artifacts & normal variants in eeg
TRANSCRIPT
ARTIFACTS & NORMAL VARIANTS
DR.SHAHANAZ AHAMED.MPAEDIATRIC NEUROLOGIST
GMC,TVM
ArtifactsArtifacts are pen deflections not
caused by cerebral activity.They may be due to 1)physiological activity originating
from the patient2) Interference from power lines or
other electrical sources3)Malfunction of the recording system
which includes recording electrodes, amplifiers, pen motors & paper drive.
Artifacts from patient Blinking & other eye movements Muscle artifacts Movement artifacts Heart beat artifacts Pulse wave artifact. Perspiration artifact oropharyngeal artifacts
Interference artifacts
60 HzArtifact cardiac pacemaker artifact mobile phone artifact
Artifacts from Equipment
Electrode poping Paper stop artifact Impedence artifact
ALPHA
EPILEPTIFORM TRANSIENTS
SPIKES
SHARP WAVES
SLOW SHARP WAVES – not always epileptiform
Blinking & other eye movements These are picked up by frontal
electrodes Vertical eye movements cause
potentials in electrode pairs in paramedical positions-Fp1-F3, Fp2-f4 etc
Lateral eye movements Deflections in transverse chains=Fp1-F7, Fp2-F8, F7-F3
F8-F4 etc
Blinking or eye closure causes large down going deflections,
Eye opening produces upward deflections.
EYE BLINK
EYE OPENING
EYE CLOSURE
Lateral eye movements Lateral eye movements may be
preceded by a single sharp muscle potential which may resemble a cerebral spike & in combination with eye movements form a spike & wave artifact
LATERAL EYE MOVEMENTS
Eye movement artifacts Eye movement artifacts can be
identified by their frontal distribution, their symmetry on the 2 sides & their characteristic shape.
Slow repetitive rhythmic eye movements may closely resemble B/L synchronous frontal slow waves like FIRDA
They are stopped by asking the patient to place his fingertip on the eyes,tapping cotton balls over the eyes etc
Muscle artifacts Muscle activity causes very brief
potentials which usually recur. If they recur they resemble cerebral
discharges except that most cerebral spikes are of much longer duration than muscle potentials.
Moreover epileptiform activity usually has a aftercoming slow wave associated with a spike.
MUSCLE ARTIFACT
MUSCLE ARTIFACT They are usually recorded predominantly
from frontal/ temporal electrodes but can occur in any electrodes
They can be eliminated by asking the patient to relax, drop the jaw or open the mouth slightly/change position.
Repetititive movements such as chewing, blinking & tremor may give rise to fast & slow artifacts which resemble cerebral discharges
Movement artifacts Movement of head /body or other rhythmic
movements such as chewing & sucking can lead to irregular high amplitude wave forms which can be easily recognized.
They do not have consistency & characteristics of cerebral discharges
They can occur in instances such as restless/ confused patients,infants & children , patients having seizures,tremors/other movt disorders
Heart beat artifacts Mainly in recording with wide
interelectrode distances& in subjects with short necks.
Small artifacts reflect mainly the Rwave of ECG. Large artifacts may reflect additional components of ecg
EKG ARTIFACT
Heart beat artifacts If necessary heart beat can be
eliminated by using a balanced noncephalic reference.
They can be identified by the equal intervals & rate corresponding to heart rate.
Simultaneous recording of ecg can be done in doubtful cases
Pulse wave artifact Periodic waves of smooth
/triangular shape may be picked up by electrode on or near a scalp artery
.More likely to occur in frontal/temporal areas. It can be eliminated by reapplying electrode
Perspiration artifact Sweating causes very slow drifting
of electrodes lasting several seconds .
Usually common in frontal/temporal electrodes & occurs in several channels at the same time
Glossokinetic & oropharyngeal artifacts. They produce intermittent
/repetitive slow waves in a wide distribuition often with a maximum in the middle of the head
They occur in speaking, chewing, swallowing ,sucking,coughing,hiccoughs.
Palatal myoclonus causes rhythmical artifacts at the rates of 100-200 /mt.
Dental spike like artifacts are produced by dental fillings /dissimilar metals rubbing against each other
GLOSSOKINETIC ARTIFACT
Interference artifacts
Artifacts due to electrical interference from power lines & equipments.
