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EEG and Evoked Potentials
Speaker : Dr. Tushar kumar
What is EEG
It is the study , observation and recording
of brain, which are measureable expression
of electrical activity of brain.
It could be Unipolar or bipolar
Unipolar : where reference point is fixed
Bipolare : voltage recorded lies near to
scalp or brain.
EEG
Standard EEG:
has 10 – 20 standardized placement system.
Electrodes placed on anatomic location.
8 – 20 wave lines are graphed.
EEG signals
There are 3 basic parameters that are recorded
in EEG waves:
1. Amplitude : measurement of electrical
height. Ranges from 0 -200 μV.
2. Frequency : it is no of times per second the
wave touches the zero voltage line. It is the
degree of activity of cerebral cortex.
3. Time : plotted on x axis
Type of waves
1. Alpha waves: 8 – 13 Hz
occurs during awake and quiet resting state.
2. Beta waves: > 13 Hz
excited and specific mental activity.
3. Theta waves: 4 -8 Hz
4. Delta waves: 0 – 4 Hz
occurs during deep sleep, infancy, organic
brain disease.
EEG ! How does it look like?
EEG interpretations
• Sudden development of delta waves
coincident with surgical manuver  injury
warning
• In penumbra region, EEG poorly predict
brain damage
• Anesthetics & hypothermia causes EEG
changes  multifactorial interpretation
Uses of EEG
1. Monitoring the functional integrity of neural
structures that may be at risk.
2. Identification of specific structures
3. Monitoring the effects of specific agents and
other drugs that affect nervous system
4. Diagnosis and monitoring of
pathophysiological conditions.
Anaesthesia and EEG
a. Ketamine : rhythmic theta activity…. 4 -8
Hz increased epileptiform activity.
b. Opioid : dose related decrease in frequency.
but if further doses are not given then
there is return of higher wave occurs.
Epileptiform waves occur with large doses
of opioid such as during fentanyl induction.
c. Nitrous oxide : when used in combination
with other anaesthetic agents it depresses
EEG pattern.
Anaesthesia and EEG….
d. Isoflurane: depression in EEG pattern with
increase in depth of anaesthesia. Electrical
silence at 2 – 2.5 MAC.
e. Halothane: smaller degree of EEG
suppression. At 3 – 4 MAC EEG suppression
occurs to greater extent but cardiac depression
also occurs.
f. Sevoflurane: smaller degree of EEG
suppression but greater epileptiform activity.
Surgery and EEG
1. No change in anaesthetic technique should be
made during critical period of monitoring.
for eg. Induced hypotension, carotid
clamping, aneurysm clipping.
2. Avoid major changes in anaesthetic gas level/
bolus opioid or benzodiazepines near time of
increases ischemic risk.
3. If drug must be given then monitor it with
Evoked Potential.
Why Evoked potentials…
The EEG machine records tiny electrical
voltages from the brain, representing the
averaged electrical activity of millions of
neurons. ……… non specific.
But evoked potentials are triggered.
They are very specific.
Evoked potentials
What are evoked potentials:
• The electrical potentials produced after
stimulation of specific neural tracts.
• The recorded plot of voltage versus time
Initial artifact representing the stimulation
of the tract followed by the neuronal
response, which is recorded as a series of
peaks and valleys
Evoked potentials
Many times signals are small  an amplifier
reduces the electrical noise by subtracting
the signal at a referenceelectrode from the
recording electrode.
• Filtering of this signal we focus on the
evoked response of interest.
Evoked potentials
• The evoked response always occurs at a set
time after stimulation.
• Summating all those responses increases
the time-locked response, whereas the
background activity acts as a random signal
and averages out to zero.
• So we get only the evoked potential
response.
CLASIFICATION OF EVOKED POTENTIALS
SENSORY
EVOKED
POTENTIALS
MOTOR
EVOKED
POTENTIALS
EVENT
RELATED
POTENTIALS
VISUAL
EVOKED
POTENTIAL
AUDITORY
EVOKED
POTENTIAL
SOMATOSENSORY
EVOKED
POTENTIAL
EVOKED POTENTIALS
Visual Evoked potential:
Visual evoked potential:
Visual evoked potential:
Primary visual system is arranged to emphasize
the edges and movements so shifting patterns with
multiple edges and contrasts are the most
appropriate method to assess visual function.
