A 52-year-old man with oat-cell carcinoma of the lung develops progressive proximal muscle weakness, depressed deep tendon reflexes, and autonomic dysfunction. A 48-year-old woman with ovarian cancer presents with progressive distal sensory loss, ataxia, and nystagmus with anti-Hu antibodies detected. Which laboratory/clinical finding best distinguishes Lambert-Eaton myasthenic syndrome (LEMS) from paraneoplastic cerebellar degeneration (PCD)?
A. Association with small-cell lung cancer
B. Presence of anti-neuronal antibodies (anti-Hu, anti-Yo, anti-CRMP5)
C. Involvement of the cerebellum on MRI brain
D. Electromyographic findings showing incremental response to high-frequency repetitive nerve stimulation
Explanation
Discriminating LEMS from Paraneoplastic Cerebellar Degeneration
Overview
LEMS and PCD are both paraneoplastic neurological syndromes, but they affect different anatomical sites (neuromuscular junction vs. cerebellum) and have distinct electrophysiological signatures.
Pathophysiology
Key Point
LEMS is a disorder of the presynaptic neuromuscular junction (calcium channel dysfunction); PCD is a disorder of cerebellar Purkinje cells (antibody-mediated destruction).
The incremental response on high-frequency RNS is PATHOGNOMONIC for LEMS and is the single most discriminating feature. In LEMS, repetitive stimulation at high frequency (20–50 Hz) causes progressive increase in compound muscle action potential (CMAP) amplitude by >100% — this is opposite to myasthenia gravis (decremental response).
Mnemonic
LEMS = Incremental (goes UP with high-frequency stim) because calcium channels are partially blocked; repeated stimulation opens more channels. Myasthenia = Decremental (goes DOWN) because acetylcholine receptors are blocked.
PCD: Cerebellar degeneration does NOT affect neuromuscular transmission → EMG is normal or shows only non-specific myopathic changes. No incremental or decremental response.
Clinical Pearl
Clinical Pearl
LEMS patients often improve dramatically with 3,4-diaminopyridine (3,4-DAP), which blocks potassium channels and prolongs action potentials, increasing calcium influx. PCD does not respond to 3,4-DAP because the problem is not at the neuromuscular junction.
Harrison 21e Ch 381; Robbins 10e Ch 7
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