Circulating microaggregates during cardiac surgery precedes postoperative stroke.

Postoperative stroke and encephalopathy are potentially serious complications associated with coronary artery bypass grafting. In this case report a 78-year-old male patient receiving routine elective cardiac surgery presented with microaggregations in the sublingual microcirculation while on cardiopulmonary bypass that was undetected by routine intraoperative anticoagulation assessment. Microaggregates identified using video microscopy on his sublingual microcirculation during the procedure preceded a stroke postoperatively. Postoperative cerebral and carotid artery examination with computed tomography scanning revealed a left watershed cerebral infarct with carotid stenosis. This report presents intraoperative microcirculation-based evidence suggesting that observations of microaggregations, otherwise undetected by conventional anticoagulation assessment techniques, could serve as an early warning in elderly patients at high risk for postoperative cerebrovascular events.

Consensus paper: management of degenerative cerebellar disorders.

Treatment of motor symptoms of degenerative cerebellar ataxia remains difficult. Yet there are recent developments that are likely to lead to significant improvements in the future. Most desirable would be a causative treatment of the underlying cerebellar disease. This is currently available only for a very small subset of cerebellar ataxias with known metabolic dysfunction. However, increasing knowledge of the pathophysiology of hereditary ataxia should lead to an increasing number of medically sensible drug trials. In this paper, data from recent drug trials in patients with recessive and dominant cerebellar ataxias will be summarized. There is consensus that up to date, no medication has been proven effective. Aminopyridines and acetazolamide are the only exception, which are beneficial in patients with episodic ataxia type 2. Aminopyridines are also effective in a subset of patients presenting with downbeat nystagmus. As such, all authors agreed that the mainstays of treatment of degenerative cerebellar ataxia are currently physiotherapy, occupational therapy, and speech therapy. For many years, well-controlled rehabilitation studies in patients with cerebellar ataxia were lacking. Data of recently published studies show that coordinative training improves motor function in both adult and juvenile patients with cerebellar degeneration. Given the well-known contribution of the cerebellum to motor learning, possible mechanisms underlying improvement will be outlined. There is consensus that evidence-based guidelines for the physiotherapy of degenerative cerebellar ataxia need to be developed. Future developments in physiotherapeutical interventions will be discussed including application of non-invasive brain stimulation.

Magnetization transfer MRI demonstrates spinal cord abnormalities in adrenomyeloneuropathy.

BACKGROUND: In adrenomyeloneuropathy (AMN) conventional MRI detects only spinal cord atrophy in the late stages. OBJECTIVE: To apply a magnetization transfer-weighted (MTw) imaging to patients with AMN and AMN-like syndrome in order to visualize and quantitatively assess the pathology of white matter tracts in the cervical spinal cord. METHODS: MTw studies were conducted in nine men with AMN, eight symptomatic heterozygous women, and 10 age- and sex-matched controls and compared to the Expanded Disability Status Scale (EDSS) and quantitative tests of vibratory sense and postural sway. MTw data sets were obtained at the level of C1 to C3 using a three-dimensional gradient echo acquisition technique, these images were then standardized between subjects by using the in-slice CSF signal as a normalization reference, allowing a quantitative assessment of the MTw signal. RESULTS: In contrast to conventional MRI, MTw images showed signal hyperintensities in the lateral and dorsal columns of all patients. The MT signal quantified in the dorsal column showed significant differences between patients with AMN, X-linked adrenoleukodystrophy heterozygotes, and controls. MT hyperintensity in the dorsal column correlated with EDSS, vibratory sense, and postural sway. CONCLUSION: Magnetization transfer-weighted imaging is a sensitive modality for the visual and quantitative assessment of spinal cord pathology in adrenomyeloneuropathy, and is a potential tool for evaluation of new therapies.

How do strength, sensation, spasticity and joint individuation relate to the reaching deficits of people with chronic hemiparesis?

