Modeling of Brain Tissue Heating Caused by Direct Cortical Stimulation for Assessing the Risk of Thermal Damage

Impact of intraoperative direct cortical stimulation dynamics on perioperative seizures and afterdischarge frequency in patients undergoing awake craniotomy

OBJECTIVE

Intraoperative stimulation is used as a crucial adjunct in neurosurgical oncology, allowing for greater extent of resection while minimizing morbidity. However, limited data exist regarding the impact of cortical stimulation on the frequency of perioperative seizures in these patients.

METHODS

A retrospective chart review of patients undergoing awake craniotomy with electrocorticography data by a single surgeon at the authors' institution between 2013 and 2020 was conducted. Eighty-three patients were identified, and electrocorticography, stimulation, and afterdischarge (AD)/seizure data were collected and analyzed. Stimulation characteristics (number, amplitude, density [stimulations per minute], composite score [amplitude × density], total and average stimulation duration, and number of positive stimulation sites) were analyzed for association with intraoperative seizures (ISs), ADs, and postoperative clinical seizures.

RESULTS

Total stimulation duration (p = 0.005), average stimulation duration (p = 0.010), and number of stimulations (p = 0.020) were found to significantly impact AD incidence. A total stimulation duration of more than 145 seconds (p = 0.04) and more than 60 total stimulations (p = 0.03) resulted in significantly higher rates of ADs. The total number of positive stimulation sites was associated with increased IS (p = 0.048). Lesions located within the insula (p = 0.027) were associated with increased incidence of ADs. Patients undergoing repeat awake craniotomy were more likely to experience IS (p = 0.013). Preoperative antiepileptic drug use, seizure history, and number of prior resections of any type showed no impact on the outcomes considered. The charge transferred to the cortex per second during mapping was significantly higher in the 10 seconds leading to AD than at any other time point examined in patients experiencing ADs, and was significantly higher than any time point in patients not experiencing ADs or ISs. Although the rate of transfer for patients experiencing ISs was highest in the 10 seconds prior to the seizure, it was not significantly different from those who did not experience an AD or IS.

CONCLUSIONS

The data suggest that intraoperative cortical stimulation is a safe and effective technique in maximizing extent of resection while minimizing neurological morbidity in patients undergoing awake craniotomies, and that surgeons may avoid ADs and ISs by minimizing duration and total number of stimulations and by decreasing the overall charge transferred to the cortex during mapping procedures.

Numerical analysis of transcranial magnetic stimulation application in patients with orofacial pain

In this paper, we monitored the accuracy of non-navigated application of repetitive Transcranial Magnetic Stimulation (rTMS) in 10 patients suffering from orofacial pain by using functional magnetic resonance (fMRI), computer modeling and numerical simulation. Through a unique process, each fMRI scan was used to define a Region of Interest (ROI) where the source of the orofacial pain was located, which was to be stimulated using rTMS. For each patient, MRI scans with a spatial resolution of 0.7 mm were converted into an anatomically accurate head model. The head model including the ROI was then co-registered with a model of the stimulation coil in an electromagnetic field numerical simulator. The accuracy of rTMS application was evaluated based on the calculations of electric field intensity distribution in the ROI. The research has yielded unique insight into ROIs (with average volume 904mm3) in patients with orofacial pain and has also extended further possibilities of human head MRI image semi-automatic segmentation. According to the calculations performed, the average ROI volume that was stimulated by an electric field with an intensity of over 80 V/m was only 4.4%, with the maximum ROI volume being 20.5%. Furthermore, a numerical study of the impact of coil rotation and translation was performed. It demonstrated a) the optimal placement of the stimulation coil can significantly increase the volume of the stimulated ROI up to 60% and b) patients with orofacial pain would need precise coil positioning with a navigation error lower than 10 mm. Due to an acceptable proccessing time of up to 6 hours, described numerical simulation opens up new options for precise rTMS treatment planning. This planning platform together withpatient-specific navigated rTMS, could lead to significant increase of treatment outcomes in patients suffering from orofacial pain.

Tissue Temperature Increases by a 10 kHz Spinal Cord Stimulation System: Phantom and Bioheat Model

OBJECTIVE

A recently introduced Spinal Cord Stimulation (SCS) system operates at 10 kHz, faster than conventional SCS systems, resulting in significantly more power delivered to tissues. Using a SCS heat phantom and bioheat multi-physics model, we characterized tissue temperature increases by this 10 kHz system. We also evaluated its Implanted Pulse Generator (IPG) output compliance and the role of impedance in temperature increases.

