Physiologic and essential tremor

We studied physiologic and essential hand tremor using inertial loading; hand acceleration and forearm EMG data were analyzed by auto- and cross-spectral analysis. Early essential tremor was qualitatively similar to the 8- to 12-Hz component of physiologic tremor, suggesting that this tremor component is a forme fruste of essential tremor. Advanced essential tremor had a frequency of 4 to 8 Hz. Patients with tremor frequencies in both ranges were observed in each of 10 families. In antagonistic forearm muscles, both synchronous and alternating tremor bursts were observed in 11 of 44 patients. Essential tremor should not be classified solely on the basis of frequency or EMG pattern.     

Characteristics of physiologic tremor in young and elderly adults.

OBJECTIVE: To determine the prevalence of tremor-related motor-unit entrainment in young and elderly adults. METHODS: Postural hand tremor in neurologically healthy people, age ranges 20-42 (59 women and 41 men) and 70-92 (50 women and 50 men) years, was studied with accelerometry and forearm electromyogram (EMG). Tremor was recorded with and without a 300 g load distributed over the distal half of the horizontally extended hand and was analyzed with Fourier spectral techniques. RESULTS: No tremor-related spectral peak was found in the EMG of 59 young and 65 elderly controls, and inconsistent EMG peaks were observed in 29 young and 21 elderly. Twelve young and 14 elderly people exhibited a well-defined tremor-coherent EMG peak with and without 300 g loading, and the frequency of the EMG peak decreased less than 1 Hz in 8 young and 7 elderly. The EMG peak frequency was 9-12 Hz during mass loading in all 8 young adults but in only two elderly adults. The other 5 elderly people had peak frequencies at 5-7 Hz. Age had no significant effect on the frequency and amplitude of the mechanical-resonant component of hand tremor. CONCLUSIONS: Approximately 8% of young and elderly adult controls have an EMG-acceleration pattern that is indistinguishable from mild essential tremor. SIGNIFICANCE: These results provide a framework for the interpretation of electrophysiologic studies in patients with suspected essential tremor.     

Phase resetting and frequency entrainment of essential tremor.

Step and sinusoidal forcings were delivered to the wrists of 10 patients with advanced essential tremor using a computer-controlled torque motor. Consistent (type 0) phase resetting was produced in only 5 patients, whereas all 10 patients exhibited harmonic and 2:1 subharmonic entrainment of essential tremor. These results indicate that the oscillator of essential tremor is coupled to the stretch-reflex arc. Irregularity in the tremor rhythm and postperturbation mechanical-reflex oscillations obscured the phase of essential tremor, thereby impeding the computation of phase resetting curves. Sinusoidal forcings circumvented these problems, but frequency entrainment was produced only by forcings at frequencies within +/- 1 Hz of the tremor frequency and its second harmonic. Resonance accompanied frequency entrainment and probably plays a role in the clinical expression of tremor.     

Coherence analysis differentiates between cortical myoclonic tremor and essential tremor.

Familial cortical myoclonic tremor with epilepsy (FCMTE) is characterized by a distal kinetic tremor, infrequent epileptic attacks, and autosomal dominant inheritance. The tremor is thought to originate from the motor cortex. In our patient group, a premovement cortical spike could not be established on electroencephalogram (EEG) back-averaging. Corticomuscular and intermuscular coherence analysis can demonstrate a cortical common drive to muscles. We carried out coherence analysis of electromyography (EMG) of forearm muscles and EEG of contralateral motor cortex in 7 FCMTE patients, 8 essential tremor (ET) patients, and 7 healthy controls. Results showed strong cortico- and intermuscular coherence in the 8- to 30-Hz range in the FCMTE patients, with EEG preceding EMG. Healthy controls and ET patients showed normal weak coherence around 20 Hz. The ET patients showed some additional coherence at tremor frequency (6 Hz), probably the result of sensory information flowing back to the sensorimotor cortex. These findings point to a pathological cortical drive in FCMTE patients leading to tremulous movements. Coherence analysis is an easy and useful method to differentiate FCMTE from ET. Coherence analysis is helpful when investigating a cortical common drive in cortical tremor and other movement disorders.     

