Computerized hand grip myometry reliably measures myotonia and muscle strength in myotonic dystrophy (DM1)

Antisense oligonucleotide targeting DMPK in patients with myotonic dystrophy type 1: a multicentre, randomised, dose-escalation, placebo-controlled, phase 1/2a trial

BACKGROUND

Myotonic dystrophy type 1 results from an RNA gain-of-function mutation, in which DM1 protein kinase (DMPK) transcripts carrying expanded trinucleotide repeats exert deleterious effects. Antisense oligonucleotides (ASOs) provide a promising approach to treatment of myotonic dystrophy type 1 because they reduce toxic RNA levels. We aimed to investigate the safety of baliforsen (ISIS 598769), an ASO targeting DMPK mRNA.

METHODS

In this dose-escalation phase 1/2a trial, adults aged 20-55 years with myotonic dystrophy type 1 were enrolled at seven tertiary referral centres in the USA and randomly assigned via an interactive web or phone response system to subcutaneous injections of baliforsen 100 mg, 200 mg, or 300 mg, or placebo (6:2 randomisation at each dose level), or to baliforsen 400 mg or 600 mg, or placebo (10:2 randomisation at each dose level), on days 1, 3, 5, 8, 15, 22, 29, and 36. Sponsor personnel directly involved with the trial, participants, and all study personnel were masked to treatment assignments. The primary outcome measure was safety in all participants who received at least one dose of study drug up to day 134. This trial is registered with ClinicalTrials.gov (NCT02312011), and is complete.

FINDINGS

Between Dec 12, 2014, and Feb 22, 2016, 49 participants were enrolled and randomly assigned to baliforsen 100 mg (n=7, one patient not dosed), 200 mg (n=6), 300 mg (n=6), 400 mg (n=10), 600 mg (n=10), or placebo (n=10). The safety population comprised 48 participants who received at least one dose of study drug. Treatment-emergent adverse events were reported for 36 (95%) of 38 participants assigned to baliforsen and nine (90%) of ten participants assigned to placebo. Aside from injection-site reactions, common treatment-emergent adverse events were headache (baliforsen: ten [26%] of 38 participants; placebo: four [40%] of ten participants), contusion (baliforsen: seven [18%] of 38; placebo: one [10%] of ten), and nausea (baliforsen: six [16%] of 38; placebo: two [20%] of ten). Most adverse events (baliforsen: 425 [86%] of 494; placebo: 62 [85%] of 73) were mild in severity. One participant (baliforsen 600 mg) developed transient thrombocytopenia considered potentially treatment related. Baliforsen concentrations in skeletal muscle increased with dose.

INTERPRETATION

Baliforsen was generally well tolerated. However, skeletal muscle drug concentrations were below levels predicted to achieve substantial target reduction. These results support the further investigation of ASOs as a therapeutic approach for myotonic dystrophy type 1, but suggest improved drug delivery to muscle is needed.

FUNDING

Ionis Pharmaceuticals, Biogen.

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.

Quantitative muscle MRI as a sensitive marker of early muscle pathology in myotonic dystrophy type 1

BACKGROUND

Quantitative muscle MRI as a sensitive marker of early muscle pathology and disease progression in adult-onset myotonic dystrophy type 1. The utility of muscle MRI as a marker of muscle pathology and disease progression in adult-onset myotonic dystrophy type 1 (DM1) was evaluated.

METHODS

This prospective, longitudinal study included 67 observations from 36 DM1 patients (50% female), and 92 observations from 49 healthy adults (49% female). Lower-leg 3T magnetic resonance imaging (MRI) scans were acquired. Volume and fat fraction (FF) were estimated using a three-point Dixon method, and T2-relaxometry was determined using a multi-echo spin-echo sequence. Muscles were segmented automatically. Mixed linear models were conducted to determine group differences across muscles and image modality, accounting for age, sex, and repeated observations. Differences in rate of change in volume, T2-relaxometry, and FF were also determined with mixed linear regression that included a group by elapsed time interaction.

RESULTS

Compared with healthy adults, DM1 patients exhibited reduced volume of the tibialis anterior, soleus, and gastrocnemius (GAS) (all, P < .05). T2-relaxometry and FF were increased across all calf muscles in DM1 compared to controls. (all, P < .01). Signs of muscle pathology, including reduced volume, and increased T2-relaxometry and FF were already noted in DM1 patients who did not exhibit clinical motor symptoms of DM1. As a group, DM1 patients exhibited a more rapid change than did controls in tibialis posterior volume (P = .05) and GAS T2-relaxometry (P = .03) and FF (P = .06).

