24.1 Overview and Technique

With the introduction of new technology, the surgical treatment of epilepsy continues to evolve. Laser interstitial thermal therapy (LITT) guided by magnetic resonance imaging (MRI) is a minimally invasive treatment for stereotactic ablation of epileptogenic tissue or focal lesions. It has the advantages of producing less tissue damage, reduced perioperative pain and a shorter hospital stay when compared to open surgery. It has a high level of precision, and provides for real-time imaging feedback.

In LITT, laser energy is delivered using a long, flexible optical fiber stereotactically placed into the parenchymal region or lesion intended for ablation. Several software solutions already available to functional neurosurgeons for planning stereotactic procedures may also be used to plan targets and trajectories for LITT. A stab incision in the skin and small burr hole are required at the entry site, and an anchor bolt is used to fix the laser probe. Precise probe placement is essential and can be achieved with a traditional stereotactic frame, a patient-specific 3D-printed platform and an adjustable navigated micropositioner system, or using frameless stereotaxy with neuronavigation. MRI-guided surgical platforms are also available with which a probe is inserted while the patient is inside the MRI scanner. This gives near immediate confirmation of placement accuracy.

Once MRI verifies probe position, laser energy creates thermal coagulation of tissue at the probe tip. Serial ablations may be done after partial withdrawal of the laser probe. During the ablation, MRI thermography imaging using the water proton resonance frequency shift is done to monitor tissue temperature. ¹ A recent FDA letter sent to physicians however has warned that magnetic resonance thermometry may be inaccurate in certain conditions and lead to overheating. ² A cooling cannula controls thermal spread within tissue and protects the probe tip from damage caused by overheating. If a pre selected temperature threshold is reached adjacent to critical structures, ablation can be aborted. ³ The FDA recommends setting the low temperature targets on nearby critical structures to 43 degrees Celsius or less. After probe removal, the incision can be closed with a single stitch. Hospital discharge the next morning is feasible.

24.2 Treatment of Mesial Temporal Lobe Epilepsy

Thermal ablation of the amygdalohippocampal complex in mesial temporal lobe epilepsy (MTLE) is the most common use of LITT for epilepsy treatment. The hippocampus is approached from an occipital or posterolateral trajectory. By reaching it along its longitudinal axis a large portion of the structure can be engaged while the medial basal cisterns and the inferior horn of the lateral ventricle lateral to the target create a “heat sink” to avert thermal injury to nearby structures and vessels. ⁴ Example MRI images from a LITT case for MTLE are depicted in ▶ Fig. 24.1.

In one early series, Willie and others described 13 adult patients who underwent LITT for MTLE with or without mesial temporal sclerosis. ⁶ The investigators observed a 60% mean ablation volume for the amygdalohippocampal complex with a median length of stay of one day. After a median of 14 months follow-up, 54% of patients were seizure free, including 67% (6 of 9) of individuals with mesial temporal sclerosis. All seizure recurrences occurred within the first six months. Neither the ablation volume nor length correlated with the clinical outcome. There was one adverse event. An aberrant insertion of a stereotactic aligning rod caused a visual defect. The probe trajectory was corrected before ablation. A follow-up investigation using a prospective, non-randomized, parallel-group designed protocol compared neuropsychological outcome in 19 patients undergoing LITT for MTLE with 39 patients who underwent standard resection. ⁷ Naming in patients with dominant hemisphere MTLE with LITT was improved compared to open surgical resection. There was better object recognition in individuals with MTLE of the non-dominant hemisphere. Overall, no patient had a decline in object recognition and naming tasks after LITT.

Another group evaluated LITT outcomes in 20 MTLE patients and measured ablation volumes. ⁸ After six months 53% of 15 patients were seizure free; 36% of 11 patients were free after one year, and 60% of 5 individuals were free after two years (median follow-up was 13 months). No differences were found in the ablated volumes of the hippocampus, amygdala, parahippocampal gyrus, entorhinal cortex, or fusiform gyrus in patients with favorable versus unfavorable seizure outcomes. Contextual verbal memory performance was preserved, although declines in non-contextual memory task scores occurred, and no major complications were observed. Finally, a recent review of all published LITT series for MTLE estimated an overall seizure free rate of 53% (follow-up range 6 to 39 months). Morbidity was reported in 16% of 74 procedures on 68 patients. ⁹ Adverse events included visual field deficits (6), cranial nerve III or IV injury (2), and intracranial hemorrhage (3). There were no deaths.

Overall, published results to date suggest favorable seizure outcomes after LITT for MTLE, although seizure free rates remain below those published for open resection with either anterior temporal lobectomy or selective amygdalohippocampectomy. ¹⁰ Advantages of LITT over resection for MTLE are: the minimally invasive nature of the procedure, and potentially improved neuropsychological outcomes, although the latter will require further study in larger cohorts.