4.4 Myotonic Dystrophy

Question 1

HistoRY You are asked to preassess a 32-year-old man with myotonic
dystrophy, booked for wisdom tooth removal.

What is the definition of myotonic dystrophy?

Answer 1

Progressive, hereditary neuromuscular disorder characterised by

• Myotonia
  (prolonged contraction/delayed relaxation of the skeletal
  muscles after voluntary stimulation)
• Dystrophy (progressive weakness and muscular atrophy)

Question 2

Inheritance

Incidence

Answer 2

Autosomal dominant disorder with an

incidence of 2.4–5.5 cases per 100 000 in the UK.

Question 3

What is the pathophysiology of myotonic dystrophy?

Answer 3

1. Locus for myotonic dystrophy is found on chromosome 19.
2. The underlying pathophysiology is related
   to abnormal sodium or chloride channels,
3. which results in the muscle being
   in an abnormal hyperexcitable state.
4. This leads to repetitive action potentials
   and sustained muscle contraction,
   manifesting in the inability to relax.

Question 4

What are the complications and system manifestations of this disease?

Answer 4

1. Facial feature
2. Cardiac
3. Respiratory
4. Neurological
5. Endocrine
6. GI

Question 5

1. Facial feature

Answer 5

Frontal balding,
muscle wasting,
ptosis,
cataracts

Question 6

2. Cardiac

Answer 6

• Conduction defects
  (heart block, bundle branch block, wide QRS,
  increased QTc, PR intervals)
• Heart failure
• Cardiomyopathy
• Mitral valve prolapse

Question 7

3. Respiratory

Answer 7

• Respiratory muscle weakness
• OSA
• Decreased hypercapnic drive, hypoxaemia, cor pulmonale
• Mucus/sputum retention, poor cough, risk of respiratory infection

Question 8

4. Neurological

Answer 8

• Bulbar palsy, dysphagia
• Intellectual impairment

Question 9

5. Endocrine

Answer 9

• Hypothyroidism
• Diabetes

Question 10

6. GI

Answer 10

• Delayed gastric emptying, constipation
• Aspiration

Question 11

How could you pre optimise him?

Answer 11

He would require a thorough preoperative assessment
including a full history and examination
looking for the multisystem involvement
as listed above, with special mention of:

1. • Bulbar problems:
     dysphagia, slurred speech (aspiration risk)
2. • Cardiac abnormalities:
     conduction defects (may need pacing)
3. • Respiratory muscle fatigue:
     poor cough (risk of chest infection),
     OSA (need for NIV/overnight ventilation)
4. • Endocrine:
     presence of diabetes/hypothyroidism

Question 12

investigations

Answer 12

1. • 12-lead ECG
     (check for conduction abnormalities
     and consider need for pacing intraoperatively)
2. • FBC, U&Es, Blood glucose
     (may have anaemia of chronic disease,
     polycythaemia associated with lung disease,
     hyperkalaemia due to muscle dysfunction,
     raised blood glucose secondary to associated
     diabetes mellitus)
3. • Pulmonary function tests
     (to look for restrictive lung disease)
4. • ABGs
     (may have chronic hypoxaemia)
5. • CXR
     (may have evidence of aspiration pneumonitis,
     evidence of cardiac failure)
6. • Echocardiogram
     (to exclude structural abnormality,
     e.g. mitral valve prolapse)

Question 13

Pre counselling

Answer 13

• Local versus general anaesthesia (see below)
• Discussion regarding factors that precipitate myotonia
• Risk of deterioration of disease with anaesthesia
  and need for overnight stay

Question 14

He states that anything in his mouth precipitates his myotonia and that he
might bite the surgeon’s fingers off (if done under local anaesthesia).

How could you prevent this?

Answer 14

• Sedation may be an option but has a risk
  of inducing severe respiratory depression
• Local anaesthetic infiltration of the masseter
  has been shown to reduce myotonia

He refuses the option of local anaesthetic.

Question 15

How would you proceed with regard to giving a general anaesthetic?

Pre-op

Answer 15

Preoperative

1. • Ensure preoperative optimisation (as above),
     take a good history, and explain potential complications
2. • Premedication:
     Avoid respiratory depressants and give antacids

Question 16

How would you proceed with regard to giving a general anaesthetic?

Intraoperative

Answer 16

1. • Full monitoring as per AAGBI guidelines,
     consider invasive monitoring if history of cardiomyopathy
     or arrhythmias.

Have pacing capability available

2. • Avoid precipitation of myotonias:
     hypothermia, shivering, mechanical and electrical stimulation.

Use warming blankets, warm fluid, monitor temperature

3. • Induction:
     Etomidate, thiopentone, and propofol
     have all been shown to be safe,

though propofol is associated with less postoperative ventilation

4. • Muscle relaxation:
     Depolarizing neuromuscular blocking agents
     (suxamethonium) may induce generalized muscular contractures

and are therefore not recommended.

Non depolarizing neuromuscular blocking agents are
not associated with myotonia,

but the use of anticholinesterases may precipitate contractures,
due to increased sensitivity of acetylcholine

5. Maintenance:
   Avoid volatiles as they may induce shivering,
   and therefore myotonia, at high concentrations.

Propofol and remifentanil total intravenous anaesthesia

has been shown to be very effective and avoids
the need for muscle relaxation

6. • Airway:
     Will require intubation as significant risk of aspiration and possibly
     nasal route to facilitate surgical access
7. • Emergence:
     Avoid anticholinesterase agents (precipitate myotonia),
     extubated with care to prevent aspiration

Question 17

How would you proceed with regard to giving a general anaesthetic?

Answer 17

Postoperative

1. • Consider need for prolonged ventilation,

at high risk of delayed onset apnoea
and should have ECG and oxygen saturations
monitored for 24 hours postop

2. Consider need for ICU/HDU.
   Try to avoid depressant analgesics.

May require chest physiotherapy postoperatively
depending on prior lung function

Question 18

What other muscular dystrophies are you aware of and how do they differ?

Answer 18

Duchene Muscular Dystrophy

It is most common in childhood
affecting males and
is inherited as X linked recessive disorder.

It is characterized by proximal muscle wasting and
weakness associated with contractures, scoliosis, restrictive lung disease,
and cardiomyopathy.

Death occurs usually in second or third decade from cardiorespiratory failure.

The causative factor is shown to be the lack of
dystrophin (protein that anchors muscle to extracellular matrix).

Question 19

Becker’s Muscular Dystrophy

Answer 19

1. It is a milder form of muscular dystrophy affecting 1 in 30,000 men.
2. In this condition, dystrophin is only partially absent.
3. It presents in teenage years and has a protracted course,
4. Death happens at fourth or fifth decade from cardiorespiratory failure