Corticomuscular coherence is increased in the small postural tremor of Parkinson's disease.

The mechanisms and electrophysiological characteristics of the postural tremor in Parkinson's disease (PD) have not been defined. We hypothesized that PD subjects with small amplitude postural tremor would show increased corticomuscular coherence at certain frequencies compared to PD subjects without visible tremor. Four groups of participants were studied: (1) Control without postural tremor, (2) Control with small amplitude postural tremor, (3) PD without postural tremor, and (4) PD with small amplitude postural tremor. Accelerometry and electroencephalography-electromyography fast-fourier transform and corticomuscular coherence spectra were generated. Findings showed (1) elevated corticomuscular coherence centered at 12-18 Hz in PD with small amplitude postural tremor; (2) 5-12 Hz accelerometer frequency peaks that did not shift with increasing weight loads in some individuals; and (3) 5-8 Hz accelerometer peaks that shifted frequency with increasing weight loads, consistent with a peripheral-mechanical oscillator in all groups. The small amplitude postural tremor in PD arises from heterogeneous oscillator mechanisms. The discovery of increased corticomuscular coupling shows cortical involvement in the small amplitude postural tremor of PD.     

Effects of thalamic stimulation frequency on intention and postural tremor

Deep brain stimulation (DBS) of the ventral intermediate nucleus (VIM) of the thalamus improves essential tremor. Suppression of the amplitude of the postural tremor component with VIM DBS depends on stimulation frequency. The purpose of this study was to determine the effect of DBS frequency on the intention tremor component, that is, tremor that is enhanced by target-directed movement, and to compare it to the effect of DBS frequency on postural tremor in people with essential tremor. We measured tremor frequency and amplitude during trials of postural holding and voluntary reaching between two targets at 10 different stimulation frequency settings between 0 and 185 Hz. Tremor frequency did not change with changes in stimulation frequency. Amplitude suppression of both intention and postural tremor depended on stimulation frequency. Maximal tremor reduction occurred at approximately 130 Hz for both forms of tremor. However, at optimal frequencies, the percent reduction in tremor amplitude relative to the DBS OFF condition was greater for postural than for intention tremor. These results suggest that VIM DBS stimulation frequencies near 130 Hz may provide maximal control of intention and postural tremor. Identification of optimal stimulation settings should consider assessment of intention tremor, not just postural tremor, as intention tremor may not be as well controlled as postural tremor but may be a better gauge for functional benefit.     

Essential tremor frequency decreases with time

OBJECTIVE: To quantify the extent to which tremor frequency changes with time in patients with essential tremor. BACKGROUND: Tremor frequency tends to be lower in older patients. The author's previous study of 18 patients with essential tremor produced evidence that tremor frequency decreases slowly over a period of 4 to 8 years. A decrement in frequency will increase tremor amplitude because there is less attenuation of lower-frequency tremor by the low-pass filtering properties of muscle and limb mechanics. METHODS: Nineteen women and 25 men with essential tremor and no other neurologic conditions were followed for 4 years. Accelerometry and surface electromyography (EMG) were used to measure hand tremor and motor unit entrainment in the extensor carpi radialis brevis every 2 years. Tremor frequency was computed from the spectral peak in the rectified filtered EMG spectrum under the condition of 300-gram loading. RESULTS: The patients' mean +/- SD age was 68.0+/-9.95 years. The mean tremor frequency at baseline was 5.79+/-1.32 Hz. The mean decrement in tremor frequency over 4 years was 0.332 Hz (95% CI = 0.141 to 0.523) and was 0.270 Hz (95% CI = 0.122 to 0.418) when a 61-year-old outlier patient was excluded. Tremor frequency and patient age were linearly related: frequency = -0.061(age) + 9. 94 (r = 0.459; p<0.002). CONCLUSIONS: The frequency of essential tremor decreases by approximately 0.06 to 0.08 Hz/year. This decrement in frequency is consistent with the linear relationship between age and tremor frequency.     