CONCLUSIONS

Muscle MRI renders sensitive, early markers of muscle pathology and disease progression in DM1. T2 relaxometry may be particularly sensitive to early muscle changes related to DM1.

Muscle Ultrasound Shear Wave Elastography as a Non-Invasive Biomarker in Myotonia

Myotonia, i.e., delayed muscle relaxation in certain hereditary muscle disorders, can be assessed quantitatively using different techniques ranging from force measurements to electrodiagnostics. Ultrasound shear wave elastography (SWE) has been proposed as a novel tool in biomechanics and neuromuscular medicine for the non-invasive estimation of muscle elasticity and, indirectly, muscle force. The aim of this study is to provide ‘proof-of-principle’ that SWE allows a quantitative measurement of the duration of delayed muscle relaxation in myotonia in a simple clinical setting. In six myotonic muscle disorder patients and six healthy volunteers, shear wave velocities (SWV) parallel to the fiber orientation in the flexor digitorum superficialis muscle in the forearm were recorded with a temporal resolution of one per second during fist-clenching and subsequent relaxation; the relaxation time to 10% of normalized shear wave velocity (RT_0.1) was calculated. Forty-six SWE imaging sequences were acquired, yielding a mean RT_0.1 of 7.38 s in myotonic muscle disorder patients, significantly higher than in healthy volunteers (1.36 s), which is comparable to data obtained by mechanical dynamometry. SWV measurements during the baseline relaxation and voluntary contraction phases did not differ significantly between groups. We conclude that SWE is a promising, non-invasive, widely available tool for the quantitative assessment of myotonia to aid in diagnosis and therapeutic monitoring.

Mexiletine in Myotonic Dystrophy Type 1: A Randomized, Double-Blind, Placebo-Controlled Trial

OBJECTIVE

To assess mexiletine's long-term safety and effect on 6-minute walk distance in a well-defined cohort of patients with myotonic dystrophy type 1 (DM1).

METHODS

We performed a randomized, double-blind, placebo-controlled trial of mexiletine (150 mg 3 times daily) to evaluate its efficacy and safety in a homogenous cohort of adult ambulatory patients with DM1. The primary outcome was change in 6-minute walk distance at 6 months. Secondary outcomes included changes in hand grip myotonia, strength, swallowing, forced vital capacity, lean muscle mass, Myotonic Dystrophy Health Index scores, and 24-hour Holter and ECG results at 3 and 6 months.

RESULTS

Forty-two participants were randomized and 40 completed the 6-month follow-up (n = 20 in both groups). No significant effects of mexiletine were observed on 6-minute walk distance, but hand grip myotonia was improved with mexiletine treatment. There were no differences between the mexiletine and placebo groups with respect to the frequency or type of adverse events. Changes in PR, QRS, and QTc intervals were similar in mexiletine- and placebo-treated participants.

CONCLUSIONS

There was no benefit of mexiletine on 6-minute walk distance at 6 months. Although mexiletine had a sustained positive effect on objectively measured hand grip myotonia, this was not seen in measures reflecting participants' perceptions of their myotonia. No effects of mexiletine on cardiac conduction measures were seen over the 6-month follow-up period.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that for ambulatory patients with DM1, mexiletine does not significantly change 6-minute walk distance at 6 months.

Allele length of the DMPK CTG repeat is a predictor of progressive myotonic dystrophy type 1 phenotypes

Myotonic dystrophy type 1 (DM1) is an autosomal dominant inherited disorder caused by expansion of a germline and somatically unstable CTG repeat in the DMPK gene. Previously, CTG repeat length at birth has been correlated to patient age at symptom onset. Attempts to correlate CTG repeat length with progressive DM1 phenotypes, such as muscle power, have proven difficult. To better correlate genotype with progressive phenotypes, we have measured CTG repeat tract length and screened for interrupting variant repeats in 192 study participants from a well-characterized Canadian cohort. We have assessed genotype-phenotype correlations with nine progressive measures of skeletal muscle power and respiratory function. We have built statistical models that include confounding factors such as sex, age, height and weight to further explain variation in muscle power. Our analysis reveals a strong correlation between DM1 genotype and respiratory function and skeletal muscle power, as part of a complex model that includes additional modulators such as sex, age, height, weight and the presence or absence of interrupting variant repeats. Distal skeletal muscle measurements, such as hand pinch and grip strength, show the strongest correlation with disease genotype. Detailed analysis of CTG repeat length, and incorporation of confounding factors, greatly improves the predictive ability of these models. They reveal a greater genetic influence on individual progressive phenotypes than on age at symptom onset and for clinical trials will help optimize stratification and explain patient variability. They will also help practitioners prioritize assessment of the muscular power measurements that correlate best with disease severity.

