This document provides a comprehensive overview of EEG interpretation, detailing normal and abnormal waveforms, key EEG features, artifacts, and diagnostic uses in neurological disorders. It emphasizes the importance of understanding normal EEG patterns to avoid misdiagnosis, especially in cases of epilepsy. The document also discusses various EEG frequency bands and their clinical significance in diagnosing different epileptic conditions and other neurological events.

1. Interpretation of EEG
Dr G. Sree Ranga Lakshmi
Professor & HOD of Neurology
Osmania Medical College

2.  Clinicians face patients who
develop spells of altered responsiveness/remain
drowsy/have recurring episodes of neurological
dysfunction.
 Can be sorted out by thorough history taking and
clinical examination followed by some investigations.
 EEG can provide some clues in such situations.

3.  Hans Berger ---- recorded first human EEG, 1924
 American EEG Society---- 1947
 First EEG machine in India----Dr. Anil D . Desai in Mumbai and
housed at King Edward hospital
 In the 1980s EEG became digitalized and recorded, allowing
ambulatory procedures.
 By the 1990s the first-generation commercial digital EEG
systems were introduced

4.  A clear understanding of a normal EEG is mandatory
prior to studying abnormalities.
 Recognizing variations of a normal tracing ---
challenging.
 Interpreting rhythmic or sharply contoured normal
discharges as abnormal and epileptiform can lead to
the erroneous diagnosis of epilepsy and years of
unnecessary treatment.
 “Over reading” EEG is more common than “under
reading”, and can lead to more patient distress

5. Normal Awake EEG
 There are many features of a normal awake EEG that
should be sought in every recording.
 Not all features will be seen in each EEG.

6. Alpha rhythm with a 10 Hz background.
Blocking with eye opening . Immediately after eye closure, the alpha rhythm is 11
Hz (alpha squeak).

7.  The morphology of the alpha rhythm is usually sinusoidal
and regular.
 It may appear peaked at the top or bottom of the
waveform if there are superimposed beta frequencies;
 this is referred to as apiculate alpha activity

8. The Apiculate Alpha
It is differentiated from
sharp waves by its
association with similar
shaped waveforms (i.e.
“does not disrupt the
background”),
location,
disappearance during
sleep, and
absence of an after going
slow wave.

9.  Amplitude of the alpha activity varies during the tracing
and between the hemispheres.
 Often the amplitude on the right side is higher---
Occipital bone thickness
 Amplitude asymmetries are best assessed in an ear
reference montage.
 Waxing and waning of the amplitude can also occur
when two frequencies (i.e. 10 and 11 Hz) occur together.
This is referred to as “beating” of the alpha activity.

10.  Paradoxical alpha rhythm:
 In some patients alpha activity decrease with eye
closure but appears with eye opening.
 No pathological significance.

11. Beta activity
 Greater than 13 Hz. ( between 14 -25 Hz).
 It commonly occurs in the frontal and central regions in
awake individuals.
 Benzodiazepines, barbiturates, and other sedatives cause
an increase in the amplitude of beta activity,
 Persists in light sleep and REM sleep,
 With age beta activity tends to increase. its amplitude
also increases
 As individuals become very old, the beta activity may
decrease. This change is also associated with cerebral
atrophy;

12. Theta activity 4 – 7 Hz.
 Can be seen in wakefulness in the frontocentral area in
young individuals.
 This activity is present in states of heightened attention or
vigilance
 In sleep----- this activity disappears.
High amplitude theta activity can also occur with
hyperventilation
 Like the alpha rhythm, temporal theta activity is reactive
to eye opening and stimulation.
 Intermittent temporal theta activity mixed with alpha is
normal after 60 yrs
 it is abnormal when it is persistent and of high amplitude

13. Delta activity-- less than 4 Hz.
 Seen in a normal sleep EEG stage III
 They are less common than theta frequencies in the normal awake
adult EEG.
 They occur only in the elderly in the same distribution as temporal
theta activity.
 When present these delta waves should be of the same amplitude
as the alpha rhythm, occur as single wave, and occupy less than 1%
of the record
If delta waves are more frequent or of higher amplitude, they represent
an abnormality.

