Peripheral neuropathy in myotonic dystrophy type 1

Mouse models of non-dystrophic and dystrophic myotonia exhibit nociplastic pain-like behaviors

Pain is a prominent and debilitating symptom in myotonic disorders, yet its physiological mechanisms remain poorly understood. This study assessed preclinical pain-like behavior in murine models of pharmacologically induced myotonia and myotonic dystrophy type 1 (DM1). In both myotonia congenita and DM1, impairment of the CLCN1 gene, which encodes skeletal muscle voltage-gated CLC-1 chloride channels, reduces chloride ion conductance in skeletal muscle cells, leading to prolonged muscle excitability and delayed relaxation after contraction. We used the CLC-1 antagonist anthracene-9-carboxylic acid (9-AC) at intraperitoneal doses of 30 or 60 mg/kg and HSA LR20b DM1 mice to model CLC-1-induced myotonia. Our experimental approach included in vivo pain behavioral testing, ex vivo calcium imaging, and whole-cell current-clamp electrophysiology in mouse dorsal root ganglion (DRG) neurons. A single injection of 9-AC induced myotonia in mice, which persisted for several hours and resulted in long-lasting allodynic pain-like behavior. Similarly, HSA LR20b mice exhibited both allodynia and hyperalgesia. Despite these pain-like behaviors, DRG neurons did not show signs of hyperexcitability in either myotonic model. These findings suggest that myotonia induces nociplastic pain-like behavior in preclinical rodents, likely through central sensitization mechanisms rather than peripheral sensitization. This study provides insights into the pathophysiology of pain in myotonic disorders and highlights the potential of using myotonic mouse models to explore pain mechanisms and assess novel analgesics. Future research should focus on the central mechanisms involved in myotonia-induced pain and develop targeted therapies to alleviate this significant clinical burden.

Small and large fiber neuropathy in adults with myotonic dystrophy type 1

Introduction

Myotonic dystrophy type 1 (DM1) is an inherited neuromuscular disorder that affects multiple organs. In this study, we investigated symptoms of pain and presence of small and large fiber neuropathy in the juvenile and adult form of DM1.

Method

Twenty genetically verified DM1 patients were included. Pain was assessed, and neurological examination and investigations of the peripheral nervous system by quantification of small nerve fibers in skin biopsy, quantitative sensory testing and nerve conduction studies were performed. Results from skin biopsies were compared to healthy controls.

Result

Seventeen patients reported chronic pain. Large and/or small fiber abnormalities were present in 50% of the patients. The intraepidermal nerve fiber density was significantly lower in the whole group of patients compared to healthy controls.

Conclusion

Small-fiber neuropathy might be an important cause of pain in DM1.

Afterdischarges in myotonic dystrophy type 1

OBJECTIVE

Electrodiagnostic testing is an important screening test for myotonic dystrophy type 1 (DM1). Although myotonic discharges are observed on electromyography in cases of DM1, it is difficult to distinguish DM1 from other myotonic disorders clinically. In the present study, afterdischarges, another type of pathological potential revealed by electrodiagnostic testing, were analyzed, and their role in distinguishing DM1 from other myotonic disorders was explored.

METHODS

Data from 33 patients with myotonic discharges on electromyography were analyzed retrospectively. According to gene testing, the patients were divided into DM1 (n = 20) and non-DM1 myotonia (n = 13) groups. Afterdischarges were investigated by retrospectively evaluating the electrodiagnostic findings of motor nerve conduction studies, F-waves, and repetitive nerve stimulations.

RESULTS

Afterdischarges were observed in 17 of the 20 patients with DM1, with an occurrence rate of approximately 85%. However, afterdischarges were absent in all patients with non-DM1 myotonia. There were significant differences in the occurrence rate between the two groups (P < 0.01).

CONCLUSION

Afterdischarges may serve as a suggestive role in clinical diagnosis of DM1. The discovery that DM1 can present with afterdischarges may pave a new way to study the pathogenesis of DM1.

