Physiologic analysis of the myoclonus of Alzheimer's disease

Rethinking the neurophysiological concept of cortical myoclonus

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

Myoclonus generators in sialidosis

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The cortical origin of myoclonus in sialidosis does not fully explain the phenomena.

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We used electrophysiology to show a possible subcortical source for the myoclonus.

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Correct understanding of this physiopathology may help improve treatment.

Objective

Sialidosis is an inborn error of metabolism. There is evidence that the myoclonic movements observed in this disorder have a cortical origin, but this mechanism does not fully explain the bilaterally synchronous myoclonus activity frequently observed in many patients. We present evidence of a subcortical basis for synchronous myoclonic phenomena.

Methods

Electromyographic investigations were undertaken in two molecularly and biochemically confirmed patients with sialidosis type-1.

Results

The EMG recordings showed clear episodes of bilaterally synchronous myoclonic activity in contralateral homologous muscles. We also observed a high muscular-muscular coherence with near-zero time-lag between these muscles.

Conclusion

The absence of coherence phase lag between the right-and-left homologous muscles during synchronous events indicates that a unilateral cortical source cannot fully explain the myoclonic activity. There must exist a subcortical mechanism for bilateral synchronization accounting for this phenomenon.

Significance

Understanding this mechanism may illuminate cortical-subcortical relationships in myoclonus.

Bilateral Representation of Sensorimotor Responses in Benign Adult Familial Myoclonus Epilepsy: An MEG Study

Patients with cortical reflex myoclonus manifest typical neurophysiologic characteristics due to primary sensorimotor cortex (S1/M1) hyperexcitability, namely, contralateral giant somatosensory-evoked potentials/fields and a C-reflex (CR) in the stimulated arm. Some patients show a CR in both arms in response to unilateral stimulation, with about 10-ms delay in the non-stimulated compared with the stimulated arm. This bilateral C-reflex (BCR) may reflect strong involvement of bilateral S1/M1. However, the significance and exact pathophysiology of BCR within 50 ms are yet to be established because it is difficult to identify a true ipsilateral response in the presence of the giant component in the contralateral hemisphere. We hypothesized that in patients with BCR, bilateral S1/M1 activity will be detected using MEG source localization and interhemispheric connectivity will be stronger than in healthy controls (HCs) between S1/M1 cortices. We recruited five patients with cortical reflex myoclonus with BCR and 15 HCs. All patients had benign adult familial myoclonus epilepsy. The median nerve was electrically stimulated unilaterally. Ipsilateral activity was investigated in functional regions of interest that were determined by the N20m response to contralateral stimulation. Functional connectivity was investigated using weighted phase-lag index (wPLI) in the time-frequency window of 30–50 ms and 30–100 Hz. Among seven of the 10 arms of the patients who showed BCR, the average onset-to-onset delay between the stimulated and the non-stimulated arm was 8.4 ms. Ipsilateral S1/M1 activity was prominent in patients. The average time difference between bilateral cortical activities was 9.4 ms. The average wPLI was significantly higher in the patients compared with HCs in specific cortico-cortical connections. These connections included precentral-precentral, postcentral-precentral, inferior parietal (IP)-precentral, and IP-postcentral cortices interhemispherically (contralateral region-ipsilateral region), and precentral-IP and postcentral-IP intrahemispherically (contralateral region-contralateral region). The ipsilateral response in patients with BCR may be a pathologically enhanced motor response homologous to the giant component, which was too weak to be reliably detected in HCs. Bilateral representation of sensorimotor responses is associated with disinhibition of the transcallosal inhibitory pathway within homologous motor cortices, which is mediated by the IP. IP may play a role in suppressing the inappropriate movements seen in cortical myoclonus.

BacAv, a new free online platform for clinical back-averaging

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Back-averaging is a useful technique to correlate the activity of the motor cortex with a muscle jerk.

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Back-averaging is a tool for the diagnosis of conditions such as functional movement disorders and cortical myoclonus.

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BacAv, a new online platform, can be used for back-average analysis.

Objective

The back-average technique is very useful to study the relation between the activity in the cortex and the muscles. It has two main clinical applications, Bereitschaftspotential (BP) recording and myoclonus studies. The BP is a slow wave negativity originating in the supplementary motor cortex and premotor cortex that precedes voluntary movements. This wave also precedes involuntary movements in functional movement disorders (FMD), and it can be used as a helpful diagnostic tool. For the myoclonus studies, the back-average technique is very important to help localizing the source of the myoclonus. The hardware needed to do BP or myoclonus studies is standard and available in any electrophysiology lab, but there are not many software solutions to do the analysis. In this article together with describing the methodology that we use for recording clinical BPs and myoclonus, we present BacAv, an online free application that we developed for the purpose of doing back-average analysis.

Methods

BacAv was developed in “R” language using Rstudio, a free integrated development environment. The recommended parameters for the data acquisition for BP recording and myoclonus studies are given in this section.

Results

The platform was successfully developed, is able to read txt files, look for muscle bursts, segment the data, and plot the average. The parameters of the algorithm that look for the muscle bursts can be adapted according to the characteristics of the dataset.

Conclusion

We have developed software for clinicians who do not have sophisticated equipment to do back-averaging.

Significance

This tool will make this useful analysis method more available in a clinical environment.

Myoclonic Disorders

Few movement disorders seem to make a straightforward approach to diagnosis and treatment more difficult and frustrating than myoclonus, due to its plethora of causes and its variable classifications. Nevertheless, in recent years, exciting advances have been made in the elucidation of the pathophysiology and genetic basis of many disorders presenting with myoclonus. Here, we provide a review of all of the important types of myoclonus encountered in pediatric and adult neurology, with an emphasis on the recent developments that have led to a deeper understanding of this intriguing phenomenon. An up-to-date list of the genetic basis of all major myoclonic disorders is presented. Randomized studies are scarce in myoclonus therapy, but helpful pragmatic approaches at diagnosis as well as treatment have been recently suggested.

