II. Epidemiology
- Prevalance: 1 in 8,000 to 10,000
- Prevalance as common as 1 in 2100 in some references
- Most common form of Muscular Dystrophy in adults
- Age of onset
- DM1 is typically diagnosed in infancy
- DM2 is frequently identified at age 20 to 30 years (but may be diagnosed in later adulthood)
- Race
- Rare in non-white groups
III. Pathophysiology
- Genetic mutation is Autosomal Dominant inheritance
- Subsequent generations experience worse symptoms
IV. Types: Genetic Mutations
- Myotonic Dystrophy 1 (DM1, Steinert Disease)
- DMPK Gene Mutation
- Typically identified near birth (congenital DM1) and is more severe than DM2
- Congenital DM1 is associated with the most significant manifestations
- Myotonic Dystrophy 2 (DM2)
V. Findings: Muscular
-
Muscle Weakness
- Proximal Muscles are most affected (e.g. neck flexors, hip flexors and extensors) in DM2
- Facial Muscle Weakness including Ptosis, Dysarthria, and weak Muscles of Mastication
- Myotonia (esp. DM1)
- Delayed Muscle relaxation after contraction
- Myalgias
- Muscle pain is more common in DM2
- Muscle Atrophy
VI. Findings: Extramuscular
- Cataracts (early onset, before age 50 years old)
- Insulin Resistance
- Hypersomnia (esp. DM1)
- Obstructive Sleep Apnea
- Cognitive Deficits (esp. DM1)
- Executive Function deficits (e.g. word finding, organization)
- Cardiovascular effects
- Conduction abnormalities and Blocks
- Arrhythmias
- Dilated Cardiomyopathy
- Men
- Premature Balding
- Hypogonadism and Infertility
VII. Findings: Congenital DM1
- Infancy (diagnosis of congenital DM1)
- Hypotonia
- Feeding difficulty
- Respiratory Failure
- Club Foot
- Associated Findings at later ages
- See Muscular and Extramuscular findings above
- Gastrointestinal symptoms (e.g. Diarrhea, Constipation, Encopresis)
- Attention Deficit Disorder
- Autism Spectrum Disorder
- Intellectual Disability
VIII. Diagnosis
- Genetic Testing
- Lab abnormalities
- Serum Creatinine Kinase may be mildly increased
- Liver Function Tests elevated in up to 50% of patients
- Electrodiagnostic Tests
- Motor Nerve Conduction Studies (NCS)
- Decreased amplitude, but normal lactency and conduction velocity
- Electromyography (EMG)
- Rapid spontaneous discharges, waxing and waning in frequency and amplitude (esp. distal Muscles)
- Motor Nerve Conduction Studies (NCS)
- Muscle Biopsy
IX. Management
- Monitoring
- Baseline Electrocardiogram (EKG) and then yearly
- Pulmonary Function Tests at baseline and repeat at intervals
- Obstructive Sleep Apnea Screening and consider Sleep Study
- Endocrinopathy screening
- Diabetes Mellitus Screening
- Hypothyroidism screening
- Myotonia Management with Sodium Channel Blockers (avoid in AV Block)
- Mexiletene
- Tricyclic Antidepressants
- Benzodiazepines
- Referrals as needed
- Physical Therapy
- Occupational Therapy
- Orthotics and Prosthetics
- Speech and Language Pathology (Dysarthria)
X. References
- Vydra (2022) Myotonic Dystrophy, Stat Pearls, Treasure Island, FL, accessed 7/11/2023
- Thornton (2014) Neurol Clin 32(3):705-19 +PMID: 25037086 [PubMed]