Clinical and Molecular Spectrum of Degenerative Cerebellar Ataxia: A Single Centre Study

Tremor in Spinocerebellar Ataxia: A Scoping Review

Background

Spinocerebellar ataxia (SCA) denotes an expanding list of autosomal dominant cerebellar ataxias. Although tremor is an important aspect of the clinical spectrum of the SCAs, its prevalence, phenomenology, and pathophysiology are unknown.

Objectives

This review aims to describe the various types of tremors seen in the different SCAs, with a discussion on the pathophysiology of the tremors, and the possible treatment modalities.

Methods

The authors conducted a literature search on PubMed using search terms including tremor and the various SCAs. Relevant articles were included in the review after excluding duplicate publications.

Results

While action (postural and intention) tremors are most frequently associated with SCA, rest and other rare tremors have also been documented. The prevalence and types of tremors vary among the different SCAs. SCA12, common in certain ethnic populations, presents a unique situation, where the tremor is typically the principal manifestation. Clinical manifestations of SCAs may be confused with essential tremor or Parkinson's disease. The pathophysiology of tremors in SCAs predominantly involves the cerebellum and its networks, especially the cerebello-thalamo-cortical circuit. Additionally, connections with the basal ganglia, and striatal dopaminergic dysfunction may have a role. Medical management of tremor is usually guided by the phenomenology and associated clinical features. Deep brain stimulation surgery may be helpful in treatment-resistant tremors.

Conclusions

Tremor is an elemental component of SCAs, with diverse phenomenology, and emphasizes the role of the cerebellum in tremor. Further studies will be useful to delineate the clinical, pathophysiological, and therapeutic aspects of tremor in SCAs.

An Update on the Adult-Onset Hereditary Cerebellar Ataxias: Novel Genetic Causes and New Diagnostic Approaches

The hereditary cerebellar ataxias (HCAs) are rare, progressive neurologic disorders caused by variants in many different genes. Inheritance may follow autosomal dominant, autosomal recessive, X-linked or mitochondrial patterns. The list of genes associated with adult-onset cerebellar ataxia is continuously growing, with several new genes discovered in the last few years. This includes short-tandem repeat (STR) expansions in RFC1, causing cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS), FGF14-GAA causing spinocerebellar ataxia type 27B (SCA27B), and THAP11. In addition, the genetic basis for SCA4, has recently been identified as a STR expansion in ZFHX3. Given the large and growing number of genes, and different gene variant types, the approach to diagnostic testing for adult-onset HCA can be complex. Testing methods include targeted evaluation of STR expansions (e.g. SCAs, Friedreich ataxia, fragile X-associated tremor/ataxia syndrome, dentatorubral-pallidoluysian atrophy), next generation sequencing for conventional variants, which may include targeted gene panels, whole exome, or whole genome sequencing, followed by various potential additional tests. This review proposes a diagnostic approach for clinical testing, highlights the challenges with current testing technologies, and discusses future advances which may overcome these limitations. Implementing long-read sequencing has the potential to transform the diagnostic approach in HCA, with the overall aim to improve the diagnostic yield.

Genetic Testing of Movements Disorders: A Review of Clinical Utility

Currently, pathogenic variants in more than 500 different genes are known to cause various movement disorders. The increasing accessibility and reducing cost of genetic testing has resulted in increasing clinical use of genetic testing for the diagnosis of movement disorders. However, the optimal use case(s) for genetic testing at a patient level remain ill-defined. Here, we review the utility of genetic testing in patients with movement disorders and also highlight current challenges and limitations that need to be considered when making decisions about genetic testing in clinical practice.

Highlights

The utility of genetic testing extends across multiple clinical and non-clinical domains. Here we review different aspects of the utility of genetic testing for movement disorders and the numerous associated challenges and limitations. These factors should be weighed on a case-by-case basis when requesting genetic tests in clinical practice.

Diagnostic Yield of NGS Tests for Hereditary Ataxia: a Systematic Review

Next-generation sequencing (NGS), comprising targeted panels (TP), exome sequencing (ES), and genome sequencing (GS) became robust clinical tools for diagnosing hereditary ataxia (HA). Determining their diagnostic yield (DY) is crucial for optimal clinical decision-making. We conducted a comprehensive systematic literature review on the DY of NGS tests for HA. We searched PubMed and Embase databases for relevant studies between 2016 and 2022 and manually examined reference lists of relevant reviews. Eligible studies described the DY of NGS tests in patients with ataxia as a significant feature. Data from 33 eligible studies showed a median DY of 43% (IQR = 9.5-100%). The median DY for TP and ES was 46% and 41.9%, respectively. Higher DY was associated with specific phenotype selection, such as episodic ataxia at 68.35% and early and late onset of ataxia at 46.4% and 54.4%. Parental consanguinity had a DY of 52.4% (p = 0.009), and the presumed autosomal recessive (AR) inheritance pattern showed 62.5%. There was a difference between the median DY of studies that performed targeted sequencing (tandem repeat expansion, TRE) screening and those that did not (p = 0.047). A weak inverse correlation was found between DY and the extent of previous genetic investigation (rho = - 0.323; p = 0.065). The most common genes were CACNA1A and SACS. DY was higher for presumed AR inheritance pattern, positive family history, and parental consanguinity. ES appears more advantageous due to the inclusion of rare genes that might be excluded in TP.