14. Low voltage EEG
 In some individuals a clear alpha rhythm cannot be
identified.
 Instead their background activity is a low amplitude
activity with beta, alpha, and theta frequencies.
 The amplitude of this activity is usually less than 20 µV
 A low voltage EEG is seen more often in older individuals
than children and is not considered an abnormality
unless a previous EEG in the same patient showed clear
alpha rhythm.
 When all activity is less than 10 µV it may be abnormal

15. Artifacts in EEG

16.  EEG records activities arising from brain and
other sites also
 The extra cerebral activity is called artifact.

17. Artifacts in EEG
 May obscure / be confused with EEG activity.
 Physiological : EKG ,EMG, Tongue movement ,
 Eye movement
Other movements
Sweat artifact
Non Physiological : instrumental /electrode/environment

18. Eye movement artifact
the cornea is positively charged compared to the retina
(Negative)
whenever the cornea moves closer to one electrode, a
positive deflection occurs in that electrode.
When the eyes move upward, (eye closure) a downward
deflection is noted in the frontal leads; (FP1 & FP2)
when the eyes move laterally, out of phase deflections are
noted in F7/F8 electrodes.
Eye movement artifact ----- helpful in identifying REM sleep
and wakefulness.

20.  Asymmetry of eye movement artifact : Placement of
frontal leads which is not symmetric is a common cause
of such asymmetry.
 Enucleation of one eye will result in absence of artifact
on that side
 A skull defect over one frontal region will cause the eye
movement artifact on that side to have higher
amplitude.

21. Unilateral
eye blinks in
a patient
with
enucleation
of right eye.

22. Breach rhythm after a
right frontotemporal
in the right
midtemporal region
that are spiky (box)

23. EKG artifact
 EKG artifact – infants /short neck
 Referential montage.
 Poorly formed QRS complex
 Maximum amplitude T3/T4
 Neck extension can eliminate

25. Pulse artifact
 Electrode placed on an artery
slow wave,
usually confined o single electrode
follows QRS after 100ms

27. Muscle artifact
 High amplitude and frequency. Frontal/temporal leads
 Obscure EEG patterns
 High frequency filters eliminate them

28. Muscle artifact over the bitemporal leads

29. Glossokinetic artifact
 occurs when there is tongue movement.
 The tip of the tongue is negatively charged compared
to its base.
 Thus when the tongue moves, the artifact is best seen
along the temporal chain
 It consists of a burst of delta activity accompanied by
EMG artifact

30. Glossokinetic artifact

31. Sweat artifact
 Perspiration produces artifact that consists of very slow
potentials, often lasting several seconds.
 Sweating produces a very slow discharge (less than 1 Hz)
that can often be reduced with low frequency filters.
 Cooling the room may help in reducing sweat artifact.

32. Sweat artifact

34. Repetitive body movements
produce changing electrical fields
produce rhythmic depolarization mimicking
electrographic seizures

35. Repetitive artifact from scratching simulating an electrographic seizure
.

36. Environmental artifact
 Nonbiologic artifact that can be very challenging to
isolate.
 They are often seen in hostile recording environments
such as intensive care units or the operating room.
 Common examples include
60-Hz line artifact,
drip artifact from intravenous bags,
respirator artifact

37. Electrode artifact
 also known as electrode pop,
 Nonbiologic artifact
 Typically confined to the single electrode that is at fault.
 At times it can produce a high amplitude negative
phase reversal between two channels in a bipolar
montage, resembling a spike

