Early ReportTremor-correlated cortical activity in essential tremor
Introduction
Essential tremor is a common movement disorder characterised by a postural tremor of the arms that can be accompanied by tremor in other body parts such as the head, tongue, larynx, trunk, or legs. Other neurological signs are absent.1 Essential tremor is generally assumed to be generated by a central oscillatory neuronal network.2 However, the structural components involved in this network are largely unknown. Two main assumptions about the origin of essential tremor have been put forward: the involvement of the olivocerebellar-spinal loop and the predominant role of thalamic nuclei.
The suggestion that the olivocerebellar system is involved is based on the results of animal experiments with harmaline-induced tremor, which is similar to essential tremor in many respects.3, 4 After injection of harmaline, animals develop oscillatory activity in the inferior olive, which is accompanied by a tremor of the same frequency. In human beings, the olivocerebellar hypothesis is supported by studies that used positron emission tomography or functional magnetic resonance imaging. Patients with essential tremor are characterised by having increased glucose metabolism in the medulla5 or increased blood flow in the cerebellum.6, 7
Thalamic involvement in the oscillatory network that generates essential tremor is suggested by neurophysiological and clinical evidence. Neuronal activity in the thalamus is strongly correlated with forearm electromyography (EMG) signals recorded in essential tremor.8 Moreover, stereotactic lesions or high-frequency stimulation in the nucleus ventralis intermedius of the ventrolateral thalamus have been shown to suppress essential tremor.9, 10
Given the presence of strong thalamocortical projections in human beings, the cerebral cortex might also contribute to tremor generation in essential tremor. In parkinsonian resting tremor, such cortical involvement has indeed been shown. Coherence analysis has revealed tremor-correlated activity in the contralateral sensorimotor cortex by magnetoencephalography (MEG) and electroencephalography (EEG).11, 12 However, MEG studies in patients with essential tremor have failed to detect cortical involvement in tremor generation.13 This finding was regarded as evidence that central oscillatory activity related to essential tremor is imposed on the muscles by descending pathways other than the corticospinal tract. Moreover, it was suggested that cortical involvement in the tremor-generating network might be useful in the differential diagnosis between parkinsonian and essential tremor.14
We aimed to find out whether EEG recordings assessed by coherence analysis could identify tremor-correlated cortical activity in patients with essential tremor and patients with an enhanced physiological tremor. This enhanced tremor is a physiological tremor with a prominent 8–12 Hz component which has been regarded as a forme fruste of essential tremor.15
Section snippets
Patients
Seven patients (four women, three men) with essential tremor, and three patients (all men) with enhanced physiological tremor participated in the study. All patients were selected because they showed a postural tremor of the arms without significant head tremor. Patients with essential tremor were on average 60·3 years of age (range 45–73 years). Tremor had been present for at least 5 years (mean 18·7 years; range 5–45 years). Apart from the postural tremor in the arms, there was no evidence of
Results
Figure 2 illustrates results from a patient with essential tremor who activated postural tremor on the left side. Two EEG traces (electrodes C3 and C4), one EMG trace (left wrist extensor), the autospectra of the EEG and EMG traces, and the corresponding coherences are displayed. Inspection of EEG and EMG traces did not reveal any correlation between tremor and cortical activity. Although the autospectrum of the EMG trace showed peaks at the tremor frequency (about 5 Hz) and its higher
Discussion
Clinical and experimental studies suggest that the central oscillator that generates essential tremor involves the inferior olive, the cerebellum, and the thalamus. However, the complex pathophysiological mechanisms leading to the clinical expression of essential tremor are still far from being understood. For instance, whether the cerebral cortex contributes to tremor generation via the corticospinal tract is questionable.
In parkinsonian resting tremor, there is clear evidence for the
References (29)
- et al.
Glucose metabolism in the brains of patients with essential tremor
J Neurol Sci
(1993) - et al.
Long-term suppression of tremor by chronic stimulation of the ventral intermediate thalamic nucleus
Lancet
(1991) - et al.
Tremor-correlated cortical activity detected by electroencephalography
Clin Neurophysiol
(2000) - et al.
Coherence between low-frequency activation of the motor cortex and tremor in patients with essential tremor
Lancet
(2000) - et al.
Electroencephalographic analysis of corticomuscular coherence: reference effect, volume conduction and generator mechanism
Clin Neurophysiol
(1999) - et al.
Using electroencephalography to study functional coupling between cortical activity and electromyograms during voluntary contractions in humans
Neurosci Lett
(1998) - et al.
Electroencephalographic measurement of motor cortex control of muscle activity in humans
Clin Neurophysiol
(2000) - et al.
Quantitative analysis of tremor time series
Electroenceph Clin Neurophysiol
(1996) - et al.
Central mechanisms in human enhanced physiological tremor
Neurosci Lett
(1998) Origins of tremor
Lancet
(2000)
Essential tremor: a review
Neurology
Central mechanisms of tremor
J Clin Neurophysiol
The olivo-cerebellar system: functional properties as revealed by harmaline-induced tremor
Exp Brain Res
Animal models of physiological, essential and parkinsonian-like tremors
Cited by (167)
Using chemosensory-induced EEG signals to identify patients with de novo Parkinson's disease
2024, Biomedical Signal Processing and ControlCortical mean diffusivity is reliable in measuring brain abnormalities in drug-naïve essential tremor patients
2023, Clinical Neurology and NeurosurgeryEssential tremor: Clinical perspectives and pathophysiology
2022, Journal of the Neurological SciencesIs essential tremor a degenerative or an electrical disorder? Electrical disorder
2022, International Review of NeurobiologyEEG as a marker of brain plasticity in clinical applications
2022, Handbook of Clinical Neurology