Cortical stimulation mapping (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. 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.
History
The history of cortical stimulation mapping dates back to the late 19th century. Neurologist David Ferrier and neurosurgeon Victor Horsley were some of the first to utilize this technique. Ferrier and Horsley employed CSM to further grasp the structure and function of the pre-Rolandic and post-Rolandic areas, also known as the pre central gyrus and post central gyrus. Prior to the development of more advanced methods, in 1888 C.B. Nancrede utilized a battery operated bipolar probe in order to map the motor cortex. In 1937, Wilder Penfield and Boldrey were able to show that stimulating the precentral gyrus elicited a response contralaterally; a significant finding given that it correlated to the anatomy based on which part of the brain was stimulated. In the early 1900s Charles Sherrington began to use monopolar stimulation in order to elicit a motor response. This technique allowed Sherrington to determine that the precentral gyrus (pre-Rolandic area) is a motor cortex and the postcentral gyrus (post-Rolandic area) is a sensory cortex. These findings, which were repeated by Harvey Cushing through the early 1900s, show that the Rolandic fissure is the point of separation between the motor and sensory cortices. Cushing's work with CSM moved it from an experimental technique to one that became a staple neurosurgery technique used to map the brain and treat epilepsy. Cushing took work that had previously been done on animals, specifically chimpanzees and orangutans, and was able to utilize cortical stimulation mapping to account for the differences between these species and humans. Cushing's work dramatically increased the effectiveness of the treatment utilizing cortical stimulation mapping, as neurosurgeons were now utilizing a more updated picture of the brain.
Procedure
Cortical stimulation mapping is an invasive procedure that has to be completed during a craniotomy. Once the dura mater is peeled back, an electrode is placed on the brain to test motor, sensory, language, or visual function at a specific brain site. The electrode delivers an electric current lasting from 2 to 10 seconds on the surface of the brain, causing a reversible lesion in a particular brain location. This lesion can prevent or produce a testable response, such as the movement of a limb or the ability to identify an object. The electric current from the electrode stimulates whatever function that site in the brain is responsible for, in essence telling the surgeon or examiner what a specific locale in the brain does.
Electrodes are usually made of stainless steel or platinum-iridium embedded in a silastic material, and are usually circular with diameters of 2 to 3 mm. Electrode positioning varies from patient to patient, and electrodes can come in rows, in a grid array, or can be individually arranged. The number of electrodes necessary and their exact spatial arrangement is often determined in the operating room. Cortical stimulation mapping allows electrodes to be placed in exact locations to test brain function and identify if stimulation of the brain location causes a functional impairment in the patient. CSM can be completed using anesthetized patients or awake patients.
Electrodes can either be placed directly on brain areas of interest or can be placed in the subdural space of the brain. Subdural electrodes can shift slightly and can be affected by cerebrospinal fluid in the subdural space, which could interfere with the current used to stimulate the brain from the electrodes and possibly cause shunting and dissipate the current, making the stimulation's effect less accurate. However, an advantage of subdural electrode grids is that they can be left in the brain for multiple days, and allow functional testing during stimulation outside the operating room.