Myotonic dystrophy (also called dystrophia myotonica) is an autosomal dominant inherited myopathy with features developing at around 20-30 years old.

It is a multisystem disease and affects skeletal, cardiac and smooth muscle.

There are two main types of myotonic dystrophy, DM1 and DM2.

Genetics

• autosomal dominant
• a trinucleotide repeat disorder
• DM1 is caused by a CTG repeat at the end of the DMPK (Dystrophia Myotonica-Protein Kinase) gene on chromosome 19
• DM2 is caused by a repeat expansion of the ZNF9 gene on chromosome 3

General features

• myotonic facies (long, 'haggard' appearance)
• frontal balding
• bilateral ptosis
• cataracts
• dysarthria

Endocrine features:

• diabetes mellitus