EEG

Technical 1

Normal EEG.. 1

Sleep. 1

Activating procedures. 1

Normal Variants. 1

Focal abnormalities. 1

Generalised abnormalities. 1

Seizures. 1

Encephalopathies. 1

Neonatal EEG.. 1

Technical

Electrodes

• Made from gold or silver (or compounds thereof)

• Skin abrasion prior to application

• Attached with electroconductive substance

o Gel

o Collodian

- Dries hard for long lasting attachment

- Requires acetone for removal

- ?Potentially teratogenic – avoid use in pregnancy

• Impedance should be less than 5 kOhms

Electrode Placement

• 10-20 system

o Measured from nasion at front and inion at back

o Measured from tragus of ear on each side

Lead descriptions

Lead	Description
Fp1 and Fp2	Frontopolar
F7 and F8	Anterior temporal (or frontal)
F3 and F4	Superior frontal (or midfrontal)
Fz	Frontal midline
T3 and T4 (T7, T8)	Midtemporal
C3 and C4	Central (Rolandic)
Cz	Vertex or central midline
T5 and T6 (P7, P8)	Posterior temporal
P3 and P4	Parietal
Pz	Parietal midline
O1 and O2	Occipital
Sp1 and Sp2	Sphenoidal
A1 and A2	Ears

Filters

Time constant = R x C

R = resistance

C = capacitance

Time constant is the amount of time to reduce high frequency waves by 63% or low frequency by 37%

It should be noted that digital filters do not use resistor/capacitor circuit and use digital filters

Digital processing

Nyquist theorem

Required sampling rate = 2 x fmax

This will only sample a wave twice – so in reality to properly represent a wave this needs to be further multiplied by 2x-5x

Most machines have a sampling rate of att least 200Hz ( and so can accurately represent 50Hz and below)

Montages

Referential

Bipolar

• When input 1 is negative compared to input 2 – upward (negative) deflection

• When input 1 is positive compared to input 2 – downward (positive) deflection

Input 1	Input 2	Deflection
Neg	-	Upward (Negative)
Pos	-	Downward (Positive)
-	Neg	Downward (Positive)
-	Pos	Upward (Negative)

LINEUP (Lead 1 – Negative – Up)

Referential	Good for assessing symmetry Avoid electrode cancelling	Reference can be involved/contaminated by discharge
Average
Source (Laplacian)
Ipsilateral Ear		Electrodes are variable distance from reference Temporal activity can contaminate reference
Contralateral Ear	Does not have problem of contamination of ipsilateral temporal contamination	Electrodes are are very long and variable distance from reference
Vertex
Bipolar		Lead will not register response if large area area of discharge End of chain issues

Normal EEG

Frequencies

Name	Frequency	Notes
Delta	1-3Hz (<4Hz)	High amplitude, slow
Theta	4-7Hz (<8)	Normal in children up to 13 and in sleep
Alpha	8-13Hz*	Posterior regions Occur with eye closing and relaxation Present during most of life
Beta	>13 Hz- 25Hz	Mainly frontal Normal rhythm in most awake people with eyes open.
Gamma	>25Hz	(Controversial term)

* The alpha band is the only one to include the upper boundary frequency. i.e. It includes 13 (but not 13.1)

Background frequencies

Alpha

• Posterior dominant

• Attenuates with eye opening

• Is bilaterally absent in 5% of people

• Unilateral absence is pathological - indicative of complete dysfunction of underlying brain (e.g. infarction)

• If a person in very tense alpha will be will evident

• If a person is drowsy then alpha will be more common – may also be present more frontally

• Dementia states can result in more universal alpha

• Slow alpha variant – normal variant (see below)

• Asynchrony

o Up to 1Hz difference in alpha frequency is allowable between sides

• Alpha symmetry

o The amplitude of right sided alpha is usually ~20% higher than left.

o For asymmetry to be abnormal:

- Left amplitude >2x right (= right <50% of left)

- Right amplitude >3x left (= left <33% of right)

Beta

• Normal background rhythm

• Predominant in fronto-central regions

• Does not attenuate with eye closure

• Asymmetrical beta is sign of underlying brain dysfunction

Theta

• Present in normal EEG’s however in general should not be present in >5% of waking adult trace

• More predominant with drowsiness – seen in the occipital and temporal regions in particular

• Can occur in rhythmic, hypersynchronous runs during drowsiness (hypnagogic hypersynchronies) and during arousal (hypnapompic hypersynchronies)

Delta

• Generally not present in normal adult EEG

• Temporal delta in the elderly it may be acceptable as long as it is <1% of the record.

