Continuous EEG - Indication and Utilizations

Authors:

Filzah Faheem MD,

Junaid Kalia MD

Introduction

• Defining Continuous EEG?
• Continuous EEG monitoring device with a video camera for at least 24 hours in a conscious patient.
• Reflects cortical synaptic activity
• Continuous EEG monitoring terminology was first used in 2005
• Advocated by the American Clinical Neurophysiology Society in 2012.
• Provides data on cerebral activity, brain function and any seizure activity.
• Summarizes data such as
• amplitude,
• frequencies,
• rhythm and
• Power

Technique

Electrode Placement

• The International 10-20 system is considered to be the gold standard to monitor CEEG.
• Electrodes are placed at 10% to 20% distance from anatomical landmarks
• Anatomical landmarks are
• Nasion
• Inion
• Left Preauricular
• Right Preauricular
• Minimum number of 21 electrodes are recommended
• Extra electrons can be added
• Terminology used is
• F for frontal
• P for parietal
• T for temporal
• C for central
• Z for midline
• Even number indicates right hemisphere
• Odd number indicates left hemisphere

• A Headset-type continuous video EEG monitor (Figure: 1)
• Featuring eight electrodes
• Three left electrodes
• Frontal
• Central
• Temporal
• Three right electrodes
• Frontal
• Central
• Temporal
• Two O1 and O2 electrodes
• Gel type electrodes are used to feature this eight channel EEG.
• Developed recently
• Uses Bluetooth to transmit data

• Sub-dermal wire electrodes are also used to monitor CEEG for several days to obtain artifact free recording.

Figure: 1 Headset type CEEG monitor a: Frontal view showing frontal, central and temporal electrode (black dots) b: Lateral view showing position of occipital electrode

EEG recording, storage, review

• EEG recording along with video camera

• Recording is done over hours to weeks

• Recordings may include graphical displays of EEG trends

• Power spectrographic displays help to review large quantity of EEG
• Highlight areas of changes in EEG
• Compress several hours EEG into single image

• Digital EEG systems are in use nowadays which includes
• Amplifier
• Monitor and
• Processor
• Benefits of EEG system
• gets information from 128 channels
• Greater than 10kHz sampling rate
• 24-bit resolution at each amplifier

• Advantages
• Quantitative displays are time saving
• Improved detection
• Subtle seizure activities such as blinking, eye twitching and deviation may get unnoticed but can be confirmed on CEEG
• Has greater sensitivity in identifying clinical and subclinical seizures
• Helps to review ample amount of data efficiently without wasting time on raw EEG
• Helps in assessing prognosis in conditions
• cardiac arrest
• Stroke
• Traumatic brain injury
• Excluding seizures by continuous EEG monitoring prevents unnecessary administration of anti-seizure drugs

• Disadvantages
• Increased cost
• Patient movement can result in false tracings due to electrodes malposition
• Time consuming
• Multiple factors causing artifacts
• Body movement such as eye blinking
• Muscle activity such as talking, swallowing
• Blood vessel pulsation
• Respiration
• Bed displacement
• Electrode displacement
• Increased sweating
• Pacemakers

Automated Processing (Persyst)

• Automated displays help to interpret raw EEG effectively

• These display the EEG according to different frequencies along with amplitude

• Commonly used techniques for these displays are
• Color density spectral array (Figure: 2)
• Amplitude-integrated EEG

Figure 2: Colour density spectral array in status epilepticus

Indication

Non-Convulsive status epilepticus

• Defined as change of mental status from patient’s baseline for minimum of 30 minutes associated with ictal discharges on EEG such as
• Periodic discharges for > 2.5/s (Figure: 3.1)
• Repeated spike and waves for > 10s (Figure: 3.2)
• Rhythmic delta activity

• About 90 % of patients show seizure activity during the first 24 hours of CEEG.

• Prevalence of NCSE in ICU patients is 8-20 percent.

• Continuous EEG monitoring done
• After control of convulsive status epilepticus to detect nonconvulsive seizures
• In patients having non convulsive seizures without any evident clinical correlation

• Symptoms associated with NCSE
• Altered mental status of unknown etiology
• Disturbed consciousness after generalized seizures
• Subtle eye movement
• Delirium
• Aphasia
• Conjugate deviation
• nystagmus
• Facial myoclonus
• Limb myoclonus

• Predictors of epileptic seizures in critically ill patients
• Altered mental status
• Coma

• Higher probability of NCSE in patients with
• Generalized periodic discharges
• Localized periodic discharges
• Intensive Care Unit

Figure: 3 (left) CEEG of the patient shows lateralized periodic discharges in the Right temporal region. (Right) CEEG of the patient showing spike and wave activity thus fulfils the criteria for non-convulsive status epilepticus.

