Myasthenia Gravis

Question 1

Presentation of myasthenia gravis

Answer 1

Slowly increasing or relapsing muscular fatigue 
Ptosis
Diploplia
Weak face and jaw
Dysarthria 
Myasthenic snarl on smiling 
'Peek sign' of orbicularis fatigability 
Dysphonia 
Dysphagia 
Limb weakness (proximal shoulder and hips)
Neck and trunk weakness 
Respiration problems 
Tendon reflexes are normal

Question 2

What is myasthenia gravis

Answer 2

Autoimmune disease medicated by antibodies to nicotinic acetylcholine receptors (ACh) on the post-synaptic side of the neuromuscular junction
Both B and T cells are implicated

Question 3

Which muscle groups are affected in myasthenia gravis?

Answer 3

In order

• extraocular
• bulbar (swallowing and chewing affected)
• face
• neck
• limb girdle
• trunk

Question 4

What exacerbates myasthenia gravis symptoms?

Answer 4

Pregnancy
Hypokalaemia 
Infection
Over-treatment
Change of climate 
Emotion
Exercise
Drugs
- gentamicin 
- opiates 
- tetracycline
- quinine 
- beta blockers

Question 5

Differentials for myasthenia gravis

Answer 5

Polymyositis/other myopathies
SLE
Takayasu’s arteritis (fatigability of the extremities)
Botulism

Question 6

How is myasthenia gravis diagnosed?

Answer 6

Antibodies
- increased anticholinergic receptor antibodies (90%)
- if anti-AChR negative, look for MuSK antibodies (muscle specific tyrosine kinase)
EMG
- Decremental muscle response to repetitive nerve stimulation with/without single fibre jitter
Imaging
- CT excludes thyroma
Ptosis improves by >2mm after ice application to the eyelid for >2 mins
- not diagnostic

Question 7

What are the three areas of management of myasthenia gravis

Answer 7

Symptom control
Immunosuppression
Thymectomy

Question 8

How is symptom control achieved in myasthenia gravis

Answer 8

Anticholinesterase

• prevents the breakdown of acetylcholine, so more is free to act on receptors
• e.g. pyridostigmine

Question 9

Describe the role of immunosupression in myasthenia gravis

Answer 9

Relapses
- prednisolone
- 5mg on alternate days, and increase by 5mg a week until dosage is 1mg/kg on treatment days
- decrease once remission achieved (may take months)
- azathioprine, ciclosporin and mycophenolate mofetil can also be used
- osteoporosis prophylaxis
IV immunoglobulins or plasma exchange may be used if above ineffective

Question 10

What is the purpose of thymectomy in the management of myasthenia gravis

Answer 10

Beneficial effects, even in patients without a thymoma
- consider especially in younger patients with onset <5 years previously and poor response to medical therapy
Also prevents local invasion in thymoma is present

Question 11

What is a myasthenic crisis

Answer 11

Life-threatening weakness of respiratory muscles during a relapse
- can be difficult to differentiate from cholinergic crisis (i.e. overtreatment), although this is rare and usually only occurs in doses of pyridostigmine >960mg/d

Question 12

How is a myasthenic crisis managed

Answer 12

Monitor forced vital capacity
Ventilatory support may be needed
Plasmaphoresis (removes anti-AChR antibodies from circulation)
OR
IV immunoglobulins - same theory as immunosupression, but has faster action
Identify and treat trigger of event