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

The blink reflex and its modulation - Part 2: Pathophysiology and clinical utility

The blink reflex (BR) is integrated at the brainstem; however, it is modulated by inputs from various structures such as the striatum, globus pallidus, substantia nigra, and nucleus raphe magnus but also from afferent input from the peripheral nervous system. Therefore, it provides information about the pathophysiology of numerous peripheral and central nervous system disorders. The BR is a valuable tool for studying the integrity of the trigemino-facial system, the relevant brainstem nuclei, and circuits. At the same time, some neurophysiological techniques applying the BR may indicate abnormalities involving structures rostral to the brainstem that modulate or control the BR circuits. This is a state-of-the-art review of the clinical application of BR modulation; physiology is reviewed in part 1. In this review, we aim to present the role of the BR and techniques related to its modulation in understanding pathophysiological mechanisms of motor control and pain disorders, in which these techniques are diagnostically helpful. Furthermore, some BR techniques may have a predictive value or serve as a basis for follow-up evaluation. BR testing may benefit in the diagnosis of hemifacial spasm, dystonia, functional movement disorders, migraine, orofacial pain, and psychiatric disorders. Although the abnormalities in the integrity of the BR pathway itself may provide information about trigeminal or facial nerve disorders, alterations in BR excitability are found in several disease conditions. BR excitability studies are suitable for understanding the common pathophysiological mechanisms behind various clinical entities, elucidating alterations in top-down inhibitory systems, and allowing for follow-up and quantitation of many neurological syndromes.

Dystonia Diagnosis: Clinical Neurophysiology and Genetics

Dystonia diagnosis is based on clinical examination performed by a neurologist with expertise in movement disorders. Clues that indicate the diagnosis of a movement disorder such as dystonia are dystonic movements, dystonic postures, and three additional physical signs (mirror dystonia, overflow dystonia, and geste antagonists/sensory tricks). Despite advances in research, there is no diagnostic test with a high level of accuracy for the dystonia diagnosis. Clinical neurophysiology and genetics might support the clinician in the diagnostic process. Neurophysiology played a role in untangling dystonia pathophysiology, demonstrating characteristic reduction in inhibition of central motor circuits and alterations in the somatosensory system. The neurophysiologic measure with the greatest evidence in identifying patients affected by dystonia is the somatosensory temporal discrimination threshold (STDT). Other parameters need further confirmations and more solid evidence to be considered as support for the dystonia diagnosis. Genetic testing should be guided by characteristics such as age at onset, body distribution, associated features, and coexistence of other movement disorders (parkinsonism, myoclonus, and other hyperkinesia). The aim of the present review is to summarize the state of the art regarding dystonia diagnosis focusing on the role of neurophysiology and genetic testing.

Cerebellum Involvement in Dystonia During Associative Motor Learning: Insights From a Data-Driven Spiking Network Model

Dystonia is a movement disorder characterized by sustained or intermittent muscle contractions causing abnormal, often repetitive movements, postures, or both. Although dystonia is traditionally associated with basal ganglia dysfunction, recent evidence has been pointing to a role of the cerebellum, a brain area involved in motor control and learning. Cerebellar abnormalities have been correlated with dystonia but their potential causative role remains elusive. Here, we simulated the cerebellar input-output relationship with high-resolution computational modeling. We used a data-driven cerebellar Spiking Neural Network and simulated a cerebellum-driven associative learning task, Eye-Blink Classical Conditioning (EBCC), which is characteristically altered in relation to cerebellar lesions in several pathologies. In control simulations, input stimuli entrained characteristic network dynamics and induced synaptic plasticity along task repetitions, causing a progressive spike suppression in Purkinje cells with consequent facilitation of deep cerebellar nuclei cells. These neuronal processes caused a progressive acquisition of eyelid Conditioned Responses (CRs). Then, we modified structural or functional local neural features in the network reproducing alterations reported in dystonic mice. Either reduced olivocerebellar input or aberrant Purkinje cell burst-firing resulted in abnormal learning curves imitating the dysfunctional EBCC motor responses (in terms of CR amount and timing) of dystonic mice. These behavioral deficits might be due to altered temporal processing of sensorimotor information and uncoordinated control of muscle contractions. Conversely, an imbalance of excitatory and inhibitory synaptic densities on Purkinje cells did not reflect into significant EBCC deficit. The present work suggests that only certain types of alterations, including reduced olivocerebellar input and aberrant PC burst-firing, are compatible with the EBCC changes observed in dystonia, indicating that some cerebellar lesions can have a causative role in the pathogenesis of symptoms.