Ischemia

• Most seizures are nonconvulsive in ICU patients.

• Neuronal activity depends on the blood supply hence it makes EEG monitoring reliable to check for brain ischemia.

• In patients with decreased cerebral perfusion EEG changings can be observed even before infarction occurs

• helps us to undergo any intervention, if necessary, before permanent damage occurs.

• In mild to moderate ischemia, perfusion 15-35 mL/100g per minute
• Decrease in fast activity i.e., alpha and beta
• Increase in slow activity in the delta wave

• In severe cases, perfusion less than 10mL/100g per minute
• EEG becomes isoelectric and rhythms disappear completely. (Table: 1)

• Complete rhythm suppression indicates irreversible damage

Table 1: Cerebral blood flow and EEG Changes 

Haemorrhage

• Clinical detection of seizures in intracerebral haemorrhage is 8 percent.

• Use of CEEG has increased detection to 25 percent

• Subarachnoid hemorrhage is associated with
• diffuse increase in slow activity (delta and theta)
• decrease in fast activity

• Performed for at least 24 hours in conscious patients

• For 48 hours in unconscious patients.

Critical Illness

• Critical care EEG includes
• Continuous EEG recording
• Simultaneous video recording
• Graphical displays of quantitative EEG may be included

• Goals include early Identification of
• Changes in brain function
• Non convulsive seizures
• Ischemic changes

• Assess prognosis in critically ill patients experiencing encephalopathy
• Good prognostic factors
• Variability on EEG
• Background continuity
• Reactivity to stimulus
• Normal sleep patterns
• Worse prognostic factors
• Burst suppression patterns
• Isoelectric pattern
• Periodic patterns
• Electrographic seizures

Psychogenic non-epileptic Seizure (PNES)

• Altered movement or sensation similar to epilepsy
• Results due to emotional causes
• Absence of any abnormal electrical discharges
• Often misdiagnosed as epilepsy

• 2-25 % people treated in USA for epilepsy have psychogenic non-epileptic seizures

• Early identification of PNES is necessary
• To decrease misdiagnosis
• Inappropriate anticonvulsant treatment

• Continuous EEG with Video monitoring is considered to be gold standard to diagnose PNES
• Brain's electrical activity remains normal
• Video records a seizure while EEG monitoring at the same time to capture any change in brain electrical activity
• Presence of interictal epileptiform discharges favours epilepsy over PNES
• Rhythmic movement artifacts can occur on EEG, they remain stable over time during that episode, while epileptiform discharge evolve with time

• As seizures can occur at any instant so prolonged monitoring should be done

• PNES coexists with organic diseases such as
• Epilepsy
• Head injury
• Mental retardation

Epilepsy Monitoring Unit

• Monitoring Drug Response
• Response to pharmacologic drugs given to control non-convulsive seizures and non-convulsive status epilepticus can be demonstrated by CEEG monitoring
• If patient is still unconscious for more than 30 minutes or not improving within 10 minutes of seizure cessation CEEG should be started in order to monitor for any ongoing seizure activity
• Patients on paralytic agents prevent clinical presentation of seizures thus making it essential to monitor high risk patients by using CEEG
• Triphasic waves occur due to hepatic renal or electrolyte abnormalities.

• Surgical Planning
• Carotid Surgery
• Clamping of internal carotid artery is done
• Decrease in blood flow occurs
• CEEG is sensitive in assessing cerebral ischemia
• assess ipsilateral hemisphere blood flow during test clamping
• Decrease in faster activity filled by increase in slower activity shows decreased blood flow
• No rhythmic change shows sufficient blood supply to brain
• 10 % decrease in amplitude of alpha or beta rhythm may be considered safe during the procedure
• Transcranial doppler is also use by many centers but CEEG has more sensitivity
• EEG changes during clamping are associated with postoperative stroke
• Surgeons use shunting mechanism to decrease the risk
• Use of shunting mechanism is associated with perioperative stroke due to
• Thromboembolism
• Arterial injury
• Techniques used for shunting are
• Routine shunting: perioperative stroke risk 1.4%
• Selective shunting: perioperative stroke risk 1.6%
• No shunting: perioperative stroke risk 2%

