Medical Lambert-Eaton vs. Myasthenia Gravis

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The clinical presentation of Lambert-Eaton Myasthenic Syndrome (LEMS) and Myasthenia Gravis (MG) differs primarily due to their effects on different muscle groups and the mechanisms of action at the neuromuscular junction. LEMS typically affects the pre-synaptic calcium channels, leading to muscle weakness that improves with exertion, while MG targets the post-synaptic receptors, causing weakness that worsens with activity. Symptoms in LEMS often present in the limbs first, whereas MG more commonly affects bulbar muscles responsible for swallowing and speaking. Electromyography (EMG) findings further differentiate the two, with LEMS showing a significant increase in evoked potentials compared to MG. Understanding these differences is crucial for accurate diagnosis and treatment of these autoimmune disorders.
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How comes the clinical presentation differs between LE and MG? In both cases anti-bodys attack the neuromuscular junction, LE attacks the calcium channels on the pre-synaptic axon terminal, and MG the receptor muscles Na/K-channels... Why is there difference in symptoms?
 
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Here are two links explaining the diseases and symptoms with general descriptions of the causes (you may have seen these already, but for the sake of ensuring everyone else knows what is being talked about, this should help).
http://www.emedicine.com/emerg/TOPIC325.HTM
http://www.emedicine.com/NEURO/topic181.htm

The simplest answer (and only one) I can offer on this is that they affect different muscle groups. Autoimmune disorders can be tissue specific, or more general, and I don't have any explanation for you of why they target the muscle groups they do. Otherwise, the symptoms are similar in terms of inducing muscle weakness or loss of function, it's just that one affects the limbs first while the other is more likely to affect the bulbar muscles first (the ones used for swallowing, speaking, chewing).
 
"LEMS is most often a paraneoplastic syndrome associated with small-cell carcinoma, which it may precede. Like myasthenia gravis, it is an autoimmune disorder. Unlike myasthenia gravis, the weakness improves with exertion, and oculobulbar involvement is rare except for ptosis."

"Repetitive electric stimulation at 50-Hz applied to the ulnar nerve at the wrist normally elicits from the abductor of the fifth finger an evoked response of ≥ 5 mV, which may increase no more than 50%. In the case we describe, the electromyogram (EMG) showed that the initial lowamplitude potential was at 0.54 mV and increased progressively to 1.82 mV, a change of 237%. This EMG pattern is a diagnostic feature of Lambert-Eaton myasthenic syndrome (LEMS)."
(Lambert-Eaton myasthenic syndrome: Monique D'Amour, Gilles Gariepy, Joseph Braidy. Canadian Medical Association. Journal. Ottawa: Jan 2, 2007. Vol. 176, Iss. 1; pg. 38, 1 pgs)


"This variability is highly sensitive to neuromuscular transmission abnormalities and is increased in MG patients. Neuromuscular blocking is the failure of transmission of one of the potentials, when one of the muscle fibers fails to transmit an action potential because the EPP does not reach the necessary threshold. The American Association of Neuromuscular & Electrodiagnostic Medicine has developed guidelines for electrodiagnostic testing for evaluation of MG (88)."
(Myasthenia gravis: past, present, and future: Bianca M Conti-Fine, Monica Milani, Henry J Kaminski. Journal of Clinical Investigation. Ann Arbor: Nov 2006. Vol. 116, Iss. 11; pg. 2843, 12 pgs)

It looks like End Plate Potential is inhibited in MG and increased in LEMS and fast twitch or slow twitch muscles are more or less susceptible.
 
Care to link that?
 

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