Severity, type, and distribution of myotonic discharges are different in type 1 and type 2 myotonic dystrophy

Functions of the Muscleblind-like protein family and their role in disease

Conserved proteins are characterized by their functions remaining nearly constant throughout evolutionary history, both vertically through time and horizontally across species. In this review, we focus on a class of conserved proteins known as the Muscleblind-like (MBNL) family. As RNA-binding proteins, MBNL family members interact with pre-mRNAs through evolutionarily conserved tandem zinc finger domains and play critical roles in various RNA metabolic processes, including alternative splicing, mRNA stability, trafficking, regulation of subcellular localization, and alternative polyadenylation. Dysregulation of MBNL proteins can lead to severe consequences. Initially, research primarily associated MBNL proteins with myotonic dystrophy. However, recent studies have revealed their involvement in a broad spectrum of physiological and pathological processes, such as embryonic tissue differentiation and circulatory disorders. Furthermore, the emerging role of MBNL proteins in cancer sheds light on a novel aspect of these evolutionarily ancient proteins. This review provides a comprehensive overview of the MBNL family, emphasizing its structure, the mechanisms underlying its biological functions, and its roles in various diseases.Subject terms: Muscleblind-like-like protein, RNA-binding proteins, Alternative splicing, Tumor, Myotonic dystrophy.

Pediatric Neuromuscular Diseases

The diagnostic and referral workflow for children with neuromuscular disorders is evolving, particularly as newborn screening programs are expanding in tandem with novel therapeutic developments. However, for the children who present with symptoms and signs of potential neuromuscular disorders, anatomic localization, guided initially by careful history and physical examination, continues to be the cardinal initial step in the diagnostic evaluation. It is important to consider whether the localization is more likely to be in the lower motor neuron, peripheral nerve, neuromuscular junction, or muscle. After that, disease etiologies can be divided broadly into inherited versus acquired categories. Considerations of localization and etiologies will help generate a differential diagnosis, which in turn will guide diagnostic testing. Once a diagnosis is made, it is important to be aware of current treatment options, as a number of new therapies for some of these disorders have been approved in recent years. Families are also increasingly interested in clinical research, which may include natural history studies and interventional clinical trials. Such research has proliferated for rare neuromuscular diseases, leading to exciting advances in diagnostic and therapeutic technologies, promising dramatic changes in the landscape of these disorders in the years to come.

Clinical score for early diagnosis of myotonic dystrophy type 2

INTRODUCTION

Myotonic dystrophy type 2 (DM2) is a rare, multisystemic, autosomal dominant disease with highly variable clinical presentation. DM2 is considered to be highly underdiagnosed.

OBJECTIVE

The aim of this study was to determine which symptoms, signs, and diagnostic findings in patients referred to neurological outpatient units are the most indicative to arouse suspicion of DM2. We tried to make a useful and easy-to-administer clinical scoring system for early diagnosis of DM2-DM2 early diagnosis score (DM2-EDS).

PATIENTS AND METHODS

Two hundred ninety-one patients with a clinical suspicion of DM2 were included: 69 were genetically confirmed to have DM2, and 222 patients were DM2 negative. Relevant history, neurological, and paraclinical data were obtained from the electronic medical records.

RESULTS

The following parameters appeared as significant predictors of DM2 diagnosis: cataracts (beta = 0.410, p < 0.001), myotonia on needle EMG (beta = 0.298, p < 0.001), hand tremor (beta = 0.211, p = 0.001), positive family history (beta = 0.171, p = 0.012), and calf hypertrophy (beta = 0.120, p = 0.043). In the final DM2-EDS, based on the beta values, symptoms were associated with the following values: cataracts (present 3.4, absent 0), myotonia (present 2.5, absent 0), tremor (present 1.7, absent 0), family history (positive 1.4, negative 0), and calf hypertrophy (present 1.0, absent 0). A cut-off value on DM2-EDS of 3.25 of maximum 10 points had a sensitivity of 84% and specificity of 81% to diagnose DM2.

CONCLUSION

Significant predictors of DM2 diagnosis in the neurology outpatient unit were identified. We made an easy-to-administer DM2-EDS score for early diagnosis of DM2.

Balance impairment in myotonic dystrophy type 1: Dynamic posturography suggests the coexistence of a proprioceptive and vestibular deficit

Falls are frequent in Myotonic Dystrophy type 1 (DM1), but the pathophysiology of the balance impairment needs further exploration in this disease. The current work aims to provide a richer understanding of DM1 imbalance. Standing balance in 16 patients and 40 controls was tested in two posturographic tests (EquiTest™). In the Sensory Organization Test (SOT), standstill balance was challenged by combining visual (eyes open vs. closed) and environmental conditions (fixed vs. sway-tuned platform and/or visual surround). In the “react” test, reflexes induced by sudden shifts in the support base were studied. Oscillations of the body centre of mass (COM) were measured. In the SOT, COM sway was larger in patients than controls in any condition, including firm support with eyes open (quiet standing). On sway-tuned support, COM oscillations when standing with closed eyes were larger in patients than controls even after taking into account the oscillations with eyes open. In the “react” paradigm, balance reflexes were delayed in patients. Results in both experimental paradigms (i.e., SOT and react test) are consistent with leg muscle weakness. This, however, is not a sufficient explanation. The SOT test highlighted that patients rely on vision more than controls to maintain static balance. Consistently enough, evidence is provided that an impairment of proprioceptive and vestibular systems contributes to falls in DM1. Rehabilitation programs targeted at reweighting sensory systems may be designed to improve safe mobility in DM1.