60 Hz is the frequency. Appears in all channels.
Other types of interference include signals from nearby TV stations,
radiopaging,Mobile phones telephone ringing,cardiac pacemakers,
CARDIAC PACEMAKER
mobile phone artifact Another confusing artifact is
mobile phone artifact which may look like a epileptiform discharge but can be distinguished by the lack of aftercoming slow wave & positivity & raggedness of the artifact wave
Artifacts from Equipment
They are distinguished in that They differ radically from
previously recorded activity, Do not blend with other recorded
activity but seem to be superimposed on it &
Appear only in channels connected to the faulty electrode.
Electrode poping Electrode poping is due to sudden
changes in electrode contact causing spike artifact.
PAPER STOP ARTIFACT
A1/A2 ARTIFACT
NORMAL VARIANTS
Hyperventilation changes Hypnagogic hypersynchrony Mu rhythm Lambda Positive occipital sharp transients of sleep(POSTS} Posterior slow waves Ctenoids/14 & 6hz positive spikes 6 hz spike wave Rhythmic midtemporal theta-Psychomotor variant Small sharp spikes Frontal arousal rhythm
HYPERVENTILLATION
HV produces bursts of 2—3 HZ frontally dominant delta activity.
Normal background in between. Can be admixed with sharper
components.
“ V” WAVESVERTEX WAVES
Bilaterally synchronous Maximum amplitude at vertex Extend to central, frontal & parietal May appear in sequence Shifting asymmetries may occur Higher amplitude in youth Principal component is sharply contoured
electronegative wave. Occurs in light sleep
Hypnagogic hypersynchrony
Prominent bursts of rhythmic high amplitude delta slowing maximum in the parasagittal area are a normal finding in the drowsy state in the first few years of life
HYPNOGOGIC HYPERSYNCHRONY Appears during transition from
wakefulness to drowsiness Seen in age group 5 months to 10 yrs. Bursts of 3-5 Hz , moderate to high
amplitude activity. Lasts for 1.5 to 3 secs. Can be mistaken for spike –wave
activity,when intermixed with faster frequency components.
Mu rhythm It is a 7-11 hz saw tooth shaped
rhythm seen in 15 % recordings It can be unilateral/bilateral &
seen in central area It is attenuated by touch/
movement
Lambda Lambda is low amplitude (<
20micv) sharp transient bi/triphasic activity which is surface positive
It is seen in the waking state due to visual exploration & attenuated by closing the eyes
POSTS( Positive Occipital Sharp Transients of sleep)
Occur in NREM sleep ;Esp. stages - 2 & 3 Occur in occipital region. Monophasic, sharp contoured,
electropositive waves. Similar to Lamda waves; higher in
amplitude & longer in duration. Occur singly or in trains of 4-6 Hz. Usually bisynchronous, but may be
asymmetric
POSTS( Positive Occipital Sharp Transients of sleep)
Differentiating points from spikes:
Predominant phase is surface positive
Monophasic Occur in trains of 4-5 Hz. Bilateral occurrence of POSTS
Posterior slow waves They are 1-4 hz slow waves seen
spreading from occipital to tremporal to parietal which are prominent in first decade.
Ctenoids/14 & 6hz positive spikes Seen b/w 5-15 yrs in the posterior
head region as a surface positive comb shaped rhythm
6 hz spike wave It is a bisynchronous low
amplitude 5-7hz spike wave rhythm most prevalent in drowsiness & light sleep.
It is of gradual onset & offset & maximal in midcentral/parietal electrodes
Rhythmic midtemporal theta-Psychomotor variant
Theta rhythm of 5-7 hz seen in temporal region unilateral/biateral lasting 5-10 seconds.
Commonly seen in adoloscents. Attenuated in stage 2 sleep & alerting
Small sharp spikes Medium amplitude spikes of short
duration seen in anterior & midtemporal areas associated with drowsiness & light sleep
Frontal arousal rhythm
They are bursts of rhythmic notched theta with superadded beta in midfrontal regions on arising from sleep,seen in 2-14 yrs lasts upto 20 sec.
Can resemble ictal pattern. But there is no evolution