Motor unit number estimation by MScanFit in myotonic dystrophies

BACKGROUND

MScanFit is a new motor unit number estimation (MUNE) technique applied in motor neuron diseases and polyneuropathies to assess clinical progression and underlying disease pathology. So far, its value in myopathies, especially myotonic dystrophies (MD), has not been investigated.

METHODS

Motor unit loss and characteristics of patients with genetically confirmed MD type 1 (n = 7) and type 2 (n = 5) were investigated using MScanFit of the abductor pollicis brevis muscle and compared to age-matched healthy controls. MUNE measures were correlated with muscle impairment determined by the MRC sum score and handgrip strength.

RESULTS

MScanFit detected motor unit loss in patients with MD (p = 0.017). There was no significant difference in motor unit loss between MD type 1 and type 2 (p = 0.64). CMAP-discontinuities which, when added up, exceed 50% of the CMAP amplitude were reduced in MD patients (p = 0.0284), but motor unit amplitudes were not significantly different (p = 0.0597). The motor unit loss strongly correlated with the MRC sum score (p = 0.014, Rho = 0.678).

CONCLUSIONS

Our study shows the feasibility of MScanFit in MD and its potential to serve as a surrogate marker for overall muscle impairment. Motor unit analysis indicates that neurogenic alterations in both MD subtypes might be present.

Mexiletine in spinal and bulbar muscular atrophy: a randomized controlled trial

Objective

Patients with spinal and bulbar muscular atrophy (SBMA) often experience muscular weakness under cold exposure.

Methods

In our previously conducted observational study, we assessed nerve conduction and grip strength to examine the effect of cold exposure on motor function, based on which we conducted a randomized controlled trial to evaluate the efficacy and safety of mexiletine hydrochloride in SBMA (MEXPRESS).

Results

In the observational study, 51 consecutive patients with SBMA and 18 healthy controls (HCs) were enrolled. Of the patients with SBMA, 88.0% experienced cold paresis. Patients with SBMA exhibited greater prolongation of ulnar nerve distal latency under cold (SBMA, 5.6 ± 1.1 msec; HC, 4.3 ± 0.6 msec; p <0.001); the change in the distal latencies between room temperature and cold exposure conditions correlated with the change in grip power. In the MEXPRESS trial, 20 participants took mexiletine or lactose, three times a day for 4 weeks with a crossover design. There was no difference in distal latencies at room temperature and under cold exposure between mexiletine and placebo groups as the primary endpoint. However, tongue pressure and 10‐sec grip and release test under cold exposure were improved in the mexiletine group. There were no serious adverse events throughout the study period.

Interpretation

Cold paresis is common and associated with prolongation of distal latency in SBMA. The results of the phase II clinical trial revealed that mexiletine showed short‐term safety, but it did not restore cold exposure‐induced prolongation of distal latency.

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.

Cell type-specific abnormalities of central nervous system in myotonic dystrophy type 1

Myotonic dystrophy type 1 is a multisystem genetic disorder involving the muscle, heart and CNS. It is caused by toxic RNA transcription from expanded CTG repeats in the 3′-untranslated region of DMPK, leading to dysregulated splicing of various genes and multisystemic symptoms. Although aberrant splicing of several genes has been identified as the cause of some muscular symptoms, the pathogenesis of CNS symptoms prevalent in patients with myotonic dystrophy type 1 remains unelucidated, possibly due to a limitation in studying a diverse mixture of different cell types, including neuronal cells and glial cells. Previous studies revealed neuronal loss in the cortex, myelin loss in the white matter and the presence of axonal neuropathy in patients with myotonic dystrophy type 1. To elucidate the CNS pathogenesis, we investigated cell type-specific abnormalities in cortical neurons, white matter glial cells and spinal motor neurons via laser-capture microdissection. We observed that the CTG repeat instability and cytosine–phosphate–guanine (CpG) methylation status varied among the CNS cell lineages; cortical neurons had more unstable and longer repeats with higher CpG methylation than white matter glial cells, and spinal motor neurons had more stable repeats with lower methylation status. We also identified splicing abnormalities in each CNS cell lineage, such as DLGAP1 in white matter glial cells and CAMKK2 in spinal motor neurons. Furthermore, we demonstrated that aberrant splicing of CAMKK2 is associated with abnormal neurite morphology in myotonic dystrophy type 1 motor neurons. Our laser-capture microdissection-based study revealed cell type-dependent genetic, epigenetic and splicing abnormalities in myotonic dystrophy type 1 CNS, indicating the significant potential of cell type-specific analysis in elucidating the CNS pathogenesis.

