Electroencephalography

Here are some notes about various electroencephalography (EEG) features including features of normal wakefulness, features of sleep, features of epilepsy/seizures, artifacts and non-specific features.
Notes:

  • Spike: single wave that stands out from the background, lasting < 8milliSeconds
  • Sharp: single wave that stands out from the background, lasting > 8milliSeconds
  • Any frequency may discharge

Types of Montages:

  • Double banana
  • Referential

Types of activations:

  • Three minutes hyperventilation
  • Flashing strobe light at different frequencies

 

Steps of description & interpretation:

  • Look at posterior dominant rhythm PDR ~background:
  • Rhythm & frequency
  • Response to opening & closing
  • Well, moderately or poorly developed
  • Look at other leads ~background:
    • Changes, slowing, abnormalities, asymmetry
  • Look at Sleep:
    • What stages achieved
  • Changes during them
  • Look at hyperventilation:
    • Did normal slowing occur
  • Changes during them
  • Look at photostimulation:
    • Was there photic driving (same frequency, subharmonics, asymmetry)
  • Look for changes during this
  • Look for epileptiform discharges
  • If spikes, sharps are found, confirm them with referential montage

 

Normal EEG patterns:

Wakefulness:

  • Alert, Eyes closed: Posterior (occipital) prominent alpha rhythm
  • Alert, Eyes open: disappearance or “block” of posterior prominent alpha rhythm

 
Apha rhythm:

  • Features: 8-13 Hz, occipital>> parietal, temporal regions bilaterally. Sinusoidal.
  • Correlation/comment: if it fails to block (Bancaud phenomenon) indicates structural abnormality on that side.

 
Mu rhythm (a.k.a. rolandic alpha):

  • Features: Centrally located alpha frequency (unilateral or bilaterally)
  • Correlation/comment: attenuate with movement, thought of movement and tactile stimulation. Abnormal if nonreactive

 
Beta rhythm:

  • Features: >13 Hz
  • Correlation/comment: increased by cognition or stage 1 sleep, benzodiazepines, bartiburates, chloral hydrate, and sometimes by antihistamines, antidepressants

 
Lambda waves:

  • Features: in the occipital region
  • Correlation/comment: response to a stimulus in the visual field evoking saccades

 
Sleep:
Stage I:

  • Disappearance of the “alpha rhythm“
  • Increasing Fronto-central beta activity
  • Increasing fronto-central-temporal theta activity
  • V-waves= Vertex sharp waves (High voltage single or complex diphasic sharp transients), appear centrally C3, C4. Phase reversal in midline on coronal montages
  • POSTS may occur
  • Small sharp spikes (benign epileptiform transients of sleep) may occur

Stage II:

  • V-waves= Vertex sharp waves
  • Sleep spindles occur (centrally predominant runs of sinusoidal 12 to 14 Hz activity)
  • K-complex: diphasic sharply contoured transients followed by slow waves

Slow wave sleep a.k.a. Stage III-IV:

  • Delta rhythm (Delta slow waves)

REM sleep:

  • Similar to awake EEG
  • REM, detected in frontal eye-fields
  • Muscle hypotonia on EMG

POSTS: positive occipital sharp transients

  • Features: bisynchronous sharp transients (surface-positive followed by low amplitude surface-negativity)
  • Correlation/comment: a feature of stage I-II sleep

Small sharp spikes (benign epileptiform transients of sleep) may occur

  • Features: they don’t disrupt the background
  • Correlation/comment: a feature sleep

 

Artefacts:

  • Prominent waveform confined to 1 electrode is an artefact until proven otherwise, i.e. it doesn’t have a field.
  • Waveforms that jump from nonhomologous areas or nonadjacent areas of the brain.
  • Eye movement artifact. Frontally located.
  • Muscle artifact: very short, occurs in clusters.
  • Photomyogenic response:
    • Features: muscle spikes in frontal leads (with forehead and eyelid twitch) with same frequency of flashing light
    • Correlation/comment: non-cerebral response

 

Patterns in relation to epilepsy:

Patterns with high specificity for epilepsy:
3 per second spike-wave/polyspike-wave discharge

  • Correlation/comment: a feature of generalized epilepsy such as idiopathic generalized epilepsies

 
4 per second spike-wave/polyspike-wave discharge

  • Correlation/comment: a feature of generalized epilepsy such as idiopathic generalized epilepsies

 
Hypsarrhythmia

  • Features: very high amplitude multifocal irregular spikes, variable duration and size, chaotic
  • Correlation/comment: infantile spasms

 
Generalised photoparoxysmal response

  • Features: occurs during photic stimulation
  • Correlation/comment: associated with idiopathic generalized epilepsy e.g. JME

 
Patterns with moderate correlation with epilepsy:
Focal sharp waves in centro-temporal region
 
Focal sharp waves in occipital region
 
Patterns with low predictive value for epilepsy/normal variants:
Posterior slow waves of youth (sail waves):

  • Features: single 2-4 Hz triangular-contoured slow waves interspersed with alpha activity of the posterior leads
  • Correlation/comment: Normal variant

 
Wicket spikes:

  • Features: arc-like waves that occur in the temporal leads in runs. They don’t disrupt the background.
  • Correlation/comment: normal variant

 
Temporal transients:

  • Features: episodic trains of 2-5 Hz slow waves in the temporal leads
  • Correlation/comment: normal in >40yo

 
Other patterns:

  • 14 and 6 Hz spikes
  • Phantom spike and wave
  • Rhythmic mid temporal theta
  • Photoparoxysmal response
  • Psychomotor variant
  • Subclinical rhythmic epileptiform discharge in adults (SREDA)

 

EEG patterns and their correlation:

Generalised theta and delta activity (Generalised slow activity):

  • Correlation/comment: encephalopathy

 
Triphasic waves, “triphasic delta waves” or “liver waves”:

  • Features: 3 phase waves, AP (anterior-posterior) delay from lead to lead, high amplitude slow waves
  • Correlation/comment: indicate hepatic encephalopathy, renal encephalopathy, hypoglycemia, hyponatremia, hypercalcemia, hyperthyroidism, drug intoxication, anoxic brain injury.