Hemiparetic subjects present with movement deficits including weakness, spasticity and an inability to isolate movement to one or a few joints. Voluntary attempts to move a single joint often result in excessive motion at adjacent joints. We investigated whether the inability to individuate joint movements is associated with deficits in functional reaching. Controls and hemiparetic subjects performed two different reaching movements and three individuated arm movements, all in the parasagittal plane. The reaching movements were a sagittal 'reach up' (shoulder flexion and elbow flexion) and 'reach out' (shoulder flexion and elbow extension). Joint individuation was assessed by getting each subject to perform an isolated flexion-extension movement at each of the shoulder, elbow and wrist joints. In addition, we measured strength, muscle tone and sensation using standard clinical instruments. Hemiparetic subjects showed varying degrees of impairment when performing reaching movements and individuated joint movements. Reaching impairments (hand path curvature, velocity) were worse in the reach out versus the reach up condition. Typical joint individuation abnormalities were excessive flexion of joints that should have been held fixed during movement of the instructed joint. Hemiparetic subjects tended to produce concurrent flexion motions of shoulder and elbow joints when attempting any movement, one explanation for why they were better at the 'reach up' than the 'reach out' task. Hierarchical regression analysis showed that impaired joint individuation explained most of the variance in the reach path curvature and end point error; strength explained most of the variance in reaching velocity. Sensation also contributed significantly, but spasticity and strength were not significant in the model. We conclude that the deficit in joint individuation reflects a fundamental motor control problem that largely explains some aspects of voluntary reaching deficits of hemiparetic subjects.

Cerebellar subjects show impaired coupling of reach and grasp movements.

We examined how cerebellar deficits in isolated reaching or grasping movements contribute to abnormalities in a combined reach and grasp movement, and whether people with cerebellar damage show abnormalities in the spatiotemporal relationships of reach and grasp movements. We studied subjects with cerebellar damage and matched controls as they performed a combined reach and grasp, an isolated reach, and an isolated grasp. These movements were performed under slow-accurate and fast speed conditions. Subjects were also tested for their ability to correctly estimate the target size based on visual information. We measured the three-dimensional position of the index finger, thumb and wrist joint during all tasks. Results showed that cerebellar subjects overestimated the target size to a greater extent than did controls. During movement testing, cerebellar subjects were impaired on isolated reach and isolated grasp. However, they did not worsen parameters of reach or grasp movements during the combined reach and grasp. Instead there were distinct deficits in the coupling of the reach and grasp movement. Compared with controls, cerebellar subjects showed abnormalities in the sequence of the reach and grasp movement and highly variable timing of peak grip aperture. In the slow-accurate condition, cerebellar subjects decomposed the reach and grasp movement into separate reach then grasp components, and produced multiple peaks in grip aperture. In the fast condition, cerebellar subjects did not decompose, produced a single peak grip aperture, and dropped the target more often. These results indicate that cerebellar damage can cause a specific breakdown in the coupling of reach and grasp movements. The cerebellum may be involved in combining reach and grasp movements into a single motor program.

Blood flow responses to deep brain stimulation of thalamus.

BACKGROUND AND OBJECTIVE: Deep brain stimulation (DBS) of the ventral intermediate nucleus of the thalamus (VIM) provides remarkable relief of tremor in the limbs contralateral to the side of the brain stimulated. The benefits have been sufficiently dramatic that this is now an accepted clinical treatment of essential as well as other forms of tremor. Despite this clinical benefit, the mechanism of action of DBS remains unknown. In this investigation, we sought to determine the effects of VIM DBS on neuronal function. METHODS: The authors used PET measurements of qualitative regional cerebral blood flow in patients with essential tremor to determine the effects of DBS in the left VIM. Each subject had four to six scans with the arms at rest and DBS turned either on or off during alternate scans. Continuous physiologic monitoring revealed no tremor during any of the scans. The PET images from each subject were aligned, averaged, and coregistered to a standard image oriented in stereotactic space. RESULTS: The authors used subtraction image analysis with statistical parametric mapping methods and a restricted volume search to identify a significantly increased flow response at the site of stimulation in thalamus. An exploratory analysis revealed increased flow in ipsilateral supplementary motor area, a region that receives afferents from VIM. CONCLUSIONS: The increased blood flow at terminal fields of thalamocortical projections suggests that DBS stimulates and does not inactivate projection neurons in VIM thalamus.