Quantitative myotonia assessment with a commercially available dynamometer in myotonic dystrophy types 1 and 2

INTRODUCTION

The objective of this study was to develop a simple method for quantitative assessment of myotonia in patients with myotonic dystrophy type 1 (DM1) and DM2, to compare the myotonia severity, and to correlate this objective outcome with a subjective scale, the Myotonia Behaviour Scale (MBS).

METHODS

A commercially available dynamometer was used for all measurements. The relaxation time after voluntary contraction was measured in 20 patients with DM1, 25 patients with DM2, and 35 healthy controls.

RESULTS

The average relaxation time was 0.17 s in controls, 2.96 s in patients with DM1, and 0.4 s in patients with DM2. The correlation between relaxation time and MBS score was significant, 0.627 in patients with DM1 and 0.581 in patients with DM2.

DISCUSSION

Our method provides a valid and reliable quantitative measure of grip myotonia suitable as an outcome measure in clinical trials and as part of routine examinations to gather data on the natural history of myotonic disorders. Muscle Nerve 59:431-435, 2019.

Ankle Strength Impairments in Myotonic Dystrophy Type 1: A Five-Year Follow-up

BACKGROUND

In Myotonic Dystrophy type 1 (DM1) patients, ankle muscles are affected early and this impairment is reported to be a good biological marker for longitudinal studies.

OBJECTIVE

To characterize the ankle dorsiflexion (DF) and eversion (EV) maximal isometric muscle strength changes in adult DM1 patients over 5 years using a standardized handheld dynamometer protocol and the Myoankle method, compare the changes measured with both methods and to the standard error of measurement, and verify the relationship between ankle muscle strength and gait performance.

METHODS

The maximal isometric muscle strength of ankle DF and EV in DM1 patients from Quebec and Lyon was assessed at baseline, 18, 36 and 60 months using a handheld dynamometer (HHD) protocol and the MyoAnkle method.

RESULTS

There was a decrease of torque in DF/EV of 36.0% /31.3% and 27.7% /35.5% for the Quebec and Lyon cohorts respectively (p≤0.01), but not in a linear way. In most cases (82.5%), the changes observed were greater than the standard error of measurement. DF torque measures taken by the two methods (HHD and MyoAnkle) were highly correlated (rp = 0.97-0.98, p < 0.001).

CONCLUSION

Muscle strength ankle impairments are clinically meaningful in DM1 and can be accurately monitored using quantitative testing to measure the efficacy of therapeutic trials.

Repeatability and reliability of muscle relaxation properties induced by motor cortical stimulation

Impaired muscle relaxation is a feature of many neuromuscular disorders. However, few tests are available to quantify muscle relaxation. Transcranial magnetic stimulation (TMS) of the motor cortex can induce muscle relaxation by abruptly inhibiting corticospinal drive. The aim of our study was to investigate whether repeatability and reliability of TMS-induced relaxation are greater than voluntary relaxation. Furthermore, effects of sex, cooling, and fatigue on muscle relaxation properties were studied. Muscle relaxation of deep finger flexors was assessed in 25 healthy subjects (14 men and 11 women, age 39.1 ± 12.7 and 45.3 ± 8.7 yr, respectively) with handgrip dynamometry. All outcome measures showed greater repeatability and reliability in TMS-induced relaxation compared with voluntary relaxation. The within-subject coefficient of variability of normalized peak relaxation rate was lower in TMS-induced relaxation than in voluntary relaxation (3.0% vs. 19.7% in men and 6.1% vs. 14.3% in women). The repeatability coefficient was lower (1.3 vs. 6.1 s^-1 in men and 2.3 vs. 3.1 s^-1 in women) and the intraclass correlation coefficient was higher (0.95 vs. 0.53 in men and 0.78 vs. 0.69 in women) for TMS-induced relaxation compared with voluntary relaxation. TMS enabled demonstration of slowing effects of sex, muscle cooling, and muscle fatigue on relaxation properties that voluntary relaxation could not. In conclusion, repeatability and reliability of TMS-induced muscle relaxation were greater compared with voluntary muscle relaxation. TMS-induced muscle relaxation has the potential to be used in clinical practice for diagnostic purposes and therapy effect monitoring in patients with impaired muscle relaxation. NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation demonstrates greater repeatability and reliability compared with voluntary relaxation, represented by the ability to demonstrate typical effects of sex, cooling, and fatigue on muscle relaxation properties that were not seen in voluntary relaxation. In clinical practice, TMS-induced muscle relaxation could be used for diagnostic purposes and therapy effect monitoring. Furthermore, fewer subjects will be needed for future studies when using TMS to demonstrate differences in muscle relaxation properties.