Normal background for age

	Typical (Hz)	Limit of normal (Hz)
4 months	3-4	-
5 months	5	-
1 year	6	5
2 years	7
3 years	8	6
6 years	9	7
8 years		8
13 years	10

• “Rules”

o Rule of 3’s and 8’s - 3Hz by 3 months, 6Hz by 1year, 8Hz by 3years, 9Hz by 8 years

o 8 by 8 – 8Hz by 8years

Amplitude

• Measured peak to peak

• Low 0-25uv

• Moderate 25-75uV

• High >75uV

Reactivity

• Change in EEG activity due to sensory stimulus or sudden change in internal state

Includes:

• Alpha attenuation with eye opening

• Slowing with hyperventilation

• Cessation of Mu with moving contralateral upper limb

• Response to painful stimulus

• Photic driving response

Sleep

Drowsiness	Decreased background voltage Posterior rhythm slows - theta slowing can be seen (sometimes in runs 4-8Hz) Diffuse increase in fast activity Slow lateral eye movement
Stage I	Vertex waves
Stage II	Vertex waves Sleep spindles 11-15Hz K-complexes (usually seen in stage II but do not define it)
Stage III	Delta activity >20% of epoch
Stage IV	Delta activity >50% of epoch
REM	Resembles stage I with some slower elements, low voltage background. Rapid eye movements. Theta waves (saw-tooth waves in central areas)

K-complex

• Moderate to high amplitude, diffuse/centrally predominant biphasic, slow wave transients

• At least 0.5sec long and are at least 75uv in amplitude.

• Initial phase usually negative

• Can also be triggered by partial arousal – e.g. lightly tapping noise near patient during stage 2 sleep can induce.

• Unknown what the origin of the name “K” is from

Sleep spindles

• Regular, rhythmic, sinusoidal or spindle shaped waves at 12-14Hz

• Low amplitude

• Predominant in frontal or central regions

• Typically 1-3 seconds

Sleep in children

(See below for Neonatal EEG including sleep)

Vertex waves

Spindles

REM sleep

Activating procedures

Eye opening and closing

• To see PDR and determine its reactivity

• May induce epileptiform activity in some childhood epilepsies

Mental Alerting

• Getting the patient to do tasks to ensure they are alert

• Ensures any background slowing is real – rather than due to drowsiness

Hyperventilation

• Usually done for 3-5min

• Induces respiratory alkalosis

• Normal response is

o Slowing of PDR

o Gradually increasing theta, often polymorphic and widespread

o Followed by delta, often high amplitude

o Referred to as – hyperventilation hypersynchrony OR hyperventilation induced, high amplitude rhythmic slowing.

• Initially frontal in adults

• Subsides 60-90sec after HV

• More prominent in younger patients and with low BSL

• Contraindications:

o Chronic hear to lung conditions

o Pregnancy

o Recent stroke or sub-arachnoid haemorrhage

o Sickle cell disease

o Moya-moya

Abnormalities that can be induced:

• Generalised spike and waves discharges

• Focal spikes

• Lateralised slowing

• Normal HV changes have gradual build-up whereas induced spikes will be sudden

• During hyperventilation hypersynchrony children may have reduced responsiveness – so this should be used with caution in interpretation

Photic Stimulation

• Strobe light flashed at 1-35Hz for 5-10sec each frequency

Visual evoked potentials/Photic driving response

• Positive wave seen in occipital region ~100msec after flash

• Photic driving response is train of VEPs

• Flash can often entrain at low frequencies but not at high

• Can sometimes be seen at subharmonic frequency (e.g. 21Hz flash induces 10.5Hz driving response) or harmonic (i.e. twice the frequency

• The ability to produce a driving response at high frequency (H reponse) has been said to be associated with migraine

• Absence of response is not abnormal

• A very high amplitude driving reponse has been associated with some neurodegenerative disorders (e.g. neuronal ceroid lipofuscinosis)

Photomyoclonic response

• Brief repetitive muscle spikes over anterior regions of the head.