Deciphering the mechanisms underlying brain alterations and cognitive impairment in congenital myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is a multisystemic and heterogeneous disorder caused by the expansion of CTG repeats in the 3' UTR of the myotonic dystrophy protein kinase (DMPK) gene. There is a congenital form (CDM1) of the disease characterized by severe hypotonia, respiratory insufficiency as well as developmental delays and intellectual disabilities. CDM1 infants manifest important brain structure abnormalities present from birth while, in contrast, older patients with adult-onset DM1 often present neurodegenerative features and milder progressive cognitive deficits. Promising therapies targeting central molecular mechanisms contributing to the symptoms of adult-onset DM1 are currently in development, but their relevance for treating cognitive impairment in CDM1, which seems to be a partially distinct neurodevelopmental disorder, remain to be elucidated. Here, we provide an update on the clinical presentation of CDM1 and review recent in vitro and in vivo models that have provided meaningful insights on its consequences in development, with a particular focus on the brain. We discuss how enhanced toxic gain-of-function of the mutated DMPK transcripts with larger CUG repeats and the resulting dysregulation of RNA-binding proteins may affect the developing cortex in utero. Because the methylation of CpG islets flanking the trinucleotide repeats has emerged as a strong biomarker of CDM1, we highlight the need to investigate the tissue-specific impacts of these chromatin modifications in the brain. Finally, we outline promising potential therapeutic treatments for CDM1 and propose future in vitro and in vivo models with great potential to shed light on this disease.

Electrodiagnostic Assessment of Myopathy

Electrodiagnostic testing is a useful tool in the evaluation of suspected myopathy and helps to confirm the presence of a myopathy and exclude disease mimickers. The electrodiagnostic pattern of findings during testing guides subsequent laboratory evaluation, genetic testing, and in identifying potential muscle biopsy targets. It also provides a baseline for subsequent assessment of disease progression or response to therapy. This article summarizes the approach to electrodiagnostic assessment in various myopathic disorders.

Current Treatment Options for Patients with Myotonic Dystrophy Type 2

Purpose of the review

Myotonic dystrophy types 1 and 2 are frequent forms of muscular dystrophies in adulthood. Their clinical differences need to be taken into account for the most appropriate treatment of patients. The aim of this article is to provide an overview on the current and upcoming therapeutic options for patients with myotonic dystrophy type 2 (DM2).

Recent findings

At the moment, no disease-modifying therapies are available for DM2; next-generation therapies may however be available in the near future. In the meanwhile, the symptomatic management of patients has greatly improved, thank to the production of consensus-based standards of care and the growing evidence of efficacy of anti-myotonic drugs, promising employment of cannabinoids for symptom’s relief, regular monitoring, and early detection of treatable extra-muscular manifestations.

Summary

The treatment of DM2 is currently symptomatic and relies on the coordinated intervention of a multidisciplinary team. It remains to be determined whether upcoming causal therapies for myotonic dystrophy type 1 will be applicable also in DM2.

New diagnostic and therapeutic modalities in neuromuscular disorders in children

Pediatric neuromuscular disorders are a diverse group of conditions that affect how muscle and nerve function. They involve the motor neurons, nerves, neuromuscular junction and muscles. Pathology of any of these regions leads to the inability to perform voluntary movements. Over time, the natural progression of most of these disorders is followed by significant disability, and at the most extreme, almost complete paralysis and death secondary to complications. Diagnostic measures for pediatric neuromuscular disorders, like that of most medical conditions, relies heavily on clinical presentation, history and a detailed physical examination. Primary additional diagnostic measures have included serum creatine kinase (CK) levels, electromyography (EMG), nerve conduction studies (NCS) and muscle or nerve biopsies, which has historically been the gold standard. In the last several decades less invasive testing has become more common such as muscle magnetic resonance imaging (MRI) and genetic testing.  The advances of molecular genetics, such as next generation sequencing (NGS) which includes whole-exome sequencing (WES) and whole-genome sequencing (WGS), enable clinicians to pinpoint more accurately exact gene mutations. The advent of genetic testing enhances personalized medicine. The field of pediatric neuromuscular disorders is also undergoing a remarkable evolution in therapeutic modalities including novel targeted therapies such as exon skipping/inclusion and gene replacement therapies. This is a review of the initial approach to suspected neuromuscular disorders in children as well as up to date diagnostic and therapeutic modalities for the most common pediatric neuromuscular disorders. As the world enters the new decade, there are encouraging therapeutic results. However, there remain key challenges to these modalities including limitations in its applicability, optimization for delivery of gene replacement therapies and in its effectiveness.