Dystonia, chorea, hemiballismus and other dyskinesias

Hyperkinesias are heterogeneous involuntary movements that significantly differ in terms of clinical and semeiological manifestations, including rhythm, regularity, speed, duration, and other factors that determine their appearance or suppression. Hyperkinesias are due to complex, variable, and largely undefined pathophysiological mechanisms that may involve different brain areas. In this chapter, we specifically focus on dystonia, chorea and hemiballismus, and other dyskinesias, specifically, levodopa-induced, tardive, and cranial dyskinesia. We address the role of neurophysiological studies aimed at explaining the pathophysiology of these conditions. We mainly refer to human studies using surface and invasive in-depth recordings, as well as spinal, brainstem, and transcortical reflexology and non-invasive brain stimulation techniques. We discuss the extent to which the neurophysiological abnormalities observed in hyperkinesias may be explained by pathophysiological models. We highlight the most relevant issues that deserve future research efforts. The potential role of neurophysiological assessment in the clinical context of hyperkinesia is also discussed.

Low-Intensity Transcranial Ultrasound Stimulation: Mechanisms of Action and Rationale for Future Applications in Movement Disorders

Low-intensity transcranial ultrasound stimulation (TUS) is a novel non-invasive brain stimulation technique that uses acoustic energy to induce changes in neuronal activity. However, although low-intensity TUS is a promising neuromodulation tool, it has been poorly studied as compared to other methods, i.e., transcranial magnetic and electrical stimulation. In this article, we first focus on experimental studies in animals and humans aimed at explaining its mechanisms of action. We then highlight possible applications of TUS in movement disorders, particularly in patients with parkinsonism, dystonia, and tremor. Finally, we highlight the knowledge gaps and possible limitations that currently limit potential TUS applications in movement disorders. Clarifying the potential role of TUS in movement disorders may further promote studies with therapeutic perspectives in this field.

A Critical Investigation of Cerebellar Associative Learning in Isolated Dystonia

Background

Objective

Methods

Results

Conclusions

Bayesian Interpretation of Essential Tremor Plus

Essential tremor (ET) plus is a new tremor classification that was introduced in 2018 by a task force of the International Parkinson and Movement Disorder Society. Patients with ET plus meet the criteria for ET but have one or more additional systemic or neurologic signs of uncertain significance or relevance to tremor (“soft signs”). Soft signs are not sufficient to diagnose another tremor syndrome or movement disorder, and soft signs in ET plus are known to have poor interrater reliability and low diagnostic sensitivity and specificity. Therefore, the clinical significance of ET plus must be interpreted probabilistically when judging whether a patient is more likely to have ET or a combined tremor syndrome, such as dystonic tremor. Such a probabilistic interpretation is possible with Bayesian analysis. This review presents a Bayesian analysis of ET plus in patients suspected of having ET versus a dystonic tremor syndrome, which is the most common differential diagnosis in patients referred for ET. Bayesian analysis of soft signs provides an estimate of the probability that a patient with possible ET is more likely to have an alternative diagnosis. ET plus is a distinct tremor classification and should not be viewed as a subtype of ET. ET plus covers a more-comprehensive phenotyping of people with possible ET, and the clinical interpretation of ET plus is enhanced with Bayesian analysis of associated soft signs.