 
Slow waves with low amplitudes:

  • Correlation/comment: indicate hypoxic encephalopathy

 
Frontal intermittent rhythmic delta activity (FIRDA):

  • Correlation/comment:
    • In young patients may indicate raised ICP
    • In elderly, non-specific.

 
Alpha coma:

  • Features: monotonous unreactive alpha, frontally predominant
  • Correlation/comment: poor prognosis in anoxic brain injury. Can occur in drug overdose and vascular lesions (ventral pons, sparing the tegmentum).

 
Polymorphic delta activity (PDA):

  • Correlation/comment: usually indicates a focal lesion (involving grey & white matter)

 
Increased beta activity:

  • Correlation/comment: Caused by drugs e.g. Alcohol, benzodiazepines, barbituates, neuroleptics,

 
Causes of Generalized periodic epileptiform discharges (GPEDs):

  • Metabolic encephalopathy
  • Infectious encephalopathy
  • Subacute sclerosing panencephalitis (SSPE)
  • Creutzfeld–Jakob disease (CJD)
  • Structural lesions
  • Status epilepticus

 
Periodic lateralised epileptiform discharges PLEDs:

  • Features: Periodic high-voltage sharp waves
  • Correlation/comment: Indicate unilateral lesion e.g. herpes encephalitis, brain tumour

 
SIRPIDs Stimulus-induced rhythmic, periodic, or ictal discharges:

  • Features: Rhythmic, periodic or ictal-appearing EEG patterns elicited by stimulation
  • Correlation/comment: encephalopathy

 
Photomyoclonic response:

  • Features: muscle contraction of facial muscles 50-60ms after each flash of photic stimulation. The amplitude increases as flashes continue
  • Correlation/comment: increased by alcohol withdrawal

 

Patterns by etiology:

  • Toxic encephalopathy: generalised slowing and beta activity

 

Pediatric EEG (<18 years old):

EEG in prematurity:
Delta brushes:

  • Features: delta waves with superimposed rhythmic fast activity. Initially rolandic then, occipito-temporal with age.
  • Correlation/comment: a pattern of prematurity, may also be seen in full term neonates disappear by 1 month

 
Rate temporal sharp waves:

  • Correlation/comment: seen a premature infants

 
Sleep in pediatric EEG:
Age specific rhythm:

  • 24-28 weeks: discontinuous EEG during wake and during sleep. No changes on stimulation
  • 32-36 weeks: Trace discontinue: discontinuous EEG during sleep only. The interburst activity is low voltage (<25 microV) but gradually increased until it becomes trace alternans pattern (>25 microV) of full term neonates

Trace discontinue:

  • Features: discontinuous EEG during sleep only. The interburst activity is low voltage (<25 microV) but gradually increased until it becomes trace alternans pattern (>25 microV) of full term neonates
  • Correlation/comment: sleep in premature neonates.

EEG in infants:
Wakefulness:
Alpha rhythm:

  • Features: similar to adult
  • Correlation/comment: first seen at 3-4 Months (3.5-4.5Hz), the frequency changes: At 1 year 5-7 Hz, 2 years 6-8 Hz, 3 years 7-9 Hz, 7 years 9 Hz,

 
Activite moyenne

  • Features: continuous low to medium voltage mixed-frequency activity
  • Correlation/comment: Wakefulness in 36-40 weeks

 
Sleep:
Trace discontinue

  • Features: full term neonates have a discontinuous rhythm in quiet sleep, 3-6Hz fast waves lasting 3-8 seconds. Occur from 36-44 weeks.
  • Correlation/comment: Trace alternans is a feature of sleep in full-term neonates.

 
Sleep pattern in >44 weeks:

  • Features: continuous slow wave sleep. No longer discontinuous (trace discontinue or trace alternans)
  • Correlation/comment: sleep pattern in mature infants

 
Vertex waves:

  • Features: similar to adult
  • Correlation/comment: first seen at 2-3 months

 
K complexes:

  • Features: same as adults
  • Correlation/comment: appear at age 5 months

 
Sleep spindles:

  • Features: same as adults
  • Correlation/comment: appear at age 2 months (asynchronous until age 18 months)

 
Rhythmic anterior slow waves:

  • Correlation/comment: can be seen during sleep in full-term infants

 
Abnormal patterns:
Interhemispheric asynchronous bursts

  • Features: delay of 1.5 seconds between bursts in each hemisphere
  • Correlation/comment: prematurity, diffuse encephalopathy, cerebral dysgenesis

 
EEG in children:
Hyperventilation in children under the age of 8 usually causes slowing in the posterior head regions and in children over 8 in the anterior head regions.
 

References:

  1. Husain AM, Mebust KA, Radtke RA. Generalized periodic epileptiform discharges: etiologies, relationship to status epilepticus, and prognosis. J Clin Neurophysiol. 1999 Jan;16(1):51-8.
  2. Yemisci M, Gurer G, Saygi S, Ciger A. Generalised periodic epileptiform discharges: clinical features, neuroradiological evaluation and prognosis in 37 adult patients. Seizure. 2003 Oct;12(7):465-72.