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Myasthenia gravis (MG)

Overview

MG is a neuromuscular disorder, whose prevalence peaks at 20–30 years or 60–70 years of age. MG is characterized by fatiguability, where initial part of movement can be done smoothly and then the power decreases on continued activity.

  • Generation of autoantibodies against acetylcholine receptors at the neuromuscular junction  and MuSK, LRP, ryanodine, and titin proteins. This leads to failure of conduction of action potential from nerve to muscle, leading to weakness.
  • Thymus is abnormal in 75% of patients: hyperplasia (85%) and thymoma (15%)
  • Other autoimmune conditions associated with MG are thyroiditis, Graves’s disease, rheumatoid arthritis, systemic lupus erythematosus, pernicious anaemia, Addison’s disease, and vitiligo.

  • Certain genetic markers, such as HLA-B8 and DR3
  • Autoimmune disorders such as lupus, rheumatoid arthritis, or thyroid disease
  • Infants of mothers with myasthenia gravis may develop neonatal myasthenia gravis
  • Smoking
  • Medications including antimalarials, certain antibiotics, and medications used to treat heart rhythm irregularities

  • Painless muscle weakness following exercise
  • In 15–20% cases, only the ocular muscles are involved, whereas in 85% of the cases, the weakness is generalized
  • Other presenting features include dysphagia (6%), dysarthria and dysphonia (5%), jaw weakness (4%), and neck weakness (1%).
  • Rarely, respiratory failure and isolated foot drop may be presented
  • Exacerbation of weakness by drugs (aminoglycosides, quinine, anti-arrhythmic drugs)

Neurologist

  • Clinical suspicion should guide further investigations
  • Antibody panel- Anti acetylcholine receptor antibody, anti-Musk antibody, anti LRP antibodies
  • Repetitive nerve stimulation- sensitive in 50–60% of cases (see chapter 6)
  • Single fibre EMG studies- detect delay or failed neurotransmission in pairs of muscle fibres supplied by a single nerve fibre
  • Tensilon (edrophonium) test- uses a rapid onset (30 seconds) short-acting (5 minutes) cholinesterase inhibitor drug. An unequivocal improvement in a muscle is considered a positive result
  • Post-contrast CT or MRI of the mediastinum to check for thymoma
  • Other tests such as thyroid function and thyroid antibodies test among others may be needed

  • Cholinesterase inhibitors
  • Immunomodulators
  • Supportive treatment with calcium, bisphosphonates, vitamin D, and antacids
  • Immunosuppressants may be used as steroid sparing agents
  • Blood tests (FBC and LFT) necessary every week for 2 months and then 3 monthly for the duration of treatment to monitor immunosuppression
  • Plasma exchange and IV immunoglobulin given in cases of sudden deterioration with impending respiratory failure, dysphagia, severe worsening of weakness for immediate relief of symptoms (myasthenia crisis)
  • Thymectomy when thymoma is found in CT chest to eliminate the source of abnormal antibodies.
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