The function of the chloride channel ClC-1 is crucial for the control of muscle excitability. Thus, reduction of ClC-1 functions by CLCN1 mutations leads to myotonia congenita. Many different animal models have contributed to understanding the myotonia pathophysiology. However, these models do not allow in vivo screening of potentially therapeutic drugs, as the zebrafish model does. In this work, we identified and characterized the two zebrafish orthologues (clc-1a and clc-1b) of the ClC-1 channel. Both channels are mostly expressed in the skeletal muscle as revealed by RT-PCR, western blot, and electrophysiological recordings of myotubes, and clc-1a is predominantly expressed in adult stages. Characterization in Xenopus oocytes shows that the zebrafish channels display similar anion selectivity and voltage dependence to their human counterparts. However, they show reduced sensitivity to the inhibitor 9-anthracenecarboxylic acid (9-AC), and acidic pH inverts the voltage dependence of activation. Reduction of clc-1a/b expression hampers spontaneous and mechanically stimulated movement, which could be reverted by expression of human ClC-1 but not by some ClC-1 containing myotonia mutations. Treatment of clc-1-depleted zebrafish with mexiletine, a typical drug used in human myotonia, improves the motor behaviour. Our work extends the repertoire of ClC channels to evolutionary structure-function studies and proposes the zebrafish clcn1 crispant model as a simple tool to find novel therapies for myotonia. KEY POINTS: We have identified two orthologues of ClC-1 in zebrafish (clc-1a and clc-1b) which are mostly expressed in skeletal muscle at different developmental stages. Functional characterization of the activity of these channels reveals many similitudes with their mammalian counterparts, although they are less sensitive to 9-AC and acidic pH inverts their voltage dependence of gating. Reduction of clc-1a/b expression hampers spontaneous and mechanically stimulated movement which could be reverted by expression of human ClC-1. Myotonia-like symptoms caused by clc-1a/b depletion can be reverted by mexiletine, suggesting that this model could be used to find novel therapies for myotonia.

Non-dystrophic myotonias include several entities with possible clinical overlap, i.e. myotonia congenita caused by CLCN1 gene mutations, as well as paramyotonia congenita and sodium channel myotonia caused by SCN4A gene mutations. Herein, we describe the clinical features of five relatives affected by clinical and neurophysiological myotonia, with an aspecific and mixed phenotype. Next-generation sequencing identified the novel p.K1302R variant in SCN4A and the p.H838P variant in CLCN1. Segregation of the two mutations with the disease was confirmed by genotyping affected and non-affected family members. Patch-clamp experiments showed that sodium currents generated by p.K1302R and WT hNav1.4 were very similar. Mutant channel showed a small negative shift (5 mV) in the voltage-dependence of activation, which increased the likelihood of the channel to open at more negative voltages. The p.H838P mutation caused a reduction in chloride current density and a small voltage-dependence shift towards less negative potentials, in agreement with its position into the CBS2 domain of the C-terminus. Our results demonstrated that the mild functional alterations induced by p.K1302R and p.H838P in combination may be responsible for the mixed myotonic phenotypes. The K1302R mutant was sensitive to mexiletine and lamotrigine, suggesting that both drugs might be useful for the K1302R carriers.

Myotonic Dystrophies (DM, Dystrophia Myotonia) are autosomal dominant inherited myopathies with a high prevalence across different ethnic regions. Despite some differences, mainly due to the pattern of muscle involvement and the age of onset, both forms, DM1 and DM2, share many clinical and genetic similarities. In this study, we retrospectively analyzed the medical record files of 561 Greek patients, 434 with DM1 and 127 with DM2 diagnosed in two large academic centers between 1994-2020. The mean age at onset of symptoms was 26.2 ± 15.3 years in DM1 versus 44.4 ± 17.0 years in DM2 patients, while the delay of diagnosis was 10 and 7 years for DM1 and DM2 patients, respectively. Muscle weakness was the first symptom in both types, while myotonia was more frequent in DM1 patients. Multisystemic involvement was detected in the great majority of patients, with cataracts being one of the most common extramuscular manifestations, even in the early stages of disease expression. In conclusion, the present work, despite some limitations arising from the retrospective collection of data, is the first record of a large number of Greek patients with myotonic dystrophy and emphasizes the need for specialized neuromuscular centers that can provide genetic counseling and a multidisciplinary approach.