• Often increase gradually in amplitude as stimulation continues and cease promptly when the stimulus is withdrawn.

• Frequently associated with eyelid flutter, a vertical oscillation of the eyelids and eyeballs; sometimes it is associated with discrete jerking, mostly involving musculature of the face and head

• Overall <1% of EEG’s

• No association with epilepsy.

• More common in patients who are anxious or have a psychiatric disorder

Photoparoxysmal response

• Generalised spike wave discharges elicited by photic stimulation

• Photoconvulsive response if photic stimulation triggers convulsion

• Most often seen in the generalised epilepsies such as JME, but can be seen in patients with focal epilepsy

• Features that correlate with increased association with epilepsy:

o Frontal predominant

o High voltage discharge

o Spike wave outlasting the photic stimulus

• Waltz et al. classified PPR into four types (of increasing significance):

o Type 1, spikes with occipital rhythm;

o Type 2, parieto-occipital spikes with a biphasic slow wave;

o Type 3, parieto-occipital spikes with a biphasic slow wave and spread to the frontal region;

o Type 4, generalized spikes and waves or polyspikes and waves.

• There are other classification systems

• The presence of a PPR does not necessarily mean the patient with have photosensitive seizures

	PPR	Photic induced Sz	Other seizures
Epilepsy with PPR	Yes	No	Yes
Epilepsy with mixed seizures	Yes	Yes	Yes
Pure photosensitive epilepsy	Yes	Yes	No

• With a generalised (type 4) PPR the risk of epilepsy is 70-90%

o Likelihood of visually induced seizures is:

- 60% overall

- 30% if there are no epileptiform discharges, except in PPR

• A PPR can be seen in non-epileptic patients and is more common in relatives of epilepsy patients

• More common in younger patients with epilepsy group:

Aurlien et al Clinical Neurophysiology 2009

•

Artifacts

• Electrode ‘pop’

• Muscle

• Chewing/tongue

• Perspiration

• ECG artefact

• Pulse artefact

• Electrical Artifact (50Hz)

• Eye blink

• Eye movements

Normal Variants

Variant	Morphology	Location	State	Age	Reactivity
Age dependent variants
Posterior slow waves of youth	Theta and delta (2-4Hz) mixed with the posterior dominant rhythm	Bilateral, asymmetrical	?	Age 7-20
Hyperventilation induced slowing	Rhythmic high amplitude theta/delta	Generalised – more posterior in children, anterior in adolescents	During HV – should cease within 1 min of HV	<30 years
Temporal theta	4-5Hz theta	Temporal leads		Elderly	Hyperventilation
Rhythmic variants
Alpha variant – slow	Half frequency of standard alpha	Posterior dominant	Wake		Attenuate with eye opening
Alpha variant - fast	Twice alpha frequency, notched or bifurcated
Mu	Arciform 8-11Hz	Central Uni or Bilateral	Wake	Young Decreases with age	Attenuate with contralateral arm movement
RMTD	5-7Hz theta Monomorphic, fixed frequency.	Mid temporal Unilateral, can shift laterality	Drowsy/light sleep	Young adults
SREDA	Runs of diffuse, monomorphic theta (5-6Hz)	Bilateral but can be asymmetrical or more focal	Wake/ Drowsy/ light sleep	Adults and elderly
Midline theta rhythms	5-7Hz, smooth or arc shaped (mu like)	Cz and nearby	Wake/drowsy
Epileptiform patterns
Wicket spikes	Arciform 90-150ms, <200uV Isolated or in trains	Mid temporal	Drowsy/light sleep	?Any
SSS/BETS	Mono or biphasic spikes Duration <50ms Amplitude >50uV	Mid temporal, oblique dipole Uni or bilateral Synchronous or asynchronous	Stage I or II sleep	?Any
PSW/6Hz spike wave	5-7Hz burst of spike and slow wave Small spikes buried in slow wave Last 1-2 sec	Bilaterally synchronous	Drowsiness/light sleep	Young adults	WHAM and FOLD varieties
14 and 6Hz positive waves	0.5-1sec run of positive, arciform sharp waves	Posterior temporal and occipital	Drowsiness/light sleep	Children/young adults
Lamba/POSTS
Lambda	Single, positive, sawtooth/ triangular waves 160-250ms	Occipital Bilateral but asymmetrical	Wake	Young Decreases with age	Promoted by scanning /saccadic eye movements
POSTS	Similar to lambda, may be biphasic May occur in trains	Occipital Usually synchronous	Stage I and II sleep	Young Decreases with age

