Unravelling of the paroxysmal dyskinesias

BK Channelopathies and KCNMA1-Linked Disease Models

Novel KCNMA1 variants, encoding the BK K+ channel, are associated with a debilitating dyskinesia and epilepsy syndrome. Neurodevelopmental delay, cognitive disability, and brain and structural malformations are also diagnosed at lower incidence. More than half of affected individuals present with a rare negative episodic motor disorder, paroxysmal nonkinesigenic dyskinesia (PNKD3). The mechanistic relationship of PNKD3 to epilepsy and the broader spectrum of KCNMA1-associated symptomology is unknown. This review summarizes patient-associated KCNMA1 variants within the BK channel structure, functional classifications, genotype-phenotype associations, disease models, and treatment. Patient and transgenic animal data suggest delineation of gain-of-function (GOF) and loss-of-function KCNMA1 neurogenetic disease, validating two heterozygous alleles encoding GOF BK channels (D434G and N999S) as causing seizure and PNKD3. This discovery led to a variant-defined therapeutic approach for PNKD3, providing initial insight into the neurological basis. A comprehensive clinical definition of monogenic KCNMA1-linked disease and the neuronal mechanisms currently remain priorities for continued investigation.

Clinical Reasoning: A 3-Year-Old Boy With Abnormal Movements During Sleep

We report a case of a 3-year-old boy who presented with abnormal movements that initially occurred only during sleep. Three years later, he went on to develop hyperkinetic movements during the daytime while awake. There was a strong family history of various paroxysmal neurologic disorders. In this report, we discuss the clinical approach, differential diagnosis, investigation, and treatment options for nocturnal hyperkinetic movements and paroxysmal movement disorders.

Functional Movement Disorder: Assessment and Treatment

Functional movement disorder (FMD) is a common, potentially reversible source of disability in neurology. Over the last two decades, there have been major advances in our understanding of the clinical picture, diagnosis, and management of this condition. Motor presentation is heterogeneous and several non-motor symptoms (e.g., pain, fatigue) are part of the clinical spectrum. The diagnosis should be made by neurologists or neuropsychiatrists based on the presence of positive signs of inconsistency and incongruence with neurological diseases. Promising evidence has accumulated for the efficacy of physiotherapy, psychotherapy, or both in the management of FMD, for a majority of patients.

Genetic Movement Disorders Commonly Seen in Asians

The increasing availability of molecular genetic testing has changed the landscape of both genetic research and clinical practice. Not only is the pace of discovery of novel disease-causing genes accelerating but also the phenotypic spectra associated with previously known genes are expanding. These advancements lead to the awareness that some genetic movement disorders may cluster in certain ethnic populations and genetic pleiotropy may result in unique clinical presentations in specific ethnic groups. Thus, the characteristics, genetics and risk factors of movement disorders may differ between populations. Recognition of a particular clinical phenotype, combined with information about the ethnic origin of patients could lead to early and correct diagnosis and assist the development of future personalized medicine for patients with these disorders. Here, the Movement Disorders in Asia Task Force sought to review genetic movement disorders that are commonly seen in Asia, including Wilson's disease, spinocerebellar ataxias (SCA) types 12, 31, and 36, Gerstmann-Sträussler-Scheinker disease, PLA2G6-related parkinsonism, adult-onset neuronal intranuclear inclusion disease (NIID), and paroxysmal kinesigenic dyskinesia. We also review common disorders seen worldwide with specific mutations or presentations that occur frequently in Asians.

Translating Genetic Discovery into a Mechanistic Understanding of Pediatric Movement Disorders: Lessons from Genetic Dystonias and Related Disorders

The era of next-generation sequencing has increased the pace of gene discovery in the field of pediatric movement disorders. Following the identification of novel disease-causing genes, several studies have aimed to link the molecular and clinical aspects of these disorders. This perspective presents the developing stories of several childhood-onset movement disorders, including paroxysmal kinesigenic dyskinesia, myoclonus-dystonia syndrome, and other monogenic dystonias. These stories illustrate how gene discovery helps focus the research efforts of scientists trying to understand the mechanisms of disease. The genetic diagnosis of these clinical syndromes also helps clarify the associated phenotypic spectra and aids the search for additional disease-causing genes. Collectively, the findings of previous studies have led to increased recognition of the role of the cerebellum in the physiology and pathophysiology of motor control-a common theme in many pediatric movement disorders. To fully exploit the genetic information garnered in the clinical and research arenas, it is crucial that corresponding multi-omics analyses and functional studies also be performed at scale. Hopefully, these integrated efforts will provide us with a more comprehensive understanding of the genetic and neurobiological bases of movement disorders in childhood.

MRM2 variants in families with complex dystonic syndromes: evidence for phenotypic heterogeneity

BACKGROUND

Dystonia involves repetitive movements and muscle contractions leading to abnormal postures. We investigated patients in two families, DYAF11 and M, exhibiting dystonic or involuntary movement disorders.

