Summary of the clinical findings, phenotype and CTG repeat length in myotonic dystrophy type 1
| Phenotype | Clinical signs | CTG repeat size‡ | Age of onset (years) | Age of death (years) |
| Pre-mutation | None | 38–49 | N/A | N/A |
| Mild/late onset/asymptomatic | Cataracts mild myotonia | 50–100 | 20–70 | 60 to normal life span |
| Classic | Weakness | 50–1000 | 10–30 | 48–60 |
| Myotonia | ||||
| Cataracts | ||||
| Conduction defects | ||||
| Insulin insensitivity | ||||
| Balding | ||||
| Respiratory failure | ||||
| Childhood onset | Facial weakness, myotonia, psychosocial problems, low IQ, conduction defects | 50–1000 | 1–10 | N/A |
| Congenital | Infantile hypotonia | >1000* | Birth | 45† |
| Respiratory failure | ||||
| Learning disability | ||||
| Cardiorespiratory complications in third and fourth decade | ||||
↵* Redman et al24 reported a few individuals with congenital myotonic dystrophy type 1 with repeats between 730 and 1000.
↵† Does not include neonatal deaths.
↵‡ The correlation of CTG length and clinical phenotype can vary, with some patients severely affected with CTGs between 50 and 100 and asymptomatic middle age patients with repeats between 300 and 500. Adapted from Die-Smulders et al,25 Mathieu et al,26 Cobo et al27 and Second IDMC 2000.3