Alpha variants

• Harmonics of normal alpha

• Fast alpha variant (harmonic) – twice normal alpha frequency

• Slow alpha variant (subharmonic) – 4-5Hz, notched appearance

• Reactive to eye opening, stimulation

• Same features as normal alpha activity

Mu rhythm

• Sharply contoured, arch shaped

• 8-11Hz (tend to occur at similar frequency to patients background)

• Central regions – unilateral or bilateral

• Attenuate with arm movement or thought of movement

• Does not attenuate with eye opening (to differentiate from alpha)

• Disappear with sleep

• Seen in younger individuals and decreases with age

Lambda waves

• Single, occipitally based positive waves, sawtooth

• Usually bilateral, but often significant asymmetry

• Probably a form of visual evoked potentials (increased by visual scanning tasks)

• Occur when eyes open, not seen in sleep

• Seen in younger individuals and decreases with age

Positive Occipital Sharp Transients of Sleep (POSTS)

• Monophasic, triangular waveforms, positive (similar to lambda waves)

• Occipital regions

• Occur in stage I and II of sleep (not in wakefulness)

• Usually synchronous

• May occur in trains of 4 or 5, but not rhythmic

Wicket spikes/ Wicket Rhythms

• Sharp waves 90-150ms duration, <200uV amplitude

• Appear similar to mu – i.e. arciform

• Occur in trains or isolated

• Often when drowsy and light sleep

• Mid-temporal

• Symmetrical (c.f. epileptic discharges)

Rhythmic (Mid)Temporal Theta Bursts of Drowsiness (RMTD)

• Also previously known as Psychomotor variant

• 5-7Hz discharge in the temporal area, occurs in relaxed wakefulness or drowsiness

• Sharp on one side, rounded or flat topped on the other

• Monomorphic, constant frequency, does not evolve

• Can shift laterality

• No after-going slow waves

• Young adults

SREDA (Subclinical Rhythmical EEG Discharges of Adults)

• Bilateral, diffuse discharges, temporal (and parietal) regions, 5-6Hz

• Can be asymmetric or appear more focal

• Usually 20-40sec, but can last minutes

• Relaxed wakefulness or drowsiness

• Occur in adults and elderly

• Monomorphic theta sharp waves

• Compared to true epileptiform discharge:

o No clear cut spikes

o No recruitment

o No alteration in consciousness

o Does not (?rarely) occur in sleep

o No post-ictal slowing

Small Sharp Spikes/ BETS (benign epileptiform transients of sleep)

• Mono or biphasic spikes, amplitude <50uV, duration <50ms

• Sleep stages I and II

• Mid temporal

• Complex polarity, oblique dipole extending over both hemishperes (atypical for epileptifrom discharge)

