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ORIGINAL ARTICLE

Mechanical suppression of essential tremor

EDUARDO ROCON 1 , MARIO MANTO

2 , JOSE PONS

1 , STEPHANE CAMUT

2 &

JUAN MANUEL BELDA 3

1 IAI-CSIC, Madrid, Spain,

2 Service de Neurologie, Hôpital Erasme-ULB, Bruxelles, Belgium, and

3 Institute of

Biomechanics (IBV), Valencia, Spain

Abstract This paper describes a new treatment for essential tremor. A wearable orthosis, which can be adapted to each configuration of each joint of the upper limb, is able to apply effective dynamic force between consecutive segments of the upper limb and change its biomechanical characteristics. The orthosis is controlled by a computer with a dedicated software application that distinguishes between real time tremor and voluntary movement. The wearable orthosis is able to detect position, rate and acceleration of rotation of the joint by means of a chip gyroscope. This technology was evaluated in six patients suffering from essential tremor. The technique is non invasive and represents an alternative to medication and deep brain stimulation.

Key words: Essential tremor, human, muscle, side effects, robotics, viscosity

Introduction

Essential tremor (ET) is defined by a bilateral

postural tremor involving the upper limbs (1). A

kinetic tremor, increasing in amplitude when the

limb approaches a target, is often observed, and a

head tremor may be associated. ET, a heterogeneous

disorder, is the most common movement disorder in

adults, with an estimated prevalence of 4% (2,3).

Possible non-motor manifestations of ET, such as

mood fluctuations or personality disorders, require

further neuropsychological work assuming that they

could share similar mechanisms with motor symp-

toms (4). Symptoms typically are progressive and

disabling. The link between ET and Parkinson’s

disease is often a matter of debate (2).

The mechanism of ET remains to be elucidated

(5). According to animal models of ET, an abnormal

olivo-cerebellar activity would result in enhanced

oscillatory activity in the cerebellum and its target

nuclei in the thalamic circuits. However, recordings

of neurons in human are not consistent with the

theory of continuous olivo-cerebellar driving of the

motor cortex via thalamic connections (5). ET might

be induced by networks enabling tremor-related

activity during voluntary movement and by

enhanced access of sensory feedback. Given the role

of the cerebellum in sensorimotor processing, a

pathological coupling between sensory inputs and

motor output may be considered (6,7). There is

evidence from behavioural studies and imaging

techniques that the cerebellar circuit is a plausible

site for the genesis of ET. The clinical similarities

between symptoms of ET and the symptoms in

classical cerebellar disorders is obvious (1,8,9).

Kinematic studies are highly suggestive of a genuine

cerebellar disorder, and magnetic resonance spectro-

scopy reveals decreased N-acetylaspartate/creatine

ratios (NAA/Cr) and N-acetylaspartate/choline

ratios NAA/Cho ratios in the cerebellum of ET

patients, indicating a possible degenerative phenom-

enon with neuronal dysfunction (10,11). An inverse

association between cerebellar cortical NAA/Cr

ratios and arm tremor severity has been observed.

The favourable effect of ethanol reported in ET

could be due to an effect on neuronal activity of

inferior olivary cells or an effect on alcohol-sensitive

GABA receptors within the cerebellum (10,12).

Ethanol administration decreases complex spikes

and increases simple spike activity generated in the

inferior olive.

Current treatments of ET include drugs (mainly

primidone and propranolol), and surgery (thalamot-

omy and deep brain stimulation) in patients refrac-

tory to medications, in which the response is

considered insufficient or only partial (3,13).

