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.
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
7. EEG interpretations
• Sudden development of delta waves
coincident with surgical manuver injury
• In penumbra region, EEG poorly predict
• 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
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
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
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
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
• 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
16. CLASIFICATION OF 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.
• 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
24. Limitations of VEP
Less useful in surgical monitoring.
Do not measure the pathways of useful
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
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
27. Anatomical & Physiological basis
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
28. Dorsal column pathways:
They carry sensory inputs to
brain for vibration and
First synapse near
the nucleus cuneatus and nucleus
gracilis and then decussates near
the cervicomedullary junction,
ascending via the contralateral
Second synapse occurs in the
ventroposterolateral nucleus of
the thalamus, from which it
projects to the contralateral
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.
Recording electrodes are placed at the
• 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)
Stimulation activates predominantly the large-
diameter nerve fibers.
Anatomically, peripheral nerve stimulation
produces both orthodromic and antidromic
The orthodromic motor stimulation elicits a
muscle response, which is seen as a twitch
(verifying stimulation), and the orthodromic
sensory stimulation produces the SSEP.
• STIMULUS: Electrical – square wave pulse by
surface or needle electrode
• DURATION: 100 – 200 msec at a rate of 3 – 7
• INTENSITY: for producing observable
muscle twitch or 2.5 – 3 times the threshold for
Unilateral stimulation for localization
Bilateral stimulation for intra-operative
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
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
Clinical application of
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
• 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
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
• 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
2. Cranial nerves IX and X : in head & neck
surgeries when injured produces
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
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
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%
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
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
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
Opioids cause only mild depression of all
responses, with loss of late sensory evoked
response peaks. ( at sedative dose)
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.
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
• so to monitor with SSEP is best.
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
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
59. Impairs or prevents MEP and EMG
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
Effect of neuromuscular blocking
Evoked potential monitoring require
training, financial obligations and not 100
percent full proof in minimizing
Have risk involved.
On the other hand they have been shown to
be cost effective and have become part of
For many surgeons and anesthesiologists,
these methods have become an indispensable
intraoperative diagnostic tool.
fast conducting Ia muscle afferent and group II cutaneous nerve fibers.
Orthodromic :(propagating in the normal direction)
Antidromic : (propagating in the reverse direction)
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.
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
This technique is used in
procedures such as neck dissections, thyroid and parathyroid
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
Acts on nucleus cuneatus in brain stem and thalamic projections to slower the response.
occasional case reports have appeared suggesting that it may
prevent MEP monitoring in some cases.
; 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