54-year-old woman with progressive weakness and areflexia
A 54-year-old woman comes to the clinic for follow-up of several months of progressive fatigue and difficulty with daily activities. She is a librarian and notes particular trouble rising from a seated position and climbing stairs at work. She quit smoking 15 years ago after a 15-pack-year history. On review of systems, she endorses a persistently dry mouth and constipation. Her temperature is 37.0°C (98.6°F), blood pressure is 128/74 mmHg, pulse is 78/min, and respirations are 14/min. On examination, strength is 4/5 in the proximal hip and shoulder girdles, and 5/5 distally. Deep tendon reflexes are absent in the lower extremities. After 10 seconds of sustained quadriceps contraction, the patellar reflex becomes 1+. Sensation is intact. Laboratory studies, including a complete blood count and comprehensive metabolic panel, are unremarkable. Which of the following is the most appropriate diagnostic test?
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A
Explanation
Clinical Pearl:
Lambert-Eaton Myasthenic Syndrome (LEMS) is a presynaptic neuromuscular junction disorder characterized by proximal weakness, autonomic dysfunction, and areflexia with post-exercise facilitation. Over half of cases are paraneoplastic, most commonly due to small cell lung cancer.
Explanation:
This patient's presentation of progressive proximal muscle weakness, autonomic symptoms (dry mouth, constipation), and examination findings of areflexia with post-exercise facilitation is characteristic of Lambert-Eaton Myasthenic Syndrome (LEMS). LEMS is strongly associated with an underlying malignancy, most frequently small cell lung cancer (SCLC). Therefore, the most appropriate initial diagnostic test is a CT scan of the chest to search for a primary lung tumor. Serum acetylcholine receptor antibody testing is used to diagnose myasthenia gravis, which typically presents with weakness that worsens with activity and has preserved reflexes. While a whole-body PET scan is used to search for an unknown primary cancer, a targeted chest CT is the preferred initial study given the specific and strong association of LEMS with SCLC. A muscle biopsy would be considered for a primary myopathy, but the prominent reflex changes point toward a disorder of the neuromuscular junction. Although voltage-gated calcium channel antibody testing can confirm LEMS, identifying the highly associated and life-threatening underlying malignancy is an immediate priority that will critically influence management.
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