Myasthenia gravis

Myasthenia gravis is an autoimmune condition where antibodies target postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction. Antibodies to acetylcholine receptors are seen in 85-90% of cases*. Myasthenia 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
  • In 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
• Single fibre electromyography has the highest sensitivity (92-100%) for diagnosing myasthenia gravis and would eventually confirm the diagnosis if antibody testing were negative. However, it is a more invasive, technically demanding, and less widely available investigation. In clinical practice, serological testing is performed first as it is simpler and less burdensome for the patient. Single fibre EMG is typically reserved for cases where there is strong clinical suspicion but negative antibody tests, or when the diagnosis remains uncertain after initial investigations.

Management

• long-acting acetylcholinesterase inhibitors
  • pyridostigmine is first-line