Memantine-induced Myoclonus in a Patient with Alzheimer Disease

BACKGROUND

Myoclonus can be a clinical manifestation of numerous neurodegenerative disorders and an adverse drug reaction to medications used in their treatment.

CASE REPORT

Herein, we report memantine-induced myoclonus in a patient with Alzheimer disease. The myoclonus seen in our patient was generalized (proximal limbs and trunk), present at rest and with action, and stimulus sensitive. A structured evaluation with the Unified Myoclonus Rating Scale showed that the myoclonus had no significant effect on functional capacity. After discontinuation of memantine, myoclonus slowly resolved over the course of several weeks.

DISCUSSION

Memantine may cause myoclonus in susceptible individuals.

Myoclonus

Myoclonus can be classified as physiologic, essential, epileptic, and symptomatic. Animal models of myoclonus include DDT and posthypoxic myoclonus in the rat. 5-Hydrotryptophan, clonazepam, and valproic acid suppress myoclonus induced by posthypoxia. The diagnostic evaluation of myoclonus is complex and involves an extensive work-up including basic electrolytes, glucose, renal and hepatic function tests, paraneoplastic antibodies, drug and toxicology screens, thyroid antibody and function studies, neurophysiology testing, imaging, and tests for malabsorption disorders, assays for enzyme deficiencies, tissue biopsy, copper studies, alpha-fetoprotein, cytogenetic analysis, radiosensitivity DNA synthesis, genetic testing for inherited disorders, and mitochondrial function studies. Treatment of myoclonus is targeted to the underlying disorder. If myoclonus physiology cannot be demonstrated, treatment should be aimed at the common pattern of symptoms. If the diagnosis is not known, treatment could be directed empirically at cortical myoclonus as the most common physiology. In cortical myoclonus, the most effective drugs are sodium valproic acid, clonazepam, levetiracetam, and piracetam. For cortical-subcortical myoclonus, valproic acid is the drug of choice. Here, lamotrigine can be used either alone or in combination with valproic acid. Ethosuximide, levetiracetam, or zonisamide can also be used as adjunct therapy with valproic acid. A ketogenic diet can be considered if everything else fails. Subcortical-nonsegmental myoclonus may respond to clonazepam and deep-brain stimulation. Rituximab, adrenocorticotropic hormone, high-dose dexamethasone pulse, or plasmapheresis have been reported to improve opsoclonus myoclonus syndrome. Reticular reflex myoclonus can be treated with clonazepam, diazepam and 5-hydrotryptophan. For palatal myoclonus, a variety of drugs have been used.

Senile myoclonic epilepsy: delineation of a common condition associated with Alzheimer's disease in Down syndrome

In Down syndrome (DS), epilepsy is frequent in all age classes and is recognized as a significant cause of additional handicap and morbidity. Longer life expectancy has led to the recognition of the high incidence of both Alzheimer's disease and seizures in elderly persons with DS. Neuropathological markers of AD are found in all DS brains and clinical symptoms of AD become apparent by the age of 60 years and above in over 50% of DS subjects. Following preliminary description of myoclonic seizures and/or myoclonic epilepsy in isolated cases or small series, we wish to report the diagnostic criteria, treatment and prognosis of a specific and recognizable form of epilepsy associated with AD in a larger group of middle-aged to elderly DS patients. This markedly under-recognized entity may indeed concern an already large and steadily increasing number of patients. We reviewed all medical records of patients with DS referred to our centers (Centre Saint Paul-Gastaut, Marseille; Epilepsy Unit, Montpellier University Hospital; Department of Neurology, Hospital General de Asturias, Oviedo) since 1995. DS had been diagnosed in all at birth, and all presented with the typical morphological changes associated with DS. We selected all cases (18) referred as adults with new onset of myoclonic jerks (MJ) and/or behavioral or cognitive deterioration (CD).

Human motor associative plasticity induced by paired bihemispheric stimulation

Paired associative stimulation (PAS) is an effective non-invasive method to induce human motor plasticity by the repetitive pairing of peripheral nerve stimulation and transcranial magnetic stimulation (TMS) at the primary motor cortex (M1) with a specific time interval. Although the repetitive pairing of two types of afferent stimulation might be a biological basis of neural plasticity and memory, other types of paired stimulation of the human brain have rarely been studied. We hypothesized that the repetitive pairing of TMS and interhemispheric cortico-cortical projection or paired bihemispheric stimulation (PBS), in which the right and left M1 were serially stimulated with a time interval of 15 ms, would produce an associative long-term potentiation (LTP)-like effect. In this study, 23 right-handed healthy volunteers were subjected to a 0.1 Hz repetition of 180 pairings of bihemispheric TMS, and physiological and behavioural measures of the motor system were compared before, immediately after, 20 min after and 40 min after PBS intervention. The amplitude of the motor evoked potential (MEP) induced by the left M1 stimulation and its input-output function increased for up to approximately 20 min post-PBS. Fine finger movements were also facilitated by PBS. Spinal excitability measured by the H-reflex was insensitive to PBS, suggesting a cortical mechanism. The associative LTP-like effect induced by PBS was timing dependent, occurring only when the interstimulus interval was 5-25 ms. These findings demonstrate that using PBS in PAS can induce motor cortical plasticity, and this approach might be applicable to the rehabilitation of patients with motor disorders.

