Transcranial Motor Evoked Potentials
corticospinal/pyramidal tract and motor neurons
transcranial electrical stimulation, record from the upper extremity and lower extremity muscles
large signals, no averaging required; sometimes facilitation
produces gross movement
sensitive to spinal ischemia, may need to increase blood pressure
corticospinal tract distribution UE>LE, distal>proximal muscles
most constraint on anesthetic management of commonly used monitoring techniques
transcranial electrical stimulation, record from the upper extremity and lower extremity muscles
large signals, no averaging required; sometimes facilitation
produces gross movement
sensitive to spinal ischemia, may need to increase blood pressure
corticospinal tract distribution UE>LE, distal>proximal muscles
most constraint on anesthetic management of commonly used monitoring techniques
*LH*
Transcranial motor evoked potentials
traditional techniques for evaluating motor function; the stagnara wake-up test, neurogenic motor evoked potentials, transcranial electrical and magnetic motor
Wake-Up Test; the wake up test was the original test of spinal cord function during spinal surgery, the patient is brought to a level of consciousness and asked to move the arms or legs on command. he disadvantages is it is somewhat traumatic for the patient, it cannot be repeated during the surgery, intervention may not be effective because the injury occurred much earlier than the wake-up test
Neurogenic Motor Evoked Potentials; popularized in the early 90's; technique involves spinal cord stimulation rostral to the surgical site while recording the descending potential from the nerve, sensitive to spinal cord injury, multiple studies have demonstrated this descending potential is largely an antidromic sensory conducted response
Antidromic sensory potentials are unimpeded while motor conduction must conduct through a synapse that is being suppressed by anesthesia
orthodromic; nerve action potential traveling from periphery toward the head
antidromic goes away from where supposed to
direct spinal stimulation, spinous process, lamina
normal dnep, pf and cortical
dnep parameters; stimulus repetition rate 4.47/sec, stimulus pulse duration 200us, stimulus intensity, 25-75 mA for translaminar (JO5 spinal needles) 0-25mA for epidural stimulation
Record Cortical Cz'-Fpz and PF (L&R) filters 30-500 Hz and 10-2000 Hz, sweep of 50 ms or 100 ms
Involve stimulation of motor fibers in the cerebrum recording is from the spinal cord or muscle; considered to be pure motor potentials since any somatosensory fibers activated in the brain will not be able to conduct past the synapses in the thalamus or medulla
Rationale for TCMEP; SSEPs only monitor dorsal column function, posterior spinal artery
motor tracts have separate blood supply and can be independently injured; anterior spinal artery
other techniques such as spinally elicited descending evoked potentials are not specific to motor tracts
spinal cord vascular supply
anatomy of the pyramidal system; response in the motor cortex which lies anterior to the central sulcus in the precentral gyrus of the frontal lobe; corticospinal tract fibers have their cell body in the motor cortex; the axons descend through the white matter of the brain, through the posterior limb of the internal capsule
the fibers then enter the midbrain at the level of the cerebral peduncle and continue through the pons and the pyramid of the medulla
anatomy of the pyramidal system; upon descending to the level of the medulla, 90% of the corticospinal fibers decussate to form the lateral corticospinal tract; the remaining fibers continue as the anterior corticospinal tract
functional areas of the brain; pyramidal motor pathways; lateral corticospinal tract 90% anterior corticospinal tract 10%
lateral corticospinal tract fibers terminate at the anterior horn of each level in the spinal cord where they form synaptic connections to lower motor neurons; decussation at the medulla to synapse in the anterior horn to the lower motor neuron
anterior corticospinal tract fibers cross within the spinal cord segments and form synaptic connections to lower motor neurons; these fibers only contribute to proximal muscles of the spine and pelvis; decussation at the spinal cord
TCMEP recordings; transcranial motor evoked potential protocol; transcranial electrical stimulation from a single stimulus, from a train of 5 stimuli from a train of 9 with an inter stimulus pulse duration interval
