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					EEG : Basic Principles and Montages




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



                                      1
    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

                                                    2
    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

                                                 3
Electrode Placement
Conventional 10-20 system                                                  Expanded
                                                                           10% system

T7                                     T8




 P7                                    P8


 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


                                                                                         4
    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

                                                    5
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 (A1+A2
    linked ear reference or
    laplacian reference varies
    from channel to channel)

                                     6
Localization of voltage peaks
   F4 max. electronegativity: F4 peak > 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)



                                                      7
    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



                                                 8
     Background EEG in Adults
   Alpha:   8-13 Hz, posterior predominant, symmetric,
             Amp 30-60 V, R>L by 20-50%,  by EO, drowsiness
             Age 60-80, =9.5 Hz;  < 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 < 20 V and lower in elderly,
              by benzodiazepine, light sleep and skull defect


                                                                   9
    Response to Photic stimulation
   Asymmetrical  unilateral destructive
    occipital lesion

   Photomyoclonic response at f=12-18 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
                                                  10
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 > 200ms outlasting flash stimulus


                                                    11
    Photic driving response
   Rhythmic occipital
    dominant waveform
   Occurs at stimulus
    frequency 5-30 Hz,
    especially at 8-13 Hz
   Associated with
    lamda and POST




                              12
    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


                                                 13
Common EEG artifacts

   Electrical mains
   Eye movement
   ECG
   Head movement
   Muscle
   Sweating
   Electrode
                       14
Application of EEG in Epilepsy




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


                            15
     ? 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


                                                                                      16
    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 < 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
                                                                                   17
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


                                                         18
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

                                                                19

				
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