[Neurophysiologic study of tremor]

Myoclonus and extrapyramidal diseases

Parkinsonism or dystonia are associated with myoclonus in several extrapyramidal diseases. Although the latter symptom is not always prominent, stimulus-sensitive, distal, or focal reflex myoclonus is frequently observed. This review will consider the clinical and electrophysiological features of myoclonus in Parkinson's disease, multiple system atrophy, corticobasal degeneration, progressive supranuclear palsy, Huntington's disease, dentatorubral-pallidoluysian atrophy, Lewy body dementia, and myoclonus with dystonia. The evidence of a long-latency reflex response, the presence of giant somatosensory evoked potentials, and the demonstration of a back-averaged premyoclonus focal cortical EEG activity often lead to classify myoclonus as a cortical phenomenon. However, a subcortical origin cannot always be ruled out.

Neurophysiology of myoclonus

Myoclonus may be generated by any area in the central nervous system. Finding its generator is helpful in the diagnostic process. Although clinical features have to be carefully analyzed as they may give a first idea, neurophysiologic study of myoclonus provides the most important clues for the determination of the generator. Surface electromyography (EMG) allows analyzing the recruitment order in generalized myoclonus, thereby suggesting either a cortical, brainstem, or spinal origin. It also reveals whether myoclonus is positive (jerks that are caused by muscle activation) or negative (jerks that are caused by brief muscle inhibition). In non-generalized myoclonus the EMG burst duration gives an idea of the level of the generator. Repetitive peripheral nerve stimulation is required to record somatosensory evoked potentials (SEPs) as well as long latency reflexes (LLR), especially the C reflex. The presence of giant cortical SEPs is an indirect argument for cortical myoclonus. Similarly the existence of LLR at rest orientates towards cortical reflex (sensitive to sensory stimuli) myoclonus. Finally EEG-EMG polygraphy is the only test which is able to prove directly the cortical origin of myoclonus. This is the case when focal cortical events precede myoclonus with a fixed delay. These premyoclonic cortical potentials may either be seen directly on raw recordings or require the use of jerk-locked back averaging (JLBA). This technique allows the averaging of the EEG prior to myoclonus onset (as determined by EMG) in order to reveal a premyoclonic spike that otherwise would remain undetected in the global EEG.

Identification of psychogenic, dystonic, and other organic tremors by a coherence entrainment test

The differentiation of psychogenic from organic tremors, particularly those of a dystonic nature, can be difficult on clinical grounds. Entrainment of tremulous movements of different body parts into a single rhythm has been used clinically as a means of distinguishing these tremor forms, based on the inability of a patient with hysterical tremor to generate voluntary tapping oscillations independent of their ongoing tremor oscillation. The coherence entrainment test is a quantified electrophysiological entrainment test performed on accelerometer or surface EMG tremor signals. This test was carried out on 25 patients referred with suspected psychogenic tremor or dystonic tremor and on 10 normal subjects attempting to tap two independent voluntary oscillations. Using established and new clinical diagnostic criteria, patients were assigned the following final clinical diagnoses: 6 cases of clinically definite dystonic tremor, 5 cases of probable dystonic tremor, 2 cases of classic essential tremor, 5 cases of clinically definite psychogenic tremor, 3 cases of probable psychogenic tremor and 4 uncertain cases. On comparing these clinical diagnoses with those reached by a coherence entrainment test subsequently carried out on each patient, there was 100% concordance in both clinically definite and clinically probable patients. In uncertain cases, when later clinical information came to light, this also corroborated with the coherence entrainment diagnosis. No normal subjects were able to "mimic" organic tremor. The coherence entrainment test appears to be a sensitive and specific means of distinguishing psychogenic tremor from dystonic and other organic tremors.

Intérêt et limites de l’examen polymyographique dans l’étude des dystonies

INTRODUCTION

Diagnosis of dystonia is based on well-known clinical findings, but electrophysiological explorations can help to establish the diagnosis.

METHODS

We report 179 patients presenting primary or secondary dystonia. Polymyographic recordings were available for all. We studied activation patterns of agonist and antagonists muscles (prolonged bursts, co-contraction), presence of antagonistic gestures and other associated movement disorders such as tremor or myoclonus.

RESULTS

83.9 percent of patients presented abnormalities which evoked dystonia (prolonged bursts: 75.3 percent, co-contraction: 48.7 percent, successful antagonistic gesture successful with reduction or suppression of EMG activity, 8.7 percent). For the patients who presented no EMG abnormality, diagnosis was maintained in 22 cases because of the limitations of the exploration: intermittent symptoms, dystonia involving deep muscles which were not recorded by the surface electromyogram, or difficulties to reproduce the conditions of apparition of the dystonic symptoms during examination or to recognize a pathologic EMG pattern in hand cramps for instance. Another diagnosis was retained because of the polymyographic findings in 16.2 p.cent of these patients, particularly those who had tremor associated with dystonia (essentially diagnosis of primary writing tremor).

CONCLUSION

Polymyography is an effective tool for confirming clinical findings.

[Primary orthostatic tremor]

INTRODUCTION

Tremor is frequent in neurologic practice but primary orthostatic tremor was first described in 1984. Its prevalence accounts for around 4% for tremors explored in neurophysiology; in contrast, essential and parkinsonian tremors represent respectively 28 and 22% of these cases.

EXEGESIS

Orthostatic tremor is characterized by its appearance while standing. Walking, sitting, and lying down are unaffected. Clinical examination is normal except for unsteadiness disappearing when walking. Surface electromyography in the standing position is necessary for the diagnosis and shows a regular rapid tremor (frequency around 14 to 18 Hz). Its pathophysiology is unknown.

CONCLUSION

Clonazepam is the first-line treatment for orthostatic tremor. In cases of resistance or side effects of this drug orthostatic tremor may be improved by primidone or, as in our case, gabapentin.

[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.