METHODS

Clinical investigations were performed for all patients. Genetic analyses included genome-wide linkage analysis and exome sequencing followed by Sanger sequencing validation. MRM2-specific transcripts were analysed from participants' blood samples in Family DYAF11 after cloning of gene-specific cDNA.

RESULTS

Four affected siblings in Family DYAF11 had progressive dystonic features. Two patients in Family M exhibited a neurodevelopmental disorder accompanied by involuntary movements. In Family DYAF11, linkage was detected to the telomere at chromosome 7p22.3, spanning <2 Mb. Exome sequencing identified a donor splice-site variant, c.8+1G>T in MRM2, which segregated with the phenotype, corresponding to the linkage data since all affected individuals were homozygous while the obligate unaffected carriers were heterozygous for the variant. In the MRM2 c.8+1G>T allele, an aberrant alternative acceptor splice-site located within exon 2 was used in a subset of the transcripts, creating a frameshift in the open reading frame. Exome sequencing in Family M revealed a rare missense variant c.242C>T, p.(Ala81Val), which affected a conserved amino acid.

CONCLUSIONS

Our results expand the clinical and allelic spectrum of MRM2 variants. Previously, these descriptions were based on observations in a single patient, diagnosed with mitochondrial DNA depletion syndrome 17, in whom movement disorder was accompanied by recurrent strokes and epilepsy. We also demonstrate a subset of correctly spliced tt-ag MRM2 transcripts, raising the possibility to develop treatment by understanding the disease mechanism.

Genetic Links to Episodic Movement Disorders: Current Insights

Episodic or paroxysmal movement disorders (PxMD) are conditions, which occur episodically, are transient, usually have normal interictal periods, and are characterized by hyperkinetic disorders, including ataxia, chorea, dystonia, and ballism. Broadly, these comprise paroxysmal dyskinesias (paroxysmal kinesigenic and non-kinesigenic dyskinesia [PKD/PNKD], paroxysmal exercise-induced dyskinesias [PED]) and episodic ataxias (EA) types 1-9. Classification of paroxysmal dyskinesias has traditionally been clinical. However, with advancement in genetics and the discovery of the molecular basis of several of these disorders, it is becoming clear that phenotypic pleiotropy exists, that is, the same variant may give rise to a variety of phenotypes, and the classical understanding of these disorders requires a new paradigm. Based on molecular pathogenesis, paroxysmal disorders are now categorized as synaptopathies, transportopathies, channelopathies, second-messenger related disorders, mitochondrial or others. A genetic paradigm also has an advantage of identifying potentially treatable disorders, such as glucose transporter 1 deficiency syndromes, which necessitates a ketogenic diet, and ADCY5-related disorders, which may respond to caffeine. Clues for a primary etiology include age at onset below 18 years, presence of family history and fixed triggers and attack duration. Paroxysmal movement disorder is a network disorder, with both the basal ganglia and the cerebellum implicated in pathogenesis. Abnormalities in the striatal cAMP turnover pathway may also be contributory. Although next-generation sequencing has restructured the approach to paroxysmal movement disorders, the genetic underpinnings of several entities remain undiscovered. As more genes and variants continue to be reported, these will lead to enhanced understanding of pathophysiological mechanisms and precise treatment.

Paroxysmal movement disorders: Paroxysmal dyskinesia and episodic ataxia

Paroxysmal movement disorders have traditionally been classified into paroxysmal dyskinesia (PxD), which consists in attacks of involuntary movements (mainly dystonia and/or chorea) without loss of consciousness, and episodic ataxia (EA), which features spells of cerebellar dysfunction with or without interictal neurological manifestations. In this chapter, PxD will be discussed first according to the trigger-based classification, thus reviewing clinical, genetic, and molecular features of paroxysmal kinesigenic dyskinesia, paroxysmal nonkinesigenic dyskinesia, and paroxysmal exercise-induced dyskinesia. EA will be presented thereafter according to their designated gene or genetic locus. Clinicogenetic similarities among paroxysmal movement disorders have progressively emerged, which are herein highlighted along with growing evidence that their pathomechanisms overlap those of epilepsy and migraine. Advances in our comprehension of the biological pathways underlying paroxysmal movement disorders, which involve ion channels as well as proteins associated with the vesical synaptic cycle or implicated in neuronal energy metabolism, may represent the cornerstone for defining a shared pathophysiologic framework and developing target-specific therapies.

The Choreoathetotic Movement of Paroxysmal Nonkinesigenic Dyskinesia

This case report describes a 14-year-old boy with paroxysmal involuntary movement attacks that were diagnosed as paroxysmal nonkinesigenic dyskinesia.