Flash VEP
• Greater variability of response with multiple
positive and negative peaks
• Primarily use when an individual cannot
cooperate or for gross determination of visual
pathway. Ex in infants / comatose patients
VEP response:
Clinical application of VEP:
VEPs can be used to monitor:
• the anterior visual pathways during
a. craniofacial procedures,
b. pituitary surgery,
c. surgery in the retrochiasmatic visual
tracts and occipital cortex.
Their specificity ranges from 95% to > 99%
Clinical application of VEP:
Other medical applications:
1. Multiple sclerosis
2. Demylinating disorders
3. Axonal loss disorders
4. Migraine headaches
VEP
Limitations of VEP
Less useful in surgical monitoring.
Do not measure the pathways of useful
clinical vision
The large, bulky “goggles” usually used for
stimulation pose technical problems,
The bilateral nature of the response may
obscure focal changes,
Anaesthetic sensitivity makes recording
difficult.
Somatosensory evoked
potentials (SSEP) :
SSEP :
• Evoked potentials of sensory nerves in the
peripheral & central nervous system
• Used to diagnose nerve damage or
degeneration in the spinal cord
• Can distinguish central Vs peripheral nerve
lesion
Anatomical & Physiological basis
of SSEP
SENSE ORGANS – PACINIAN AND GOLGI COMPLEXES
IN JOINTS, MUSCLES AND TENDONS
DORSAL ROOT GANGLIA
TYPE A FIBRES
GRACILE AND CUNEATE Nu. IN MEDULLA
Nu POSTEROLATERALIS OF THALAMUS
MEDIAL LEMNISCUS
SENSORY CORTEX
THALAMOPARIETAL
RADIATYIONS
Dorsal column pathways:
They carry sensory inputs to
brain for vibration and
proprioception.
First synapse near
the nucleus cuneatus and nucleus
gracilis and then decussates near
the cervicomedullary junction,
ascending via the contralateral
medial lemniscus.
Second synapse occurs in the
ventroposterolateral nucleus of
the thalamus, from which it
projects to the contralateral
sensory cortex
SSEP:
Peripheral sensory nerves are stimulated
electrically and the response is measured
along the sensory pathway.
The large, mixed motor and sensory nerves
are usually monitored:
1. Median (C6-T1),
2. ulnar (C8-T1),
3. Common peroneal (L4-S1),
4. posterior tibial (L4-S2) nerves.
SSEP
Median SEP:
Recording electrodes are placed at the
following locations:
• Erb’s point on each side (EP)
• Over the second or fifth cervical spine
process (C2S or C5S)
• Scalp over the contralateral cortex (CPc)
and ipsilateral cortex (CPi)
• Noncephalic electrode (Ref)
SSEP
Stimulation activates predominantly the large-
diameter nerve fibers.
 Anatomically, peripheral nerve stimulation
produces both orthodromic and antidromic
neural transmission.
The orthodromic motor stimulation elicits a
muscle response, which is seen as a twitch
(verifying stimulation), and the orthodromic
sensory stimulation produces the SSEP.
SSEP:
• STIMULUS: Electrical – square wave pulse by
surface or needle electrode
• DURATION: 100 – 200 msec at a rate of 3 – 7
/ sec
• INTENSITY: for producing observable
muscle twitch or 2.5 – 3 times the threshold for
sensory stimulus.
Unilateral stimulation for localization
Bilateral stimulation for intra-operative
monitoring
Stimulus is given as vibration that ascends via
ipsilateral dorsal column.
Monitoring of SSEP:
Monitoring is estimated to reduce the
morbidity in spinal surgery by 50% to 80%.
sensitivity of 52% and a specificity of 100%
 Blood flow: clinical functional neurologic
findings become abnormal at a cortical blood
flow of about 25 mL/min/100 g, but the SSEP
remains normal until cortical blood flow is
reduced to about 20 mL/min/100 g. SSEP
responses are lost at a local blood flow of
between 15 and 18 mL/min/100 g.
1. Monitoring during temporary clipping in
aneurysm surgery has shown that a very
prompt loss of cortical SSEP response (less
than 1 minute after clipping) is associated
with development of permanent neurologic
deficit.
2. SSEP in spinal surgeries has become standard
care of monitoring.
3.Monitoring during carotid endarterectomy.
Intraoperative SSEP changes are used as an
indication for shunt placement and to predict
postoperative morbidity.
