Dystrophia Myotonica

Question 1

Dystrophia myotonica presentation

Answer 1

This man complains of worsening weakness in his hands. Please examine him.

Question 2

Face Clinical signs in Dystrophia myotonica

Answer 2

1. Myopathic facies: long, thin and expressionless
2. Wasting of facial muscles and sternocleidomastoid
3. Bilateral ptosis
4. Frontal balding
5. Dysarthria: due to myotonia of tongue and pharynx

Question 3

Hands Clinical signs in Dystrophia myotonica

Answer 3

1. Myotonia: ‘Grip my hand, now let go’ (may be obscured by profound weakness). ‘Screw up your eyes tightly shut, now open them’.
2. Wasting and weakness of distal muscles with areflexia.
3. Percussion myotonia: percuss thenar eminence and watch for involuntary thumb flexion.

Question 4

Additional signs in Dystrophia myotonica

Answer 4

1. Cataracts
2. Cardiomyopathy, brady‐ and tachy‐arrhythmias (look for pacemaker scar)
3. Diabetes (ask to dip urine)
4. Testicular atrophy
5. Dysphagia (ask about swallowing)

Question 5

Genetics in Dystrophia myotonica

Answer 5

1. Dystrophia myotonica (DM) can be categorised as type 1 or 2 depending on the underling genetic defect.
   ⚬⚬⚬⚬> DM1: expansion of CTG trinucleotide repeat sequence within DMPK gene on chromosome 19
   ⚬⚬⚬⚬> DM2: expansion of CCTG tetranucleotide repeat sequence within ZNF9 gene on chromosome 3
2. Genetic anticipation: worsening severity of the condition and earlier age of presentation within successive generations.
   - Seen in Trinucleotide repeat expansion diseases:
3. Both DM1 and 2 are autosomal dominant
4. DM1 usually presents in 20s–40s (DM2 later), but can be very variable depending on number of triplet repeats.

Question 6

Genetic anticipation and Trinucleotide repeat expansion diseases

Answer 6

Genetic anticipation: worsening severity of the condition and earlier age of presentation within successive generations.
Seen in Trinucleotide repeat expansion diseases:
⚬⚬⚬> DM1,
⚬⚬⚬> Huntington’s chorea (autosomal dominant),
⚬⚬⚬> Friedrich’s ataxia (autosomal recessive),
⚬⚬⚬> Fragile X syndrome (X-linked defect)

Question 7

Diagnosis of Dystrophia myotonica

Answer 7

1. Clinical features
2. EMG: ‘dive‐bomber’ potentials
3. Genetic testing

Question 8

Management of Dystrophia myotonica

Answer 8

1. Affected individuals die prematurely of respiratory and cardiac complications
2. Weakness is major problem – no treatment
3. Phenytoin may help myotonia
4. Advise against general anaesthetic (high risk of respiratory/cardiac complications)

Question 9

Common causes of ptosis

Answer 9

*** Bilateral ============== *** Unilateral
⚬⚬> Myotonic dystrophy =====⚬⚬> Third nerve palsy
⚬⚬> Myasthenia gravis =======⚬⚬> Horner’s syndrome
⚬⚬> Congenital