38. Electrode artifact in T5 electrode manifesting as negative (short
arrow) and positive (long arrow) phase reversals.

39. Electrode pop
 It should be differentiated from a spike by
its very restricted field (confined to a singe electrode),
its association with other bizarre-looking discharges in the same
channels,
and disappearance with reapplication of the electrode.
These artifacts most commonly result from
poorly applied electrodes
can also occur due to a broken electrode wire,
drying of electrode gel, or
change in the scalp-lead interface.
When an electrode artifact is seen, the technologist should reapply the
electrode and if it does not disappear, replace it

40. Electrode “pop” artifact at P7 simulates rhythmic seizure activity

41. Electrode “pop” artifact at F3

42. Periodic single electrode artifact mimicking periodic lateralized
epileptiform discharges

44. Features of EEG s/o Artifact

45.  A normal adult EEG is often considered the easiest type
of EEG to interpret.
 The simplicity can be deceptive, however.
 The wide range of normal, rhythmic and sharply
contoured normal variants, and artifacts that resemble
cerebral activity make interpretation of these studies
much more challenging

46.  When normal patterns are interpreted as epileptiform, patients are
inappropriately subjected to years of antiepileptic drug therapy.
 It should be remembered that it is not the absence of normal
features but the presence of abnormal ones that makes an EEG
abnormal.
 A normal EEG report does not prevent treatment, however an
abnormal EEG report often mandates it!

47. The role of EEG in epilepsy
Differential diagnosis of paroxysmal neurological events(spells)
Distinction between a focal and generalized seizure disorder
(classification)
Diagnosis of epileptic syndromes and predicting the prognosis
Recognition of photosensitivity (triggering factors)
Identification of the refractory epilepsies
Detection of non convulsive status

48. Additional indications
 Rapidly progressive dementia ----CJD
 Other degenerative dementias
 Declaration of brain death----- ancillary
 SSPE
 Autoimmune encephalitis--- Delta brushes in NMDA
receptor encephalitis

49. When interpreting EEG
 Conservative approach is generally recommended.
 When interpreting an EEG, if a waveform in question is
equivocally epileptiform, a nonepileptiform
interpretation is preferred

50. Careful analysis of a waveform's
 location,
 frequency, and
 morphology in relationship to the background electrocerebral
activity and
 to accurately discern normal variations /benign variants / abnormal
waveforms

51. Abnormal EEG

52. Abnormal Adult EEG
 Focal or Generalized
 Subdivided into Nonepileptiform,
Interictal epileptiform, and
Ictal patterns.

53. Nonepileptiform EEG abnormalities
 Include
focal slow activity,
regional or generalized bisynchronous slow activity,
generalized asynchronous slow activity, and
focal or generalized suppression of the background
activity.

54. Interictal epileptiform discharges
 are characterized by the presence of
spikes and sharp waves,
with or without after-going slow waves.
 Associated with epilepsy usually
 These discharges may also be seen up to 6.6% of
healthy adult volunteers without epilepsy

55. International Federation of Societies of
Electroencephalography and Clinical
Neurophysiology
 Epileptiform Discharges
 These are transient potentials that are
clearly distinguishable from background activity,
have a pointed peak, and
are described as a spike or a sharp wave.

56. Criteria for Inter Ictal Spikes and
Sharp Waves
Paroxysmal and clearly distinguished from background activity
An abrupt change in polarity occurring over several milliseconds
Duration less than 200 ms:
70–200 ms for a sharp wave and
20–70 ms for a spike
Asymmetric contour with steep upslope, with down stroke usually
being less steep and deepening below the baseline
Has a physiologic field (seen in ≥ 2 nearby electrode sites) with
voltage gradient
Are typically negative in polarity
Aftergoing slow wave
Appears in a location with an associated area of abnormality (e.g.,
focal slowing)
Persists during slow wave sleep (in contrast to benign variants)

57.  Dr. Shah,
 “If it looks like you would sit on it and it would hurt,
it’s probably a spike”.

58.  In the 1930s, Gibbs, Lennox, and Jasper defined
the interictal patterns reporting
generalized spike-and-wave activity and
focal epileptiform discharges (EDs) as markers of
epilepsy

59.  The difference between spikes and sharp waves is based only on the
duration.
 Have the same clinical significance
 On the EEG, the slopes of interictal epileptiform discharges are often
asymmetric with the initial negative component typically steeper,
followed by a slower positive component
 The asymmetric morphology could be used to differentiate an EEG
artifact or a physiologic waveform with a sharply contoured
morphology.
 The field of interictal epileptiform discharges is typically detected by
at least a few electrodes.

