Lambert-Eaton myasthenic 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
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