DM1 has an overall estimated worldwide prevalence of 1 in 20,000.

DM1 is caused by a CTG repeat expansion in the noncoding region of the DMPK gene located on chromosome 19. Our DNA is composed of four bases: adenine (A), thymine (T), guanine (G), and cytosine ( C ). It exhibits an autosomal dominant inheritance pattern, which requires only one mutated copy to be passed down from either parent (see RareShare Guide on Genetic Inheritance). DM1 manifests with 50 or more consecutive CTG repeats in the gene region. As the repeat count increases, the severity of disease increases and the age of onset decreases. Up to 34 repeats is considered within normal range. Between 35 and 49 repeats is considered a mutable normal range where patients generally aren’t affected. However, these patients are at risk of having children with the disease due to the anticipation phenomenon. In anticipation, the number of repeats increases from one generation to the next because the cellular processes of making our reproductive cells (egg, sperm) causes the repeat stretch to expand. Due to this, each successive generation to inherit the disease experiences increasingly severe symptoms at an increasingly earlier age of onset. Therefore, patients without clinical disease but possessing 35-49 CTG repeats can pass down a faulty DMPK gene with over 50 repeats, resulting in clinical DM1 disease. A count between 50-150 CTG repeats results in mild DM1, 100-1000 CTG repeats results in classic DM1, and over 1000 CTG repeats causes congenital DM1.

• Myotonia: Delayed muscle relaxation after contraction. It can cause stiffness, especially in the hands and other muscles.

• Progressive muscle weakness: Typically affects distal muscles (hands, feet), facial muscles, and eventually proximal muscles (shoulders, hips).

• Cataracts (clouding of the lens in your eye): Early-onset cataracts are common.

• Cardiac problems: Conduction defects, arrhythmias, and even heart failure.

• Respiratory issues: Weakness of respiratory muscles can lead to breathing difficulties.

• Endocrine issues: Insulin resistance, thyroid problems, and testicular atrophy in men.

• Cognitive and behavioral problems: Some patients have mild intellectual disability or learning difficulties

• A thin, sharp face (myopathic face) due to wasting of facial muscles

• Daytime sleepiness

• Low testosterone (male hypogonadism)

• Erectile dysfunction

Genetic testing: detect total number of CTG repeats in the DMPK gene

There is no cure for DM1. Treatment generally focuses on managing symptoms and improving quality of life. This includes:

• Medications that reduce sustained myotonia, including sodium channel blockers such as mexiletine, tricyclic antidepressants, benzodiazepines or calcium antagonists.

• A CPAP machine for sleep apnea.

• Neurostimulants such as methylphenidate for excessive daytime sleepiness.

• Cataract surgery for cataracts that impair your vision.

• Diabetes Management. People with DM1 are at an increased risk of diabetes from insulin resistance.

• Synthetic testosterone for the treatment of low testosterone 

• Physical therapy

• Cardiac and respiratory monitoring

• Genetic counseling

DM1 is a chronic progressive condition for which there is no cure. Management focuses on alleviating symptoms and extending life as long as possible. Life expectancy can be reduced, particularly in the classic and congenital forms, due to respiratory or cardiac complications. Generally speaking, the earlier the onset, the poorer the prognosis. Congenital DM1 has a neonatal mortality rate of 18% with around 25% of infants dying before 18 months. 50% of congenital DM1 sufferers don’t make it to their mid 30’s. The classic form of DM1 has an average life expectancy of 48-55 years. The mild form of DM1 has the best prognosis as it has a near normal life expectancy. The leading causes of death are neuromuscular respiratory failure and cardiac issues. Healthcare providers need to stay vigilant in these areas in order to prevent complications. Patients also benefit from genetic counseling as the nature of DM1’s autosomal dominant inheritance pattern has a high likelihood of affecting the next generation.

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