normal eeg

1. Approaching t he EEG
Dr rzgar hamed abdwl

2. Approaching t he EEG: An
Introduction to Visual Analysis
EEG activity is described in the following terms:
1- frequency
2- Amplitude
3- distribution or location
4- Symmetry
5- Synchrony
6- reactivity
7- Morphology
8- Rhythmicity
9- regulation

3. 1- Frequency
• Frequency is defined as the number of complete waveforms that occur per
second, expressed as cycles per second (cps) or Hz.
• we can determine the frequency by counting the number of waves between the
1-s vertical bars.

5. 2- Amplitude
• Amplitude is the size of the waveforms, measured in microvolts (µV).
• Amplitude is often measured “peak to peak,” i.e., from the highest to the lowest
point of the sinusoidal wave. However, this can be misleading if there is a drift of
the waveform due to slow oscillations (e.g., alpha waves superimposed on delta
waves, as in Fig. 2-1B).
• Amplitude can be reported as a numerical range (e.g., 20 to 40 µV) or in
descriptive terms as low (0 to 25 µV), moderate (25 to 75 µV), or high (>75 µV)
amplitude.
• Some pathological conditions are associated with enormous amplitudes of
hundreds of microvolts, such as hypsarrhythmia, a chaotic pattern seen in severe
infantile epilepsies.
• Amplitude asymmetry ……. Confirmed by montage change

6. 3- Distribution or Location

7. FIGURE 1- 13. Commonly used montages for 10–20
electrode placement.
A:Standard 10–20 electrode positions.
B:Ipsilateral ear electrode referential montage. In this
and subsequent figures, the exploring electrode
(input 1) is at the origin of the arrowand the
reference electrode (input 2) is at the arrow tip;
numbers represent the channel at which that
derivation is recorded.
C:Vertex (Cz) referential montage.
D:Longitudinal bipolar (“double banana”) montage .
E:Transverse bipolar montage. F:“Hatband” bipolar
montage.

8. 4- Symmetry
• Symmetry refers to a
comparison of the amplitudes
and frequencies on either side
of the midline.
• Activity may be asymmetrical
due to differences in frequency
components between
hemispheres, with similar
overall amplitudes, or due to a
significant difference in
amplitude (defined as >50%
difference between sides) but
similar frequencies, or both.

9. 5- Synchrony
• Synchrony is the simultaneous occurrence of similar waveforms over each
hemisphere.
• Normal EEG activity is usually synchronous over the left and right hemispheres,
both for relatively continuous background activities and more sporadic
waveforms (e.g., “K complexes,” the large amplitude biphasic slow waves seen in
stage 2 sleep, should have simultaneous onset over both hemispheres).
• Loss of synchrony can occur when communication between the hemispheres is
impaired by damage to the corpus callosum or in severe disorders of cortical
function.
• Lack of synchrony can also indicate that the location where the waveform
appears first may be closer to the origin of that activity, and thus help with the
localization of abnormal or epileptiform activity.

10. 6- Reactivity
• Reactivity is a change in EEG activity in response to sensory stimulation or a
sudden change in the internal state. Examples:
>> When the eyes open, the alpha activity in the occipital leads disappears, and
when the eyes close again, it reappears.
>> slowing of the background frequencies during hyperventilation
>> blocking of the mu rhythm (an alpha frequency activity over the centrotemporal
region often unmasked by eye opening) by moving, or even thinking about
moving, the opposite arm.
>> changes in EEG activity in comatose patients induced by painful stimulation and
occipital waveforms induced by a flashing strobe light known as the posterior
photic driving response.
• Reactivity is usually a normal response, but its absence is not always abnormal
unless it is asymmetrical. For example, absence of the photic driving response
over one hemisphere suggests a lesion interrupting the optic tract on that side,
which is known as Bancaud’s phenomenon

14. 7- Morphology
• Morphology is a physical
description of :
>> the waveform, which
includes its shape (e.g.,
sinusoidal, saw-toothed, cone-
shaped, spindle-shaped, and
epileptiform)
>> the number of phases (e.g.,
biphasic, triphasic, polyphasic)
>> the polarity of those
phases.