Electrophysiologic assessments of involuntary movements: tremor and myoclonus

Tremor is defined as a rhythmical, involuntary oscillatory movement of a body part. Although neurological examination reveals information regarding its frequency, regularity, amplitude, and activation conditions, the electrophysiological investigations help in confirming the tremor, in differentiating it from other hyperkinetic disorders like myoclonus, and may provide etiological clues. Accelerometer with surface electromyogram (EMG) can be used to document the dominant frequency of a tremor, which may be useful as certain frequencies are more characteristic of specific etiologies than others hyperkinetic disorders. It may show rhythmic bursts, duration and activation pattern (alternating or synchronous). Myoclonus is a quick, involuntary movement. Electrophysiological studies may helpful in the evaluation of myoclonus, not only for confirming the clinical diagnosis but also for understanding the underlying physiological mechanisms. Electroencephalogram (EEG)-EMG correlates can give us important information about myoclonus. Jerk-locked back-averaging and evoked potentials with recording of the long-latency, long-loop reflexes are currently available to study the pathophysiology of myoclonus.

Parkinsonism & related disorders. Myoclonus

Myoclonus has now been recognized to have many possible etiologies, anatomical sources, and pathophysiologic features. Classification schemes may be based on clinical syndromes and etiology, neurophysiology properties, or exam findings. In recent years, many myoclonus case reports and short series have been published. However, this article will group new developments into three areas: (1) Myoclonus in parkinsonian disorders, (2) Concepts in myoclonus generation, and (3) Treatment. Current findings do not allow one to conclude whether or how parkinsonism contributes to the myoclonus mechanism in parkinsonian disorders. Therefore, it seems unlikely that the myoclonus in Lewy body disorders is mostly caused by abnormal basal ganglia input to motor areas of the neocortex. The exact source of cortical myoclonus generation is controversial. Increased corticomuscular coherence represents a robust phenomenon that will need to be explained by any model that offers a putative explanation for cortical myoclonus generation. Myoclonus treatment is still limited, and more research on basic mechanisms before truly effective treatment will be available. The best approach for myoclonus is based on the physiological classification of the myoclonus.

Speech-activated myoclonus: An uncommon form of action myoclonus

We describe an unusual form of facial myoclonus activated by speech in 3 patients with different underlying neurological diseases and present the electrophysiological investigations and results of structural and functional imaging. In 1 of 2 patients in whom jerk-locked electroencephalogram (EEG) back-averaging was done, a cortical potential clearly preceded the facial jerks. In the second patient, a cortical potential preceding the jerk was not certain. In the third patient, the resting EEG contained outbursts of symmetric, slower frequencies of indeterminate significance. An epileptiform disorder was suspected in this patient.

Electrophysiological studies of myoclonus

As myoclonus is often associated with abnormally increased excitability of cortical structures, electrophysiological studies provide useful information for its diagnosis and classification, and about its generator mechanisms. The electroencephalogram-electromyogram polygraph reveals the most important information about the myoclonus of interest. Jerk-locked back-averaging and evoked potential studies combined with recording of the long-latency, long-loop reflexes are useful to investigate the pathophysiology of myoclonus further, especially that of cortical myoclonus. Recent advances in magnetoencephalography and transcranial magnetic stimulation have contributed significantly to the understanding of some of the cortical mechanisms underlying myoclonus. Elucidation of physiological mechanisms underlying myoclonus in individual patients is important for selecting the most appropriate treatment.

Sensorimotor integration in patients with parkinsonian type multisystem atrophy

Sensorimotor integration is an essential feature of the central nervous system that contributes to the accurate performance of motor tasks. Some patients with multiple system atrophy with parkinsonian features (MSAp) exhibit clinical signs compatible with an abnormal central nervous system excitability to somatosensory inputs, such as action myoclonus or enhanced cutaneo-muscular reflexes. To investigate further the site where such dysfunction in sensorimotor integration takes place, we examined the inhibitory effects of a cutaneous afferent volley at two different levels of the motor system in 10 MSAp patients and in 10 age-matched healthy volunteers. Electrical digital nerve stimuli were given as the conditioning stimulus for the motor evoked potentials (MEP) elicited by transcranial magnetic stimulation in hand muscles, and for the blink reflex responses obtained in the orbicularis oculi muscles by supraorbital nerve stimulation. Intervals for the conditioning were 20 to 50 ms for the MEP and 90 to 110 ms for the blink reflex. The MEP was significantly inhibited in test trials in healthy volunteers, reaching a mean of 32% of the baseline values at the ISI of 35 ms. Significant inhibition occurred also in the blink reflex, in which the R2 response was a mean of 12% of baseline values at the ISI of 100 ms. The inhibitory effects were abnormally reduced in 8 patients on the MEP, and in 7 patients on the blink reflex. There were significant group differences between patients and control subjects in the size of the conditioned MEP and blink reflex. These results suggest that sensorimotor integration is abnormal in patients with MSAp in at least two central nervous system sites: the sensorimotor cortex, and the brainstem reticular formation.

Myoclonus and neurodegenerative disease--what's in a name?

Myoclonus is a clinical symptom (or sign) defined as sudden, brief, shock-like, involuntary movements caused by muscular contractions or inhibitions. It may be classified by examination findings, etiology, or physiological characteristics. The main physiological categories for myocolonus are cortical, cortical-subcortical, subcortical, segmental, and peripheral. Neurodegenerative syndromes are potential causes of symptomatic myoclonus. Such syndromes include multiple system atrophy, corticobasal degeneration, progressive supranuclear palsy, frontotemporal dementia and parkinsonism linked to chromosome 17, Huntington's disease, dentato-rubro-pallido-luysian atrophy, Alzheimer's disease, and Parkinson's disease, and other Lewy body disorders. Each neurodegenerative syndrome can have overlapping as well as distinctive clinical neurophysiological properties. However, claims of differentiating between neurodegenerative disorders by using the presence or absence of small amplitude distal action myclonus appear unwarranted. When the myoclonus is small and repetitive, it may not be possible to distinguish it from tremor by phenotypic appearance alone. In this case, clinical neurophysiological offers an opportunity to provide greater differentiation of the phenomenon. More study of the myoclonus in neurodegenerative disease will lead to a better understanding of the processes that cause phenotypic variability among these disorders.

