Cortical Mapping
Goal of cortical mapping is to locate functional motor cortical surfaces with direct cortical stimulation, location of the primary sensory cortex with direct cortically recorded SSEPs (Phase Reversal) location of eloquent speech centers through direct cortical stimulation; localization of epileptic foci using electrocorticography; to avoid these areas during tumor resection, to identify and remove epileptic foci
Homunculus
Exposed Cortex
Tumors can often distort the cortical anatomy
Phase reversal to locate the central sulcus
Theories for phase reversal; dipole theory; due to the folding of the cortex in the central sulcus the cortical response generator (post-central gyrus) produces a positive-negative dipole across the central sulcus; Independent Generator Theory, separate sensory and motor generators exist as a result of ascending thalamocortical fibers activating thalamocortical fibers activating motor cortex via a synapse
Direct Cortical SSEP parameters; stimulating, contralateral median nerve and posterior tibial nerve, rep rate 4.47/sec, can be increased to 923/sec, 200 usec pulse, supramaximal stimulation. Recording, grid contacts referenced to Fpz, 10-10,000 Hz, 50-100 msec sweep, 100 mS may be superior, sensitivity at 1-20 uv/div, average as needed (usually < 100)
anesthesia for sensory-motor mapping; desire 0.5 MAC or less inhalational agent, 4/4 twitches, narcotics as needed, supplemental propofol infusion as necessary, dexmetitomidine (precedex) is also acceptable
Motor Cortex Mapping; purpose is to positively identify cortical tissue essential to motor function. METHOD; stimulate the cortex, record muscle potentials in the periphery, label the cortex.
Two techniques of cortical stimulation for motor mapping; ojemann technique, long pulse train, low frequency; short pulse train, high frequency train similar to tcmep, described by tanaguchi 1993
Ojemann technique; long pulse train 1-4 seconds, biphasic pulses (to minimize deposition of toxic metal salts) of 1 ms duration, 50-60 Hz, 1-20 mA peak to peak intensity
Ojemann stimulator; delivers constant current biphasic pulses, runs on four 9 volt batteries, can control frequency, duration and intensity, probe uses banana plug connectors
Electrical safety of cortical stimulation; acceptable charge density per phase is 40 uC/cm2
if the patient has a seizure, ice-cold irrigation of the exposed cortex will stop the electrical activity (without harm) and allow mapping to continue, irrigation should be applied for 5-10 seconds, pharmaceutical intervention (barbiturates or paralytics) will delay mapping and may reduce reliability.
Cortical Language Function; wernicke's area, responsible for recognition of the words we see and hear, a lesion to this area would have fluent speech and grammatical construction, inability to find the correct words, may omit words, substitute words or use words without precise meanings broca's area responsible for production of the words we speak, a lesion would result in good comprehension, difficulty producing spoken language, executive aphasia, many prepositions, nouns and verbs are deleted, parietal operculum
Direct Cortical EEG recording methods
Variability of language sites; patients will typically have 2-3 cortical sites essential to language function; some will have several, considerable variation in the location of areas important to language has been described; patients with a non english primary language will have separate language areas and should be mapped in both languages
Hemisphere Dominance for language; 95% of the population is left-hemisphere dominant, essentially all right handed people have left hemisphere language dominance, 1/2 of the left handed people have left hemisphere language dominance
Terminology; aphasia; inability to speak; anomia, inability to name objects; agnosia failure to recognize stimuli; apraxia, inability to carry out movements; alexia, inability to read
Homunculus
Exposed Cortex
Tumors can often distort the cortical anatomy
Phase reversal to locate the central sulcus
Theories for phase reversal; dipole theory; due to the folding of the cortex in the central sulcus the cortical response generator (post-central gyrus) produces a positive-negative dipole across the central sulcus; Independent Generator Theory, separate sensory and motor generators exist as a result of ascending thalamocortical fibers activating thalamocortical fibers activating motor cortex via a synapse
Direct Cortical SSEP parameters; stimulating, contralateral median nerve and posterior tibial nerve, rep rate 4.47/sec, can be increased to 923/sec, 200 usec pulse, supramaximal stimulation. Recording, grid contacts referenced to Fpz, 10-10,000 Hz, 50-100 msec sweep, 100 mS may be superior, sensitivity at 1-20 uv/div, average as needed (usually < 100)
anesthesia for sensory-motor mapping; desire 0.5 MAC or less inhalational agent, 4/4 twitches, narcotics as needed, supplemental propofol infusion as necessary, dexmetitomidine (precedex) is also acceptable
Motor Cortex Mapping; purpose is to positively identify cortical tissue essential to motor function. METHOD; stimulate the cortex, record muscle potentials in the periphery, label the cortex.
Two techniques of cortical stimulation for motor mapping; ojemann technique, long pulse train, low frequency; short pulse train, high frequency train similar to tcmep, described by tanaguchi 1993
Ojemann technique; long pulse train 1-4 seconds, biphasic pulses (to minimize deposition of toxic metal salts) of 1 ms duration, 50-60 Hz, 1-20 mA peak to peak intensity
Ojemann stimulator; delivers constant current biphasic pulses, runs on four 9 volt batteries, can control frequency, duration and intensity, probe uses banana plug connectors
Electrical safety of cortical stimulation; acceptable charge density per phase is 40 uC/cm2
if the patient has a seizure, ice-cold irrigation of the exposed cortex will stop the electrical activity (without harm) and allow mapping to continue, irrigation should be applied for 5-10 seconds, pharmaceutical intervention (barbiturates or paralytics) will delay mapping and may reduce reliability.
Cortical Language Function; wernicke's area, responsible for recognition of the words we see and hear, a lesion to this area would have fluent speech and grammatical construction, inability to find the correct words, may omit words, substitute words or use words without precise meanings broca's area responsible for production of the words we speak, a lesion would result in good comprehension, difficulty producing spoken language, executive aphasia, many prepositions, nouns and verbs are deleted, parietal operculum
Direct Cortical EEG recording methods
Variability of language sites; patients will typically have 2-3 cortical sites essential to language function; some will have several, considerable variation in the location of areas important to language has been described; patients with a non english primary language will have separate language areas and should be mapped in both languages
Hemisphere Dominance for language; 95% of the population is left-hemisphere dominant, essentially all right handed people have left hemisphere language dominance, 1/2 of the left handed people have left hemisphere language dominance
Terminology; aphasia; inability to speak; anomia, inability to name objects; agnosia failure to recognize stimuli; apraxia, inability to carry out movements; alexia, inability to read