Cerebral gliomas are rare and highly heterogeneous tumors that originate from the cells that make up the cerebral cortex and the white matter.

By their very nature, they tend to infiltrate the brain, blending with it, although in some cases they can be clearly distinguished from healthy brain tissue. This infiltrative behavior makes surgical removal particularly complex, and surgery alone rarely ensures a cure. Moreover, gliomas often infiltrate areas of the brain that are essential for verbal expression, reading, writing, motor coordination, and body sensation.

Traditionally, certain areas of the brain are defined as “eloquent” (that is, highly functional), while others are considered non-eloquent. In reality, all brain areas have functions — the difference lies in the fact that some regions act as major hubs where countless neural connections converge. Damage to one of these hubs can therefore cause neurological deficits that are difficult to recover from. Other areas are less interconnected, and their functions can, over time, be taken over by other brain regions.

Another crucial characteristic of gliomas is the wide variability in their growth rate. Some grow very slowly, without significantly affecting the patient’s brain functions, and often the patient becomes aware of the tumor only when it reaches a considerable size (see images below). When, on the other hand, the growth rate is high, it is more likely that the patient will experience symptoms related to the malfunction of the brain area irritated by the tumor’s presence.

One of the main goals of neurosurgery is to achieve the most extensive possible tumor removal while minimizing damage to brain functions. It has been clearly demonstrated that the extent of resection correlates with longer survival.

Because anatomical variability and the distribution of functions in the cortex and white matter are enormous from one person to another, it is impossible to predict with absolute precision whether an area infiltrated by the tumor can be safely removed — and to what extent its removal would produce permanent neurological deficits.

Fortunately, today there are diagnostic technologies that allow visualization of the activity of specific brain regions and help determine whether they are involved in a particular function. The most commonly used test for this purpose is Functional Magnetic Resonance Imaging (fMRI), which shows, in the form of colored “spots,” the areas of the cerebral cortex that are activated while the subject performs certain tasks.

Similarly, but through different techniques, it is possible to reconstruct a model of the connections between neurons (at least the larger fiber bundles). Certain MRI sequences exploit the diffusion of water molecules along neural connections to produce highly reliable 3D images that can help the surgeon understand the relationship between the tumor and the white matter tracts.

Despite these major advances in neuroradiology, there are still limitations that prevent surgeons from relying solely on these methods when operating on brain tumors located in critical areas.

For this reason, brain mapping through direct cortical stimulation in awake patients, along with intraoperative monitoring, has become the gold standard for brain tumor surgery in critical areas.

The purpose of applying a non-damaging electric current to the cerebral cortex or white matter is to excite or inhibit groups of neurons, temporarily creating a short circuit that simulates what would happen if those neurons were removed. In this way, a functional map of the exposed brain area can be drawn.

Using this technique, it is possible to study various brain functions such as sensory processing, verbal language, writing, reading, calculation, and vision.

Sensory and motor areas can also be mapped while the patient is asleep. The choice of which technique to use is discussed with the surgeon on a case-by-case basis. Conversely, mapping of the other functions mentioned above requires the patient’s full cooperation during the surgical procedure.