Sub-movement cueing and motor sequence execution in patients with Huntington's disease

Emerging concepts on bradykinesia in non-parkinsonian conditions

BACKGROUND AND PURPOSE

Bradykinesia is one of the cardinal motor symptoms of Parkinson's disease. However, clinical and experimental studies indicate that bradykinesia may also be observed in various neurological diseases not primarily characterized by parkinsonism. These conditions include hyperkinetic movement disorders, such as dystonia, chorea, and essential tremor. Bradykinesia may also be observed in patients with neurological conditions that are not seen as "movement disorders," including those characterized by the involvement of the cerebellum and corticospinal system, dementia, multiple sclerosis, and psychiatric disorders.

METHODS

We reviewed clinical reports and experimental studies on bradykinesia in non-parkinsonian conditions and discussed the major findings.

RESULTS

Bradykinesia is a common motor abnormality in non-parkinsonian conditions. From a pathophysiological standpoint, bradykinesia in neurological conditions not primarily characterized by parkinsonism may be explained by brain network dysfunction.

CONCLUSION

In addition to the pathophysiological implications, the present paper highlights important terminological issues and the need for a new, more accurate, and more widely used definition of bradykinesia in the context of movement disorders and other neurological conditions.

Characterising Upper Limb Movements in Huntington's Disease and the Impact of Restricted Visual Cues

BACKGROUND

Voluntary motor deficits are a common feature in Huntington's disease (HD), characterised by movement slowing and performance inaccuracies. This deficit may be exacerbated when visual cues are restricted.

OBJECTIVE

To characterize the upper limb motor profile in HD with various levels of difficulty, with and without visual targets.

METHODS

Nine premanifest HD (pre-HD), nine early symptomatic HD (symp-HD) and nine matched controls completed a motor task incorporating Fitts' law, a model of human movement enabling the quantification of movement timing, via the manipulation of task difficulty (i.e., target size, and distance between targets). The task required participants to make reciprocal movements under cued and blind conditions. Dwell times (time stationary between movements), speed, accuracy and variability of movements were compared between groups.

RESULTS

Symp-HD showed significantly prolonged and less consistent movement times, compared with controls and pre-HD. Furthermore, movement planning and online control were significantly impaired in symp-HD, compared with controls and pre-HD, evidenced by prolonged dwell times and deceleration times. Speed and accuracy were comparable across groups, suggesting that group differences observed in movement time, variability, dwell time and deceleration time were evident over and above simple performance measures. The presence of cues resulted in greater movement time variability in symp-HD, compared with pre-HD and controls, suggesting that the deficit in movement consistency manifested only in response to targeted movements.

CONCLUSIONS

Collectively, these findings provide evidence of a deficiency in both motor planning, particularly in relation to movement timing and online control, which became exacerbated as a function of task difficulty during symp-HD stages. These variables may provide a more sensitive measure of motor dysfunction than speed and/or accuracy alone in symp-HD.

The detection and measurement of locomotor deficits in a transgenic mouse model of Huntington's disease are task- and protocol-dependent: influence of non-motor factors on locomotor function

Locomotor performance of transgenic R6/2 mice carrying the Huntington's disease (HD) mutation was assessed using four different tasks, fixed speed rotarod, accelerating rotarod, Digigait and footprint test. The tasks were compared directly in age- and CAG repeat-matched R6/2 mice. Accelerating rotarod was more sensitive than fixed speed rotarod for detecting early motor deficits in R6/2 mice. The sensitivity of accelerating rotarod increased with the acceleration rate and/or the start speed from which the rod accelerated. Differences between tasks were not due to inability of R6/2 mice to maintain balance at high speeds or increased fatigue on accelerating rotarod, but to difficulties in coordinating gait changes required by the constant change in speed on accelerating rotarod. The footprint test was sensitive to gait disturbances. However, surprisingly, R6/2 mice did not show major gait abnormalities on an automated treadmill task (Digigait), even though they showed overt gait deficits in the home cage. The fact that the sensitivity for detecting motor deficits depended strongly on the individual task, and on the protocol used, suggests that non-motor factors were differentially engaged in the different paradigms. We thus recommend that more than one task should be used for detecting and tracking different aspects of motor decay in animal models of HD. Since deficits in non-motor factors such as executive function and motivation may differentially influence motor outcome in each task, our results call for a more thorough investigation of the importance of higher level control of locomotion in animal models of HD.

