Lambert-Eaton syndrome
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 pre-synaptic voltage gated calcium channel in the peripheral nervous system
Features
EMG
Management
*in reality this is seen in only 50% of patients and following prolonged muscle use muscle strength will eventually decrease
**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
Features
- repeated muscle contractions lead to increased muscle strength* (in contrast to myasthenia gravis)
- limb girdle weakness (affects lower limbs first)
- hyporeflexia
- autonomic symptoms: dry mouth, impotence, difficultly micturating
- ophthalmoplegia and ptosis not commonly a feature (unlike in myasthenia gravis)
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**
- intravenous immunoglobulin therapy and plasma exchange may be beneficial
*in reality this is seen in only 50% of patients and following prolonged muscle use muscle strength will eventually decrease
**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