Different neurophysiologic patterns of myoclonus characterize Lennox-Gastaut syndrome and myoclonic astatic epilepsy

PURPOSE

To study the neurophysiologic characteristics of epileptic myoclonus in patients with Lennox-Gastaut syndrome (LGS) and myoclonic astatic epilepsy (MAE).

METHODS

Three patients with symptomatic LGS (mean age, 15 years +/- 4) and three with cryptogenic MAE (mean age, 9 years +/- 3) were studied. Temporal relationships between electroencephalographic (EEG) and electromyographic activity were studied by analyzing latencies of EEG activity related to the onset of single myoclonic jerks, by using burst-locked EEG averaging where necessary.

RESULTS

LGS: neurophysiologic analysis indicated that jerks and the accompanying premyoclonic spikes showed latency differences between sides (mean +/- SD, 18 +/- 5 ms for both) with a constant leading side in each patient. The premyoclonic spike latency was 20 +/- 10 ms (mean +/- SD). Topographic voltage mapping of the premyoclonic spike peak showed a unilateral frontal distribution. MAE: muscles from both sides were activated synchronously, and the EEG correlate was a generalized spike-wave, in which the negative peak of the spike preceded the generalized jerks by 30 +/- 2 ms (mean +/- SD). Topographic voltage mapping of the premyoclonic spike peak showed a diffuse distribution of the electrical field, predominating over the anterior regions, but not lateralized.

CONCLUSIONS

These neurophysiologic findings indicate that epileptic myoclonus in LGS originates from a stable generator in the frontal cortex, to spread to contralateral and ipsilateral cortical areas, whereas myoclonus in MAE appears to be a primary generalized epileptic phenomenon.

Interhemispheric interaction between the hand motor areas in patients with cortical myoclonus

OBJECTIVE

To study interhemispheric interaction between the hand motor areas of both hemispheres through the corpus callosum in myoclonus epilepsy.

SUBJECTS

Five patients with benign myoclonus epilepsy and ten age matched normal volunteers.

METHODS

We studied effects of a medially directed conditioning stimulus over the right hand motor area on responses in the right first dorsal interosseous muscle to a posteriorly directed test stimulus over the left hand motor area.

RESULTS

In normal subjects, inhibition was evoked at interstimulus intervals (ISIs) of 8-20ms (late inhibition). In contrast, facilitation occurred in patients at ISIs of 4-6ms (early facilitation) with no late inhibition.

CONCLUSIONS

The lack of late inhibition in the patients is consistent with the idea that cortical inhibitory interneurones are affected in myoclonus epilepsy. We propose that this releases interhemispheric facilitation from powerful surround inhibition. The consequence is a predominant early facilitation between the hemispheres in patients with myoclonus epilepsy.

Interhemispheric facilitation of the hand motor area in humans

1. We investigated interhemispheric interactions between the human hand motor areas using transcranial cortical magnetic and electrical stimulation. 2. A magnetic test stimulus was applied over the motor cortex contralateral to the recorded muscle (test motor cortex), and an electrical or magnetic conditioning stimulus was applied over the ipsilateral hemisphere (conditioning motor cortex). We investigated the effects of the conditioning stimulus on responses to the test stimulus. 3. Two effects were elicited at different interstimulus intervals (ISIs): early facilitation (ISI = 4-5 ms) and late inhibition (ISI > or = 11 ms). 4. The early facilitation was evoked by a magnetic or anodal electrical conditioning stimulus over the motor point in the conditioning hemisphere, which suggests that the conditioning stimulus for early facilitation directly activates corticospinal neurones. 5. The ISIs for early facilitation taken together with the time required for activation of corticospinal neurones by I3-waves in the test hemisphere are compatible with the interhemispheric conduction time through the corpus callosum. Early facilitation was observed in responses to I3-waves, but not in responses to D-waves nor to I1-waves. Based on these results, we conclude that early facilitation is mediated through the corpus callosum. 6. If the magnetic conditioning stimulus induced posteriorly directed currents, or if an anodal electrical conditioning stimulus was applied over a point 2 cm anterior to the motor point, then we observed late inhibition with no early facilitation. 7. Late inhibition was evoked in responses to both I1- and I3-waves, but was not evoked in responses to D-waves. The stronger the conditioning stimulus was, the greater was the amount of inhibition. These results are compatible with surround inhibition at the motor cortex.

[Myoclonus in the adult: diagnostic approach]

Myoclonus, defined as shock-like involuntary movement, may be physiological or caused by a very wide variety of hereditary and acquired conditions. Because myoclonus can originate from different disorders and lesions affecting quite varied levels of the central and peripheral nervous systems, it represents from many points of view a diagnostic challenge. Moreover, new entities have been recently individualized, such as cortical tremor, which deserve renewed attention. The aim of this review is to propose a rationale for a diagnostic approach based on clinical and electrophysiological grounds. In this setting, we successively address 1) the clinical features allowing a positive diagnosis of myoclonus; 2) the clinical clues to the etiology; 3) the relevance of the clinical context to the diagnosis; and 4) the contribution of neurophysiology. Differentiating myoclonus from tics, spasm, chorea and dystonia can be difficult, and a careful reappraisal of clinical features allowing precise identification is presented. Moreover, the topographical distribution of myoclonus, the temporal pattern of muscle recruitment, the condition of occurrence and the rhythm of the event, may provide clinical clues relevant to the diagnosis. Myoclonus without associated epilepsy, myoclonus with epilepsy, myoclonus with encephalopathy, parkinsonism and/or dementia represent overlapping clinical categories, although they remain useful for the diagnostic approach. Using electrophysiology (including back-averaging EEG, MEG, SEP, C-reflex studies) to determine the origin of myoclonus may not allow us to focus on the underlying condition. Indeed, in many instances, the myoclonus is cortical in origin, but the pathology is found elsewhere.