17. 8- Rhythmicity
• is the continuous repetition of similar waveforms and frequencies over time.
• The rhythmic repetition of waves creates the background activity upon which
sporadic waveforms are superimposed.
• When such superimposed waves occur at regular intervals, they are called
periodic activity(if the period is irregular, the term pseudoperiodicis used).
• Waveforms that occur without any regular period can be called aperiodicor
arrhythmic. Such activity is sometimes also called polymorphic
9- Regulation
• is the degree to which amplitude and frequency change over time.
• A rhythm can be described as “well-regulated” if the amplitude varies smoothly
in a waxing and waning pattern and the average frequency does not vary more
than ±0.5 Hz during a 2-sepoch (without an obvious change of state).
• The gradual variation in amplitude and frequency is also known as
“modulation,”

19. 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

20. Normal Alpha Rhythm

21. Normal Alpha Rhythm

22. 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

23. Normal Alpha Reactivity

24. Normal Alpha Reactivity
Eyes Closed

26. Beta Activity
 Frequency of over 13 Hz; if >30-35 Hz  gamma
activity or exceedingly fast activity by Gibbs.
 Average voltage is 10-20 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.

27. Beta Activity
 Frequency of over 13 Hz; if >30-35 Hz  gamma activity or exceedingly
fast activity by Gibbs.
 Average voltage is 10-20 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.

28. Generalized Beta Activity

29. Beta Activity
 Frequency of over 13 Hz; if >30-35 Hz  gamma
activity or exceedingly fast activity by Gibbs.
 Average voltage is 10-20 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.

30. 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.

31. 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.

32. 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.

33. 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 60-70 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

34. Normal awake EEG:
1- review technical aspect of recording:
 Low frequency filter (LFF) is set to 1 Hz
 High frequency filter (HFF) is set to 70
 Notch filter is off
 Sensetivity = 7 microvolt/mm
 Time base = 30 mm/sec
2- review montage: Anterior posterior bipolar montage is the standard one.
3- interpreting the alpha rhythm
-Reactivity to eye closure and opening
- Take amplitude in different regions then
Say the range.
- Calculate amplitude symmetry between
Left and right.
Which is called PDR

38. They are
generated by
inspection of the
dots on the
ceiling

39. They found
in the
occipital
region

40. lambda waves are blocked
by eye closure and replaced
by alpha rhythm

41. Resembles
letter mu

42. Normal Wakefulness: The Posterior Dominant Rhythm
• The most important EEG pattern of wakefulness is the posterior dominant
rhythm, or PDR.
• This activity is primarily located at the occipital poles but can be prominent more
anteriorly, particularly as the patient becomes drowsy or with a sudden arousal
from sleep.
• It is rhythmic and usually sinusoidal in character.
• it is often the highest amplitude activity observed in normal awake subjects.
• The frequency of the PDR gradually increases during development from infancy
through childhood and reaches a plateau in the early teen years.
• In adults and children older than 9 years of age, this activity should be in the
alpha frequency range (8 to 12.5 Hz).
• Thus, PDR frequencies slower than 9 Hz in young adults represent an abnormality
• > 13 Hz or faster are occasionally seen, and are not considered abnormal.

43. • The PDR frequency should ideally be measured at two symmetrical occipital derivations
(e.g., T5–O1 and T6–O2) within the same 1-s period, and should be counted “by hand,”
not relying on digital frequency measurements
• If the subject is drowsy, the technician should stimulate alertness by asking the patient
to perform a mental alerting task (count from one to ten, name the Great Lakes, etc.)
with eyes closed.
• Other faster and slower frequencies may be represented in the waking EEG.
• Faster beta frequencies are sometimes seen, particularly over frontal and central
regions, but they should be low in amplitude.
• Beta activity of more than 25 µV is considered abnormal (the benzodiazepines or
barbiturates)
• Theta activity is frequently present, often in the context of a transition to drowsiness,
and is almost never abnormal.
• Slow wave (delta) activity is generally considered abnormal in awake adults.
• A certain amount of delta activity is acceptable in younger children through the teenage
years, particularly in the occipital region, where individual delta slow waves with
overriding alpha are frequently observed in normal children.