• Unilateral or bilateral, synchronous or asynchronous

• Some lingering argument as to an association with epilepsy

Phantom Spike-Wave Discharges

• Also called: 6Hz Spike and Waves Bursts OR Six per second spike wave

• 5-7Hz, bilaterally synchronous burst of spike and slow wave

• Spike often low amplitude and buried in slow wave

• 1-2sec bursts

• Seen in drowsiness and light sleep

• Adolescents and young adults

• FOLD – Female, occipital, low amplitude, drowsy - Benign

• WHAM – wake, high amplitude, anterior, male - Some association with epilepsy

14 and 6Hz Positive Spikes

• 0.5-1sec runs of positive sharp waves

• Arciform or comb shaped

• Frequency either ~14Hz (most common) or ~6Hz

• Widespread field, best seen over posterior temporal and occipital areas

• Usually bilateral but asynchronous

• Drowsiness and light sleep, usually up to young adulthood

• Positivity best seen in a referential montage

Lateral Rectus Spikes

• Occur with contraction of the lateral rectus muscles

• Low amplitude, short duration

• Seen best in F7/F8

• Very sharp

• Disappear in non-REM sleep when eyes relaxed

Posterior slow waves of Youth

• Theta or Delta waves that intermix with the posterior rhythm

• From about age 7 to 20

• Occipital areas

• Suppress with eye opening

• Should not exceed PDR amplitude by more than 50%

• Can be asymmetrical

Abnormal EEG

Focal abnormalities

Epileptiform discharges – see epileptiform section below

Lateralised Periodic Discharges (LPDs) [Previously Periodic lateralized epileptiform discharges (PLEDS)]

• Periodic – occurring with a interdischarge interval i.e. occurring regularly every second or so.

• Usually broad, high voltage sharply contoured slow wave or sharp and slow wave complex (sometimes more triphasic appearance)

• Indicative of underlying acute pathology

• Associated with

o HSV encephalitis

o Stroke

o Tumour

o Abscess

o

PLEDS Plus +

• PLEDS with admixed faster rhythms/low amplitude polyspike

• Stronger association with epilepsy, 70% will go on to have a seizure

Poly-PLEDS

• Polyphasic/polyspike discharges

• Very high association with seizures

Bilateral independent Periodic Discharges (BIPDs) [Previously BiPLEDS]

• LPDs in both hemispheres that are not occurring synchronously (if synchronous would be GPDs – see below)

• Global or multifocal cerebral insult

Generalised LPDs (GPDs)- discussed below

Unilateral Independent Periodic Discharges (UIPDs)

• Similar to LPDs but where there are more than one periodic discharge (e.g. if you have a left temporal LPD and also a left parietal LPD occurring at a different frequency).

Brief potentially ictal rhythmic discharges (BIRDs)

• Brief (<10 sec) runs of high frequency rhythmic activity

• “Too short” to be a seizure, however highly predive of seizures

Focal Polymorphic Slowing

Focal intermittent Polymorphic Delta activity

• Arrhythmic delta with changing frequency, amplitude and morphology

• Associated with focal structural abnormality affecting the underlying white matter (cortical deafferentation)

• Localising value is increased with the fast beta frequencies are also attenuated in the same region.

Persistent (Continuous) Polymorphic Delta Activity

• Irregular delta activity (as above)

• Delta activity present for >50% of record

• Typically seen in:

o White matter lesions

o Post-ictal states

o Ipsilateral thalamic lesions

Focal monomorphic slowing

Said to more likely represent underlying grey matter dysfunction

N.B. cortical

Intermittent Rhythmic Delta Activity

• Thought to arise from dysfunction of the subcortical centres influencing the cortex

• Similar finding is seen in hyperventilation

o Adults tend to show frontally predominant – like FIRDA

o Children show occipitally predominant – like ORIDA

GRDA (Generalised intermittent rhythmic delta activity) [previously FIRDA (frontal intermittent rhythmic delta activity)]

• Short bursts of, rhythmic, synchronous bifrontal delta activity

• Often high amplitude

• Lasts only a few seconds

• Present in wakefulness and drowsiness, disappears in sleep (may return during REM)

• Non-specific aetiology - some type of cerebral disturbance – focal or diffuse

o E.g. Toxic, metabolic encephalopathy, raised ICP, deep focal lesion

OIRDA (occipital intermittent rhythmic delta activity)

