<p>Epidural cortical stimulation (ECS) is a developing therapy for many neurological disorders. However, the mechanisms by which ECS has its effects are unknown, and this lack of understanding has limited the development and optimization of this promising therapy. This dissertation investigates the effects of ECS on the neurons in the cortex and how these effects vary with electrode geometry and location as well as the electrical and geometrical properties of the anatomy.</p><p>The effects of ECS on cortical neurons were investigated using a three dimensional computational model of the human precentral gyrus and surrounding anatomy. An epidural electrode was placed above the gyrus, and the model was solved using the finite element method. The outputs of the model included distributions of electric potential, the second spatial derivative of potential (activating function), and current density. The distributions of electric potential were coupled to compartmental models of cortical neurons to quantify the effects of ECS on cortical neurons. A sensitivity analysis was performed to assess how thresholds and distributions of activating function were impacted by changes in the geometrical and electrical properties of the model. In vivo experiments of epidural electrical stimulation of cat motor cortex were performed to measure the effects of stimulation parameters and electrode location on thresholds for evoking motor responses.</p><p>During ECS, the region of cortex directly underneath the electrode was activated at the lowest thresholds, and neurons deep in the sulcus could not be directly activated without coactivation of neurons located on the crowns or lips of the gyri. The thresholds for excitation of cortical neurons depended on stimulation polarity as well as the orientation and position of the neurons with respect to the electrode. In addition, the patterns and spatial extent of activation were influenced by the geometry of the cortex and surrounding layers, the dimensions of the electrodes, and the positioning of the lead. In vivo thresholds for evoking motor responses were dependent on electrode location and stimulation polarity, and bipolar thresholds were often different from monopolar thresholds through the respective anode and cathode individually. The effects of stimulation polarity and electrode location on thresholds for evoking motor responses paralleled results of the computational model. Experimental evidence of indirect effects of ECS, mediated by synaptic interactions between neural elements, revealed an opportunity for further development of the computational model. The outcome of this dissertation is an improved understanding of the factors that influence the effects of ECS on cortical neurons, and this knowledge will help facilitate the rational implantation and programming of ECS systems.</p> / Dissertation

A estimulação cerebral profunda do núcleo subtalâmico (NST) é um tratamento bem estabelecido para os sintomas refratários à medicação em paciente com doença de Parkinson avançada. Além do procedimento de implante, a programação dos eletrodos é uma etapa fundamental para atingir os resultados desejados. A primeira etapa da programação é estabelecer os limiares para efeitos adversos. Contudo, a correlação entre a localização do eletrodo e o limiar para efeitos adversos associados à estimulacao das estruturas adjacentes ainda não é bem estabelecida. Características neuroanatômicas e a localização dos eletrodos foram identificadas utilizando-se um programa de planejamento de cirurgia estereotáxica, enquanto os parâmetros de estimulação e os efeitos adversos foram obtidos dos prontuários médicos. As correlações entre estas variáveis foram testadas através de análises univariadas e análises multivariadas. Estimulação monopolar produziu efeitos adversos capsulares (EA-C) em 208 dos 316 contatos (65,8%) e efeitos adversos não-capsulares (EA-NC) em 223 dos 316 contatos (70,6%). A ocorrência de EA-C esteve associada com o número do contato (p = 0,009) e com a coordenada \"Z\" (p = 0,03), enquanto o limiar de voltagem para EA-C esteve correlacionado com o ângulo da cápsula interna (p = 0,035). A ocorrência de EA-NC esteve associada com o número do contato (p = 0,005), \"X\" (p = 0,03), \"Y\" (p = 0,004) e com a distância para o núcleo rubro (p = 0,001 e p = 0,003). Houve correlação entre o limiar de voltagem para EA-NC e o ângulo da cápsula interna (p = 0,006), o ângulo coronal do eletrodo (p = 0,02), \"X\" (p = 0,001), \"Y\" (p < 0,001), \"Z\" (p < 0,001) e com as distâncias para a cápsula interna (p = 0,02) e para o núcleo rubro (p = 0,004 e p < 0,001). EA-C estiveram associados com os contatos mais distais do eletrodo e com localização mais profunda, bem como com maior angulação da cápsula interna. EA-NC estiveram associados com os contatos mais distais do eletrodo, localizados mais medial, posterior e inferiormente e mais próximos do núcleo rubro. Ademais, houve associação entre EA-NC e eletrodos implantados com maior ângulo coronal, bem como com maior angulação da cápsula interna. Estes achados poderão ser úteis no desenvolvimento de novas estratégias para o planejamento do implante de eletrodos de estimulação cerebral profunda. / Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a well-established treatment for medically refractory motor symptoms of patients with advanced Parkinson\'s disease. Programming of the device is as relevant to patient outcome as accurate implantation of the electrodes. The first step of DBS programming is to identify the thresholds to side effects. However, the relationship between lead location and the threshold to adverse effects is not fully understood. Anatomical measurements and electrode location were evaluated on a stereotactic surgical planning software, whereas stimulation parameters and side effects were obtained from medical records. Correlations among these variables were tested using univariate and multivariable analyses. Monopolar stimulation elicited capsular side effects (CSEs) in 208 of 316 contacts (65.8%) and noncapsular side effects (NCSEs) in 223 of 316 contacts (70.6%). The occurrence of CSEs was correlated with contact number (p = 0,009) and with the \"Z\" coordinate (p = 0,03), whereas voltage threshold to CSEs exhibited correlation with the internal capsule angle (p = 0,035). The occurrence of NCSEs was correlated with contact number (p = 0,005), \"X\" (p = 0,03), \"Y\" (p = 0,004), and the distance to the red nucleus (p = 0,001 and p = 0,003). There was correlation between voltage threshold to NCSEs and the internal capsule angle (p = 0,006), electrode\'s coronal angle (p = 0,02), \"X\" (p = 0,001), \"Y\" (p < 0,001), \"Z\" (p < 0,001), and the distances to the internal capsule (p = 0,02) and to the red nucleus (p = 0,004 and p < 0,001). CSEs were associated with more distally contacts, with deeper localization, as well as with greater internal capsule angles. NCSEs were associated with more distally contacts, with localization more medial, posterior and inferior, and closer to the red nucleus. Moreover, there was a correlation between NCSEs and electrodes implanted with greater coronal angles, as well as with greater internal capsule angles. These findings can be useful to inform novel targeting strategies for deep brain stimulation lead implantation.