Outcome measures frequently used to assess muscle strength in patients with myotonic dystrophy type 1: a systematic review

Measurement of muscle strength is fundamental for the management of patients with myotonic dystrophy type 1 (DM1). Nevertheless, guidance on this topic is somewhat limited due to heterogeneous outcome measures used. This systematic literature review aimed to summarize the most frequent outcome measures to assess muscle strength in patients with DM1. We searched on Pubmed, Web of Science and Embase databases. Observational studies using measures of muscle strength assessment in adult patients with DM1 were included. From a total of 80 included studies, 24 measured cardiac, 45 skeletal and 23 respiratory muscle strength. The most common method and outcome measures used to assess cardiac muscle strength were echocardiography and ejection fraction, for skeletal muscle strength were quantitative muscle test, manual muscle test and maximum isometric torque and medical research council and for respiratory muscle strength were manometry and maximal inspiratory and expiratory pressure. We successfully gathered the more consensual methods and measures to evaluate muscle strength in future clinical studies, particularly to test muscle strength response to treatments in patients with DM1. Future consensus on a set of measures to evaluate muscle strength (core outcome set), is important for these patients.

Pain in adult myotonic dystrophy type 1: relation to function and gender

Background

Pain is prevalent in myotonic dystrophy 1 (DM1). This study investigated whether CTG repeat size, disease duration, BMI and motor and psychological function were related to pain in adult patients with DM1, and if there were gender differences regarding intensity and location of pain.

Method

Cross-sectional design. Pain was investigated in 50 genetically confirmed DM1 patients by combining clinical assessment and self-reports of pain intensity and locations. Pain scoring results were related to CTG size, disease duration, muscle strength, walking capacity measured by 6-min walk test, activity of daily life by Katz ADL Index, respiratory function by Forced Vital Capacity and BMI. In addition, the degree of reported pain was related to Quality of life measured by WHOQOL-BREF; fatigue was measured by Fatigue severity scale; psychological functions were measured by Beck Depression Inventory, Beck Anxiety Inventory, IQ and Autism spectrum Quotient.

Results

Pain was reported in 84% of the patients and was significantly correlated with CTG size (r = 0.28 p = 0.050), disease duration (r = 0.38 p = 0.007), quality of life (r = − 0.37 p = 0.009), fatigue (r = 0.33 p = 0.02) and forced vital capacity (r = − 0.51, p = 0.005). Significant gender differences, with higher scores for females, were documented. In male subjects the number of pain locations was significantly correlated with quality of life and the autism quotient. In females, pain intensity was significantly correlated with activity, respiratory function and BMI.

Conclusions

Pain in DM1 was prevalent, with a strong association to lung function and other aspects of the disease. Significant gender differences were present for pain intensity and number of pain locations. How pain was related to other symptoms differed between male and female subjects. Our findings highlight the importance of assessments of pain in DM1 patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02124-9.

Myotonic dystrophy type 1 alters muscle twitch properties, spinal reflexes, and perturbation-induced trans-cortical reflexes

BACKGROUND

Neurophysiologic biomarkers are needed for clinical trials of therapies for myotonic dystrophy (DM1). We characterized muscle properties, spinal reflexes (H-reflexes), and trans-cortical long-latency reflexes (LLRs) in a cohort with mild/moderate DM1.

METHODS

Twenty-four people with DM1 and 25 matched controls underwent assessment of tibial nerve H-reflexes and soleus muscle twitch properties. Quadriceps LLRs were elicited by delivering an unexpected perturbation during a single-limb squat (SLS) visuomotor tracking task.

RESULTS

DM1 was associated with decreased H-reflex depression. The efficacy of doublet stimulation was enhanced, yielding an elevated double-single twitch ratio. DM1 participants demonstrated greater error during the SLS task. DM1 individuals with the least-robust LLR responses showed the greatest loss of spinal H-reflex depression.