Clinical application of
SSEP
SSEP
Auditory evoked potentials
AUDITORY BRAINSTEM RESPONSES
I. organ of Corti and extracranial cranial nerve VII; II. cochlear
nucleus; III. superior olivary complex; IV. lateral
lemniscus; V. inferior colliculus; VI. medial geniculate body;
VII. auditory radiation.
• Breif electrical pulse “ click”
• Intensity – 65 – 70 dB above threshold
• Rate – 10 – 50 clicks / sec
• Averaging of 1000 – 4000 Hz range
• Read after first 10 msec of stimulus
AUDITORY BRAINSTEM RESPONSES
Findings:
• Absolute waveform latencies
• Interpeak latencies ( I – III, I – V & III – V )
• Amplitude ratio of wave V / I
In general I – V peak are preserved then hearing
is preserved.
AUDITORY BRAINSTEM RESPONSES
Clinical utility of AER:
1. Acoustic neurinoma
2. Vertebral-basilar aneurysm
3. Cerebellar vascular malformations
4. Other posterior fossa procedure , eg. during
microvascular decompression for relief of
hemifacial spasm or trigeminal neuralgia.
Clinical utility of AER:
Electromyographic monitoring
• EMG: Muscle responses to monitor neural
tracts
• Two types of EMG monitoring :
1. recording spontaneous activity
2. recording responses subsequent to stimulation
of the motor nerves or motor pathways
Clinical utility: EMG
All cranial nerves with a motor component
are monitored from their EMG activity in
their respectively innervated muscles.
1. Facial nerve monitoring: most common
cranial nerve monitored (e.g., acoustic
neuroma)
2. Cranial nerves IX and X : in head & neck
surgeries  when injured produces
cardiovascular changes.
Continue….
3. Cranial nerve XI: harmful head movement due
to trapezius and sternocleidomastoid ms.
4. Vagus nerve (CN X) monitoring by vocal cord
EMG is for skull base (large brainstem tumors)
and anterior neck procedures.
Monitoring of the recurrent laryngeal and
superior laryngeal branches is done with
electrodes in the cricothyroid or vocalis muscles
or contact electrodes mounted on an ET Tube.
Cont…
ET tube with
electrodes
Motor evoked potentials
More sensitive to ischemic vascular insults.
MEP monitoring has become important during
surgery for:
1. Correction of axial skeletal deformity,
2. Intramedullary spinal cord tumors,
3. Intracranial tumors,
4. Vascular lesions.
5. Assessment of outcome in stroke and spinal
cord function during thoracoabdominal
• The MEP allows rapid detection of ischemia
because the gray matter, with its higher
metabolic rate, is more sensitive to
hypoperfusion.
During middle and anterior cerebral aneurysm
clipping several neural structures in the motor
pathway are at risk for hypoperfusion.
The sensitivity and specificity of MEP is 100%
and 97.4%
Motor evoked potentials
MEPs can :
1. Identify vasospasm
2. Incorrect placement of the aneurysm clip,
3. Inadequate clip placement.
KINDLING: Inherent risk with MEP.
it is cortical thermal injury.
Motor evoked potentials
Evoked potentials & Anaesthesia
Monitoring of EP: The anesthesiologist must
consider the various effects of anaesthesia
management on the different modalities used.
In general, the inhalational anaesthetic agents
which have drug effects at multiple synaptic
receptor types appear to have the most
profound effect on monitoring.
A. SSEP: Volatile anaesthetics produce an increase
in latency and a decrease in amplitude of cortical
sensory responses.
Continue…
B. MEP: No synapses are involved in the
production of the D wave, so recordings of the
D wave in the epidural space are altered little
by anaesthetics.
I waves are generated by synaptic mechanisms,
so they are progressively reduced by increasing
doses of anaesthetic agents.
Nitrous oxide (N2O) also produces amplitude
reduction and latency increases in cortical
sensory responses or MEP responses when
used alone or when combined with
halogenated inhalational agents or opioid
agents.
Opioids cause only mild depression of all
responses, with loss of late sensory evoked
response peaks. ( at sedative dose)
Continue…
Ketamine : subcortical and peripheral responses
are also minimal.
Preferred drug for monitoring responses that are
usually difficult to record during anaesthesia
and in children.
Use of ketamine with propofol reduces the
depressant effect of propofol while providing
an enhancement effect on responses.
Continue…
Barbiturates and benzodiazepines:
• Thiopental and midazolam produce mild
depression of cortical sensory responses but
long-lasting depression of MEPs.
