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Difficulty: MediumNeoplastic Processes

A 62-year-old man presents to the clinic with a 1-month history of progressive proximal muscle weakness, primarily affecting his thighs and hips. He reports significant difficulty climbing stairs and rising from a seated position, but notes that his strength temporarily improves after a few minutes of continuous walking. He has a 45-pack-year smoking history. Physical examination reveals decreased deep tendon reflexes in the lower extremities and proximal muscle weakness that transiently improves following repetitive muscle contraction. A basic metabolic panel, including serum calcium, is within normal limits. A chest radiograph demonstrates a 4.54.5-cm, centrally located left lung mass with associated hilar adenopathy. Which of the following best describes the underlying pathogenesis of this patient's neurologic symptoms and the most appropriate next step in his care?

  1. Production of autoantibodies against presynaptic voltage-gated calcium channels; obtain tissue biopsy of the hilar mass to confirm the suspected neuroendocrine neoplasmAnswer
  2. B
    Autoimmune destruction of postsynaptic acetylcholine receptors; initiate therapy with an acetylcholinesterase inhibitor and monitor symptom progression
  3. C
    Ectopic secretion of parathyroid hormone-related peptide by squamous cell carcinoma; administer aggressive intravenous hydration and bisphosphonates
  4. D
    Hematogenous dissemination of tumor cells to the central nervous system; order immediate magnetic resonance imaging of the brain and complete spine
  5. E
    Age-related neuromuscular decline combined with chronic tobacco use; schedule a low-dose computed tomography scan in 12 months as part of lung cancer screening

Answer

Production of autoantibodies against presynaptic voltage-gated calcium channels; obtain tissue biopsy of the hilar mass to confirm the suspected neuroendocrine neoplasm
The correct answer accurately identifies the pathogenesis of Lambert-Eaton myasthenic syndrome (LEMS) as autoantibodies against presynaptic voltage-gated calcium channels. The clinical triad of proximal muscle weakness, autonomic signs (often implied or accompanied by decreased reflexes), and post-tetanic potentiation (improvement with repetitive use) is the hallmark of LEMS. In a patient with a central lung mass and heavy smoking history, LEMS is highly indicative of an underlying paraneoplastic process driven by small cell lung cancer (SCLC). The most appropriate management is to diagnose the primary tumor, which necessitates a tissue biopsy of the lung mass to confirm its neuroendocrine origin.

Step-by-Step Solution

1
Analyze the neurologic presentation.
The patient has progressive proximal muscle weakness and decreased deep tendon reflexes that improve with repetitive muscle use.
This specific pattern differentiates Lambert-Eaton myasthenic syndrome (LEMS) from Myasthenia Gravis (which worsens with use) and central nervous system lesions.
2
Correlate the neurologic findings with the patient's risk factors and imaging.
The patient has a heavy smoking history and a central lung mass with hilar adenopathy.
LEMS is a well-known paraneoplastic syndrome that is highly correlated with small cell lung cancer (SCLC), which typically presents as a central mass in smokers.
3
Determine the pathophysiologic mechanism.
LEMS is caused by autoantibodies directed against presynaptic voltage-gated calcium channels at the neuromuscular junction.
Understanding the precise molecular mechanism is required to identify the paraneoplastic process.
4
Select the most appropriate next step in management.
Tissue biopsy of the central lung mass is required.
The definitive treatment for paraneoplastic syndromes is identifying and treating the underlying malignancy. Without tissue confirmation of the neuroendocrine tumor (SCLC), appropriate oncologic management cannot begin.

Key Concept

Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic syndrome associated with small cell lung cancer, caused by autoantibodies against presynaptic voltage-gated calcium channels, requiring evaluation of the primary tumor.
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Neoplastic Processes

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