CONCLUSIONS

DM1 is associated with abnormalities of muscle twitch properties. Co-occurring alterations of spinal and trans-cortical reflex properties underscore the central nervous system manifestations of this disorder and may assist in gauging efficacy during clinical trials.

Assessment of small sensory fiber function in myotonic dystrophy type 1

BACKGROUND

Myotonic dystrophy type 1 (DM1) is a multisystem disorder affecting the peripheral nervous system. However, studies evaluating somatic small fiber sensory nerve function, which may contribute to pain in DM1, are lacking.

METHODS

Using quantitative sensory testing of the hand and foot, we evaluated Aδ and C-fiber function. Of 20 adult DM1 patients recruited, 16 were analyzed. Their results were compared with those of 32 age- and sex-matched controls.

RESULTS

No DM1 patient had diabetes mellitus or clinical evidence of small fiber neuropathy. In DM1, hand (P < .01) and foot (P = 0.02) warm detection thresholds were higher than those of controls. Cool detection thresholds were lower in the foot (P < .001).

CONCLUSIONS

Subclinical small sensory fiber dysfunction occurs in DM1 patients without large fiber neuropathy. Further research with other modalities is required to characterize these disturbances as disease modifying therapies are developed.

Myotonic Dystrophies: Targeting Therapies for Multisystem Disease

Myotonic dystrophy is an autosomal dominant muscular dystrophy not only associated with muscle weakness, atrophy, and myotonia but also prominent multisystem involvement. There are 2 similar, but distinct, forms of myotonic dystrophy; type 1 is caused by a CTG repeat expansion in the DMPK gene, and type 2 is caused by a CCTG repeat expansion in the CNBP gene. Type 1 is associated with distal limb, neck flexor, and bulbar weakness and results in different phenotypic subtypes with variable onset from congenital to very late-onset as well as variable signs and symptoms. The classically described adult-onset form is the most common. In contrast, myotonic dystrophy type 2 is adult-onset or late-onset, has proximal predominant muscle weakness, and generally has less severe multisystem involvement. In both forms of myotonic dystrophy, the best characterized disease mechanism is a RNA toxic gain-of-function during which RNA repeats form nuclear foci resulting in sequestration of RNA-binding proteins and, therefore, dysregulated splicing of premessenger RNA. There are currently no disease-modifying therapies, but clinical surveillance, preventative measures, and supportive treatments are used to reduce the impact of muscular impairment and other systemic involvement including cataracts, cardiac conduction abnormalities, fatigue, central nervous system dysfunction, respiratory weakness, dysphagia, and endocrine dysfunction. Exciting preclinical progress has been made in identifying a number of potential strategies including genome editing, small molecule therapeutics, and antisense oligonucleotide-based therapies to target the pathogenesis of type 1 and type 2 myotonic dystrophies at the DNA, RNA, or downstream target level.

Core Clinical Phenotypes in Myotonic Dystrophies

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) represent the most frequent multisystemic muscular dystrophies in adulthood. They are progressive, autosomal dominant diseases caused by an abnormal expansion of an unstable nucleotide repeat located in the non-coding region of their respective genes DMPK for DM1 and CNBP in DM2. Clinically, these multisystemic disorders are characterized by a high variability of muscular and extramuscular symptoms, often causing a delay in diagnosis. For both subtypes, many symptoms overlap, but some differences allow their clinical distinction. This article highlights the clinical core features of myotonic dystrophies, thus facilitating their early recognition and diagnosis. Particular attention will be given to signs and symptoms of muscular involvement, to issues related to respiratory impairment, and to the multiorgan involvement. This article is part of a Special Issue entitled “Beyond Borders: Myotonic Dystrophies—A European Perception.”