A estimulação cerebral profunda do núcleo subtalâmico (NST) é um tratamento bem estabelecido para os sintomas refratários à medicação em paciente com doença de Parkinson avançada. Além do procedimento de implante, a programação dos eletrodos é uma etapa fundamental para atingir os resultados desejados. A primeira etapa da programação é estabelecer os limiares para efeitos adversos. Contudo, a correlação entre a localização do eletrodo e o limiar para efeitos adversos associados à estimulacao das estruturas adjacentes ainda não é bem estabelecida. Características neuroanatômicas e a localização dos eletrodos foram identificadas utilizando-se um programa de planejamento de cirurgia estereotáxica, enquanto os parâmetros de estimulação e os efeitos adversos foram obtidos dos prontuários médicos. As correlações entre estas variáveis foram testadas através de análises univariadas e análises multivariadas. Estimulação monopolar produziu efeitos adversos capsulares (EA-C) em 208 dos 316 contatos (65,8%) e efeitos adversos não-capsulares (EA-NC) em 223 dos 316 contatos (70,6%). A ocorrência de EA-C esteve associada com o número do contato (p = 0,009) e com a coordenada \"Z\" (p = 0,03), enquanto o limiar de voltagem para EA-C esteve correlacionado com o ângulo da cápsula interna (p = 0,035). A ocorrência de EA-NC esteve associada com o número do contato (p = 0,005), \"X\" (p = 0,03), \"Y\" (p = 0,004) e com a distância para o núcleo rubro (p = 0,001 e p = 0,003). Houve correlação entre o limiar de voltagem para EA-NC e o ângulo da cápsula interna (p = 0,006), o ângulo coronal do eletrodo (p = 0,02), \"X\" (p = 0,001), \"Y\" (p < 0,001), \"Z\" (p < 0,001) e com as distâncias para a cápsula interna (p = 0,02) e para o núcleo rubro (p = 0,004 e p < 0,001). EA-C estiveram associados com os contatos mais distais do eletrodo e com localização mais profunda, bem como com maior angulação da cápsula interna. EA-NC estiveram associados com os contatos mais distais do eletrodo, localizados mais medial, posterior e inferiormente e mais próximos do núcleo rubro. Ademais, houve associação entre EA-NC e eletrodos implantados com maior ângulo coronal, bem como com maior angulação da cápsula interna. Estes achados poderão ser úteis no desenvolvimento de novas estratégias para o planejamento do implante de eletrodos de estimulação cerebral profunda. / Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a well-established treatment for medically refractory motor symptoms of patients with advanced Parkinson\'s disease. Programming of the device is as relevant to patient outcome as accurate implantation of the electrodes. The first step of DBS programming is to identify the thresholds to side effects. However, the relationship between lead location and the threshold to adverse effects is not fully understood. Anatomical measurements and electrode location were evaluated on a stereotactic surgical planning software, whereas stimulation parameters and side effects were obtained from medical records. Correlations among these variables were tested using univariate and multivariable analyses. Monopolar stimulation elicited capsular side effects (CSEs) in 208 of 316 contacts (65.8%) and noncapsular side effects (NCSEs) in 223 of 316 contacts (70.6%). The occurrence of CSEs was correlated with contact number (p = 0,009) and with the \"Z\" coordinate (p = 0,03), whereas voltage threshold to CSEs exhibited correlation with the internal capsule angle (p = 0,035). The occurrence of NCSEs was correlated with contact number (p = 0,005), \"X\" (p = 0,03), \"Y\" (p = 0,004), and the distance to the red nucleus (p = 0,001 and p = 0,003). There was correlation between voltage threshold to NCSEs and the internal capsule angle (p = 0,006), electrode\'s coronal angle (p = 0,02), \"X\" (p = 0,001), \"Y\" (p < 0,001), \"Z\" (p < 0,001), and the distances to the internal capsule (p = 0,02) and to the red nucleus (p = 0,004 and p < 0,001). CSEs were associated with more distally contacts, with deeper localization, as well as with greater internal capsule angles. NCSEs were associated with more distally contacts, with localization more medial, posterior and inferior, and closer to the red nucleus. Moreover, there was a correlation between NCSEs and electrodes implanted with greater coronal angles, as well as with greater internal capsule angles. These findings can be useful to inform novel targeting strategies for deep brain stimulation lead implantation.