Intraoperative Neurophysiological Monitoring Techniques for the Resection of Malignant Brain Tumors Located in Eloquent Cortical Areas

Review Article

Austin Neurosurg Open Access. 2015; 2(4): 1038.

Intraoperative Neurophysiological Monitoring Techniques for the Resection of Malignant Brain Tumors Located in Eloquent Cortical Areas

Lorena Vega-Zelaya and Jesús Pastor*

Clinical Neurophysiology, Hospital Universitario “La Princesa”, Spain

*Corresponding author: Jesús Pastor, Clinical Neurophysiology, Hospital Universitario “La Princesa”, C/Diego de León 62, 28006, Madrid, Spain

Received:August 10, 2015; Accepted: September 21, 2015Published: September 23, 2015


Glioblastoma Multiforme (GBM) is the most common central nervous system tumor. Despite progress in both medical and surgical treatments for this disease, the life expectancy associated with GBM is short; only a limited number of patients survive more than three years following diagnosis. When tumors are located in eloquent areas, the achievement of Gross Tumor Resection (GTR) is limited by the risk of permanent neurological deficits, restricting patients’ quality of life. Mapping techniques have enabled clinicians to localize eloquent cortical and subcortical fibers and Intraoperative Neurophysiological Monitoring (IONM) allows to monitor the function of at-risk neurological structures during the surgery. We have identified several criteria that may provide us with both reliable and efficient means of monitoring said structures. The efficacy of such techniques has improved, as have their sensitivity, specificity and safety. Tumor resection is more successful when guided via fluorescence but carries the risk of permanent neurological deficits. IONM minimizes said risk without compromising the chances of a successful resection. Diligent monitoring may also enable clinicians to avoid performing a surgery in which the patient awake provided that the language areas of the brain are not involved. The achievement of maximum GTR is the most important prognostic factor with respect to patient survival in the setting of high-grade gliomas. IONM and monitoring techniques maximize the effectiveness of GTR and are associated with reduced rates of surgery-related deficits.

Keywords: Direct cortical stimulation, High-grade gliomas, Motor-evoked potentials, Somatosensory-evoked potentials, Visual-evoked potentials, Anesthetized craniotomy, 5-aminolevulenic acid


Glioblastoma Multiforme (GBM) is the most common and lethal primary malignancy of the Central Nervous System (CNS). Several adjuvant therapies have been developed to improve progression-free survival, including surgical resection, local radiotherapy and systemic chemotherapy. Despite these innovations, the median survival time following diagnosis is only 14.6 months [1]. Nevertheless, 3–5% of patients survive more than 3 years; said patients are known as longterm survivors [2]. Younger age and good Karnofsky Performance Scores (KPSs) at the time of diagnosis are both associated with longer survival [3], but the first and most important step in the treatment of any primary malignant brain tumor is Gross Total Resection (GTR) [4]. To preserve patients’ quality of life, the primary goal of surgery is the achievement of GTR without compromising neurological function. Advances in surgical techniques such as Intraoperative Neurophysiological Monitoring (IONM), intraoperative Magnetic Resonance Imaging (MRI), Diffusion Tensor Imaging (DTI), stereotactic guidance, and fluorescence-guided resection have facilitated the delineation of tumor borders and may help to optimize maximal safe surgical resection [5-7].

In patients with tumors located in eloquent brain areas (areas responsible for carrying out basic neurological functions) such as the sensorimotor cortex and the language cortex and subcortical structures such as the basal ganglia and the internal capsule, the proper identification of relevant tracts is necessary to preserve adequate neurological function [8]. Tractographyvia Diffusion Tensor Imaging (DTI) and intraoperative Magnetic Resonance Imaging (MRI) have proven useful in the identification of Internal Capsule (IC) and Thalamocortical Fibers (ThCF); dynamic changes during surgery (loss of cerebrospinal fluid and low-grade brain edema) and a lack of reliability with respect to the identification of small and functional tracts pose potential risks to patients [9,10]. IONM enables clinicians to access the function of patients’ motor and sensory systems during surgery to preserve neurological function and increases the success of radical tumor resection.

This article encompasses a review of the most commonly used techniques available for the mapping and monitoring of neural function in the setting of glioma surgery involving eloquent brain areas, the warning criteria for each modality, and the intrinsic technical limitations of each technique.

Intraoperartive Neurophysiological Monitoring Modalities

Mapping to localize eloquent areas

Functional areas: Intraoperative electroencephalographic recordings obtained directly from the cortical surface and Electrocorticography (ECoG) are used to monitor stimulationtriggered epileptiform discharges and after discharges before and during electrophysiologic functional mapping in the setting of glioma surgery (Figure 1). Said procedures are performed before electrical stimulation of the cortex is using either grid or strip electrodes with bandwidths of 1.5–1000 Hz. Weutilize spectral analyses using a Fast Fourier Transform (FFT), using non-overlapping windows of 8 s in length, for each ECoG recording. These results are depicted as a Density Spectral Array (DSA) for frequencies ranging from 1 to 50 Hz, which allows us to define three functional areas as follows: i) an activity loss area, defined based on decreases in the appearances of all frequencies, particularly the faster alpha and beta bands; ii) an irritative area, defined based on the presence of interictal epileptiform discharges appearing either as spikes (<80 ms) or as sharp waves (80- 200 ms) with amplitudes greater than 3 standard deviations above basal activity; and iii) normal cortex, a region devoid of either irritative elements or the abnormal loss of bioelectrical activity [11]. The identification of complex spectral changes over time enables clinicians to observe local cortical excitability [12], which is important for the following reason: said identification allows clinicians to determine the risk of stimulation-triggered seizure activity and to determine the locations of the irritative zones relative to the site of stimulation, which makes it possible to estimate the risk of triggering Ads [13]. It is also important to identify cortical areas with larger amounts of lesions (denoted by the loss of cortical rhythm) to preserve cortical areas with fewer lesions.