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The effect of valproate on silent period and corticomotor excitability.

OBJECTIVE
To investigate, by transcranial magnetic stimulation, the effects of valproate on silent period and corticomotor excitability.


METHODS
thirty patients with generalized epilepsy were studied at baseline, and re-examined 4 (S1) and 25 (S2) weeks after the administration of valproate (mean dose: 1040 +/- 284 mg). Transcranial magnetic stimulation was performed with a figure of eight coil (recording, first dorsal interosseous). Threshold was measured at 1% steps. Silent period was measured using a recently described protocol. Briefly, silent periods were elicited at 5% increments from 0 to 100% maximum stimulus intensity. At each stimulus intensity, 4 silent periods were obtained and the average value of silent period duration was used to construct a stimulus/response curve of stimulus intensity versus silent period. The resulting curves were then fitted to a Boltzman function and were statistically compared. The motor-evoked potential recruitment curve was constructed under active conditions and analyzed in a similar way.


RESULTS
Valproate increased threshold from 36.5 +/- 5.99% at baseline to 41.02 +/- 7.84% at S1 (p < 0.0001, paired t-test). The maximum value of the silent period curve decreased from 257.5 +/- 3.9 ms at baseline to 230.3 +/- 3.9 ms at S1 (p < 0.0001, F-test and AIC) while the other best-fit values (V(50), slope, threshold) were not significantly affected. Regarding the motor-evoked potential recruitment curve, the maximum value decreased significantly post-drug (from 0.449 +/- 0.007 to 0.392 +/- 0.009, p < 0.01, F-test and AIC test), whereas the rest of the best-fit values remained unaffected.


CONCLUSION
In patients with idiopathic generalized epilepsy, valproate increases threshold and reduces the maximum values of the silent period curve and the motor-evoked potential recruitment curve. These findings probably reflect valproate's effects on voltage-dependent Na(+) channels, as well as an activation of GABA(A) receptors.