflow.

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.

Enhanced associated movements in the contralateral limbs elicited by brisk voluntary contraction in choreic disorders

OBJECTIVES

To study the deficit of inhibition of excessive motor drive generated in the central nervous system in chorea.

METHODS

Identical associated movements in the contralateral limb elicited by rapid hand squeezing were measured in 6 patients with Huntington's disease, 7 patients with peak-dose dyskinesia, 10 patients with Parkinson's disease, 8 patients with spinocerebellar degeneration and in 8 normal subjects. The intensity of associated movements was assessed by the EMG amplitude ratio of associated contractions to active contractions.

RESULTS

The associated movement ratios were larger in Huntington's disease and peak-dose dyskinesia as compared to other groups. The ratios in akinetic "off" phase were smaller than those in dyskinetic "on" phase in all peak-dose dyskinesia patients.

CONCLUSIONS

Enhanced associated movements support a possible common mechanism that chorea may result from failure in inhibition of phasic neural activity pathologically generated in the brain.

Neurophysiological characterization of parkinsonian syndromes

Electrophysiological examination can provide relevant information on functional abnormalities in patients with parkinsonism. The combined use of various electrodiagnostic techniques can contribute to the diagnosis of the illness, to its correct classification and differentiation from other diseases with a clinically similar presentation, and in particular the identification of the pathophysiological processes underlying some of the signs and symptoms characterizing the movement disorder. Tests which are useful in the differential diagnosis of various parkinsonian syndromes can now be performed in most electrodiagnostic laboratories. This article reviews some of the most relevant observations provided by neurophysiological studies on patients with parkinsonism, with a special focus on those that could be of more value in neurological clinical practice through their contribution to the characterization of the disease or to the recognition of underlying pathophysiological processes.

Reproducibility of intracortical inhibition and facilitation using the paired-pulse paradigm

We have evaluated the reproducibility of intracortical inhibition (ICI) and facilitation (ICF) studied with paired-pulse focal transcranial magnetic stimulation. Three investigators studied the same subjects (n = 4) in three different sessions. A high variability was shown across subjects [coefficient of variation, (cv) 67.3% for ICI and 21.2% for ICF]. Intersession variability was up to 37.1% for ICI and 22.7% for ICF. Interinvestigator variability was 17.3% for ICI and negligible for ICF. Our results may have implications for planning future studies.

Exploration of motor cortex excitability in a diabetic patient with hemiballism-hemichorea

Hemiballism-hemichorea in older patients with hyperglycemia, associated with high signal intensity in the contralateral striatum on T1-weighted magnetic resonance scans, is now an accepted clinical entity. We present an additional patient with this disorder. Using transcranial magnetic stimulation, we show that intracortical inhibition in the motor cortex contralateral to hemiballism-hemichorea is increased. This finding is discussed in the context of current models of basal ganglia-thalamo-cortical connectivity.

Intracortical inhibition of the motor cortex in movement disorders

We have studied the function of inhibitory interneurons within the motor cortex in several movement disorders using a paired-pulse magnetic stimulation technique. Their function was disturbed in patients with dystonia or focal lesions of the basal ganglia. On the other hand, the inhibition was normal in patients with chorea or essential tremor. The inhibitory circuit in the motor cortex must be functionally involved in some movement disorders probably because of changes of the inputs from basal ganglia to motor cortices, but not involved in the others. This difference in the functional involvement of inhibitory interneurons of the motor cortex may reflect different pathogenesis of these movement disorders.

Spinal and cortical inhibition in Huntington's chorea

In this article we studied spinal and cortical inhibitory mechanisms in patients with Huntington's disease. To evaluate spinal cord inhibitory circuitries, we assessed reciprocal inhibition between antagonist forearm muscles and the recovery cycle of the H reflex in the flexor carpi radialis. Patients showed a significant decrease in the presynaptic phase of reciprocal inhibition reaching a minimum at the conditioning-test interval of 20 msec and an abnormal facilitation of the test H reflex at the conditioning test interval of 40 to 60 msec. Throughout its time course (10-200 msec), the H reflex recovery cycle showed a more prominent facilitation in patients than in control subjects. To assess whether the observed pathophysiological abnormalities might have arisen from an abnormal motor cortical excitability, we examined the recovery cycle of the motor potentials evoked by paired transcranial magnetic stimuli. We found that the inhibitory mechanisms controlling motor cortical excitability were normal. An interpretation of the spinal cord abnormalities is that the intrinsically normal but deafferentated motor cortex in Huntington's disease partly loses its inhibitory control, thus disinhibiting spinal cord circuitry. Our findings from paired transcranial magnetic stimulation suggest that cortical motor areas are not hyperexcitable in Huntington's disease. Hence, the postulated thalamocortical overactivity in experimental models of Huntington's disease needs to be reappraised.