47. PDR Frequency Changes in Childhood

50. PDR Amplitude, Synchrony, and Symmetry
• The PDR amplitude : 15 - 45 µV
• Higher in children and lower in elderly
• The waveforms should be synchronous between hemispheres and symmetrical
in amplitude
• amplitude differences of up to 20% are not uncommon in normal patients.
• Most often, such differences in amplitude are due to variations in skull thickness,
which is usually greater on the left side causing right side amplitudes to be
slightly higher

51. Slow Alpha Versus Slow Alpha Variant
• If the PDR is slower than 9 Hz in young adults, this is generally considered
abnormal but nonspecific and suggests a mild diffuse disturbance of cortical
function, as seen in metabolic encephalopathies and primary neuronal disorders.
• However, patients with normal alpha frequencies will sometimes have brief
episodes (several seconds) in which the PDR is suddenly reduced by half and
increased in amplitude.
• This is “slow alpha variant,” a subharmonic of the normal alpha frequency, which
is thought to be of no clinical significance.

53. Mu Rhythm
• Mu is an alpha frequency activity with arciform (arc-shaped) appearance located over
the central regions that is not blocked by eye opening.
• It may be unilateral or bilateral, may be synchronous or asynchronous, and may or may not be
present at any given time.
• It is likely generated in the sensorimotor cortex and can be suppressed by moving a
contralateral extremity (or sometimes by thinking of moving a contralateral extremity)

55. Lambda Waves
• Lambda waves are sharply contoured surface-positive waves observed in the
occipital leads during wakefulness with eyes open, which correlate with visual
fixation to a target after a saccadic eye movement.
• They usually have a prominent initial positive component followed by a
downstroke that may go past the baseline giving it a biphasic appearance
• Since they occur with eyes open, when the alpha PDR is suppressed, they stand
out clearly from the background. They have no clinical significance.

58. EEG in Sleep

61. Drowsiness and Sleep ( stage 1
sleep): characterized by
1- Slowing and anterior spread of
alpha activity, followed by
Dropout of the posterior
dominant rhythm
2- Slow lateral eye movements
seen in the lateral eye and frontal
leads
3- Vertex sharp waves
4- Occurrence of theta activity
over the posterior regions
5- Positive Occipital Sharp
Transients of Sleep (POSTs)

62. Best appreciated in
occipital region

63. 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

65. 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

69. Stage II Sleep
 Sleep Spindles
 K Complex

70. 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 0.5-1.5 seconds in duration
 More prolonged trains or continuous spindle activity may be
seen in some patients on medication, particularly
benzodiazepams

73. 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

75. Slow-Wave Sleep (stage 3 and stage 4 sleep)

78. Arousal Patterns
• Arousals from light drowsiness (stage 1) are often marked only by return of the
alpha PDR.
• From stage 2 or deeper NREM sleep, there is often a high-amplitude biphasic or
triphasic transient lasting 0.5 s or longer, reminiscent of an exaggerated K
complex often followed by a burst of diffuse alpha activity and intermixed
muscle artifact
• In children, arousals tend to occur more slowly with the diffuse alpha activity
gradually slowing into the theta and delta range before being replaced by the
PDR.

80. Mental Alerting
• Alerting is performed by the technician when the patient appears to be drowsy and the
background alpha frequency is slower than expected.
• To enhances the PDR alpha frequency.
• This procedure is thus designed to determine whether a slower-than-expected PDR is
due to drowsiness (i.e., state-dependent) or is pathological.
• Vertex waves followed by an arousal.
• Sharply contoured repetitive vertex waves in light sleep, reversing at the Cz electrode,
are followed by a high-amplitude slow transient and then a diffuse alpha frequency
pattern indicative of arousal.
• Calibration bar is 1 s, 50 µV.
• Counting slowly from one to ten, serial subtractions, spelling “world” forward and
backward, or simply calling the patient’s name may each be appropriate for different
patients.
• The verbal answer will create speech related artifacts involving temporal and tongue
muscles, but between words or after completion, the patient should be at maximal
alertness.

