Station 3.13: Dystrophia myotonica

Question 1

Clinical signs in three sections Face, hands and additional signs

What are the clinical signs of the face for Dystrophia myotonica?

Dystrophia myotonica

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

Answer 1

• Myopathic facies: long, thin and expressionless
• Wasting of facial muscles and sternocleidomastoid
• Bilateral ptosis
• Frontal balding
• Dysarthria: due to myotonia of tongue and pharynx

Question 2

Clinical signs in three sections Face, hands and additional signs

What are the clinical signs of the hands for Dystrophia myotonica?

Dystrophia myotonica

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

Answer 2

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

Question 3

Clinical signs in three sections Face, hands and additional signs

What are the clinical signs of the additional signs for Dystrophia myotonica?

Dystrophia myotonica

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

Answer 3

• Cataracts
• Cardiomyopathy, brady‐ and tachy‐arrhythmias (look for pacemaker scar)
• Diabetes (ask to dip urine)
• Testicular atrophy
• Dysphagia (ask about swallowing)

Question 4

Dscussion - genetics

What is the clinical genetics of Dystrophia myotonica?

Dystrophia myotonica

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

Answer 4

• 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
• Genetic anticipation: worsening severity of the condition and earlier age of presentation within successive generations. Seen in DM1 and also occurs in Huntington’s chorea (autosomal dominant) and Friedrich’s ataxia (autosomal recessive).
• Both DM1 and 2 are autosomal dominant
• DM1 usually presents in 20s–40s (DM2 later), but can be very variable depending on number of triplet repeats.

Question 5

Dscussion - diagnosis

What is the clinical diagnosis of Dystrophia myotonica?

Dystrophia myotonica

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

Answer 5

• Clinical features
• EMG: ‘dive‐bomber’ potentials
• Genetic testing

Question 6

Dscussion - management

What is the clinical management of Dystrophia myotonica?

Dystrophia myotonica

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

Answer 6

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

Question 7

Dscussion - common causes of ptosis

What are the common causes of Ptosis?

Dystrophia myotonica

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

Answer 7

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