60.  The convention is: “First input more negative: pen goes
up. First input more positive: pen goes down
 In most areas of mathematics and science, values are
graphed above the x axis when they are positive and
below the x axis when they are negative.
 Alas, in electroencephalography, the opposite is true;
this “upside-down”

66. Focal Interictal Epileptiform Discharges
 Focal epileptiform discharges occur in a focal
distribution, commonly in one lobe or region.
 Temporal lobe epilepsy is the most common focal
epilepsy in adolescents and adults, and therefore,
temporal spikes or sharp waves are the most commonly
observed interictal epileptiform discharges

67. Temporal spikes with
an electrographic
maximum at the right
anterior temporal
electrode (T8)
followed by low-
voltage slow waves.

68. Generalized Interictal Epileptiform Discharges
 Present in a bilateral symmetric fashion.
 Generalized spike and slow waves consist of bilaterally
synchronous spikes followed by a slow wave of high
amplitude
 Typically, the diffuse spike and slow wave often has
frontal predominance and may occur in rhythmic runs at
various frequencies
 This electrographic pattern is consistent with the
diagnosis of generalized epilepsy.

70. 3 HZ
Spike &
wave

79. Rhythm Frequency Physiologic (normal) Pathologic
Infra-slow (DC-shift) 0–0.5 Hz
Sweat and perspiration
artifact
Interictal and ictal
rhythm with seizures
Delta 0.5–< 4 Hz
Stage 2 and slow wave
sleep
Buildup with
hyperventilation
Severe degree of diffuse
or focal
encephalopathy; ictal
pattern with neocortical
onset temporal lobe
seizures
Theta 4– 7 Hz
Drowsiness
Benign variants (RTTD,
wicket waves)
Positive occipital sharp
transients (sleep)
Mild to moderate
degree of diffuse or
focal encephalopathy;
ictal pattern with mesial
temporal lobe seizures;
theta coma

80. Rhythm Frequency Physiologic
(normal)
Pathologic
Alpha 8–13 Hz
Posterior dominant
rhythm
Mu rhythm
Ictal rhythm in
temporal and
extratemporal
seizures; alpha
coma
Beta 14–30 Hz
Drowsiness and light
sleep
Medication effect
(benzodiazepines)
Breach rhythm;
ictal rhythm
associated with
tonic seizures; as
generalized
paroxysmal fast
activity in LGS
Gamma 30–80 Hz
Voluntary motor
movement
Task related
(speech and motor)
Ictal rhythm with
seizures (mostly
seen during
intracranial
recording)

81. Rhythm Frequency Physiologic
(normal)
Pathologic
High-
frequency
oscillations
(HFOs)
> 80 Hz
Cognitive
processing
Localize
epileptogenic
zone when
accompanying
spike and sharp
waves
Ripples 80–250 Hz
Episodic memory
consolidation
Ictal onset zone
Fast Ripples 250–500 Hz
Acquisition of
sensory
information
Brain tumor??

83.  EEG can be viewed on computer monitors as a series of
separate pages ---usually 10 s/page
 Display includes ---voltage on the vertical axis and time
on the horizontal axis.
 Sensitivity (microvolts per millimeter) may be increased or
decreased to change the display height of waveforms.

84.  Display speed --------typically 30 mm , may be modified.
 Fast display speeds may enhance the ability to identify
waveforms that appear generalized on routine
recording by identifying a “lead-in” suggestive of a focal
mechanism.
 Slow display speeds may identify slowly evolving
rhythmic discharges to help identify seizures when
gradual or subtle changes are present, by “condensing”
the EEG.