Relationships between grip strength, myotonia, and CTG expansion in myotonic dystrophy type 1

In myotonic dystrophy type 1, several studies have suggested causal relationships between CTG repeat length and the severity of symptoms, such as weakness or myotonia. We aimed to explore these relationships in a large population of 144 DM1 patients. All patients underwent clinical and functional assessments using a standardized test for grip strength and myotonia assessment. Myotonia was assessed using a fully automatic software based on mathematical modeling of relaxation force curve. CTG repeat length was statistically correlated with both myotonia and grip strength, which are two major primary neuromuscular symptoms of DM1 patients. However, these relationships are not clinically meaningful and not predictive at the individual level.

A review of the use of mexiletine in patients with myotonic dystrophy and non-dystrophic myotonia

Introduction

Myotonia is found in a number of muscle diseases, including myotonic dystrophy and non-dystrophic myotonia. The resulting symptoms of myotonia can interfere with daily activities such as walking or climbing the stairs. Due to the rarity of both these conditions, pharmacological treatment of myotonia is largely anecdotal and is led by specialist clinicians who tend to favour the use of mexiletine, a class 1b antiarrhythmic sodium antagonist.

Objective

To identify and review randomised controlled trials in order to assess the efficacy and safety of use of mexiletine in myotonic dystrophy and non-dystrophic myotonia for two different patient cases.

Search methods

The literature search was conducted using MEDLINE, EMBASE and The Cochrane Library (from January 1990 to December 2014). Specialist neurology centres were also contacted.

Selection criteria

All randomised controlled trials between January 1990 and December 2014 which compared the use of mexiletine for the treatment of myotonia in patients who suffer from myotonic dystrophy and non-dystrophic were included in this review. Primary outcome: reduction of clinical myotonia.

Results

Two randomised controlled trials were included for review. Both studies are underpowered; however, there is evidence to support the use of mexiletine for the improvement of clinical myotonia.

Conclusions

Larger randomised controlled trials are required, which look at the functional effect of myotonia as a primary outcome (ie, stair test) and the long-term use of mexiletine. This is needed to establish the ongoing efficacy and safety of the long-term use of mexiletine in the management of myotonia.

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.

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.

Splicing biomarkers of disease severity in myotonic dystrophy

OBJECTIVE

To develop RNA splicing biomarkers of disease severity and therapeutic response in myotonic dystrophy type 1 (DM1) and type 2 (DM2).

METHODS

In a discovery cohort, we used microarrays to perform global analysis of alternative splicing in DM1 and DM2. The newly identified splicing changes were combined with previous data to create a panel of 50 putative splicing defects. In a validation cohort of 50 DM1 subjects, we measured the strength of ankle dorsiflexion (ADF) and then obtained a needle biopsy of tibialis anterior (TA) to analyze splice events in muscle RNA. The specificity of DM-associated splicing defects was assessed in disease controls. The CTG expansion size in muscle tissue was determined by Southern blot. The reversibility of splicing defects was assessed in transgenic mice by using antisense oligonucleotides to reduce levels of toxic RNA.

RESULTS

Forty-two splicing defects were confirmed in TA muscle in the validation cohort. Among these, 20 events showed graded changes that correlated with ADF weakness. Five other splice events were strongly affected in DM1 subjects with normal ADF strength. Comparison to disease controls and mouse models indicated that splicing changes were DM-specific, mainly attributable to MBNL1 sequestration, and reversible in mice by targeted knockdown of toxic RNA. Splicing defects and weakness were not correlated with CTG expansion size in muscle tissue.

INTERPRETATION

Alternative splicing changes in skeletal muscle may serve as biomarkers of disease severity and therapeutic response in myotonic dystrophy.