Case Report: Migraine-Induced Dystonia of the Lower Extremities

Migraine is a highly prevalent neurological disorder characterized by recurrent, unilateral, or bilateral throbbing severe headaches. Currently, there are extremely rare cases of migraine-induced dystonia. A 52-year-old woman was admitted for intractable migraine for about 5 days and walking difficulties for 1 day. The symptom of an inability to walk appeared on the fourth day of the headache attack lasting for 1 day and resolved on its own as the headache subsided. The same symptoms appeared once 6 years ago. Neurological examination, brain Magnetic resonance imaging (MRI), laboratory tests of blood and cerebrospinal fluid (CSF) were normal. The contrast transcranial Doppler echocardiography (cTCD) revealed a latent and massive right-to-left shunt (RLS) after the release of the Valsalva maneuver. The patient was diagnosed with migraine-induced dystonia of the lower limbs. Oral ibuprofen and flunarizine and avoidance of increased chest pressure maneuvers were used for treatment and prevention. During the 6-month follow-up, the patient was free of headaches and walking difficulties. Our study reported a rare case of migraine-induced dystonia of the lower extremities.

Paroxysmal Dyskinesias Revealing 3-Hydroxy-Isobutyryl-CoA Hydrolase (HIBCH) Deficiency

Paroxysmal dyskinesias (PD) are rare movement disorders characterized by recurrent attacks of dystonia, chorea, athetosis, or their combination, with large phenotypic and genetic heterogeneity. 3-Hydroxy-isobutyryl-CoA hydrolase (HIBCH) deficiency is a neurodegenerative disease characterized in most patients by a continuous decline in psychomotor abilities or a secondary regression triggered by febrile infections and metabolic crises.We describe two PD patients from two pedigrees, both carrying a homozygous c.913A > G, p.Thr305Ala mutation in the HIBCH gene, associated with an unusual clinical presentation. The first patient presented in the second year of life with right paroxysmal hemidystonia lasting for 30 minutes, without any loss of consciousness and without any triggering factor. The second patient has presented since the age of 3 recurrent exercise-induced PD episodes which have been described as abnormal equinovarus, contractures of the lower limbs, lasting for 1 to 4 hours, associated with choreic movements of the hands. Their neurological examination and metabolic screening were normal, while brain magnetic resonance imaging showed abnormal signal of the pallidi.We suggest that HIBCH deficiency, through the accumulation of metabolic intermediates of the valine catabolic pathway, leads to a secondary defect in respiratory chain activity and pyruvate dehydrogenase (PDH) activity and to a broad phenotypic spectrum ranging from Leigh syndrome to milder phenotypes. The two patients presented herein expand the spectrum of the disease to include unusual paroxysmal phenotypes and HIBCH deficiency should be considered in the diagnostic strategy of PD to enable adequate preventive treatment.

Pediatric Paroxysmal Exercise-Induced Neurological Symptoms: Clinical Spectrum and Diagnostic Algorithm

Paroxysmal exercise-induced neurological symptoms (PENS) encompass a wide spectrum of clinical phenomena commonly presenting during childhood and characteristically elicited by physical exercise. Interestingly, few shared pathogenetic mechanisms have been identified beyond the well-known entity of paroxysmal exercise-induced dyskinesia, PENS could be part of more complex phenotypes including neuromuscular, neurodegenerative, and neurometabolic disease, epilepsies, and psychogenetic disorders. The wide and partially overlapping phenotypes and the genetic heterogeneity make the differential diagnosis frequently difficult and delayed; however, since some of these disorders may be treatable, a prompt diagnosis is mandatory. Therefore, an accurate characterization of these symptoms is pivotal for orienting more targeted biochemical, radiological, neurophysiological, and genetic investigations and finally treatment. In this article, we review the clinical, genetic, pathophysiologic, and therapeutic landscape of paroxysmal exercise induced neurological symptoms, focusing on phenomenology and differential diagnosis.

PRRT2 Related Epilepsies: A Gene Review

PRRT2 encodes for proline-rich transmembrane protein 2 involved in synaptic vesicle fusion and presynaptic neurotransmitter release. Mutations in human PRRT2 have been related to paroxysmal kinesigenic dyskinesia (PKD), infantile convulsions with choreoathetosis, benign familial infantile epilepsies, and hemiplegic migraine. PRRT2 mutations cause neuronal hyperexcitability, which could be related to basal ganglia or cortical circuits dysfunction, leading to paroxysmal disorders. PRRT2 is expressed in the cerebral cortex, basal ganglia, and cerebellum. Approximately, 90% of pathogenic variants are inherited and 10% are de novo. Paroxysmal attacks in PKD are characterized by dystonia, choreoathetosis, and ballismus. In the benign familial infantile epilepsy (BFIE), seizures are usually focal with or without generalization, usually begin between 3 and 12 months of age and remit by 2 years of age. In 30% of cases of PRRT2-associated PKD, there is an association with BFIE, and this entity is referred to as PKD with infantile convulsions (PKD/IC). PRRT2 mutations are the cause of benign family childhood epilepsy and PKD/IC. On the other hand, PRRT2 mutations do not seem to correlate with other types of epilepsy. The increasing incidence of hemiplegic migraine in families with PRRT2-associated PKD or PKD/IC suggests a common disease pathway, and it is possible to assert that BFIE, paroxysmal kinesigenic dyskinesia, and PKD with IC belong to a continuous disease spectrum of PRRT2-associated diseases.

