Mysathenia Gravis

Question 1

Definition of myasthenia gravis

Answer 1

Is a chronic autoimmune disease that affects the post-synaptic membrane at the neuromuscular junction in skeletal muscle

Question 2

Aetiology/Pathophysiology of myasthenia gravis

Answer 2

• Is an antibody mediated autoimmune disease.
◦ The antibodies would be present at the NMJ and repsond against the nicotinic acetylcholine receptors on the post-synaptic muscle membrane at the NMJ
◦ Most patients have detectable antibodies

• The cause of the presence of antibodies is unclear, but likely due to certain genotypes
• Thymus may also be involved in aetiology of MG

• The antibody attack on the acetylcholine receptors would reduce the number of binding sites for acetylcholine; hence lead to an inconsistent generation of muscle action potential.
◦ This would manifest as muscle weakness

Question 3

History and examination of myasthenia gravis

Answer 3

• Muscle strength fatigability: The weakness WORSENS with activity or repetition (fatigable weakness) and improves on rest. Better in the morning compared to the evening
• Ptosis and diplopia: very common symptoms in many patients. Pupil is spared
• Dysphagia: difficulty in chewing or swallowing occurs when the facial and oropharyngeal muscles are affected. May worsen as they eat
• Dysarthria: Oropharyngeal muscles affected can affect speech and lead to nasal speech. Speech may become more slurred as they talk
• Facial paresis: changes in muscles of facial expression. May show characteristic flattened smile
• Proximal limb weakness: difficulty climbing stairs etc, difficulty sustaining upper limb functions

• SOB: in a MYASTHENIC CRISIS, the muscles of respiration can be weakened, may need mechanical ventilation

Question 4

Investigations for myasthenia gravis

Answer 4

• Serum acetylcholine receptor antibody analysis: Antibodies are positive in the majority of patients
• If ^ negative, then check for muscle-specific tyrosine kinase antibodies: may be positive

• Pulmonary function test if SOB

Question 5

Treatment for myasthenia gravis

Answer 5

Patients with mild and occasional symptoms don’t need treatment

1) Pyridostigmine: Is a cholinesterase inhibitor. Dose depends on severity. Can have cholinergic side effects (increased salivation, lacrimation, sweats etc)

CONSIDER
Corticosteroids, then immunosuppressants
May be considered for a thymectomy

Myasthenic crisis:
1)Intubation and mechanical ventilation
+ Plasma exchange or IV immunoglobulin: Plasma exchange has rapid response

Question 6

Prevention and complications of myasthenia gravis

Answer 6

• Infections are major provoking factors for a myasthenic crisis

• Respiratory failure
• Impaired swallowing
• Acute aspiration