Myotonic Muscular Dystrophy Type 2 in CT, USA: A Single-Center Experience With 50 Patients

ABSTRACT

Myotonic dystrophy type 2 (DM2) is an autosomal dominant disorder due to a (CCTG)n repeat expansion in intron 1 of the CNBP gene. In this article, we report the clinicopathologic findings in 50 patients seen at a single site over a 27 year period. DM2 was the fifth most common type of muscular dystrophy seen at our center with a 5-fold lower frequency as compared to DM1. Age of symptom onset ranged from 15 to 72 years, and the mean duration between symptom onset and diagnosis was 7.4 years. Weakness referable to the proximal lower extremities was the presenting symptom in 62% of patients. The degree of generalized weakness varied from severe in 30% to no weakness in 20% of patients. Clinical myotonia was noted in 18% and myotonic discharges on electromyography in 97% of patients. Pain symptoms were uncommon in our cohort. A significant correlation was noted between limb weakness and degree of muscle pathologic changes. There was no correlation between CCTG repeat size and other clinicopathologic findings. Six patients (12%) had cardiac abnormalities including one who developed progressive nonischemic dilated cardiomyopathy ultimately leading to cardiac transplantation. In 21 patients followed for 2 or more years, we noted a mean rate of decline in total Medical Research Council score of about 1% per year.

Guidelines on clinical presentation and management of nondystrophic myotonias

The nondystrophic myotonias are rare muscle hyperexcitability disorders caused by gain-of-function mutations in the SCN4A gene or loss-of-function mutations in the CLCN1 gene. Clinically, they are characterized by myotonia, defined as delayed muscle relaxation after voluntary contraction, which leads to symptoms of muscle stiffness, pain, fatigue, and weakness. Diagnosis is based on history and examination findings, the presence of electrical myotonia on electromyography, and genetic confirmation. In the absence of genetic confirmation, the diagnosis is supported by detailed electrophysiological testing, exclusion of other related disorders, and analysis of a variant of uncertain significance if present. Symptomatic treatment with a sodium channel blocker, such as mexiletine, is usually the first step in management, as well as educating patients about potential anesthetic complications.

Myotonic Muscular Dystrophies

PURPOSE OF REVIEW

This article describes the clinical features, pathogenesis, prevalence, diagnosis, and management of myotonic dystrophy type 1 and myotonic dystrophy type 2.

RECENT FINDINGS

The prevalence of myotonic dystrophy type 1 is better understood than the prevalence of myotonic dystrophy type 2, and new evidence indicates that the risk of cancer is increased in patients with the myotonic dystrophies. In addition, descriptions of the clinical symptoms and relative risks of comorbidities such as cardiac arrhythmias associated with myotonic dystrophy type 1 have been improved.

SUMMARY

Myotonic dystrophy type 1 and myotonic dystrophy type 2 are both characterized by progressive muscle weakness, early-onset cataracts, and myotonia. However, both disorders have multisystem manifestations that require a comprehensive management plan. While no disease-modifying therapies have yet been identified, advances in therapeutic development have a promising future.

rbFOX1/MBNL1 competition for CCUG RNA repeats binding contributes to myotonic dystrophy type 1/type 2 differences

Myotonic dystrophy type 1 and type 2 (DM1, DM2) are caused by expansions of CTG and CCTG repeats, respectively. RNAs containing expanded CUG or CCUG repeats interfere with the metabolism of other RNAs through titration of the Muscleblind-like (MBNL) RNA binding proteins. DM2 follows a more favorable clinical course than DM1, suggesting that specific modifiers may modulate DM severity. Here, we report that the rbFOX1 RNA binding protein binds to expanded CCUG RNA repeats, but not to expanded CUG RNA repeats. Interestingly, rbFOX1 competes with MBNL1 for binding to CCUG expanded repeats and overexpression of rbFOX1 partly releases MBNL1 from sequestration within CCUG RNA foci in DM2 muscle cells. Furthermore, expression of rbFOX1 corrects alternative splicing alterations and rescues muscle atrophy, climbing and flying defects caused by expression of expanded CCUG repeats in a Drosophila model of DM2.

