The clinical spectrum of myotonic disorders is broad. Myotonic dystrophy type 1, or DM1, is the most common of them, with prevalence estimates based on clinical diagnosis ranging from one in six to ten thousand, although genetic screening data suggest as many as one in 2,100 people may be affected. DM1 results from an unstable expansion of CTG repeats in the DMPK gene on chromosome 19. The expanded transcripts exert a toxic gain of function, sequestering RNA-binding proteins and disrupting alternative splicing across the genome, which explains why the disease touches so many organ systems. Patients may develop progressive muscle wasting and weakness, cardiac conduction disease and cardiomyopathy, respiratory failure, cataracts, diabetes, thyroid dysfunction and cognitive impairment. Presentation varies enormously: some individuals live nearly normal lives and remain undiagnosed well into adulthood, while others lose ambulation and face shortened life expectancy from cardiorespiratory complications. Myotonic dystrophy type 2, caused by a CCTG expansion in the CNBP gene on chromosome 3, shares the same RNA-mediated spliceopathy mechanism but tends to produce a milder, proximal myopathy without the congenital forms or genetic anticipation seen in DM1.

Distinct from the dystrophies are the non-dystrophic myotonias, pure skeletal muscle channelopathies caused by variants in ion channel genes, with no systemic involvement and a normal life expectancy. Myotonia congenita, the most common with a prevalence of roughly two to seven per 100,000, stems from variants in CLCN1, the gene encoding the ClC-1 voltage-gated chloride channel essential for membrane repolarisation. It is the same channel whose function is secondarily disrupted in myotonic dystrophy. Paramyotonia congenita and sodium channel myotonia are allelic autosomal dominant disorders of SCN4A, the gene for the Nav1.4 sodium channel that generates the action potentials driving contraction. Paramyotonia is notorious for worsening with cold and repeated activity, and can be accompanied by episodic paralysis, while sodium channel myotonia may show both warm-up and paradoxical worsening. For many patients, pain and fatigue are as troublesome as the stiffness itself, and the visible normality of their appearance can make the condition a hidden disability.

The therapeutic mainstay for myotonia has long been sodium channel blockade. In paramyotonia and sodium channel myotonia the mechanism is direct, since the underlying SCN4A variants confer a gain of channel function that the drugs inhibit. In myotonia congenita and the dystrophies the effect is indirect: reducing persistent sodium current partially compensates for impaired chloride-mediated repolarisation. Mexiletine emerged as first-line therapy on the strength of clinical impression, and in 2012 the first randomised controlled trial confirmed its efficacy in non-dystrophic myotonia, with subsequent studies, including aggregated N-of-1 trials, consolidating the evidence. Secondary outcomes consistently showed benefits extending to quality of life, pain, fatigue, independence and emotional well-being. Lamotrigine joined the arsenal in 2017 when a phase 2 crossover trial in 22 patients demonstrated efficacy on the myotonia behaviour score, making these two drugs the only anti-myotonic agents with randomised trial evidence. A head-to-head phase 3 trial failed to show lamotrigine was non-inferior to mexiletine, but it still delivered considerable benefit, giving clinicians two credible options.

Both drugs have also shown efficacy in DM1, with favourable cardiac safety profiles in trials, though these studies were small, short and potentially subject to selection bias in a population where cardiac conduction defects are common. Evidence in DM2 remains far thinner, with no dedicated large randomised trials, forcing treatment strategies to be extrapolated from other populations. Cardiac caution has historically shadowed mexiletine because of its potential pro-arrhythmic properties, yet neither the trials nor 302.4 years of real-world follow-up in non-dystrophic myotonia demonstrated significant electrocardiographic changes, and observational data in myotonic dystrophy show no safety signal on conduction parameters. The FDA has issued a warning on lamotrigine based on in vitro class 1B sodium channel blocker activity, but this has not been replicated by other regulators, including the European Medicines Agency, and has not translated into observable effects on human cardiac conduction. A joint ILAE/AES task force now recommends baseline electrocardiography only for those over 60 or with multiple cardiovascular risk factors, with specialist review mandatory for known cardiac disease. Clinicians must also weigh the rare but serious risk of Stevens-Johnson syndrome with lamotrigine, which a 55-year review of the FDA adverse event database found accounted for nearly ten percent of reported cases.

Practical considerations are also reshaping prescribing. Mexiletine, licensed as Namuscla for adults with non-dystrophic myotonia in several countries and used off-label in children and in myotonic dystrophy, typically requires dosing up to three times daily, which can undermine adherence. Prolonged-release once-daily formulations are now in phase 3 trials: the HERCULES study in DM1 and DM2 and the ACHILLES study in non-dystrophic myotonia. Meanwhile, basic science continues to probe alternative targets. No specific chloride channel activators exist, but intriguing work has shown that niflumic acid, which acutely blocks the ClC-1 channel in transfected cells, paradoxically acts as a pharmacological chaperone that increases channel protein expression when applied over 24 hours. Potassium channel openers such as retigabine, withdrawn from the market in 2017 over blue pigmentation concerns, have inspired next-generation compounds in development.