Electrophysiological studies of myoclonus

As myoclonus is often associated with abnormally increased excitability of cortical structures, electrophysiological studies provide useful information for its diagnosis and classification and about its generator mechanisms. The EEG-EMG polygraph provides the most essential information about the myoclonus of interest. Jerk-locked back averaging and evoked potential studies combined with recording of the long latency, long loop reflexes are useful to further investigate the pathophysiology of myoclonus, especially that of cortical myoclonus. A recent advance in magnetoencephalographic techniques has contributed significantly to the elucidation of some of the cortical mechanisms underlying myoclonus. Elucidation of physiological mechanisms underlying myoclonus in each individual patient is important for selecting the most appropriate treatment of choice.

Postural and action myoclonus in patients with parkinsonian type multiple system atrophy

Patients with a parkinsonian syndrome and features of multisystem atrophy (pMSA) may exhibit abnormal movements of the hands and fingers, which are reported in the literature either as "jerky" tremor or myoclonus. We studied clinically and electrophysiologically these movements in 11 consecutive patients with pMSA. No abnormal movements were observed when the patients were at complete rest, except for a characteristic parkinsonian "pill-rolling" tremor in one patient. Abnormal small-amplitude, nonrhythmic movements involving just one or a few fingers, or more rarely the whole hand, were observed in nine patients when holding a posture or at the beginning of an action. Accelerometric recordings showed small-amplitude irregular oscillations which, contrary to those of patients with tremor, had no predominant peak in the Fast Fourier frequency spectrum analysis. Electromyographic recordings in the forearm and hand muscles showed brief jerks of less than 100 ms duration which were synchronous in antagonist muscles of the forearm and alternated with brief periods of silence. Electrical stimulation of the digital nerves evoked consistent reflex responses in the wrist flexor and extensor muscles at a latency of 55.3+/-4.1 ms (range, 50-63 ms). Routine electroencephalographic (EEG) and somatosensory evoked potentials to median nerve stimulation were normal. Back-averaging of the EEG activity time-locked to the jerks was performed in two patients with no evidence of abnormal cortical activity. Two patients had episodes of transient respiratory failure related to pneumonia. This caused a long-lasting enhancement of the abnormal hand and finger movements, which became larger and more widespread, with features of posthypoxic myoclonus. We conclude that the abnormal hand and finger movements of patients with pMSA are a form of postural and action myoclonus, and can be described as mini-polymyoclonus.

[Evoked motor potentials obtained with double magnetic cortical stimulation: techniques and interpretation]

The technique of motor evoked potentials (MEP) obtained with single and double magnetic stimulation of the motor cortex in man has considerably improved over the past decade. We present the techniques and parameters involved in double magnetic stimulation for clinical purposes.

METHOD

The conditioning-test design is used to study modifications in the amplitudes of the muscular responses to the "test" shock, recorded on the first dorsal interosseus muscle. Enhanced amplitudes of conditioned responses indicate facilitation, reduced response inhibition.

RESULTS

The effects vary according to the shock intensity, the delay between shocks and the position of the conditioning coil. The latter may be located at the same place as the test shock (to test interneural circuitry related to pyramidal tract), on the hand area opposite the test shock (to test interhemispheric influences), or over the cerebellar area contralateral to the test side (to test the effect of cerebellar stimulations over the motor cortex). When the coils were located on the same cortical hand area there was facilitation when the intensities were both set at the threshold with an interstimulus interval (ISI) between 1 and 2.5-3 ms. At conditioning shock intensities below the threshold and the test shock 150% above, inhibition occurred at ISI 1-5 ms followed by facilitation at ISI 15-35 ms. When the intensities of both shocks were 150% above threshold, there were two clear cut individual responses at ISI above 10 ms; facilitation was recorded at ISI 15-35 ms, and inhibition between 55 and 255 ms. When the conditioning coil was located on the opposite hand area from the test shock (conditioning shock intensity supramaximal, test shock intensity above the threshold), ISI 1-5 ms facilitation occurred followed by inhibition up to ISI 30 ms. When the conditioning shock (intensity supramaximal) was located on the cerebellar area contralateral to the test side (intensity above the threshold), inhibition occurred at ISI 5 ms.

CONCLUSIONS

Double magnetic stimulations delivered over the same cortical area reflect facilitatory and inhibitory influences over the pyramidal tract controlled by interneurons, i.e., these tests investigate the intrinsic circuitry of the motor strip. Double magnetic stimulations delivered on each motor area study interhemispheric influences mediated by the corpus callosum, which are facilitatory and inhibitory. Double magnetic stimulations delivered on the cerebellar area demonstrates inhibitory influences over the contralateral cerebral motor cortex.