However, (a) ET is not managed effectively or

sufficiently in about 25% of patients, (b) the drugs

Correspondence: Eduardo Rocon, PhD, Instituto de Automática Industrial, Carreterra Campo Real, km 0.200, La Poveda – Arganda del Rey, 28500 Madrid,

Spain. E-mail: [email protected]

The Cerebellum. 2007; 6: 73–78

ISSN 1473-4222 print/ISSN 1473-4230 online # 2007 Taylor & Francis

DOI: 10.1080/14734220601103037

used often induce side effects or may be contra-

indicated, and (c) surgery is associated with a risk of

hemorrhage and psychiatric manifestations (14). In

particular, a high rate of suicide (4.3%) has been

observed recently in patients treated with deep brain

stimulation (15). Paradoxically though, patients

developing psychiatric symptoms often experience

significant motor improvement following a surgical

procedure (15). Therefore, further research and new

therapeutic options are required to manage ET more

effectively. It has been established in the literature

that essential tremor responds to biomechanical

loading. In particular, it has been clinically tested

that the increases of damping and/or inertia in the

upper limb leads to a reduction of the tremorous

motion, i.e., the change in impedance characteristics

of the upper limb has a direct effect on the tremor

characteristics (16,17). This phenomenon gives rise

to the possibility of an orthotic management of

tremor. Biomechanical loading for tremor reduction

can be approached either by ambulatory robotics

based orthotic devices or by non-ambulatory table or

wheelchair mounted devices. The former approach

is characterized by selective tremor suppression

through internal forces at particular joints, while

the later relies on global application of external

forces that leads to the overall tremor reduction.

While wearable tremor suppression exoskeletons are

already a matter of research, non-ambulatory sys-

tems have lead to commercial products, see, for

instance, the so-called Neater Eater (18). In addi-

tion, the MIT damped joystick (19), the controlled

Energy-Dissipation Orthosis, CEDO (20), or the

Modulated Energy Dissipation Arm, MED, [cited in

21], are implementations of non-ambulatory, wheel-

chair-mounted tremor suppression prototypes. As

far as wearable tremor suppression concepts are

concerned only the well-known wearable tremor-

suppression orthosis (21) has been reported in the

literature. This is a passive damping loading device,

which acts mechanically in parallel to the wrist in

flexion-extension. It completely constrains both

wrist abduction-adduction and prono-supination.

This paper discusses the effects of a new ther-

apeutic strategy based on biomechanical loading of

the tremor in six patients exhibiting a definite

essential tremor (1). Biomechanical loading of tremor

was achieved through a robotic wearable device called

WOTAS (Wearable Orthosis for Tremor Assessment

and Suppression), which acts in parallel to the

affected limb and is able to apply damping or inertial

load to a selected set of limb articulations.

Innovations of this technique are: (i) the orthosis

ability to apply forces between segments of the upper

limb, and (ii) the intelligent detection of tremor vs.

voluntary motion, in such a way that the orthosis

does not load the voluntary movement of the patient.

WOTAS will be briefly described in the following

section. This will be followed by the presentation of

the materials and methods used during the clinical

trials. Next, the results obtained are presented.

Finally, the discussion and the conclusions of this

study are given.

Description of the technique

The active orthosis (exoskeleton) WOTAS was

developed under the framework of the European

project DRIFTS (22). The concept of WOTAS is to

develop an active upper limb exoskeleton based on

robotic technologies capable of applying forces to

cancel tremor and retrieve kinematic information

from the tremorous upper limb. This active orthosis

is equipped with kinematics (angular position,

velocity and acceleration) and kinetic (interaction

force between limb and orthosis) sensors. The

WOTAS active orthosis has built-in gyroscopes

(ENC-03J manufactured by Murata Inc.) for tremor

measurement in each of the controlled joints.

Moreover, it could also apply dynamic force to the

articulations of the upper limb by means of a set of

flat brushless DC motors + pancake gears (23) selected to be a compact and light weight solution

suitable for a wearable device. The orthosis is

adaptable to each configuration of the joint between

different patients owing to the use of thermoplastics.

This robotic device spans the elbow and wrist joints,

being able to apply independent tremor suppression

strategies to elbow flexion-extension, wrist flexion-

extension and wrist pronation-supination, see

Figure 1. The total weight of the final system is

roughly 850 g. Innovations of the WOTAS exoske-

leton are the following: its portability, it is a non

invasive system, and provides direct information

from each joint of the upper limb, allowing the

estimation of the contribution of each joint to the

Figure 1. The active orthosis WOTAS placed on a patient.

