Intraoperative neurophysiological mapping: the effect of general anaesthetic impact on brain cortex affectability

Neuron irritation lies at the heart of intraoperative motor mapping and varies with the general anaesthetic type and dose. Basing on the analysis of 63 cases (male/female 25/38, aged 21–69) of brain tumors (glial tumors, metastasis, cavernous angiomas) the study explores the role of propofol and sevoflurane in the affectability of cortex neurons during the intraoperative neurophysiological mapping. The study has clearly demonstrated that the liminal current strength is notably higher when inhalation anaesthesia (sevoflurane) is used, than in the case of TIVA (propofol). The propofol activity in the doses causing brain activity depression results in a sharp increase in the excitability threshold. In contrast, sevoflurane causes a steady dose-related rise in the liminal current strength during the motor area galvanic stimulation due to the suppression of affectability and conduction in the nervous system.

When imaging meets neurophysiology: the value of navigated transcranial magnetic stimulation for preoperative neurophysiological mapping prior to brain tumor surgery

Maximal safe resection is the modern goal for surgery of intrinsic brain tumors located in or close to brain eloquent areas. Nowadays different neuroimaging techniques provide important anatomical and functional information regarding the brain functional organization that can be used to plan a customized surgical strategy to preserve functional networks, and to increase the extent of tumor resection. Among these techniques, navigated transcranial magnetic stimulation (nTMS) has recently gained great favor among the neurosurgical community for preoperative mapping and planning prior to brain tumor surgery. It represents an advanced neuroimaging technique based on the neurophysiological mapping of the functional cortical brain organization. Moreover, it can be combined with other neuroimaging techniques such as diffusion tensor imaging tractography, thus providing a reliable reconstruction of brain eloquent networks. Consequently, nTMS mapping may provide reliable noninvasive brain functional mapping, anticipating information that otherwise may be available to neurosurgeons only in the operating theater by using direct electrical stimulation. The authors describe the reliability and usefulness of the preoperative nTMS-based approach in neurosurgical practice, and briefly discuss their experience using nTMS as well as currently available evidence in the literature supporting its clinical use. In particular, special attention is reserved for the discussion of the role of nTMS as a novel tool for the preoperative neurophysiological mapping of motor and language networks prior to surgery of intrinsic brain tumors located in or close to eloquent networks, as well as for future and promising applications of nTMS in neurosurgical practice.

Electrodiagnostics

Clinical neurophysiology provides valuable information in neurosurgery, serving as: a diagnostic tool that can quantify type and severity of damage to the central and peripheral nervous system, a means of monitoring the safety of structures within and around the surgical site, and a method to map structures. As such it aides in identifying structures (e.g. finding sacral nerve roots within a spinal lipoma or nuclei within the thalamus), assessing functional integrity (e.g. motor pathways from cortex to any relevant accessible muscle), and monitoring their function while surgery occurs near to structures (e.g. VII while retraction during trigeminal microvascular decompression, and in scoliosis surgery) and provide guidance to technical operative steps (e.g. for selective dorsal rhizotomy). Intraoperative monitoring is not new, though the advances in equipment and technique of recent years have seen an explosion in the useful ways that neurophysiology can aid the neurosurgeon and protect the patient. The development of techniques to localize epileptic foci and map eloquent cerebral cortex in the 1950s produced major scientific advances as well as revolutionizing epilepsy surgery. Since the 1960s Tasker in Toronto, and Gillingham in Edinburgh, were recording from microelectrodes in the human thalamus to guide movement disorder surgery. Pioneers such as Møller have extended the applications of neurophysiological monitoring in skull base surgery. This chapter describes neurophysiological mapping and monitoring, and the different tools that are useful in different situations.