American Board of Internal Medicine (ABIM) Certification Practice Exam

For patients with Lambert-Eaton Myasthenic Syndrome, how is diagnosis confirmed?

1. Visual acuity testing

2. Electrocardiogram

3. Magnetic resonance imaging

4. Serum anti-voltage-gated calcium channel antibodies testing

The correct answer is: Serum anti-voltage-gated calcium channel antibodies testing

Lambert-Eaton Myasthenic Syndrome (LEMS) is characterized by muscle weakness due to an autoimmune reduction of presynaptic release of acetylcholine at the neuromuscular junction, often associated with an underlying malignancy, particularly small cell lung cancer. The definitive diagnosis of LEMS is confirmed through serological testing for anti-voltage-gated calcium channel antibodies. The presence of these antibodies is critical as they are associated with the pathophysiological mechanism leading to the symptoms seen in LEMS. The testing outcome, showing elevated levels of these antibodies, provides strong evidence for the diagnosis. Other diagnostic tools like visual acuity testing, electrocardiogram, or magnetic resonance imaging may help rule out other conditions or evaluate associated issues, but they do not confirm LEMS. Therefore, serological testing for anti-voltage-gated calcium channel antibodies is the gold standard for confirming the diagnosis of Lambert-Eaton Myasthenic Syndrome.