Motor learning deficits in cervical dystonia point to defective basal ganglia circuitry

Dystonia is conceptualized as a network disorder involving basal ganglia, thalamus, sensorimotor cortex and the cerebellum. The cerebellum has been implicated in dystonia pathophysiology, but studies testing cerebellar function in dystonia patients have provided equivocal results. This study aimed to further elucidate motor network deficits in cervical dystonia with special interest in the role of the cerebellum. To this end we investigated motor learning tasks, that differ in their dependence on cerebellar and basal ganglia functioning. In 18 cervical dystonia patients and 18 age matched healthy controls we measured implicit motor sequence learning using a 12-item serial reaction time task mostly targeting basal ganglia circuitry and motor adaptation and eyeblink conditioning as markers of cerebellar functioning. ANOVA showed that motor sequence learning was overall impaired in cervical dystonia (p = 0.01). Moreover, unlike healthy controls, patients did not show a learning effect in the first part of the experiment. Visuomotor adaptation and eyeblink conditioning were normal. In conclusion, these data lend support to the notion that motor learning deficits in cervical dystonia relate to basal ganglia-thalamo-cortical loops rather than being a result of defective cerebellar circuitry.

Contemporary clinical neurophysiology applications in dystonia

The complex phenomenological understanding of dystonia has transcended from the clinics to genetics, imaging and neurophysiology. One way in which electrophysiology will impact into the clinics are cases wherein a dystonic clinical presentation may not be typical or a "forme fruste" of the disorder. Indeed, the physiological imprints of dystonia are present regardless of its clinical manifestation. Underpinnings in the understanding of dystonia span from the peripheral, segmental and suprasegmental levels to the cortex, and various electrophysiological tests have been applied in the course of time to elucidate the origin of dystonia pathophysiology. While loss of inhibition remains to be the key finding in this regard, intricacies and variabilities exist, thus leading to a notion that perhaps dystonia should best be gleaned as network disorder. Interestingly, the complex process has now spanned towards the understanding in terms of networks related to the cerebellar circuitry and the neuroplasticity. What is evolving towards a better and cohesive view will be neurophysiology attributes combined with structural dynamic imaging. Such a sound approach will significantly lead to better therapeutic modalities in the future.

Cerebellar Direct Current Stimulation (ctDCS) in the Treatment of Huntington's Disease: A Pilot Study and a Short Review of the Literature

Introduction: In recent years, a growing body of literature has investigated the use of non-invasive brain stimulation (NIBS) techniques as a putative treatment in Huntington's Disease (HD). Our aim was to evaluate the effects of cerebellar transcranial Direct Current Simulation (ctDCS) on the motor outcome in patients affected by HD, encompassing at the same time the current knowledge about the effects of NIBS both on motor and non-motor dysfunctions in HD. Materials and Methods: Four patients (two females) were enrolled and underwent ctDCS (both anodal or sham, elapsed by at least 3 months: 2.0 mA, 20 min per day, 5 days a week). Clinical scores were assessed by using the Unified Huntington's Disease Rating Scale - part I (UHDRS-I), immediately before ctDCS (T₀), at the end of the 5-days treatment (T₁) and 4 weeks later (T₂). Results: Anodal ctDCS improved motor scores compared to baseline (p = 0.0046), whereas sham stimulation left them unchanged (p = 0.33, Friedman test). In particular, following anodal ctDCS, UHDRS-I score significantly improved, especially regarding the subitem "dystonia," both at T₁ and T₂ compared to sham condition (p < 0.05; Wilcoxon matched-pairs signed test). Conclusions: ctDCS improved motor scores in HD, with effects lasting for about 4 weeks after tDCS completion. This is the first study discussing the putative role of cerebellar non-invasive simulation for the treatment of HD.

Defective Somatosensory Inhibition and Plasticity Are Not Required to Develop Dystonia

BACKGROUND

Dystonia may have different neuroanatomical substrates and pathophysiology. This is supported by studies on the motor system showing, for instance, that plasticity is abnormal in idiopathic dystonia, but not in dystonia secondary to basal ganglia lesions.

OBJECTIVE

The aim of this study was to test whether somatosensory inhibition and plasticity abnormalities reported in patients with idiopathic dystonia also occur in patients with dystonia caused by basal ganglia damage.

METHODS

Ten patients with acquired dystonia as a result of basal ganglia lesions and 12 healthy control subjects were recruited. They underwent electrophysiological testing at baseline and after a single 45-minute session of high-frequency repetitive somatosensory stimulation. Electrophysiological testing consisted of somatosensory temporal discrimination, somatosensory-evoked potentials (including measurement of early and late high-frequency oscillations and the spatial inhibition ratio of N20/25 and P14 components), the recovery cycle of paired-pulse somatosensory-evoked potentials, and primary motor cortex short-interval intracortical inhibition.

