This document discusses various EEG artifacts and benign variants that can appear on an EEG reading. It describes mechanical artifacts such as those caused by electromagnetic interference, electrostatic interference, radio frequency interference, and instrument artifacts. It also discusses biological artifacts including those from eye movements, cardiac/pulse activity, electromyography, movement, skin potentials, and tongue movements. Some benign rhythmic activities and epileptiform variants are also outlined such as frontal arousal rhythm, midline theta rhythm, and 14-16Hz positive bursts. Proper identification and differentiation of artifacts and variants is important for accurate EEG interpretation.

1. EEG- Artefacts
and Benign
Variants
Dr.Roopchand.PS
Senior Resident Academic
Dept. of Neurology, TDMC, Alappuzha

2. Artefacts:
• Recorded signals of non cerebral origin.
• Poses great problem in EEG reading.
• Recognition and elimination are therefore
important.
• Mechanical
o Electromagnetic, electrostatic, radio frequency, mains born, external
electrical interference.
o Instrument artefact.
o IV drip artefact
o Respirator artefact.
• Biological
o Eye movement, cardiac/pulse, respiratory, electromyographic,
movement, cutaneous, glossokinetic, Msic

3. Electro magnetic
interference:
• Due to AC current.
• Induces fluctuating magnetic fields to EEG leads.
• Opposite fields may cancel out.
• Proper earth connection of apparatus.
• Patient bed may be connected to earth socket.

4. Electrostatic Interference:
• Due to capacitance property of objects.
• Patient or electrode may pick up capacitance
potentials from sources in their vicinity.
• Reduced by moving the patient from the source.
• Proper earth connection.

5. Radio frequency
Interference:
• Signal between 120-400Hz
• Especially diathermy equipment's.
• Radio frequency filters may be used.

6. • Mains born interference: due to fluctuating power
supply.
o Stabilized power supply can avoid this artefact.
• External electrical interference: fluorescent light ,
ac, refrigerator…

7. Instrument Artefact:
• Electrode artefact: due to change in resistance,
capacitance and inductive reactance of two
electrodes compared to others.
o Individual electrode impedance should be less than 5kΩ.
• Bizarre potentials are seen
o Confined to two adjacent channels in bipolar chain .
o Confined to one channel or one hemisphere in reference recording.
• Waves are markedly different from background
activity.
• Appear as spike like potentials.

8. • Ground electrode artefact: due to defective
grounding.
o Produce 60Hz AC artefact.
• Machine fault: loss of main, blown fuse, selective
malfunctioning of one system.
• Intravenous drip artefact: may produce
stereotyped spike like potentials at fixed intervals.
o Infusion pumps produce stereotyped brief spike
like transients.
• Respirator artefact: single high voltage or multiple
high/low voltage transients of 2 to 40 Hz.

10. Biological Artefact:

11. Eye movement Artefacts:
• From eye ball or muscles around orbit.
• Eye ball act as a dipole.
• Eye movements produce AC fields.
• Detectable in electrodes near globe
o Fp1, Fp2, F7, F8
• Negative deflection when Fp1 and Fp2 are positive.
• Eye ball movement monitoring.
• Blinking produces upward movement of eye ball.
o Fp1 and Fp2 are positive.
• Repetitive blinking can mimic FRIDA(Frontal
Intermittent Rhythmic Delta Activity) or triphasic
wave.

12. • More frequent blinking can simulate theta activity.
• Horizontal or vertical nystagmus can produce
aretfacts simulating theta activity.
• Asymmetric eyeball artifacts can be seen in
unilateral ophthalmoplegia and enucleation.
• Can be abolished by keeping the eye closed or
simultaneously recording eye ball movements.
• IPS may produce photomyoclonic repose from
orbicularis occuli and frontalis.

14. Cardiac and respiratory
artefacts:
• Normally electrical field of heart extends up to base
of the skull.
• In short necked persons can extend up to vertex.
• Normally can extend up to ear.
• Cardiac artefacts are mainly due to QRS
complexes.
• Positive in A1 and negative in A2.
• Recognized by characteristic from and regularity.
• Interfere in diagnosing electro cerebral silence.
• Respiration may cause change is electrical axis of
the heart.
o Produce fluctuation in amplitude of waves.

16. Pulse artefact:
• Electrode near or overlying a small scalp artery.
• Systolic pulse alter the impedance.
• Waves are periodic
• Smooth and sharply contoured
• Time locked to ECG by 200msec delay in peak.

