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Cortical Stimulation
Mapping
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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. 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.
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
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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
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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.
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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.
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