RESULTS

Unlike previous reports of patients with idiopathic dystonia, patients with acquired dystonia did not differ from healthy control subjects in any of the electrophysiological measures either before or after high-frequency repetitive somatosensory stimulation, except for short-interval intracortical inhibition, which was reduced at baseline in patients compared to control subjects.

CONCLUSIONS

The data show that reduced somatosensory inhibition and enhanced cortical plasticity are not required for the clinical expression of dystonia, and that the abnormalities reported in idiopathic dystonia are not necessarily linked to basal ganglia damage. © 2020 International Parkinson and Movement Disorder Society.

Delineating the electrophysiological signature of dystonia

Over the last 30 years, the concept of dystonia has dramatically changed, from being considered a motor neurosis, to a pure basal ganglia disorder, to finally reach the definition of a network disorder involving the basal ganglia, cerebellum, thalamus and sensorimotor cortex. This progress has been possible due to the collaboration between clinicians and scientists, and the development of increasingly sophisticated electrophysiological techniques able to non-invasively investigate pathophysiological mechanisms in humans. This review is a chronological excursus of the electrophysiological studies that laid the foundation for the understanding of the pathophysiology of dystonia and delineated its electrophysiological signatures. Evidence for neurophysiological abnormalities is grouped according to the neural system involved, and a unifying theory, bringing together all the hypothesis and evidence provided to date, is proposed at the end.

Abnormal cerebellar connectivity and plasticity in isolated cervical dystonia

There is increasing evidence that supports the role of the cerebellum in the pathophysiology of dystonia. We used transcranial magnetic stimulation to test the hypothesis that patients with cervical dystonia may have a disrupted cerebellar cortical connectivity at rest, and that cerebellar plasticity is altered too. We enrolled 12 patients with isolated cervical dystonia and 13 controls. A paired-pulse transcranial magnetic stimulation protocol was applied over the right cerebellum and the left primary motor area. Changes in the amplitude of motor evoked potentials were analysed. Continuous and intermittent Theta Burst Stimulation over the cerebellum was also applied. The effects of these repetitive protocols on cortical excitability, on intra-cortical circuits and on cerebellar cortical inhibition were analysed. In healthy subjects, but not in dystonic patients, a conditioning stimulus over the cerebellum was able to inhibit the amplitude of the motor evoked potentials from primary motor cortex. In healthy subjects continuous and intermittent cerebellar Theta Burst Stimulation were able to decrease and increase respectively motor cortex excitability. Continuous Theta Burst Stimulation was able to abolish the cerebellar cortical inhibition observed in basal condition. These effects were not observed in patients with cervical dystonia. Cerebellar cortical connectivity and cerebellar plasticity is altered at rest in patients with cervical dystonia.

The Use of Botulinum Toxin for Treatment of the Dystonias

Dystonias are characterized by involuntary muscle contractions, twisting movements, abnormal postures, and often tremor in various body regions. However, in the last decade several studies have demonstrated that dystonias are also characterized by sensory abnormalities. While botulinum toxin is the gold standard therapy for focal dystonia, exactly how it improves this disorder is not entirely understood. Neurophysiological studies in animals and humans have clearly demonstrated that botulinum toxin improves dystonic motor manifestations by inducing chemodenervation, therefore weakening the injected muscles. In addition, neurophysiological and neuroimaging evidence also suggests that botulinum toxin modulates the activity of various neural structures in the CNS distant from the injected site, particularly cortical motor and sensory areas. Concordantly, recent studies have shown that in patients with focal dystonias botulinum toxin ameliorates sensory disturbances, including reduced spatial discrimination acuity and pain. Overall, these observations suggest that in these patients botulinum toxin-induced effects encompass complex mechanisms beyond chemodenervation of the injected muscles.

