Lambert-Eaton syndrome (myasthenic syndrome) is a disorder with symptoms very similar to those of myasthenia gravis. There is muscle weakness associated with disturbed communication between nerves and muscles.

In myasthenia gravis, the neurotransmitter acetylcholine (the chemical that transmits impulses between nerves and muscles) is blocked by antibodies to its receptor. In Lambert-Eaton syndrome, however, the signal distortion is caused by an insufficient release of neurotransmitter by the nerve cell. As muscle contraction is continued, the amount of neurotransmitter may build up in sufficient quantities and result in increased strength.

The disorder may be associated with small-cell carcinoma of the lung and other malignancies as a paraneoplastic syndrome or with autoimmune disorders.