Cortical stimulation mapping

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. 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. The history of cortical stimulation mapping dates back to the late 19th century. Neurologists David Ferrier and 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. 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. Current levels and density are an important consideration in all cortical stimulation mapping procedures. Current density, that is the amount of current applied to a defined area of the brain, must be sufficient to stimulate neurons effectively and not die off too quickly, yet low enough to protect brain tissue from damaging currents. Currents are kept at levels that have been determined safe and are only given as short bursts, typically bursts that slowly increase in intensity and duration until a response (such as a muscle movement) can be tested. Current intensity is usually set around bursts of 1 mA to begin and gradually increased by increments of 0.5 to 1 mA, and the current is applied for a few seconds. If the current applied causes afterdischarges, nerve impulses that occur after stimulation, then the levels are lowered. Studies on patients who have received cortical stimulation mapping have found no cortical damage in the tested areas. The different types and administration techniques for anesthesia have been shown to affect cortical stimulation mapping. CSM can be done performed on awake patients, called an awake craniotomy or in patients who have been placed under general anesthesia. If the patient is under general anesthesia, the depth of the anesthesia can affect the outcome because if the levels of muscle relaxation are too high due to neuromuscular blocking drugs, then the results from the mapping can be incorrect. For the awake procedure there are different considerations for patient care that the anesthesiologist must take into account. Rather than simply ensuring that the patient is asleep, the doctor can follow what is called the asleep-awake-asleep technique. In this technique the patient is anesthetized using a general anesthesia during the opening and closing portions of the procedure, but during the interim the patient is maintained utilizing local anesthesia. The local anesthesia techniques can be either a local field block or a regional nerve block of the scalp. The more common technique for the awake craniotomy is conscious sedation. In conscious sedation, the patient is only sedated during the opening and closing process, but never fully anesthetized, eliminating the need for breathing tubes, lessening the chances of complications, and lessening the chances of problems with motor response. Patients who undergo the procedure with an awake craniotomy instead of general anesthesia have better preservation of language function, a prediction of their seizure-free outcome based on corticography, a shorter hospitalization (which corresponds to a reduced cost of care), a decreased usage of invasive monitors, and decreased number of postoperative complications due to anesthesia such as nausea and vomiting. Cortical stimulation mapping is used for somatotopy to determine the areas of the cerebral cortex that connect through nerve fibers with different body parts. Cortical stimulation identifies which regions of the brain are vital for certain functions, thereby allowing a 'map' to be made which can be used to decide if brain areas are safe to remove. Cortical stimulation mapping led to the development of a homunculus for the motor and sensory cortices, which is a diagram showing the brain's connections to different areas of the body. An example is the cortical homunculus of the primary motor cortex and the somatosensory cortex, which are separated by the central sulcus. The diagram starts in the longitudinal fissure and continues out laterally from the center of the brain, representing the general pattern from lower extremities and genitals in the fissure up to the hands and face on outer edges of the brain. Functional testing of movement during cortical stimulation includes looking for active movement and inhibition of movement. When the precentral gyrus of the frontal lobe is stimulated, specific muscles in the body will contract based on the location of the brain that receives the electric signal. Stimulation on one side of the brain will cause a contraction on the contralateral, or opposite, side of the body.