81. Hyperventilation
• The underlying mechanisms for the EEG changes remain controversial.
• The effects on EEG may be subtle or quite dramatic, particularly in children.
• After 30 to 60 s of hyperventilation, there can be slowing of the PDR into the
theta range with diffuse spread of rhythmic polymorphic theta activity.
• With continued hyperventilation, the amplitude of the rhythmic slowing may
increase dramatically to several hundred µV, and waves may slow into the
delta frequency range (known as “buildup,” hyperventilation hypersynchrony, or
hyperventilation-induced high-amplitude rhythmic slowing (HIHARS).
• This effect persists for tens of seconds after hyperventilation ceases
• evidence of abnormality:
• epileptiform discharges
• clear-cut focal or lateralized slowing or asymmetry of activity

82. • The movements associated with breathing may cause artifacts ( the patient leans
forward and back during deep breaths, and increased temporalis muscle tone)
• The technician will encourage the patient to continue overbreathing for up
to 3 min
Contraindications:
• COPD
• chronic lung or heart disease
• Pregnancy
• recent stroke
• subarachnoid hemorrhage
• sickle cell disease
• moyamoya disease

84. Sleep
• Sleep is activating for nearly all forms of epilepsy, The most striking example is
electrical status epilepticus of sleep, in which spike-and-wave discharges occur
during more than 85% of slow-wave sleep.
• Children with benign childhood epilepsy with centrotemporal spikes may have a
normal EEG in wakefulness, but characteristic centrotemporal spikeand-wave
discharges appear once the patient is asleep.
• Seizures are also more likely during sleep or upon awakening, particularly in
some epilepsy syndromes like juvenile myoclonic epilepsy.
• Chloral hydrate was often the drug of choice due to its relatively minor effect on
EEG patterns compared to other sedatives (benzodiazepines, etc.).
• Chloral hydrate was fairly well tolerated in most patients, but rare cases of
respiratory depression and even mortality have occurred with accidental
overdoses
• the use of behavioral techniques including sleep deprivation prior to the study
and timing studies to correspond to nap times has largely replaced the use of
sedatives, with nearly the same success rate.

85. Eye Opening and Closure
• We have already mentioned the
ability of eye opening to block the
PDR and the return of the PDR with a
slightly faster frequency in the first
second after eye closure (called
“squeak”).
• Eye opening and closure should be
performed several times during the
course of the study to assess
reactivity of the PDR.
• In children who have childhood
epilepsy with occipital paroxysms
(CEOP), the occipital spikes are
sometimes suppressed by eye
opening.

86. • perform this twice, once with eyes open and once with eyes closed
• the absence of a response is only abnormal if it is unilateral.
• It is important to distinguish photic driving from the photomyoclonic response
(stimulated blinking or facial twitching in response to each flash, seen in the frontal
electrodes, which is usually not epileptic) and the photoelectric response (a rare finding
in which the light itself triggers an electrical impulse from its interaction with the frontal
electrodes).

88. FIGURE 2- 14.Photic driving and
photomyoclonic response.
A:Posterior photic driving response seen at
10-Hz, 14-Hz, and 18-Hz stimulation rates.
Note that although the posterior dominant
rhythm is close to 10 Hz, driving is still
clearly evident as synchronization of the
alpha activity with the light stimulus
(marked by the bottom trace).
B:Low-frequency photic stimulation (3 Hz)
evokes a repetitive frontal slow transient
associated with flash-induced blinking,
termed a photomyoclonic response. No
posterior photic driving is observed.
Calibration bar is 1 s, 20 µV.

89. Photoparoxysmal Response
• Photic stimulation can induce epileptiform activity, termed a
photoparoxysmal response (PPR), particularly associated with idiopathic
generalized epilepsies (IGE).
• The PPR consists of flash-induced spike-and-wave or polyspike-and-wave
discharges tracking each flash, initially in the occipital leads but often
spreading anteriorly as photic stimulation continues .
• Such discharges can evolve into a clinical seizure, known as a photoconvulsive
response, with spike discharges that outlast the photic stimulation and clinical
features ranging from loss of awareness to convulsions.
• Patients with juvenile myoclonic epilepsy and the progressive myoclonic
epilepsies appear to be particularly susceptible to photic stimulation.

91. Painful Stimulation
• In unresponsive or comatose patients, sternal rub or forceful pinching of the fingernail or
toenail may cause a partial arousal that alters the EEG background.
• The change can be subtle or dramatic and varies from patient to patient.
• Reactivity to pain is usually a favorable prognostic sign suggesting that the cortex can respond
to sensory stimuli.