Cerebral function revealed by transcranial magnetic stimulation

Although transcranial magnetic stimulation (TMS) has been introduced only recently, it is safe and provides a painless, inexpensive noninvasive method for the evaluation of brain function. Determining central motor conduction time (CMCT) permits assessment of the corticospinal pathways. Mapping the central representation of muscles provides a method for investigating the cortical reorganization that follows training, amputation and injury to the central nervous system. Such studies of human plasticity may have important implications for neurorehabilitation. TMS also provides a method whereby cortical excitability can be noninvasively evaluated, which is likely to have important implications in the study of epilepsy, movement disorders and related conditions. TMS is useful in tracking the flow of information from one brain region to another and in investigations of cognition and functional localization, thereby complementing information obtained using functional imaging techniques, which have superior spatial but inferior temporal resolution. Finally, TMS is currently being investigated as a method for establishing cerebral dominance and as a therapeutic tool in the treatment of depression. Investigations for treatment of other neurologic and psychiatric conditions are likely to be undertaken.

Crossed reduction of human motor cortex excitability by 1-Hz transcranial magnetic stimulation

Electrophysiological studies have shown that 1-Hz repetitive transcranial magnetic stimulation (rTMS) of the primary motor area (M1) can produce a local decrease in excitability. Functional imaging data suggest that this change may be bilateral. In normal subjects, we measured motor evoked potential (MEP) amplitude at a series of stimulation intensities in the contralateral M1 before and after 15 min of active or sham rTMS at just above the MEP threshold. The slope of the curve relating MEP amplitude and stimulation intensity was decreased in the unstimulated hemisphere by active but not sham rTMS. This demonstrates that rTMS can condition cortical excitability at a distance of one or more synapses and suggest that decreased excitability to TMS is a correlate of decreased blood flow and metabolism.

Pathogenesis of cortical myoclonus studied by magnetoencephalography

Myoclonus-associated cortical activities were studied by simultaneous recording of a magnetoencephalogram and an electroencephalogram in 6 patients with cortical myoclonus due to various causes. Cortical activities were averaged, with respect to the precise onset of the myoclonic jerk, to evaluate the myoclonus-associated cortical magnetic fields. The estimated generator of their earliest peak was localized at the contralateral precentral gyrus in all patients. As judged from the direction of the electrical current, surface positive activity preceding the electromyographic discharge was detected in 3 patients with cortical reflex myoclonus and in 1 patient with possible corticobasal degeneration. In contrast, in the remaining 2 patients (Lennox-Gastaut syndrome and Alzheimer's disease), magnetic fields time-locked to the myoclonic jerk were associated with surface negative activity at the precentral cortex. The present study, applying for the first time an off-line jerk-locked back-averaging analysis to magnetoencephalography, demonstrated the important role of the precentral cortex in generating spontaneous myoclonus. It is most likely that the differing polarity of the electromagnetic activity reflects the differing activation patterns within the cortical laminar structure in the precentral area, underlying the generation of various types of myoclonus.

Cortical reflex myoclonus in Rett syndrome

Rett syndrome (RS) is one of the most frequent causes of mental retardation in females. As there are no known biochemical, genetic, or morphological markers, diagnosis is based on clinical phenotype including severe dementia, autism, truncal ataxia/apraxia, loss of purposeful hand movements, breathing abnormalities, stereotypies, seizures, and extrapyramidal signs. Myoclonus, although reported in some series, has never been characterized. We studied 10 RS patients, age 3 to 20 years, and observed myoclonus in 9. Severity of myoclonus did not correlate with that of the other symptoms or with age. Multifocal, arrhythmic, and asynchronous jerks mainly involved distal limbs. Electromyographic bursts lasted 48 +/- 12 msec. Burst-locked electroencephalographic averaging generated a contralateral centroparietal premyoclonus transient preceding the burst by 34 +/- 7.2 msec. Motor evoked potentials showed normal latencies, indicating integrity of the corticospinal pathway. Somatosensory evoked potentials were enlarged. The C-reflex was hyperexcitable and markedly prolonged (62 +/- 4.3 msec), mainly due to increase in cortical relay time (28.4 +/- 4.5 msec). We conclude that RS patients show a distinctive pattern of cortical reflex myoclonus with prolonged intracortical delay of the long-loop reflex.

Stimulus-evoked electrographic patterns in neonates: an abnormal form of reactivity

Stimulus-evoked electrographic patterns are described for 12 neonates, coincident with tactile or painful stimulation. Four newborns had stimulus-evoked electrographic seizures with and without concomitant clinical seizure behaviors. Eight neonates had generalized or focal stimulus-evoked discharges, with or without coincident movements, which did not evolve into electrographic seizures. Nine infants were less than 36 weeks estimated gestational age at the time stimulus-evoked discharges were noted. Eleven neonates were comatose or phamacologically paralyzed at the time the stimulus-evoked patterns were initially noted. All neonates had moderate or severe interictal EEG background abnormalities at the time these patterns were observed. All patients had brain lesions documented by either cranial imaging or neuropathological examinations. Eleven patients died or have significant neurological handicaps. These phenomena represent an abnormal form of cortical reactivity to sensory stimuli in the developing brain. Most neonates with these patterns have significant diffuse or multifocal damage to the neocortex.

Ipsilateral median somatosensory evoked potentials recorded from human somatosensory cortex

Somatosensory evoked potentials (SEP) to ipsilateral and contralateral median nerve stimulations were recorded from subdural electrode grids over the perirolandic areas in 41 patients with medically refractory focal epilepsies who underwent evaluation for epilepsy surgery. All patients showed clearly defined, high-amplitude contralateral median SEPs. In addition, four patients showed ipsilateral SEPs. Compared with the contralateral SEPs, ipsilateral SEPs were very localized, had a different spatial distribution, were of considerably lower amplitude, had a longer latency (1.2-17.8 ms), did not show an initial negativity, and were markedly attenuated during sleep. Stimulation of the subdural electrodes overlying the sensory hand area was associated with contralateral hand paresthesias, but no ipsilateral hand paresthesias, occurred. It was concluded that subdurally recorded cortical SEPs to ipsilateral stimulation of the median nerve (M) reflect unconscious sensory input from the hand possibly serving fast bimanual hand control. The anatomical pathway of these ipsilateral short-latency MSEPs is not yet known. Transcallosal transmission seems unlikely because of the short delay between the ipsilateral and contralateral responses in selected cases. The infrequent occurrence of ipsilateral subdurally recorded SEPs and their low amplitude and limited distribution suggest that they contribute very little to the short-latency ipsilateral median SEPs recorded on the scalp.