Thalamic stimulation reduces essential tremor but not the delayed antagonist muscle timing.

BACKGROUND: Electrical stimulation of the thalamus dramatically reduces essential tremor (ET). It has been hypothesized that the cerebellum and inferior olive are involved in the generation of ET, and thalamic stimulation is presumed to dampen ET through interactions with cerebellar output to the thalamus. Evidence suggests that abnormal timing of agonist and antagonist muscle responses contribute to cerebellar tremor (CbT); however, this relationship has not been investigated for ET. The mechanisms of the tremor and improvement are unknown. OBJECTIVE: To measure the effect of ventral intermediate thalamic stimulation in controlling the ET response to sudden stretch of an agonist muscle and to determine whether, in ET, the timing relationships between the initial agonist and antagonist electromyography (EMG) responses show abnormalities similar to those seen in CbT. METHODS: The authors studied ET subjects (with implanted thalamic stimulators turned off and on) and normal controls as they responded to mechanical torque pulses given at the wrist joint. The wrist joint angle, wrist agonist, and antagonist EMG were recorded. RESULTS: Like CbT, patients with ET showed delayed onsets of antagonist EMG and excessive rebound. Thalamic stimulation reduced the tremor but did not alter the antagonist delay or the rebound. CONCLUSIONS: In ET, antagonist muscle responses to a torque pulse are similar to that in CbT. However, benefit from thalamic stimulation did not alter these EMG responses; therefore, suppression of tremor must be caused by mechanisms other than the re-establishment of normal agonist-antagonist timing.

Additional somatosensory information does not improve cerebellar adaptation during catching.

OBJECTIVE: People with cerebellar damage are impaired in their ability to adapt anticipatory muscle activity during catching. We asked whether prior or on-line information about ball weight and drop height could improve the impaired adaptation of people with cerebellar damage. METHODS: Cerebellar and control subjects caught a series of balls of different weights under two conditions. The first condition provided subjects with information about ball weight prior to the series of trials. The second condition provided subjects with information about ball weight, drop height, and time of ball release during the series of trials. Subjects had to maintain their hand within a vertical spatial 'window' during the catch. We measured 3-dimensional position and electromyography (EMG) from the catching arm. RESULTS: With prior information, controls required a few trials to adapt to a new ball weight. Cerebellar subjects were slow, or unable, to adapt. With on-line information, controls were able to catch the ball within the window immediately, showing that they did not require practice to make this adjustment. Cerebellar subjects remained slow or unable to adapt to the changed ball weight even with on-line information. CONCLUSIONS: These results suggest that other, intact central nervous system structures cannot compensate for the role of the cerebellum in generating and adjusting anticipatory muscle activity across multiple joints.

Selection and coordination of human locomotor forms following cerebellar damage.

We have previously shown that control subjects use two distinct temporal strategies when stepping on an inclined surface during walking: one for level and 10 degrees surfaces and another for 20 and 30 degrees surfaces. These two temporal strategies were characterized by systematic shifts in the timing of muscle activity and peak joint angles. We examined whether cerebellar subjects with mild to moderate gait ataxia were impaired in their ability to select these two temporal strategies, adjust peak joint angle amplitudes, and/or adjust one joint appropriately with respect to movements and constraints at another joint. Subjects walked on a level surface and on different wedges (10, 20, and 30 degrees ) presented in the context of level walking. In a single trial, a subject walked on a level surface in approach to a wedge, took a single step on the wedge, and continued walking on an elevated level surface beyond the wedge. Cerebellar subjects used two temporal strategies, one for the level and 10 degrees surfaces and another for 20 and 30 degrees surfaces. Cerebellar strategies were similar to those used by controls except for the timing of ankle-joint movement on the steeper wedges. Cerebellar subjects adjusted the peak amplitudes of individual joint angles normally, with the exception of peak ankle plantarflexion. However, they exhibited greater trial-to-trial variability of peak hip and knee joint angles that increased as a function of wedge inclination. The most substantial deficit noted in the cerebellar group was in the relative movement of multiple joints. Cerebellar subjects demonstrated multijoint coordination deficits in all conditions, although these deficits were most pronounced during stance on the steeper wedges. On the 30 degrees wedge, cerebellar subjects showed abnormal relative movement of hip, knee, and ankle joints and the most substantial decomposition of movement. We speculate that to simplify multijoint control, cerebellar subjects decomposed their movement by fixing the ankle joint in a dorsiflexed position on the steepest wedges. Our results suggest that the cerebellum may not be critical in selecting the basic motor patterns for the two temporal strategies because cerebellar subjects produced appropriate timing shifts at most joints. Instead, our data suggest that the cerebellum is most critical for adjusting the relative movement of multiple joints, especially to accommodate external constraints.