• Hence not good for MEP monitoring.
• The ABR is virtually unaffected by doses of
phenobarbital that produce coma, and the
SSEP is unaffected at doses that produce a
silent EEG.
• so to monitor with SSEP is best.
Continue…
• Etomidate Produces an amplitude
increase of cortical sensory components
following injection, with no changes in
subcortical and peripheral sensory
responses. As consistent with myoclonus.
• SSEP recording commonly done but not
MEP monitoring.
Continue…
Propofol Most commonly used sedative
component of total intravenous anesthesia
when SSEPs and MEPs are monitored.
• Induction produces amplitude depression
in cortical SSEPs, with rapid recovery after
termination of infusion.
• Recordings in the epidural space are
unaffected, consistent with the site of
anaesthetic action of propofol in the cerebral
cortex.
Continue…
Droperidol and Dexmedetomidine
• Have minimal effects on responses
Dexmedetomidine has been successfully
used with MEP.
Continue…
Impairs or prevents MEP and EMG
monitoring.
Muscle relaxants are generally thought to have
no effect on the SSEPs
Partial NMB reduces the amplitude of motor
unit potentials and might therefore change the
ability to detect impending axonal injury from
nerve irritation.
Effect of neuromuscular blocking
agents
Cont….
It is preferred to avoid NMB after
anaesthesia induction and during EMG
and MEP monitoring.
Cont…
Effect of isoflurane on VEP
Effects on SSEP
Conclusion:
Evoked potential monitoring require
training, financial obligations and not 100
percent full proof in minimizing
complications.
Have risk involved.
On the other hand they have been shown to
be cost effective and have become part of
routine management.
For many surgeons and anesthesiologists,
these methods have become an indispensable
intraoperative diagnostic tool.
THANK
YOU

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EEG & Evoked potentials

  • 1. EEG and Evoked Potentials Speaker : Dr. Tushar kumar
  • 2. What is EEG It is the study , observation and recording of brain, which are measureable expression of electrical activity of brain. It could be Unipolar or bipolar Unipolar : where reference point is fixed Bipolare : voltage recorded lies near to scalp or brain.
  • 3. EEG Standard EEG: has 10 – 20 standardized placement system. Electrodes placed on anatomic location. 8 – 20 wave lines are graphed.
  • 4. EEG signals There are 3 basic parameters that are recorded in EEG waves: 1. Amplitude : measurement of electrical height. Ranges from 0 -200 μV. 2. Frequency : it is no of times per second the wave touches the zero voltage line. It is the degree of activity of cerebral cortex. 3. Time : plotted on x axis
  • 5. Type of waves 1. Alpha waves: 8 – 13 Hz occurs during awake and quiet resting state. 2. Beta waves: > 13 Hz excited and specific mental activity. 3. Theta waves: 4 -8 Hz 4. Delta waves: 0 – 4 Hz occurs during deep sleep, infancy, organic brain disease.
  • 6. EEG ! How does it look like?
  • 7. EEG interpretations • Sudden development of delta waves coincident with surgical manuver  injury warning • In penumbra region, EEG poorly predict brain damage • Anesthetics & hypothermia causes EEG changes  multifactorial interpretation
  • 8. Uses of EEG 1. Monitoring the functional integrity of neural structures that may be at risk. 2. Identification of specific structures 3. Monitoring the effects of specific agents and other drugs that affect nervous system 4. Diagnosis and monitoring of pathophysiological conditions.
  • 9. Anaesthesia and EEG a. Ketamine : rhythmic theta activity…. 4 -8 Hz increased epileptiform activity. b. Opioid : dose related decrease in frequency. but if further doses are not given then there is return of higher wave occurs. Epileptiform waves occur with large doses of opioid such as during fentanyl induction. c. Nitrous oxide : when used in combination with other anaesthetic agents it depresses EEG pattern.
  • 10. Anaesthesia and EEG…. d. Isoflurane: depression in EEG pattern with increase in depth of anaesthesia. Electrical silence at 2 – 2.5 MAC. e. Halothane: smaller degree of EEG suppression. At 3 – 4 MAC EEG suppression occurs to greater extent but cardiac depression also occurs. f. Sevoflurane: smaller degree of EEG suppression but greater epileptiform activity.
  • 11. Surgery and EEG 1. No change in anaesthetic technique should be made during critical period of monitoring. for eg. Induced hypotension, carotid clamping, aneurysm clipping. 2. Avoid major changes in anaesthetic gas level/ bolus opioid or benzodiazepines near time of increases ischemic risk. 3. If drug must be given then monitor it with Evoked Potential.