Dystonia

Dystonia is a neurological condition characterized by abnormal involuntary movements or postures owing to sustained or intermittent muscle contractions. Dystonia can be the manifesting neurological sign of many disorders, either in isolation (isolated dystonia) or with additional signs (combined dystonia). The main focus of this Primer is forms of isolated dystonia of idiopathic or genetic aetiology. These disorders differ in manifestations and severity but can affect all age groups and lead to substantial disability and impaired quality of life. The discovery of genes underlying the mendelian forms of isolated or combined dystonia has led to a better understanding of its pathophysiology. In some of the most common genetic dystonias, such as those caused by TOR1A, THAP1, GCH1 and KMT2B mutations, and idiopathic dystonia, these mechanisms include abnormalities in transcriptional regulation, striatal dopaminergic signalling and synaptic plasticity and a loss of inhibition at neuronal circuits. The diagnosis of dystonia is largely based on clinical signs, and the diagnosis and aetiological definition of this disorder remain a challenge. Effective symptomatic treatments with pharmacological therapy (anticholinergics), intramuscular botulinum toxin injection and deep brain stimulation are available; however, future research will hopefully lead to reliable biomarkers, better treatments and cure of this disorder.

Recent advances in understanding and managing dystonia

Within the field of movement disorders, the conceptual understanding of dystonia has continued to evolve. Clinical advances have included improvements in recognition of certain features of dystonia, such as tremor, and understanding of phenotypic spectrums in the genetic dystonias and dystonia terminology and classification. Progress has also been made in the understanding of underlying biological processes which characterize dystonia from discoveries using approaches such as neurophysiology, functional imaging, genetics, and animal models. Important advances include the role of the cerebellum in dystonia, the concept of dystonia as an aberrant brain network disorder, additional evidence supporting the concept of dystonia endophenotypes, and new insights into psychogenic dystonia. These discoveries have begun to shape treatment approaches as, in parallel, important new treatment modalities, including magnetic resonance imaging-guided focused ultrasound, have emerged and existing interventions such as deep brain stimulation have been further refined. In this review, these topics are explored and discussed.

Genetic Dystonia-ataxia Syndromes: Clinical Spectrum, Diagnostic Approach, and Treatment Options

BACKGROUND

Dystonia and ataxia are manifestations of numerous disorders, and indeed, an ever-expanding spectrum of genes causing diseases that encompass dystonia and ataxia are discovered with the advances of genetic techniques. In recent years, a pathophysiological link between both clinical features and the role of the cerebellum in the genesis of dystonia, in some cases, has been proposed. In clinical practice, the genetic diagnosis of dystonia-ataxia syndromes is a major issue for genetic counseling, prognosis and, occasionally, specific treatment.

METHODS

For this pragmatic and educational review, we conducted a comprehensive and structured literature search in Pubmed, OMIM, and GeneReviews using the key words "dystonia" and "ataxia" to identify those genetic diseases that may combine dystonia with ataxia.

RESULTS

There are a plethora of genetic diseases causing dystonia and ataxia. We propose a series of clinico-radiological algorithms to guide their differential diagnosis depending on the age of onset, additional neurological or systemic features, and imaging findings. We suggest a sequential diagnostic approach to dystonia-ataxia syndromes. We briefly highlight the pathophysiological links between dystonia and ataxia and conclude with a review of specific treatment implications.

CONCLUSIONS

The clinical approach presented in this review is intended to improve the diagnostic success of clinicians when faced with patients with dystonia-ataxia syndromes.

Abnormal blink reflex recovery cycle in manifesting and nonmanifesting carriers of the DYT1 gene mutation

The aim of this study is to evaluate the brainstem function in DYT1 carriers manifesting clinical dystonia (MDYT1) and those without clinical symptoms (NMDYT1). Motor cortical inhibition and plasticity were found to be abnormal in MDYT1, whereas these were less abnormal in NMDYT1. However, the spinal reciprocal inhibition was abnormal in MDYT1, but normal in NMDYT1. Moreover, protein accumulation and perinuclear inclusion bodies were found in the brainstem, but not in other brain areas, in DYT1 patients. Therefore, we designed this study to investigate the brainstem physiology using the blink reflex (BR) recovery cycle test in MDYT1 and NMDYT1. We recruited eight MDYT1, five NMDYT1, and nine age-matched healthy controls. The BR recovery cycle was assessed with paired stimuli that induced the BR in a random order at interstimulus intervals of 250, 500, and 1000 ms. A two-way analysis of variance showed a significant difference between MDYT1, NMDYT1, and the healthy control (P=0.004). Post-hoc analysis showed that this was because of a significantly lower inhibition of R2 in MDYT1 and NMDYT1 compared with the controls (two-way analysis of variance: P=0.003 and 0.021, respectively). There was no difference between MDYT1 and NMDYT1 (P=0.224). The tested brainstem circuits were equally involved in MDYT1 and NMDYT1. The finding is in agreement with the pathological findings in DYT1 carriers. Together with previous findings in the motor cortex and the spinal cord, the brainstem may lie closer to the pathogenesis of dystonia than the motor cortex in DYT1 gene carriers.

