Station 3.25: Myasthenia gravis

Question 1

What are the clinical signs of Myasthenia gravis?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 1

• Bilateral ptosis (worse on sustained upward gaze)
• Complicated bilateral extra‐ocular muscle palsies
• Myasthenic snarl (on attempting to smile)
• Nasal speech, palatal weakness and poor swallow (bulbar involvement)
• Demonstrate proximal muscle weakness in the upper limbs and fatiguability. The reflexes are normal
• Look for sternotomy scars (thymectomy)
• State that you would like to assess respiratory muscle function (FVC)

Question 2

Discussion

What are the associations and cause of Myasthenia gravis?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 2

• Associations: other autoimmune diseases, e.g. diabetes mellitus, rheumatoid arthritis, thyrotoxicosis, SLE and thymomas
• Cause: Anti‐nicotinic acetylcholine receptor (anti‐AChR) antibodies affect motor end‐plate neurotransmission

Question 3

Investigations

What diagnostics tests would you prescribe Myasthenia gravis?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 3

Anti‐AChR antibodies positive in 90% of cases
Anti‐MuSK (muscle‐specific kinase) antibodies often positive if anti‐AChR negative
EMG: decremented response to a titanic train of impulses
Edrophonium (Tensilon) test: an acetylcholine esterase inhibitor increases the concentration of ACh at the motor end plate and hence improves the muscle weakness. Can cause heart block and even asystole.

Other tests
* CT or MRI of the mediastinum (thymoma in 10%)
* TFTs (Grave’s present in 5%)

Question 4

Investigations

What treatments would you prescribe for acute Myasthenia gravis?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 4

• IV immunoglobulin or plasmapheresis (if severe)

Question 5

Investigations

What treatments would you prescribe for Chronic Myasthenia gravis?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 5

• Acetylcholine esterase inhibitor, e.g. pyridostigmine
• Immunosuppression: steroids and azathioprine
• Thymectomy is beneficial even if the patient does not have a thymoma (usually young females)

Question 6

Investigations

What treatments would you prescribe for Lambert–Eaton myasthenic syndrome (LEMS) Myasthenia gravis?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 6

• Diminished reflexes that become brisker after exercise
• Lower limb girdle weakness (unlike myasthenia gravis)
• Associated with malignancy, e.g. small‐cell lung cancer
• Antibodies block pre‐synaptic calcium channels
• EMG shows a ‘second wind’ phenomenon on repetitive stimulation

Question 7

Investigations

What are the Causes of bilateral extra‐ocular palsies?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 7

• Myasthenia gravis
• Graves’ disease
• Mitochondrial cytopathies, e.g. Kearns–Sayre syndrom
• Miller–Fisher variant of Guillain–Barré syndrome
• Cavernous sinus pathology

Question 8

Investigations

What are the Causes of bilateral ptosis?

Myasthenia gravis

Examine this patient’s cranial nerves. She has been suffering with double vision.

Answer 8

• Congenital
• Senile
• Myasthenia gravis
• Myotonic dystrophy
• Mitochondrial cytopathies, e.g. Kearns–Sayre syndrome
• Bilateral Horner’s syndrome