The most dramatic shift, however, is in DM1, where an era of molecularly targeted clinical trials is underway. Several companies are developing therapies designed to reduce the mutant DMPK transcript or to prevent the sequestration of muscleblind-like protein 1, the key splicing regulator whose loss drives the downstream transcriptional chaos. Delivery strategies range from naked oligonucleotides to targeted conjugates and adeno-associated virus vectors, with studies progressing from early-phase work to phase 3. The most advanced candidate is del-desiran, an antibody-oligonucleotide conjugate that hijacks the transferrin receptor to ferry the drug into muscle. In the phase 2 MARINA trial it demonstrated target engagement and amelioration of aberrant alternative splicing in some patients, a proof of principle that has electrified the field. None of these approaches is curative, and the extent of achievable molecular and clinical correction remains uncertain, but they represent the first credible assault on the underlying pathology rather than its symptoms.

Translational challenges loom large. Many trials use video hand opening time as the primary endpoint, a measure that captures skeletal muscle excitability and likely reflects modulation of MBNL-dependent mis-splicing of ion channel transcripts, making it a useful proof-of-mechanism marker, but its relevance to broader clinical benefit in a multisystem disease is debated. Effects on the heart and brain may not be established for years and will depend on post-marketing surveillance. Delivery poses further difficulties: repeated intravenous infusions at six- to eight-week intervals demand substantial healthcare infrastructure and may limit scalability and central nervous system penetration. Alongside the genetic approaches, repurposed small molecules are advancing. Metformin showed signals of efficacy on the six-minute walk test in the MYOMET phase 2 study, though the primary endpoint was missed in the intention-to-treat population, and the larger phase 3 METFORMYO trial is now underway with the MFM-32 as its primary measure. Tideglusib, a GSK3-beta inhibitor tested in congenital and childhood-onset DM1, showed signals in cognitive and neuromuscular ratings without significant safety concerns, and a phase 2/3 study in children is recruiting. Erythromycin, which modulates MBNL-dependent splicing preclinically, was well tolerated in a phase 2 trial with promising improvements in splicing biomarkers and creatine kinase levels.

Symptomatic care continues to evolve in parallel. Excessive daytime sleepiness affects the majority of people with DM1, yet randomised evidence for psychostimulants such as modafinil remains thin. A phase 2 trial of pitolisant in 30 participants suggested improvements in subjective sleepiness but was not powered for statistical significance, and a small study of intravenous flumazenil found no benefit over placebo. The overall picture that emerges from the review is not one of replacement but of layering: mechanism-based therapies are being built on top of a symptomatic framework that already works reasonably well for myotonia, even as it leaves progressive weakness and multisystem degeneration unaddressed. The recurring theme, the authors note, is the gap between molecular target engagement and clinically meaningful benefit, a gap that applies equally to established drugs and to the new genetic agents. Whether the emerging therapies will ultimately change disease trajectory remains uncertain, but for patients who have waited decades for anything more than symptomatic relief, the pipeline now offers genuine hope, and clinicians are being urged to prepare for a practice landscape that may look radically different within a few years.

Subject of Research: Treatment advances for myotonia in myotonic dystrophy and non-dystrophic muscle channelopathies

Article Title: Treatment updates in myotonic disorders

Article References: Matthews, E., Specterman, M. J., & Mul, K. (2026). Treatment updates in myotonic disorders. Journal of Neurology, 273(10), Article 562. https://doi.org/10.1007/s00415-026-14073-9

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14073-9

Keywords: myotonia, myotonic dystrophy type 1, myotonia congenita, sodium channel blockers, mexiletine, lamotrigine, RNA splicing, oligonucleotide therapy, clinical trials, muscle channelopathies, DMPK gene, disease-modifying therapy

Arrhythmias arise when the heart’s electrical impulses become irregular, leading to potentially life-threatening conditions such as atrial fibrillation and ventricular tachycardia. The ionic imbalances and miscommunications within cardiac cells frequently trigger these disturbances. The recent study asserts that disrupting pathological calcium cycling through the inhibition of the late sodium current (I_Na-L) and ryanodine receptors (RyR2) showcases an innovative avenue for therapeutic intervention.