The cerebellum and dystonia

Dystonia is a heterogeneous disorder characterized by involuntary muscle contractions, twisting movements, and abnormal postures in various body regions. It is widely accepted that the basal ganglia are involved in the pathogenesis of dystonia. A growing body of evidence, however, is challenging the traditional view and suggest that the cerebellum may also play a role in dystonia. Studies on animals indicate that experimental manipulations of the cerebellum lead to dystonic-like movements. Several clinical observations, including those from secondary dystonia cases as well as neurophysiologic and neuroimaging studies in human patients, provide further evidence in humans of a possible relationship between cerebellar abnormalities and dystonia. Claryfing the role of the cerebellum in dystonia is an important step towards providing alternative treatments based on noninvasive brain stimulation techniques.

Disrupted superior collicular activity may reveal cervical dystonia disease pathomechanisms

Cervical dystonia is a common neurological movement disorder characterised by muscle contractions causing abnormal movements and postures affecting the head and neck. The neural networks underpinning this condition are incompletely understood. While animal models suggest a role for the superior colliculus in its pathophysiology, this link has yet to be established in humans. The present experiment was designed to test the hypothesis that disrupted superior collicular processing is evident in affected patients and in relatives harbouring a disease-specific endophenotype (abnormal temporal discrimination). The study participants were 16 cervical dystonia patients, 16 unaffected first-degree relatives with abnormal temporal discrimination, 16 unaffected first-degree relatives with normal temporal discrimination and 16 healthy controls. The response of participant’s superior colliculi to looming stimuli was assessed by functional magnetic resonance imaging. Cervical dystonia patients and relatives with abnormal temporal discrimination demonstrated (i) significantly reduced superior collicular activation for whole brain and region of interest analysis; (ii) a statistically significant negative correlation between temporal discrimination threshold and superior collicular peak values. Our results support the hypothesis that disrupted superior collicular processing is involved in the pathogenesis of cervical dystonia. These findings, which align with animal models of cervical dystonia, shed new light on pathomechanisms in humans.

Abnormalities of Eye-Hand Coordination in Patients with Writer's Cramp: Possible Role of the Cerebellum

Background

Writer's cramp (WC) is one of the commonly observed focal dystonias. The pathophysiology of WC has not been fully understood. The role of the cerebellum has been increasingly recognized in the pathogenesis of dystonia. As the cerebellum is crucial for maintaining accurate eye-hand coordination (EHC), its role in the pathogenesis of WC can be investigated by studying the EHC in patients with WC.

Methods

Fifteen patients with WC (women:men, 3:12) and 15 age- and gender-matched controls performed oculomotor and EHC tasks. A visually guided stimulus (VGS) task was first performed with eye-only condition (EOC) and then with EHC.

Results

A significant interaction between the groups (controls and patients) and tasks (EOC and EHC) with age as a covariate confirmed that the two groups reacted differently to the tasks in saccadic latency (F(1,27) = 4.8; p = 0.039) and average saccade acceleration (F(1,27) = 10.6; p = 0.003). The curvature index of acceleration of the hand was significantly more in patients compared to controls (patients vs. controls, 2.4±0.4 vs. 1.8±0.2, p = 0.01). While performing the EHC task, there was a significant correlation of the Writer's Cramp Rating Score with the average saccadic speed (-0.61, p = 0.016), peak saccadic deceleration (0.59, p = 0.019) and average saccadic acceleration (-0.63, p = 0.012).

