Clinical characteristics: Myotonic dystrophy type 1 (DM1) is a multisystem disorder that affects skeletal and smooth muscle as well as the eye, heart, endocrine system, and central nervous system. The clinical findings, which span a continuum from mild to severe, have been categorized into three somewhat overlapping phenotypes: mild, classic, and congenital.

1. Mild DM1 is characterized by cataract and mild myotonia (sustained muscle contraction); life span is normal.

2. Classic DM1 is characterized by muscle weakness and wasting, myotonia, cataract, and often cardiac conduction abnormalities; adults may become physically disabled and may have a shortened life span.

3. Congenital DM1 is characterized by hypotonia and severe generalized weakness at birth, often with respiratory insufficiency and early death; intellectual disability is common.

Diagnosis/testing: DM1 is caused by expansion of a CTG trinucleotide repeat in the noncoding region of DMPK. The diagnosis of DM1 is suspected in individuals with characteristic muscle weakness and is confirmed by molecular genetic testing of DMPK. CTG repeat length exceeding 34 repeats is abnormal. Molecular genetic testing detects pathogenic variants in nearly 100% of affected individuals.

Management: Treatment of manifestations: Use of ankle-foot orthoses, wheelchairs, or other assistive devices; special education support for affected children; treatment of hypothyroidism; management of pain; consultation with a cardiologist for symptoms or EKG evidence of arrhythmia; removal of cataracts if vision is impaired; hormone replacement therapy for males with hypogonadism; surgical excision of pilomatrixoma and basal cell carcinomas.

Prevention of secondary complications: Choice of induction agents, airway care, local anesthesia, and neuromuscular blockade to minimize complications during surgery; cardiac pacemakers or implantable cardioverter-defibrillators may prevent life-threatening arrhythmias; continue physical activity and maintain appropriate weight.

Surveillance: Annual EKG or 24-hour Holter monitoring; annual measurement of fasting serum glucose concentration and glycosylated hemoglobin concentration; ophthalmology examination every two years; attention to nutritional status; polysomnography for sleep disturbances.

Agents/circumstances to avoid: Cholesterol-lowering medications (i.e., statins), which can cause muscle pain and weakness; the anesthetic agent vecuronium; succinylcholine, propofol, and doxorubicin; smoking; obesity; illicit drug use; excessive alcohol intake.

Evaluation of relatives at risk: Molecular genetic testing for early diagnosis of relatives at risk to allow treatment of cardiac manifestations, diabetes mellitus, and cataracts.

Genetic counseling: DM1 is inherited in an autosomal dominant manner. Offspring of an affected individual have a 50% chance of inheriting the expanded allele. Pathogenic alleles may expand in length during gametogenesis, resulting in the transmission of longer trinucleotide repeat alleles that may be associated with earlier onset and more severe disease than that observed in the parent. Prenatal testing and preimplantation genetic testing are possible when the diagnosis of DM1 has been confirmed by molecular genetic testing in an affected family member.