E. Prolonged PR interval – Explanation
A prolonged PR interval is seen in around 20-40% of patients
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