Remote study visits (RSVs) are emerging as important tools for clinical research. We tested the feasibility of using RSVs to evaluate patients with myotonic dystrophy type 1 (DM1), including remote quantitative assessment of muscle function, and we assessed correlations of remote assessments with patient-reported function.
Twenty three subjects with DM1 were consented remotely. Toolkits containing a tablet computer, grip dynamometer, and spirometer were shipped to participants. The tablets were loaded with software for video-conferencing and questionnaires about functional impairment, patient experience with technology, and willingness to participate in future remote studies. Grip strength, forced vital capacity, peak cough flow, timed-up-and-go (TUG), and grip myotonia (hand opening time) were determined during RSVs. We assessed correlations of remote assessments with patient-reported outcomes of muscle function and with CTG repeat size.
All 23 subjects completed RSVs. 95% of participants were able to complete all components of the remote study. All toolkit components were returned upon completion. Grip strength and TUG demonstrated moderate to strong correlations with self-reported inventories of upper and lower extremity impairment, respectively (ρ = 0.7 and ρ = -0.52). A total of 91% of subjects expressed interest in participating in future RSVs.
Results of this study support the feasibility of using portable devices and video-conferencing for remote collection of patient-reported outcomes and quantitative assessment of muscle function in DM1.

The treatment of myotonic dystrophy type 1 (DM1) focuses on reducing symptom burden. However, since medication often fails to produce satisfying symptom relief, some patients seek alternatives, such as cannabis, to help reduce some of these symptoms. The aim of this study was to provide an accurate profile of cannabis use among DM1 patients. Phone interviews were conducted to identify current and former users, and to assess reasons for cannabis use. Characteristics of cannabis use were also investigated. Briefly, among the 72 study participants, 22.2% currently used cannabis and a majority of them (56.9%) reported using it to relieve symptoms associated with DM1. These users, classified as therapeutic users, reported poorer health status (EQ-5D index scores: 0.532±0.230 vs. 0.823±0.208, p = 0.020; EQ-VAS scale 50.56±10.74 vs. 75.57±21.50, p = 0.009) than non-therapeutic users. Finally, differences among sex were also highlighted. While the therapeutic effects of cannabis were not explored in our study, our results support the potential role of cannabis and cannabinoids in the treatment of DM1-associated symptoms which will need to be thoroughly investigated.

Myotonic dystrophy (DM) is a complex multisystem disease with specific clinical and electrodiagnostic findings. Myotonia can be seen in the distal and proximal muscle groups in upper and lower limbs. There is no established guideline to demonstrate the sensitivity of muscles in the diagnosis of myotonic dystrophy. The aims of this study are to describe common electrodiagnostic findings in patients with DM; and to assess the electrodiagnostic sensitivity of muscles in the diagnosis of DM. In this retrospective study, patients\' age, sex, nerve conduction study findings including common upper and lower limbs nerve functions, and needle examination findings were collected and analyzed. A descriptive analysis (with percentage) was performed on the data obtained from the charts. NCS analysis showed more than half of patients had normal sensory and motor NCS findings. In 11 over 12 patients, sensory NCSs were within normal limits. Only one patient showed abnormal sensory responses. The most common abnormal NCS findings were decreased amplitude with normal latency and normal conduction velocity. The needle analysis showed distal muscles including first dorsal interosseous, abductor policies brevis, tibialis anterior, medial gastrocnemius and peroneal longus muscles are more sensitive in detecting myotonic discharges than proximal muscles including deltoid, triceps, vastus medialis and vastus lateralis. Our findings showed sensory nerve responses were usually within normal limits. The most common NCS abnormality showed decreased motor nerve amplitudes. The needle test showed myotonic discharges were more prominent in the distal muscles in upper and lower limbs.

The prevalence and impact of symptoms affecting individuals with pediatric forms of myotonic dystrophy type-1 (DM1) are not well understood.
Patients from the United States, Canada, and Sweden completed a survey that investigated 20 themes associated with pediatric-onset DM1. Participants reported the prevalence and importance of each theme affecting their lives. Surveys from participants were matched with surveys from their caregivers for additional analysis.
The most prevalent symptomatic themes included problems with hands or fingers (79%) and gastrointestinal issues (75%). Problems with urinary/bowel control and gastrointestinal issues were reported to have the greatest impact on patients\' lives. Responses from participants and their caregivers had varying levels of agreement among symptomatic themes.
Many symptoms have meaningful impact on disease burden. The highest levels of agreement between caregivers and individuals with pediatric forms of myotonic dystrophy were found for physical activity themes.

