Myotonic dystrophy type 1 is a common form of muscular dystrophy
The prevalence of myotonic dystrophy type 1 ranges between 0.5 and 18.1 per 100 000 population, making it the most common muscular dystrophy, ahead of Duchenne and facioscapulohumeral muscular dystrophies.1 Myotonic dystrophy type 1 is especially prevalent in certain regions of Quebec.2 It is an autosomal dominant nucleotide repeat disorder. The mutant messenger RNA with expanded repeats has a toxic gain of function, resulting in widespread splicing dysregulation.
Myotonic dystrophy type 1 is multisystemic
Myotonic dystrophy type 1 can present at any age but is typically diagnosed in adults. Although this disorder can present in many different ways (Appendix 1, available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.151384/-/DC1), facial and distal muscle weakness, along with grip myotonia, are key findings on examination. Given the risk of sudden death (described below), dominant inheritance and high penetrance,3 first-degree relatives and women of childbearing age who are at risk of carrying an affected child should be referred for genetic counselling.
Myotonic dystrophy type 1 shows genetic anticipation
Genetic anticipation, the earlier and more severe presentation in offspring, relates to the unstable trinucleotide repeat mutation expanding in subsequent generations. Congenital myotonic dystrophy type 1 is the most severe manifestation of this disorder. Congenitally affected neonates with this manifestation are weak and hypotonic, and often require ventilation and feeding support. Mortality is about 25% in the first year.4 Strength and ability to feed and breathe improve in those who live beyond the first year.
Cardiac arrythmia is a frequent cause of death
Unlike other dystrophies, arrhythmias and other conduction abnormalities are the primary cardiac manifestation in myotonic dystrophy type 1 and require regular electrocardiographic (ECG) monitoring and referral to cardiology.5 Sudden cardiac death may be predicted by severe abnormalities detected through ECG monitoring and a diagnosis of symptomatic atrial tachyarrhythmias.5
Treatments are symptomatic
No treatment to modify disease currently exists. However, mexiletine is effective in reducing myotonia based on evidence from randomized controlled trials (RCTs).6 In a small RCT, methylphenidate was found to decrease excessive somnolence seen in this disorder.7 There are some promising disease-modifying therapies entering clinical trials.
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Acknowledgements
Shannon Venance and Cynthia Gagnon provided valuable feedback on this manuscript.
Footnotes
Competing interests: Craig Campbell has received a grant from Valerion Therapeutics and is a site investigator for Ionis Pharmaceuticals. Cam-Tu Emilie Nguyen is a site sub-investigator for Ionis Pharmaceuticals.
This article has been peer reviewed.