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Lambert-Eaton myasthenic syndrome (LEMS) is frequently mistaken for myasthenia gravis (MG), but subtle differences in weakness patterns, reflexes, and eye signs can set them apart. Unlike MG, LEMS starts in the legs, spares the eyes until later, and comes with autonomic symptoms like dry mouth. Crucially, more than half of LEMS cases are linked to small cell lung cancer — making an accurate, timely diagnosis potentially life-saving.
Lambert-Eaton myasthenic syndrome (LEMS) and myasthenia gravis (MG) both attack the neuromuscular junction, but they do so at opposite ends — LEMS targets the presynaptic membrane, MG the postsynaptic side. That anatomical difference drives a host of clinical contrasts that can help clinicians tell them apart. LEMS classically begins with proximal leg weakness and absent tendon reflexes, while MG typically starts with droopy eyelids and double vision. Notably, ptosis in LEMS may improve with sustained upgaze — the opposite of MG — a paradoxical clue that shouldn't be missed.
The stakes are high: over 50% of LEMS cases are paraneoplastic, most often tied to small cell lung cancer (SCLC). Neurologic symptoms can precede tumor detection by years, meaning eye care providers and neurologists may be the first to spot the disease. Once LEMS is confirmed, chest CT or PET imaging is recommended with repeat screening every 3–6 months for at least 2 years.
Key Takeaways:
Why it matters: LEMS is rare and its eye signs are subtle, making misdiagnosis common. Recognizing the distinguishing features — especially ptosis that improves with effort, proximal leg weakness, and autonomic symptoms — can prompt the right tests and uncover an associated lung cancer at an earlier, more treatable stage.