74 E. Rocon et al.

overall tremorous motion in the kinematic chain of

the upper limb.

WOTAS’ control architecture is mainly composed

of three elements, namely, the exoskeleton structure

with its sensors and actuators; a microprocessor

which executes the real-time control algorithms of

the orthosis and a standard desktop PC that works as

a host computer and implements the interface with

the clinician. The active orthosis is controlled by a

computer with a dedicated software application that

implements an algorithm able to distinguish in real

time tremorous from voluntary movement and to

calculate the force applied by the active orthosis over

the upper limb in order to change its biomechanical

characteristics and, consequently, suppress tremor.

The algorithm developed for the WOTAS orthosis

is based on a two-stage method that estimates

voluntary and tremorous motion in real-time (23).

After a comparison between several tracking algo-

rithms (23), the Benedict-Bordner filter was selected

for the estimation of voluntary motion. The esti-

mated voluntary movement is removed from the

overall movement and the resulting movement is

supposed to be the tremorous movement. After this,

we use the Weighted-Frequency Fourier Linear

Combiner (WFLC) in order to estimate tremor

parameters. The WFLC is an adaptive algorithm

that estimates tremor using a sinusoidal model,

estimating its time-varying frequency, amplitude and

phase (17). The filtering on the first stage is

implemented in order to estimate and use filter

equations to distinguish tremorous from overall

motion with reduced phase lag, but without con-

sidering high frequency movements (such as tre-

mors). The second filtering stage estimates both the

amplitude and frequency of the tremorous move-

ment on the assuming that the remaining movement

from stage one is not voluntary. Experimental results

with 33 subjects presenting different tremor diseases

showed that the algorithm is capable of estimating in

real-time (roughly 1 ms of time delay introduced)

the voluntary and tremor components from the

overall movement.

Material and methods

Patients

All patients were right-handed (Mean age 72.3

years; Females: 3; Males: 3). ET was moderate in

patients 1, 3, and 4 and severe in patients 2, 5, and

6. Patients were still exhibiting a bilateral postural

tremor in upper limbs despite regular intake of

medication (mysoline, propranolol or a combination

of both at usual doses for ET). The protocol of the

experiments was informed to the patients before

the trials and signed-up an informed consent. The

investigation was approved by the ethical committee

of Hôpital Erasme-ULB, Bruxelles (ULB).

Protocol

Three different people were present during the

measurements:

(1) A computer operator for setting the parameters

of the system and recording the signals.

(2) A clinician for supervision of the condition of

the trials and the state of the patient.

(3) An experimenter to aid the patient don and doff

the orthoses and interact with them to perform

the trials.

The active orthosis WOTAS is able to operate in

two different modes: the suppression mode in which

the system is able to simulate different parameters

of viscosity and inertia to the joints of the upper

limb and the monitoring mode in which WOTAS

operates in free mode (no force is applied to the

upper limb) and monitors tremor parameters of

the patients. During the experiments neither the

patient nor the experimenters knew when the

system was applying a suppressing strategy or when

it was operating in monitoring mode. Only the

computer operator knew when the systems were

applying the suppression strategy. This experimen-

tal design was used to reduce the placebo effect in

the experimentation. All the experiments were

recorded on video.

Tasks

Three different tasks were selected: keep the upper

limbs outstretched, point to the nose with a finger,

and keep the arm in a rest position. These tasks have

been previously used to characterize tremor move-

ment (24). These tasks were selected:

(1) To have a representation of the different forms

of tremor (24).

(2) To cover tasks that can be meaningful from the

point of view of function. For this second issue,

a good correlation was found (25) between

these tasks and the functional scale for the

assessment of tremor severity (Tolosa and

Fahn’s scale) (33)

The experiments were balanced to avoid interactions

in the analysis, as well as learning effects (25). The

order in which the modes were applied was

alternated, as well as the order in which the patients

executed the tasks. In each experimental session, 3

repetitions of each task were realized. The number of

repetitions was chosen to have an experimental

session not longer than 1 h.

