Abnormal movement-related suppression of sensory evoked potentials in upper limb dystonia.

BACKGROUND: Gating of sensory evoked potentials (SEPs) around the onset of a voluntary movement is a physiological phenomenon with centripetal and central components, and may reflect sensorimotor integration required for normal movement control. OBJECTIVE: Our objective was the investigation of SEP suppression at the onset of movement and the interaction between SEP suppression and vibration of the limb. METHODS: Fourteen patients with primary focal/segmental dystonia and 17 age-matched healthy volunteers were studied. SEPs were elicited after electrical stimulation of the median nerve at the wrist. Electroencephalograms (EEGs) were recorded over the scalp at three sites according to the International 10-20 System (F3, C3 and P3). SEPs were recorded in four conditions: at rest, at the onset of movement (a self-paced abduction movement of the right thumb), both in the absence and in the presence of vibration of the limb. RESULTS: Repeated measures anova revealed that there was a significant main effect of group [F(1, 11.1) = 0.471, P = 0.002]. Post hoc exploration of this effect revealed it to be due to an absence of SEP suppression at movement onset in patients (mean ratio SEP movement onset/rest 1.15 at F3, 1.13 at C3, 1.01 at P3) compared to controls, who had SEP suppression at movement onset (mean ratio SEP movement onset/rest 0.79 at F3, 0.78 at C3, 0.77 at P3). With vibration, SEP suppression reduced in both patients and controls to a similar extent. CONCLUSION: These results demonstrate abnormal SEP suppression at the onset of movement in patients with primary dystonia, and in addition that vibration of the limb reduces SEP suppression in patients and controls.

All in the blink of an eye: new insight into cerebellar and brainstem function in DYT1 and DYT6 dystonia.

BACKGROUND AND PURPOSE: Traditionally dystonia has been considered a disorder of basal ganglia dysfunction. However, recent research has advocated a more complex neuroanatomical network. In particular, there is increasing interest in the pathophysiological role of the cerebellum. Patients with cervical and focal hand dystonia have impaired cerebellar associative learning using the paradigm eyeblink conditioning. This is perhaps the most direct evidence to date that the cerebellum is implicated in patients. METHODS: Eleven patients with DYT1 dystonia and five patients with DYT6 dystonia were examined and rates of eyeblink conditioning were compared with age-matched controls. A marker of brainstem excitability, the blink reflex recovery, was also studied in the same groups. RESULTS: Patients with DYT1 and DYT6 dystonia have a normal ability to acquire conditioned responses. Blink reflex recovery was enhanced in DYT1 but this effect was not seen in DYT6. CONCLUSIONS: If the cerebellum is an important driver in DYT1 and DYT6 dystonia our data suggest that there is specific cerebellar dysfunction such that the circuits essential for conditioning function normally. Our data are contrary to observations in focal dystonia and suggest that the cerebellum may have a distinct role in different subsets of dystonia. Evidence of enhanced blink reflex recovery in all patients with dystonia was not found and recent studies calling for the blink recovery reflex to be used as a diagnostic test for dystonic tremor may require further corroboration.

Non-invasive cerebellar stimulation--a consensus paper.

The field of neurostimulation of the cerebellum either with transcranial magnetic stimulation (TMS; single pulse or repetitive (rTMS)) or transcranial direct current stimulation (tDCS; anodal or cathodal) is gaining popularity in the scientific community, in particular because these stimulation techniques are non-invasive and provide novel information on cerebellar functions. There is a consensus amongst the panel of experts that both TMS and tDCS can effectively influence cerebellar functions, not only in the motor domain, with effects on visually guided tracking tasks, motor surround inhibition, motor adaptation and learning, but also for the cognitive and affective operations handled by the cerebro-cerebellar circuits. Verbal working memory, semantic associations and predictive language processing are amongst these operations. Both TMS and tDCS modulate the connectivity between the cerebellum and the primary motor cortex, tuning cerebellar excitability. Cerebellar TMS is an effective and valuable method to evaluate the cerebello-thalamo-cortical loop functions and for the study of the pathophysiology of ataxia. In most circumstances, DCS induces a polarity-dependent site-specific modulation of cerebellar activity. Paired associative stimulation of the cerebello-dentato-thalamo-M1 pathway can induce bidirectional long-term spike-timing-dependent plasticity-like changes of corticospinal excitability. However, the panel of experts considers that several important issues still remain unresolved and require further research. In particular, the role of TMS in promoting cerebellar plasticity is not established. Moreover, the exact positioning of electrode stimulation and the duration of the after effects of tDCS remain unclear. Future studies are required to better define how DCS over particular regions of the cerebellum affects individual cerebellar symptoms, given the topographical organization of cerebellar symptoms. The long-term neural consequences of non-invasive cerebellar modulation are also unclear. Although there is an agreement that the clinical applications in cerebellar disorders are likely numerous, it is emphasized that rigorous large-scale clinical trials are missing. Further studies should be encouraged to better clarify the role of using non-invasive neurostimulation techniques over the cerebellum in motor, cognitive and psychiatric rehabilitation strategies.

Cerebellum-dependent associative learning deficits in primary dystonia are normalized by rTMS and practice.