the d-wave is recorded from the spinal epidural space and represents direct activation of descending corticospinal fibers, the d-wave in a moderately or deeply anesthetized patient
the d wave; requires placement of spinal epidural recording electrode, should be monitored when access to epidural space is available (Spinal Cord Tumor or AVM), amplitude of D-wave correlates with the number of synchronous functional corticospinal tract fibers in the spinal cord, the d-wave is impervious to anesthetic effects
the d wave amplitude and latency depend on the spinal level of recording the number of CST axons decrease the lower in the cord; the D-wave may be too small to measure below 10
The D-Wave latency is also affected by stimulus intensity; and increased intensity activates the descending axons at a deeper level
Effect of increasing stimulus
D wave parameters; stimulating use standard TCMEP stimulus sites C3-4, C1-2 etc using both polarities; 50-500uS duration Single Pulse stimulation; a few averages may be needed for small responses; obtain d-wave as frequently as needed
Recording Recording Sweep set at 20mS (2mS/div) or 50 ms as necessary, set filter settings at 10-1000 Hz; Anesthesia no restrictions on use of anesthetic drugs or neuromuscular blockade
Monitoring the D wave record caudal to the surgical site (A rostral electrode could also be useful); acquire the D-wave frequently during tumor resections (as high as 1/sec); single pulse produces minimal movement, test both polarities of stimulation (may represent left and right dominant D-waves))
D-Wave Changes; amplitude is the most important criteria; a 30% or greater reduction should be reported; > 50% reduction is correlated with post-surgical motor deficits; intervention should include pausing surgical activity, copious irrigation, elevating BP or stopping surgery
I waves are recorded from the spinal epidural space, represents indirect activation of descending corticospinal fibers via trans-synaptic neuronal activation in the cortex
spinal cord recordings to cortical stimulation at various depths
cortical evoked spinal responses to cortical cooling
i waves recorded (along with d-waves) from the epidural space; i-waves are a series of waves following the d-wave that are a result of cortical neuronal activation at multiple sites within the network; i-waves are extremely susceptible to anesthesia (present only in lightly anesthetized or awake patients), presence of I-waves facilitates successful activation of the lower motoneuron
How do I-waves facilitate activation of the lower motoneurons? Anser: Temporal Summation at the Anterior Horn Synapse; corticospinal fiber to lower motoneuron
What if the patient is moderately anesthetized? Answer then there are no I waves present to facilitate activation of the lower motoneuron (ie No muscle response with single pulse)
how then do we elicit a muscle response in an anesthetized patient? Answer: by producing a series of D-waves which is achieved by using a train of stimuli;
Muscle tcmep; comple multiphasic response recorded from distal muscles, elicited using a train of stimulus puslses, represents motor conduction from cortex to muscle, sensitive to anesthetic drugs (affected at both cortex and anterior horn); sensitive to neuromuscular blocking agents
stimulus electrode placement; c3 & c4; alternate montages c1&C2, Cz and Fz; only lower MEPs; C1/C3 and C2/C4, C3, Cz, C4 to large forehead cathode
on cnim corkscrews because they have a larger surface area
tcmep stimulus polarity; motor activation is most favorable at the site of the anode (due to the orientation of the electrical field of the stimulus path), both stimulus polarities should be used during monitoring, occasionally one polarity will activate all four extremities
* show the change of C3+ to C4- and from C4+ to C3-, on page 15 in LH towards the back right side, left side
TCMEP CMAP Stimulus Parameters; single train stimulus of 2-7 pulses, ISI of 2-4 ms (250-500Hz), 50-500 uS pulse duration, intensity up to 1000 V, stimulus parameters should be optimized
effect of pulse number on tcmep, effect of number of pulses, the number of pulses
cmap recording parameters; 100 ms sweep, 10-1000 hz filters, record from the hand and leg, or foot
anesthesia for cmap; tiva is optimal; propofol, remifentanil; when TIVA is not available use 0.5 MAC halogenated agent, propofol, narcotics as needed; absence of neuromuscular blockade is optimal, although cmap can still be recorded with as low as 1-2 twitches
monitoring cmap; collect responses periodically during case, collect responses frequently around critical events; check tcmep whenever changes are noted in ssep's
