12.2.1 Neuropsychological Assessment

In the preoperative work-up of glioma patients, neuropsychological evaluation is very important. The commonly used neuropsychological assessment scales are Edinburgh Handedness Inventory, Karnofsky Performance Score (KPS), Mini Mental State Examination (MMSE), Boston Naming Test (BNT), and the Aphasia Battery test. All patients must undergo neuropsychological assessment before and after surgery, since a comprehensive understanding of all types of dominant or recessive neurological dysfunctions before surgery allows surgeons to provide individualized treatment, predict possible temporary postoperative deficits, develop postoperative rehabilitation program, and provide a baseline for continuous assessment of postoperative neurological function.

12.2.2 Functional Magnetic Resonance Imaging

Functional MRI (fMRI) is the most common functional neuroimaging technique. When neurons are excited, the neurovascular coupling mechanism leads to a local increase in the ratio of oxyhemoglobin (antimagnetic) to deoxyhemoglobin (paramagnetic), which can be detected by echo-planner imaging, a fast MRI imaging technique. Although fMRI is fairly accurate in localizing primary motor ³ and sensory cortices, it is less sensitive in localizing language. ⁴ There are some important considerations underlying fMRI usage in brain tumor surgery. First, fMRI only shows areas of increased cortical blood flow which is an indirect measure of neuronal activity, and is susceptible to errors in the peritumoral brain. Furthermore, there is a temporal delay between neuronal activity and changes in blood flow; therefore, the transient activation of brain regions cannot be detected. In essence, fMRI best reveals areas of the gray matter involved in a particular task but does not indicate areas that are functionally necessary. Therefore, relying on fMRI ^{3 ,​ 4} to guide surgery may result in excision of critical functional cortical regions (false negative) or hinder neurosurgeons from removing noncritical cortical regions (false positive) that patients can actually compensate for. More appropriate uses of fMRI include ¹ preoperative surgical planning and rehearsal ² ; screening for functional brain regions that need to be verified by intraoperative direct electrical stimulation; or ³ providing cortical motor network information, such as SMA, for patients unable to undergo awake craniotomy.

12.2.3 Transcranial Magnetic Stimulation

This is a newer noninvasive preoperative cortical function mapping technique that involves altering the action potential of cortical neurons through a time-varying magnetic field and combines with navigation technology to accurately localize functional cortical areas. Recent studies have shown that transcranial magnetic stimulation (TMS) localization of cortical motor areas concurs with results of intraoperative direct electrical stimulation. ^{5 ,​ 6} TMS can be used in healthy subjects and patients with brain lesions. It can also be used to study cortical function remodeling due to its ability of regulating cortical excitability.

12.2.4 Diffusion Tensor Imaging

This technique is based on anisotropic imaging of the diffusion of water molecules in white matter fibers and can be used to form three-dimensional models of subcortical neural conduction pathways. Tracking imaging (tractography) shows the morphology, structure, and projection of these pathways. In addition to diffusion tensor imaging (DTI) tractography being important for research and teaching, it provides information about virtual structure rather than real function. Brain tumors are often accompanied by edema resulting in distortion and erosion of DTI white matter tracts. Results from our division’s clinical trial demonstrated that DTI tractography is clinically effective but not completely reliable in delineating descending motor pathways (corticospinal tract) and must be integrated with direct cortical stimulation for functional glioma surgery (Fig. 12‑2). ⁷ Despite the utility of fMRI and DTI in localizing functional cortical areas and subcortical neural pathways, respectively, a multimodal imaging approach comprising IONM is essential to ensure the accuracy and reliability of intraoperative functional brain mapping.

12.3.1 Somatosensory Evoked Potential

SSEP involves the stimulation of action potentials in peripheral nerves that are recorded in the cerebral cortex which, to some extent, reflect the afferent pathways of specific somatic sensations, the reticular structure of the brainstem, and the functional state of the cerebral cortex. SSEP recording is characterized by continuity, repeatability, and identifiable waveforms, and is advantageous in terms of anatomical monitoring and technique. The stimulation technique involves repetitive stimuli of intensity 15 to 25 mA and duration 0.10 to 0.30 ms at a rate of 2.10 to 4.70 Hz.

An important initial step in sensorimotor function mapping is identifying the central sulcus by SSEP phase reversal technique (Fig. 12‑3). A strip electrode with multiple contacts is placed perpendicularly on the surface of the presumed central sulcus to record SSEP by stimulating the median nerve (Fig. 12‑4). The strip electrode is moved until it demonstrates polarity reversal. ⁸ From this, the surgeon and neurophysiologist can estimate the position of the central sulcus which lies between the contacts where polarity reversal occurs. Although it is possible to identify the central sulcus by analyzing anatomical landmarks on neuroimaging, ⁹ anatomical and neurophysiological discrepancies resulting from large rolandic and pre-rolandic brain lesions can significantly impact the reliability of direct visual identification of the central sulcus.