Discussion

Saccadic acceleration and latency are abnormal while performing EHC tasks in patients with WC. Our study gives further insights into the possible role of the cerebellum in the pathogenesis of WC.

The Anatomical Basis for Dystonia: The Motor Network Model

Background

The dystonias include a clinically and etiologically very diverse group of disorders. There are both degenerative and non-degenerative subtypes resulting from genetic or acquired causes. Traditionally, all dystonias have been viewed as disorders of the basal ganglia. However, there has been increasing appreciation for involvement of other brain regions including the cerebellum, thalamus, midbrain, and cortex. Much of the early evidence for these other brain regions has come from studies of animals, but multiple recent studies have been done with humans, in an effort to confirm or refute involvement of these other regions. The purpose of this article is to review the new evidence from animals and humans regarding the motor network model, and to address the issues important to translational neuroscience.

Methods

The English literature was reviewed for articles relating to the neuroanatomical basis for various types of dystonia in both animals and humans.

Results

There is evidence from both animals and humans that multiple brain regions play an important role in various types of dystonia. The most direct evidence for specific brain regions comes from animal studies using pharmacological, lesion, or genetic methods. In these studies, experimental manipulations of specific brain regions provide direct evidence for involvement of the basal ganglia, cerebellum, thalamus and other regions. Additional evidence also comes from human studies using neuropathological, neuroimaging, non-invasive brain stimulation, and surgical interventions. In these studies, the evidence is less conclusive, because discriminating the regions that cause dystonia from those that reflect secondary responses to abnormal movements is more challenging.

Discussion

Overall, the evidence from both animals and humans suggests that different regions may play important roles in different subtypes of dystonia. The evidence so far provides strong support for the motor network model. There are obvious challenges, but also advantages, of attempting to translate knowledge gained from animals into a more complete understanding of human dystonia and novel therapeutic strategies.

Cerebellum: An explanation for dystonia?

Dystonia is a movement disorder that is characterized by involuntary muscle contractions, abnormal movements and postures, as well as by non-motor symptoms, and is due to abnormalities in different brain areas. In this article, we focus on the growing number of experimental studies aimed at explaining the pathophysiological role of the cerebellum in dystonia. Lastly, we highlight gaps in current knowledge and issues that future research studies should focus on as well as some of the potential applications of this research avenue. Clarifying the pathophysiological role of cerebellum in dystonia is an important concern given the increasing availability of invasive and non-invasive stimulation techniques and their potential therapeutic role in this condition.

Using the shared genetics of dystonia and ataxia to unravel their pathogenesis

In this review we explore the similarities between spinocerebellar ataxias and dystonias, and suggest potentially shared molecular pathways using a gene co-expression network approach. The spinocerebellar ataxias are a group of neurodegenerative disorders characterized by coordination problems caused mainly by atrophy of the cerebellum. The dystonias are another group of neurological movement disorders linked to basal ganglia dysfunction, although evidence is now pointing to cerebellar involvement as well. Our gene co-expression network approach identified 99 shared genes and showed the involvement of two major pathways: synaptic transmission and neurodevelopment. These pathways overlapped in the two disorders, with a large role for GABAergic signaling in both. The overlapping pathways may provide novel targets for disease therapies. We need to prioritize variants obtained by whole exome sequencing in the genes associated with these pathways in the search for new pathogenic variants, which can than be used to help in the genetic counseling of patients and their families.

Impaired eye blink classical conditioning distinguishes dystonic patients with and without tremor

INTRODUCTION

Tremor is frequently associated with dystonia, but its pathophysiology is still unclear. Dysfunctions of cerebellar circuits are known to play a role in the pathophysiology of action-induced tremors, and cerebellar impairment has frequently been associated to dystonia. However, a link between dystonic tremor and cerebellar abnormalities has not been demonstrated so far.

METHODS

Twenty-five patients with idiopathic isolated cervical dystonia, with and without tremor, were enrolled. We studied the excitability of inhibitory circuits in the brainstem by measuring the R2 blink reflex recovery cycle (BRC) and implicit learning mediated by the cerebellum by means of eyeblink classical conditioning (EBCC). Results were compared with those obtained in a group of age-matched healthy subjects (HS).