Genetic updates on paroxysmal dyskinesias

The paroxysmal dyskinesias are a diverse group of genetic disorders that manifest as episodic movements, with specific triggers, attack frequency, and duration. With recent advances in genetic sequencing, the number of genetic variants associated with paroxysmal dyskinesia has dramatically increased, and it is now evident that there is significant genotype-phenotype overlap, reduced (or incomplete) penetrance, and phenotypic variability. In addition, a variety of genetic conditions can present with paroxysmal dyskinesia as the initial symptom. This review will cover the 34 genes implicated to date and propose a diagnostic workflow featuring judicious use of whole-exome or -genome sequencing. The goal of this review is to provide a common understanding of paroxysmal dyskinesias so basic scientists, geneticists, and clinicians can collaborate effectively to provide diagnoses and treatments for patients.

Challenges in Clinicogenetic Correlations: One Gene - Many Phenotypes

Background

Progress in genetics – particularly the advent of next‐generation sequencing (NGS) – has enabled an unparalleled gene discovery and revealed unmatched complexity of genotype–phenotype correlations in movement disorders. Among other things, it has emerged that mutations in one and the same gene can cause multiple, often markedly different phenotypes. Consequently, movement disorder specialists have increasingly experienced challenges in clinicogenetic correlations.

Objectives

To deconstruct biological phenomena and mechanistic bases of phenotypic heterogeneity in monogenic movement disorders and neurodegenerative diseases. To discuss the evolving role of movement disorder specialists in reshaping disease phenotypes in the NGS era.

Methods

This scoping review details phenomena contributing to phenotypic heterogeneity and their underlying mechanisms.

Results

Three phenomena contribute to phenotypic heterogeneity, namely incomplete penetrance, variable expressivity and pleiotropy. Their underlying mechanisms, which are often shared across phenomena and non‐mutually exclusive, are not fully elucidated. They involve genetic factors (ie, different mutation types, dynamic mutations, somatic mosaicism, intragenic intra‐ and inter‐allelic interactions, modifiers and epistatic genes, mitochondrial heteroplasmy), epigenetic factors (ie, genomic imprinting, X‐chromosome inactivation, modulation of genetic and chromosomal defects), and environmental factors.

Conclusion

Movement disorders is unique in its reliance on clinical judgment to accurately define disease phenotypes. This has been reaffirmed by the NGS revolution, which provides ever‐growing sequencing data and fuels challenges in variant pathogenicity assertions for such clinically heterogeneous disorders. Deep phenotyping, with characterization and continual updating of “core” phenotypes, and comprehension of determinants of genotype–phenotype complex relationships are crucial for clinicogenetic correlations and have implications for the diagnosis, treatment and counseling.

Paroxysmal Genetic Movement Disorders and Epilepsy

Paroxysmal movement disorders include paroxysmal kinesigenic dyskinesia, paroxysmal non-kinesigenic dyskinesia, paroxysmal exercise-induced dyskinesia, and episodic ataxias. In recent years, there has been renewed interest and recognition of these disorders and their intersection with epilepsy, at the molecular and pathophysiological levels. In this review, we discuss how these distinct phenotypes were constructed from a historical perspective and discuss how they are currently coalescing into established genetic etiologies with extensive pleiotropy, emphasizing clinical phenotyping important for diagnosis and for interpreting results from genetic testing. We discuss insights on the pathophysiology of select disorders and describe shared mechanisms that overlap treatment principles in some of these disorders. In the near future, it is likely that a growing number of genes will be described associating movement disorders and epilepsy, in parallel with improved understanding of disease mechanisms leading to more effective treatments.

Homozygous ADCY5 mutation causes early-onset movement disorder with severe intellectual disability

BACKGROUND

Mutations of theADCY5 have been identified in patients with familial dyskinesia, early-onsetautosomal dominant chorea and dystonia, and benign hereditary chorea. Most ofthe ADCY5 mutations are de novo or transmitted in an autosomal dominantfashion. Only two pedigrees are known to show autosomal recessive inheritance.

OBJECTIVES

We report twosiblings with severe ID, dystonic movement, and growth failure with unknownetiology.

METHODS

We planned a proband-parentapproach using whole exome sequencing.

RESULTS

Homozygous mutationin exon 21 of the ADCY5 (p.R1238W) was identified in the siblings. Althoughtheir parents were heterozygous for the mutation, they were free from clinicalmanifestations.

CONCLUSIONS

Our results furtherexpand the phenotype/genotype correlations of the ADCY5-related disorders.Mutations of ADCY5 should be considered in pediatric patients with ID andinvoluntary movement.