Eyeblink classical conditioning (EBCC) is a cerebellum-dependent paradigm of associative motor learning, and abnormal EBCC is a neurophysiological indicator of cerebellar dysfunction. We have previously demonstrated impaired EBCC in patients with primary dystonia, but it remains uncertain if this represents actual cerebellar pathology or reflects a functional cerebellar disruption. We examined this further by: (1) studying acquisition and retention of EBCC in a second session in eight patients with cervical dystonia (CD) who had a first session 7-10 days earlier; and (2) by investigating the potential of continuous theta burst stimulation (cTBS) over the right cerebellar hemisphere to modify a first-ever EBCC session in 11 patients with CD. EBCC data of eight healthy controls previously studied were used for additional between-group comparisons. We observed an improvement of EBCC in a second session in patients with CD, which is in contrast to patients with proven cerebellar pathology who do not show further improvement of EBCC in additional sessions. We also found that cerebellar cTBS paradoxically normalized EBCC in patients with CD, while we previously showed that it disrupts EBCC in healthy volunteers. Combined, these two experiments are in keeping with a functional and reversible disruption of the cerebellum in dystonia, a phenomenon that is probably secondary to either cerebellar compensation or to cerebellar recruitment in the abnormal sensorimotor network.

Adaptation of surround inhibition in the human motor system.

Adaptation of a rapid ballistic movement requires that commands for the next movement are updated on the basis of sensory error signals from the current movement. Previous experiments, mostly using visual feedback, have demonstrated that adaptation is highly sensitive to the timing of feedback and can be substantially impaired by delays of 100 ms or so. Here, we use the phenomenon of surround inhibition (SI) to explore the consequences of somatosensory feedback delay in a task requiring participants to flex the index finger without generating any electromyographical (EMG) activity in other fingers. Participants were requested to perform brief isolated flexion movements of the index finger. After a short period of practice, SI in the distant abductor digiti minimi (ADM) muscle was quantified by measuring the amplitude of EMG responses evoked by a standard pulse of transcranial magnetic stimulation to the contralateral motor cortex at the onset of flexion. SI indicates that the response during flexion was smaller than the response at rest. After this, two training blocks were performed in which the ADM muscle was vibrated (80 Hz, 100 ms) either at the onset (VIB(onset)) of finger flexion or with a delay of 100 ms (VIB(100)). SI was reassessed after training. SI measured after VIB(onset) training was transiently more effective than at baseline. In contrast, SI was unchanged compared to baseline after VIB(100). The present study demonstrates that SI can be modified by experience. The timing of the sensory stimulation was found to be critical for the modification of SI, suggesting that only sensory signals closely related to the movement onset can induce adaptive changes, presumably through a feed-forward process.

Inpatient treatment of functional motor symptoms: a long-term follow-up study.

Functional neurological disorders are common, disabling and often difficult to treat. There is little consensus on the best approach to management. Multidisciplinary inpatient approaches are employed in some centres for patients with severe refractory symptoms, but their efficacy and, in particular, long-term outcomes are uncertain. We conducted a study using questionnaires completed retrospectively by patients treated at a specialised multidisciplinary inpatient programme at the National Hospital for Neurology and Neurosurgery. Consecutive patients with functional motor symptoms admitted to this centre between 2006 and 2008 were invited to participate. Questionnaires were sent at least 2 years after discharge. We contacted 32 patients, and 26 responded. The majority had symptoms for at least 3 years prior to admission; 58 % of patients reported benefit from the programme on discharge. This self-reported benefit to symptoms and function was after a 2-year follow-up period in the majority of patients, but return to work or cessation of health-related financial benefits was uncommon even in those who improved. Seventy-four percent of those questioned stated they would recommend the programme to others with similar symptoms. Attribution of symptoms to stress or emotional state was correlated with favourable outcome. Our data suggest that multidisciplinary inpatient treatment for patients with refractory functional motor symptoms provides self-reported benefit in the long-term. Prospective analysis of such interventions and the determinants of benefit need assessment in order to improve the service and target treatment to patients most likely to benefit.

Functional reorganization of sensorimotor cortex in early Parkinson disease.

OBJECTIVE: Compensatory reorganization of the nigrostriatal system is thought to delay the onset of symptoms in early Parkinson disease (PD). Here we sought evidence that compensation may be a part of a more widespread functional reorganization in sensorimotor networks, including primary motor cortex. METHODS: Several neurophysiologic measures known to be abnormal in the motor cortex (M1) of patients with advanced PD were tested on the more and less affected side of 16 newly diagnosed and drug-naive patients with PD and compared with 16 age-matched healthy participants. LTP-like effects were probed using a paired associative stimulation protocol. We also measured short interval intracortical inhibition, intracortical facilitation, cortical silent period, and input/output curves. RESULTS: The less affected side in patients with PD had preserved intracortical inhibition and a larger response to the plasticity protocol compared to healthy participants. On the more affected side, there was no response to the plasticity protocol and inhibition was reduced. There was no difference in input/output curves between sides or between patients with PD and healthy participants. CONCLUSIONS: Increased motor cortical plasticity on the less affected side is consistent with a functional reorganization of sensorimotor cortex and may represent a compensatory change that contributes to delaying onset of clinical symptoms. Alternatively, it may reflect a maladaptive plasticity that provokes symptom onset. Plasticity deteriorates as the symptoms progress, as seen on the more affected side. The rate of change in paired associative stimulation response over time could be developed into a surrogate marker of disease progression in PD.