Myotonic dystrophy type 1 is caused by a trinucleotide (CTG) expansion in the DMPK gene. Myotonic dystrophy type 2 is less common and is caused by a tetranucleotide (CCTG) expansion in the CCTG gene.
Clinical manifestations of myotonic dystrophy type 1 usually begin in adolescence or early adult life with distal muscle weakness and myotonia. Given the phenomenon of anticipation the disease may manifest in childhood if the inherited repeat size is large.
Muscle wasting about the face and sternocleidomastoids, in combination with facial weakness, leading to the distinctive “hatchet-face” appearance
Partial ptosis, swan-like posture of the neck, enlarged paranasal sinuses, early prominent male-pattern balding in both sexes
Cataracts, cardiac conduction abnormalities, hypogonadism with testicular atrophy, and abnormal glucose tolerance