Orthostatic tremor: essential and symptomatic cases

We studied clinically and electrophysiologically 8 patients affected by orthostatic tremor (OT), which is an unusual movement disorder consisting of shaking movements of the legs and trunk in the standing position. We failed to find any cause in 6 of the 8 cases. In 2 patients OT was clearly secondary to neurologic disease: hydrocephalus due to non-tumoral aqueduct stenosis and chronic relapsing polyradiculoneuropathy. The findings obtained suggest a relationship between OT and essential tremor (ET).     

Activity of muscle spindles, motor cortex and cerebellar nuclei during action tremor

Repeated electrode penetration of the dentate and interpositus nuclei in a rhesus monkey transformed an 11-13 Hz physiologic tremor into a much larger action tremor at 5-7 Hz. This tremor was associated with muscle spindle spike train modulation and reflexly evoked tremor modulation of interpositus and motor cortex neurons as well as electromyogram (EMG). No tremor modulation was observed in spike trains recorded from dentate. The timing relationships of the spindle, EMG, and interpositus tremor discharges suggest that the interpositus plays a direct role in tremor suppression. Dentate, by contrast, may function indirectly by setting optimal transcortical long loop reflex dynamics concerned with intended voluntary movement.     

Orthostatic tremor: Clinical and electrophysiologic characteristics

Orthostatic tremor, sometimes known as ?shaky legs syndrome,”? is a disorder of middle-aged or elderly people characterized by feelings of unsteadiness in the legs and a fear of falling when standing. Patients stand on a wide base but walk normally. These symptoms are due to high-frequency (13–18 Hz) burst firing in weight-bearing muscles. They are attenuated by walking and are abolished immediately by sitting. Some authors believe that the disorder is a variant of essential tremor. This study reports the clinical and electrophysiologic features of orthostatic tremor in 30 patients. The findings indicate that orthostatic tremor is distinct from essential tremor, both clinically and electrophysiologically. The major differences are the frequency of electromyographic burst firing, the invariable involvement of lower limb and paraspinal muscles, and the task-specific nature of the tremor in orthostatic tremor. The study shows that the diagnosis can be established rapidly with surface electromyographic recordings. © 1993 John Wiley & Sons, Inc.     

Tuning the power spectrum of physiological finger tremor frequency with flickering light

Fast Fourier Transform analyses were performed on finger tremor movements at 0.2-Hz intervals from 0.4 to 40 Hz in 10 human subjects, under a flickering light condition of 4-15 Hz and an unstimulated control condition. Under the control condition, the power spectrum showed an essentially normal curve distribution, except for an early frequency component in the histogram. In contrast, when the flickering light stimulus was presented, the power of specific frequency components at 8-11 Hz was strongly enhanced. This effect was induced exclusively at a frequency of 8, 9, or 11 Hz of flickering light, and this flickering frequency producing the enhancement effect differed from subject to subject. There existed a significant correlation between the frequencies of flicker and tremor at the tuned frequency. These findings demonstrate that a specific frequency of flickering light can intensify a specific frequency of physiological finger tremor, and that different individuals exhibit different optimal "tuning" frequencies.     

Resetting of essential tremor and postural tremor in Parkinson's disease with transcranial magnetic stimulation

We studied the effects of transcranial motor cortex stimulation on the electromyographic characteristics of tremor in 9 patients with familial essential tremor and in 12 patients with postural tremor associated with Parkinson's disease. Transcranial magnetic stimulation reset both types of tremor equally. The resetting depended on the stimulus intensity, but was most closely correlated with the duration of the electromyographic silent period that followed the stimulus-induced motor evoked potential. Tremor resetting was present bilaterally even after focal, unilateral stimulation. Transcranial electrical stimulation failed to reset the tremor in either patient group. These results emphasize the role of central, intracortical structures in the generation of essential tremor and postural tremor in Parkinson's disease. © 1994 John Wiley & Sons, Inc.     

