5.1 Introduction

Stereotactic procedures have been proposed with success in the treatment of many neurological and psychiatric disorders, starting from the 1950s. ¹ The bulk of these operations was carried out to selectively lesion a brain structure, aiming at symptoms improvement. Several techniques have been considered, such as injection of an oil–wax mixture, ² implantation of radio-active isotopes, ³ cooling, ⁴ and thermocoagulation. ⁵

The hypothesis that selective destruction of epileptogenic foci or critical nodes in the epileptogenic networks may result in control of seizures has been advanced over those years. ² , ³ , ⁴ , ⁵ The vast majority of the reported series used radiofrequency-thermocoagulations (RF-TCs), and in many of them intracerebral electrical recording was performed before coagulation. ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹²

Most of the series reported patients with a diagnosis of temporal lobe epilepsy (TLE) or “psychomotor seizures,” ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹³ while others reported patients with unspecified epileptic disorders. ¹⁰ , ¹¹ , ¹² Given that, mesial temporal lobe structures were the most employed target, unilateral or bilateral stereotactic amygdalotomy, including a unilateral hippocampectomy in selected cases, and lesioning of other limbic structures, were described in patients with epilepsy with or without behavioral disorders, with conflicting results regarding seizures. The rate of seizure-free or markedly improved patients ranged between 6 and 63%. ² , ⁴ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ This high interstudy variability is probably due to the different outcome assessment parameters used, the length of the postoperative follow-up, and the lack of controlled trials.

Possibly due to their not encouraging results, these kinds of procedures were abandoned for more than a decade. Due to the advent of modern imaging techniques, namely magnetic resonance imaging (MRI), allowing an easier and more accurate targeting, RF-TC stereotactic procedures gained new popularity. ¹⁸ , ¹⁹ Other stereotactic lesional experiences, such as radiosurgery ²⁰ and MR-guided laser ablation, ²¹ were reported, with promising results on seizure outcomes.

RF-TC of amygdala and hippocampus was supposed to be highly effective in the treatment of the mesial TLE secondary to hippocampal sclerosis, ²² with results similar to those of microsurgical selective amygdalo-hippocampectomy. ²³ It was claimed that RF-TC might be a valuable alternative to open microsurgery in selected patients with mesial TLE. ²⁴ Other types of epilepsy, such as those generated by hypothalamic hamartoma, ¹⁹ , ²⁵ focal cortical dysplasia, ²⁶ and gray matter heterotopia, ²⁷ could benefit from RF-TC procedures.

RF-TC through intracerebral electrodes implanted for stereoelectroencephalography (SEEG) was first reported in 2004. In a series of 41 patients, 1 patient became seizure free while a reduction of more than 50% of seizure frequency was observed in 19 patients, with a mean follow-up of 43 months. The best results were observed in those with malformation of cortical development (MCD). ²⁸ , ²⁹ The sites considered for thermocoagulation were those indicated by the SEEG monitoring as involved in the epileptogenic zone (EZ). Functional mapping by means of electrical stimulation and recording (evoked potentials) also allowed to avoid critical structures. The use of SEEG electrodes for coagulation eliminated the risk of further implantations. Moreover, the possibility to coagulate several structures allowed to extend the area to be lesioned, when required. An ictal low-voltage fast activity and the initiation of seizures after cortical stimulation were positive predictor factors for good seizure outcome, especially in MCD cases. ³⁰ Cossu et al obtained seizure freedom in four out of five patients with a diagnosis of nodular gray matter heterotopia using the same methodology. ³¹ A larger series of SEEG-guided RF-TC cases from the same center subsequently confirmed that patients with gray matter nodular heterotopia are ideal candidates for a successful procedure, with 66.7% of the patients rendered seizure free, with a mean follow-up of 33.5 months. ³² The same study revealed that a good outcome could be obtained in patients with lesions seen in the preoperative brain MR, including hippocampal sclerosis. MR-negative patients could also be considered for treatment.

RF-TC is an alternative option to open surgery in patients with nodular gray matter heterotopia, and in selected MCD and medial temporale lobe epilepsy cases. Patients who would not be eligible for resective surgery based on SEEG data can be treated by RF-TC as a palliative procedure, which could lead to a relevant improvement and, in some cases, seizure freedom.

5.2 Technical Details of RF-TC

Details about indications for SEEG evaluation and technique have been provided elsewhere. ³³ , ³⁴

A tailored SEEG approach is planned based on the presurgical noninvasive electroclinical and anatomical data. SEEG planning makes use of multimodal imaging techniques, which are coregistered in the same reference space. The accuracy of implantation is guaranteed by the use of a stereotactic robot (NeuroMate, Renishaw-Mayfield SA, Nyon, Switzerland).

RF-TCs are usually offered to the majority of patients undergoing a SEEG monitoring in our center. Patients not eligible for resective surgery are openly informed that RF-TC is the only treatment option available. RF-TCs are performed at the end of the recording period, before electrode removal. The procedure is generally well tolerated without anesthesia, enabling an adequate clinical monitoring. A different approach (deep sedation) is considered only in children. Each thermocoagulation lesion is placed between a pair of contiguous electrode contacts. The contacts used for RF-TC are selected according to the presence of one or more of the following criteria:

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  Involvement in the onset of ictal discharges.

  
  
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  Induction of habitual ictal clinical phenomena by electrical stimulation.

  
  
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  Intralesional location.

Whenever the selected contacts sample an eloquent area, as documented by SEEG functional mapping, they are excluded from treatment. Similarly, no coagulation is performed near vascular structures (<2 mm from the geometric center of the selected contacts). Therefore, the original plan of treatment might be incompletely performed.

The electrodes are connected to a radiofrequency lesion generator equipment (NeuroN50 and NeuroN100 Stryker Leibinger, Freiburg, Germany) modified to be used with SEEG electrodes like Dixie (Microdeep Intracerebral Electrodes-D08; Dixi Medical, Besançon, France) or Alcis electrodes (Depth Electrodes Range 2069; Alcis, Besançon, France). Since no local temperature monitoring is possible when employing SEEG electrodes, temperature feedback was excluded from the generator circuitry. Current power is progressively raised from 1.5 W up to 8.32 W within 60 seconds; current intensity (usually around 25 mA) is strictly dependent on impedance values. These parameters are defined to increase tissue temperature to 78 to 82°C, which had been previously reported to induce a lesion around the selected contacts within 40 to 50 seconds. ³⁵ Using these parameters, the lesion produced between two adjacent contacts is an ovoid with approximating 6 mm in the long axis and a maximal diameter of 3.5 mm.

The number of coagulations is tailored to the individual requirement. In our series, an average of 10.6 (SD: 7.2; range: 1–33) coagulations per patient were placed. ³² The radiofrequency current propagates between the two adjacent contacts generating an electric field. The oscillations of the tissue ions within the field enable heat generation, followed by necrosis which occurs at temperatures ranging from 50 to 100°C. ³⁶ At the end of the procedure, the electrodes and supportive screws are removed.

Patients are discharged within 1 to 2 days. Whenever possible, an MR study is obtained 1 to 6 months following the procedure.

Related posts:

15 Multiple Subpial Transections 6 Temporal Lobectomy and Amygdalohippocampectomy 16 Laser-Induced Thermal Therapy for Medically Refractory Epilepsy 13 Surgery of the Insula 10 Frontal Lobe Resection in Refractory Epilepsy 14 Treatment Strategies for Hypothalamic Hamartomas: Microsurgery versus Endoscopy

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