Inter- and intra-limb generalization of adaptation during catching.

We have previously shown that healthy adults require a few trials to adapt to a changed ball weight during catching. It is not known whether this adaptation generalizes to the opposite arm or to different configurations of the same arm. We tested healthy adult subjects catching balls of different weight while maintaining the hand within a vertical spatial "window." In experiment 1, subjects caught a series of light and heavy balls, first with one hand and then with the other. In experiment 2, subjects caught a series of light and heavy balls, first with the catching arm in either a "bent" or a "straight" configuration and then with the same arm in the other configuration. A percentage transfer value was calculated to determine the degree to which previous experience with a given ball weight in one context affected performance of the same task in a new context (i.e., different arm or different arm configuration). Results showed that generalization occurred both between arms and within an arm. However, the subjects who switched arms showed less generalization than those who switched arm positions. Specifically, the percentage transfer value for subjects who switched arms was 58%, while the percentage transfer for those who switched arm positions was 100%. These results support the idea that the motor system is able to generalize adaptive control of ball catching to the contralateral arm and to different arm configurations. Our findings are also in agreement with the recent notion that multiple internal representations of a task may exist in the CNS. Because there was partial generalization between the two arms, we conclude that there must be a representation stored and used for catching that is not effector specific, but rather can be utilized by brain regions controlling either arm. However, because generalization was only complete within an arm, we conclude that another sensorimotor representation exists, which might only be stored in brain regions specific to a single arm.

Form switching during human locomotion: traversing wedges in a single step.

We examined the neural control strategies used to accommodate discrete alterations in walking surface inclination. Normal subjects were tested walking on a level surface and on different wedges (10 degrees, 15 degrees, 20 degrees, and 30 degrees ) presented in the context of level walking. On a given trial, a subject walked on a level surface in approach to a wedge, took a single step on the wedge, and continued walking on an elevated level surface beyond the wedge. As wedge inclination increased, subjects linearly increased peak joint angles. Changes in timing of peak joint angles and electromyograms were not linear. Subjects used two distinct temporal strategies, or forms, to traverse the wedges. One form was used for walking on a level surface and on the 10 degrees wedge, another form for walking on the 20 degrees and 30 degrees wedges. In the level/10 degrees form, peak hip flexion occurred well before heel strike (HS) and peak dorsiflexion occurred in late stance. In the 20 degrees /30 degrees form, peak hip flexion was delayed by 12% of the stride cycle and peak dorsiflexion was reached 12% earlier. For the level/10 degrees form, onsets of the rectus femoris, gluteus maximus, and vastus lateralis muscles were well before HS and offset of the anterior tibialis was at HS. For the 20 degrees /30 degrees form, onsets of the rectus femoris, gluteus maximus, and vastus lateralis and offset of the anterior tibialis were all delayed by 12% of the stride cycle. Muscles shifted as a group, rather than individually, between the forms. Subjects traversing a 15 degrees wedge switched back and forth between the two forms in consecutive trials, suggesting the presence of a transition zone. Differences between the forms can be explained by the differing biomechanical constraints imposed by the wedges. Steeper wedges necessitate changes in limb orientation to accommodate the surface, altering limb orientation with respect to gravity and making it necessary to pull the body forward over the foot. The use of different forms of behavior is a common theme in neural control and represents an efficient means of coordinating and adapting movement to meet changing environmental demands. The forms of locomotion reported here are likely used on a regular basis in real-world settings.

Motor benefit from levodopa in spastic quadriplegic cerebral palsy.

