Abstracts

RATIONALE: Objective -To determine if there is an interhemispheric difference in motor cortex excitability that can be detected by transcranial magnetic stimulation (TMS).
Introduction - There have been many reports in the literature that have suggested that epileptogenic discharges have a tendency to originate more frequently from the left hemisphere compared to the right (Dean et al., 1997, Foy et al., 1992). We hypothesized that the interhemispheric difference in epileptogenicity may be because the left hemisphere is physiologically predisposed to being more epileptogenic and that there may be an interhemispheric difference in cortical excitability which may be amenable to detection by TMS.
METHODS: 5 right handed normal volunteers were subjected to transcranial magnetic stimulation of their two hemispheres separately. After determining the resting motor threshold (RMT), the cortical silent period (C-SP) was measured while recording from the appropriate abductor pollicis brevis. The C-SP obtained on right hemispheric stimulation was compared to the C-SP measured on left hemispheric stimulation by a paired t-test. Informed consent was obtained from all the volunteers and the study protocol was approved by the institutional review board of the Weill- Cornell Medical Center.
RESULTS: All the subjects had a longer C-SP on stimulation of their right hemisphere compared to that on stimulation of their left hemisphere. The mean interhemispheric difference in C-SP was 37.74 +_ 8.09 ms. The difference was significant with a p =.009.
CONCLUSIONS: In our study although the sample size is small, we demonstrated a statistically significant difference in motor cortical inhibition between the two hemispheres with the inhibition being more pronounced on right hemispheric stimulation. This suggests a physiological asymmetry between the two hemispheres and is concordant with the data in the literature on increased left hemispheric epileptogenicty (Dean et al., 1997, Foy et al., 1992).
References-
Dean AC, Solomon G, Harden CL, Papakostas G & Labar DR. Left hemispheric dominance of epileptiform discharges. Epilepsia 1997;38:503-505.
Foy PM, Chadwick DW, Rajgopalan N, Johnson AL & Shaw MDM. Do prophylactic anticonvulsant drugs alter the pattern of seizures after craniotomy. J Neurol. Neurosurg. Psychiatry 1992;55:753-757.