What is the pathophysiology of Lambert-Eaton myasthenic syndrome (LEMS)?

Updated: May 23, 2019
  • Author: David E Stickler, MD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

Physiologic studies of neuromuscular transmission demonstrate that ACh release from the motor nerve terminal is impaired in the LEMS muscle. An autoimmune attack directed against the voltage-gated calcium channels (VGCCs) on the presynaptic motor nerve terminal results in a loss of functional VGCCs at the motor nerve terminals.

The number of quanta released by a nerve impulse is diminished. However, because presynaptic stores of ACh and the postsynaptic response to ACh remain intact, rapid repetitive stimulation or voluntary activation that aids in the release of quanta will raise the endplate potential above threshold and permit generation of muscle action potential.

As neuromuscular transmission is completed at additional neuromuscular junctions, a transient increase will occur in the strength of the muscle. Parasympathetic, sympathetic, and enteric neurons are all affected. Clinically, this phenomenon is noted by the appearance of previously absent tendon reflexes following a short period of strong muscle contraction by the patient.


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