We report on a 16-year-old girl with spastic quadriplegic cerebral palsy associated with premature birth and typical periventricular leukomalacia, who had a dramatic improvement in motor function after treatment with carbidopa/levodopa. Kinematic and electromyographic analyses of reaching movements demonstrate that levodopa decreased muscle co-contraction, decreased unwanted movements, and improved her ability to maintain a steady arm posture. These findings suggest that levodopa be considered as an adjunct therapy for the treatment of spastic quadriplegic cerebral palsy.

Cerebellar ataxia: torque deficiency or torque mismatch between joints?

Prior work has shown that cerebellar subjects have difficulty adjusting for interaction torques that occur during multi-jointed movements. The purpose of this study was to determine whether this deficit is due to a general inability to generate sufficient levels of phasic torque inability or due to an inability to generate muscle torques that predict and compensate for interaction torques. A second purpose was to determine whether reducing the number of moving joints by external mechanical fixation could improve cerebellar subjects' targeted limb movements. We studied control and cerebellar subjects making elbow flexion movements to touch a target under two conditions: 1) a shoulder free condition, which required only elbow flexion, although the shoulder joint was unconstrained and 2) a shoulder fixed condition, where the shoulder joint was mechanically stabilized so it could not move. We measured joint positions of the arm in the sagittal plane and electromyograms (EMGs) of shoulder and elbow muscles. Elbow and shoulder torques were estimated using inverse dynamics equations. In the shoulder free condition, cerebellar subjects made greater endpoint errors (primarily overshoots) than did controls. Cerebellar subjects' overshoot errors were largely due to unwanted flexion at the shoulder. The excessive shoulder flexion resulted from a torque mismatch, where larger shoulder muscle torques were produced at higher rates than would be appropriate for a given elbow movement. In the shoulder fixed condition, endpoint errors of cerebellar subjects and controls were comparable. The improved accuracy of cerebellar subjects was accompanied by reduced shoulder flexor muscle activity. Most of the correct cerebellar trials in the shoulder fixed condition were movements made using only muscles that flex the elbow. Our findings suggest that cerebellar subjects' poor shoulder control is due to an inability to generate muscle torques that predict and compensate for interaction torques, and not due to a general inability to generate sufficient levels of phasic torque. In addition, reducing the number of muscles to be controlled improved cerebellar ataxia.

Mirror movements complicate interpretation of cerebral activation changes during recovery from subcortical infarction.

In recovered stroke patients, performance of motor tasks with the affected limb has been reported to activate cortical areas ipsilateral to the affected side. The better to determine the causal role these areas play in recovery of motor function, we assessed cerebral activation during motor activity longitudinally after hemiparesis due to cerebral infarction. A secondary goal was to ascertain the relation between mirror movements and activation ipsilateral to motor activity. Positron emission tomography with oxygen-15 water measured regional cerebral blood flow during wrist movement early and late in the course of recovery from hemiparesis. Surface electromyography recorded muscular activity, and computer-assisted video analysis quantified movement during the scans. Mirror movements, movements contralateral to the instructed movement of the hemiparetic arm, were often seen. Activation of motor areas in the hemisphere ipsilateral to the affected limb roughly correlated with presence of mirror movements. Other changes in cerebral activation were small, when the task was controlled for rate, but high-rate-specific recruitment of ipsilateral cortical areas occurred in one case. However, the common occurrence of mirror movements, particularly with effortful tasks, complicates interpretation of data regarding the role of the ipsilateral hemisphere in recovery.

Cerebellar subjects show impaired adaptation of anticipatory EMG during catching.