Investigation of an unusual,high‐frequency jaw tremor with coherence analysis

Normal physiological tremor of the jaw has a frequency of 6 to 8 Hz. A patient is described with jaw tremor at frequencies of 12 Hz during jaw movement and 15 Hz when the jaw was relaxed. The 15 Hz tremor was driven by synchronous, bilateral bursts of activity in the temporalis and masseter muscles, which alternated with digastric bursts. Coherence analysis indicated the tremor was highly correlated with both opening and closing muscle activity, and that the opening and closing muscles were about 180° out of phase. The existence of two tremors with different, nonphysiological peak frequencies and the influence of attention, relaxation, and movement in switching from one tremor frequency to the other, suggest that more than one generator may be operating. © 2007 Movement Disorder Society     

Postural tremor in X‐linked spinal and bulbar muscular atrophy

Postural tremor is a common initial symptom in spinal and bulbar muscular atrophy (SBMA), but its pathophysiological mechanisms remain to be studied. This study was undertaken to examine the physiological mechanisms underlying postural tremor in SBMA. For eight patients (36–63 years old) with genetically confirmed SBMA, we recorded surface electromyograms (EMGs) from the forearm muscles and hand movements with an accelerometer (ACC) while maintaining a posture with and without a weight load. We then analyzed their power spectra and coherence. The peak tremor frequency was 6–9 Hz in seven patients and 2–3 Hz in one patient. Oscillatory movements were associated with EMG activity in five patients, but not in three patients. Weight loads and postural changes affected the tremor frequency in all patients. Tremor was classified as “reflex tremor” in five patients and “mechanical tremor” in three patients. These results suggest that peripheral factors play important roles in tremor genesis in SBMA, although its clinical features resemble essential tremor. Subclinical sensory disturbance or a decrease of motor unit numbers might be candidates for such peripheral factors contributing to tremor genesis in SBMA. © 2009 Movement Disorder Society     

Orthostatic tremor: report of two cases and an electrophysiological study

Two patients with legs tremor present on standing, but none on walking or sitting, are reported. Tremor was not exclusive or orthostatism and was also evoked by strong tonic contraction of leg muscles. Synchronous EMG bursts were recorded in antagonistic muscle groups at 8-10 Hz in the first patient and at 16 Hz in the second. EMG activity was synchronous in corresponding muscles of both legs. The occurrence of EMG activity was not influenced by stimulation of nerve afferent fibers. We suggest that this movement disorder may be an exaggeration of physiological tremor due to synchronization of motor units by spontaneous oscillations in central structures.     

The dynamics of resting and postural tremor in Parkinson's disease.

OBJECTIVE: The study examines the time and frequency structure of Parkinson's disease tremor in patients that exhibit no clinical signs of tremor. METHODS: Eight mild to moderate Parkinson's disease and 8 matched control subjects maintained their limb in a constant position (30 s) under a postural finger, postural hand and resting tremor condition. Finger acceleration from the middle phalange, electromyographic (EMG) activity from extensor digitorum communis and flexor digitorum superficialis (FDS) were recorded. RESULTS: The data confirmed that there were no differences in the amount of limb motion and the modal frequency was around 9 Hz for each subject group. The time-dependent organization of tremor was more regular (lower approximate entropy [ApEn]) in Parkinson's disease. Both time and frequency analyses between the acceleration and extensor EMG signals demonstrate a reduction in the 20-25 Hz tremor component and an increase in the 8-12 Hz region of tremor. CONCLUSIONS: The results are discussed in relation to the proposal that increased regularity results from an increase in motor unit synchronization at 8-12 Hz and a reduction in the amplitude of the 20-25 Hz tremor component. The time and frequency structure of tremor may be useful in assessing individuals with Parkinson's disease.     