Longitudinal in vivo muscle function analysis of the DMSXL mouse model of myotonic dystrophy type 1

Myotonic dystrophy is the most common adult muscle dystrophy. In view of emerging therapies, which use animal models as a proof of principle, the development of reliable outcome measures for in vivo longitudinal study of mouse skeletal muscle function is becoming crucial. To satisfy this need, we have developed a device to measure ankle dorsi- and plantarflexion torque in rodents. We present an in vivo 8-month longitudinal study of the contractile properties of the skeletal muscles of the DMSXL mouse model of myotonic dystrophy type 1. Between 4 and 12 months of age, we observed a reduction in muscle strength in the ankle dorsi- and plantarflexors of DMSXL compared to control mice although the strength per muscle cross-section was normal. Mild steady myotonia but no abnormal muscle fatigue was also observed in the DMSXL mice. Magnetic resonance imaging and histological analysis performed at the end of the study showed respectively reduced muscle cross-section area and smaller muscle fibre diameter in DMSXL mice. In conclusion, our study demonstrates the feasibility of carrying out longitudinal in vivo studies of muscle function over several months in a mouse model of myotonic dystrophy confirming the feasibility of this method to test preclinical therapeutics.

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.

A quantitative measure of handgrip myotonia in non-dystrophic myotonia

INTRODUCTION

Non-dystrophic myotonia (NDM) is characterized by myotonia without muscle wasting. A standardized quantitative myotonia assessment (QMA) is important for clinical trials.

METHODS

Myotonia was assessed in 91 individuals enrolled in a natural history study using a commercially available computerized handgrip myometer and automated software. Average peak force and 90% to 5% relaxation times were compared with historical normal controls studied with identical methods.

RESULTS

Thirty subjects had chloride channel mutations, 31 had sodium channel mutations, 6 had DM2 mutations, and 24 had no identified mutation. Chloride channel mutations were associated with prolonged first handgrip relaxation times and warm-up on subsequent handgrips. Sodium channel mutations were associated with prolonged first handgrip relaxation times and paradoxical myotonia or warm-up, depending on underlying mutations. DM2 subjects had normal relaxation times but decreased peak force. Sample size estimates are provided for clinical trial planning.

CONCLUSION

QMA is an automated, non-invasive technique for evaluating myotonia in NDM.

Muscle phenotype in patients with myotonic dystrophy type 1

INTRODUCTION

The pathogenesis of muscle involvement in patients with myotonic dystrophy type 1 (DM1) is not well understood. In this study, we characterized the muscle phenotype in patients with confirmed DM1.

METHODS

In 38 patients, muscle strength was tested by hand-held dynamometry. Myotonia was evaluated by a handgrip test and by analyzing the decrement of the compound muscle action potential. Muscle biopsies were assessed for morphological changes and Na(+)-K(+) pump content.

RESULTS

Muscle strength correlated with a decline in Na(+)-K(+) pump content (r = 0.60, P < 0.001) and with CTG expansion. CTG expansion did not correlate with severity of myotonia, proximal histopathological changes, or Na(+)-K(+) pump content. Histopathologically, we found few centrally placed nuclei (range 0.2-6.9%).

CONCLUSIONS

The main findings of this study are that muscle weakness correlated inversely with CTG expansion and that central nuclei are not a prominent feature of proximal muscles in DM1.

Mexiletine for symptoms and signs of myotonia in nondystrophic myotonia: a randomized controlled trial

CONTEXT

Nondystrophic myotonias (NDMs) are rare diseases caused by mutations in skeletal muscle ion channels. Patients experience delayed muscle relaxation causing functionally limiting stiffness and pain. Mexiletine-induced sodium channel blockade reduced myotonia in small studies; however, as is common in rare diseases, larger studies of safety and efficacy have not previously been considered feasible.

OBJECTIVE

To determine the effects of mexiletine for symptoms and signs of myotonia in patients with NDMs.

DESIGN, SETTING, AND PARTICIPANTS

A randomized, double-blind, placebo-controlled 2-period crossover study at 7 neuromuscular referral centers in 4 countries of 59 patients with NDMs conducted between December 23, 2008, and March 30, 2011, as part of the National Institutes of Health-funded Rare Disease Clinical Research Network.

INTERVENTION

Oral 200-mg mexiletine or placebo capsules 3 times daily for 4 weeks, followed by the opposite intervention for 4 weeks, with 1-week washout in between.

MAIN OUTCOME MEASURES

Patient-reported severity score of stiffness recorded on an interactive voice response (IVR) diary (scale of 1 = minimal to 9 = worst ever experienced). Secondary end points included IVR-reported changes in pain, weakness, and tiredness; clinical myotonia assessment; quantitative measure of handgrip myotonia; and Individualized Neuromuscular Quality of Life summary quality of life score (INQOL-QOL, percentage of maximal detrimental impact).