interpreting cmap; criterion is primarily absence vs presence due to high intra patient variability; use hand responses as a control; report absence of response or a dramatic decrease > 90% associated with a surgical even (not technical or anesthetic cause)
stimulation threshold technique;
calancies's threshold level monitoring technique; advantages uses minimal stimulus to obtain response; threshold shift is a common early sign of problems; disadvantages; each muscle has its own threshold to be monitored; threshold can vary significantly due to non surgical factors (anesthesia, physiologic, technical)
troubleshooting absence of tcmep in baseline; check train of four and anesthetic levels, increase intensity to maximum, work with anesthesia to reduce depth and increase number of twitches, modify stimulus parameters (pulse rate, number of pulses, pulse duration), stimulate several trains in a row, adjust head electrodes, record from the foot, raise blood pressure, ensure electrodes have not been dislodged (impedance test)
always get patient consent form signed by the patient or legal guardian, explain what the monitoring will involve in lamens terms, explain the potential complications (even though they are unlikely) and how we deal with those contingencies; if you cannot get a signature, surgeon must authorize us to perform tcmeps with the knowledge that no consent was obtained. this must be documented in the log
safety/patient factors; history of seizure, epileptics may be at higher rist for seizure; history of previous craniotomies; presence of burr holes or other skull defects; presence of intracranial metal implants; pacemaker, defibulator, electrical stimulator
placement of bite block; laceration of tongue can occur with stimulation du to activation of temporalis; patient movement; surgeon must be aware of potential movement in the field caused by tcmep stimulation get surgeon's permission to stimulate; patients with unstable cervical spines may be at risk, watch out for excessive movement; seizure monitor eeg for evidence of seizure activity
tcmeps are safe; only 1 report of any tcmep induced seizure out of 10,000 + cases; tcmep stimulation is two orders of magnitude smaller than used in electroconvulsive therapy; kindling of seizures from electrical brain stimulation involves a very different set of stimulus parameters performed repetitively over a period of days or weeks; bite blocks will reduce incidence of tongue lacerations
effect of IV agents on single pulse mep; propofol big, midazolam big, etomidate not so big, fentanyl not very much if at all
effect of isoflurane on 1 2 and 3 pulse tcmeps; need more stimulus pulses
Transcranial motor evoked potentials
traditional techniques for evaluating motor function; the stagnara wake-up test, neurogenic motor evoked potentials, transcranial electrical and magnetic motor
Wake-Up Test; the wake up test was the original test of spinal cord function during spinal surgery, the patient is brought to a level of consciousness and asked to move the arms or legs on command. he disadvantages is it is somewhat traumatic for the patient, it cannot be repeated during the surgery, intervention may not be effective because the injury occurred much earlier than the wake-up test
Neurogenic Motor Evoked Potentials; popularized in the early 90's; technique involves spinal cord stimulation rostral to the surgical site while recording the descending potential from the nerve, sensitive to spinal cord injury, multiple studies have demonstrated this descending potential is largely an antidromic sensory conducted response
Antidromic sensory potentials are unimpeded while motor conduction must conduct through a synapse that is being suppressed by anesthesia
orthodromic; nerve action potential traveling from periphery toward the head
antidromic goes away from where supposed to
direct spinal stimulation, spinous process, lamina
normal dnep, pf and cortical
dnep parameters; stimulus repetition rate 4.47/sec, stimulus pulse duration 200us, stimulus intensity, 25-75 mA for translaminar (JO5 spinal needles) 0-25mA for epidural stimulation
Record Cortical Cz'-Fpz and PF (L&R) filters 30-500 Hz and 10-2000 Hz, sweep of 50 ms or 100 ms
Involve stimulation of motor fibers in the cerebrum recording is from the spinal cord or muscle; considered to be pure motor potentials since any somatosensory fibers activated in the brain will not be able to conduct past the synapses in the thalamus or medulla