  • 12. Why Evoked potentials… The EEG machine records tiny electrical voltages from the brain, representing the averaged electrical activity of millions of neurons. ……… non specific. But evoked potentials are triggered. They are very specific.
  • 13. Evoked potentials What are evoked potentials: • The electrical potentials produced after stimulation of specific neural tracts. • The recorded plot of voltage versus time Initial artifact representing the stimulation of the tract followed by the neuronal response, which is recorded as a series of peaks and valleys
  • 14. Evoked potentials Many times signals are small  an amplifier reduces the electrical noise by subtracting the signal at a referenceelectrode from the recording electrode. • Filtering of this signal we focus on the evoked response of interest.
  • 15. Evoked potentials • The evoked response always occurs at a set time after stimulation. • Summating all those responses increases the time-locked response, whereas the background activity acts as a random signal and averages out to zero. • So we get only the evoked potential response.
  • 16. CLASIFICATION OF EVOKED POTENTIALS SENSORY EVOKED POTENTIALS MOTOR EVOKED POTENTIALS EVENT RELATED POTENTIALS VISUAL EVOKED POTENTIAL AUDITORY EVOKED POTENTIAL SOMATOSENSORY EVOKED POTENTIAL EVOKED POTENTIALS
  • 19. Visual evoked potential: Primary visual system is arranged to emphasize the edges and movements so shifting patterns with multiple edges and contrasts are the most appropriate method to assess visual function. Flash VEP • Greater variability of response with multiple positive and negative peaks • Primarily use when an individual cannot cooperate or for gross determination of visual pathway. Ex in infants / comatose patients
  • 21. Clinical application of VEP: VEPs can be used to monitor: • the anterior visual pathways during a. craniofacial procedures, b. pituitary surgery, c. surgery in the retrochiasmatic visual tracts and occipital cortex. Their specificity ranges from 95% to > 99%
  • 22. Clinical application of VEP: Other medical applications: 1. Multiple sclerosis 2. Demylinating disorders 3. Axonal loss disorders 4. Migraine headaches
  • 23. VEP
  • 24. Limitations of VEP Less useful in surgical monitoring. Do not measure the pathways of useful clinical vision The large, bulky “goggles” usually used for stimulation pose technical problems, The bilateral nature of the response may obscure focal changes, Anaesthetic sensitivity makes recording difficult.
  • 26. SSEP : • Evoked potentials of sensory nerves in the peripheral & central nervous system • Used to diagnose nerve damage or degeneration in the spinal cord • Can distinguish central Vs peripheral nerve lesion
  • 27. Anatomical & Physiological basis of SSEP SENSE ORGANS – PACINIAN AND GOLGI COMPLEXES IN JOINTS, MUSCLES AND TENDONS DORSAL ROOT GANGLIA TYPE A FIBRES GRACILE AND CUNEATE Nu. IN MEDULLA Nu POSTEROLATERALIS OF THALAMUS MEDIAL LEMNISCUS SENSORY CORTEX THALAMOPARIETAL RADIATYIONS
  • 28. Dorsal column pathways: They carry sensory inputs to brain for vibration and proprioception. First synapse near the nucleus cuneatus and nucleus gracilis and then decussates near the cervicomedullary junction, ascending via the contralateral medial lemniscus. Second synapse occurs in the ventroposterolateral nucleus of the thalamus, from which it projects to the contralateral sensory cortex
  • 29. SSEP: Peripheral sensory nerves are stimulated electrically and the response is measured along the sensory pathway. The large, mixed motor and sensory nerves are usually monitored: 1. Median (C6-T1), 2. ulnar (C8-T1), 3. Common peroneal (L4-S1), 4. posterior tibial (L4-S2) nerves.
  • 30. SSEP Median SEP: Recording electrodes are placed at the following locations: • Erb’s point on each side (EP) • Over the second or fifth cervical spine process (C2S or C5S) • Scalp over the contralateral cortex (CPc) and ipsilateral cortex (CPi) • Noncephalic electrode (Ref)
  • 31.
  • 32. SSEP Stimulation activates predominantly the large- diameter nerve fibers.  Anatomically, peripheral nerve stimulation produces both orthodromic and antidromic neural transmission. The orthodromic motor stimulation elicits a muscle response, which is seen as a twitch (verifying stimulation), and the orthodromic sensory stimulation produces the SSEP.