Huntington's disease affects movement termination

Huntington's disease (HD) is a neurodegenerative disease affecting the striatum and associated with deficits in voluntary movement in early stages. The final portion of aiming movements is particularly affected in HD and one hypothesis is that this deficit is linked to attention or terminal control requirements. Sixteen patients with early HD and 16 age-matched controls were examined in aiming movements. Four conditions manipulated movement termination requirements (discrete movements with a complete stop vs. cyclical back-and-forth movements) and the presence of flankers around the target. Reducing movement termination requirements significantly attenuated deficits in the final movement phase in patients. The presence of flankers around the target affected the initial portion of movements but did not affect the two groups differentially. These results indicate that terminal control requirements affect voluntary movements in HD. This suggests that frontostriatal systems are involved in movement termination.

A biomechanical study of gait initiation in Huntington's disease

BACKGROUND

Akinesia in basal ganglia disorders is essentially defined by delayed movement initiation; the reaction time increases and it becomes difficult (or even impossible) for the subject to initiate movement. A biomechanical study of gait initiation would help evaluate the role of akinesia in early stage Huntington's disease (HD) patients.

METHODS

We recorded kinematic, spatiotemporal and angular parameters (using video motion analysis, a force platform and an optoelectronic system) for the first two steps taken by 15 HD patients and 15 gender- and age-matched controls. In order to evaluate the influence of an external cue on gait initiation parameters, we studied two movement paradigms: self-triggered initiation and initiation triggered (cued) by a "beep" sound. We analyzed kinematic, spatiotemporal (the speed, length and duration of the two first steps) and angular parameters (range of joint angles) as well as kinetic data (the trajectory of the centre of pressure (COP); the speed and trajectory of the centre of mass (COM)).

RESULTS

HD patients presented akinesia in both externally triggered and self-triggered conditions. Patients had more difficulties with self-triggered gait than with triggered gait. In HD, anticipatory postural adjustments (APAs) were more impaired in self-triggered gait initiation than in cued initiation. Indeed, an alteration in the kinetic parameters revealed a reduction in first step speed in both conditions. Hypokinesia (as assessed by a reduction in the range of angle joints) played an important role in this reduction.

CONCLUSION

Akinesia is a major feature of impaired gait initiation in HD. The deficiencies in self-triggered initiation in HD seen here fit with a hypothesis whereby deficient internal cueing can be replaced by an external trigger.

Firing rates of pallidal neurons are similar in Huntington's and Parkinson's disease patients

According to the now classical basal ganglia-thalamocortical circuitry model, the chorea of Huntington's disease (HD) and the hypokinesia in Parkinson's disease (PD) are explained by a decrease in the inhibitory output (reduced firing rates) from the globus pallidus internus (GPi) in HD and increased output in PD. Differences between firing patterns might also be a factor contributing to the different symptoms, however. To test the predictions of the model we examined neuronal firing rates and patterns in two HD patients and 14 PD patients. Single-cell, microelectrode recordings were obtained from awake patients undergoing stereotactic surgery for implantation of deep brain stimulating (DBS) electrodes in the GPi. The mean neuronal firing rate in the GPi of HD patients was 81.8+/-4.3 Hz (mean+/-SEM), which was not significantly different from that in PD patients (89.9+/-3.0 Hz). Firing pattern analyses using measurements of burst index, coefficient of variation, and percentage participation of spikes in bursts revealed, however, that GPi neurons in HD patients fired in a more regular pattern (fewer "bursts") than in PD patients. These results suggest that the rate-based model does not adequately explain the motor abnormalities present in the two HD patients studied. Furthermore, the findings did reveal a difference between firing patterns in the HD and PD groups, thereby supporting the role of altered firing patterns in the pathophysiology of these diseases.

Differential effect of Huntington's and Parkinson's diseases in programming motor sequences of varied lengths

BACKGROUND

Parkinson's disease (PD) and Huntington's disease (HD) patients have difficulties executing sequential movements. Attention control and short-term memory probably play an important role in programming sequential movements. To investigate the contribution of these cognitive factors to programming and executing visuomotor sequences in HD and PD patients a computerized version of the Corsi Block Tapping-Test was employed.

METHODS

the performance of 11 patients with early stage PD, 11 HD patients with borderline to mild caudate atrophy and 20 healthy subjects was compared. The task was a reaction time task where targets were illuminated in groups of sequences increasing from 2 items to 5 items. Subjects reproduced the sequence (pressing the illuminated target) in the same order of appearance. Reaction Times and movement times were recorded.

RESULTS

PD patients had increasing difficulties in programming and executing series greater than three components. HD patients did not differ significantly from the controls, although they showed a tendency to lose accuracy in the longer series. Both patient groups did not differ in their attention span.