MATERIALS AND METHODS

The 10 kHz SCS system output was characterized under resistive loads (1-10 KΩ). Separately, fiber optic temperature probes quantified temperature increases (ΔTs) around the SCS lead in specially developed heat phantoms. The role of stimulation Level (1-7; ideal pulse peak-to-peak of 1-7mA) was considered, specifically in the context of stimulation current Root Mean Square (RMS). Data from the heat phantom were verified with the SCS heat-transfer models. A custom high-bandwidth stimulator provided 10 kHz pulses and sinusoidal stimulation for control experiments.

RESULTS

The 10 kHz SCS system delivers 10 kHz biphasic pulses (30-20-30 μs). Voltage compliance was 15.6V. Even below voltage compliance, IPG bandwidth attenuated pulse waveform, limiting applied RMS. Temperature increased supralinearly with stimulation Level in a manner predicted by applied RMS. ΔT increases with Level and impedance until stimulator compliance was reached. Therefore, IPG bandwidth and compliance dampen peak heating. Nonetheless, temperature increases predicted by bioheat multi-physic models (ΔT = 0.64°C and 1.42°C respectively at Level 4 and 7 at the cervical segment; ΔT = 0.68°C and 1.72°C respectively at Level 4 and 7 at the thoracic spinal cord)-within ranges previously reported to effect neurophysiology.

CONCLUSIONS

Heating of spinal tissues by this 10 kHz SCS system theoretically increases quickly with stimulation level and load impedance, while dampened by IPG pulse bandwidth and voltage compliance limitations. If validated in vivo as a mechanism of kHz SCS, bioheat models informed by IPG limitations allow prediction and optimization of temperature changes.

Novel Paradigm of Subdural Cortical Stimulation Does Not Cause Thermal Damage in Brain Tissue: A Simulation-Based Study

The thermal effect of a novel effective electrical stimulation mapping (ESM) technique using an Ojemann's stimulation electrode in open craniotomy areas causes a nondestructive local increase in temperature. Another type of stimulating electrode is a subdural strip, routinely used in intraoperative electrocorticography (ECoG), which applies ESM in a covered subdural area over the motor cortex. ECoG electrode geometry produces a different electrical field, causing a different Joule heat distribution in tissue, one that is impossible to measure in subdural space. Therefore, the previous safety control study of the novel ESM technique needed to be extended to include an assessment of the thermal effect of ECoG strip electrodes. We adapted a previously well-validated numerical model and performed coupled complex electro-thermal transient simulations for short-time (28.4 ms) high-frequency (500 Hz) and hyperintense (peak 100 mA) ESM paradigm. The risk of heat-induced cellular damage was assessed by applying the Arrhenius equation integral on the computed time-dependent spatial distribution of temperature in the brain tissue during ESM stimulation and during the cooldown period. The results showed increases in temperature in the proximity around ECoG electrode discs in a safe range without destructive effects. As opposed to open craniotomy, subdural space is not cooled by the air; hence a higher - but still safe - induced temperature was observed. The presented simulation agrees with the previously published histopathological examination of the stimulated brain tissue, and confirms the safety of the novel ESM technique when applied using ECoG strip electrodes.

A novel effective paradigm of intraoperative electrical stimulation mapping in children

OBJECTIVE

Resective epilepsy surgery is an established treatment method for children with focal intractable epilepsy, but the use of this method introduces the risk of postsurgical motor deficits. Electrical stimulation mapping (ESM), used to define motor areas and pathways, frequently fails in children. The authors developed and tested a novel ESM protocol in children of all age categories.

METHODS

The ESM protocol utilizes high-frequency electric cortical stimulation combined with continuous intraoperative motor-evoked potential (MEP) monitoring. The relationships between stimulation current intensity and selected presurgical and surgery-associated variables were analyzed in 66 children (aged 7 months to 18 years) undergoing 70 resective epilepsy surgeries in proximity to the motor cortex or corticospinal tracts.

RESULTS

ESM elicited MEP responses in all children. Stimulation current intensity was associated with patient age at surgery and date of surgery (F value = 6.81, p < 0.001). Increase in stimulation current intensity predicted postsurgical motor deficits (F value = 44.5, p < 0.001) without effects on patient postsurgical seizure freedom (p > 0.05).

CONCLUSIONS

The proposed ESM paradigm developed in our center represents a reliable method for preventing and predicting postsurgical motor deficits in all age groups of children. This novel ESM protocol may increase the safety and possibly also the completeness of epilepsy surgery. It could be adopted in pediatric epilepsy surgery centers.