The Spectrum of PRRT2-Associated Disorders: Update on Clinical Features and Pathophysiology

Mutations in the PRRT2 (proline-rich transmembrane protein 2) gene have been identified as the main cause of an expanding spectrum of disorders, including paroxysmal kinesigenic dyskinesia and benign familial infantile epilepsy, which places this gene at the border between epilepsy and movement disorders. The clinical spectrum has largely expanded to include episodic ataxia, hemiplegic migraine, and complex neurodevelopmental disorders in cases with biallelic mutations. Prior to the discovery of PRRT2 as the causative gene for this spectrum of disorders, the sensitivity of paroxysmal kinesigenic dyskinesia to anticonvulsant drugs regulating ion channel function as well as the co-occurrence of epilepsy in some patients or families fostered the hypothesis this could represent a channelopathy. However, recent evidence implicates PRRT2 in synapse functioning, which disproves the "channel hypothesis" (although PRRT2 modulates ion channels at the presynaptic level), and justifies the classification of these conditions as synaptopathies, an emerging rubric of brain disorders. This review aims to provide an update of the clinical and pathophysiologic features of PRRT2-associated disorders.

Levetiracetam-responsive paroxysmal exertional dyskinesia in a Welsh Terrier

A 5-and-a-half-year old, 9-kg, spayed, female Welsh Terrier presented with a 12 month history of paroxysmal exertion-induced dyskinesia (PED) characterized by recurrent episodes of involuntary hyperkinetic movements, abnormal muscle tone, and contractions triggered by exercise. A single episode occurred within 2 hours after exercise, lasted from 7 to 10 minutes, and resolved without treatment. The owner sought treatment for the dog when the episodes began to last longer (20-30 minutes), and occurred as long as 2.5 to 8 hours after exercise. Diazepam administered intranasally at the start of an episode promptly alleviated the symptoms. Maintenance therapy with levetiracetam proved effective, such that the dog was gradually returned to exercise. However, attempts to wean the dog off the drug resulted in reoccurrence. Although the pathophysiology of PED is not fully understood, the clinical presentation and the positive response to antiepileptic therapy highlight the overlap between disease pathways in epilepsy and PED in dogs.

Recommendations for the diagnosis and treatment of paroxysmal kinesigenic dyskinesia: an expert consensus in China

Paroxysmal dyskinesias are a group of neurological diseases characterized by intermittent episodes of involuntary movements with different causes. Paroxysmal kinesigenic dyskinesia (PKD) is the most common type of paroxysmal dyskinesia and can be divided into primary and secondary types based on the etiology. Clinically, PKD is characterized by recurrent and transient attacks of involuntary movements precipitated by a sudden voluntary action. The major cause of primary PKD is genetic abnormalities, and the inheritance pattern of PKD is mainly autosomal-dominant with incomplete penetrance. The proline-rich transmembrane protein 2 (PRRT2) was the first identified causative gene of PKD, accounting for the majority of PKD cases worldwide. An increasing number of studies has revealed the clinical and genetic characteristics, as well as the underlying mechanisms of PKD. By seeking the views of domestic experts, we propose an expert consensus regarding the diagnosis and treatment of PKD to help establish standardized clinical evaluation and therapies for PKD. In this consensus, we review the clinical manifestations, etiology, clinical diagnostic criteria and therapeutic recommendations for PKD, and results of genetic analyses in PKD patients performed in domestic hospitals.

The Identification of a Novel Fucosidosis-Associated FUCA1 Mutation: A Case of a 5-Year-Old Polish Girl with Two Additional Rare Chromosomal Aberrations and Affected DNA Methylation Patterns

Fucosidosis is a rare neurodegenerative autosomal recessive disorder, which manifests as progressive neurological and psychomotor deterioration, growth retardation, skin and skeletal abnormalities, intellectual disability and coarsening of facial features. It is caused by biallelic mutations in FUCA1 encoding the α-L-fucosidase enzyme, which in turn is responsible for degradation of fucose-containing glycoproteins and glycolipids. FUCA1 mutations lead to severe reduction or even loss of α-L-fucosidase enzyme activity. This results in incomplete breakdown of fucose-containing compounds leading to their deposition in different tissues and, consequently, disease progression. To date, 36 pathogenic variants in FUCA1 associated with fucosidosis have been documented. Among these are three splice site variants. Here, we report a novel fucosidosis-related 9-base-pair deletion (NG_013346.1:g.10233_10241delACAGGTAAG) affecting the exon 3/intron 3 junction within a FUCA1 sequence. This novel pathogenic variant was identified in a five-year-old Polish girl with a well-defined pattern of fucosidosis symptoms. Since it is postulated that other genetic, nongenetic or environmental factors can also contribute to fucosidosis pathogenesis, we performed further analysis and found two rare de novo chromosomal aberrations in the girl’s genome involving a 15q11.1-11.2 microdeletion and an Xq22.2 gain. These abnormalities were associated with genome-wide changes in DNA methylation status in the epigenome of blood cells.

Current challenges in the pathophysiology, diagnosis, and treatment of paroxysmal movement disorders

INTRODUCTION

Paroxysmal movement disorders mostly comprise paroxysmal dyskinesia and episodic ataxia, and can be the consequence of a genetic disorder or symptomatic of an acquired disease.