Myotonic Dystrophy and Developmental Regulation of RNA Processing

Myotonic dystrophy (DM) is a multisystemic disorder caused by microsatellite expansion mutations in two unrelated genes leading to similar, yet distinct, diseases. DM disease presentation is highly variable and distinguished by differences in age-of-onset and symptom severity. In the most severe form, DM presents with congenital onset and profound developmental defects. At the molecular level, DM pathogenesis is characterized by a toxic RNA gain-of-function mechanism that involves the transcription of noncoding microsatellite expansions. These mutant RNAs disrupt key cellular pathways, including RNA processing, localization, and translation. In DM, these toxic RNA effects are predominantly mediated through the modulation of the muscleblind-like and CUGBP and ETR-3-like factor families of RNA binding proteins (RBPs). Dysfunction of these RBPs results in widespread RNA processing defects culminating in the expression of developmentally inappropriate protein isoforms in adult tissues. The tissue that is the focus of this review, skeletal muscle, is particularly sensitive to mutant RNA-responsive perturbations, as patients display a variety of developmental, structural, and functional defects in muscle. Here, we provide a comprehensive overview of DM1 and DM2 clinical presentation and pathology as well as the underlying cellular and molecular defects associated with DM disease onset and progression. Additionally, fundamental aspects of skeletal muscle development altered in DM are highlighted together with ongoing and potential therapeutic avenues to treat this muscular dystrophy. © 2018 American Physiological Society. Compr Physiol 8:509-553, 2018.

The Dystrophic and Nondystrophic Myotonias

PURPOSE OF REVIEW

This article describes clinical and electrical myotonia and provides an update on the classification, diagnosis, and management of myotonic disorders.

RECENT FINDINGS

In the myotonic dystrophies, antisense oligonucleotides provide a general strategy to correct RNA gain of function and modulate the expression of CTG expanded repeats; they are currently being tested in a phase 1-2 randomized controlled trial in patients with adult-onset myotonic dystrophy type 1. New genetic mutations are continuously being identified in the nondystrophic myotonias involving sodium and chloride channels. This contributes to the difficulty in describing genotype-phenotype correlations as the same mutations can give rise to different phenotypes, and the same phenotypes can arise from different mutations. Pharmacologic therapy is moving toward mutation-targeted treatments.

SUMMARY

This article describes the clinical and diagnostic characteristics and management of the myotonic dystrophies and the nondystrophic myotonias. Clinical features of the congenital, juvenile, and classic adult forms of myotonic dystrophy type 1 are reviewed, and for the adult form, reference is made to the main diagnostic and follow-up tests for which general consensus exists. The different clinical presentations of myotonic dystrophy type 2 and its main differential diagnostic options are also discussed. The clinical spectrum of the sodium and chloride channelopathies is described, and clinical diagnostic clues to differentiate between these two groups are provided. Therapeutic options for patients with nondystrophic myotonias are also presented with reference to literature review and the author's personal experience.

Biomolecular diagnosis of myotonic dystrophy type 2: a challenging approach

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are the most common adult form of muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia, and multiorgan involvement. The onset and symptoms of the myotonic dystrophies are diverse, complicating their diagnoses and limiting a comprehensive approach to their clinical care. Diagnostic delay in DM2 is due not only to the heterogeneous phenotype and the aspecific onset but also to the unfamiliarity with the disorder by most clinicians. Moreover, the DM2 diagnostic odyssey is complicated by the difficulties to develop an accurate, robust, and cost-effective method for a routine molecular assay. The aim of this review is to underline by challenging approach the diagnostic limits and pitfalls that could results in failure to recognize the presence of DM2 disease. Understanding and preventing delays in DM2 diagnosis may facilitate family planning, improve symptom management in the short term, and facilitate more specific treatment in the long term.

Electromechanical delays during a fatiguing exercise and recovery in patients with myotonic dystrophy type 1

PURPOSE

The partitioning of the electromechanical delay by an electromyographic (EMG), mechanomyographic (MMG) and force combined approach can provide further insight into the electrochemical and mechanical processes involved with skeletal muscle contraction and relaxation. The aim of the study was to monitor by this combined approach the changes in delays' electrochemical and mechanical components throughout a fatiguing task and during recovery in patients with myotonic dystrophy type 1 (DM1), who present at the skeletal muscle level fibres rearrangement, muscle weakness and myotonia, especially in the distal muscles.

METHODS

After assessing maximum voluntary contraction (MVC), 14 male patients with DM1 and 14 healthy controls (HC) performed a fatiguing exercise at 50% MVC until exhaustion. EMG, MMG, and force signals were recorded from tibialis anterior and vastus lateralis muscles. The electromechanical delay during contraction (Delay_TOT) and relaxation (R-Delay_TOT) components, EMG and MMG root mean square (RMS) and mean frequency (MF) were calculated off-line.

RESULTS

The fatiguing exercise duration was similar in both groups. In patients with DM1, delays components were significantly longer compared to HC, especially in the distal muscle during relaxation. Delays components recovered quickly from the fatiguing exercise in HC than in patients with DM1 in both muscles.

CONCLUSIONS

The alterations in delays observed in DM1 during the fatiguing exercise may indicate that also the lengthening of the electrochemical and mechanical processes during contraction and relaxation could play a role in explaining exercise intolerance in this pathology.

