Myasthenia gravis

Myasthenia gravis

Myasthenia gravis is an autoimmune disorder characterised by impaired neuromuscular transmission due to antibodies targeting acetylcholine receptors at the neuromuscular junction. Anti-acetylcholine receptor antibodies

Myasthenia gravis - antibodies against acetylcholine receptors

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Importance: 51

are present in approximately 85–90% of cases*. It is more common in women (2:1).


The key feature is muscle fatigability

fatigability

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- muscles become progressively weaker during periods of activity and slowly improve after periods of rest:

• extraocular muscle weakness: diplopia
• proximal muscle weakness: face, neck, limb girdle
• ptosis
• dysphagia

Associations

• thymomas in 15%
  thymomas in 15%
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• autoimmune disorders: pernicious anaemia, autoimmune thyroid disorders, rheumatoid, SLE
• thymic hyperplasia in 50-70%

Investigations

• single fibre electromyography: high sensitivity (92-100%)
• CT thorax to exclude thymoma
• CK normal
• antibodies to acetylcholine receptors
  Testing for acetylcholine receptor antibodies is a key step in investigating patients with suspected myasthenia gravis
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  Importance: 66
  • positive in around 85-90% of patients
  • n the remaining patients, about about 40% are positive for anti-muscle-specific tyrosine kinase antibodies
• Tensilon test: IV edrophonium reduces muscle weakness temporarily - not commonly used any more due to the risk of cardiac arrhythmia

Management

• long-acting acetylcholinesterase inhibitors
  Pyridostigmine is a long-acting acetylcholinesterase inhibitor that reduces the breakdown of acetylcholine in the neuromuscular junction, temporarily improving symptoms of myasthenia gravis
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  Importance: 57
  • pyridostigmine
    pyridostigmine
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    is first-line
• immunosuppression is usually not started at diagnosis, but the majority of patients eventually require it in addition to long-acting acetylcholinesterase inhibitors:
  • prednisolone initially
  • azathioprine, cyclosporine, mycophenolate mofetil may also be used
• thymectomy

Management of myasthenic crisis

Management of myasthenic crisis

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• plasmapheresis
• intravenous immunoglobulins

*antibodies are less commonly seen in disease limited to the ocular muscles

+PassMedicine Notes

Myasthenia gravis is an autoimmune disorder characterised by impaired neuromuscular transmission due to antibodies targeting acetylcholine receptors at the neuromuscular junction. Anti-acetylcholine receptor antibodies are present in approximately 85–90% of cases*. It is more common in women (2:1).

The key feature is muscle fatigability - muscles become progressively weaker during periods of activity and slowly improve after periods of rest:

• extraocular muscle weakness: diplopia
• proximal muscle weakness: face, neck, limb girdle
• ptosis
• dysphagia

Associations

• thymomas in 15%
• autoimmune disorders: pernicious anaemia, autoimmune thyroid disorders, rheumatoid, SLE
• thymic hyperplasia in 50-70%

Investigations

• single fibre electromyography: high sensitivity (92-100%)
• CT thorax to exclude thymoma
• CK normal
• antibodies to acetylcholine receptors
• positive in around 85-90% of patients
• n the remaining patients, about about 40% are positive for anti-muscle-specific tyrosine kinase antibodies
• Tensilon test: IV edrophonium reduces muscle weakness temporarily - not commonly used any more due to the risk of cardiac arrhythmia

Management

• long-acting acetylcholinesterase inhibitors
• pyridostigmine is first-line
• immunosuppression is usually not started at diagnosis, but the majority of patients eventually require it in addition to long-acting acetylcholinesterase inhibitors:
• prednisolone initially
• azathioprine, cyclosporine, mycophenolate mofetil may also be used
• thymectomy

Management of myasthenic crisis

• plasmapheresis
• intravenous immunoglobulins

*antibodies are less commonly seen in disease limited to the ocular muscles