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Laser Interstitial Thermal Therapy for Epilepsy
Introduction
Laser interstitial thermal therapy (LITT) is a minimally invasive percutaneous procedure that involves the stereotactic insertion of a fiberoptic catheter and delivery of non-ionized photons into a predetermined intracranial location to thermally ablate specific anatomical structures or lesions. Laser technology for intracranial ablation has been available since 1960, with the use of ruby and CO 2 lasers for the treatment of malignant gliomas. , This technology was cumbersome, given the bulky nature of the systems, and was not practical for the ablation of deep brain structures. LITT was first described by Bown in 1983 and used in a clinical setting by Sugiyama et al. in 1990, with the development of a thin fiberoptic catheter that allowed for an intracranial insertion to reach deep brain structures, becoming more practical than previous attempts. Despite these improvements, the lack of real-time monitoring and accurate control over the ablation volume limited the acceptance of this technique. The development of magnetic resonance thermography (MRT) allows for continuous monitoring of thermal damage in near real time. LITT guided by MRT is known as magnetic resonance guided LITT (MRgLITT) and has been gaining acceptance among surgeons, given its accuracy and near real-time monitoring. MRgLITT is currently used as a minimally invasive option in the treatment of gliomas, brain metastasis, radiation necrosis, deep-seated high-grade gliomas (HHG), and epilepsy. Although surgical resection of an intracranial lesion is still generally considered the first line of treatment, there are scenarios in which MRgLITT can be beneficial. Patients with low preoperative functional scores, medical comorbidities with a high risk of peri- and postoperative complications, deep-seated lesions, and patients with medically refractory epilepsy that are hesitant to pursue a craniotomy for open resection of the epileptogenic tissue often benefit from MRgLITT, with less associated risk than an open resection. , The need for a minimally invasive approach for epilepsy has arisen from the underutilization of surgery due to multiple factors, one of which is the perceived complication rates and morbidity associated with open surgical resection. There are approximately 300,000 patients with medically refractory epilepsy in the United States; however, only 7000 procedures for the surgical treatment of epilepsy are performed annually and less than 10% of potentially eligible patients currently receive an intervention. The role of minimally invasive approaches is to limit the morbidity and recovery associated with open resections, with the goal of offering treatment to additional cohorts of eligible patients. This chapter explores the technical aspects and the science behind MRgLITT technology, reviews the most recent data for cases and scenarios in which LITT is indicated as a treatment for epilepsy, and provides a detailed technical description of the surgical procedure.
The Science of Laser Interstitial Thermal Therapy
LASER (light amplification by stimulated emission of radiation) is the main component of this technique, in which photons are emitted from a fiberoptic probe and are absorbed by tissue molecules called chromophores . , Photon energy is absorbed by chromophores in the tissue, resulting in a release of thermal energy causing heating of the target area. , Temperatures between 46°C and 60°C will cause irreversible enzyme induction, DNA and protein denaturation, membrane dissolution, vessel sclerosis, and coagulative necrosis, leading to cell death. , Temperatures above 60°C cause instantaneous coagulation necrosis. , Absorption coefficients vary between tissue types and composition; the higher the absorption coefficient of the target zone, the faster the tissue temperature rises. The dissipation of heat in the targeted regions is affected by the close proximity of blood vessels and cerebrospinal fluid (CSF) that function as heat sinks. In most cases, pathological tissue has a greater absorption coefficient that translates to faster denaturation/ablation than healthy parenchyma, favoring surrounding healthy tissue preservation. Temperature limits are often set, with an automatic shut-off mechanism as high as 90°C at the tip of the catheter (enough to perform an appropriate ablation but also low enough to avoid carbonization/vaporization) and to 50°C at the periphery of the target zone, to avoid damage to the adjacent parenchyma. , This mechanism is utilized to optimize the lesioning near the tip of the laser fiber while preserving the integrity of the surrounding structures. , , The most commonly used and commercially available LITT systems are the Visualase (Medtronic, US, Minneapolis, MN), which uses a 15W, 980-nm diode laser, and the NeuroBlate (Monteris Medical, Plymouth, MN), which uses a 12W, 1064-nm Nd:YAG laser. The specifications of and differences between these two systems are summarized in Table 98.1 . The NeuroBlate probe comes with a tip that can be rotated, retracted, and advanced with a calibrated driver, which can be particularly helpful for targets that have an irregular geometry. A potential advantage of the Visualase is its capability to generate a uniform ellipsoid field of heating around the tip of the probe that produces a constant area of coverage ideal for lesions with a homogenous shape. MRT is utilized for a continuous monitoring of the area of ablation. Temperature data is incorporated into a mathematical model (Arrhenius model) that provides an estimation of the ablation/necrosis zone in near real time (with imaging updated every 3 to 9 seconds, depending upon the number of planes that are being monitored) by displaying a colored zone of heating overlaid on the brain MRT image. The usage of MRT in LITT procedures is revolutionizing the acceptance of this technique, due to the capability to monitor the ablation in near real time.
