0-EEG waveforms are defined by frequency.

  • Frequency means the number of waves per unit of time (sec)
  • The frequency of the EEG waves run from 0.5/sec to hundreds/second
  • Machines usually show frequencies up to 25-32/sec
  • EEG waves are defined by their frequency and are divide into 2 groups and 4 bands.

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High Frequency

Alpha

  • Between 8 – 13Hz
  • Usually seen in the posterior regions of the head on each side, higher in amplitude on the dominant side
    • Often 50% greater in amplitude over the right hemisphere
  • Brought out by closing the eyes and relaxation
  • Abolished by opening the eyes or thinking/calculating
  • Major rhythm seen in normal relaxed adults (13 yrs and older)

Beta

  • 13 – 35Hz
  • Usually seen on both sides in symmetrical distribution and most evident frontally
  • Accentuated by sedative/hypnotic drugs (benzo’s and barbs)
  • May be absent or reduced in areas of cortical damage
  • Generally regarded as a normal rhythm
  • Dominant rhythm in patients who are alert with their eyes open
  • Associated with intense mental activity

Gamma

  • 35Hz or above
  • Associated with conscious perception and attention processing

Low Frequency

Theta

  • 4 – 7Hz
  • Classified as slow activity
  • Abnormal in awake adults, but normal in children up to 13 yo and in sleep
  • Can be seen as a focal distribution in focal subcortical lesions
  • Can be seen in generalized distribution in metabolic encephalopathy, diffuse disorders, or midline disorders

Delta

  • Below 4Hz
  • Tends to be the highest amplitude and the slowest waves
  • Quite normal and is the dominant rhythm in infants up to 1 yr and in stages of 3 &4 of sleep
  • May occur focally with subcortical lesions
  • May be seen in general distribution with diffuse lesions, metabolic encephalopathy, hydrocephalus, or deep midline lesions
  • Cardiac activity may appear as delta on CSA/DSA displays
  • Usually most prominent frontally in adults
    • FIRDA – Frontal Intermittent Rhythmic Delta
      • Seen in hydrocephalus and some toxic/metabolic states
  • Usually most prominent posteriorly in children
    • OIRDA – Occipital Intermittent Rhythmic Delta

Defined by Morphology

  • Certain waves have characteristic forms irrespective of their frequency and are recognized by their shape
  • In other instances pair or groups of waves have typical appearances
  • Single waves that are specially shaped include spikes or sharp waves that rise rapidly to a point and fall away equally dramatically with a base that is small compared to the wave’s amplitude
    • Spikes are narrow-based waves that have a relatively high amplitude giving them a narrow, high form, and a sharp top
    • Sharp waves are slightly broader than spikes

Artifacts

  • Disturbances caused by technical defects, usually transitory

Caused by:

  • Electrode movement
  • Loss of contact
  • Muscle activity obscuring the EEG
  • Movements of the head
  • Scratching the scalp
  • Sweating

Spike and Waves

Unknown

  • Seen in all ages, but mostly children
  • Consist of a spike and a large amplitude slow wave (usually delta)
  • May occur spontaneously and symmetrically in the generalized epilepsies or focally in the partial ones

Polyspike and Waves

Unknown

  • Each slow wave is accompanies by 2 or more spikes
  • Often associated with myoclonus or myoclonic seizures.

Burst Suppression

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  • A pattern of burst of slow mixed waves ofter at high amplitude alternating with a flat baseline
  • The pattern is bilateral, but not always symmetrical
  • Usually seen after severe brain injury or in deep anesthesia prior to total flattening of the EEG

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