Directionality of corticomuscular coupling in essential tremor and cortical myoclonic tremor

ObjectiveA role of the motor cortex in tremor generation in essential tremor (ET) is assumed, yet the directionality of corticomuscular coupling is unknown. Our aim is to clarify the role of the motor cortex. To this end we also study ‘familial cortical myoclonic tremor with epilepsy’ (FCMTE) and slow repetitive voluntary movements with a known cortical drive.MethodsDirectionality of corticomuscular coupling (EEG-EMG) was studied with renormalized partial directed coherence (rPDC) during tremor in 25 ET patients, 25 healthy controls (mimicked) and in seven FCMTE patients; and during a self-paced 2 Hz task in eight ET patients and seven healthy controls.ResultsEfferent coupling around tremor frequency was seen in 33% of ET patients, 45.5% of healthy controls, all FCMTE patients, and, around 2 Hz, in all ET patients and all healthy controls. Ascending coupling, seen in the majority of all participants, was weaker in ET than in healthy controls around 5–6 Hz.ConclusionsPossible explanations are that tremor in ET results from faulty subcortical output bypassing the motor cortex; rate-dependent transmission similar to generation of rhythmic movements; and/or faulty feedforward mechanism resulting from decreased afferent (sensory) coupling.SignificanceA linear cortical drive is lacking in the majority of ET patients.     

Coupling between limb tremor and postural sway in Parkinson's disease

Increased tremor and postural instability are motor problems commonly associated with Parkinson's disease (PD). Despite the similarity between these oscillatory forms, little is known about the relation between them, especially for individuals with enhanced tremor. This study was designed to examine the nature of any relation between center of pressure (COP) excursions and postural/resting limb tremor of young, older individuals, and Parkinsonian participants in their different medication states. The resting and postural tremor for the PD participants was characterized by a single, prominent peak frequency between 4 and 7 Hz. The postural tremor for young/older participants contained smaller peaks between 1 to 4 and 7 to 12 Hz although no prominent peak was seen in their resting tremor. The AP and ML COP dynamics of all participants was characterized by a major peak between 0.1 and 0.5 Hz. An additional peak was observed in the COP output of the PD participants between 4 and 7 Hz. While no tremor‐COP coupling was observed for the young/old groups, coherence analysis revealed a significant degree of coupling between COP motion and tremor between 4 and 7 Hz for PD participants. These results highlight that the amplified tremor in PD can manifest itself in COP dynamics. This finding may have implications for postural stability for this patient group. © 2007 Movement Disorder Society     

Orthostatic tremor: an essential tremor variant?

Three patients with a clear-cut history of essential tremor of the upper limbs presented with the clinical features reported by Heilman as orthostatic tremor. Electromyographic findings included 6-8 Hz postural tremor in all four limbs. Highly synchronized 16 Hz rhythmic discharges were found in the legs upon standing. This peculiar pattern of discharge was also observed in the upper limbs and spinal muscles. High frequency rhythmic bursts, either alternating or co-contracting were present in specific postures not necessarily related to standing. An additional group of 12 patients with postural tremor of the legs was studied; seven of these showed modification in the frequency and synchronization of the muscle discharges upon standing. Although none of them had the full-blown clinical syndrome of orthostatic tremor, they complained of mild unsteadiness upon standing, together with a vague sensation of stiffness in the lower limbs. The present findings induce us to think that there might be a link between essential tremor and the so-called orthostatic tremor. Orthostatic tremor might be an essential-tremor-related entity that may be caused by a derangement in the central mechanism in charge of the organization of certain motor activities, not necessarily controlling the standing position.     

Effect of stimulation frequency on tremor suppression in essential tremor.

We sought to determine the effect of deep brain stimulation (DBS) frequency on tremor suppression in essential tremor (ET) patients with deep brain stimulators implanted in the ventral intermediate nucleus (VIM) of the thalamus. A uniaxial accelerometer was used to measure tremor in the right upper extremity of subjects with a diagnosis of ET who had DBS electrodes implanted in the left VIM. The root-mean-square acceleration was used as the index of tremor magnitude and normalized to the OFF DBS condition. There was a highly significant inverse sigmoidal relationship between stimulation frequency and normalized tremor acceleration (X(2)/DoF = 0.42, r(2) = 0.997). Tremor acceleration had a nearly linear response to stimulation frequencies between 45 and 100 Hz with little additional benefit above 100 Hz. These findings have two important implications. Clinically, frequency of thalamic stimulation is an important variable for optimal tremor control with maximal benefit achieved with 100 to 130 Hz in most patients. Second, thalamic DBS provides tremor benefit in a graded manner and is not an all-or-nothing phenomenon.