Cognitive potentials: ipsilateral corticocortical interconnections in prefrontal human cortex ablations

The research deals with the possible role of the essentially monosynaptic bidirectional corticocortical connections between occipito-temporo-parietal association cortical areas and frontal areas in the genesis of some contingent negative variation (CNV) components, especially on the supramodal dorsolateral prefrontal regions. With standard and topographic mapping methods of analysis, the multicomponent CNV complex formation was examined in 7 patients with extensive frontal cortex ablations exactly identified through CT/MRI examinations, and in 10 normal subjects. On the scalp over the ablated frontocortical areas, no consistent post-warning auditory N100 a-b-c, P200, P300, early and late CNV components were recordable. The hypothesis is proposed that the bidirectional ipsilateral long-distance pathways which interconnect uni-polymodal occipito-temporo-parietal cortical areas to prefrontal ones, in particular the arcuate-superior longitudinal and superior/inferior occipito-frontal fasciculi, play an important role in the genesis of several CNV complex components, especially the multicomponent post-S1 auditory N100. The posteroanterior sequential latency differences of these neurocognitive components, roughly measured along the scalp or on MRI imagings, is probably accounted for by the transcortical ipsilateral conduction time of about 1 cm/ms (10 m/s).

Levodopa-induced dyskinesias in Parkinson's disease phenomenology and pathophysiology

The aim of this study was to provide further insight into the phenomenology and pathophysiology of monophasic and biphasic dyskinesias induced by levodopa in Parkinson's disease. For this purpose, the type, localization, severity, and timing of dyskinesias were evaluated in 15 parkinsonian patients in relation to motor disability after administration of levodopa using a video-electromyographic recording device. Foot-dystonia, myoclonus, and akathisia were observed in most patients. The dyskinesias started in the foot, usually on the side most affected by the disease, and spread in an "ascending wave" to the contralateral side, the trunk, and upper extremities. In a few patients, onset was axial, spreading almost instantaneously to all limbs. The dyskinesias were dystonic and ballistic at the start, and became increasingly choreic as they attained the upper limbs. Their intensity was maximal in the lower limbs, then progressively decreased, while increasing in upper limbs and head. The results indicate that there is no strict dichotomy between biphasic and monophasic dyskinesias. In other words, there is a "continuum" between the first dyskinesias and those observed during the period of maximal clinical improvement. These dyskinesias can also appear in reverse order, as if there were an "oscillator" determining a sequence of alternating patterns.

Interhemispheric inhibition of the human motor cortex

1. Using two magnetic stimulators, we investigated the effect of a conditioning magnetic stimulus over the motor cortex of one hemisphere on the size of EMG responses evoked in the first dorsal interosseous (FDI) muscle by a magnetic test stimulus given over the opposite hemisphere. 2. A single conditioning shock to one hemisphere produced inhibition of the test response evoked from the opposite hemisphere when the conditioning-test interval was 5-6 ms or longer. We shall refer to this as interhemispheric inhibition. However, the minimum latency of inhibition observed using surface EMG responses may have underestimated the true interhemispheric conduction time. Single motor unit studies suggested values 4-7 ms longer than the minimum interval observed with surface EMG. 3. Interhemispheric inhibition was seen when the test muscle was active or relaxed. Increasing the intensity of the conditioning stimulus increased the duration of inhibition: increasing the intensity of the test stimulus reduced the depth of inhibition. 4. The conditioning coil had to be placed on the appropriate area of scalp for inhibition to occur. The effect of the conditioning stimulus was maximal when it was applied over the hand area of motor cortex, and decreased when the stimulus was moved medial or lateral to that point. 5. The inhibitory effect on the test stimulus probably occurred at the level of the cerebral cortex. In contrast to the inhibition of test responses evoked by magnetic test stimuli, test responses evoked in active FDI by a small anodal electric shock were not significantly inhibited by a contralateral magnetic conditioning stimulus. Similarly, H reflexes in relaxed forearm flexor muscles were unaffected by conditioning stimuli to the ipsilateral hemisphere. However, inhibition was observed if the experiment was repeated with the muscles active.

Scalp topography of giant SEP and pre-myoclonus spike in cortical reflex myoclonus

Scalp topography of the giant SEP and the pre-myoclonus spike demonstrated by jerk-locked back averaging was studied by using a computer-assisted evoked potential mapping technique in 5 patients with cortical reflex myoclonus. The initial positive peak of giant SEP was localized to the postcentral region contralateral to the stimulus and was associated with a negative potential field of the same latency at the frontal region in all cases. The main positive peak of pre-myoclonus spike was localized to the postcentral region contralateral to the myoclonus in 4 cases, and maximal at the midline postcentral region extending contralaterally with respect to the myoclonus in 1 case. The postcentral positive peak was associated with a frontal negativity in 2 of the 5 cases. In those 2 cases, the main components of giant SEP and the pre-myoclonus spike showed a similar scalp distribution with respect to the hand which was stimulated or myoclonic jerks were recorded from, although the latter was much smaller and less sharp than the former. These findings support our previous hypothesis that those 2 activities might be generated, at least in part, by common physiological mechanisms. In 3 other cases, however, the postcentral positive peak of the pre-myoclonus spike was not associated with a frontal negativity.

