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Letter: A Review of Passive Brain Mapping Techniques in Neurological Surgery.

To the Editor: We read with great interest the article recently published in your distinguished journal, Neurosurgery, by Roland et al,1 entitled “A Review of Passive Brain Mapping Techniques in Neurological Surgery.” The authors conducted a review on brain mapping used by neurosurgeons for surgical planning involving both invasive and noninvasive mapping, emphasizing the properties and advantages of passive mapping.1 The brain mapping technique in neurological surgery has to be accurate to identify the eloquent areas, in particular the motor homunculus, to access those areas, avoiding postoperative deficits. The awake craniotomy with electrocortical stimulation has evidence-based advantages of delineating the homunculus, identifying the superior longitudinal fasciculus (SLF) with the understanding of the unbiased response such as picture naming, and text reading languages from direct communication with the patient.2 The evolution of cortical stimulation will preserve the functional status with neurocognitive improvement in brain mapping.3 Although direct cortical electrical stimulation continues to be considered the method of choice for brain mapping and eloquent cortex location, primarily when the edges of the planned surgical resection are overlapping, and it has excellent spatial (5 mm) and temporal (instantaneous) resolution, which is why it is compared to other noninvasive techniques, the advance of technology and the development of procedures with less exposure and need for patient cooperation have diminished its role at the intraoperative level.4,5 Likewise, the intracreaneal electroencephalography (electrocorticography and stereoencephalography), despite continuing to be used as the gold standard for locating epileptogenic foci, is a highly invasive procedure; the refinement of techniques such as the electroencephalogram (EEG), blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI), and magnetoencephalography (MEG) have provided viable alternatives that do not expose the patient to an intervention and can be performed preoperatively, as well as provide reliable information, demonstrating that they are tools capable of successfully guiding epilepsy and brain tumor surgeries, in addition to being more accessible, less demanding, and presenting the advantage of mapping the entire brain and not only the areas exposed by the craniotomy.4-6 However, when noninvasive procedures do not sufficiently help locate the epileptic area or do not provide the information necessary for adequate surgery planning, due to their lower resolution, the use of invasive mapping can be beneficial. Although new technologies represent more and more precise tools, with a decrease in the risks for the patient and the time dedicated to the procedure, and also presenting greater accessibility, invasive techniques continue to be a step forward in terms of location and delineation of epileptic zones and cortical function, as well as their practical application has been documented throughout various studies demonstrating that it is useful to eliminate and minimize postoperative linguistic or sensorimotor deficits, in addition to evoking reliable and very specific results, without artifacts, that are associated with the anarchic propagation of the current. However, we cannot turn a blind eye when it comes to its deficiencies in terms of lack of standardization, risk of seizure episodes, and after-discharge evoking, which greatly limits the stimulation cycles of patients with an epilepsy monitor unit.4,5,7 Considering the advantages and disadvantages of both brain mapping alternatives, we want to emphasize that each case must be evaluated separately taking into account risks and contraindications, as well as the socioeconomic aspect of the country, since the new mapping techniques usually have a high maintenance cost and are not available in all centers6; consequently, their use is limited to developing and first world countries, being excluded as a brain mapping alternative in countries and entities with fewer economic resources. Let us remember that the gold standard of brain mapping continues to be invasive techniques, and that, in the case of using passive mapping procedures, if these do not meet the required precision, either because of measurement artifacts or resolution limitations, invasive alternatives should be used to ensure correct planning of the surgery and successful mapping, since noninvasive options have in no way supplanted them, being considered only promising tools with lower risk and higher efficiency.1