WBR0766
| Author | [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Will Gibson)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Pathology |
| Sub Category | SubCategory::Musculoskeletal/Rheumatology |
| Prompt | [[Prompt::A 58-year-old man with a past medical history significant for small cell lung cancer (SCLC), presents to the physician's office complaining 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 shows an increment in compound muscle action potential amplitude. Which of the following is the most likely pathophysiology of the patient's condition?]] |
| Answer A | AnswerA::Antibodies against the presynaptic sodium (Na) channels |
| Answer A Explanation | [[AnswerAExp::Lambert-Eaton myasthenic syndrome (LEMS) is not due to formation of antibodies against the presynaptic sodium (Na) channels. The presynaptic sodium channels are normally responsible for depolarization of the presynaptic neuron of the neuromuscular junction.]] |
| Answer B | AnswerB::Antibodies against the presynaptic acetylcholine receptors |
| Answer B Explanation | AnswerBExp::LEMS is not caused by antibodies against presynaptic acetylcholine receptors. |
| Answer C | AnswerC::Antibodies against postsynaptic acetylcholine receptors |
| Answer C Explanation | AnswerCExp::LEMS is not due to antibodies against postsynaptic acetylcholine receptors. Myasthenia gravis is due to formation of auto-antibodies against the postsynaptic acetylcholine receptors. |
| Answer D | AnswerD::Antibodies against presynaptic calcium channels |
| Answer D Explanation | AnswerDExp::LEMS is due to formation of auto-antibodies against the presynaptic calcium (Ca) channels. |
| Answer E | AnswerE::Antibodies against the postsynaptic calcium channels |
| Answer E Explanation | AnswerEExp::LEMS is not due to antibodies against postsynaptic calcium channels. |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune neuromuscular disorder that may present in approximately half of the patients as a paraneoplastic syndrome in patients who have SCLC. LEMS is characterized by the presence of autoimmune antibodies against voltage-gated calcium channels (VGCC) that are located on the presynaptic nerve terminal.
LEMS, similar to myasthenia gravis (MG), is also a neuromuscular junction disease. Unlike MG, LEMS is characterized by incremental improvement of the junction following repetitive stimulation. In contrast, MG is characterized by junctional fatigue; loss of action potential generation is seen following repetitive stimulation. The most common presentation of patients with LEMS is proximal muscle weakness, which often starts in the lower extremities and progresses to involve the upper extremities. Ocular involvement, similar to patients with MG, is also frequently implicated in LEMS. On physical examination, muscle weakness and tendinous areflexia are common findings. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::LEMS, WBRKeyword::Lambert-eaton, WBRKeyword::Myasthenia, WBRKeyword::Myasthenic, WBRKeyword::Myasthenia gravis, WBRKeyword::Lambert, WBRKeyword::Eaton, WBRKeyword::Presynaptic, WBRKeyword::pre-synaptic, WBRKeyword::VGCC, WBRKeyword::voltage, WBRKeyword::gated, WBRKeyword::calcium, WBRKeyword::channel, WBRKeyword::channels, WBRKeyword::autoimmune, WBRKeyword::autoantibody, WBRKeyword::autoantibodies, WBRKeyword::auto-antibody, WBRKeyword::auto-antibodies, WBRKeyword::small, WBRKeyword::cell, WBRKeyword::lung, WBRKeyword::cancer, WBRKeyword::carcinoma, WBRKeyword::paraneoplastic, WBRKeyword::acetylcholine, WBRKeyword::receptor, WBRKeyword::receptors, WBRKeyword::antibody, WBRKeyword::antibodies |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |