Abstracts

Rationale: Patients with unilateral hippocampal sclerosis (HS) represent the most suitable candidates for epilepsy surgery; however, the risk of cognitive decline and the invasiveness of an open craniotomy is a limiting concern - particularly with HS of the dominant-hemisphere. Minimally invasive stereotactic laser ablation (SLA) using a 980nm diode laser may enable precise ablation of seizure foci with sparing of eloquent neocortical structures. Methods: Patients were candidates for SLA if EEG revealed seizures originating in the temporal lobe while MRI showed concordant mesial temporal sclerosis. Preoperative evlauation also included neuropsychological evaluation. Frame-based navigation was used to introduce a 1.6mm diameter MR-compatible laser applicator housing a 1cm long diffusing tip optical fiber into the amygdalohippocampal complex (AHC) from a temporo-occipital trajectory. The near-infrared laser ablation system (Visualase, Inc, Houston, TX) used consists of a 15W 980nm diode laser, a light-diffusing tip, and magnetic-resonance thermal imaging (MRTI) software. This laser produces a cylindrical to ellipsoid light distribution in the tissue along the axis of the diffusing element. AHC ablation was performed with 2 exposures of 10-12W for 90-130sec. MRTI was performed using proton resonance frequency (PRF) phase difference imaging techniques using a fast field echo (FFE) sequence. Temperature was monitored and safety limits (>50degC) were placed near critical structures such as the optic tract, cerebral arteries, and hypothalamus. Temperatures at these pre-designated locations were set to terminate laser delivery if heating was excessive. Post-ablation T1- weighted gadolinium series confirmed areas of thermal ablation. Results: Five patients at 3 institutions (2 females and 3 males), ages 16-65, presented with 2-20 seizures/month. Preoperative MRI demonstrated HS alone in 4 patients, and HS in addition to a second lesion in 1 patient. Trajectories used were appropriate for controlled thermal ablation of desired structures and volumes as confirmed by MRI. Post-ablation T1-weighted gadolinium series demonstrated a mean ablation volume of 3.65+/-0.93cc (2.18-4.91cc). Average length of stay was 1 day. Asymptomatic delayed hemorrhage within the ablation site was observed in 1 patient. There were no other surgical related complications. After follow-up of 3-13 months, all patients were improved with regard to seizure control with some experiencing seizure remission. Poorer outcomes were associated with smaller volumes of ablation, the presence of another radiographic lesion or seizure focus, or abrupt discontinuation of AEDs. Conclusions: SLA has promise as a treatment for refractory epilepsy secondary to HS. Seizures appear to respond to this minimally invasive therapy; and the procedure is associated with much less discomfort and a shorter hospital stay than conventional resective surgery. A larger study appears warranted by these preliminary results.