[Multiple cortical photolesions and penicillin epileptogenic focus. Study of a model in rabbits]

Inactivation of an epileptogenic focus by dividing it into sub-unities unable to maintain epileptic activity has been demonstrated in animals. Based on these experiments, multiple subpial transections have been performed in patients suffering from partial drug-resistant epilepsies when resection of the epileptogenic cortex was not possible. In order to develop a new surgical technique for such epilepsies, the authors present an initial study of multiple cortical laser photolesions on the rabbit brain. The aim of this study is to assess the histological lesions created on the cortex with a laser at a chronic stage, and to compare the electrical paroxysmal activity of a penicillin focus on the laser irradiated cortex and on the non radiated cortex in the same animal. Twenty-five adult albino rabbits were used for this study. Both hemispheres of 19 rabbits have been exposed; 14 to 35 photolesions in a network were performed on one hemisphere using a 1.064 microns wavelength Nd-YAG laser. Six to twenty-one days later, a penicillin-induced focus was created on both hemispheres, a corticographic study was performed, and each animal was sacrificed for histological study of the brain. For the 6 last animals a high-power 0.805 micron wavelength diode laser was used with the same protocol. Histological study was solely performed in order to compare the effects of both lasers. In 11 animals electrocorticographic control was reliable. Time to first spike occurrence after penicillin application was significantly increased on the treated hemisphere as compared to the untreated one (1' to 14'30" and 10" to 6', respectively; p < 0.01) and the number of spikes per minute at early and late counts was significantly smaller on the treated hemisphere as compared to the untreated one (1 to 29 and 2 to 70, respectively; p < 0.02). Histologically the lesions appeared as cone-shaped coagulation necrosis surrounded by an area of macrophagic reaction, angiogenesis and gliosis. With the diode laser, some lesions included ischemic changes extended in the white matter. This study demonstrated the possibility of creating limited and reproducible photolesions using the laser light energy, without extensive destruction of the cortex. These lesions were reaching from one third to the totality of the cortical depth, depending on laser exposure parameters. This study also demonstrated that these photolesions arranged in a network and at a chronic stage were associated with a significantly reduced paroxysmal activity of a penicillin focus when compared to intact cortex. Since such lesions did not totally isolate cortical sub-unities, spiking was still recorded from the irradiated cortex, but at a lower rate, suggesting a disorganization of the local neuronal network responsible for paroxysmal activity.