The Lambert-Eaton myasthenic syndrome (also known as the Eaton-Lambert syndrome) usually occurs in association with small-cell carcinoma of the lung.
It is caused by an antibody directed against the calcium channels in the presynaptic membrane of the neuromuscular junction, resulting in impaired release of acetylcholine. It is characterized by proximal muscle weakness, which initially involves the legs. It may progress to involve the arms and the extraorbital and bulbar muscles in some patients. EMG examination shows characteristic abnormalities.
There may be transient improvement in strength following exercise, distinguishing it from true myasthenia.
The neurological symptoms may improve with successful control of the underlying malignancy, corticosteroids, immunosuppression, or intravenous immunoglobulin.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. 4th ed. Oxford University Press, 2004. p. 338.