The CA LEMS Underdiagnosis Gap in Small Cell Lung Cancer: Prevalence, Attribution Bias, and the Neurology Referral Problem

Dr. Sen addresses the striking gap between CA LEMS prevalence in prospective studies (3-6% of SCLC) and real-world diagnosis rates (under 1%), citing the Drapkin et al. published dataset. This is a detection problem, not a prevalence problem. Two structural forces drive underdiagnosis: first, post-exercise facilitation testing and reflex assessment are not built into routine oncology workflows, so even textbook presentations may be missed; second, attribution bias leads oncologists to assign progressive weakness to disease burden, chemotherapy, or deconditioning rather than considering a distinct paraneoplastic process. A key clinical trigger that should be recognized is the mismatch between radiographic response and the patient's functional status; when a patient is responding on imaging but becoming progressively weaker, something else is going on.

Published data confirm that neurologists perform most of the diagnostic heavy lifting for CA LEMS, meaning patients are entirely dependent on timely oncology-to-neurology referral. When that referral is delayed, patients may go undiagnosed for months while strength and independence decline, with real consequences not only for quality of life but for ability to tolerate cancer-directed therapy.

Dr. Primdahl adds that neurophobia, defined as discomfort with neurologic examination among non-neurologists, and access barriers (neurology wait times exceeding 6 months in some settings) compound diagnostic delays. In SCLC, where median survival is measured in months, this delay is clinically unacceptable. She emphasizes that community oncologists without immediate neurology access can initiate the workup themselves by ordering serum VGCC antibody testing, which does not require a neurology referral first, and that a positive result typically accelerates specialist access.