RESULTS

Statistical analysis did not disclose any significant clinical differences among dystonic patients with and without tremor. Patients with dystonia (regardless of the presence of tremor) showed decreased inhibition of R2 blink reflex by conditioning pulses compared with HS. Patients with dystonic tremor showed a decreased number of conditioned responses in the EBCC paradigm compared to HS and dystonic patients without tremor.

CONCLUSION

The present data show that cerebellar impairment segregates with the presence of tremor in patients with dystonia, suggesting that the cerebellum might have a role in the occurrence of dystonic tremor.

Neural correlates of abnormal sensory discrimination in laryngeal dystonia

Aberrant sensory processing plays a fundamental role in the pathophysiology of dystonia; however, its underpinning neural mechanisms in relation to dystonia phenotype and genotype remain unclear. We examined temporal and spatial discrimination thresholds in patients with isolated laryngeal form of dystonia (LD), who exhibited different clinical phenotypes (adductor vs. abductor forms) and potentially different genotypes (sporadic vs. familial forms). We correlated our behavioral findings with the brain gray matter volume and functional activity during resting and symptomatic speech production. We found that temporal but not spatial discrimination was significantly altered across all forms of LD, with higher frequency of abnormalities seen in familial than sporadic patients. Common neural correlates of abnormal temporal discrimination across all forms were found with structural and functional changes in the middle frontal and primary somatosensory cortices. In addition, patients with familial LD had greater cerebellar involvement in processing of altered temporal discrimination, whereas sporadic LD patients had greater recruitment of the putamen and sensorimotor cortex. Based on the clinical phenotype, adductor form-specific correlations between abnormal discrimination and brain changes were found in the frontal cortex, whereas abductor form-specific correlations were observed in the cerebellum and putamen. Our behavioral and neuroimaging findings outline the relationship of abnormal sensory discrimination with the phenotype and genotype of isolated LD, suggesting the presence of potentially divergent pathophysiological pathways underlying different manifestations of this disorder.

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Abnormal temporal but not spatial discrimination is an LD mediational endophenotype.

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Penetrance of abnormal TDT is higher in familial than sporadic LD.

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No differences in penetrance between clinical phenotypes of LD

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Distinct neural phenotype/genotype relations of TDT reflect divergent pathophysiology.

Dissecting the links between cerebellum and dystonia

Dystonia is a common movement disorder characterized by sustained muscle contractions. These contractions generate twisting and repetitive movements or typical abnormal postures, often exacerbated by voluntary movement. Dystonia can affect almost all the voluntary muscles. For several decades, the discussion on the pathogenesis has been focused on basal ganglia circuits, especially striatal networks. So far, although dystonia has been observed in some forms of ataxia such as dominant ataxias, the link between the cerebellum and dystonia has remained unclear. Recent human studies and experimental data mainly in rodents show that the cerebellum circuitry could also be a key player in the pathogenesis of some forms of dystonia. In particular, studies based on behavioral adaptation paradigm shed light on the links between dystonia and cerebellum. The spectrum of movement disorders in which the cerebellum is implicated is continuously expanding, and manipulation of cerebellar circuits might even emerge as a candidate therapy in the coming years.

Sensory abnormalities in focal hand dystonia and non-invasive brain stimulation

It has been proposed that synchronous and convergent afferent input arising from repetitive motor tasks may play an important role in driving the maladaptive cortical plasticity seen in focal hand dystonia (FHD). This hypothesis receives support from several sources. First, it has been reported that in subjects with FHD, paired associative stimulation produces an abnormal increase in corticospinal excitability, which was not confined to stimulated muscles. These findings provide support for the role of excessive plasticity in FHD. Second, the genetic contribution to the dystonias is increasingly recognized indicating that repetitive, stereotyped afferent inputs may lead to late-onset dystonia, such as FHD, more rapidly in genetically susceptible individuals. It can be postulated, according to the two factor hypothesis that dystonia is triggered and maintained by the concurrence of environmental factors such as repetitive training and subtle abnormal mechanisms of plasticity within somatosensory loop. In the present review, we examine the contribution of sensory-motor integration in the pathophysiology of primary dystonia. In addition, we will discuss the role of non-invasive brain stimulation as therapeutic approach in FHD.