Stereotactic Laser Ablation for Medically Intractable Epilepsy: The Next Generation of Minimally Invasive Epilepsy Surgery

Abstract

Epilepsy is a common, disabling illness that is refractory to medical treatment in approximately one-third of patients, particularly among those with mesial temporal lobe epilepsy. While standard open mesial temporal resection is effective, achieving seizure freedom in most patients, efforts to develop safer, minimally invasive techniques have been underway for over half a century. Stereotactic ablative techniques, in particular, radiofrequency (RF) ablation, were first developed in the 1960s, with refinements in the 1990s with the advent of modern computed tomography and magnetic resonance-based imaging. In the past 5 years, the most recent techniques have used MRI-guided laser interstitial thermotherapy (LITT), the development of which began in the 1980s, saw refinements in MRI thermal imaging through the 1990s, and was initially used primarily for the treatment of intracranial and extracranial tumors. The present review describes the original stereotactic ablation trials, followed by modern imaging-guided RF ablation series for mesial temporal lobe epilepsy. The developments of LITT and MRI thermometry are then discussed. Finally, the two currently available MRI-guided LITT systems are reviewed for their role in the treatment of mesial temporal lobe and other medically refractory epilepsies.

Keywords: epilepsy; laser ablation; magnetic resonance thermal imaging; minimally invasive; stereotactic.

Figure 1

Visualase Cooled Laser Applicator System (VCLAS). (A) The cooled laser applicator. The ports are for inflow and outflow of saline coolant. Inset: titanium anchor for insertion. (B) VCLAS with 10 mm (top) and 3 mm (bottom) diffuser tips and simulated ablation zones. The VCLAS is a two-channel cannula: the optical fiber with diffuser tip is passed through the central channel, whereas the coolant circulates through the outer channel, as depicted in (C). Images provided by Medtronic, Louisville, CO, USA.

Figure 2

Visualase workstation. (A) Entire workstation is shown, consisting of the computer and two monitors (at top), the 15 W, 980 nm diode laser (bottom right) and the peristaltic pump for the saline coolant (bottom left). (B) The top monitor screen is shown and magnified in (C), with the axial image plane being visualized. The screen can be quickly toggled between two or three chosen image planes, as desired; moreover, one plane can be visualized on the top monitor and concurrently a different one on the bottom monitor. The left side of the top monitor is a sliding window that can depict the four types of information being acquired all at once: phase image (arrow), i.e., “thermal map”; magnitude image (^#) – a rapidly acquired but low-resolution anatomical image from the same sequence (fast spoiled gradient echo, SPGRE) that produces the phase image; irreversible damage zone estimate (arrowhead), calculated from the Arrhenius equation; and high-resolution anatomical image (*) in same plane as the phase image, acquired prior to the phase mapping. Also shown in sliding window are temperatures of the user-defined safety points. Images provided by Medtronic, Louisville, CO, USA.

Figure 3

Visualase ablation. Left, target region in the hippocampus, with Visualase cooling catheter laser applicator in place. Center, irreversible damage zone estimate (red) during the laser ablation. Right, post-ablation contrast-enhanced T1 image depicting the actual damage zone surrounded by region of blood–brain barrier breakdown (contrast enhancement).

Figure 4

NeuroBlate catheter. (A) The AXiiiS stereotactic miniframe is attached to the skull with spikes and bone screws. (B) The NeuroBlate 12 W, 1064 nm Nd:YAG laser delivery probe has a depth stop adjustment with a locking interface. (C) The SideFire fiber emits laser energy through a clear sapphire lens at a 78° angle. CO₂ gas cools the tip and the thermocouple transmits temperature data to the M*Vision software. FullFire diffusing, rather than directional, tip available (not shown). ©Monteris Medical. Used by permission. The use of any Monteris Medical photo or image does not imply Monteris’ review or endorsement of any article or publication.

Figure 5

NeuroBlate software. The M*Vision software allows the target volume to be contoured slice-by-slice, thermal dose threshold lines are calculated, and the laser probe trajectory can be planned. After planning, the software executes the treatment allowing the user to monitor real-time thermal imaging. Laser adjustments can be made during treatment. Once the ablation is complete, MR images are obtained and compared with thermal dose thresholds. ©Monteris Medical. Used by permission.