Data analysis

The data analysed were the output voltage coming

from the gyroscopes placed on the active orthosis.

An alternative to drugs and surgery to treat tremor 75

The figure of merit adopted to quantify the reduc-

tion achieved by the active orthosis is the ratio

between the signal analysed in monitoring mode

(Pmm), and the signal analysed in suppression mode

(Psm). Therefore, the reduction of tremor was

measured with the patients under the same condi-

tions: with the orthosis placed on the upper limb. As

a result, the reduction estimated is the remaining

tremor in suppression mode referred as a percentage

of the tremor with respect to the monitoring mode.

Results

The effects of adding effective viscosity were investi-

gated for the upper limb during the execution of the

different clinical tasks. During the trials, some patients

were able to identify when the system was operating in

suppression mode, relating to the clinician ‘now the

system is suppressing my tremor’, and ‘now it is not’.

Figure 2 illustrates the performance of WOTAS

when operating in suppression mode showing the

efficiency of the active orthosis increases as tremor

power increases. This shows a lower limit for

efficient tremor suppression, this limit is roughly

0.16 (rad/s) 2 /Hz. These lower limits for tremor

suppression should be determined by the physical

interaction between the orthosis and the upper limb.

This physical interaction is bidirectional: on the one

hand, the orthosis must transmit the loads to the

bones for tremor suppression. This transmission is

mediated by the soft tissues between the supports

and the actuator. Usually, the effect of load

transmission is negligible in common orthotics.

However, tremor has a pure dynamic component.

Therefore the characteristics of the transmissions

through the soft tissues play an important role in the

efficiency of tremor suppression. On the other hand,

the orthosis should be able to measure tremor

activity. This process is also affected by the

transmission of movements from the upper limb to

the orthosis since this transmission is also performed

through soft tissues. Both factors are more crucial

for lower amplitude tremors. These results indicate

that there is a physical limitation for tremor

suppression through wearable devices due to force

transmission through soft tissues.

According to the results, the range of tremor

suppression of the signals above this orthosis opera-

tional limit ranges from 2.9% (percentile 5) to 78.5%

(percentile 95) in relation to energy in the monitoring

mode. The results also indicated that the device could

achieve a consistent 40% of tremor power reduction.

Figure 3 illustrates the effects of WOTAS in the

tremorous movement when operating in suppression

mode. Figure 3 (upper traces) shows the time series

corresponding to the tremorous movement of the wrist

joint during the arm outstretched task of patient 2. The

top left part of the figure shows the time signal with

WOTAS in the monitoring mode. The top right part

illustrates the time series when the orthosis is operating

in suppression mode. Notice that the amplitude of

tremor is clearly lower than in the monitoring mode.

Figure 3 (bottom) shows the representation of the

Power Spectrum Density of the same time series

represented on the upper panels. The PSD was

obtained from the part of the signal with tremor. The

left part of the figure illustrates the PSD of the

tremorous movement with WOTAS operating in

monitoring mode. It is possible to see a clear peak of

tremor activity close to 6 Hz. In the right part of the

figure the peak of energy corresponding to the

tremorous activity when WOTAS is operating in

suppression mode presents a clear reduction. The

Figure 2. Tremor reduction achieved by WOTAS.

76 E. Rocon et al.

reduction of the power spectral density (PSD) was

80.4% in this patient. Results indicate that reductions

of tremor can be as high as 98% for severe cases of

tremor. In addition, the analysis of the video record

indicated that in the majority of patients there was no

displacement of tremor movement to proximal joints.

Nevertheless the authors believe that it is important to

investigate and define the profile of the users affected

by this new phenomenon.

