Introduction

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.

Epidemiology

  • Incidence: 0.50 cases per 100,000 person-years
  • Peak incidence: 70+ years
  • Sex ratio: 1:1
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Clinical features

Features
  • 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

EMG
  • incremental response to repetitive electrical stimulation

Management

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