Paramyotonia congenita (PMC) is a nondystrophic myotonic disorder that is believed to be caused by a defect in Nav 1.4 sodium channel inactivation. Ranolazine, which acts by enhancing slow inactivation of sodium channels, has been proposed as a therapeutic option, but in vivo studies are lacking.
We conducted an open-label, single-center trial of ranolazine to evaluate efficacy and tolerability in patients with PMC. Subjective symptoms of stiffness, weakness, and pain as well as clinical and electrical myotonia were evaluated. Baseline measures were compared with those after 4 weeks of treatment with ranolazine.
Ranolazine was tolerated well without any serious adverse events. Both subjective symptoms and clinical myotonia were significantly improved. Duration of myotonia was reduced according to electromyography, but this change was not statistically significant in all tested muscles.
Our findings support the use of ranolazine as a treatment for myotonia in PMC and suggest that a randomized, placebo-controlled trial is warranted. Muscle Nerve 59:240-243, 2019.

Mexiletine is the only drug with proven effect for treatment of non-dystrophic myotonia, but mexiletine is expensive, has limited availability and several side effects. There is therefore a need to identify other pharmacological compounds that can alleviate myotonia in non-dystrophic myotonias. Like mexiletine, lamotrigine is a sodium channel blocker, but unlike mexiletine, lamotrigine is available, inexpensive, and well tolerated. We investigated the potential of using lamotrigine for treatment of myotonia in patients with non-dystrophic myotonias. In this, randomized double-blind, placebo-controlled, two-period cross-over study, we included adult outpatients recruited from all of Denmark with clinical myotonia and genetically confirmed myotonia congenita and paramyotonia congenita for investigation at the Copenhagen Neuromuscular Center. A pharmacy produced the medication and placebo, and randomized patients in blocks of 10. Participants and investigators were all blinded to treatment until the end of the trial. In two 8-week periods, oral lamotrigine or placebo capsules were provided once daily, with increasing doses (from 25 mg, 50 mg, 150 mg to 300 mg) every second week. The primary outcome was a severity score of myotonia, the Myotonic Behaviour Scale ranging from asymptomatic (score 1) to invalidating myotonia (score 6), reported by the participants during Weeks 0 and 8 in each treatment period. Clinical myotonia was also measured and side effects were monitored. The study was registered at ClinicalTrials.gov (NCT02159963) and EudraCT (2013-003309-24). We included 26 patients (10 females, 16 males, age: 19-74 years) from 13 November 2013 to 6 July 2015. Twenty-two completed the entire study. One patient withdrew due to an allergic reaction to lamotrigine. Three patients withdrew for reasons not related to the trial intervention. The Myotonic Behaviour Scale at baseline was 3.2 ± 1.1, which changed after treatment with lamotrigine by 1.3 ± 0.2 scores (P < 0.001), but not with placebo (0.2 ± 0.1 scores, P = 0.4). The estimated effect size was 1.0 ± 0.2 (95% confidence interval = 0.5-1.5, P < 0.001, n = 22). The standardized effect size of lamotrigine was 1.5 (confidence interval: 1.2-1.8). Number needed to treat was 2.6 (P = 0.006, n = 26). No adverse or unsuspected event occurred. Common side effects occurred in both treatment groups; number needed to harm was 5.2 (P = 0.11, n = 26). Lamotrigine effectively reduced myotonia, emphasized by consistency between effects on patient-related outcomes and objective outcomes. The frequency of side effects was acceptable. Considering this and the high availability and low cost of the drug, we suggest that lamotrigine should be used as the first line of treatment for myotonia in treatment-naive patients with non-dystrophic myotonias.

This exploratory study aimed to evaluate the electrophysiological profiles of patients with myotonic dystrophy type 1 (DM1) and to assess their correlations with genotype and phenotype.
Twenty-two patients with genetically confirmed DM1 were included. Global motor testing score, severity of myotonia, occurrence of cardiac disturbances, and CTG repeat number were recorded. All patients underwent repeated short exercise tests after 7 min of cooling.
Two trajectories could be distinguished following 3 periods of exercise, although most clearly following the third exercise period. Cardiac disturbances were more common among patients who had a B-type trajectory (larger decrement in compound muscle potential amplitude and slower recovery) following the third exercise period.
While the electrophysiological pattern in each profile appeared to confirm chloride muscle channel impairment, the B-type trajectory may suggest dysfunction of other muscle channels in DM1 and their link with cardiac disturbances. Muscle Nerve 54: 104-109, 2016.