CONCLUSIONS

In PD although the spatial information may be well stored, they have difficulty accessing it when their attention is overloaded, leading to poor encoding and slow information processing. This process interferes with programming and execution of movement sequences. HD patients in the early stages of the illness seem to have more attention resources than PD patients, so that they start to show more problems in executing visuomotor sequences with longer movement sequences than PD patients.

Early Huntington's disease affects movements in transformed sensorimotor mappings

This study examined the effect of transformed visual feedback on movement control in Huntington's disease (HD). Patients in the early stages of HD and controls performed aiming movements towards peripheral targets on a digitizing tablet and emphasizing precision. In a baseline condition, HD patients were slower but showed few precision problems in aiming. When visual feedback was inverted in both vertical and horizontal axes, patients showed problems in initial and terminal phases of movement where feedback is most critical. When visual feedback was inverted along a single axis as in a mirror-inversion, HD patients showed large deviations and over-corrections before adaptation. Adaptation was similar in both groups. These results suggest that HD impairs on-line error correction in novel movements.

The timing and intensity of transcranial magnetic stimulation, and the scalp site stimulated, as variables influencing motor sequence performance in healthy subjects

OBJECTIVE

The increasing therapeutic use of transcranial magnetic stimulation (TMS) in disorders of cortical excitability raises the need for reliable stimulus variables. Stimulation of cortical motor areas influences motor programming and execution. We investigated the effects of TMS delivered over various cortical motor areas during the reaction time (RT) on the execution of sequential rapid arm movements in healthy subjects.

METHODS

Subjects performed a five-submovement (S1-S5) motor sequence mainly involving upper limb proximal muscles. RT and movement time (MT) were measured. We delivered late (close to movement onset) and early (close to the go signal) TMS over the primary motor area (M1-FDI hot-spot for the first dorsal interosseus, M1-D hot-spot for the deltoid muscle), the premotor (PM) area, and the supplementary motor area (SMA), using subthreshold and suprathreshold intensity, single and triple pulses.

RESULTS

The motor sequence showed a characteristic pattern of submovement duration, S2-S3-S4 being faster than S1 and S5. Late TMS prolonged RT only when high-intensity pulses were delivered over M1-FDI. Single- and triple-pulse TMS over M1-D or M1-FDI significantly prolonged MT with a dose-related effect. Suprathreshold triple-pulse TMS over the PM-but not over the SMA-also lengthened the MT but did not change RT. Early triple-pulse TMS reduced the RT independently from the stimulus intensity and scalp site. SMA and PM-but not M1-D-stimulation also reduced the MT. Single-pulse TMS over the SMA, despite being delivered through a double-cone coil, did not change RT or MT.

CONCLUSIONS

TMS-induced changes in the kinematics of a sequential arm movement depend closely on the timing of TMS interference, the scalp site stimulated, and the intensity (and number) of stimuli delivered. Late TMS interference inhibits, whereas early interference facilitates, motor performance. The cortical motor region most sensitive to TMS-induced inhibition is that below the scalp site for M1-FDI. In contrast, TMS-induced facilitation has no strict topographic organization. Particularly for MT (although inhibitory and facilitatory effects both depend on stimulation at high intensities) intensity is less crucial than timing of interference and scalp site.

Sensorimotor mapping affects movement correction deficits in early Huntington's disease

Huntington's disease (HD) is associated with early voluntary movement problems linked to striatal dysfunction. In pointing movements, HD increases the irregularity of the terminal part of movements, suggesting a dysfunction in error feedback control. We tested this hypothesis in movements requiring continuous feedback control. Patients in the early stages of HD and controls traced as fast and accurately as possible circles within a 5-mm annulus on a digitizing tablet when visual feedback of the hand and the circle was direct or indirect (through a monitor). Patients deviated more often from the annulus and showed larger corrections toward the circle than controls when using indirect visual feedback but not with direct visual feedback. When velocity requirements were removed, patients showed little change in these control problems. These results suggest that HD does not affect error feedback control in all movements and that the striatal contribution to voluntary movement is sensitive to sensorimotor mapping.

