1. Senior Consultant Neurologist Singapore General Hospital
Normal EEG Patterns
Dr Lim Shih Hui
Senior Consultant Neurologist
Singapore General Hospital

2. EEG Interpretation Normal Abnormal Lack of Abnormality
Non-epileptiform Patterns
Epileptiform Patterns

3. Alpha Rhythm The starting point of analysing awake EEG
8-13 Hz activity occurring during wakefulness
20-60 mV, max over posterior head regions
Present when eyes closed; blocked by eye opening or alerting the patient
8 Hz is reached by 3 years of age and progressively increases in a stepwise fashion until 9-12 Hz is reached by adolescence
Very stable in an individual, rarely varying by more than 0.5 Hz.
With drowsiness, alpha activity may decrease by 1-2 Hz
A difference of greater than 1 Hz between the two hemispheres is significant.
10% of adult have little or no alpha

4. Normal Alpha Rhythm

5. Normal Alpha Rhythm

6. Alpha Rhythm: Reactivity
Should attenuate bilaterally with
eye opening
alerting stimuli
mental concentration
Some alpha may return when eyes remain open for more than a few seconds.
Failure of the alpha rhythm to attenuate on one side with either eye opening or mental alerting indicates an abnormality on the side that fails to attenuate

7. Normal Alpha Reactivity

8. Normal Alpha Reactivity
Eyes Closed
Normal Alpha Reactivity

10. Beta Activity
Frequency of over 13 Hz; if >30-35 Hz  gamma activity or exceedingly fast activity by Gibbs.
Average voltage is microvolts
Two main types in adults:
Often enhanced during drowsiness or when present over a skull defect
Should not be misinterpreted as a focus of abnormal fast activity.

11. Beta Activity Two main types in adults:
Frequency of over 13 Hz; if >30-35 Hz  gamma activity or exceedingly fast activity by Gibbs.
Average voltage is microvolts
Two main types in adults:
The precentral type: predominantly over the anterior and central regions; related to the functions of the sensorimotor cortex and reacts to movement or touch.
The generalized beta activity: induced or enhanced by drugs; may attain amplitude over 25 microvolts.
Often enhanced during drowsiness or when present over a skull defect
Should not be misinterpreted as a focus of abnormal fast activity.

12. Generalized Beta Activity

13. Beta Activity
Frequency of over 13 Hz; if >30-35 Hz  gamma activity or exceedingly fast activity by Gibbs.
Average voltage is microvolts
Two main types in adults:
Often enhanced during drowsiness or when present over a skull defect
Should not be misinterpreted as a focus of abnormal fast activity.

14. Theta Activity
The term theta was coined by Gray Walter in 1944 when it was believed that this rhythm was related to the function of the thalamus.
Occurs as a normal rhythm during drowsiness
In young children between age 4 months  8 years: predominance over the fronto-central regions during drowsiness
In adolescents: sinusoidal theta activity can occur over the anterior head regions during drowsiness.
In adults, theta components can occur diffusely or over the posterior head regions during drowsiness.
Single transient theta waveforms or mixed alpha-theta waves can be present over the temporal regions in older adults.

15. Theta Activity
The term theta was coined by Gray Walter in 1944 when it was believed that this rhythm was related to the function of the thalamus.
Occurs as a normal rhythm during drowsiness
In young children between age 4 months  8 years: predominance over the fronto-central regions during drowsiness
In adolescents: sinusoidal theta activity can occur over the anterior head regions during drowsiness.
In adults, theta components can occur diffusely or over the posterior head regions during drowsiness.
Single transient theta waveforms or mixed alpha-theta waves can be present over the temporal regions in older adults.

16. Theta Activity
The term theta was coined by Gray Walter in 1944 when it was believed that this rhythm was related to the function of the thalamus.
Occurs as a normal rhythm during drowsiness
In young children between age 4 months  8 years: predominance over the fronto-central regions during drowsiness
In adolescents: sinusoidal theta activity can occur over the anterior head regions during drowsiness.
In adults: theta components can occur diffusely or over the posterior head regions during drowsiness.
Single transient theta waveforms or mixed alpha-theta waves can be present over the temporal regions in older adults.

