Abstracts

Rationale: Structural maturation of brain and fiber tracts in the human brain may play a role in developments of human behavior and cognitive function. On MRI, the growth late of the corpus callosum (CC) or myelination change of commissural fiber systems in the white matter dependent on the aging has been reported, but electrophysiological study was not performed. In this study, we recorded transcallosal response (TCRs) in patients with intractable epilepsy during corpus callosotomy and estimated an interhemispheric transmission time (ITT) and conduction velocity (CV) of callosal fiber from infants to adults. Methods: 77 patients were the subjects of this study. All patients and families consented to this study. Age at operation ranged from 0.5 to 52 years old. Preoperative diagnosis was symptomatic generalized epilepsy, frontal lobe epilepsy, severe myoclonic epilepsy in infant, and multifocal epilepsy. During operation, sevoflurane or enflurane was used under the concentration of anesthetics to induce epileptic discharges on electrocorticograms. After the exposure of CC, bipolar stimulating electrodes with inter-electrode distance of 3mm placed on CC 2-3cm caudally from tip of the genu. For CC stimulation, a monphasic square wave with 0.2 msec pulse duration at supra-threshold intensity was delivered. TCRs were recorded from strip electrode over the unilateral frontal cortical surface. TCR was characterized by positive (P1) and negative (N1) component, and we measured the peak latency of P1. Subsequently, all patients underwent bisection of CC involving stimulated point according to the preoperative plan. The length of callosal fiber was measured on the coronal plane through the foramen Monro on preoperative MRI. Results: P1 latency was mean 31.2msec under 1 year old and rapid shortening of the latency was observed until 5 years old. Although the shortening rate of P1 latency decreased over 10 years old, it was 10.4msec in the second decade, 9.4msec in the third decade and 8.8msec in patients with over 30 years old. ITT is estimated as a twice of these value in each age. The mean length of callosal fiber was 40.8mm under one year old and 47.0mm at 4 to 6 years old, but the structural growth of callosal fiber was limited in elder patients in contrast with shortening rate of P1 latency. CV was calculated from P1 and a length of callosal fiber. Mean CV was 1.36 m/sec under one year old and became fast rapidly by 5 years old. In the third decade, CV was 5.42m/sec (mean) and gradual increase of CV was also observed on more aged patients. There was no significant relationship between sex, type of epilepsy and CV. Conclusions: Rapid shortening of ITT and CV of callosal fiber were observed before 3 years old simultaneously with the growth of callosal fiber. Furthermore, gradual physiological maturation continues to develop over the third decade suggest that these process may account for the increasing effectiveness in cognition and skills. These results are also important for understanding of bilateral synchronization mechanism in epileptic patients.