RESULTS

Mexiletine significantly improved patient-reported severity score stiffness on the IVR diary. Because of a statistically significant interaction between treatment and period for this outcome, primary end point is presented by period (period 1 means were 2.53 for mexiletine and 4.21 for placebo; difference, -1.68; 95% CI, -2.66 to -0.706; P < .001; period 2 means were 1.60 for mexiletine and 5.27 for placebo; difference, -3.68; 95% CI, -3.85 to -0.139; P = .04). Mexiletine improved the INQOL-QOL score (mexiletine, 14.0 vs placebo, 16.7; difference, -2.69; 95% CI, -4.07 to -1.30; P < .001) and decreased handgrip myotonia on clinical examination (mexiletine, 0.164 seconds vs placebo, 0.494 seconds; difference, -0.330; 95% CI, -0.633 to -0.142; P < .001). The most common adverse effect was gastrointestinal (9 mexiletine and 1 placebo). Two participants experienced transient cardiac effects that did not require stopping the study (1 in each group). One serious adverse event was determined to be not study related.

CONCLUSION

In this preliminary study of patients with NDMs, the use of mexiletine compared with placebo resulted in improved patient-reported stiffness over 4 weeks of treatment, despite some concern about the maintenance of blinding.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00832000.

Development of a disability scale for myotonic dystrophy type 1

OBJECTIVES

Myotonic dystrophy type 1 (DM1) is a multisystem disorder. Many tests in the literature have evaluated single aspects of DM1 patients, mainly focusing on muscular impairment, without an overall quantification of the different disease-specific neurological features. We developed and validated a new functional scale for DM1 patients based on neuromuscular impairment (NI) and disability.

MATERIALS AND METHODS

Thirty-three patients were tested in basal condition, 18 were re-evaluated after therapeutic intervention with mexiletine, and 13 at one year follow-up without treatment. The scale includes 21 ordinal items in four areas: neuropsychology, motricity, myotonia and daily life activities. We evaluated inter- and intra-observer reliability (intraclass correlation coefficient, ICC and Spearman correlations, respectively), internal consistency (Cronbach's alpha), external validity (Spearman correlations between each area and other clinical and objective measurements and scales), and sensitivity to clinical changes after treatment or at follow-up.

RESULTS

Our analysis provided good results for inter-observer agreement (ICC = 0.72-0.97), intra-observer reliability, and internal consistency for all areas (Cronbach's α > 0.73). Total score and single area subscores were significantly correlated to objective measurements, disease duration and multisystem involvement. Finally, the scale was sensitive to clinical changes disclosing a significant improvement after treatment in the items assessing myotonia, and also to disease progression showing a significant worsening in all areas but myotonia in untreated patients.

DISCUSSION

Our scale provides a new practical measure to evaluate NI and disability of DM1 patients. Further longitudinal studies are warranted to confirm its reliability in tracking disease progression and severity over a longer period of time.

Endocrine function in 97 patients with myotonic dystrophy type 1

The aim of this study was to investigate the endocrine function and its association to number of CTG repeats in patients with myotonic dystrophy type 1 (DM1). Concentration of various hormones and metabolites in venous blood was used to assess the endocrine function in 97 patients with DM1. Correlation with CTG(n) expansion size was investigated with the Pearson correlation test. Eighteen percent of the DM1 patients had hyperparathyroidism with increased PTH compared with 0.5% in the background population. Of these, 16% had normocalcemia and 2% had hypercalcemia. An additional 3% had hypercalcemia without elevation of PTH; 7% had abnormal TSH values (2% subnormal and 5% elevated TSH levels); 5% of the patients had type 2 diabetes mellitus; 17% of the male DM1 patients had increased LH and low levels of plasma testosterone indicating absolute androgen insufficiency. Another 21% had increased LH, but normal testosterone levels, indicating relative insufficiency. Numbers of CTG repeats correlated directly with plasma PTH, phosphate, LH, and tended to correlate with plasma testosterone for males. This is the largest study of endocrine dysfunction in a cohort of Caucasian patients with DM1. We found that patients with DM1 have an increased risk of abnormal endocrine function, particularly calcium metabolism disorders. However, the endocrine dysfunction appears not to be of clinical significance in all of the cases. Finally, we found correlations between CTG(n) expansion size and plasma PTH, phosphate, and testosterone, and neck flexion strength.

An interactive voice response diary for patients with non-dystrophic myotonia

INTRODUCTION

Non-dystrophic myotonia (NDM) is caused by mutations in muscle chloride and sodium channels. Currently, there is no standardized instrument for documenting symptom frequency and severity in NDM.