• Typical ORIDA lasts only a few seconds

• Occurs during wakefulness

• In general non-specific aetiology – as per FIRDA

• Association with absence epilepsy

o Present in 15-30% of young patients with absence epilepsy – usually at 3-4Hz

o Correlated with increased sensitivity to hyperventilation

o Unlike ‘typical ORIDA’ can have very long runs

LRDA (Lateralised rhythmic delta activity) [Previously TIRDA (Temporal intermittent rhythmic delta activity)]

• Focal rhythmic delta slowing

• Unlike ORIDA/FIRDA it is a localising abnormality – suggestive of temporal lobe dysfunction/epilepsy

• Strong association with the presence of temporal lobe spikes

• Almost always ipsilateral to epileptiform focus

• Rarely can be ‘false localising’ and associated with extratemporal lobe focus

Generalised abnormalities

Generalised slowing

• Three forms (levels of severity)

o Background slowing

- Rhythm too slow for patients age

o Intermittent slowing

- Bursts of generalized slowing

o Continuous slowing

- Polymorphic delta activity for >80% of tracing

• Graduated from best to worst

• Most commonly associated with encephalopathy (= diffuse cerebral disturbance/dysfunction)

• No specific aetiology

Periodic patterns – GPDs (Generalised periodic discharges) [previously GPEDS]

• Periodic (i.e. regular) bursts of activity

• Complex, epileptiform

• Similar to PLEDs/Poly-PLEDS but generalized, can be called GPEDs

• Some reserve the term GPEDS for when there is no normal background between the discharges

• If there is normal background between the discharges then they may be better described as frequent interictal discharges or status epilepticus

• GPEDS are associated with a poor prognosis in obtunded patient (particularly post anoxic brain injury)

• Aetiology

o Any severe cerebral injury

o A variety of GPEDs/triphasic waves may be seen in CJD

o SSPE – discharges every 3-20seconds

Burst-suppression

• Bursts on quiet background

• Indicative of severe encephalopathy

• Prognostic significance depends on clinical scenario

o May indicate medication effect/overdose – esp. barbiturate – in which case full recovery may be expected

o In setting of post-anoxic injury it is a very poor prognostic marker

Background suppression

•

Electrocerebral inactivity

• Brain death

Triphasic waves

• Triphasic morphology – typically negative, positive, negative

• May have anterior-posterior lag

• Were specifically associated with hepatic encephalopathy but can occur in multiple encephalopathies

• A variety of triphasic waves is seen in CJD

CJD Pattern

• Periodic ~1Hz

• Triphasic waves – however may be sharper, more epileptiform (similar to GPEDs)

• May be more posterior in variant disease

Alpha-pattern coma

• Diffuse alpha pattern

• Occurs after arrest, pontine stroke and some drug overdose

• Had been thought to predict poor prognosis

Spindle Coma

Epileptiform abnormalities

Definition

• Spike (<80msec)

• Sharp wave (80-200msec)

• Spike-wave

IFCN definition

(1) Di- or tri-phasic wave with pointed peak;

(2) different wave duration than the ongoing background activity;

(3) asymmetry of the waveform;

(4) followed by a slow after-wave;

(5) the background activity is disrupted by the presence of the IEDs

(6) voltage map with distribution of the negative and positive potentials suggesting a source in the brain corresponding to a radial, oblique, or tangential orientation of the source

(From Kural MA, Duez L, Sejer Hansen V, et al. Criteria for defining interictal epileptiform discharges in EEG: A clinical validation study. Neurology. 2020;94(20):e2139-e2147. doi:10.1212/WNL.0000000000009439)

(From: Kural MA, Qerama E, Johnsen B, Fuchs S, Beniczky S. The influence of the abundance and morphology of epileptiform discharges on diagnostic accuracy: How many spikes you need to spot in an EEG. Clin Neurophysiol. 2021;132(7):1543-1549. doi:10.1016/j.clinph.2021.03.045)

Focal

Generalised

Spike wave

• Spike – negative polarity 30-60ms, followed by dome shaped wave of negative polarity 150-200ms

• Analysing aEEG one study found 96% of individual discharges are symmetric but only 24% typical morphology

• Maximal amplitude typically over fronto-central region

• Field maxima during absence seizures Fz, with spread to F3, F4 and Cz.

