Vivian Weerdesteyn

Are postural responses to backward and forward perturbations processed by different neural circuits

Startle pathways may contribute to rapid accomplishment of postural stability. Here we investigate the possible influence of a startling auditory stimulus (SAS) on postural responses. We formulated four specific questions: (1) can a concurrent SAS shorten the onset of automatic postural responses?; and if so (2) is this effect different for forward versus backward perturbations?; (3) does this effect depend on prior knowledge of the perturbation direction?; and (4) is this effect different for low- and high-magnitude perturbations? Balance was perturbed in 11 healthy participants by a movable platform that suddenly translated forward or backward. Each participant received 160 perturbations, 25% of which were combined with a SAS. We varied the direction and magnitude of the perturbations, as well as the prior knowledge of perturbation direction. Perturbation trials were interspersed with SAS-only trials. The SAS accelerated and strengthened postural responses with clear functional benefits (better balance control), but this was only true for responses that protected against falling backwards (i.e. in tibialis anterior and rectus femoris). These muscles also demonstrated the most common SAS-triggered responses without perturbation. Increasing the perturbation magnitude accelerated postural responses, but again with a larger acceleration for backward perturbations. We conclude that postural responses to backward and forward perturbations may be processed by different neural circuits, with influence of startle pathways on postural responses to backward perturbations. These findings give directions for future studies investigating whether deficits in startle pathways may explain the prominent backward instability seen in patients with Parkinsons disease and progressive supranuclear palsy.

Evidence for bilaterally delayed and decreased obstacle avoidance responses while walking with a lower limb prosthesis

OBJECTIVE To examine whether the increased failure rates in obstacle avoidance of patients with lower limb amputation can be understood on the basis of increased delay and/or decreased amplitudes of obstacle avoidance responses. METHODS Subjects performed obstacle avoidance on a treadmill while EMG recordings were made of several major muscles of the leg. RESULTS It was found that subjects with a lower limb amputation have delayed responses (e.g. delays of 20 ms for the Biceps Femoris) and have decreased response amplitudes (36-41% smaller). Furthermore, such changes were observed not only on the prosthetic side, but also on the sound side. The decreased amplitudes were associated with increased failure rates in the obstacle avoidance task. CONCLUSIONS It is concluded that the bilaterally delayed and reduced responses in persons with a lower limb prosthesis reflect a basic reorganization within the central nervous system aimed at providing synchronized activity in both lower limbs, even though the peripheral deficit involves only one limb. SIGNIFICANCE The present results on obstacle avoidance responses can be used to evaluate future prosthetic training involving obstacle crossings for amputee rehabilitation.

What startles tell us about control of posture and gait

Recently, there has been an increase in studies evaluating startle reflexes and StartReact, many in tasks involving postural control and gait. These studies have provided important new insights. First, several experiments indicate a superimposition of startle reflex activity on the postural response during unexpected balance perturbations. Overlap in the expression of startle reflexes and postural responses emphasizes the possibility of, at least partly, a common substrate for these two types of behavior. Second, it is recognized that the range of behaviors, susceptible to StartReact, has expanded considerably. Originally this work was concentrated on simple voluntary ballistic movements, but gait initiation, online step adjustments and postural responses can be initiated earlier by a startling stimulus as well, indicating advanced motor preparation of posture and gait. Third, recent experiments on StartReact using TMS and patients with corticospinal lesions suggest that this motor preparation involves a close interaction between cortical and subcortical structures. In this review, we provide a comprehensive overview on startle reflexes, StartReact, and their interaction with posture and gait.

Subcortical Structures in Humans Can Be Facilitated by Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that alters cortical excitability. Interestingly, in recent animal studies facilitatory effects of tDCS have also been observed on subcortical structures. Here, we sought to provide evidence for the potential of tDCS to facilitate subcortical structures in humans as well. Subjects received anodal-tDCS and sham-tDCS on two separate testing days in a counterbalanced order. After stimulation, we assessed the effect of tDCS on two responses that arise from subcortical structures; (1) wrist and ankle responses to an imperative stimulus combined with a startling acoustic stimulus (SAS), and (2) automatic postural responses to external balance perturbations with and without a concurrent SAS. During all tasks, response onsets were significantly faster following anodal-tDCS compared to sham-tDCS, both in trials with and without a SAS. The effect of tDCS was similar for the dominant and non-dominant leg. The SAS accelerated the onsets of ankle and wrist movements and the responses to backward, but not forward perturbations. The faster onsets of SAS-induced wrist and ankle movements and automatic postural responses following stimulation provide strong evidence that, in humans, subcortical structures - in particular the reticular formation - can be facilitated by tDCS. This effect may be explained by two mechanisms that are not mutually exclusive. First, subcortical facilitation may have resulted from enhanced cortico-reticular drive. Second, the applied current may have directly stimulated the reticular formation. Strengthening reticulospinal output by tDCS may be of interest to neurorehabilitation, as there is evidence for reticulospinal compensation after corticospinal lesions.

