This presentation looks at abnormal EEG patterns with examples for each. Benign variants, artifacts and focal ictal patterns are not part of this presentation.

1. Abnormal Focal EEG
patterns
Dr Pramod Krishnan, MD (Int Med), DM Neurology (NIMHANS)
Fellowship in Epilepsy (SCTIMST) (LMU, Munich)
World Sleep Federation Certified Sleep Medicine Specialist.
Consultant Neurologist and Epileptologist
Head of the Department of Neurology,
Manipal Hospital, Bengaluru.

2. Introduction
• Focal epileptiform discharges
• Focal slowing
• Amplitude asymmetry
• Focal ictal patterns are not part of this presentation.

3. Abnormal EEG
• An EEG is considered abnormal if it shows:
1. Epileptiform activity
2. Slow waves
3. Amplitude asymmetries.
4. Certain patterns resembling normal activity but deviating from it
in frequency, reactivity, distribution or other features.
• Usually the abnormal patterns are intermittent, and in certain head
regions.

4. Focal epileptiform activity
• Spikes or sharp waves that
appear at one or a few
neighbouring electrodes.
• Usually asymmetric, initial
half (baseline to peak) has
shorter duration than the
second half (peak to
baseline).
• Non-epileptiform transients
are approximately symmetric.

5. Spike morphology
• May be followed by a slow
wave, which has a longer
duration than the predominant
background waveforms.
• Have more than one phase
(usually 2-3) and the duration
of each phase differs from the
durations of the phases of the
surrounding background
waveforms.

6. Referential montage showing frequent spikes with maximum amplitude at T6, followed by a slow
wave which disturbs the background. Also, in O2.

7. Spikes
• Abrupt increase in the amplitude of sharply contoured waveforms
that are part of the ongoing background activity should not be
mistaken for spike.
• Epileptiform activity often interrupts the ongoing background
beyond the duration of the spike/ sharp wave due to the aftergoing
slow wave.
• Epileptiform activity should be detected at more than one
electrode site. Spikes in a single electrode may be non-cerebral in
origin.

8. Bipolar longitudinal montage showing sharply contoured (spiky) alpha waves. This does not
disturb the background and there is no aftercoming slow waves. This morphology is therefore
benign.

9. Spikes
• In practice, epileptiform activity may not show all these points;
non-epileptiform transients may show some of these patterns.
• Spikes are intermittent and repeat without any variation in shape.
• The distribution of spikes is limited to a few electrodes over an
area (irritative zone), but, may be hemispheric or generalised.
• The minimum area of cortical surface involved in the generation
of an interictal spike visible at one scalp electrode site is
approximately 6 cm2, but most epileptiform spikes arise from a
larger cortical area of atleast 10-20 cm2.

10. MD YOUSUF ALI(4820545)
EEG Longitudinal bipolar montage of a 56 year old gentleman, showing frequent right temporal
spikes, along with slowing. MRI brain showed right MTS.

11. Bipolar longitudinal montage showing instrumental phase reversal across C3, and also frequent
spikes at P3 in a 36 year old lady with left parietal gliosis and seizures.

12. Referential montage in the same patient showing frequent spikes with maximum amplitude at
P3 and C3.

13. Bipolar longitudinal montage showing instrumental phase reversal across F3, in a 28 year lady
with nocturnal seizures and normal MRI brain.

14. Spikes, mirror foci
• More than one focus may occur in the same patient and the shape
of the discharges from each focus may be different.
• Pairs of foci are often located in corresponding parts of the
hemispheres, especially in the temporal areas (mirror foci).
• One focus may fire only when the other one occurs, suggesting
that it is triggered by the other, or both foci may occur
independent of each other.

15. YESHRAJ SINGH-8 Y (4759113) BIFRONTAL
SPIKES
EEG Longitudinal bipolar montage of a 45 year old gentleman showing frequent bifrontal
epileptiform discharges. He had frequent nocturnal seizures and was on VPA.

16. INDIRA ANAVATTI(2222355)
Focal Spikes
EEG Longitudinal bipolar montage of a 55 year old lady, left centro-parietal spikes. She presented
with first episode of seizures in life. MRI brain showed left parietal gliosis.

