Combined Use of DES, EMG and MEP Monitoring, ECoG and EEG for Surgical Resection of Gliomas
Combined Use of DES, EMG and MEP Monitoring, ECoG and EEG for Surgical Resection of Gliomas
Surgery for gliomas aims to remove as much of the tumour mass as possible while at the same time preserving the patient’s functional integrity. This policy particularly applies to resection of gliomas located close to or within the so-called ‘eloquent’ areas, i.e. areas that are involved in motor, language or visuospatial functions. In these cases, extended resection and maximal functional integrity can be achieved by using a cohort of procedures that make up the so-called brain-mapping techniques. These include neuropsychological evaluation, imaging techniques such as functional magnetic resonance imaging (fMRI) and diffusion tensor imaging–fibre tracking (DTI-FT) – generally loaded into the neuronavigation system so as to be available peri-operatively – and a series of neurophysiological techniques available at the time of surgery such as cortical and subcortical direct electrical stimulation (DES), motor-evoked potentials (MEPs), multichannel electromyogram (EMG), electroencephalogram (EEG) and electrocorticogram (ECoG) recordings. In this article we will focus on these neurophysiological adjuncts by describing the protocol in use at our institution for resection of gliomas, and by discussing the rationale and indications for and the results of our experience.
Brain-mapping Protocol
The brain-mapping protocol is part of a large management protocol that includes pre- and peri-operative components. The pre-operative component comprises a comprehensive neuropsychological evaluation and an extensive neuroradiological examination. The neuropsychological evaluation consists of a large number of tests aimed at the evaluation of various functions such as cognitive, emotional, intelligence and language functions.1–5 This broad evaluation provides information on how the tumour has influenced the social, emotional and cognitive life of the patient, who is generally intact at the neurological examination. In addition, it allows us to build up a series of tests, composed of various items, that will be used intra-operatively for the evaluation and mapping of various functions, among which memory, language in its various components and visuospatial abilities are some of the most important. The neuroradiological examination is composed of basic examinations, such as morphological T1, T2 and flair images, as well as postcontrast T1 images. These images, together with volumetric sequences, provide information on the location of the tumour and allow its relationship with various structures – such as major vessels – to be determined and tumour volume to be measured. Other MR studies include MR spectroscopy, which provides information on the metabolic characteristics of the tumour, and perfusion MR, which is useful for designating the perfusion map of the tumour (regional blood flow dependent on tumour angiogenesis). These studies provide additional and complementary information about the biological behaviour of the tumour and help in tissue collection for histological and molecular purposes at the time of surgery.
Additional neuroradiological studies include functional studies such as fMRI and anatomical studies such as DTI-FT.6 The former provides functional information on the location of cortical sites that are activated in response to motor or various language tasks. Motor fMRI is generally used to create a map of the cortical motor sites and to establish their relationship with the tumour. Language fMRI provides a map of the cortical sites that are activated during various language tasks, such as object naming, famous face naming, verb generation and verbal fluency. Together, they form a complex map of how the various components of language are organised at the cortical level and allow the spatial relationship between these cortical areas of language activation and the tumour mass to be established. DTI-FT enables the connectivity around and inside a tumour to be depicted by reconstructing and visualising the fibre tracts that run around or inside the tumour mass. DTI-FT also provides anatomical information on the location of motor tracts, mainly the corticospinal tract, and various language tracts involved in either the phonological or semantic components of language, such as the superior longitudinalis, the inferior fronto occipital and the inferior longitudinalis tracts.
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