We evaluated the role of the cerebellum in adapting anticipatory muscle activity during a multijointed catching task. Individuals with and without cerebellar damage caught a series of balls of different weights dropped from above. In Experiment 1 (light-heavy-light), each subject was required to catch light balls (baseline phase), heavy balls (adaptation phase), and then light balls again (postadaptation phase). Subjects were not told when the balls would be switched, and they were required to keep their hand within a vertical spatial "window" during the catch. During the series of trials, we measured three-dimensional (3-D) position and electromyogram (EMG) from the catching arm. We modeled the adaptation process using an exponential decay function; this model allowed us to dissociate adaptation from performance variability. Results from the position data show that cerebellar subjects did not adapt or adapted very slowly to the changed ball weight when compared with the control subjects. The cerebellar group required an average of 30.9 +/- 8.7 trials (mean +/- SE) to progress approximately two-thirds of the way through the adaptation compared with 1.7 +/- 0.2 trials for the control group. Only control subjects showed a negative aftereffect indicating storage of the adaptation. No difference in performance variability existed between the two groups. EMG data show that control subjects increased their anticipatory muscle activity in the flexor muscles of the arm to control the momentum of the ball at impact. Cerebellar subjects were unable to differentially increase the anticipatory muscle activity across three joints to perform the task successfully. In Experiment 2 (heavy-light-heavy), we tested to see whether the rate of adaptation changed when adapting to a light ball versus a heavy ball. Subjects caught the heavy balls (baseline phase), the light balls (adaptation phase), and then heavy balls again (postadaptation phase). Comparison of rates of adaptation between Experiment 1 and Experiment 2 showed that the rate of adaptation was unchanged whether adapting to a light ball or a heavy ball. Given these findings, we conclude that the cerebellum is important in generating the appropriate anticipatory muscle activity across multiple muscles and modifying it in response to changing demands though trial-and-error practice.

Posterior vermal split syndrome.

We have studied a battery of movements in 5 children (age, 6-15 years) after transection of the posterior inferior cerebellar vermis. In each case, the surgery destroyed the midline vermis only (ranging from lobules VI-X). Tandem gait was badly impaired in all subjects. No subjects had impairments of kicking, reaching, pinching, or speech. Regular gait, standing, and hopping on one leg were relatively unimpaired. Cutting the parallel fibers that cross the midline may be the critical variable causing incoordination in tandem gait.

Mechanisms of ataxia.

Ataxia, or incoordination of movement, is a disorder that can be caused by damage to several different nervous system structures. Common causes of ataxia include damage of the cerebellum and damage of sensory structures. Sensory ataxia is distinguishable from cerebellar ataxia, because the sensory ataxia causes symptoms to worsen when movements are made with the eyes closed. The basic mechanism underlying ataxia is not yet understood, although studies indicate that ataxia may be due in part to an inability to coordinate the relative activity of multiple muscles and adjust movements at a given joint for the effects of other moving joints (interaction torques). Based on these findings, it could be reasoned that treatments focusing on strategies to reduce the complexity of a movement by minimizing the number of moving joints or by stabilizing against the inertial effects of limb movement will improve function. 2,12-14,21-23 Further testing of treatments for ataxia, however, is needed. Ataxia may be best treated by teaching people to avoid rapid multijoint movements and instead make slower movements limited to single joints.

Throwing while looking through prisms. I. Focal olivocerebellar lesions impair adaptation.

Normal human subjects and patients with lesions of the olivocerebellar system threw balls of clay at a visual target while wearing wedge prism spectacles. Normal subjects initially threw in the direction of prism-bent gaze, but with repeated throws adapted to hit the target. Patients with generalized cerebellar atrophy, inferior olive hypertrophy, or focal infarcts in the distribution of the posterior inferior cerebellar artery, in the ipsilateral inferior peduncle, in the contralateral basal pons or in the ipsilateral middle cerebellar peduncle had impaired or absent prism adaptation. Patients with infarcts in the distribution of the posterior inferior cerebellar artery usually had impaired or absent adaptation but little or no ataxia. By contrast, patients with damage in the distribution of the superior cerebellar artery or in cerebellar thalamus usually had ataxia but preserved adaptation. These results implicate climbing fibres from the contralateral inferior olive via the ipsilateral inferior cerebellar peduncle, mossy fibres from the contralateral pontocerebellar nuclei via the ipsilateral middle cerebellar peduncle, and posterior inferior cerebellar artery territory cortex as being critical for this adaptation. The dentatothalamic projection and the superior cerebellar artery territory cortex are not necessary for this adaptation.

Throwing while looking through prisms. II. Specificity and storage of multiple gaze-throw calibrations.