Postural inflexibility in PD: does it affect compensatory stepping?

Parkinsons disease (PD) impairs the ability to shape postural responses to contextual factors. It is unknown whether such inflexibility pertains to compensatory steps to overcome balance perturbations. Participants were instructed to recover balance in response to a platform translation. A step was necessary to recover balance when the translation was large, whereas a feet-in-place (FiP) response was sufficient when the translation was small (i.e. no step). We compared step trials that required a switch away from the current postural set (switch trials: step trials that were preceded by FiP trials) with non-switch trials (i.e. step trials were preceded by identical step trials). 51 PD patients (59 ± 7 years) were compared with 22 healthy controls (60 ± 6 years). In a second analysis, we compared a subgroup of 14 freezers (PD-FOG) with a subgroup of 14 non-freezers (PD-noFOG; matched for age, gender and disease severity). Compared to non-switch trials, switch trials resulted in poorer step execution and more steps needed to recover balance. These switching effects were similar in PD patients and controls, and in PD-FOG and PD-noFOG patients. Overall, PD patients demonstrated poorer stepping performance than controls. PD-FOG had a worse performance than PD-noFOG. Moreover, PD patients, and particularly PD-FOG patients, were less able to improve step performance with repetitive step trials, in contrast to controls. Thus, there was no PD-related deficit to switch to an alternative response strategy, neither in patients with FOG nor in patients without FOG. Difficulty to adapt the step trial-by-trial might have contributed to the absence of switch deficits in PD.

Unraveling the mechanisms underlying postural instability in Parkinson’s disease using dynamic posturography

Postural instability, one of the cardinal symptoms of Parkinson’s disease (PD), has devastating consequences for affected patients. Better strategies to prevent falls are needed, but this calls for an improved understanding of the complex mechanisms underlying postural instability. We must also improve our ability to timely identify patients at risk of falling. Dynamic posturography is a promising avenue to achieve these goals. The latest moveable platforms can deliver ‘real-life’ balance perturbations, permitting study of everyday fall circumstances. Dynamic posturography studies have shown that PD patients have fundamental problems in scaling their postural responses in accordance with the need of the actual balance task at hand. On-going studies evaluate the predictive ability of impaired posturography performance for daily life falls. We also review recent work aimed at exploring balance correcting steps in PD, and the presumed interaction between startle pathways and postural responses.

Gait adaptability training improves obstacle avoidance and dynamic stability in patients with cerebellar degeneration

Balance and gait problems in patients with cerebellar degeneration lead to reduced mobility, loss of independence, and frequent falls. It is currently unclear, however, whether balance and gait capacities can be improved by training in this group of patients. Therefore, the aim of this study was to examine the effects of gait adaptability training on obstacle avoidance and dynamic stability during adaptive gait. Ten patients with degenerative cerebellar ataxia received 10 protocolized gait adaptability training sessions of 1 h each during 5 weeks. Training was performed on a treadmill with visual stepping targets and obstacles projected on the belts surface. As the primary outcome, we used an obstacle avoidance task while walking on a treadmill. We determined avoidance success rates, as well as dynamic stability during the avoidance manoeuvre. Clinical ratings included the scale for the assessment of ataxia (SARA), 10 m walking test, timed up-and-go test, berg balance scale, and the obstacle subtask of the emory functional ambulation profile (EFAP). Following the intervention, success rates on the obstacle avoidance task had significantly improved compared to pre-intervention. For successful avoidance, participants allowed themselves smaller stability margins in the sagittal plane in the (shortened) pre-crossing step. However, in the subsequent steps they returned to baseline stability values more effectively than before training. SARA scores and the EFAP obstacle subtask improved significantly as well. This pilot study provides preliminary evidence of a beneficial effect of gait adaptability training on obstacle avoidance capacity and dynamic stability in patients with cerebellar degeneration.