Myotonic dystrophy health index: Correlations with clinical tests and patient function

INTRODUCTION

The Myotonic Dystrophy Health Index (MDHI) is a disease-specific patient-reported outcome measure. Here, we examine the associations between the MDHI and other measures of disease burden in a cohort of individuals with myotonic dystrophy type-1 (DM1).

METHODS

We conducted a cross-sectional study of 70 patients with DM1. We examined the associations between MDHI total and subscale scores and scores from other clinical tests. Participants completed assessments of strength, myotonia, motor and respiratory function, ambulation, and body composition. Participants also provided blood samples, underwent physician evaluations, and completed other patient-reported outcome measures.

RESULTS

MDHI total and subscale scores were strongly associated with muscle strength, myotonia, motor function, and other clinical measures.

CONCLUSIONS

Patient-reported health status, as measured by the MDHI, is associated with alternative measures of clinical health. These results support the use of the MDHI as a valid tool to measure disease burden in DM1 patients.

Use of Clinical and Electrical Myotonia to Differentiate Childhood Myopathies

We retrospectively reviewed 2030 childhood electromyograms performed over an 11-year period (2004-2014). Twenty children (1%) with myotonic discharges were identified and placed into 2 groups. Group A (electrical and clinical myotonia) comprised 9 children (8 with myotonia congenita and 1 with paramyotonia congenita); all of them had diffuse myotonic discharges without clinical weakness or elevated creatine kinase. Group B (electrical myotonia without clinical myotonia) comprised 11 children (4 with inflammatory myopathy; 3, congenital myopathy, 3, muscular dystrophy; and 1, congenital muscular dystrophy). Clinical weakness was demonstrated in all of them and elevated creatine kinase in 6; all had a myopathic electromyogram and scattered myotonic discharges. We conclude that myotonic discharges are a rare but characteristic spontaneous discharge identified during electrodiagnostic studies in children. The presence of electrical and clinical myotonia provides helpful clues to differentiate between various muscle disorders in children.

Peripheral nerve involvement in myotonic dystrophy type 2 - similar or different than in myotonic dystrophy type 1?

INTRODUCTION

Multisystem manifestations of myotonic dystrophies type 1 (DM1) and 2 (DM2) are well known. Peripheral nerve involvement has been reported in DM1 but not in genetically confirmed DM2. The aim of our study was to assess peripheral nerve involvement in DM2 using nerve conduction studies and to compare these results with findings in DM1.

METHODS

We prospectively studied patients with genetically confirmed DM2 (n=30) and DM1 (n=32). All patients underwent detailed neurological examination and nerve conduction studies.

RESULTS

Abnormalities in electrophysiological studies were found in 26.67% of patients with DM2 and in 28.13% of patients with DM1 but the criteria of polyneuropathy were fulfilled in only 13.33% of patients with DM2 and 12.5% of patients with DM1. The polyneuropathy was subclinical, and no correlation was found between its presence and patient age or disease duration.

CONCLUSIONS

Peripheral nerves are quite frequently involved in DM2, but abnormalities meeting the criteria of polyneuropathy are rarely found. The incidence of peripheral nerve involvement is similar in both types of myotonic dystrophy.

Electromyographic findings in 37 patients with adult-onset acid maltase deficiency

INTRODUCTION

In acid maltase deficiency (AMD), electrical myotonia (EM) may be restricted to paraspinal muscles. A comprehensive description of the electromyographic (EMG) findings in AMD is lacking. The purpose of this study is to describe the EMG features in adult-onset AMD, focusing on the distribution of EM.

METHODS

A retrospective chart review of AMD patients diagnosed at Mayo Clinic over age 18 years.

RESULTS

Thirty-seven patients were included. Twenty-eight (76%) had EM in at least 1 muscle, and EM was more common in paraspinal and proximal limb muscles. The tensor fasciae latae (TFL) was equally sensitive to the paraspinals for EM. Three of 4 patients had EM identified in the diaphragm.

CONCLUSIONS

Approximately three-quarters of adult-onset AMD patients display EM on EMG. The paraspinal muscles and TFL are the most likely to demonstrate EM, and EM can be detected in the diaphragm of adult onset AMD patients.

Myotonic discharges discriminate chloride from sodium muscle channelopathies

Non-dystrophic myotonic syndromes represent a heterogeneous group of clinically quite similar diseases sharing the feature of myotonia. These syndromes can be separated into chloride and sodium channelopathies, with gene-defects in chloride or sodium channel proteins of the sarcolemmal membrane. Myotonia has its basis in an electrical instability of the sarcolemmal membrane. In the present study we examine the discriminative power of the resulting myotonic discharges for these disorders. Needle electromyography was performed by an electromyographer blinded for genetic diagnosis in 66 non-dystrophic myotonia patients (32 chloride and 34 sodium channelopathy). Five muscles in each patient were examined. Individual trains of myotonic discharges were extracted and analyzed with respect to firing characteristics. Myotonic discharge characteristics in the rectus femoris muscle almost perfectly discriminated chloride from sodium channelopathy patients. The first interdischarge interval as a single variable was longer than 30 ms in all but one of the chloride channelopathy patients and shorter than 30 ms in all of the sodium channelopathy patients. This resulted in a detection rate of over 95%. Myotonic discharges of a single muscle can be used to better guide toward a molecular diagnosis in non-dystrophic myotonic syndromes.

