Minimally Invasive Neurosurgery for Epilepsy Using Stereotactic MRI Guidance




Medically refractory epilepsy is associated with significant morbidity and mortality. Surgery is a safe and effective option for some patients, however the opportunity exists to develop less invasive and more effective surgical options. To this end, multiple minimally invasive, image-guided techniques have been applied to the treatment of epilepsy. These techniques can be divided into thermoablative and disconnective techniques. Each has been described in the treatment of epilepsy only in small case series. Larger series and longer follow up periods will determine each option’s place in the surgical armamentarium for the treatment of refractory epilepsy but early results are promising.


Key points







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    New, innovative, and minimally invasive, image-guided techniques for the surgical management of medically refractory epilepsy can be divided into thermoablative options and image-guided, robotic-assisted disconnective techniques.


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    The most well described of these new minimally invasive options is laser thermoablation, which uses thermal energy delivered to a patient’s unique seizure focus to locally destroy tissue under real-time magnetic resonance (MR) guidance.


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    Larger series and longer follow-up periods will determine each individual option’s long-term role in the surgical armamentarium for the treatment of medically refractory epilepsy but preliminary results seem promising.






Introduction


Approximately 1,000,000 Americans live with medically refractory epilepsy, and current surgical techniques address only a subset of this population’s epileptic pathologies. Although efficacious, there is an opportunity for improvement in the manner that the seizure focus is addressed. Surgical management typically involves an open craniotomy and either a removal of tissue or a disconnection procedure. As with all craniotomies, there are the attendant morbidities of bleeding, infection, postoperative pain, and wound complications. There has always a drive to reduce the invasiveness of these procedures to optimize the balance between the benefit of the surgery and the associated risks. Additionally, the risks of surgery may plan an adverse role in the perceptions of surgery as a therapeutic approach for epilepsy, thus potentially limiting the number of patients referred for surgical therapy despite level I evidence to the contrary. These factors synergistically encourage the development of new and innovative treatment options that can diversify the surgical armamentarium for the treatment of medically refractory epilepsy. Several new minimally invasive treatment options have developed from this treatment gap and will hopefully contribute to its narrowing as they become more widely recognized and patient outcomes prove their long-term efficacy for the management of medically refractory epilepsy.




Introduction


Approximately 1,000,000 Americans live with medically refractory epilepsy, and current surgical techniques address only a subset of this population’s epileptic pathologies. Although efficacious, there is an opportunity for improvement in the manner that the seizure focus is addressed. Surgical management typically involves an open craniotomy and either a removal of tissue or a disconnection procedure. As with all craniotomies, there are the attendant morbidities of bleeding, infection, postoperative pain, and wound complications. There has always a drive to reduce the invasiveness of these procedures to optimize the balance between the benefit of the surgery and the associated risks. Additionally, the risks of surgery may plan an adverse role in the perceptions of surgery as a therapeutic approach for epilepsy, thus potentially limiting the number of patients referred for surgical therapy despite level I evidence to the contrary. These factors synergistically encourage the development of new and innovative treatment options that can diversify the surgical armamentarium for the treatment of medically refractory epilepsy. Several new minimally invasive treatment options have developed from this treatment gap and will hopefully contribute to its narrowing as they become more widely recognized and patient outcomes prove their long-term efficacy for the management of medically refractory epilepsy.




Patient evaluation


Patients with medically refractory epilepsy referred to a tertiary-care epilepsy center for surgical consideration are reviewed by a multidisciplinary epilepsy team. This group typically includes the following members: (1) epilepsy neurologists who coordinate care and oversee video electroencephalogram (EEG) monitoring and interpretation; (2) specialized neuroradiologists trained in the interpretation of high-resolution MRI, typically including hippocampal volumetric analysis as well as PET and frequently single-photon emission CT (SPECT) imaging; (3) dedicated neuropsychologists who administer a battery of neuropsychological tests used to evaluate patients’ cognitive function and predict cognitive outcomes across various surgical techniques based on preoperative performance; and (4) neurosurgeons with subspecialized training in the surgical treatment options for the management of medically refractory epilepsy. Patients who are reviewed by this multidisciplinary team may be found to warrant further work-up with intracranial monitoring, including either subdural and/or depth electrode placement if there is a discordance between seizure semiology, imaging, or EEG findings. Alternatively, if there is concordance across these various categories, patients are referred directly for surgical treatment of their medically refractory epilepsy.




Surgical treatment options


Historically, treatment options were divided into 2 categories: resective or disconnective. Resective treatment strategies included removal of the epileptogenic focus. Some of the most common surgical procedures include temporal lobectomy, selective amygdalohippocampectomy, and topectomy of the neocortical tissue, which was believed to be a patient’s epileptogenic focus. Disconnective treatment strategies include disconnection of epileptogenic brain from nonepileptogenic brain by corpus callosotomy or hemispherotomy, both of which are typically reserved for palliative treatment of pediatric epilepsy. Additional accepted surgical treatment options for medically refractory epilepsy include neuromodulatory techniques, including vagal nerve stimulation and more recently responsive neurostimulation. Additional stereotactic MR-guided treatment options for the management of medically refractory epilepsy have also been recently introduced. These newer techniques are thermoablative in nature, such that rather than removing the tissue, the site is destroyed in situ.


Thermoablative Techniques


Multiple thermoablative treatment strategies have developed over the past 25 years and all rely on the basic principle of heating tissue to result in protein denaturation and tissue destruction. The energy source by which this thermoablative process occurs varies across the different techniques and includes laser, radiofrequency, and ultrasound-induced forms of thermal tissue destruction.


Laser thermoablation


Since its introduction to neurosurgery in 1990 and with an increasing presence since 2007, laser ablative techniques have been applied to the management of unresectable tumors, mesial temporal lobe epilepsy, neocortical dysplasia, poststroke neocortical seizure foci, periventricular nodular heterotopia, and hypothalamic hamartomas, among others. There are currently 2 commercially available devices used to carry out these ablative procedures. The first of these to receive Food and Drug Administration approval was the Visualase system in 2007 (Medtronic, Minneapolis, Minnesota) and more recently the NeuroBlate System in 2010 (Monteris Medical, Plymouth, Minnesota). Both systems offer stereotactic access to deep and superficial lesions and provide concentric laser ablation capabilities. Conformal laser ablation is additionally provided by the NeuroBlate System, which offers the option of a directionally oriented laser source to guide delivery of thermal energy for more irregularly shaped surgical targets.


Regardless of which system is used, the surgical technique includes the patient undergoing a preablation localizing MRI from which a volumetric target is selected for ablation. The thermal energy source is then placed under stereotactic guidance, allowing delivery of the thermoablative energy. Live intraprocedural repetitive measurements of a T1-weighted 2-D– fast low-angle shot (FLASH) sequence provide temporally sensitive thermometry measurements necessary to inform and control the energy delivered to create controlled and conformal lesions ( Fig. 1 ). These lesions are verifiable with postablation diffusion-weighted MRI, evidence of hemorrhagic necrosis on T1-weighted imaging, and a new distribution of gadolinium enhancement ( Fig. 2 ).




Fig. 1


Real-time heat map of thermal energy delivered under MR guidance.

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Oct 12, 2017 | Posted by in NEUROSURGERY | Comments Off on Minimally Invasive Neurosurgery for Epilepsy Using Stereotactic MRI Guidance

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