Myasthenia gravis is an acquired autoimmune disorder characterised by weakness, typically of the periocular, facial, bulbar, and girdle muscles
- associated with serum IgG antibodies to acetylcholine receptors in the postsynaptic membrane of the neuromuscular junction. Classically, the muscles are easily fatigued.
It affects 5 people in every 100 000
- non-thymoma cases have a peak incidence at 10-30 years and again, at 60-70 years of age; those associated with thymoma have a peak incidence at 40-50 years of age
- females are affected more often in the under 40 year old age group whereas men predominate in cases which develop in older age groups. About 10% of cases develop during childhood
Myasthenia gravis causes fluctuating weakness that worsens with activity and as the day progresses, and ocular weakness, causing ptosis and diplopia
- in 15% of patients, life-threatening respiratory weakness can occur, called myasthenic crisis
- ocular symptoms are the most common presenting symptoms
- around two-thirds of patients progressing to generalized disease - generally within the first 2 years
Diagnosis is based on clinical history and neurologic examination and confirmed by electrodiagnostic testing and the presence of serum autoantibodies directed at proteins in the neuromuscular junction
- vast majority of patients with generalized myasthenia gravis (approx 85%) and pure ocular myasthenia gravis (approx 50%) will have antibodies to the skeletal muscle nicotinic acetylcholine receptor (AChR)
- additionally 8%-10% of patients with generalized disease have antibodies to muscle-specific tyrosine kinase receptor (MuSK), an enzyme involved in acetylcholine receptor clustering in the synaptic cleft
Management:
- majority of patients respond well to immunosuppressive therapies, including prednisone and immunosuppressants such as azathioprine, cyclosporine, mycophenolate mofetil
- cholinesterase inhibiting agents provide temporary relief from symptoms but do not alter the disease course
- if evidence for thymoma on chest CT then patient will benefit from thymectomy
- for patients in myasthenic crisis, with severe disease not responding to immunosuppressive therapy, or preparing for thymectomy
- rapid improvement can be achieved with plasmapheresis or IV immunoglobulin (IVIg)
An increase in symptoms of MG can be seen in pregnancy, but with proper multidisciplinary management, pregnancy is generally not associated with adverse pregnancy outcomes.
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