The authors studied two patients with Lambert-Eaton myasthenic syndrome (LEMS) in whom the repeated examination did not find specific of LEMS P/Q type voltage-gates calcium channel autoantibodies. The results of clinical testing and electrophysiological examination showed the typical character of movement disorders with the absence of tendon reflexes and signs of disautonomia as well as a decrease in M-response amplitude and phenomena of decrement with low frequency- and increment with high frequency stimulation. Both patients revealed no signs of paraneoplastic process. Autoimmune character of the damage was confirmed by the effectiveness of treatment with glucocorticoid hormones.
The decrease of safety factor of neuromuscular transmission was equal in both groups. At the same time, a significant difference in the decrease of pattern of the amplitude compound of muscle action potential (CMAP) was found. In LEMS, by contrast with MG, another sequence of amplitude variability of CMAP from the second stimulus to the first and from the fifth stimulus to the fourth was noted. In LEMS patients, progressive decrement, manifesting by increasing ratios of late A9/A1 to early A4/A1 was found, whereas the MG patients showed retrogressive decrement expressed by the reduction in decrement ratio (from late to early). These differences reflect the mechanisms and status of acetylcholine mobilization and release from the axon terminal.
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