Myotonic dystrophy

Myotonic dystrophy (dystrophia myotonica, DM) is one of the most common lethal monogenic disorders in populations of European descent. DM type 1 was first described over a century ago. More recently, a second form of the disease, DM type 2 was recognized, which results from repeat expansion in a different gene. Both disorders have autosomal dominant inheritance and multisystem features, including myotonic myopathy, cataract, and cardiac conduction disease. This article reviews the clinical presentation and pathophysiology of DM and discusses current management and future potential for developing targeted therapies.

The diagnosis and treatment of myotonic disorders

Myotonia is a defining clinical symptom and sign common to a relatively small group of muscle diseases, including the myotonic dystrophies and the nondystrophic myotonic disorders. Myotonia can be observed on clinical examination, as can its electrical correlate, myotonic discharges, on electrodiagnostic testing. Research interest in the myotonic disorders continues to expand rapidly, which justifies a review of the scientific bases, clinical manifestations, and numerous therapeutic approaches associated with these disorders. We review the pathomechanisms of myotonia, the clinical features of the dystrophic and nondystrophic myotonic disorders, and the diagnostic approach and treatment options for patients with symptomatic myotonia.

Electrodiagnosis of myotonic disorders

Clinical and electrical myotonia is caused by a small group of neuromuscular disorders. This article reviews myotonia and its differential diagnosis. The use of electrodiagnostic testing to evaluate the primary myotonic disorders (myotonic dystrophy and the nondystrophic myotonias) is also discussed.

Electrodiagnostic evaluation of myopathies

Electrodiagnostic studies play an important role in the evaluation of patients suspected of having a myopathic disorder. They are used to exclude alternative diagnoses, confirm the presence of muscle disease, narrow down the differential, and identify an appropriate biopsy site. The most informative part of the electrodiagnostic study is needle electromyography. This allows for the analysis of spontaneous activity and motor unit action potential morphology and recruitment patterns. This article proposes a practical electrodiagnostic approach and describes the electrophysiologic patterns of the most commonly encountered myopathies.

Needle electromyography: basic concepts and patterns of abnormalities

Needle electromyography (EMG) records electrical signals generated from muscle fibers and interprets the signals to characterize underlying pathologic changes that are occurring in motor units within muscles. Different types of spontaneously firing waveforms and motor unit potential changes occur with different neuromuscular disorders. The performance of reliable EMG studies depends on the technical skills of the physician in inserting, moving, recording with a needle electrode, and analyzing electric signals recorded from muscle. This article reviews the technique of needle EMG and recognition and interpretation of various EMG waveforms. The author presents several demonstrative videos at www.neurologic.theclinics.com.

Myotonic dystrophy types 1 and 2

Myotonic dystrophies (dystrophia myotonica, or DM) are inherited disorders characterized by myotonia and progressive muscle degeneration, which are variably associated with a multisystemic phenotype. To date, two types of myotonic dystrophy, type 1 (DM1) and type 2 (DM2), are known to exist; both are autosomal dominant disorders caused by expansion of an untranslated short tandem repeat DNA sequence (CTG)(n) and (CCTG)(n), respectively. These expanded repeats in DM1 and DM2 show different patterns of repeat-size instability. Phenotypes of DM1 and DM2 are similar but there are some important differences, most conspicuously in the severity of the disease (including the presence or absence of the congenital form), muscles primarily affected (distal versus proximal), involved muscle fiber types (type 1 versus type 2 fibers), and some associated multisystemic phenotypes. The pathogenic mechanism of DM1 and DM2 is thought to be mediated by the mutant RNA transcripts containing expanded CUG and CCUG repeats. Strong evidence supports the hypothesis that sequestration of muscle-blind like (MBNL) proteins by these expanded repeats leads to misregulated splicing of many gene transcripts in corroboration with the raised level of CUG-binding protein 1. However, additional mechanisms, such as changes in the chromatin structure involving CTCN-binding site and gene expression dysregulations, are emerging. Although treatment of DM1 and DM2 is currently limited to supportive therapies, new therapeutic approaches based on pathogenic mechanisms may become feasible in the near future.