  • 33. SSEP: • STIMULUS: Electrical – square wave pulse by surface or needle electrode • DURATION: 100 – 200 msec at a rate of 3 – 7 / sec • INTENSITY: for producing observable muscle twitch or 2.5 – 3 times the threshold for sensory stimulus. Unilateral stimulation for localization Bilateral stimulation for intra-operative monitoring
  • 34. Stimulus is given as vibration that ascends via ipsilateral dorsal column.
  • 35. Monitoring of SSEP: Monitoring is estimated to reduce the morbidity in spinal surgery by 50% to 80%. sensitivity of 52% and a specificity of 100%  Blood flow: clinical functional neurologic findings become abnormal at a cortical blood flow of about 25 mL/min/100 g, but the SSEP remains normal until cortical blood flow is reduced to about 20 mL/min/100 g. SSEP responses are lost at a local blood flow of between 15 and 18 mL/min/100 g.
  • 36. 1. Monitoring during temporary clipping in aneurysm surgery has shown that a very prompt loss of cortical SSEP response (less than 1 minute after clipping) is associated with development of permanent neurologic deficit. 2. SSEP in spinal surgeries has become standard care of monitoring. 3.Monitoring during carotid endarterectomy. Intraoperative SSEP changes are used as an indication for shunt placement and to predict postoperative morbidity. Clinical application of SSEP
  • 37. SSEP
  • 39. AUDITORY BRAINSTEM RESPONSES I. organ of Corti and extracranial cranial nerve VII; II. cochlear nucleus; III. superior olivary complex; IV. lateral lemniscus; V. inferior colliculus; VI. medial geniculate body; VII. auditory radiation.
  • 40. • Breif electrical pulse “ click” • Intensity – 65 – 70 dB above threshold • Rate – 10 – 50 clicks / sec • Averaging of 1000 – 4000 Hz range • Read after first 10 msec of stimulus AUDITORY BRAINSTEM RESPONSES
  • 41. Findings: • Absolute waveform latencies • Interpeak latencies ( I – III, I – V & III – V ) • Amplitude ratio of wave V / I In general I – V peak are preserved then hearing is preserved. AUDITORY BRAINSTEM RESPONSES
  • 42. Clinical utility of AER: 1. Acoustic neurinoma 2. Vertebral-basilar aneurysm 3. Cerebellar vascular malformations 4. Other posterior fossa procedure , eg. during microvascular decompression for relief of hemifacial spasm or trigeminal neuralgia.
  • 44. Electromyographic monitoring • EMG: Muscle responses to monitor neural tracts • Two types of EMG monitoring : 1. recording spontaneous activity 2. recording responses subsequent to stimulation of the motor nerves or motor pathways
  • 45. Clinical utility: EMG All cranial nerves with a motor component are monitored from their EMG activity in their respectively innervated muscles. 1. Facial nerve monitoring: most common cranial nerve monitored (e.g., acoustic neuroma) 2. Cranial nerves IX and X : in head & neck surgeries  when injured produces cardiovascular changes.
  • 46. Continue…. 3. Cranial nerve XI: harmful head movement due to trapezius and sternocleidomastoid ms. 4. Vagus nerve (CN X) monitoring by vocal cord EMG is for skull base (large brainstem tumors) and anterior neck procedures. Monitoring of the recurrent laryngeal and superior laryngeal branches is done with electrodes in the cricothyroid or vocalis muscles or contact electrodes mounted on an ET Tube.
  • 48. Motor evoked potentials More sensitive to ischemic vascular insults. MEP monitoring has become important during surgery for: 1. Correction of axial skeletal deformity, 2. Intramedullary spinal cord tumors, 3. Intracranial tumors, 4. Vascular lesions. 5. Assessment of outcome in stroke and spinal cord function during thoracoabdominal
  • 49. • The MEP allows rapid detection of ischemia because the gray matter, with its higher metabolic rate, is more sensitive to hypoperfusion. During middle and anterior cerebral aneurysm clipping several neural structures in the motor pathway are at risk for hypoperfusion. The sensitivity and specificity of MEP is 100% and 97.4% Motor evoked potentials
  • 50. MEPs can : 1. Identify vasospasm 2. Incorrect placement of the aneurysm clip, 3. Inadequate clip placement. KINDLING: Inherent risk with MEP. it is cortical thermal injury. Motor evoked potentials
  • 51. Evoked potentials & Anaesthesia Monitoring of EP: The anesthesiologist must consider the various effects of anaesthesia management on the different modalities used. In general, the inhalational anaesthetic agents which have drug effects at multiple synaptic receptor types appear to have the most profound effect on monitoring. A. SSEP: Volatile anaesthetics produce an increase in latency and a decrease in amplitude of cortical sensory responses.
