Stereotactic laser amygdalohippocampotomy (SLAH) uses laser interstitial thermal therapy guided by magnetic resonance thermography. This novel intervention can achieve seizure freedom while minimizing collateral damage compared to traditional open surgery, in patients with mesial temporal lobe epilepsy. An algorithm is presented to guide treatment decisions for initial and repeat procedures in patients with and without mesial temporal sclerosis. SLAH may improve access by medication-refractory patients to effective surgical treatments and thereby decrease medical complications, increase productivity, and minimize socioeconomic consequences in patients with chronic epilepsy.
Key points
- •
Stereotactic laser amygdalohippocampotomy (SLAH) is a minimally invasive approach to the treatment of medication-resistant mesial temporal lobe epilepsy that accomplishes ablation of the seizure focus with real-time magnetic resonance thermal mapping.
- •
Seizure-free rates in early series suggest that SLAH approaches the effectiveness of open resection for patients with mesial temporal sclerosis (MTS).
- •
SLAH avoids neurocognitive adverse effects of open resection on naming (dominant side) and object recognition (nondominant side).
- •
Although early data suggest more preserved memory function after SLAH, further research is required. Thus, patients with relatively preserved memory in the absence of MTS are offered SLAH only after careful considerations of risks, benefits, and alternative procedures, including neuromodulation.
- •
Secondary benefits of SLAH include decreased length of stay, elimination of intensive care unit stay, reduced procedure-related discomfort, and improved access to surgical treatment for patients less likely to consider an open resective procedure.
Video of laser ablation accompanies this article at www.neurosurgery.theclinics.com/
Introduction
Mesial temporal lobe epilepsy (MTLE) is the most common cause of medication-resistant epilepsy, and decades of targeted drug discovery in epilepsy have not decreased the percentage of patients with MTLE suffering disabling seizures. For these patients, resective surgical treatment, as shown in a recent meta-analysis, leads to approximately 75% of patients becoming seizure free. The adverse surgical effects of open resection, whether anterior temporal lobectomy (ATL) or selective amygdalohippocampectomy (SAH), may be well tolerated in most circumstances. In the absence of other options, it has been emphasized that the benefits of seizure freedom outweigh the adverse effects, resulting in overall improvement in quality of life. Nevertheless, both ATL and SAH have significant effects on neurocognitive function(s), some related to the mesial resection (eg, declarative memory impairment), but some more related to the surgical approach to the mesial structures, that is, anterior-lateral temporal resection, division, or retraction during ATL or SAH. Cognitive declines related to the approach (collateral damage) impair naming and verbal learning (dominant hemisphere) or object recognition and figural learning (nondominant hemisphere). These impairments can be impactful and permanent, although the seizures become a distant memory. The potential of alternative, minimally invasive stereotactic procedures to accomplish the therapeutic goal of preventing seizures in the absence of such collateral damage may therefore increase the quality of life in surgical MTLE patients compared with traditional open resective procedures.
Stereotactic approaches to the mesial temporal lobe hold the promise of target ablation in the absence of collateral damage by avoiding injury as a result of the approach to the mesial temporal structures ( Table 1 ). Several techniques have been explored over the last several decades.
| Technique | Pros | Cons |
|---|---|---|
| SRFA | Reduced collateral damage Immediate benefit Low cost | Temperature monitored only at tip Best results with string electrode not available in United States |
| Stereotactic radiosurgery (SRS) | Noninvasive | Delayed benefit after initial increase in seizures/risk of sudden unexpected death from epilepsy Potential radiation injury, dose limitations High cost |
| Laser interstitial thermal therapy (LITT) | Minimal collateral damage Immediate benefit Near real-time magnetic resonance thermography guides therapy and confirms ablation zone | High cost/disposables |
With respect to stereotactic radiofrequency ablation (SRFA):
- •
Parrent and Blume achieved a rate of 27% (4 of 15) seizure freedom in a subgroup of patients with many confluent lesions within the amygdala and hippocampus from a transtemporal approach, orthogonal to the hippocampal long axis, spanning 21.5 mm (mean) of the hippocampus.