Dysautonomia as Onset Symptom of Myotonic Dystrophy Type 2

Myotonic dystrophy type 2 (DM2) is an autosomal dominant muscular dystrophy caused by the expansion of an intronic tetranucleotide CCTG repeat in <i>CNBP</i> on chromosome 3. As DM1, DM2 is a multisystem disorder affecting, beside the skeletal muscle, various other tissues, including peripheral nerves. Indeed, a subclinical involvement of peripheral nervous system has been described in several cohorts of DM2 patients, whereas DM2 patients manifesting clinical signs and/or symptoms of neuropathy have been only rarely reported. Here, we describe 2 related DM2 patients both of whom displayed an atypical disease onset characterized by dysautonomic symptoms, possibly secondary to peripheral neuropathy.

Peripheral neuropathy in patients with myotonic dystrophy type 2

OBJECTIVES

Myotonic dystrophy type 2 (dystrophia myotonica type 2-DM2) is an autosomal dominant multi-organ disorder. The involvement of the peripheral nervous system was found in 25%-45% of patients with myotonic dystrophy type 1, although limited data are available concerning polyneuropathy in patients with DM2, which was the aim of this study with a thorough presentation of the cases with peripheral neuropathy.

MATERIAL AND METHODS

Patients with genetically confirmed DM2 underwent motor nerve conduction studies of the median, ulnar, tibial and fibular nerves and sensory nerve conduction studies of the median (second finger), ulnar (fifth finger), radial (forearm) and sural nerves.

RESULTS

Seventeen adult patients with DM2 participated in the study. Fifty-three percent (9/17) of our patients had abnormality of one or more attributes (latency, amplitude or conduction velocity) in two or more separate nerves. Four types of neuropathies were found: (i) predominantly axonal motor and sensory polyneuropathy, (ii) motor polyneuropathy, (iii) predominantly demyelinating motor and sensory polyneuropathy and (iv) mutilating polyneuropathy with ulcers. The most common forms are axonal motor and sensory polyneuropathy (29%) and motor neuropathy (18% of all examined patients). No correlations were found between the presence of neuropathy and age, CCTG repeats, blood glucose or HbA1C.

CONCLUSIONS

Peripheral neuropathy is common in patients with DM2 and presents one of the multisystemic manifestations of DM2.

Association of peripheral neuropathy with sleep-related breathing disorders in myotonic dystrophies

BACKGROUND

Myotonic dystrophy (DM) type 1 and type 2 are inherited diseases characterized by myotonia and myopathy. Additional symptoms include, among others, peripheral neuropathy and sleep-related breathing disorders (SRBDs). There is growing evidence for a complex association between DM1 and DM2, which was described in patients with diabetes mellitus and in the general population. In this study, we investigated whether there is an association between peripheral neuropathy and SRBDs also in the population of patients with DM.

METHODS

The study included 16 patients with DM1 (mean age, 37.9±14.1 years; 20-69 years) and eight patients with DM2 (mean age, 47.6±14.1 years; 20-65 years), who underwent a sensory and motor nerve conduction study (NCS) and diagnostic screening for SRBDs. In both groups, the NCS parameters were correlated with respiratory parameters.

RESULTS

In both groups, the amplitude of the ulnar sensory nerve action potential (SNAP) correlated with the mean arterial oxygen saturation (SaO₂). In addition, in the DM2 group, the median SNAP correlated with the mean SaO₂. In the DM1 group, the median SNAP and the distal motor latency (DML) of the ulnar nerve correlated with the apnea-hypopnea index, while the oxygen desaturation index correlated with the DML of the tibial nerve and with conduction velocity in the sural nerve.

CONCLUSION

Our results indicate a complex association between neuropathy and SRBDs in DM1 and DM2. Axonal degeneration may contribute to nocturnal hypoxemia and vice versa. Neuropathy may contribute to muscle weakness, which in turn may cause respiratory events.

Neuromuscular transmission abnormalities in myotonic dystrophy type 1: A neurophysiological study

OBJECTIVES

Weakness and fatigue are frequent symptoms in myotonic dystrophy type 1 (DM1), mainly as a result of muscle impairment. However, neuromuscular junction (NMJ) abnormalities could play an additional role in determining these manifestations. We aimed to document the possible NMJ involvement in DM1.

PATIENTS AND METHODS

In order to substantiate this hypothesis we performed low rate repetitive nerve stimulation (RNS) and single fiber electromyography (SFEMG), in 14 DM1 subjects.