85.  The different states during which the EEG was then
recorded
-------- Awake, Drowsy, Sleep, Eyes closed, etc. should be
noted.
Comment on the background activity or the dominant
(usually posterior) in Hertz (Hz) or cycles / second.
All frequencies in turn will be described regarding
their frequency, location, persistence (continuous /
intermittent), amplitude in microvolts,
symmetry (comparing side-to-side and anterior vs
posterior)

86.  State which forms of stimulation or arousal were performed to
evaluate EEG reactivity, particularly in cases of encephalopathy or
coma.
 Forms of stimuli include --- touch, sound, eye opening, nasal tickle,
mouth or tracheal suctioning, or sternal pressure.
 comment on the EKG, notably if there is a significant abnormality,
avoiding interpretative statements such as ischemia.
 When epileptiform discharges occur in the EEG, note whether they
are congruent with the EKG complexes.

87. EEG Report

88. PITFALLS OF PATTERN RECOGNITION
 The EEG is a unique measure of electrical brain function
and is widely used in patients with seizures.
 The interpretation of EEG is associated with a poor inter
oberserver reliability (Williams et al. 1985) which makes it
even more important to follow a systematic approach to
the classification of EEG abnormalities

90.  Normal burst of theta in the EEG of an adult during
drowsiness
 Note the prominent intermixed beta activity
 and electrode artifact at F7 and F3 that combine to
make the appearance “spikier.”

91. Normal “suspicious” EEG
 Many normal variants and variations of normal EEG have
a predilection for the temporal lobe and mimic
epileptiform discharges.
 The high prevalence of temporal lobe epilepsy and the
propensity for normal variants to occupy the temporal
lobe may result in an undesired bias, leading to
misidentification of normal waveforms

93. American Epilepsy Society 2015 E Slide 93

94. Test EEGs

95. Q1

96. A 1
 Normal EEG

97. Q 2

98. A 2
 Eye opening and closing , aipha reactivity

99. Q 3 . 8yrs child EEG during HV

100. A 3
 Normal HV response , generalized slowing
 A normal HV response is characterized by diffuse high-
amplitude theta or delta background slowing (buildup)
 exaggerated during fasting (relative hypoglycemia).
 The EEG usually returns to baseline within 2 min after HV
ends.
 Focal changes on the EEG during HV that do not resolve
afterward are considered pathological

101. American Epilepsy Society 2015 E Slide 101
Q 5
Q 5

102.  photic driving response,
 A series of stimulus trains of increasing frequency is
presented for 4–10 s, with a pause of 4 s between each
train, up to a frequency of 30 Hz.
 Normal findings with IPS are either no change in the
background or a symmetrical photic “driving”
response
 The photic driving response represents repetitive visual
evoked potentials produced in response to the photic
flash.
 The response occurs in a time-locked fashion 100 ms
after the stimulus.

103. Q 6

104. A 6
 Photo myogenic response
 A photomyogenic response consists of repetitive
contractions of the frontalis muscle synchronized to the
light flash at a delay of 50–60 ms.

105. Q 4
).

106. A 4 photo paroxysmal response
The recording shows a burst of
generalized, polyspike wave discharges
occurring during and outlasting
photic stimulation,
defining a photo paroxysmal response.
The clinical and electrographic findings
are consistent with juvenile myoclonic
epilepsy

107. American Epilepsy Society 2015 E Slide 107

108.  Rhythmic temporal theta of drowsiness

109. American Epilepsy Society 2015 E Slide 109

110.  6 hertz (“phantom”) spike and wave discharges-- normal

111. Question 7
A 5-year-old girl
has recent onset of numerous brief spells of
inattention or staring noted at school and at home.
Occasionally, she has limited eye fluttering with spells,
but no other automatisms are noted.
Her developmental history is normal

112. Q 7

113. A 6
 Absence seizures 3 HZ SW

115. Q- 8
 12-year-old girl
 has a generalized tonic-clonic seizure while watching
cartoons on TV the morning after a late bedtime and
sleep over at a friend's house.
 She notes habitual and frequent jerks of the upper
extremities, clustered in the morning during breakfast.
 Her mother also has morning myoclonus.

