WBR0766
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| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Will Gibson and Yazan Daaboul, M.D.)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Pathophysiology |
| Sub Category | Musculoskeletal/Rheumatology |
| Prompt | A 58-year-old man with a recent diagnosis of small cell lung carcinoma (SCLC) presents to the physician's office with complaints of proximal muscle weakness and eyelid ptosis. He explains that his lower extremities were first affected, but then his upper extremities became involved. Physical examination is remarkable for tendinous areflexia. Electromyographic repetitive nerve stimulation using high-frequency stimulation demonstrates an increment in compound muscle action potential amplitude. What is the pathogenesis of the disease responsible for the patient's condition? |
| Answer A | Antibodies against the pre-synaptic sodium (Na+) channels |
| Answer A Explanation | Lambert-Eaton myasthenic syndrome (LEMS) is not caused by formation of antibodies against the pre-synaptic sodium (Na+) channels. The pre-synaptic sodium channels are normally responsible for depolarization of the pre-synaptic neuron of the neuromuscular junction. |
| Answer B | Antibodies against the pre-synaptic acetylcholine receptors |
| Answer B Explanation | LEMS is not caused by the formation of antibodies against the pre-synaptic acetylcholine receptors. |
| Answer C | Antibodies against post-synaptic acetylcholine receptors |
| Answer C Explanation | Myasthenia gravis is caused by formation of auto-antibodies against the post-synaptic acetylcholine receptors. |
| Answer D | Antibodies against pre-synaptic calcium (Ca++) channels |
| Answer D Explanation | LEMS is caused by the formation of auto-antibodies against the pre-synaptic calcium (Ca++) channels. |
| Answer E | Antibodies against the post-synaptic calcium (Ca++) channels |
| Answer E Explanation | LEMS is not caused by the formation of antibodies against post-synaptic calcium (Ca++) channels. |
| Right Answer | D |
| Explanation | Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic, autoimmune neuromuscular disorder that is present in approximately half of patients with SCLC. LEMS is characterized by the formation of auto-antibodies against voltage-gated calcium (Ca++) channels (VGCC) on the pre-synaptic nerve terminal. Similar to myasthenia gravis (MG), LEMS is also a neuromuscular junction disease. However, LEMS is characterized by incremental improvement of the junction following repetitive stimulation, while MG is characterized by junctional fatigue (loss of action potential generation) following repetitive stimulation. LEMS typically manifests with proximal muscle weakness, which often starts in the lower extremities and progresses to involve the upper extremities. Similar to MG, ocular involvement is also common with LEMS. On physical examination, muscle weakness and tendinous areflexia are classic findings. Educational Objective: Lambert-Eaton myasthenic syndrome (LEMS) is characterized by formation of auto-antibodies that target voltage-gated calcium (Ca++) channels (VGCC) in the pre-synaptic terminal of the neuromuscular junction. |
| Approved | Yes |
| Keyword | LEMS, Lambert-eaton myasthenic syndrome, Myasthenia gravis, Neuromuscular junction, VGCC, Voltage gated calcium channel, Autoimmune disease, Autoantibodies, Small cell lung carcinoma, SCLC, Paraneoplastic syndrome |
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