Spider dystrophy as an ocular manifestation of myotonic dystrophy

BACKGROUND

Myotonic dystrophy is the most common adult-onset muscular dystrophy. It is an autosomal dominant inherited neuromuscular disease that is characterized by myotonia, muscle weakness, and atrophy. It affects multiple systems including skeletal muscular, gastrointestinal, cardiac, respiratory, central nervous, endocrine, and ocular. Ocular manifestations of myotonic dystrophy include cataract, ocular muscle changes, hypotony, and retinal pigmentary changes in the periphery or in the macula (known as pigment pattern dystrophy). This report presents and discusses the case of a pigmented pattern dystrophy known as spider dystrophy as an ocular manifestation of myotonic dystrophy.

CASE REPORT

A 44-year-old man with myotonic dystrophy presented to the eye clinic for routine examination. Ocular history included previous bilateral cataract surgery and mild bilateral ptosis for the last "few years." Dilated fundus examination was remarkable for bilateral macular pigmentary changes in an irregular "spider"-shaped pattern. The patient was asymptomatic without decrease in vision or Amsler grid defects. Optical coherence tomography was normal. A retinal consult concurred with the diagnosis of spider dystrophy. Photo documentation was obtained, and the patient is being monitored annually.

CONCLUSION

Pigmented pattern dystrophies, including spider dystrophy, have been associated with myotonic dystrophy. They are set apart from other retinal dystrophies because they rarely affect visual acuity, and the majority of patients are asymptomatic. Progression may lead to reduced vision and in rare cases choroidal neovascularization. Annual dilated examinations, photo documentation, optical coherence tomography, and home Amsler grid monitoring are recommended for follow-up care.

Absent, unrecognized, and minimal myotonic discharges in myotonic dystrophy type 2

The purpose of this study was to describe the frequency of absent, unrecognized, or minimal myotonic discharges (MDs) in myotonic dystrophy type 2 (DM2). We performed a retrospective review of needle electromyography (EMG) data prior to genetic diagnosis in 49 DM2 patients at the Mayo Clinic. MDs were not reported on first or repeat EMG studies (n = 8) and not found in archived recordings of 4 patients (8%); archived EMG recordings (n = 4) confirmed the absence of MDs (n = 2), including 1 patient with normal insertional activity in all muscles, and misinterpretation of MDs as slow fibrillation potentials (n = 1) and complex repetitive discharge (CRD) activity (n = 1). Eight (16%) patients had minimal classic MDs with diffusely increased insertional activity, including waning-only MDs in all patients in this group with archived EMG recordings (n = 5). Diffuse MDs were found in 33 (67%) patients. Absent or minimal MDs do not exclude DM2. Over-reliance on diffuse MDs in patients who present with myopathy may lead to delay in genetic diagnosis of DM2.

HTS-Compatible Patient-Derived Cell-Based Assay to Identify Small Molecule Modulators of Aberrant Splicing in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a genetic disorder characterized by muscle wasting, myotonia, cataracts, cardiac arrhythmia, hyperinsulinism and intellectual deficits, and is caused by expansion of a CTG repeat in the 3’UTR of the Dystrophia Myotonica-Protein Kinase (DMPK) gene. The DMPK transcripts containing expanded CUG repeats accumulate in nuclear foci and ultimately cause mis-splicing of secondary genes through the dysregulation of RNA-binding proteins including Muscleblind 1 (MBNL1) and CUG binding protein 1 (CUGBP1). Correction of mis-splicing of genes such as the Skeletal muscle-specific chloride channel 1 (CLCN1), Cardiac troponin T (TNNT2), Insulin receptor (INSR) and Sarcoplasmic/endoplasmic reticulum Ca²⁺ATPase 1 (SERCA1) may alleviate some of the symptoms of DM1; hence identification of small molecule modulators is an important step towards a therapy for DM1 patients. Here we describe the generation of immortalized myoblast cell lines derived from healthy (DMPK CTG₅) and DM1 patient (DMPK CTG₁₀₀₀) fibroblasts by constitutive overexpression of human telomerase reverse transcriptase (hTERT) and inducible overexpression of the Myoblast determination factor (MYOD). MBNL1-containing nuclear foci, mis-splicing events and defective myotube differentiation defects characteristic of DM1 were observed in these cells. A CLCN1 luciferase minigene construct (CLCN1-luc) was stably introduced to monitor intron 2 retention in the DM1 cellular context (a reported splicing defect in DM1). The assay was validated by performing a high-throughput screen (HTS) of ~13,000 low molecular weight compounds against the CLCN1-luc DM1 myoblast cell line, providing an ideal system for conducting HTS to better understand and treat DM1.