- •
More recently, Liscak and colleagues working in the Czech Republic reported seizure-free rates of 78%, comparable with open resection, by using an occipital approach along the axis of the hippocampus and a string electrode that provides more extensive lesions radial to that axis. Longer (35 mm mean) and wider ablation zones, resulting from 16 to 38 lesions (mean 25), likely contributed to the increased success rate.
Noting the recently reported successes with SRFA, we explored the use of laser interstitial thermal therapy (LITT) for MTLE. During LITT, laser light is delivered fiber-optically into tissue (interstitial) via a stereotactic approach, where photonic energy causes local heating and thermocoagulation. Several critical advances have made modern LITT platforms into elegant and powerful neurosurgical tools ( Box 1 ). The most critical advance is the ability to use MRI thermometry to measure not only the temperature at the device tip (the only temperature monitored in radiofrequency [RF] ablation) but also the temperature of tissue any distance from the tip during heating, thus providing near real-time confirmation of the ablation zone relative to off-target structures.
- •
Optical fiber with diffusing tip (10 or 3 mm)
- •
Cooling cannula to control thermal spread within tissue and protect device tip
- •
984-nm diode laser causes rapid heating of water within tissue
- •
Magnetic resonance thermography reads temperature change in all voxels
- •
Six safety points to automatically shut off laser if temperature limits exceeded
- •
Irreversible damage zone estimate at each voxel based on Arrhenius equation
The mechanism of heating, that is, photonic, is different from RF ablation, but the effects of temperature on tissue are the same ( Fig. 1 ). However, with LITT, magnetic resonance (MR) thermography allows temperature monitoring beyond the laser tip, whereas in RF the thermocouple provides only temperature adjacent to the device tip. This technology allows protection of off-target tissue at risk in a way that is not possible with standard use of RF ablation. At present, 2 LITT devices are available in the United States that offer overlapping but distinctive features, only one of which (Visualase, Medtronic, Louisville, CO) we have used in our studies.
Introduction
Mesial temporal lobe epilepsy (MTLE) is the most common cause of medication-resistant epilepsy, and decades of targeted drug discovery in epilepsy have not decreased the percentage of patients with MTLE suffering disabling seizures. For these patients, resective surgical treatment, as shown in a recent meta-analysis, leads to approximately 75% of patients becoming seizure free. The adverse surgical effects of open resection, whether anterior temporal lobectomy (ATL) or selective amygdalohippocampectomy (SAH), may be well tolerated in most circumstances. In the absence of other options, it has been emphasized that the benefits of seizure freedom outweigh the adverse effects, resulting in overall improvement in quality of life. Nevertheless, both ATL and SAH have significant effects on neurocognitive function(s), some related to the mesial resection (eg, declarative memory impairment), but some more related to the surgical approach to the mesial structures, that is, anterior-lateral temporal resection, division, or retraction during ATL or SAH. Cognitive declines related to the approach (collateral damage) impair naming and verbal learning (dominant hemisphere) or object recognition and figural learning (nondominant hemisphere). These impairments can be impactful and permanent, although the seizures become a distant memory. The potential of alternative, minimally invasive stereotactic procedures to accomplish the therapeutic goal of preventing seizures in the absence of such collateral damage may therefore increase the quality of life in surgical MTLE patients compared with traditional open resective procedures.
Stereotactic approaches to the mesial temporal lobe hold the promise of target ablation in the absence of collateral damage by avoiding injury as a result of the approach to the mesial temporal structures ( Table 1 ). Several techniques have been explored over the last several decades.
| Technique | Pros | Cons |
|---|---|---|
| SRFA | Reduced collateral damage Immediate benefit Low cost | Temperature monitored only at tip Best results with string electrode not available in United States |
| Stereotactic radiosurgery (SRS) | Noninvasive | Delayed benefit after initial increase in seizures/risk of sudden unexpected death from epilepsy Potential radiation injury, dose limitations High cost |
| Laser interstitial thermal therapy (LITT) | Minimal collateral damage Immediate benefit Near real-time magnetic resonance thermography guides therapy and confirms ablation zone | High cost/disposables |
With respect to stereotactic radiofrequency ablation (SRFA):
- •
Parrent and Blume achieved a rate of 27% (4 of 15) seizure freedom in a subgroup of patients with many confluent lesions within the amygdala and hippocampus from a transtemporal approach, orthogonal to the hippocampal long axis, spanning 21.5 mm (mean) of the hippocampus.