AREAS COVERED

In this review, the authors focused on certain hot-topic issues in the field: the respective contribution of the cerebellum and striatum to the generation of paroxysmal dyskinesia, the importance of striatal cAMP turnover in the pathogenesis of paroxysmal dyskinesia, the treatable causes of paroxysmal movement disorders not to be missed, with a special emphasis on the treatment strategy to bypass the glucose transport defect in paroxysmal movement disorders due to GLUT1 deficiency, and functional paroxysmal movement disorders.

EXPERT OPINION

Treatment of genetic causes of paroxysmal movement disorders is evolving towards precision medicine with targeted gene-specific therapy. Alteration of the cerebellar output and modulation of the striatal cAMP turnover offer new perspectives for experimental therapeutics, at least for paroxysmal movement disorders due to selected causes. Further characterization of cell-specific molecular pathways or network dysfunctions that are critically involved in the pathogenesis of paroxysmal movement disorders will likely result in the identification of new biomarkers and testing of innovative-targeted therapeutics.

Treatment of Paroxysmal Dyskinesia

Paroxysmal dyskinesia (PxD) is a heterogeneous group of syndromes characterized by recurrent attacks of abnormal movements, triggered by detectable factors, without loss of consciousness. According to the precipitating factors, they are classified as paroxysmal kinesigenic dyskinesia (PKD), paroxysmal non-kinesigenic dyskinesia (PNKD), and paroxysmal exercise-induced dystonia (PED). PxD treatment is based on the combination of nonpharmacologic and pharmacologic approaches. Pharmacologic and nonpharmacologic treatments effective for PNKD and PED also are available. In PxD refractory to conventional treatment, surgery might be an alternative therapeutic option. The course of PRRT2-PKD and MR-1-PNKD is benign, and treatment might not be needed with advancing age.

Paroxysmal Cranial Dyskinesia and Nail-Patella Syndrome Caused by a Novel Variant in the LMX1B Gene

BACKGROUND

In a Danish family, multiple individuals in five generations present with early-onset paroxysmal cranial dyskinesia, musculoskeletal abnormalities, and kidney dysfunction.

OBJECTIVE

To demonstrate linkage and to identify the underlying genetic cause of disease.

METHODS

Genome-wide single-nucleotide polymorphisms analysis, Sequence-Tagged-Site marker analyses, exome sequencing, and Sanger sequencing were performed.

RESULTS

Linkage analyses identified a candidate locus on chromosome 9. Exome sequencing revealed a novel variant in LMX1B present in all affected individuals, logarithm of the odds (LOD) score of z = 6.54, predicted to be damaging. Nail-patella syndrome (NPS) is caused by pathogenic variants in LMX1B encoding a transcription factor essential to cytoskeletal and kidney growth and dopaminergic and serotonergic network development. NPS is characterized by abnormal musculoskeletal features and kidney dysfunction. Movement disorders have not previously been associated with NPS.

CONCLUSIONS

Paroxysmal dyskinesia is a heretofore unrecognized feature of the NPS spectrum. The pathogenic mechanism might relate to aberrant dopaminergic circuits. © 2020 International Parkinson and Movement Disorder Society.

Novel mutation of the PRRT2 gene in two cases of paroxysmal kinesigenic dyskinesia: Two case reports

Paroxysmal kinesigenic dyskinesia (PKD) is a rare condition characterized by recurrent brief episodes of dystonia, chorea, athetosis or any combination of these, without alterations of consciousness. The proline-rich transmembrane protein 2 (PRRT2) gene has been widely investigated as a causative gene of PKD. To date, a cluster of pathogenic variants associated with PKD have been identified in the PRRT2 gene. In the present case report, two Chinese patients with sporadic PKD are discussed. Genetic analysis revealed a de novo heterozygous missense mutation, c.955G>T (p.Val319Leu) in exon 3 of the PRRT2 gene. Compared with the commonly reported clinical manifestation of PRRT2-associated PKD, the patients in this report showed several primary distinctive features. The mutations identified in the present analysis expand upon the mutation spectrum of the PRRT2 gene, and this newly found variant further reinforces the importance of the PRR2 gene in PKD.

How have advances in genetic technology modified movement disorder nosology?

The role of genetics and its technological development have been fundamental in advancing the field of movement disorders, opening the door to precision medicine. Starting from the revolutionary discovery of the locus of the Huntington's disease gene, we review the milestones of genetic discoveries in movement disorders and their impact on clinical practice and research efforts. Before the 1980s, early techniques did not allow the identification of genetic alteration in complex diseases. Further advances increasingly defined a large number of pathogenic genetic alterations. Moreover, these techniques allowed epigenomic, transcriptomic and microbiome analyses. In the 2020s, these new technologies are poised to displace phenotype-based classifications towards a nosology based on genetic/biological data. Advances in genetic technologies are engineering a reversal of the phenotype-to-genotype order of nosology development, replacing convergent clinicopathological disease models with the genotypic divergence required for future precision medicine applications.