Objective assessment of motor slowness in Huntington's disease: clinical correlates and 2-year follow-up

Functional disability of patients with Huntington's disease (HD) is determined by impairment of voluntary motor function rather than the presence of chorea. However, only few attempts have been made to quantify this motor impairment. By using a simple reaction time paradigm, we measured the time needed for movement initiation (akinesia) and execution (bradykinesia) in 76 HD patients and 127 controls. Akinesia and bradykinesia were already evident in early stages and increased linearly with increasing disease stage. Quantified motor slowness correlated with clinical impairment of voluntary movements but also with cognitive impairment and medication use. In patients without severe cognitive impairment, quantified motor slowness reflected clinical motor impairment more purely. During 1.9 years follow-up (range, 0.8-3.8 years), quantified akinesia and bradykinesia progressed concomitantly with progression of clinical impairment of voluntary movements, cognition, and functional capacity. However, rate of change in motor slowness did not discriminate between patients whose disease stage remained stable and those whose disease stage progressed. We conclude that the reaction time paradigm may be used to quantify akinesia and bradykinesia in HD, at least in patients without severe cognitive impairment. Although reaction and movement times increased in time, these measures failed to detect functionally important changes during our follow-up period.

Impaired antagonist inhibition may contribute to akinesia and bradykinesia in Huntington's disease

OBJECTIVE

To test the hypothesis that besides impaired agonist facilitation, impaired antagonist inhibition also contributes to delayed initiation (akinesia) and slow execution (bradykinesia) of voluntary movements in Huntington's disease.

METHODS

Fifteen patients with Huntington's disease and 11 age-matched controls participated in the study. The amount of agonist facilitation was measured as the increase in soleus H-reflex amplitude prior to ballistic voluntary plantar flexion (soleus contraction). Antagonist inhibition was measured as the decrease in soleus H-reflex prior to ballistic dorsiflexion (tibialis anterior (TA) contraction). The amount of agonist facilitation and antagonist inhibition was correlated with the time needed for motor initiation (reaction time) and movement execution (movement time).

RESULTS

Starting 50ms prior to soleus contraction, soleus H-reflex increased in control subjects but less so in patients. Soleus H-reflexes decreased in controls 25ms prior to TA contraction, while this antagonist inhibition was completely lacking in patients. Thus, patients with Huntington's disease not only displayed reduced agonist facilitation, but impaired antagonist inhibition as well. Moreover, more impairment of antagonist inhibition correlated significantly with more severe akinesia and bradykinesia.

CONCLUSIONS

Antagonist inhibition prior to and during agonist contractions is markedly impaired in Huntington's disease. This impairment might contribute to motor slowness in these patients.

Effect of an attentional strategy on movement-related potentials recorded from subjects with Huntington's disease

Huntington's disease patients perform automatic movements in a bradykinetic manner, somewhat similar to patients with Parkinson's disease. Cortical activity relating to the preparation of movement in Parkinson's disease is significantly improved when a cognitive strategy is used. It is unknown whether patients with Huntington's disease can utilise an attentional strategy, and what effect this strategy would have on the premovement cortical activity. Movement-related potentials were recorded from 12 Huntington's disease patients and controls performing externally cued finger tapping movement, allowing an examination of cortical activity related to movement performance and bradykinesia in this disease. All subjects were tested in two conditions, which differed only by the presence or absence of the cognitive strategy. The Huntington's disease group, unlike controls, did not produce a rising premovement potential in the absence of the strategy. The Huntington's disease group did produce a rising premovement potential for the strategy condition, but the early slope of the potential was significantly reduced compared with the control group's early slope. These results are similar to those found previously with Parkinson's disease patients. The strategy may have put the task, which previously might have been under deficient automatic control, under attentional control.

The prolonged cortical silent period in patients with Huntington's disease

OBJECTIVES

In a group of patients with Huntington's disease and age-matched controls, we studied the cortical silent period (SP) elicited by single transcranial magnetic stimulation (TMS) pulses.

METHODS

We measured the area of the pre-stimulus electromyographic (EMG) activity, the area of the motor evoked potentials (MEPs) and the duration of the SP induced by stimuli delivered at an intensity of 150% of motor threshold with a round coil placed over the vertex. We determined the cortical SP by sampling only the 5 traces containing the shortest SPs and by collecting 10 consecutive unselected traces without selecting trials.

RESULTS

Patients and controls had normal EMG background areas, and MEP latencies and areas. Whereas data measured from selected trials gave a normal duration of the SP (patients, 154+/-58 ms; controls, 166+/-22 ms), data from unselected trials yielded a significantly longer SP duration in patients than in controls (356+/-251 vs. 159+/-44 ms) and also a larger variance and range.

CONCLUSIONS

We conclude that in Huntington's disease, an abnormal cortical SP is best sought by collecting unselected consecutive traces. We suggest that the prolonged SP in HD originates from a dysfunction of the mechanisms controlling the restart of voluntary movement after TMS.