Human subjects threw balls of clay at a visual target while looking through wedge prism spectacles. In studies of short-term adjustment, subjects threw in the direction of their prism-bent gaze, missing the target to that side. Within 10-30 throws, they gradually adapted with a wider gaze-throw angle and hit the target. Immediately after removal of the prisms the wide gaze-throw angle persisted and throws missed the target to the opposite side, the so-called 'negative after effect'. Repeated throws were required to adapt back to the normal gaze-throw angle and hit the target. The adaptation was specific both to the body parts trained and the type of throw trained: training with the right hand did not generalize to throwing with the left; overhand training seldom generalized to underhand throwing. In a study of long-term adjustment, two subjects threw with the same hand (right) and the same type of throw (overhand) alternately, with and without prisms, over a period of 6 weeks. They gradually learned to hit the target on the first throw, with and without prisms. The two gaze-throw calibrations (prism and no-prism) were retained for > 27 months. The long-term adjustment was shown to consist of a coordinated relationship of eye-in-head, head-on-trunk and trunk-on-arm angles.

Cerebellar ataxia: abnormal control of interaction torques across multiple joints.

1. We studied seven subjects with cerebellar lesions and seven control subjects as they made reaching movements in the sagittal plane to a target directly in front of them. Reaches were made under three different conditions: 1) "slow-accurate," 2) "fast-accurate," and 3) "fast as possible." All subjects were videotaped moving in a sagittal plane with markers on the index finger, wrist, elbow, and shoulder. Marker positions were digitized and then used to calculate joint angles. For each of the shoulder, elbow and wrist joints, inverse dynamics equations based on a three-segment limb model were used to estimate the net torque (sum of components) and each of the component torques. The component torques consisted of the torque due to gravity, the dynamic interaction torques induced passively by the movement of the adjacent joint, and the torque produced by the muscles and passive tissue elements (sometimes called "residual" torque). 2. A kinematic analysis of the movement trajectory and the change in joint angles showed that the reaches of subjects with cerebellar lesions were abnormal compared with reaches of control subjects. In both the slow-accurate and fast-accurate conditions the cerebellar subjects made abnormally curved wrist paths; the curvature was greater in the slow-accurate condition. During the slow-accurate condition, cerebellar subjects showed target undershoot and tended to move one joint at a time (decomposition). During the fast-accurate reaches, the cerebellar subjects showed target overshoot. Additionally, in the fast-accurate condition, cerebellar subjects moved the joints at abnormal rates relative to one another, but the movements were less decomposed. Only three subjects were tested in the fast as possible condition; this condition was analyzed only to determine maximal reaching speeds of subjects with cerebellar lesions. Cerebellar subjects moved more slowly than controls in all three conditions. 3. A kinetic analysis of torques generated at each joint during the slow-accurate reaches and the fast-accurate reaches revealed that subjects with cerebellar lesions produced very different torque profiles compared with control subjects. In the slow-accurate condition, the cerebellar subjects produced abnormal elbow muscle torques that prevented the normal elbow extension early in the reach. In the fast-accurate condition, the cerebellar subjects produced inappropriate levels of shoulder muscle torque and also produced elbow muscle torques that did not very appropriately with the dynamic interaction torques that occurred at the elbow. Lack of appropriate muscle torque resulted in excessive contributions of the dynamic interaction torque during the fast-accurate reaches. 4. The inability to produce muscle torques that predict, accommodate, and compensate for the dynamic interaction torques appears to be an important cause of the classic kinematic deficits shown by cerebellar subjects during attempted reaching. These kinematic deficits include incoordination of the shoulder and the elbow joints, a curved trajectory, and overshoot. In the fast-accurate condition, cerebellar subjects often made inappropriate muscle torques relative to the dynamic interaction torques. Because of this, interaction torques often determined the pattern of incoordination of the elbow and shoulder that produced the curved trajectory and target overshoot. In the slow-accurate condition, we reason that the cerebellar subjects may use a decomposition strategy so as to simplify the movement and not have to control both joints simultaneously. From these results, we suggest that a major role of the cerebellum is in generating muscle torques at a joint that will predict the interaction torques being generated by other moving joints and compensate for them as they occur.