EEG : Basic Principles and Montages

1
EEG Basic Principles and Montages

• Dr. Jason K.Y. Fong,
• FRCP (E), FHKAM(Med)
• President, HKES

2
Clinical Uses of EEG

• Diagnosis of epilepsy
• Classification of epileptic syndromes
• Localization of epilepsy in presurgical
  evaulation
• Management of status epilepticus
• Aids in specific neurological diagnosis
• e.g. CJD, HSV encephalitis, SSPE
• Determine progress prognosis in comatose
  conditions

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Diagnostic pitfalls of EEG

• Sensitivity of EEG ? with prolonged or repeated
  recordings (up to 80), but a normal interictal
  EEG does not exclude epilepsy
• EEG cannot replace neuroimaging in diagnosis of
  structural brain lesion
• Normal EEG variants or artifacts may be confused
  with epileptiform discharge
• Clinical correlation is always necessary for
  appropriate and meaningful interpretation

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Electrode Placement
Conventional 10-20 system
Expanded 10 system
T7
T8
P8
P7
20-30 min recording during awake resting state
with electrode placement according to 10-20
system followed by overbreathing for 3 min and
photic stimulation extension to include light
sleep will increase sensitivity of detection of
epileptiform discharges
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Basic Terminology

• Montage patterns of connection between
  electrodes usually 16 or more electrodes
• Referential background rhythm interpretation
• Bipolar adjacent electrodes are linked along
  longitudinal (parasaggital) or transverse
  (coronal) lines useful for localization
• Additional electrodes are sometimes required e.g.
  T1/T2, sphenoidal, nasopharyngeal

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Commonly used montages

• Polarity convention
• Upward negative field
• Input 2 is the nearest neighbouring electrode and
  changes from channel to channel (bipolar
  derivation)
• Input 2 is a distant electrode common to all
  channels (common reference)
• Input 2 is computed (A1A2 linked ear reference
  or laplacian reference varies from channel to
  channel)

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Localization of voltage peaks

• F4 max. electronegativity F4 peak gt Fp2, C4, Fz
  and F8 (using referential)
• Phase reversal at F4 using longitudinal montage
  (Fp2-F4, F4-C4) or transverse montage (F8-F4,
  F4-Fz)
• Not applicable for electrode at the end of the
  chain e.g. O1, Fp1
• The Laplacian source derviation is also helpful
• If voltage peaks at F4 and C4, in phase
  cancellation occurs for F4-C4, resulting in no
  output (isoelectric)

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Factors affecting EEG interpretation

• Age maturation of EEG (refer to Niedermeyer)
• Arousal refers to different sleep stages
• Medications e.g. benzodiazepines
• Pathological brain condition (e.g.craniotomy)
• Environment e.g. a.c. interference, ICU setting
• Quality of recording aware of artifacts

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Background EEG in Adults

• Alpha 8-13 Hz, posterior predominant,
  symmetric, Amp 30-60 ?V, RgtL by
  20-50, ? by EO, drowsiness Age 60-80,
  ?9.5 Hz ? lt 8 in elderly suggests A.D.
  
  Slow (sub-harmonic, 4-5 Hz) and fast alpha
  (16-20 Hz)
• Theta Usually low amplitude at frontal central
  region (6-7Hz)
• Rhythmic temporal theta bursts of drowsiness
• Midline theta rhythm (Cz max)
• Delta Diffuse in deep sleep, metabolic
  encephalopathies
• Focal in structural brain lesion
• Beta 18-25 Hz, frontal-central predominant,
• Amp lt 20 ?V and lower in elderly,
• ? by benzodiazepine, light sleep and
  skull defect

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Response to Photic stimulation

• Asymmetrical ? unilateral destructive occipital
  lesion
• Photomyoclonic response at f12-18 Hz associated
  with brainstem lesion or psychiatric disorders
  but not epilepsy
• Photoparoxysmal response most easily elicited at
  f 15-20 Hz not time locked to flash stimulation
  

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Significance of PPR

• Prevalence of PPR 2-4 in epileptic population
  peak in adolescents (10) 0.02 among normal
  population
• Associated with GTCS, myoclonic, absence seizures
  and photosensitive epilepsy
• Stronger association with epilepsy if
• 1) anterior predominant or generalized
• 2) easily and consistently elicited
• 3) duration gt 200ms outlasting flash stimulus

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Photic driving response

• Rhythmic occipital dominant waveform
• Occurs at stimulus frequency 5-30 Hz, especially
  at 8-13 Hz
• Associated with lamda and POST

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Response to Hyperventilation

• Asymmetrical slow waves imply focal pathology
• Accentuation of epileptiform discharges
• 3 Hz spike wave discharge in absence epilepsy
• Trigger seizures, generalized or focal
• Contraindicated in patients with recent stroke,
  SAH, or cardiac disease

14
Common EEG artifacts

• Electrical mains
• Eye movement
• ECG
• Head movement
• Muscle
• Sweating
• Electrode

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Application of EEG in Epilepsy

• Dr. Jason K.Y. Fong,
• FRCP (E), FHKAM(Med)
• President, HKES

16
? Abnormal EEG

• An abnormal EEG may consist of
• Abnormal changes in normal rhythm
• If asymmetrical, the side with lower
  amplitude is usually pathological
• Abnormal slow activity
• A sensitive indicator of encephalopathy if
  diffuse correlate with regional cerebral
  dysfunction if localized may appear as
  intermittent rhythmic delta (FIRDA in adult or
  OIRDA in children)
• Distinctive abnormal pattern
• Regular repetition of spikes, sharp waves,
  slow waves or any of the combination
• e.g. PLED, burst suppression, triphasic waves,
  pseudoperiodic generalized sharp waves in CJD,
  pseudoperiodic slow complexes in HSV encephalitis
• Epileptiform discharges
• Spikes, polyspikes, sharp and slow waves

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Epileptiform discharges

• An interpretive term referring to
• distinctive waves or complexes, distinguished
  from background activity, and resembling those
  recorded in a proportion of humans with epilepsy
• The distinctive waves may be
• Spikes (duration lt 70ms)
• Spikes and slow waves
• Polyspikes /- slow waves
• Sharp waves (duration 70-200ms)/- slow waves
• Periodic complexes, PLED
• Pseudoperiodic complexes
• Epileptiform discharge is highly correlated with
  epilepsy and false positive rate is about 1 in
  normal population
• Epileptiform discharge may be found in 1st degree
  relatives of patients with idiopathic epilepsy,
  or in the presence of structural
• brain lesion e.g. tumor, stroke, cerebral palsy,
  previous skull defects

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Generalised Spike Wave Complex

• 1.5-2.5 Hz Lennox Gestaut syndrome
• Atypical absence, mental retardation
• 2.5-4 Hz Idiopathic absence
• 4-6 Hz Myoclonic /- grand mal
• 6 Hz Positive spikes in adult
• Anterior discharge more epileptogenic
• Occipital form likely normal variant

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Interictal Epileptiform Discharge

• Paroxysmal
• i.e. clearly distinguished from background
  activity
• Sharp contour or spiky
• Duration 20-200ms
• Predominantly triphasic, sometimes biphasic
• Physiological field
• Differentials
• Vertex waves, mu rhythm, small sharp spikes,
  wicket spike, POST, occipital 6 Hz spike wave, 14
  and 6 Hz positive spikes, EMG/electrode artifact