A. Myotonic dystrophy – Explanation
Dystrophia myotonica – DM1
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The slow-relaxing grip may be noticed on initial hand-shake with the patient and is typical of
myotonic dystrophy.
Dysarthric speech is secondary to myotonia of the tongue and pharynx
Myotonic dystrophy
Myotonic dystrophy (also called dystrophia myotonica) is an inherited myopathy with features
developing at around 20-30 years old. It 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
The key differences are listed in table below:
| DM1 | DM2 |
| – DMPK gene on chromosome 19 – Distal weakness more prominent |
– ZNF9 gene on chromosome 3 – Proximal weakness more prominent – Severe congenital form not seen |
General features
- myotonic facies (long, ‘haggard’ appearance)
- frontal balding
- bilateral ptosis
- cataracts
- dysarthria
Other features
- myotonia (tonic spasm of muscle)
- weakness of arms and legs (distal initially)
- mild mental impairment
- diabetes mellitus
- testicular atrophy
- cardiac involvement: heart block, cardiomyopathy
- dysphagia