17. LALLU SINGH(2764693)
Bilateral Multifocal spikes.
EEG Longitudinal bipolar montage of a 63 year old gentleman with super refractory status
epilepticus due to autoimmune encephalitis, showing frequent multifocal spikes.

18. Referential montage showing frontally dominant generalised polyspike and wave discharges in a
22 year old patient with JME. Few focal frontal spikes are seen.

19. Bipolar Referential montage in the same patient showing frontally dominant generalised
polyspike and wave discharges. Few leading focal spikes (frontal) are seen.

20. PLEDs and BIPLEDs
• The EEG pattern shows complexes which consist of a di or
multiphasic spike and may include a slow wave.
• Complexes usually last for only a fraction of a second, and recur
every 1-2 seconds, separated by low amplitude slow waves or no
detectable activity at regular gain.
• They appear in a wide distribution on one side of the head.
• The background in regions not showing PLEDs is often abnormal.

21. EEG Longitudinal bipolar montage of a 42 year old lady with HSV encephalitis showing left
fronto-temporal periodic lateralising epileptiform discharges (PLEDs).

22. REKHA MADHURI(4840651)
PLEDs
Longitudinal bipolar montage of a 39 yr old lady with altered sensorium and seizures. Frequent
right anterior temporal periodic lateralising epileptiform discharges (PLEDs) are seen.

23. • Although PLEDs are often considered to be continuous and
invariant, they may transiently attenuate or disappear during state
changes, particularly during arousal.
• The natural history of PLEDs consists of a gradual simplification
of the morphology of complexes with increasingly longer
repetition intervals and decreasing amplitude.
• This may occur over a period of days, weeks or even years.
• PLEDs may also occur independently over both hemispheres a
pattern referred to as BIPLEDs.
PLEDs and BIPLEDs

24. REKHA MADHURI(4840651)
PLEDs
Lack of evolution in frequency or amplitude differentiates this from an ictal pattern. The same
PLEDs pattern continued for several days and gradually subsided.

25. BCECTS
• Spikes are frequent, with pronounced activation in sleep.
• The spikes are sometimes grouped together in short runs with a
repetition rate of 1.5-3 Hz. They are located predominantly in the
central or temporal areas, and may demonstrate a slightly shifting
distribution.
• They may be unilateral, or bilateral with varying degrees of
interhemispheric synchrony.

26. Referential montage showing frequent spikes with frontal positivity and negativity in the central,
parietal and temporal channels, consistent with BCECTS.

27. Tangential dipole in BCECTS.
• In BCECTS, the positivity projects anteriorly, and negativity
appears more posteriorly.
• The anterior positivity is typically lower in amplitude than the
posterior negativity.
• This would produce two instrumental phase reversals in a linear
bipolar chain (true phase reversal).

28. Referential montage of a 5 year old boy showing frequent spikes with frontal positivity and
negativity in the central, parietal and temporal channels, consistent with BCECTS.

29. Referential montage of the same child showing marked activation of the spikes in sleep, which is
highly characteristic of BCECTS.

30. Idiopathic occipital epilepsy
• Prominent occipital spikes that may occur in a semi rhythmic
pattern of 1-3 Hz.
• The discharges attenuate or disappear with eye opening and are
not activated by photic stimulation.
• Background is usually normal.

31. Longitudinal bipolar montage of a 9 year old child with idiopathic occipital epilepsy showing
frequent occipital spikes in sleep on both sides.

32. Longitudinal bipolar montage of a 7 year old child with idiopathic occipital epilepsy showing
frequent occipital spikes and posterior head region spikes in sleep on both sides.

33. Landau Kleffner syndrome
• Moderate to high amplitude spikes or spike and wave complexes
that are localised to the temporal head regions.
• The discharges may be strictly unilateral, but more often show
either shifting lateralisation or appear in a bilateral independent
fashion. They often become more abundant with the onset of
sleep.

34. Referential montage of a 5 year old child with acquired language regression and rare seizures
showing frequent multifocal spikes, predominantly temporal and central.

35. Longitudinal bipolar montage in the same child showing marked activation of spikes in sleep,
producing continuous spike and wave discharges in sleep.