18. Electromyographic
artefact:
• Brief single or multiple myogenic potentials.
• Located in the temporal, frontal and occipital
areas.
• Frontal epileptiform discharges can mimic them.
• Avoiding jaw clenching and frowning will abolish
the waves.
• Essential tremor and Parkinson's tremor produce 4-
6Hz sinusoidal artefacts.
• Hemi facial spasm can also produce EMG artefacts.

20. Movement Aretacts:
• Due to combination of instrument and biologic
factors.
• Related to observed activity of the subject.
• Difficult to differentiate from discharges during
GTCS.
• Significant reduction possible by proper electrode
placement and use of self retaining electrodes.

21. Cutaneous Artefacts:
• Perspiration artefact: perspiration causes slow shift
of electrical baseline due to change in impedance.
• Sweat gland produces slow changing electrical
potentials recorded by electrodes.
• Produce slow wave forms of more than 2 sec.
• Perspiration artefact + background slowing :
hypoglycemia.
• Reduced by lowering the room temparature and
wiping the brow with alcohol.

22. • Galvanic skin response: represent sympathetic skin
response produced by sweat gland and changes in
skin conductance in response to sensory or psychic
stimuli.
• Slow waves of 0.5 to 1 sec, lasting for 1.5 to 2 sec
with two to three prominent phases.
• Can be confirmed by simultaneous recording of
sympathetic response of palm.

23. Glossokinetic Artefact:
• Tongue has a DC potential.
o Tip negative compared to base
• Tongue movements produce artifacts
o Bursts of diffuse delta like activity, accompanied by muscle artefact.
• Artefacts confirmed by asking the patient to
pronounce lah lah lah.
• Sucking by infants can also produce such artefacts.
• Hiccpus, dental fillings.

24. Benign EEG variants:

25. Rhythmic activities:
• Rhythmic temporal Theta bursts of Drowsiness or
psychomotor variant pattern.
• Trains of rhythmic theta waves of 5-7Hz.
• Flat top, sharp contour or notched appearance.
• Temporal location, maximal in the mid temporal
electrodes.
• Simulates psychomotor seizure discharge.
• Seen in0.5-2% adolescent and adults in wakeful or
drowsy state.,

27. Sub clinical rhythmic
theta discharge in adults:
• Rhythmic sharp theta waves of 5-7hz.
• Widespread with highest amplitude at parietal and
post. Temporal regions.
• Lasts for 20 sec to few minutes.
• 50% adults spontaneously, > 50yrs and prominent
during HV.
• In 50% : single monophasic sharp or slow wave
followed one to several seconds later by another
sharp wave and progress to discharge at a shorter
interval reaching up to 7Hz.
• Seen in relaxed and drowsy states.

28. • Midline theta rhythm: 5-7Hz smooth sinusoidal ,
arciform waves.
• Central and vertex leads.
• Present during wakefulness and sleep.
• Benign.
• Frontal Arousal rhythm: seen in children with
minimum brain dysfunction.
• Seen during arousal from sleep.
• Trains of 7-10HZ waves
• Frontal location.
• Lasting up to 20 sec.
• Disappears when child is fully awake.

29. Benign Epileptiform
Variants:
• 14-16Hz positive bursts: seen in drowsiness and light
sleep.
• Rhythmic trains of arc shaped waveforms with
alternating positive spiky components and a
negative smooth rounded wave form.
• Sharp phase 0,5 to 1 sec
• Usually14Hz.
• Maximum amplitude in the posterior temporal
region.
• Appears by 3-4yrs, peaks in adolescence, decrease
in old age.
• Seen in
o Head ache , vertigo, emotional instability, thalamic and hypothalamic
epilepsy etc..

30. • Small sharp spikes: benign epileptiform transients of
sleep, benign sporadic sleep spikes.
• During drowsiness and light sleep.
• Monophasic or diphasic spikes, 15µV, <15msec,
abrupt ascending and steep descending.
• May have single after coming slow wave.
• Seen best in temporal and ear leads.
• Mimic temporal epileptiform discharges.
o Does not distorts background
o Not followed by rhythmic slow wave activity.
o Disappear with deeper levels of slow wave sleep.

32. • 6Hz spike and wave: 5-7Hz bursts lasting for 1-2secs.
• Called phantom spikes: brief, small amplitude.
• Appear mainly in relaxed wakeful state, drowsiness
o Disappears in deep sleep.
• Diffuse bilateral, prominent in anterior and posterior
locations.
• Wicket spikes: intermittent trains in clusters of
arciform/single spike.
• Wicket like appearance.
• 6-11Hz, temporal region B/L during drowsiness and
light sleep.
• No after coming slow component and back ground
slowing