Lambert-Eaton myasthenic syndrome is seen in association with small cell lung cancer and to a lesser extent breast and ovarian cancer. It may also occur independently as an autoimmune disorder. Lambert-Eaton myasthenic syndrome is caused by an antibody directed against presynaptic voltage-gated calcium channel in the peripheral nervous system.

PC

• repeated muscle contractions lead to increased muscle strength (in contrast to myasthenia gravis)
  • in reality, this is seen in only 50% of patients and following prolonged muscle use muscle strength will eventually decrease
• limb-girdle weakness (affects lower limbs first)
• hyporeflexia
• autonomic symptoms: dry mouth, impotence, difficulty micturating
• ophthalmoplegia and ptosis not commonly a feature (unlike in myasthenia gravis)

Investigations

• Imaging to rule out SCLC
• Serology - anti-voltage gated calcium channel antibodies
• EMG - incremental response to repetitive electrical stimulation

Management

• treatment of underlying cancer
• immunosuppression, for example with prednisolone and/or azathioprine
• 3,4-diaminopyridine is currently being trialled
  • works by blocking potassium channel efflux in the nerve terminal so that the action potential duration is increased. Calcium channels can then be open for a longer time and allow greater acetylcholine release to the stimulate muscle at the end plate
• intravenous immunoglobulin therapy and plasma exchange may be beneficial