METHODS

Subjects used an automated, interactive, telephone-based voice response diary (IVR) to record frequency and severity of stiffness, weakness, pain, and tiredness once a week for 8 weeks, after their baseline visits.

RESULTS

We describe the IVR and report data on 76 subjects for a total of 385 person-weeks. Overall there were 5.1 calls per subject. Forty-eight subjects called in 5 or more times, and 14 called in 8 times. Stiffness was both the most frequent and severe symptom. Warm-up and handgrip myotonia were associated with higher severity scores for stiffness.

CONCLUSIONS

IVR is a convenient technology to allow patient reporting of repeated and real-time symptom frequency and severity, and it is presently being used in a trial of mexiletine in NDM.

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.

Open-label trial of recombinant human insulin-like growth factor 1/recombinant human insulin-like growth factor binding protein 3 in myotonic dystrophy type 1

OBJECTIVE

To evaluate the safety and tolerability of recombinant human insulin-like growth factor 1 (rhIGF-1) complexed with IGF binding protein 3 (rhIGF-1/rhIGFBP-3) in patients with myotonic dystrophy type 1 (DM1).

DESIGN

Open-label dose-escalation clinical trial.

SETTING

University medical center.

PARTICIPANTS

Fifteen moderately affected ambulatory participants with genetically proven myotonic dystrophy type 1.

INTERVENTION

Participants received escalating dosages of subcutaneous rhIGF-1/rhIGFBP-3 for 24 weeks followed by a 16-week washout period.

MAIN OUTCOME MEASURES

Serial assessments of safety, muscle mass, muscle function, and metabolic state were performed. The primary outcome variable was the ability of participants to complete 24 weeks receiving rhIGF-1/ rhIGFBP-3 treatment.

RESULTS

All participants tolerated rhIGF-1/rhIGFBP-3. There were no significant changes in muscle strength or functional outcomes measures. Lean body muscle mass measured by dual-energy x-ray absorptiometry increased by 1.95 kg (P < .001) after treatment. Participants also experienced a mean reduction in triglyceride levels of 47 mg/dL (P = .002), a mean increase in HDL levels of 5.0 mg/dL (P = .03), a mean reduction in hemoglobin A(1c) levels of 0.15% (P = .03), and a mean increase in testosterone level (in men) of 203 ng/dL (P = .002) while taking rhIGF-1/rhIGFBP-3. Mild reactions at the injection site occurred (9 participants), as did mild transient hypoglycemia (3), lightheadedness (2), and transient papilledema (1).

CONCLUSIONS

Treatment with rhIGF-1/rhIGFBP-3 was generally well tolerated in patients with myotonic dystrophy type 1. Treatment with rhIGF-1/rhIGFBP-3 was associated with increased lean body mass and improvement in metabolism but not increased muscle strength or function. Larger randomized controlled trials would be needed to further evaluate the efficacy and safety of this medication in patients with neuromuscular disease.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00233519.

Mexiletine is an effective antimyotonia treatment in myotonic dystrophy type 1

OBJECTIVE

To determine if mexiletine is safe and effective in reducing myotonia in myotonic dystrophy type 1 (DM1).

BACKGROUND

Myotonia is an early, prominent symptom in DM1 and contributes to decreased dexterity, gait instability, difficulty with speech/swallowing, and muscle pain. A few preliminary trials have suggested that the antiarrhythmic drug mexiletine is useful, symptomatic treatment for nondystrophic myotonic disorders and DM1.

METHODS

We performed 2 randomized, double-blind, placebo-controlled crossover trials, each involving 20 ambulatory DM1 participants with grip or percussion myotonia on examination. The initial trial compared 150 mg of mexiletine 3 times daily to placebo, and the second trial compared 200 mg of mexiletine 3 times daily to placebo. Treatment periods were 7 weeks in duration separated by a 4- to 8-week washout period. The primary measure of myotonia was time for isometric grip force to relax from 90% to 5% of peak force after a 3-second maximum grip contraction. EKG measurements and adverse events were monitored in both trials.

RESULTS

There was a significant reduction in grip relaxation time with both 150 and 200 mg dosages of mexiletine. Treatment with mexiletine at either dosage was not associated with any serious adverse events, or with prolongation of the PR or QTc intervals or of QRS duration. Mild adverse events were observed with both placebo and mexiletine treatment.

CONCLUSIONS

Mexiletine at dosages of 150 and 200 mg 3 times daily is effective, safe, and well-tolerated over 7 weeks as an antimyotonia treatment in DM1.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that mexiletine at dosages of 150 and 200 mg 3 times daily over 7 weeks is well-tolerated and effective in reducing handgrip relaxation time in 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.