Comparison of human transcallosal responses evoked by magnetic coil and electrical stimulation

Human transcallosal responses (TCRs) were elicited by focal magnetic coil (MC) stimulation of homologous sites in contralateral frontal cortex and compared with those to focal anodic stimulation. With MC stimulation, the TCR consisted of an initially positive wave with an onset latency of 8.8-12.2 msec, a duration of 7-15 msec, and an amplitude which reached up to 20 microV, sometimes followed by a broad low amplitude negative wave. With anodic stimulation, a similar response was obtained in which the positive wave was similar in latency and maximum amplitude, but had a greater duration. With anodic stimulation, not only was the TCR threshold below that for contralateral movement, but it reached substantial size at intensities below motor threshold. With MC stimulation, contralateral arm movement and scalp corticomotor potentials were observed when the MC was displaced posteriorly towards the central sulcus. Unlike with anodic stimulation, the MC evoked TCR was usually not preceded by a prominent EMG potential from temporalis muscle and was not associated with subject discomfort. The TCR provides unique information concerning the functional integrity of callosal projection neurons, their axons and transsynaptic processes in recipient cortex. This information may prove useful in the evaluation of intrinsic cerebral mechanisms and in establishing cortical viability.

AAEE minimonograph #30: electrophysiologic studies of myoclonus

Definition as well as classification of myoclonus and electrophysiologic methods for investigating myoclonus were reviewed. Among the electrophysiologic techniques currently available in most laboratories, the EEG-EMG polygraph is the most essential one and can provide us with the most important information. Jerk-locked averaging and evoked potential studies are useful for further investigating the pathophysiology of myoclonus and can be performed by using the same recording electrodes as those used for the polygraph. Jerk-locked evoked potentials and double-stimulation evoked potentials can be employed only for further investigating how cerebral cortex is involved in the generation of certain myoclonia. All these techniques can be used in proper combinations depending on the clinical features of the myoclonus in question, the purpose of the study, and the facilities available in each laboratory. These techniques also will be useful for following the clinical course during the treatment with antimyoclonus agents.

Midline electrographic abnormalities and cerebral lesions in the newborn brain

Electroencephalographic (EEG) abnormalities arising from the midline region were identified in 154 of 1008 (15.2%) consecutive neonatal EEGs during a 24-month period. These records were obtained on 97 neonates with a variety of clinical diagnoses. Premature infants made up 79% (77/97) of this group. All patients received at least one cranial ultrasound at 7 to 10 days of life. Sixty-two percent (60/97) of the patients had radiographic and/or neuropathological documentation of cerebral lesions: intraventricular hemorrhage (25), periventricular leukomalacia (18), cerebral infarction (10), cerebral malformation (4), and miscellaneous lesions (3). Six types of midline EEG abnormalities are described: negative sharp waves, positive sharp waves, electrographic discharges associated with myoclonus, electrographic seizures, attenuation of background, and rhythmic monofrequencies. Approximately 90% of the patients with background attenuation, discharges with myoclonus, and positive sharp waves and 72% of patients with EEG seizures had cerebral lesions. Midline positive sharp waves were associated with periventricular leukomalacia as well as intraventricular hemorrhage. No midline positive sharp waves, attenuation, EEG seizures or discharges with myoclonus were found in 25 healthy, asymptomatic neonates. Besides positive sharp waves, other specific midline EEG abnormalities can be associated with cerebral lesions in the neonate. The rapid identification of midline EEG abnormalities in neonatal recordings can enhance the accuracy of both electrographic diagnosis and anatomic localization of associated cerebral lesions.

Myoclonus in Alzheimer's disease

Myoclonus was studied electrophysiologically in seven patients with clinically diagnosed Alzheimer's disease. There seem to be at least two physiological types of myoclonus in Alzheimer's disease. Cerebral cortical structures might participate in the generation of myoclonus in one type, while the other type is probably generated by subcortical structures.

Pathologic myoclonus of the newborn: electrographic and clinical correlations

Previous reports have described neonatal myoclonus as a benign movement in healthy premature newborns or as a clinical seizure in neonates with severe encephalopathies. The present report describes electroencephalographic-myoclonic correlations which occur independently from sustained electrical discharges in ten neonates. Cortical, reticular, and segmental types of neonatal myoclonus, similar to adult forms, are described.

Myoclonus: relation to epilepsy

Myoclonus can be divided into those types which are fragments of epilepsy and those which are nonepileptic. Epileptic myoclonus is viewed as the effect of an isolated spike in neurons of the motor system. Cortical reflex myoclonus is a fragment of partial epilepsy and represents hyperactivity of a focal area of cerebral cortex. Reticular reflex myoclonus is a fragment of generalized epilepsy with hyperactivity of medullary brainstem reticular formation. Primary generalized epileptic myoclonus is a fragment of primary generalized epilepsy and may represent a generalized hyperactive response of cortex to subcortical input.

Primary generalised epileptic myoclonus: a frequent manifestation of minipolymyoclonus of central origin

A group of patients with myoclonus is described whose jerks are preceded by a bilaterally synchronous, frontocentrally predominant, negative cerebral potential in the EEG. This potential may be a slow wave with variable timing in relation to EMG bursts, and in this circumstance the muscle jerks are usually small amplitude and multifocal ("minipolymyoclonus"). The cerebral negativity can also be shorter in duration and time-locked to limb jerks, which are larger in amplitude and more widespread. We propose that the physiology of this distinct form of myoclonus is similar to that of primary generalised epilepsy.