17. Temporal Slowing Of The Elderly
Occur chiefly over the age of 60 years
Confined to the temporal regions and are usually maximal anteriorly
Occur more frequently on the left side
Do not disrupt background activity
Usually have a rounded morphologic appearance
Voltage is usually less than microvolts
Attenuated by mental alerting and eye opening and increased by drowsiness and hyperventilation
Occur sporadically as single or double waves but not in longer rhythmic trains
Present for only a small portion of the tracing (up to 1%) of the recording time when the patient is in a fully alert state

18. EEG of Drowsiness (Stage I Sleep)
In adults, most sensitive signs of drowsiness is the disappearance of eye blinks and the onset of slow eye movements
Slowing, dropout or attenuation of the background
Occurrence of theta activity over the posterior regions

19. Drowsy

20. Drowsy

21. Drowsy

22. EEG of Drowsiness Alpha Activity Mu activity Beta activity
may be occurrence or persistence over the temporal regions after a disappearance of the occipital alpha
may be asymmetric
Mu activity
may persist
Beta activity
over the fronto-central regions
may become more prominent during drowsiness
20-30 Hz; occasional bursts of Hz activity

23. Other Activities During Stage I Sleep
Vertex Sharp Transients
Positive Occipital Sharp Transients of Sleep (POSTs)

24. Vertex Sharp Transient - V-Wave
In young adults, the V-waves may have sharp or spiky appearance and attain rather high voltages
During the earlier stages of sleep these may occur in an asymmetric fashion
Should be careful not to mistake V-waves for abnormal epileptiform activity
Sometimes trains or short repetitive series, clusters, or bursts of V-waves may occur in quick succession
In older adults the V-waves may have a more blunted appearance

25. Vertex Sharp Transients

26. Post Occipital Sharp Transients of Sleep (POSTs)
Sharp-contoured, mornophasic, surface-positive transients
Occurring singly or in trains of 4-5 Hz over the occipital head regions
May have a similar appearance to the lambda waves during the awake record but are of higher voltage and longer duration
Usually bilaterally synchronous but may be asymmetric over the two sides
Predominantly seen during drowsiness and light sleep

27. POSTs

28. Stage II Sleep
Sleep Spindles
K Complex

29. Sleep Spindles In adults, a frequency of 13-14 Hz
occur in a symmetric and synchronous fashion over the two hemispheres
Usually these occur at intervals between 5-15 seconds,
Spindle trains ranging from seconds in duration
More prolonged trains or continuous spindle activity may be seen in some patients on medication, particularly benzodiazepams

30. Sleep Spindles

31. Sleep Spindles

32. Sleep Spindles

33. K-Complex A broad diphasic or polyphasic waveform (>500 msec)
Frequently associated with spindle activity
K-complexes can occur in response to afferent stimulation and may be linked to an arousal response

34. K-Complex

35. Hyperventilation Often produces little change in the EEG in adult
If there is a change, usually consists of generalized slowing.
either gradual or abrupt onset in theta or delta range
may continue as series of rhythmic slow waves or consist of repeated bursts of slow waves at irregular intervals
Degree of response depends on the age, the vigor of hyperventilation, blood sugar levels, and posture

36. Intermittent Slow During HV

37. Intermittent Rhythmic Slow During HV

38. Persistence slowing following cessation of hyperventilation: Check if patient is still continuing to hyperventilate or if patient is hypoglycemic

39. Hyperventilation
The findings accepted as unequivocal evidence of abnormality:
epileptiform discharges
clear-cut focal or lateralized slowing or asymmetry of activity
Contraindications:
significant cardiac or cerebrovascular disease, or respiratory dysfunction.

40. Photic Stimulation
Flash rate eliciting maximum driving response increases in rough parallel with age (Niedermeyer, 1982)
Driving response may normally have a notched appearance resembling a spike-wave discharge.
It can be distinguished from spike-waves by its time-locked appearance with the flash rate and its failure to persist after the stimulation stops.
Asymmetries of photic driving probably have less clinical value and can only be interpreted in association with other significant asymmetries

44. Photoparoxysmal Response
Photic stimulation may elicit posterior dominant or generalized epileptiform discharges in patients suspected of having photosensitive seizure disorders
Photo-paroxysmal response:
complex waveform
repeat at a frequency which is independent of the flash rate
field extends beyond the usual posteriorly-situated photic driving region and may be frontally dominant
Time-locked with stimulus or not time-locked / self-sustained

45. Photoparoxysmal Response

46. Photomyoclonic Response

47. Physiologic Activities That Can Be Confused With Epileptiform Activities
Vertex transients of light sleep
Hypnagogic hypersynchrony
Positive occipital sharp transients of sleep (POST)
Mu rhythm
Lambda waves
Breach rhythms

48. Benign Variants Of Unknown Clinical Significance
Benign epileptiform transients of sleep (small sharp spikes)
6- and 14-Hz positive spikes
Wicket spikes
Psychomotor variants (rhythmic mid-temporal theta discharge of drowsiness)
Subclinical rhythmic EEG discharge of adults
Phantom spike and wave