The true incidence of LEMS is unknown. An estimated 3% of patients with SCLC have LEMS. The prevalence of SCLC is 5 cases per million population in the United States. Because only 50-70% of patients with LEMS have an identifiable cancer and because LEMS goes undiagnosed in many patients, the true total prevalence of LEMS may be considerably higher.
The overwhelming majority of cancers associated with LEMS are SCLC. However, many different malignancies may be involved. A partial list includes non-SCLC; neuroendocrine carcinomas; lymphosarcoma; malignant thymoma; cancers of the breast, stomach, colon, prostate, bladder, kidney, gallbladder, and rectum; basal cell carcinoma; leukemia; lymphoproliferative disorders such as Castleman syndrome; and Hodgkin lymphoma.
According to one estimate, there are approximately 400 cases in the United States at any given time. However, this estimate does not take into account the number of patients with LEMS who do not have SCLC or any other identifiable malignancy.
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Characteristic responses to repetitive nerve stimulation in patient with Lambert-Eaton myasthenic syndrome. (A) Responses elicited from hand muscle by stimulation of nerve at 3 Hz. Amplitude of initial response is less than normal, and response is decremental. (B) Responses as in A, immediately after voluntary activation of muscle for 10 seconds. Amplitude has increased. (C) Responses in hand muscle elicited by 20-Hz stimulation of nerve for 10 seconds. Response amplitude is less than normal initially, falls further during first few stimuli, then increases and ultimately becomes more than twice initial value.
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Compound muscle action potentials elicited from hand muscle before and immediately after maximal voluntary activation of muscle for 10 seconds. Amplitude is small initially, increasing almost 10 times after activation.
