American Board of Internal Medicine (ABIM) Certification Practice Exam

What is often the underlying cause in patients with Lambert-Eaton Myasthenic Syndrome?

1. A viral infection

2. Autoimmune reaction

3. Underlying malignancy

4. Chronic fatigue syndrome

The correct answer is: Underlying malignancy

Lambert-Eaton Myasthenic Syndrome (LEMS) is often associated with a type of cancer, particularly small cell lung carcinoma. The underlying malignancy is thought to contribute to the syndrome because the tumor produces antibodies that interfere with the normal release of acetylcholine at the neuromuscular junction. In patients with LEMS, the immune response targets the voltage-gated calcium channels, leading to impaired neuromuscular transmission. This condition presents with muscle weakness and autonomic symptoms. While autoimmune reactions play a role in LEMS, particularly in cases not associated with malignancy, the key association with underlying malignancy distinguishes this syndrome. Other options, like viral infections or chronic fatigue syndrome, do not have the same strong link to the pathophysiology of LEMS as the presence of a malignant tumor does. The correct identification of malignancy as a potential underlying cause is crucial for proper diagnosis and management strategies for patients presenting with symptoms of LEMS.