Rationale for TCMEP; SSEPs only monitor dorsal column function, posterior spinal artery
motor tracts have separate blood supply and can be independently injured; anterior spinal artery
other techniques such as spinally elicited descending evoked potentials are not specific to motor tracts
spinal cord vascular supply
anatomy of the pyramidal system; response in the motor cortex which lies anterior to the central sulcus in the precentral gyrus of the frontal lobe; corticospinal tract fibers have their cell body in the motor cortex; the axons descend through the white matter of the brain, through the posterior limb of the internal capsule
the fibers then enter the midbrain at the level of the cerebral peduncle and continue through the pons and the pyramid of the medulla
anatomy of the pyramidal system; upon descending to the level of the medulla, 90% of the corticospinal fibers decussate to form the lateral corticospinal tract; the remaining fibers continue as the anterior corticospinal tract
functional areas of the brain; pyramidal motor pathways; lateral corticospinal tract 90% anterior corticospinal tract 10%
lateral corticospinal tract fibers terminate at the anterior horn of each level in the spinal cord where they form synaptic connections to lower motor neurons; decussation at the medulla to synapse in the anterior horn to the lower motor neuron
anterior corticospinal tract fibers cross within the spinal cord segments and form synaptic connections to lower motor neurons; these fibers only contribute to proximal muscles of the spine and pelvis; decussation at the spinal cord
TCMEP recordings; transcranial motor evoked potential protocol; transcranial electrical stimulation from a single stimulus, from a train of 5 stimuli from a train of 9 with an inter stimulus pulse duration interval
the d-wave is recorded from the spinal epidural space and represents direct activation of descending corticospinal fibers, the d-wave in a moderately or deeply anesthetized patient
the d wave; requires placement of spinal epidural recording electrode, should be monitored when access to epidural space is available (Spinal Cord Tumor or AVM), amplitude of D-wave correlates with the number of synchronous functional corticospinal tract fibers in the spinal cord, the d-wave is impervious to anesthetic effects
the d wave amplitude and latency depend on the spinal level of recording the number of CST axons decrease the lower in the cord; the D-wave may be too small to measure below 10
The D-Wave latency is also affected by stimulus intensity; and increased intensity activates the descending axons at a deeper level
Effect of increasing stimulus
D wave parameters; stimulating use standard TCMEP stimulus sites C3-4, C1-2 etc using both polarities; 50-500uS duration Single Pulse stimulation; a few averages may be needed for small responses; obtain d-wave as frequently as needed
Recording Recording Sweep set at 20mS (2mS/div) or 50 ms as necessary, set filter settings at 10-1000 Hz; Anesthesia no restrictions on use of anesthetic drugs or neuromuscular blockade
Monitoring the D wave record caudal to the surgical site (A rostral electrode could also be useful); acquire the D-wave frequently during tumor resections (as high as 1/sec); single pulse produces minimal movement, test both polarities of stimulation (may represent left and right dominant D-waves))
D-Wave Changes; amplitude is the most important criteria; a 30% or greater reduction should be reported; > 50% reduction is correlated with post-surgical motor deficits; intervention should include pausing surgical activity, copious irrigation, elevating BP or stopping surgery
I waves are recorded from the spinal epidural space, represents indirect activation of descending corticospinal fibers via trans-synaptic neuronal activation in the cortex
spinal cord recordings to cortical stimulation at various depths
cortical evoked spinal responses to cortical cooling
i waves recorded (along with d-waves) from the epidural space; i-waves are a series of waves following the d-wave that are a result of cortical neuronal activation at multiple sites within the network; i-waves are extremely susceptible to anesthesia (present only in lightly anesthetized or awake patients), presence of I-waves facilitates successful activation of the lower motoneuron
How do I-waves facilitate activation of the lower motoneurons? Anser: Temporal Summation at the Anterior Horn Synapse; corticospinal fiber to lower motoneuron
What if the patient is moderately anesthetized? Answer then there are no I waves present to facilitate activation of the lower motoneuron (ie No muscle response with single pulse)