- •
More recently, Liscak and colleagues working in the Czech Republic reported seizure-free rates of 78%, comparable with open resection, by using an occipital approach along the axis of the hippocampus and a string electrode that provides more extensive lesions radial to that axis. Longer (35 mm mean) and wider ablation zones, resulting from 16 to 38 lesions (mean 25), likely contributed to the increased success rate.
Noting the recently reported successes with SRFA, we explored the use of laser interstitial thermal therapy (LITT) for MTLE. During LITT, laser light is delivered fiber-optically into tissue (interstitial) via a stereotactic approach, where photonic energy causes local heating and thermocoagulation. Several critical advances have made modern LITT platforms into elegant and powerful neurosurgical tools ( Box 1 ). The most critical advance is the ability to use MRI thermometry to measure not only the temperature at the device tip (the only temperature monitored in radiofrequency [RF] ablation) but also the temperature of tissue any distance from the tip during heating, thus providing near real-time confirmation of the ablation zone relative to off-target structures.
- •
Optical fiber with diffusing tip (10 or 3 mm)
- •
Cooling cannula to control thermal spread within tissue and protect device tip
- •
984-nm diode laser causes rapid heating of water within tissue
- •
Magnetic resonance thermography reads temperature change in all voxels
- •
Six safety points to automatically shut off laser if temperature limits exceeded
- •
Irreversible damage zone estimate at each voxel based on Arrhenius equation
The mechanism of heating, that is, photonic, is different from RF ablation, but the effects of temperature on tissue are the same ( Fig. 1 ). However, with LITT, magnetic resonance (MR) thermography allows temperature monitoring beyond the laser tip, whereas in RF the thermocouple provides only temperature adjacent to the device tip. This technology allows protection of off-target tissue at risk in a way that is not possible with standard use of RF ablation. At present, 2 LITT devices are available in the United States that offer overlapping but distinctive features, only one of which (Visualase, Medtronic, Louisville, CO) we have used in our studies.
Patient evaluation overview
Patients undergoing LITT for thermocoagulation of the mesial temporal structures, which we termed stereotactic laser amygdalohippocampotomy (SLAH), undergo the standard workup of patients being considered for epilepsy surgery, and our criteria are similar to those for the recommendation of open resective surgery ( Box 2 ). In this article, that evaluation is reviewed as it pertains specifically to SLAH. Our decision tree is described later; extensive discussion of indications for mesial temporal lobe surgery is beyond the scope of this article.
- •
Semiology: dyscognitive seizures consistent with mesial temporal onset, ± aura (typically smell, epigastric sensation, fear, déjà vu)
- •
MRI: mesial temporal sclerosis positive (MTS+) or negative (MTS−)
- •
PET: temporal lobe hypometabolism lateralized or greater on the same side as electroencephalography
- •
Long-term video electroencephalographic monitoring: localization to anterior temporal region (eg, F7/T1 or F8/T2)
- •
Neuropsychological testing: the following refers to normally organized memory function
- ○
MTS+ or MTS−: domain-specific memory decline present on side of anticipated ablation
- ○
In absence of domain-specific memory decline referable to side of ablation:
- ▪
MTS+: acceptable. If memory loss is asymmetric to contralateral side, intracarotid amobarbital test (ie, Wada test) is considered
- ▪
MTS−:
- •
Absence of visuospatial memory decline for nondominant ablation acceptable for ablation
- •
Absence of verbal memory decline for dominant-side surgery: consider also nonablative surgical options (eg, responsive neurostimulation)
- •
- ▪
- ○
- •
Intracranial electroencephalography (iEEG): in setting of ambiguity as to seizure onset zone from noninvasive studies alone, onsets with iEEG referable to ipsilateral mesial temporal lobe and absence of contralateral onsets
History
A careful history of medication usage is sufficient to identify medication-resistant epilepsy, defined as “failure of adequate trials of two tolerated, appropriately chosen, and used antiepileptic drug schedules (whether as monotherapies or in combination) to achieve sustained seizure freedom.” A history of typical mesial temporal lobe seizure semiology (dyscognitive) with or without aura is key; semiology suggestive of other onset zone(s) (eg, nighttime motor seizures suggesting frontal onsets) warrants additional investigation.