36. Secondary bilateral synchrony
• In this, a focal epileptogenic focus is thought to either trigger a
mirror image cortical area by transcallosal transmission, or
through a thalamic area that in turn produces a bilaterally
synchronous epileptiform pattern.
• Less than 2.5Hz when rhythmic.
• Morphological variability from complex to complex.
• Single site of phase reversal in transverse bipolar montages.
• May be consistently asymmetrical.
• Consistently focal epileptiform spikes or slowing may be present.

37. RAKSA KHATUN(4486610)
Left posterior quadrant epileptogenic focus with secondary bilateral synchrony.
EEG Longitudinal bipolar montage of a 27 year old gentleman, with epilepsy due to left posterior
quadrant lesion- ?FCD showing secondary bilateral synchrony.

38. Focal slow activity
• This indicates focal cerebral pathology of the underlying brain
region.
• Slowing may be intermittent or persistent, with more persistent or
consistently slower activity generally indicating more severe
underlying focal cerebral dysfunction.
• A variety of etiologies can cause slowing.

39. BRINDALEKSHMI(1887634)
Bifrontal intermittent slow activity.
Longitudinal bipolar montage of a 26 year old girl with one episode of unwitnessed loss of
consciousness, showing intermittent frontal delta range slowing (more prominent on the left).

40. BRINDALEKSHMI(1887634)
Bifrontal intermittent slow activity.
Longitudinal bipolar montage: The theta- delta range slowing over the left frontal region
correlated with the presence of a left frontal meningioma on MRI brain.

41. LAKSHMI NARAYAN (1801362)
LEFT FT SLOWING
EEG Longitudinal bipolar montage of a 58 year old gentleman showing left frontal delta range
slowing suggestive of focal electrophysiological dysfunction.

42. EEG Longitudinal bipolar montage of a 50 year old gentleman showing bilateral frontal
intermittent rhythmic delta activity (FIRDA). Patient had metabolic encephalopathy.

43. EEG Longitudinal bipolar montage bilateral frontal intermittent rhythmic delta activity (FIRDA).
This patient also had metabolic encephalopathy.

44. EEG referential montage of a 36 year old gentleman with left MTLE-HS showing left temporal
intermittent rhythmic delta activity (TIRDA). TIRDA is considered equivalent to spikes.

45. EEG Longitudinal bipolar montage of a 27 year old lady with left MTLE-HS showing left anterior
temporal spikes and temporal intermittent polymorphic delta activity (TIPDA).

46. JAYARAJ M(1563225)
Left hemispheric slowing.
EEG Longitudinal bipolar montage of a 46 year old gentleman with left MCA territory infarct and
seizures. Left hemispheric delta range slow activity is seen.

47. YASHAR RAHMAN – 7Y(4867086)
FOCAL SLOW WAVES OVER BILATERAL OCCIPITAL.
EEG Longitudinal bipolar montage of a 7 year old girl, showing frequent delta range slow waves
over both occipital regions.

48. SAI APARNA(2272689)
OIRDA
EEG Longitudinal bipolar montage of a 13 year old girl showing occipital intermittent rhythmic
delta activity (OIRDA).

49. SAI APARNA(2272689)OIRDA
EEG Longitudinal bipolar montage of a 8 year girl showing OIRDA. Rest of the EEG was
normal.

50. Amplitude asymmetry
• More than 50% amplitude difference between the right and left
side is considered abnormal.
• The right hemisphere amplitude is generally slightly more than the
left side.
• It is often difficult to decide which side is abnormal in case of
amplitude asymmetry.
• Presence of slowing, spikes/ sharp waves help in deciding the
abnormal side.

51. AUBEELUCK KAUSHIK(4816334)
Right hemispheric slow activity and right anterior temporal spikes.
EEG Longitudinal bipolar montage of a 44 year old gentleman showing amplitude asymmetry
favouring the right, with right hemispheric slowing. He had right MCA territory infarct with
seizures.

52. EEG Longitudinal bipolar montage of 28 year lady showing breech rhythm over the right
hemispheric region.

53. EEG Longitudinal bipolar montage in the same patient showing breech rhythm, causing faster
and higher amplitude discharges over the right hemispheric region.

54. THANK YOU