how then do we elicit a muscle response in an anesthetized patient? Answer: by producing a series of D-waves which is achieved by using a train of stimuli;
Muscle tcmep; comple multiphasic response recorded from distal muscles, elicited using a train of stimulus puslses, represents motor conduction from cortex to muscle, sensitive to anesthetic drugs (affected at both cortex and anterior horn); sensitive to neuromuscular blocking agents
stimulus electrode placement; c3 & c4; alternate montages c1&C2, Cz and Fz; only lower MEPs; C1/C3 and C2/C4, C3, Cz, C4 to large forehead cathode
on cnim corkscrews because they have a larger surface area
tcmep stimulus polarity; motor activation is most favorable at the site of the anode (due to the orientation of the electrical field of the stimulus path), both stimulus polarities should be used during monitoring, occasionally one polarity will activate all four extremities
* show the change of C3+ to C4- and from C4+ to C3-, on page 15 in LH towards the back right side, left side
TCMEP CMAP Stimulus Parameters; single train stimulus of 2-7 pulses, ISI of 2-4 ms (250-500Hz), 50-500 uS pulse duration, intensity up to 1000 V, stimulus parameters should be optimized
effect of pulse number on tcmep, effect of number of pulses, the number of pulses
cmap recording parameters; 100 ms sweep, 10-1000 hz filters, record from the hand and leg, or foot
anesthesia for cmap; tiva is optimal; propofol, remifentanil; when TIVA is not available use 0.5 MAC halogenated agent, propofol, narcotics as needed; absence of neuromuscular blockade is optimal, although cmap can still be recorded with as low as 1-2 twitches
monitoring cmap; collect responses periodically during case, collect responses frequently around critical events; check tcmep whenever changes are noted in ssep's
interpreting cmap; criterion is primarily absence vs presence due to high intra patient variability; use hand responses as a control; report absence of response or a dramatic decrease > 90% associated with a surgical even (not technical or anesthetic cause)
stimulation threshold technique;
calancies's threshold level monitoring technique; advantages uses minimal stimulus to obtain response; threshold shift is a common early sign of problems; disadvantages; each muscle has its own threshold to be monitored; threshold can vary significantly due to non surgical factors (anesthesia, physiologic, technical)
troubleshooting absence of tcmep in baseline; check train of four and anesthetic levels, increase intensity to maximum, work with anesthesia to reduce depth and increase number of twitches, modify stimulus parameters (pulse rate, number of pulses, pulse duration), stimulate several trains in a row, adjust head electrodes, record from the foot, raise blood pressure, ensure electrodes have not been dislodged (impedance test)
always get patient consent form signed by the patient or legal guardian, explain what the monitoring will involve in lamens terms, explain the potential complications (even though they are unlikely) and how we deal with those contingencies; if you cannot get a signature, surgeon must authorize us to perform tcmeps with the knowledge that no consent was obtained. this must be documented in the log
safety/patient factors; history of seizure, epileptics may be at higher rist for seizure; history of previous craniotomies; presence of burr holes or other skull defects; presence of intracranial metal implants; pacemaker, defibulator, electrical stimulator
placement of bite block; laceration of tongue can occur with stimulation du to activation of temporalis; patient movement; surgeon must be aware of potential movement in the field caused by tcmep stimulation get surgeon's permission to stimulate; patients with unstable cervical spines may be at risk, watch out for excessive movement; seizure monitor eeg for evidence of seizure activity
tcmeps are safe; only 1 report of any tcmep induced seizure out of 10,000 + cases; tcmep stimulation is two orders of magnitude smaller than used in electroconvulsive therapy; kindling of seizures from electrical brain stimulation involves a very different set of stimulus parameters performed repetitively over a period of days or weeks; bite blocks will reduce incidence of tongue lacerations
effect of IV agents on single pulse mep; propofol big, midazolam big, etomidate not so big, fentanyl not very much if at all
effect of isoflurane on 1 2 and 3 pulse tcmeps; need more stimulus pulses