• Classically discharges are considered regular, although studies suggest 60% are irregular.

• Frequency

o JME - >3.5Hz

o JAE – 3.25 Hz

o CAE – 3Hz

o

Seizures

Tonic-clonic

• Tonic phase

o Spikes and polyspikes

o 10Hz

o Increase in amplitude and decrease in frequency during first 10-20sec

• Clonic phase

o Bursts of generalised high amplitude spike and slow wave or polyspike coninciding with clonic jerks

o Frequency gradually decreases

• Attenuation then diffuse slowing

Absence

• 3Hz spike-wave

o The distinction between an ictal and interictal discharge can be difficult in this situation.

o A seizure might be defined by a change in clinical state or length of discharge (generally considered >2-3sec)

Atypical absence

• Slow spike and wave

• As seen in LGS

Myoclonic

• Polyspike-wave 5-6Hz

Tonic/Atonic

• Paroxysmal fast activity/electrodecrement

Infantile Spasms

• Slow wave followed by electrodecrement

Focal seizures

• Often begin with rhythmic theta

• Evolve - higher amplitude and faster or slower frequency

• Often end abruptly, but can wane

• Can be rhythmic beta (more often seen with intracranial electrode recordings)

• Sometimes rhythmic spikes or sharp waves (similar to the interictal)

Encephalopathies

• Hepatic encephalopathy
• Diffuse triphasic waves
• HSV
• PLEDs – 1-4 seconds
• Usually unilateral – temporal region
• Low sensitivity, high specificity
• sCJD
• Generalized periodic sharp wave complexs (PSWC)
• Like PLEDs but generalized (GPEDs)
• Typical ~1Hz
• Often triphasic
• Sens 65% but Spec ~90%
• Do not occur in vCJD
• SSPE
• Generalized periodic complexes lasting up to 3 sec with 3-15sec intervals.

Neonatal EEG

• Timing of EEG patterns is described in terms of post-conception age (CA)(i.e time since LNMP)

• Described as CA (i.e. CA 40 is term)

Sleep stages

• Awake

• Active sleep

o Analagous to REM sleep (except unlike adults, baby is not ‘paralysed’)

o Eye closed

o Baby moves and squirms

o Rapid eye movements

o Facial movements/grimacing

• Quiet sleep

o Deep sleep – evolves into slow wave sleep

o No eye movements

Background patterns

• Low Voltage Irregular (LVI)

o Low voltage (15-35uV), mixed frequencies

o Mainly delta and theta

o Wakefullness and active sleep

• Mixed Pattern

o Similar to LVI but with higher voltages and more slow frequencies

• High voltage slow (HVS)

o Continuous, irregular mixed frequencies but higher voltages (50-150uV)

o Delta frequencies more common

• Trace alternans

o Bursts of mixed activity 2-8 seconds

o Flatter interbursts of 4-8sec

o Bursts and interbursts are of similar length

o Pattern evolves with time:

o Interbursts gradually get shorter

o More activity fills in the interbursts

o Interhemispheric synchrony increases

• Trace discontinu

o Very high voltage polymorphic bursts, often containing sharp features/polyspikes

o Separated by flat periods of up to 10-20sec

o Looks like burst suppression

o Normal pattern of early prematurity

Synchrony

• Exists up to 30 weeks then become asynchronous (transition from trace discontinue to alternans)

• Synchrony then increases again as term approaches (transition from trace alternans to HVS)

Continuity

• Should be established in wakefulness by 34 weeks

• In quiet sleep starts to be seen from 38 weeks.

• There should be no discontinuity after 48 weeks

Graphoelements

• Specific wave forms that appear and disappear at certain CA’s

• Delta Brush

• Sharp transients

• Frontal transients

• Temporal sharp transients

• Vertex waves

o Seen from 8 weeks (CA 48)

• Spindles

o Appear at CA 44-46 (i.e. from 2^nd month of life)

o Spindles tend to be asynchronous up to age 2

o Spindles are longer in childhood and get shorter

Intensive care/Critical Care EEG

• Predictive patterns