Calcium ions play a critical role in the excitation-contraction coupling model of the heart. When the balance of calcium influx and release is perturbed, the heart’s ability to contract effectively is jeopardized. The profound impact of I_Na-L blockade stands out in this context, as excessive late sodium current can lead to calcium overload, ultimately causing cellular dysfunction and arrhythmogenic risk. This research provides compelling evidence that mitigating the late sodium current can restore homeostasis within cardiac myocytes, thus stabilizing rhythmical activity.

In addition to I_Na-L blockade, the ryanodine receptor remains a focal point in the authors’ investigation. RyR2 is responsible for calcium-induced calcium release, a fundamental process underpinning cardiac contraction. Abnormal RyR2 activity can lead to excessive calcium release during systole and insufficient calcium uptake during diastole, presenting a critical factor in the development of arrhythmias. The study underscores that targeting RyR2 signifies a double-edged sword; the correct modulation can reduce pathological calcium cycling, fostering healthier cardiac rhythms.

A pivotal aspect of the study emphasizes the normalization of CaMKII (Calcium/Calmodulin-dependent protein kinase II) signaling. CaMKII acts as a pivotal regulator in the calcium cycle and is integral to cellular response to calcium fluctuations. Pathological conditions often lead to CaMKII dysregulation, amplifying arrhythmic events through hyperphosphorylation of various substrates. By partnering the inhibition of I_Na-L with RyR2 blockade, there exists the potential to recalibrate CaMKII activity, steering it towards a balanced state that promotes cardiac health.

The implications of this research extend to clinical application, as the combination of pharmacologically targeting I_Na-L and RyR2 opens new avenues for patient-specific antiarrhythmic therapies. Current antiarrhythmic agents often yield unpredictable results due to their non-specificity or adverse effects. Thus, the refined therapeutic strategies articulated in this study stand to transform patient outcomes, providing tailored interventions that align closely with underlying pathophysiological mechanisms.

Moreover, the embrace of advanced investigative techniques, including electrophysiological assessments and molecular biology methods, underscores the robustness of this research. The authors leverage cutting-edge tools to delve deeply into the mechanistic interactions between calcium cycling, signaling pathways, and their arrhythmic consequences. This meticulous approach not only enhances the reliability of their findings but also sets a precedent for future investigations to build upon.

Understanding the multifaceted nature of cardiac arrhythmias necessitates the integration of genetic, molecular, and environmental factors. The research advocates for a holistic view, encouraging an exploration of patient-derived models that may reflect individual variability in calcium handling and signaling. This perspective is essential as it aligns therapeutic interventions with distinct patient profiles, potentially enhancing efficacy and minimizing adverse effects.

This fusion of basic science with clinical application resonates powerfully in the cardiovascular research community, fostering dialogues around innovative therapeutic avenues that tackle the complexities of arrhythmias. As researchers seek to uncover the underlying mechanisms of heart rhythm disorders, the findings presented by Ju et al. will likely catalyze further studies that interrogate the longevity and durability of these inhibition strategies.

Furthermore, the exploration of concomitant therapies may optimize results. The potential to combine I_Na-L and RyR2 blockade with lifestyle modifications or other medical therapies warrants significant exploration. Integrative approaches, where lifestyle factors bolster the effects of pharmacological interventions, could yield substantial holistic benefits for those battling arrhythmogenesis.

The significance of this research cannot be overstated. As healthcare systems are increasingly tasked with managing chronic diseases, innovations that address arrhythmias represent a substantial step towards enhanced cardiovascular health. This study posits that the strategic targeting of I_Na-L and RyR2 may not only mitigate present ailments but also serve preventative purposes for future patients, thereby reshaping the landscape of cardiac healthcare delivery.

In conclusion, the study by Ju, Qiu, Wang et al. not only elucidates the nuanced interplay between calcium signaling, sodium currents, and arrhythmias, but it also champions a transformative approach to treatment. By advocating for localized and nuanced therapeutic strategies, this research lays essential groundwork for the development of more sophisticated, patient-centered interventions that ultimately promise to enhance the quality of life for patients suffering from cardiac arrhythmias.

Subject of Research: Arrhythmias, Calcium Cycling, Sodium Channel Blockade, and Ryanodine Receptor Modulation

Article Title: Synergistic antiarrhythmic mechanism of I_Na-L and RyR2 blockade: normalization of pathological calcium cycling and CaMKII signaling.

Article References:

Ju, M., Qiu, S., Wang, Y. et al. Synergistic antiarrhythmic mechanism of I_Na-L and RyR2 blockade: normalization of pathological calcium cycling and CaMKII signaling.
J Transl Med (2025). https://doi.org/10.1186/s12967-025-07652-3

Image Credits: AI Generated

DOI: 10.1186/s12967-025-07652-3

Keywords: arrhythmias, antiarrhythmic therapy, calcium signaling, sodium channel blockers, calcium cycling, CaMKII, cardiovascular health