There are suggestions that the mechanical suppres-

sion of tremor could produce a positive feedback to

essential tremor patients. Patients reported that when

they realized that the orthosis was suppressing tremor

they felt a reduction in the tremor itself and felt

more confident to accomplish the task. This beha-

viour has been detected in patients with severe tremor

and requires further research to be confirmed.

During the trials, patients reported that the orthosis

does not limit their range of motion. In addition, no

patient felt that the orthosis applied loads on their

voluntary movement. Patient tolerance was good. No

lesions were observed on their skin, except for a

moderate and transient change in skin aspect due to

the orthosis. Slight discomfort was reported by some

patients. These results suggest this new technique is a

possible therapy for tremor suppression in humans.

The patients considered that the use of such a

device could cause social exclusion. This was expected

since the exoskeleton was developed as a platform to

evaluate the concept of mechanical tremor suppres-

sion and not as a final orthotic solution.

Discussion and conclusions

This paper reports a new solution for tremor

suppression in humans using an active orthosis

simulating viscosity. The adaptive viscous control

method is non invasive. It may be effective in

patients who are insufficiently responsive (or have

adverse reactions) to drugs or in whom surgery is

counter-indicated. This technique avoids the poten-

tial side effects of drugs and the risks of surgical

procedure. Although deep brain stimulation has

provided remarkable benefits for people with a

variety of neurological conditions (26), alternative

solutions are of interest for refractory cases. From

the motor control point of view, deep brain stimula-

tion has also the disadvantage of impairing the

adaptative control of reaching (27). In ET patients

with electrodes placed in the cerebellar thalamus, the

larger the stimulation voltage the greater the reduc-

tion in rates of adaptation. In patients with unilateral

thalamotomy, the adaptation in the contralateral

arm is also impaired as compared to the ipsilateral

arm (27). Therefore, although both central neuro-

stimulation and lesions are very effective to manage

tremor, both techniques significantly impair the

capacity of the brain to form internal models of

action (27–31). Indeed, the adaptative control of

reaching depends on the integrity of the cerebel-

lothalamocortical pathway.

Oscillations occur frequently in biological systems

(32). Interactions with the environment induce

tremors in the human body. The mechanical

Figure 3. This figure illustrates the oscillations of the elbow and the associated power spectral density (PSD) of tremor with the motor in a

free mode (left panels) and providing viscosity of 0.2 Nms/rad (right panels) in patient 2. Note the strong reduction in the PSD when

viscosity is applied. The tremor is detected by a chip gyroscope fixed on a metallic bar. Oscillations are expressed in rad/sec and PSD is

expressed in (rad/s) 2 /Hz. The solution used to actuate the orthosis is a combination of a flat brushless DC motor and a pancake gear.

Brushless servo motors have electronic commutators, isolated from high bus voltages. As a result, higher speeds and torques can be

achieved. The motor is controlled in real time. Output voltage from the gyroscopes processed at 2000 Hz. Data filtering: Kernel smoothing

algorithm. Duration of acquisition: 30 sec.

An alternative to drugs and surgery to treat tremor 77

properties of the neuromuscular system and the

effects of reflexes explain the features of these

involuntary movements. Because muscles can be

compared to viscoelastic structures, they are involved

in damping, the dissipation of mechanical energy.

Potentially, reflexes can modulate the relative magni-

tudes of the viscous and elastic components of the

mechanical impedance provided by muscle (32). The

technique described here may be seen as an adaptive

device which cancels oscillations of tremor by

governing and re-directing the movement at the

effector level. The potential compensatory mechan-

isms which might occur in the central nervous system

in the mid- or long-term remain to be defined.

Acknowledgments

This work is part of the DRIFTS project

(Dynamically Responsive Interventions for Tremor

Suppression) supported under contract QLK6-CT-

2002-00536 by the European Commission (5th

framework; RTD: ‘Quality of life and management

of living resources’; Key Action 6.4: ‘Ageing and

disabilities’). This paper does not necessarily reflect

the views of the European Commission and in no

way anticipates its future policy in the field. We

thank the patients for their cooperation.

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