Table 98.1
Visualase and NeuroBlate Characteristics
Adapted from Lagman C, Chung LK, Pelargos PE, et al. Laser neurosurgery: a systematic analysis of magnetic resonance-guided laser interstitial thermal therapies. J Clin Neurosci 2017;36:20–26.
| Component | Visualase | NeuroBlate |
|---|---|---|
| Patient interface platform | ||
| Laser probe | ||
| Composition | Silica within polymer sheath | Silica within Sapphire capsule |
| Diameter (mm) | 1.65 | 2.2 and 3.3 |
| Direction | Diffusing | Diffusing or directional |
| Output (W) | 15 CW | Up to 12, pulsed |
| Wavelength | 980 (diode) | 1064 (diose) |
| Cooling mechanism | Saline | CO 2 with temperature feedback control |
| Probe driver | N/A | Advanced (APD) and Robotic (RPD) |
| Frame | Frame-based or frameless | Frame-based or frameless |
| MRI-compatible | Yes | Yes |
| Physician workstation | ||
| Software | Medtronic | M a Vision Pro |
| Shut-off mechanism | Automatic a and manual | Automatic b and manual |
a Automatic shut-off in the Visualase System engages if a surgeon selected temperature limit is reached near a critical structure.
b Automatic shut-off in the NeuroBlate System engages if patient movement in the MRI is detected.
Surgical Procedure
Preoperative MRT imaging is obtained prior to the procedure for anatomy registration, trajectory planning, and intraoperative guidance. Trajectory assessment includes several variables, including skull thickness, dural anatomy, blood vessels, parenchyma, sulci, and ventricles. Implantation techniques vary between institutions, available equipment, and surgeon preference, including stereotactic frames, frameless systems, and robotic-assisted guidance that can provide an accurate intraoperative execution of the preplanned trajectory. In this chapter, we describe ( Fig. 98.1 ) the placement of the catheter using the ClearPoint System (a video description of the procedure is available in the online version of this chapter). MRgLITT procedure can be performed under light sedation or general anesthesia, depending on the surgeon’s preference after assessing for positioning, comfort, and procedure time length, always prioritizing safety and the patient’s comfort (see Fig. 98.1.A ) . All medications and instrumentation must be MRI compatible. Chemical paralysis is recommended in general anesthesia to avoid unexpected movements during the procedure. Special care during positioning should be taken to avoid the restriction of venous outflow. After the scalp is prepped and draped in the usual fashion, the alignment grid is placed at the region of interest (see Fig. 98.1B ) . The intraoperative MR machine is then advanced to the operating table to obtain a trajectory planning scan. A target and the entry site are chosen, and the trajectory is determined. The MR scanner is removed, and the ClearPoint base and tower are secured to the outer table of the skull, centered on the chosen entry site (see Fig. 98.1C and D). The MR scanner is again returned to the operative field for an alignment scan. The ClearPoint system is aligned along the chosen trajectory to the target, and the MR scanner removed. After infiltration with a local anesthetic, a stab incision is made and (see Fig. 98.1E ) a twist drill is made for skull penetration. The dura is coagulated with a stereotactic coagulator, and then a ceramic stylet is inserted and advanced to the target site. A new MR scan is obtained to confirm appropriate localization of the stylet to the target site. After accurate positioning is confirmed, the stylet is removed and the laser fiber is inserted into the target site. Correct positioning of the fiber is verified via MR scan (see Fig. 98.1F ) and the fiber connected to the computer system. LITT temperature limits are programmed into the monitoring system, and an initial low dose of laser energy is administered to verify the proper functioning of the system. After adequate functioning is confirmed, target ablation is carried out with continuous monitoring of the ablation volume and tissue temperatures that are visualized as a colored overlay on the MR image. After the ablation is completed, MR imaging (post-contrast T1, T2, T2-FLAIR, and diffusion images) is acquired to verify the ablation zone. The MR machine is then removed, and the skin incision is closed.