116. Q- 7

117. A 8
 Juvenile myoclonic epilepsy

118. Juvenile myoclonic epilepsy
 Background EEG activity is normal.
 There are inter ictal bursts of fronto central dominant
GSW and GPWS.
 The frequency of these bursts tends to be more irregular
than the typical 3-Hz spike-and-wave bursts and varies
between 3 and 5 Hz
 Photosensitivity is found in about 30% of males and 40%
of females

119. Juvenile myoclonic epilepsy

120. Q 9
 23 yrs old male presented with episodes of
confusion , unresponsiveness and involuntary
movements involving left UL

122. A 9
 Right temporal spikes
 Complex Partial Seizures or Focal Seizures with
Altered Awareness

123. Q 10 An 8-month-old male infant with congenital abnormalities and spells of
bilateral limb flexion
.

124. A 10
 Hypsarrhythmia
 Irregular back ground.
 Spikes , sharp and slow waves of high amplitude
 West syndrome

125. Q 11 A 7-month-old girl with
generalized seizures after
anoxic encephalopathy
(.

126. A 11 a ) Left temporal,
(b) right parietal,
(c) bifrontal regions
IEDs that occur in several different locations with no clear
relationship to each other are termed multifocal,
independent spikes
 Multifocal independent spikes usually occur in subjects
with static or progressive encephalopathies.
 Most patients have frequent seizures that are medically
intractable, and most are mentally retarded

127. Q 13 A 73-year-
old woman with
confusion and left
hemiparesis

128. A 13
 Continuous, unreactive, arrhythmic delta activity is
present across right frontal-temporal-central regions.
 Focal arrhythmic delta activity indicates a structural
lesion of the area, corresponding to the region
of hemorrhagic stroke

129. Q
15

130. A 15
 Focal slowing over the right temporal region as
the result of a right temporal brain tumor in a 35-
year-old man.

131. Q 16 elderly
man with a
metabolic
encephalopat
hy.

132. A 16
 FIRDA pattern with a slightly slower theta EEG background

133. Q 17
Elderly man
witth hepattic
encephalopa
thy

134. A 17
 Generalized triphasic wave pattern and slowing
 Will have a lag anterior to posterior in wave 2
peak

135. Q
 CHILD WITH ALTERED SENSORIUM

137.  Generalized delta activity in encephalopathic child

138. Ictal rhythms

139. Seizure Activity
 Not just epileptiform, actual seizures occurring during
record
 Seizure should be like a wave, should have a
build up and let down

140. Pre-Lorazepam Post-Lorazepam
American Epilepsy Society 2015 E Slide 140

141.  Generalized non-convulsive status epilepticus in confused patient
showing continuous generalized anterior predominant sharp activity
(labeled “Pre-Lorazepam).
 Resolution of generalized sharp activity shown post- lorazepam
infusion

142. Part 1

143. Part 2

144. American Epilepsy Society 2015 E Slide 144

145.  Left temporal onset seizure discharge with onset showing rhythmic
delta localized to the left anterior temporal region, with phase-
reversal noted over the F7-T7 and F9-T9 derivations
 Discharge evolves into rhythmic left temporal theta towards end of
the epoch shown.

146. EEG is Great !

147. THANK YOU

149. The clinical features differentiating
PNEA and ES

151. Epileptic Syndromes with Interictal Epileptifform Discharges
Activated by Sleep
1. Continuous spike wave during slow wave sleep (CSWS)
2. Benign Rolandic epilepsy
3. JME
4. Epilepsy with GTCS on awakening
5. ADNFLE