Evoked myotonia can be "dialed-up" by increasing stimulus train length in myotonic dystrophy type 1

It is unknown how evoked myotonia varies with stimulus frequency or train length, or how it compares to voluntary myotonia in myotonic dystrophy type 1 (DM1). First dorsal interosseous (FDI) tetanic contractions evoked by trains of 10-20 ulnar nerve stimuli at 10-50 HZ were recorded in 10 DM1 patients and 10 normals. For comparison, maximum voluntary handgrip contractions were also recorded. An automated computer program placed cursors along the declining (relaxation) phase of the force recordings at 90% and 5% of peak force (PF) and calculated relaxation times (RTs) between these points. For all stimulus frequencies and train lengths, evoked RTs were much shorter, and evoked PFs were much greater in normals than in DM1. In normals, evoked RT was independent of stimulus frequency and train length, while in DM1 RT was longer for train lengths of 20 stimuli (mean: 9 s in DM1; 0.20 in normals) than for 10 stimuli (mean: 3 s in DM1, 0.19 in normals), but it did not change with stimulus frequency. In both groups PF increased greatly as stimulus frequency rose from 10-50 HZ but only slightly as train length rose from 10-20 stimuli. Voluntary handgrip RT (mean: 1.9 s) was less than evoked FDI RT (mean: 9 s). In DM1, evoked RT can be "dialed up" by increasing stimulus train length. Evoked myotonia testing utilizing a stimulus paradigm of at least 20 stimuli at 30-50 HZ may be useful in antimyotonic drug trials, particularly when grip RT is normal or equivocal.

Quantitative myotonia assessment using force relaxation curve modelling

The lack of a robust quantitative measure of myotonia has been underlined in previous studies. Recent publications have proposed methods to quantify myotonia based on the measurement of force relaxation times during maximal contractions. However, they present several drawbacks mainly due to unstable force, odd peaks or digital noise. A possible solution to this issue consists in fitting the force curve with a convenient regression model. The aim of this study was, therefore, to provide a regression model in order to fit the force relaxation time curve automatically and to provide a robust index for quantitative assessment of myotonia in clinical settings. Force curves were fitted by an asymmetric sigmoidal function. The inverse function was then used to compute various absolute and relative relaxation times automatically. These variables were calculated for 16 controls and 16 patients with myotonic dystrophy type 1 (DM1). All variables were significantly increased in DM1 patients compared to controls. For instance, the relaxation time between 40 and 60% of the initial contraction level was 18.2 (SD: 3.3) ms in controls and 40.1 (SD: 17.7) ms in DM1 patients. All relaxation variables were highly discriminant. Force curve modelling provides an objective and effective quantification of myotonia.

Myotonic dystrophy type 2 with focal asymmetric muscle weakness and no electrical myotonia

Genetically proven myotonic dystrophy type 2 (DM2) was found in a 61-year-old woman with creatine kinase (CK) elevation and only isolated weakness of one triceps. There was no clinical or electrical myotonia. Electromyography (EMG) showed only scattered fibrillation potentials and short duration motor unit potentials. Muscle biopsy showed nonspecific myopathic features and highly atrophic fibers with nuclear clumps. DM2 should be considered in patients with focal proximal weakness and abnormal EMG without myotonic discharges.

Needle electromyography

Physiologic assessment of diseases of the motor unit from the anterior horn cells to the muscles relies on a combination of needle electromyography (EMG) and nerve conduction studies (NCS). Both require a unique combination of knowledge of peripheral nervous system anatomy, physiology, pathophysiology, diseases, techniques, and electricity is necessary. Successful, high-quality, reproducible EMG depends on the skills of a clinician in patient interaction during the physical insertion and movement of the needle while recording the electrical signals. These must be combined with the skill of analyzing electric signals recorded from muscle by auditory pattern recognition and semiquantitation.1052 This monograph reviews the techniques of needle EMG and waveform analysis and describes the types of EMG waveforms recorded during needle EMG.

Myotonic dystrophy: therapeutic strategies for the future

Myotonic dystrophy (DM) is a dominantly inherited neurodegenerative disorder for which there is no cure or effective treatment. Investigation of DM pathogenesis has identified a novel disease mechanism that requires development of innovative therapeutic strategies. It is now clear that DM is not caused by expression of a mutant protein. Instead, DM is the first recognized example of an RNA-mediated disease. Expression of the mutated gene gives rise to an expanded repeat RNA that is directly toxic to cells. The mutant RNA is retained in the nucleus, forming ribonuclear inclusions in affected tissue. A primary consequence of RNA toxicity in DM is dysfunction of two classes of RNA binding proteins, which leads to abnormal regulation of alternative splicing, or spliceopathy, of select genes. Spliceopathy now is known to cause myotonia and insulin resistance in DM. As our understanding of pathogenesis continues to improve, therapy targeted directly at the RNA disease mechanism will begin to replace the supportive care currently available. New pharmacologic approaches to treat myotonia and muscle wasting in DM type 1 are already in early clinical trials, and therapies designed to reverse the RNA toxicity have shown promise in preclinical models by correcting spliceopathy and eliminating myotonia. The well-defined ribonuclear inclusions may serve as convenient therapeutic targets to identify new agents that modify RNA toxicity. Continued development of appropriate model systems will allow testing of additional therapeutic strategies as they become available. Although DM is a decidedly complex disorder, its RNA-mediated disease mechanism may prove to be highly susceptible to therapy.