EEG and Evoked Potentials

Speaker : Dr. Tushar kumar

What is EEGIt 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 waves1. 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 Hz4. 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 structures3. Monitoring the effects of specific

agents and other drugs that affect nervous system

4. Diagnosis and monitoring of pathophysiological conditions.

Anaesthesia and EEGa.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 EEG1. 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 potentialsWhat 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 potentialsMany 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

VISUALEVOKED POTENTIAL

AUDITORYEVOKED POTENTIAL

SOMATOSENSORYEVOKED 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 sclerosis2. Demylinating disorders3. Axonal loss disorders4. Migraine headaches

VEP

Limitations of VEPLess useful in surgical monitoring.Do not measure the pathways of

useful clinical visionThe 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 GANGLIATYPE 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 nearthe 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.

SSEPMedian 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)

SSEPStimulation 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 localizationBilateral 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.

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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 potentialsMore 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 aneurysm repair.

• 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 vasospasm2. 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 & AnaesthesiaMonitoring 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.

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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)

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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.

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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.

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• 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.

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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.

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Droperidol and Dexmedetomidine

• Have minimal effects on responses Dexmedetomidine has been successfully used with MEP.

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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 motorunit 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.

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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.

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