Cortical stimulation mapping (often shortened to CSM) is a type of electrocorticography that involves a physically invasive procedure and aims to localize the function of specific brain regions through direct electrical stimulation of the cerebral cortex.
It remains one of the earliest methods of analyzing the brain and has allowed researchers to study the relationship between cortical structure and systemic function.
There is significant variation in how patients respond to cortical electrical stimulation. It has been hypothesized that individual demographic and pathologic factors, such as age, sex, disease duration, and MRI findings, may explain this discrepancy.
As the most accurate and reliable method of brain functional area positioning, intraoperative cortical stimulation is able to determine in real time the parts of the brain necessary for such functions as movement, sensation, language, and even memory. A recent meta-analysis suggested that it could also improve the degree of resection of glioma while reducing the incidence of permanent neurological dysfunction 1).
Intraoperative cortical stimulation increased tumor removal while preserving the functional status of the patients but also opens a window to cognitive neuroscience. Observations during such interventions and their correlation with both pre - and postoperative neuropsychological examinations and functional neuroimaging is progressively leading to new insights into the complex functional anatomy of the human brain. Furthermore, it broadens our knowledge on cerebral network reorganization in the presence of disease-with implications for all disciplines of clinical neuroscience 2).
While the fundamental and clinical contribution of direct electrical stimulation (DES) of the brain is now well acknowledged, its advantages and limitations have not been re-evaluated for a long time.
DES is highly sensitive for detecting the cortical and axonal eloquent structures. Moreover, DES also provides a unique opportunity to study brain connectivity, since each area responsive to stimulation is in fact an input gate into a large-scale network rather than an isolated discrete functional site.
DES, however, also has a limitation: its specificity is suboptimal. Indeed, DES may lead to interpretations that a structure is crucial because of the induction of a transient functional response when stimulated, whereas (1) this effect is caused by the backward spreading of the electro-stimulation along the network to an essential area and/or (2) the stimulated region can be functionally compensated owing to long-term brain plasticity mechanisms.
Direct Electrical stimulation is still the gold standard for brain mapping, its combination with new methods such as perioperative neurofunctional imaging and biomathematical modeling is now mandatory, in order to clearly differentiate those networks that are actually indispensable to function from those that can be compensated 3).
Cortical stimulation mapping is used for a number of clinical and therapeutic applications, and remains the preferred method for the pre-surgical mapping of the motor cortex and language areas to prevent unnecessary functional damage.
There are also some clinical applications for cortical stimulation mapping, such as the treatment of epilepsy. Cortical stimulation, either transcranial or by means of electrodes implanted epidurally or subdurally, is used increasingly to treat neuropsychiatric diseases. In cases where transcranial stimulation gives only short-term success, implanted electrodes can yield results that are similar but long-term.
Direct electrical stimulation (DES) at 60 Hz is used to perform real-time functional mapping of the brain during wide-awake neurosurgery. The electrophysiological effects of DES are largely unknown, locally and at a more remote distance. Here, by lowering the DES frequency to 10 Hz and by using a differential recording mode of electro-corticographic (ECoG) signals to improve the focality, we were able to record cortico-cortical evoked potentials easily with standard current amplitude of stimulation (2 mA). DES applied at 10 Hz and differential recording of ECoG could be used to map on-line the connectivity between different sub-cortical and cortical areas with a higher spatial accuracy 4).
Corley et al, retrospectively analyzed data from 92 patients with medically intractable epilepsy who had extra-operative cortical electrical stimulation. Mapping records were evaluated and information gathered about demographic data, as well as the thresholds of stimulation for motor, sensory, speech, and other responses; typical seizure behavior; and the induction of afterdischarges.
Ninety-two patient cortical stimulation mapping reports were analyzed. The average of the minimum thresholds for motor response was 4.15mA±2.67. The average of the minimum thresholds for sensory response was 3.50mA±2.15. The average of the minimum thresholds for speech response was 4.48mA±2.42. The average of the minimum thresholds for afterdischarge was 4.33mA±2.37. Most striking were the degree of variability and wide range of thresholds seen between patients and within the different regions of the same patient 5).