  • 52. Continue… B. MEP: No synapses are involved in the production of the D wave, so recordings of the D wave in the epidural space are altered little by anaesthetics. I waves are generated by synaptic mechanisms, so they are progressively reduced by increasing doses of anaesthetic agents.
  • 53. Nitrous oxide (N2O) also produces amplitude reduction and latency increases in cortical sensory responses or MEP responses when used alone or when combined with halogenated inhalational agents or opioid agents. Opioids cause only mild depression of all responses, with loss of late sensory evoked response peaks. ( at sedative dose) Continue…
  • 54. Ketamine : subcortical and peripheral responses are also minimal. Preferred drug for monitoring responses that are usually difficult to record during anaesthesia and in children. Use of ketamine with propofol reduces the depressant effect of propofol while providing an enhancement effect on responses. Continue…
  • 55. Barbiturates and benzodiazepines: • Thiopental and midazolam produce mild depression of cortical sensory responses but long-lasting depression of MEPs. • Hence not good for MEP monitoring. • The ABR is virtually unaffected by doses of phenobarbital that produce coma, and the SSEP is unaffected at doses that produce a silent EEG. • so to monitor with SSEP is best. Continue…
  • 56. • Etomidate Produces an amplitude increase of cortical sensory components following injection, with no changes in subcortical and peripheral sensory responses. As consistent with myoclonus. • SSEP recording commonly done but not MEP monitoring. Continue…
  • 57. Propofol Most commonly used sedative component of total intravenous anesthesia when SSEPs and MEPs are monitored. • Induction produces amplitude depression in cortical SSEPs, with rapid recovery after termination of infusion. • Recordings in the epidural space are unaffected, consistent with the site of anaesthetic action of propofol in the cerebral cortex. Continue…
  • 58. Droperidol and Dexmedetomidine • Have minimal effects on responses Dexmedetomidine has been successfully used with MEP. Continue…
  • 59. Impairs or prevents MEP and EMG monitoring. Muscle relaxants are generally thought to have no effect on the SSEPs Partial NMB reduces the amplitude of motor unit potentials and might therefore change the ability to detect impending axonal injury from nerve irritation. Effect of neuromuscular blocking agents
  • 60. Cont…. It is preferred to avoid NMB after anaesthesia induction and during EMG and MEP monitoring.
  • 63. Conclusion: Evoked potential monitoring require training, financial obligations and not 100 percent full proof in minimizing complications. Have risk involved. On the other hand they have been shown to be cost effective and have become part of routine management. For many surgeons and anesthesiologists, these methods have become an indispensable intraoperative diagnostic tool.

Editor's Notes

  1. Flash stimulation:
  2. (and their component spinal roots)
  3.  fast conducting Ia muscle afferent and group II cutaneous nerve fibers. Orthodromic :(propagating in the normal direction) Antidromic : (propagating in the reverse direction)
  4. unexpected ischemia (e.g., retractor pressure, hypotension, temporary clipping, and hyperventilation). In some respects, use of the EEG and SSEP in CEA are complimentary because the SSEP is able to detect ischemia in deep cortical structures, and the EEG assesses a wider area of surface cortex.
  5. response. The latency for true facial nerve response is 6 to 8 msec, whereas the latency for a trigeminal nerve response is 3 to 4 msec. Because
  6. This technique is used in procedures such as neck dissections, thyroid and parathyroid
  7. cortical thermal injury (known as “kindling”), but over the last 15 years, even though hundreds of thousand of patients have undergone MEP monitoring, only two cases of kindling have been reported.86 In a 2002 survey of the literature, published complications included
  8. Acts on nucleus cuneatus in brain stem and thalamic projections to slower the response.
  9. , but occasional case reports have appeared suggesting that it may prevent MEP monitoring in some cases.
  10. ; they may actually improve the cervically recorded sensory responses or epidural recordings of SSEPs and MEPs because EMG interference is reduced in nearby muscle groups. A new technique called post-tetanic MEPs enhances conventional