Falls in older persons with intellectual disabilities: fall rate, circumstances and consequences

BACKGROUND Falling is a common cause of injuries and reduced quality of life. Persons with intellectual disabilities (ID) are at increased risk for falls and related injuries. As the number of elderly persons with ID is growing rapidly, it is imperative to gain insight into the quantity of the problem of falling, the circumstances that precipitate falls and to better understand their aetiology in persons with ID. This is the first study to prospectively investigate fall rate, circumstances and fall consequences in older adults with mild to moderate ID. METHOD Eighty-two individuals with mild to moderate ID, 50 years and over [mean age 62.3 (SD = 7.6), 34 male], participated in this study, which was conducted at three service providers for persons with ID in the Netherlands. Falls were registered for 1 year with monthly fall registration calendars to determine the fall rate (mean number of falls per person per year). Information on fall circumstances and consequences was obtained from questionnaires completed by caregivers and study participants after each fall. RESULTS We determined that the fall rate in this sample was 1.00 fall per person per year. Thirty-seven participants reported at least one fall (range 1-6). Sex and age were not related to falls. Most falls occurred while walking (63.3%), outside (61.7%) and in familiar environments (88.9%). Importantly, 11.5% of falls resulted in severe injuries, approximately half of which were fractures. CONCLUSION The circumstances and consequences of falls in persons with ID are comparable to those of the general elderly population, but the rate is substantially higher. As such, appropriate fall prevention strategies must be developed for individuals with ID.

A Gait Paradigm Reveals Different Patterns of Abnormal Cerebellar Motor Learning in Primary Focal Dystonias

Accumulating evidence points to a role of the cerebellum in the pathophysiology of primary dystonia. The aim of this study was to investigate whether the abnormalities of cerebellar motor learning in primary dystonia are solely detectable in more pure forms of cerebellum-dependent associative motor learning paradigms, or whether these are also present in other motor learning paradigms that rely heavily on the cerebellum but in addition require a more widespread sensorimotor network. Twenty-six patients with various forms of focal dystonia and 10 age-matched healthy controls participated in a motor learning paradigm on a split-belt treadmill. By using reflective markers, three-dimensional kinematics were recorded using a 6-camera motion analysis system. Adaptation walking parameters were analyzed offline, comparing the different dystonia groups and healthy controls. Patients with blepharospasm and writer’s cramp were significantly impaired on various adaptation walking parameters. Whereas results of cervical dystonia patients did not differ from healthy controls in terms of adaptation walking parameters, differences in parameters of normal gait were found. We have here demonstrated abnormal sensorimotor adaptation with the split-belt paradigm in patients with blepharospasm and writer’s cramp. This reinforces the current concept of cerebellar dysfunction in primary dystonia, and that this extends beyond more pure forms of cerebellum-dependent associative motor learning paradigms. However, the finding of normal adaptation in cervical dystonia patients indicates that the pattern of cerebellar dysfunction may be slightly different for the various forms of primary focal dystonia, suggesting that actual cerebellar pathology may not be a primary driving force in dystonia.

StartReact effects support different pathophysiological mechanisms underlying freezing of gait and postural instability in Parkinson's disease.

Introduction The pathophysiology underlying postural instability in Parkinson’s disease is poorly understood. The frequent co-existence with freezing of gait raises the possibility of shared pathophysiology. There is evidence that dysfunction of brainstem structures contribute to freezing of gait. Here, we evaluated whether dysfunction of these structures contributes to postural instability as well. Brainstem function was assessed by studying the StartReact effect (acceleration of latencies by a startling acoustic stimulus (SAS)). Methods We included 25 patients, divided in two different ways: 1) those with postural instability (HY = 3, n = 11) versus those without (HY<3, n = 14); and 2) those with freezing (n = 11) versus those without freezing (n = 14). We also tested 15 matched healthy controls. We tested postural responses by translating a balance platform in the forward direction, resulting in backward balance perturbations. In 25% of trials, the start of the balance perturbation was accompanied by a SAS. Results The amplitude of automatic postural responses and length of the first balance correcting step were smaller in patients with postural instability compared to patients without postural instability, but did not differ between freezers and non-freezers. In contrast, the StartReact effect was intact in patients with postural instability but was attenuated in freezers. Discussion We suggest that the mechanisms underlying freezing of gait and postural instability in Parkinson’s disease are at least partly different. Underscaling of automatic postural responses and balance-correcting steps both contribute to postural instability. The attenuated StartReact effect was seen only in freezers and likely reflects inadequate representation of motor programs at upper brainstem level.