Temporal Lobe Epilepsy
Temporal lobe epilepsy (TLE) is the most common form of medically refractory epilepsy in adults, with mesial temporal lobe sclerosis (MTS; Fig. 98.2 ) being the leading cause in mesial temporal lobe epilepsy (MTLE). Surgical treatment with anterior temporal lobectomy (ATL) and selective amygdalohippocampectomy (SAH) has been demonstrated as the most effective type of treatment, with Engle I seizure-free outcomes of 55% to 70% at 1 year and 40% at 10 years for all ATL patients, and 65% to 67% and 50% at one and 10 years, respectively, for patients with MTS. , , , Although ATL surgery is considered the current gold standard for the treatment of refractory MTLE, it is also associated with variable postoperative cognitive decline, including deficits in verbal and nonverbal memory, word finding, recognition, and naming, depending on language dominance. Visual field defects (VFDs) are also commonly found after open temporal resection. A retrospective study from Schmeiser et al. reported an incidence of VFDs of 83% after ATL, 74% after selective transsylvian amygdalohippocampectomy, and 56% after subtemporal selective amygdalohippocampectomy. These reported complications, in addition to a patient’s perception of the invasiveness of an open craniotomy, have necessitated a need for less invasive procedures like MRgLITT. MR-guided selective laser amygdalohippocampectomy (SLAH) is indicated in cases of isolated mesial pathologies without further evidence of temporal alterations. , An occipital insertion of the LITT catheter is performed as described in the surgical procedure section, following an extraventricular trajectory along the axis of the hippocampus (see Fig. 98.1G ). Ablation of the hippocampus and amygdala are performed using as many ablation targets as necessary to achieve an appropriate ablation volume. At this time, there is no class I data comparing the Engle seizure outcomes and/or complication rates between ATL and SLAH. Available literature from case series studies (class III data) suggest an efficacy rate almost comparable with ATL, along with surgical complication rates for SLAH that are less than ATL ( Table 98.2 ). , Volumetric analyses of patients with persistent seizures following SLAH suggest an association of mesial hippocampal head sparing with persistent disabling seizures. SLAH can be repeated for patients who fail to obtain an Engel Class I outcome.
Table 98.2
Selective Laser Ablation Case Series Literature Review
Adapted from Petito GT, Wharen RE, Feyissa AM, et al. The impact of stereotactic laser ablation at a typical epilepsy center. Epilepsy Behav . 2018;78:37–44.