The non-dystrophic myotonias: molecular pathogenesis, diagnosis and treatment

The non-dystrophic myotonias are an important group of skeletal muscle channelopathies electrophysiologically characterized by altered membrane excitability. Many distinct clinical phenotypes are now recognized and range in severity from severe neonatal myotonia with respiratory compromise through to milder late-onset myotonic muscle stiffness. Specific genetic mutations in the major skeletal muscle voltage gated chloride channel gene and in the voltage gated sodium channel gene are causative in most patients. Recent work has allowed more precise correlations between the genotype and the electrophysiological and clinical phenotype. The majority of patients with myotonia have either a primary or secondary loss of membrane chloride conductance predicted to result in reduction of the resting membrane potential. Causative mutations in the sodium channel gene result in an abnormal gain of sodium channel function that may show marked temperature dependence. Despite significant advances in the clinical, genetic and molecular pathophysiological understanding of these disorders, which we review here, there are important unresolved issues we address: (i) recent work suggests that specialized clinical neurophysiology can identify channel specific patterns and aid genetic diagnosis in many cases however, it is not yet clear if such techniques can be refined to predict the causative gene in all cases or even predict the precise genotype; (ii) although clinical experience indicates these patients can have significant progressive morbidity, the detailed natural history and determinants of morbidity have not been specifically studied in a prospective fashion; (iii) some patients develop myopathy, but its frequency, severity and possible response to treatment remains undetermined, furthermore, the pathophysiogical link between ion channel dysfunction and muscle degeneration is unknown; (iv) there is currently insufficient clinical trial evidence to recommend a standard treatment. Limited data suggest that sodium channel blocking agents have some efficacy. However, establishing the effectiveness of a therapy requires completion of multi-centre randomized controlled trials employing accurate outcome measures including reliable quantitation of myotonia. More specific pharmacological approaches are required and could include those which might preferentially reduce persistent muscle sodium currents or enhance the conductance of mutant chloride channels. Alternative strategies may be directed at preventing premature mutant channel degradation or correcting the mis-targeting of the mutant channels.

Functional MRI changes in the central motor system in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a multisystemic disease involving multiple organ systems including central nervous system (CNS) and muscles. Few studies have focused on the central motor system in DM1, pointing to a subclinical abnormality in the CNS. The aim of our study was to investigate patterns of cerebral activation in DM1 during a motor task using functional MRI (fMRI). Fifteen DM1 patients, aged 20 to 59 years, and 15 controls of comparable age were scanned during a self-paced sequential finger-to-thumb opposition task of their dominant right hand. Functional MRI images were analyzed using SPM99. Patients underwent clinical and genetic assessment; all subjects underwent a conventional MR study. Myotonic dystrophy type 1 patients showed greater activation than controls in bilateral sensorimotor areas and inferior parietal lobules, basal ganglia and thalami, in the ipsilateral premotor area, insula and supplementary motor area (corrected P<.05). Analysis of the interaction between disease and age showed that correlation with age was significantly greater in patients than in controls in bilateral sensorimotor areas and in contralateral parietal areas. Other clinical and MR characteristics did not correlate with fMRI. Functional changes in DM1 may represent compensatory mechanisms such as reorganization and redistribution of functional networks to compensate for ultrastructural and neurochemical changes occurring as part of the accelerated aging process.

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.

The Change of Grip Strength in a Patient with Congenital Myotonic Dystrophy Over a 4-year Period

Myotonic dystrophy (MyD) is a neuromuscular disease that is autosomal dominant and the most common form of muscular dystrophy affecting adults. The clinical features of MyD include a multisystemic disorder characterized by myotonia, progressive muscle weakness and wasting, cataracts, premature balding and mental retardation. The most severe type of MyD is classified as congenital MyD (CMyD). The muscle weakness in CMyD is very severe, but muscle development can be observed in the period of growth. However, no clinical case of this type has been reported yet. Therefore, we report on a girl with CMyD who had an increase in muscle strength over a four-year period. The girl with CMyD participated in this study from the age of 9 to the age of 12. The measurement of muscle strength was recorded as the maximum score of grip strength with the use of dynamometers. Grip strength was assessed once a year by the same two physical therapists. Grip strength of CMyD for each year was markedly weak when compared with the normal controls, but muscle strength changed within some specific growth areas. The muscle weakness in CMyD was remarkable, but the result showed that specific muscle strength of CMyD in childhood was actually increased.