Cortical stimulation mapping
Cortical stimulation mapping | |
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Purpose | aims to localize the function of specific brain regions |
Cortical stimulation mapping (CSM) is a type of
History
The history of cortical stimulation mapping dates back to the late 19th century. Neurologist
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.[6]
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.[1] 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.[7] CSM can be completed using anesthetized patients or awake patients.[1]
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.[1]
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
Somatotopy
Cortical stimulation mapping is used for
Motor Mapping
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.[2]
More recent studies using CSM have shown that the motor cortex is more complex than the arrangement pictured traditional homunculus, and that motor responses occur in the frontal lobe further away from the narrow strip next to the central sulcus.[1]
Areas of the cortex that inhibit movement upon stimulation have been found in some cases to be supplemental and not vital to motor function. These areas have been removed without compromising a patient's ability to move post-operation.[1]
Language Mapping
During stimulation various language tasks are used to check brain function such as reading sentences, auditory comprehension, and spontaneous speech such as naming objects.[1] Cortical stimulation in language areas of the brain typically tests for the inhibition of some language capability, rather than a defined motor or sensory response. This can make language mapping require more complex language-related tasks to be assessed during testing, in order to determine if the site that is stimulated is essential to a certain language ability.[6]
Language mapping is normally done in the
Cortical stimulation mapping in patients with epilepsy has shown that critical language areas of the brain vary greatly in patients, highlighting the need to perform accurate mapping prior to surgeries in language areas.[9] Traditional landmarks such as Broca's and Wernicke's areas cannot be relied on to distinguish essential language cortex. Rather, experiments that have tested for vital language sites are variable, and the exact role of a specific cortex area in a language task is difficult to judge. A further complication is that many patients who have undergone language mapping have epilepsy, which often alters the localization of cortical areas due to the neuroplastic response to cortical insult caused by the patient's seizures.[10] Since the procedure is so invasive, cortical stimulation mapping for language organization is not done on healthy individuals. Additionally the distribution and abundance of specific task-related language sites have shown variation based on IQ and gender.[6]
Somatosensory mapping
Somatosensory mapping involves measuring electrical responses on the surface of the brain as the result of the stimulation of peripheral nerves, such as mechanoreceptors that respond to pressure on the skin, and stimulating the brain directly to map sensory areas. Sensation has been tested in patients through the stimulation of the postcentral gyrus, with a drop in amplitude of sensory responses occurring towards the central sulcus.[2]
Clinical Applications
Epilepsy
CSM is an effective treatment for
CSM will be considered for a patient with epilepsy when two conditions are met: the trial of anti-epileptic drugs has not controlled seizures and there is a likelihood that the surgery will benefit the patient.[4] Due to the nature of the procedure, CSM is only utilized after noninvasive procedures have not been able to fully localize and treat the patient.[12]
The invasive electrodes are stereotaxically placed electrodes or a subdural strip or grid electrode.[12] Utilizing the information obtained through CSM, limited resection of epileptogenic brain can be performed.[4] For focal epilepsy, resective surgery is one of the mainstay treatment options for medication resistant epilepsy.[11] Through the technique of CSM, generally using awake craniotomies, the neurosurgeon has the ability to monitor the functioning of the patient during the resection and stimulation of the brain.
Neuro-Oncology
Cortical stimulation mapping may be used in
Patients whose surgeon uses cortical stimulation mapping to assess the anatomy and function of rolandic areas have a greater chance and faster rate of regaining baseline function post-operatively than those who undergo surgeries that avoid this technique.[15] The same may be said for the benefits of mapping language areas with the cortical stimulation technique before a glioma resection. Assessing and minimizing the damage of operating on language-involved regions leads to the greater and faster return of overall language function.[16]
Despite the functional gain from preserving these eloquent cortical areas, benefit-to-risk factors are still considered. More complete tumor resection has been shown to possibly expand the life expectancy of glioma patients; however, increasing the amount of brain tissue removed may also cause a debilitating decrease in function. As such, cortical stimulation mapping aids in determining the maximum amount of tissue that can be removed while still maintaining the patient's quality of life.[17]
Vision
Mapping of the
For patients with glaucoma and optic nerve atrophy, existing retinal prostheses are not an option since the optic nerve is damaged, therefore a prosthesis using cortical stimulation is a remaining hope to offer some vision function. A cortical visual prosthesis is a promising subject of research because it targets neurons past the site of disease in most blind patients. However, significant challenges remain such as reproducibility in different patients, long-term effects of electrical stimulation, and the higher complexity of visual organization in the primary visual cortex versus that in the retina.[18]
Another site of research for a vision prosthesis using cortical stimulation is the optic nerve itself, which contains the nerve fibers responsible for the complete visual field. Research is still ongoing in this area and the small size of the optic nerve and the high density of nerve fibers are continuing challenges to this approach [18]
Cortical stimulation mapping vs. transcranial magnetic stimulation
Cortical stimulation mapping (CSM) is considered the gold standard for mapping functional regions of the brain to create a presurgical plan that maximizes the patient's functional outcome.[3] The history of beneficial outcomes and the amount of information already established about the CSM technique makes it advantageous in clinical and research applications. However, because it has the drawback of being an intraoperative technique, there is growing debate about its status as the preferred method. Instead, transcranial magnetic stimulation (TMS), a new[when?] procedure that does not carry the same amount of surgical risk, is being considered.[by whom?]
Transcranial magnetic stimulation has been gaining increasing interest as an alternative tool for studying the relationships between specific cortical areas and brain function, particularly because its non-invasive nature is advantageous over CSM.
Safety must also be considered with respect to both methods. So far,[when?] Food and Drug Administration (FDA) guidelines have only approved the use of TMS for the treatment of depression.[26] Although this technique has no known lasting side effects except for a few reported cases of induced seizures, it is still treated with caution due to its relative novelty in clinical use.[27] CSM has gained U.S. FDA approval for its uses regarding cortical stimulation mapping, especially in cases of seizure and glioma treatments, and for aiding in the placement of electrodes within the brain.[28]
References
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- ^ Food and Drug Administration (2009). "510(k) Summary: Nicolet Cortical Stimulator" (PDF).