Clinical and Genetic Overview of Paroxysmal Movement Disorders and Episodic Ataxias

Paroxysmal movement disorders (PMDs) are rare neurological diseases typically manifesting with intermittent attacks of abnormal involuntary movements. Two main categories of PMDs are recognized based on the phenomenology: Paroxysmal dyskinesias (PxDs) are characterized by transient episodes hyperkinetic movement disorders, while attacks of cerebellar dysfunction are the hallmark of episodic ataxias (EAs). From an etiological point of view, both primary (genetic) and secondary (acquired) causes of PMDs are known. Recognition and diagnosis of PMDs is based on personal and familial medical history, physical examination, detailed reconstruction of ictal phenomenology, neuroimaging, and genetic analysis. Neurophysiological or laboratory tests are reserved for selected cases. Genetic knowledge of PMDs has been largely incremented by the advent of next generation sequencing (NGS) methodologies. The wide number of genes involved in the pathogenesis of PMDs reflects a high complexity of molecular bases of neurotransmission in cerebellar and basal ganglia circuits. In consideration of the broad genetic and phenotypic heterogeneity, a NGS approach by targeted panel for movement disorders, clinical or whole exome sequencing should be preferred, whenever possible, to a single gene approach, in order to increase diagnostic rate. This review is focused on clinical and genetic features of PMDs with the aim to (1) help clinicians to recognize, diagnose and treat patients with PMDs as well as to (2) provide an overview of genes and molecular mechanisms underlying these intriguing neurogenetic disorders.

Generalized Dystonia and Paroxysmal Dystonic Attacks due to a Novel ATP1A3 Variant

Background

Paroxysmal movement disorders are a heterogeneous group of neurological diseases, better understood in recent years thanks to widely available genetic testing.

Case report

A pair of monozygotic twins with dystonia and paroxysmal attacks, resembling paroxysmal non-kinesigenic dyskinesias, due to a novel ATP1A3 variant are reported. The complete resolution of their paroxysms was achieved using levodopa and deep brain stimulation of the internal globus pallidus. Improvement of interictal dystonia was also achieved with this therapy.

Discussion

Paroxysmal worsening of movement disorders should be suspected as part of the ATP1A3 spectrum. Treatment outcome might be predicted based on the phenotype.

Parkinsonism in diseases predominantly presenting with dystonia

If the presence of dystonia is a well-recognized phenomenon in disorders predominantly presenting with parkinsonism, including sporadic Parkinson Disease, the term dystonia-parkinsonism usually refers to rare conditions, often genetic, in which the severity of dystonia usually equates that of parkinsonism. At variance with parkinsonian syndromes with additional dystonia, the conditions reviewed in this chapter have usually their onset in childhood and their diagnostic work-up is different. In fact, the phenotype is not usually specific of the underlying defect and additional investigations are therefore required. Here, we review the diseases predominantly presenting with dystonia where parkinsonism can develop, according to their main pathophysiological mechanism including disorders of dopamine biosynthesis, neurotransmitter transporter disorders, disorder of metal metabolism (i.e., iron, copper and manganese) and other inherited dystonia-parkinsonism conditions.

Naming Genes for Dystonia: DYT-z or Ditzy?

Dystonias are a clinically and etiologically diverse group of disorders. Numerous genes have now been associated with different dystonia syndromes, and multiple strategies have been proposed for how these genes should be lumped and split into meaningful categories. The traditional approach has been based on the Human Genome Organization’s plan for naming genetic loci for all disorders. For dystonia this involves a DYT prefix followed by a number (e.g., DYT1, DYT2, DYT3, etc.). A more recently proposed approach involves assigning multiple prefixes according to the main elements of the phenotype (e.g., DYT, PARK, CHOR, TREM, etc.) followed by the name of the responsible gene. This article describes these nomenclature systems and summarizes some of their limitations. We focus on dystonia as an example, although the concepts may be applied to all movement disorders.

Paroxysmal movement disorders - practical update on diagnosis and management

Introduction: Paroxysmal dyskinesias and episodic ataxias are often caused by mutations in genes related to cell membrane and synaptic function. Despite the exponential increase in publications of genetically confirmed cases, management remains largely clinical based on non-systematic evidence. Areas covered: The authors provide a historical and clinical review of the main types of paroxysmal dyskinesias and episodic ataxias, with recommendations for diagnosis and management of patients suffering from these conditions. Expert opinion: After secondary paroxysmal dyskinesias, the most common paroxysmal movement disorders are likely to be PRRT2-associated paroxysmal kinesigenic dyskinesias, which respond well to small doses of carbamazepine, and episodic ataxia type 2, which often responds to acetazolamide. Familial paroxysmal non-kinesigenic dyskinesias are largely caused by mutations in PNKD and have poor response to therapy but improve with age. Exercise-induced dyskinesias are genetically heterogeneous, caused by disorders of glucose transport, mitochondrial function, dopaminergic pathways or neurodegenerative conditions amongst others. GNAO1 and ADCY5 mutations can also cause paroxysmal movement disorders, often in the context of ongoing motor symptoms. Although a therapeutic trial is justified for classic cases and in limited resource settings, genetic testing may help direct initial or rescue therapy. Deep brain stimulation may be an option for severe cases.