RESULTS

RNS resulted abnormal in four patients while SFEMG revealed a pathological jitter in ten. A significative correlation was found between jitter values and decrementing response (p<0.000311; r=0.822).

CONCLUSION

These results suggest a possible involvement of NMJ in DM1.

Clinical characteristics of 37 Chinese patients with myotonic dystrophy Type 1

OBJECTIVE:

This study aims to investigate clinical characteristics of 37 Chinese patients with Myotonic dystrophy Type 1 (DM1).

METHODS:

Main clinical features of these cases were analyzed, with a focus on multi-system involvements.

RESULTS:

The median age of onset was 21.5 years, with a range from 3 to 45 years. Fourteen patients had a family history positive for DM1, whereas the other 23 were sporadic cases. Twenty-seven of the patients were male. The primary symptoms were myotonia and weakness with varying multi-system involvement including cardiac defects, cataracts, sleep disturbances, cholecystopathy, and peripheral neuropathy.

CONCLUSIONS:

This is the first report in China with the diagnosis of DM1 decisively confirmed by CTG expansion testing. Data from our study suggest that Chinese DM1 cases have different clinical characteristics compared with those of Caucasian cases, especially the prevalence of cardiac defects, cataracts, and sleep disturbances.

Gender as a Modifying Factor Influencing Myotonic Dystrophy Type 1 Phenotype Severity and Mortality: A Nationwide Multiple Databases Cross-Sectional Observational Study

Background

Myotonic Dystrophy type 1 (DM1) is one of the most heterogeneous hereditary disease in terms of age of onset, clinical manifestations, and severity, challenging both medical management and clinical trials. The CTG expansion size is the main factor determining the age of onset although no factor can finely predict phenotype and prognosis. Differences between males and females have not been specifically reported. Our aim is to study gender impact on DM1 phenotype and severity.

Methods

We first performed cross-sectional analysis of main multiorgan clinical parameters in 1409 adult DM1 patients (>18y) from the DM-Scope nationwide registry and observed different patterns in males and females. Then, we assessed gender impact on social and economic domains using the AFM-Téléthon DM1 survey (n = 970), and morbidity and mortality using the French National Health Service Database (n = 3301).

Results

Men more frequently had (1) severe muscular disability with marked myotonia, muscle weakness, cardiac, and respiratory involvement; (2) developmental abnormalities with facial dysmorphism and cognitive impairment inferred from low educational levels and work in specialized environments; and (3) lonely life. Alternatively, women more frequently had cataracts, dysphagia, digestive tract dysfunction, incontinence, thyroid disorder and obesity. Most differences were out of proportion to those observed in the general population. Compared to women, males were more affected in their social and economic life. In addition, they were more frequently hospitalized for cardiac problems, and had a higher mortality rate.

Conclusion

Gender is a previously unrecognized factor influencing DM1 clinical profile and severity of the disease, with worse socio-economic consequences of the disease and higher morbidity and mortality in males. Gender should be considered in the design of both stratified medical management and clinical trials.

Muscle wasting in myotonic dystrophies: a model of premature aging

Myotonic dystrophy type 1 (DM1 or Steinert’s disease) and type 2 (DM2) are multisystem disorders of genetic origin. Progressive muscular weakness, atrophy and myotonia are the most prominent neuromuscular features of these diseases, while other clinical manifestations such as cardiomyopathy, insulin resistance and cataracts are also common. From a clinical perspective, most DM symptoms are interpreted as a result of an accelerated aging (cataracts, muscular weakness and atrophy, cognitive decline, metabolic dysfunction, etc.), including an increased risk of developing tumors. From this point of view, DM1 could be described as a progeroid syndrome since a notable age-dependent dysfunction of all systems occurs. The underlying molecular disorder in DM1 consists of the existence of a pathological (CTG) triplet expansion in the 3′ untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene, whereas (CCTG)n repeats in the first intron of the Cellular Nucleic acid Binding Protein/Zinc Finger Protein 9(CNBP/ZNF9) gene cause DM2. The expansions are transcribed into (CUG)n and (CCUG)n-containing RNA, respectively, which form secondary structures and sequester RNA-binding proteins, such as the splicing factor muscleblind-like protein (MBNL), forming nuclear aggregates known as foci. Other splicing factors, such as CUGBP, are also disrupted, leading to a spliceopathy of a large number of downstream genes linked to the clinical features of these diseases. Skeletal muscle regeneration relies on muscle progenitor cells, known as satellite cells, which are activated after muscle damage, and which proliferate and differentiate to muscle cells, thus regenerating the damaged tissue. Satellite cell dysfunction seems to be a common feature of both age-dependent muscle degeneration (sarcopenia) and muscle wasting in DM and other muscle degenerative diseases. This review aims to describe the cellular, molecular and macrostructural processes involved in the muscular degeneration seen in DM patients, highlighting the similarities found with muscle aging.