Anaesthesia

• Sedative and comatose patients
• Used to assess the central nervous system suppression by anaesthetic agents.
• Propofol induced EEG changes
• Continuous with anteriorization of alpha rhythm
• Bursts are heterogenous i.e., appear and disappear slowly
• Absence of isoelectric activity
• Dexmedetomidine induced EEG changes which result in stage II sleep changes
• induction of phenobarbital coma
• to stop continuous seizures (status epilepticus)
• Richmond agitation Sedation Scale is used to categorize level of sedation in patients
• If patient responds to verbal stimulus then it is light sedation
• If patient responds to deeper stimulus then it is light sedation
• CEEG helps us to evaluate level of sedation without use of stimulus by calculating baseline of consciousness level.
• Done to monitor sedation in conscious quadriplegic patients by using spectral edge frequency
• such as Guillain-Barre syndrome
• CEEG patterns in comatose patients
• Diffuse synchronous or asynchronous delta or theta wave slowing
• does not lead towards any aetiology
• If focal slowing occurs
• Look for any tumour
• Infarct
• Any other focal cause
• Patterns signifying seizure in comatose patient
• Periodic discharges
• Semi-periodic discharges
• Rhythmic discharges

• Monitoring Brain activity
• Post Cardiac arrest
• CEEG has been in use to determine prognosis post cardiac arrest patients since 1960s
• Therapeutic hypothermia is considered to be standard of care in post cardiac arrest patients.
• Risk of seizure maximum during rewarming
• patients should be monitored for at least 2 hours after the achievement of normothermia.
• Done to monitor any subclinical seizure to improve the prognosis in post cardiac arrest patients
• Mortality independent risk factors are
• Seizures
• Nonconvulsive status epilepticus
• Review of CEEG should be done daily.
• Better prognosis if
• Return of Continuous rhythms on EEG within 12 hours
• Worse prognosis if
• Presence of epileptiform discharges
• EEG becomes isoelectric within 10-40 seconds of circulatory arrest
• Persistence of isoelectric, low voltage rhythms
• Burst suppression with identical burst patterns
• Lack of improvement within 24 hours
• There should be either continuous or frequent EEG for at least 48 hours monitoring in comatose patients post cardiac arrest.

Limitations and Contraindication

• No absolute contraindication

• Limitations include
• Post craniotomy
• Skin breaches in the skull
• Open wound
• Hyperventilation should be avoided in patients with
• Stroke
• Transplant surgeries
• Myocardial infarction
• Asthma
• ARDS
• Sickle cell anaemia

Artifact reduction

• Availability of technologist to reapply electrodes if displaced.

• Short acting neuromuscular blockers can be used if muscle activity disrupts EEG monitoring.

• EEG findings should be correlated with examination in order to decrease false detection of artifacts as seizures.

• Different techniques are used to reduce artifacts such as
• Regression
• Blind source separation
• Empirical mode decomposition
• Wavelength transform algorithm

Skin tear and care

• Prolonged electrode placement can result in
• Mild erythema
• Moderate erythema with sharply defined borders
• Intense erythema with or without edema
• Intense erythema with oedema and blistering

• Factors involved in skin irritation
• Acetone use during electrode removal
• Prolonged electrode placement

• Care of the skin
• Use of water soluble solutions to remove electrodes
• Small change in electrode position
• Use of tubular elastic bandage as it provides equal pressure

• Disinfection of electrodes after recording to prevent spread of contagious skin conditions.

Further Reading

• Van Putten, M. J. A. M., & Hofmeijer, J. (2016). EEG monitoring in cerebral ischemia: Basic concepts and clinical applications. Journal of Clinical Neurophysiology, 33(3), 203–210.

• Herman, S. T., Abend, N. S., Bleck, T. P., Chapman, K. E., Drislane, F. W., Emerson, R. G., Gerard, E. E., Hahn, C. D., Husain, A. M., Kaplan, P. W., LaRoche, S. M., Nuwer, M. R., Quigg, M., Riviello, J. J., Schmitt, S. E., Simmons, L. A., Tsuchida, T. N., & Hirsch, L. J. (2015). Consensus statement on continuous EEG in critically Ill adults and children, part I: Indications. Journal of Clinical Neurophysiology32(2), 87–95.

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