MRI Studies
MRI is sufficient to classify the presence or absence of mesial temporal sclerosis (MTS) and also to detect/exclude confounding lesions such as focal cortical dysplasias, secondary cortical gliosis, or other lesions that may indicate dual disease, a situation not uncommon in the temporal pole region.
Metabolic and Blood Flow Imaging
Metabolic imaging (ie, fluorodeoxyglucose-PET [FDG-PET]) is not mandatory in the setting of unilateral MTS and concordant long-term video electroencephalographic [EEG] monitoring (LTVM; see later discussion). It is corroborative when bilateral MTS is present and should be concordant with LTVM; discrepancy should prompt consideration of intracranial EEG (iEEG) monitoring. We require FDG-PET unilateral hypometabolism in non-MTS cases, which, when concordant with LTVM, may allow proceeding directly to SLAH without iEEG monitoring in nondominant cases. However, we do maintain a low threshold for iEEG in non-MTS cases. We do not routinely require subtractive ictal/interictal single-photon emission computed tomography (CT), receptor imaging, or magnetoencephalography, except in diagnostically challenging cases.
Electrographic Studies
LTVM with scalp electrodes is often sufficient to recognize the seizure onset zone in the anteromesial temporal lobe and allows for qualitative and quantitative analysis of interictal epileptiform discharges. In situations in which the seizure onset is obscured or when questions of false lateralization or bilateral onsets arise, iEEG monitoring is used. We typically use depth electrodes inserted orthogonally to provide lateral and mesial temporal coverage. Additional depth electrodes may be required to investigate other regions of interest. In certain cases, percutaneous foramen ovale electrodes may be sufficient to clarify onset laterality but should not be used alone when there is a question of lateral versus mesial or temporal versus extratemporal onsets. We insert depth electrodes through anchor bolts using standard stereotactic techniques (eg, stereotactic head frame or robotic articulating arm) to minimize the skin opening. In anticipation of the possibility of a minimally invasive therapeutic procedure, we make every attempt to similarly provide a minimally invasive diagnostic procedure (ie, minimizing the use of subdural strip and grid electrodes when possible).
Neuropsychometric Evaluation
All patients undergo extensive preoperative evaluation by specialized neuropsychologists. These tests are used both to contribute to confirmation of the onset zone as reflected in relative areas of neurocognitive weakness (eg, material specific memory dysfunction) and also to prognosticate regarding potential loss of function after various surgical approaches, including SLAH. Results with respect to cognitive domains at risk also contribute to procedural decision making, that is, destructive versus nondestructive (neuromodulatory) therapeutic procedures. Further, change from baseline after surgery is informative both academically and clinically and guides referral for cognitive rehabilitation when indicated. Intracarotid amobarbital testing or the Wada test is performed on a case-by-case basis, as described later.
Pharmacologic treatment options
Patients who are candidates for surgical procedures in epilepsy must be determined to be medication resistant, that is, they have tried at least 2 appropriately chosen drugs, appropriately used, and have not become seizure free. After 2 failed antiepileptic drug (AED) trials, Brodie and colleagues found that a third trial achieved only 8% and a fourth trial 4% long-term seizure freedom. However, this finding was recently re-evaluated in a larger cohort, in which 18.5% and 16.5% were made seizure free with a third and fourth AED, respectively, and decreased to 0% after 5 or 6 previous AED failures. This re-evaluation of seizure intractability warrants consideration in terms of surgical options. In the setting of MTS in which the seizure-free rate is high and the risk low after both ablative and resective surgery, surgery may be considered earlier, that is, after 2 drug failures. Conversely, when the chance of seizure freedom after surgery is possibly lower, such as in the absence of MTS, and risk is correspondingly higher in the setting of preserved neurocognitive functions, surgical treatment with SLAH or resection might be delayed until after a more exhaustive regimen of AEDs or neuromodulation (discussed later) have been attempted.
Related posts:
Hippocampal Transections for Epilepsy
Minimally Invasive Neurosurgery for Epilepsy Using Stereotactic MRI Guidance
Stereotactic Laser Ablation for Hypothalamic Hamartoma
Stereo-Encephalography Versus Subdural Electrodes for Seizure Localization
Responsive Direct Brain Stimulation for Epilepsy
Neuromodulation for Epilepsy
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