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.

Lower limb muscle impairment in myotonic dystrophy type 1: the need for better guidelines

In myotonic dystrophy type 1 (DM1), leg muscle weakness is a major impairment. There are challenges to obtaining a clear portrait of muscle strength impairment. A systematic literature review was conducted on lower limb strength impairment in late-onset and adult phenotypes to document variables which affect strength measurement. Thirty-two articles were reviewed using the COSMIN guidelines. Only a third of the studies described a reproducible protocol. Only 2 muscle groups have documented reliability for quantitative muscle testing and only 1 total score for manual muscle testing. Variables affecting muscle strength impairment are not described in most studies. This review illustrates the variability in muscle strength assessment in relation to DM1 characteristics and the questionable validity of the results with regard to undocumented methodological properties. There is therefore a clear need to adopt a consensus on the use of a standardized muscle strength assessment protocol.

Clinical, psychosocial, and central correlates of quality of life in myotonic dystrophy type 1 patients

AIMS

To identify sociodemographic, clinical, and central correlates of health-related quality of life (HRQoL) in DM1 patients.

METHODS

200 DM1 patients had assessments of muscular impairment, CTG repeats, and intelligence. Validated instruments were used to assess sociodemographic and clinical factors as well as social support, social participation, daytime sleepiness, fatigue, personality, mood, and quality of life. Regression analysis was used to identify correlates of SF-36 physical and mental component summary scores.

RESULTS

Patients scored lower on all SF-36 physical health subscales compared with normative data but did not differ with respect to mental health function. Regression analysis revealed that psychological distress, fatigue, severe muscular impairment, emotional stability, not having worked within the last 12 months, and lower intellectual quotient were associated with lower scores in physical health function. Moreover, neuroticism, daytime sleepiness, dissatisfaction with social participation, and lower conscientiousness were associated with lower scores in mental health function.

CONCLUSION

DM1 has an impact on SF-36 physical summary scores but not on mental summary scores. Factors such as fatigue, daytime sleepiness, psychological distress, unemployment, and social participation dissatisfaction that significantly affect HRQoL in DM1 are amenable to treatment and psychosocial interventions, namely by providing care that integrate health, social, and community services.

Peripheral neuropathy is linked to a severe form of myotonic dystrophy in transgenic mice

Myotonic dystrophy type 1 (DM1) is a multisystem disorder with a variable phenotype. The involvement of peripheral nerves in DM1 disease is controversial. The DM1 animal model DM300 transgenic mice that carry 350 to 500 CTG repeats express a mild DM1 phenotype but do not exhibit motor or sensory pathology. Here, we investigated the presence or absence of peripheral neuropathy in transgenic mice (DMSXL) that carry more than 1,300 CTG repeats and display a severe form of DM1. Electrophysiologic, histologic, and morphometric methods were used to investigate the structure and function of peripheral nerves. We observed lower compound muscle action potentials recorded from hind limb muscles and slowing of sciatic nerve conduction velocity in DMSXL versus control mice. Morphometric analyses showed an axonopathy and neuronopathy in the DMSXL mice characterized by a decrease in numbers of myelinated motor axons in sciatic nerve and in spinal cord motor neurons. Pathologic alterations in the structure of hind limb neuromuscular junctions were also detected in the DMSXL mice. These results suggest that peripheral neuropathy can be linked to a large CTG expansion and a severe form of DM1.