Paroxysmal Movement Disorders: Recent Advances

PURPOSE OF REVIEW

Recent advancements in next-generation sequencing (NGS) have enabled techniques such as whole exome sequencing (WES) and whole genome sequencing (WGS) to be used to study paroxysmal movement disorders (PMDs). This review summarizes how the recent genetic advances have altered our understanding of the pathophysiology and treatment of the PMDs. Recently described disease entities are also discussed.

RECENT FINDINGS

With the recognition of the phenotypic and genotypic heterogeneity that occurs amongst the PMDs, an increasing number of gene mutations are now implicated to cause the disorders. PMDs can also occur as part of a complex phenotype. The increasing complexity of PMDs challenges the way we view and classify them. The identification of new causative genes and their genotype-phenotype correlation will shed more light on the underlying pathophysiology and will facilitate development of genetic testing guidelines and identification of novel drug targets for PMDs.

Clinical Features and Treatment in the Spectrum of Paroxysmal Dyskinesias: An Observational Study in South-West Castilla y Leon, Spain

BACKGROUND

Paroxysmal dyskinesias (PxD) are a group of heterogeneous disorders characterized by intermittent episodes of involuntary movements. PxD include paroxysmal kinesigenic (PKD), nonkinesigenic (PNK), and exercise-induced (PED) varieties.

OBJECTIVES

To define the phenotype of primary and secondary PxD forms.

METHODS

Twenty-two patients with PxD (9 men/13 women) were evaluated in two hospitals in south-west Castilla y Leon, Spain. Clinical features of the episodes, causes, family history, and response to treatment were collected.

RESULTS

Thirteen participants with primary PxD (6 men/7 women) and 9 with secondary PxD (3 men/6 women) were recruited. Nine patients belong to three nonrelated families (2 had PKD and 1 had PED). Mean age at onset in primary PKD cases was 10 years (range 5-23 years), earlier than in PNKD (24 years) and PED (20 years). Most primary PKD cases experienced daily episodes of duration <1 minute, which are more frequent and shorter attacks than in PNKD (1-2 per month, 5 minutes) and PED (1 per day, 15 minutes). The location of the involuntary movements varied widely; isolated dystonia was more common than mixed chorea and dystonia. All PKD patients who received antiepileptic treatment significantly improved. Levodopa and ketogenic diet proved to be effective in two patients with PED. Secondary forms presented a later mean age of onset (51 years). Six cases had PNKD, 1 had PKD, 1 both PNKD and PKD, and 1 had PED. Causes comprised vascular lesions, encephalitis, multiple sclerosis, peripheral trauma, endocrinopathies, and drugs such as selective serotonin reuptake inhibitors (SSRIs).

CONCLUSION

The knowledge of the clinical features and spectrum of causes related to PxD is crucial to avoid delays in diagnosis and treatment, or even a nonorganic disorder diagnosis.

Paroxysmal Asymmetric Dystonic Arm Posturing-A Less Recognized but Characteristic Manifestation of ATP1A3-related disease

Background

ATP1A3 mutations cause a wide clinical spectrum, and are one of the "commoner rare diseases".

Methods

Case series of four patients with ATP1A3 mutations.

Results

The patients displayed characteristic episodes of dystonic arm posturing, involving a dystonic, flexed arm held in front of the body or close to the body, but with the hand raised upwards. Other attacks manifested with arm extension, either beside the body or reaching upwards. Dystonic posturing occurred paroxysmally, with no neurological signs between attacks, or combined with other signs like chorea, ataxia, and hypotonia.

Conclusions

While previous diagnostic criteria have not included paroxysmal or episodic dystonia, recent expert consensus has proposed to include alternating or paroxysmal dystonia as major feature calling for ATP1A3 genetic testing. Attacks of marked arm flexion posturing, either paroxysmal or as episodic exacerbation of mild pre-existent dystonia, are a characteristic clue to ATP1A3-related disease.

Paroxysmal movement disorders: Recent advances and proposal of a classification system

The increasing recognition of the phenotypic and genotypic heterogeneity that exists amongst the paroxysmal movement disorders (PMDs) is challenging the way these disorders have been traditionally classified. The present review aims to summarize how recent genetic advances have influenced our understanding of the nosology, pathophysiology and treatment strategies of paroxysmal movement disorders. We propose classifying PMDs using a system that would combine both phenotype and genotype information to allow these disorders to be better categorized and studied. In the era of next generation sequencing, the use of a standardized algorithm and employment of selective genetic screening will lead to greater diagnostic certainty and targeted therapeutics for the patients.