| Author | Number of Patients | Year | Design | Location in Brain of Surgery | Etiology | Follow-Up | Outcome and Complications |
|---|---|---|---|---|---|---|---|
| Cuny DJ et al. | 5 pediatrics | 2012 | Retrospective | Multiple sites | Cingulate tuber/TSC, MTS/MTLE, hypothalamic hamartoma ( n = 2), focal cortical dysplasia | 2–13 months | All seizure-free (1 patient SF after resuming ASDs at 12 months) |
| Wilfong A et al | 14 pediatrics | 201 3 | Retrospective | Hypothalamus | Hypothalamic hamartoma | 1–24 months | 79% SF initially (after 1 patient underwent repeat SLA,86% SF) |
| Willie JT et al. | 13 adults | 2014 | Retrospective | Lesional and nonlesional mTLE | MTS, mesial temporal atrophy, mesial temporal signal change, normal | 5–26 months | 8/13 patients SF |
| Esquenazi Y et al. | 2 adults | 2014 | Retrospective case | Periventricular white | Left frontal PNH, bilateral temporal PVNH | 1–16 months | Patient 1: seizures reoccurred after 1 month. Patient 2: underwent TL after failed SLA and became SF |
| Drane DL et al. | 39 resected vs. 19 SLA adults | 2014 | Retrospective compared | Temporal lobe | Amygdala and hippocampus (SLA) vs. transcortical amygdalohippocampectomy | SLAH, 6 months Resection, 1 year |
57.9% SLAH SF; stable or slight improvement in cognitive |
| Lewis Evan Cole et al. | 17 pediatrics | 2015 | Retrospective chart | Temporal and extratemporal | Mixed | 3.5–35.9 months | 7 patients SF, 1 Class II; 3 Class III, 6 Class IV, Complications: inaccurate fiber placement (2); laser cooling mechanism failure (1); post-ablation edema (1), left quadrant |
| Waseem H et al. | 7 AMTL resection 7 SLA Adults | 2015 | Prospective | Anterior mesial temporal lobe (all) | TL resection: 5/7 MTS 2/7 normal SLA 5/7 MTS 2/7 normal | 9–1 2 months | TL resection: 7/7 Class I Complications: 1 aseptic meningitis-resolved SLA: 4/7 Class I; 1/7l Class II 2/7 inadequate follow-up time (t < 12 months) Complications: 2/7 partial visual field deficit 1/7 Postop seizures resulting in repeat hospitalization 1/7 extended stay (N1 day) due to headaches |
| Kang JY et al. | 20 adults (2 pediatrics) | 2016 | Prospective review | Mesial temporal lobe | 2/7 normal SLA | 6 months | 11 patients SF; 2 Class II; 7 Class IV, 4 patients (Class IV) underwent subsequent lobectomy |
| Luedke MW et al. | 2 adults | 2016 | Retrospective case report of 2 patients Pre and postop ECoGby |
Left mesial temporal lobe | Patient 1, subtle increased T2 signal in the left mesial temporal lobe. Patient 2, left MTS | Patient 1, 1 year Patient 2, 1 year | Patient 1, Class II Patient 2, Class II Reduction in epileptiform discharges in the intra-ablation contacts compared with extra-ablation contacts |
| Rolston JD et al. | 2 adults | 2016 | Retrospective, case | Hypothalamic hamartoma | Hypothalamic hamartoma | Patient 1, most recent follow-up: 5 months | Patient 1, SF at most recent follow-up; had transient hyperphagia and amnesia after the ablation, resolved by |
| Dredla BK et al. | 2 adults | 2016 | Retrospective, case | Patients 1 and 2, left amygdala and hippocampus | Patients 1 and 2 MRI+/PET− Left TLE (unknown cause) Localized with iEEG | Patient 1–2 years Patient 2–4 months | Patient 1, SF Decline in verbal memory and semantic verbal fluency Patient 2, SF except 3 seizures in first month due to low ASD levels but remains SF since Decline in verbal and visual memory and semantic verbal fluency |
| Jeimakowicz WJ et al. | 23 adults | 201 7 | Prospective | Mesial temporal lobe | 1 8/2 3 MTS1 patient, MTS and atrial heterotopia, 2 ablations | 22.4 ± 7 months | 1 5/23 SF 4/23 Engel Class II 2/23 Engel Class III 2/23 Engel Class IV Complications: headache, nausea, unsteady gait (resolved), left homonymous hemianopia (partially resolved) |
| Gross RE et al. | 58 | 201 8 | Retrospective | Mesial temporal lobe | 43/58 MTS 3/58 signal hippocampus increase on T2 and/or FLAIR |
12 | Engel I all, 31/58 (53.4%) Non-MTS, 5/15 (33.3%) MTS, 26/43 (60.5%) Complications: 3 VFD (2 superior quadrantanopia and 1 homonymous hemianopsia after repeat SLAH, 1 acute SDH and 1 acute IPH with no neurodeficits, 4 transient partial CNP (1 third nerve and 3 fourth nerve) |
AMTL , Anterior mesial temporal lobe; ATL , anterior temporal lobectomy; ECoG , electrocorticography; MTLE , mesial temporal lobe epilepsy; MTS , mesial temporal lobe sclerosis; PVNH , periventricular nodular heterotopia; SLAH , selective laser amygdalohippocampectomy; TLE , temporal lobe epilepsy; TSC , tuberous sclerosis complex; VFD , visual field defects.
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