Version June 2024
| Adult-onset myotonic dystrophy type 1 | Myotonic dystrophy type 2 | |
| Genetics | ||
| Inheritance | Autosomal dominant | Autosomal dominant |
| Anticipation | Pronounced | Exceptionally rare |
| Congenital form | Yes | No |
| Chromosome | 19q13.3 | 3q21.3 |
| Locus | DMPK | CNBP |
| Expansion mutation | (CTG) | (CCTG) |
| Location of the expansion | 3' untranslated region | Intron 1 |
| Core features | ||
| Clinical myotonia | Typical in adult onset | Present in less than 50% |
| Myotonia on electromyography | Generally present | Absent and variable in many patients; needs detailed investigation |
| Muscle weakness | Disability often by age 30 to 50 years | Disability at age 60 to 85 years |
| Cataracts | Generally present | Present in a few patients at diagnosis |
| Localization of muscle weakness | ||
| Face or jaw | Generally present | Usually absent |
| Ptosis | Often present | Rare, mild, or moderate |
| Bulbar (dysphagia) | Generally present later in life | Not present |
| Respiratory muscles | Generally present later in life | Exceptionally rare cases |
| Distal limb muscle | Generally prominent | Flexor digitorum profundus in some patients |
| Proximal limb muscle | Can be absent for many years | Main disability in most patients, late onset |
| Sternocleidomastoid muscle | Generally prominent | Prominent in few patients |
| Muscular symptoms | ||
| Myalgic pain | Absent or moderate | Most disabling symptom in many patients |
| Muscle strength variations | Occasional | Can be considerable |
| Visible muscle atrophy | Face, temporal, distal hands, and legs | Usually absent |
| Calf hypertrophy | Absent | Present in at least 50% |
| Laboratory findings | ||
| Concentration of creatine kinase in serum | Normal-to-moderate increase | Normal-to-moderate increase |
| Muscluar biopsy findings | ||
| Fiber atrophy | Smallness of type 1 fibers | Highly atrophic type 2 fibers |
| Nuclear clump fibers | In late stage only | Scattered early before weakness |
| Sarcoplasmic masses | Very frequent in distal muscles | Very rare |
| Ring fibers | Frequent | May occur |
| Internal nuclei | Massive in distal muscle | Variable and mainly in type 2 fibers |
| Cardiac symptoms | ||
| Conduction defects | Common | Highly variable, absent to severe |
| Other neurological symptoms | ||
| Tremors | Absent | Prominent in many patients |
| Behavioral changes | Common | Not apparent |
| Hypersomnia | Prominent | Infrequent |
| Other features | ||
| Manifest diabetes | Occasional | Infrequent |
| Frontal balding in men | Generally present | Exceptional |
| Incapacity (work and activities of daily living) | Typically after age 30 to 35 years | Rarely younger than 60 years, unless severe pain |
| Life expectancy | Reduced | Normal |
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